摘要:

5 PHOTOCHEMISTRY By A. C. Day (The Dyson Perrins Laboratory Oxford) and E. J. Forbes (The Chemistry Department The University Birmingham) THISyear’s Report follows the general pattern of the last,’ except that stringent restrictions on space have necessitated some reduction in the ground covered. In addition to literature surveys for 1967’.2 and a new edition of a well- known reviews have appeared on photocycloadditions photo-oxida-tion,’ and the photochemistry of stilbenes,6 alicyclic en one^,^ aryl and vinyl esters and amides,8 and aromatic iodine compo~nds.~ Of two useful general reviews,’O?’’ one” is concerned with photochemical processes involving triplet states. E.s.r.I2 and other physical studiesI3 of the triplet state have been surveyed and isomerisation has been discussed as a route for radiation- less transitions.l4 Three books on luminescence have recently been published. ’’ The plenary lectures delivered at the 2nd IUPAC symposium on photochemis- try are now in print.I6 The Woodward-Hoffmann rules for concerted cyclic processes have been summarised,17 and a compendium of pertinent fact has appeared.’* The A. C. Day Ann. Reports 1967,64 B 161. B. Capon M. J. Perkins and C. W. Rees ‘Organic Reaction Mechanisms 1967 Interscience London 1968 p. 370. A Schonberg G. 0. Schenck and 0.-A. Neumuller ‘Preparative Organic Photochemistry’ 2nd edn. Springer-Verlag Berlin 1968. L. L. Muller and J. Hamer ‘1,ZCycloaddition Reactions’ Interscience New York 1967 p. 111; D. R Arnold Adv. Photochem.1968,6,301. ’ C. S. Foote Accounts Chem Res. 1968 1 104; T. Matsuura J. Synthetic Org. Chem (Japan) 1968,26 217; K. Gollnick Adv. Photochem 1968,6 1. M. Scholz F. Dietz and M. Miihlstadt Z. Chem. 1967,7 329. ’ P. E. Eaton Accounts Chem Res. 1968 1 50. * D. BelluS and P. HrdloviE Chem Rev. 1967,67 599. R K.Sharma and N. Kharasch Angew. Chem. 1968 80 69 (Angew. Chem Internut. Edn 1968,7 36). lo P. J. Wagner and G. S Hammond Adv. Photochem. 1968,5 21. S. T.Reid Forschr. Arzneim. 1968 11,48. l2 C. Thomson Quart. Rev. 1968,22,45. l3 M. A. El-Sayed Accounts Chem. Res. 1968 1 8. D. Phillips J. Lemaire C. S. Burton and W. A. Noyes jun.,Adv. Photochem. 1968,5 329. l5 ‘Luminescence in Chemistry’ ed. E. J. Bowen Van Nostrand London 1968; ‘Fluorescence.Theory Instrumentation and Practice’ ed. G. G. Guilbault Arnold London 1967; C. A Parker ‘Photoluminescence of Solutions’ Elsevier Amsterdam 1968. l6 2nd International Symposium on Photochemistry (Enschede The Netherlands July 1967) (a) Pure Appl. Chem. 1968 16 1 ;(b) ‘Organic Photochemistry 2’ Butterworthq London 1968. R Hoffmann and R B. Woodward Accounts Chem Rex 1968,1 17. G. B. Gill Quart. Rev. 1968 22 338. G 188 A. C.Day and E. J. Forbes rules in their classical form have now been transcended by a deeper and more satisfying generalisation which is independent of molecular symmetry in the usual sense and it is believed of any specific bonding approximation. The new postulates are a direct consequence of the nodal properties of atomic orbitals and unify the whole range of processes involving a smooth cyclic redistribution of electrons.l9 General.-A qualitative correlation diagram for the disrotatory cyclisation of butadiene to cyclobutene shows a smooth correlation between the first excited states of the two molecules and the process is thus symmetry-allowed.20 However the first excited state of cyclobutene lies at much higher energy than that of butadiene and photocyclisation of the latter would therefore be prohibitively endothermic if electronically excited cyclobutene were a necessary intermediate in the process.’l The problem is common to many photo-induced reactions which though symmetry-allowed lead to products of higher excitation energy (e.g. less extensively conjugated molecules) than the starting material.Fortunately valence-bond calculations for the butadiene- cyclobutene system reveal a crossing of the potential surfaces of the first and second excited states and van der Lugt and OosterhoffZ’ have therefore suggested that in general ‘if a conrotatory (disrotatory) process is unfavour- able in the ground state it implies the presence of a high potential barrier. . . . it follows that there will be another potential surface ofthe same symmetry which has a well not far above this barrier. The photo-induced reaction may profit from this well to bring about ring closure or opening by a conrotatory (disrotatory) process opposite to the reaction in the ground state.’ In effect the excited molecule becomes trapped in an energy well from which it undergoes a radiationless transition to the ground-state.Several instances of intramolecular energy transfer between nonconjugated chromophores have been reported,23 and also a case where owing to the rigid perpendicular orientation of the chromophores energy transfer is not ob- served.24 “on-vertical’ energy transfer in the photosensitised cis-trans-isomerisation of stilbene” has been re-investigated by Bylina.26 The efficiency of triplet- triplet transfer from a range of sensitisers closely follows the Franck-Condon probability as represented by the intensity distribution in the So -* T spectra of the stilbenes and Bylina therefore concludes that energy transfer leads l9 R B. Woodward and R Hoffmann Chemical Society Symposium on Orbital Symmetry Cambridge January 1969; H.E. Zimmerman Angew. Chem. Internat. Edn. 1969,S 1. 2o H. C. Longuet-Higgins and E. W. Abrahamson J. Amer. Chem SOC. 1965,87,2045. 21 W. G. Dauben ‘13th Chemistry Conference of the Solvay Institute. Reactivity of the Photo- excited Organic Molecule’ Interscience New York 1967 p. 171. 22 W. Th A. M. van der Lugt and L. J. Oosterhoff Chem Comm. 1968,1235. 23 D. E. Breen and R A. Keller J. Amer. Chem SOC. 1968 90 1935; R A. Keller ibid. p. 1940; R D. Rauh T. R Evans and P. A. Leermakers ibid.,p. 6897; H.Morrison and R Peiffer ibid.,p. 3428 ; H. Morrison R Brainard and D. Richardson Chem Comm. 1968 1653. 24 J. R DeMember and N. Filipescy J. Amer. Chem SOC.,1968,90,6425. 2s W. G. Herkstroeter and G.S. Hammond,J. Amer. Chem SOC.,1966,88,4769 and refs therein cited 26 A. Bylina Chem Phys. Letters 1968,1 509. Photochemistry 189 (‘vertically’) to the spectroscopic triplet of stilbene rather than to a ‘phantom’ triplet. A Schenck-type mechanism has been discussed for the cis-trans- isomerisation of simple olefins by the n,7c* triplet state ofcarbonyl corn pound^.^^ Orbital symmetry requirements have been considered for both triplet28 and singlet2” energy transfer. Singlet excited states of aromatic hydrocarbons are quenched by con-jugates dienes ; since singlet-singlet transfer would probably be highly endo- thermic in such systems it is believed that the function of the quencher is to catalyse non-radiative decay of the fluorescent species within an excited complex (‘e~ciplex’).~’ In most cases no detectable chemical change accom- panies this kind of quenching.The exceptions are (i) the isomerisation of quadricyclene (1) to norbornadiene (2) by aromatic hydrocarbon singlets ;30 (ii) the cis-trans-isomerisation of 1,2-diphenylcyclopropaneby singlet excited naphthalenes;30a (iii) the racemisation of optically active sulphoxides (Ar*SO*CH,) by singlet naphthalene;31 and (iv) the formation of an unstable 1,l-adduct from singlet excited 2-phenylthiophen and piperylene. 32 Cases (ii) and (iii) were originally interpreted as involving triplet energy transfer. 33 The second triplet state has been invoked in several recent studies employing anthracene~,~ and carbonyl compounds35 as photosensitisers.Upper triplets may also be involved in the photocycloaddition of cy~lopentenone~~ and cyclohexenone~~~ to olefins and in the photoisomerisation of certain lac- tones.28-38 The cis-trans-isomerisation of piperylene vapour by various crystalline photosensitisers gives stationary states similar to those found in homogeneous 27 N. C. Yang J. I. Cohen and A.Shani J. Amer. Chem SOC. 1968,90 3264. 28 E. F. Ullman and N. Baumann J. Amer. Chem SOC. 1968,90,4158. 29 (a)LM. Stephenson and G.S. Hammond p. 125 in ref. 16a; (b) L. M. Stephenson D. G. Whitten and G. S.Hammond in ‘The Chemistry of Ionization and Excitation’ ed G. R A. Johnson and G. Scholes Taylor and Francis London 1967 p. 35. ’O (a) S. L. Murov R S. Cole and G. S. Hammond J. Amer.Chem SOC. 1968,90 2957; (b) S. Murov and G. S.Hammond J. Phys. Chem. 1968,72,3797. ’l R S. Cooke and G. S. Hammond J. Aw. Chem SOC.,1968,90,2958. 32 R.M. Kellogg and H. Wynberg Tetrahedron Letters 1968 5895. ’’ Refs cited in refs. 30a and 31. 34 R S.H. Liu and J. R Edman J. Amer. Chem SOC.,1968,90,213; R S. H. Liu and D. M. Gale ibid. p. 1897; R S. €L Liy ibid. p. 1899. ” N. C.Yang and R L Loeschen Tetrahedron Letters 1968 2571; cf also ref. 27. 36 P. de Mayo J.-P. Pete and M. Tchir Cad. J. Chem. 1968,46 2535. ’’ 0.L. Chapman T. H. Koch F. Klein P. J. Nelson and EL Brown J. Amer. Chem SOC.,1968 90 1657. N. Baumann M. Sung,and E. F. Ullman J. Amer. Chem SOC. 1968,90,4157. I90 A. C.Day and E. J. Forbes liquid solution.39 Heterogeneous photosensitisation (gas-solid) by thin polymeric films has also been in~estigated.~~ Further photochemical studies of systems adsorbed on silica gel have been rep~rted.~~ The photodeposition of polymer between a solution and the light source and the consequent reduction in photolytic efficiency as reaction progresses can be minimised by use of a recently described falling-film apparat~s.~’ 0lefms.-The mechanism of the photosensitised protonation of olefins has been studied by deuteri~m-labelling.~~ The xylene-sensitised reaction between D,O and the octalin (3) gives the corresponding exocyclic olefin (4;40%) and the two epimeric alcohols (5a; 30%)and (5b; 20%).Thus light-induced protonation occurs from the less hindered face of the molecule to give a tertiary carbonium ion steric factors then favouring formation of the equatorial alcohol (5a).Recovered olefm (3) contained 10-20% D which a3 a2 H H (3) a R1= OH;RZ = Me b R1= Me;R2 = OH m-a3 D indicates that the intermediate carbonium ion deprotonates to give both exo- and endo-cyclic olefin. Hydration of (3) is 7 times as fast in HzO as in D,O; cJ the relative acidities Ka(H,O)/Ka(D,O) -6. In contrast the xylene- sensitised reaction of the octalin (6) with isopropyl alcohol gives cis-9-methyl- decalin as the sole hydrocarbon product.44 Like photosensitised hydration the hydrogenation of (6) is probably an ionic process; but in this case hydride transfer from isopropyl alcohol or its anion to the intermediate carbonium ion (7) is dominant.The formation of the deuteriated decalin (8) in the photo- sensitised reaction of (6) with Me,CD* OH excludes the alternative radical mechanism.44 39 R L. Daubendiek H. Magid and G. R McMillan Chem Comm. 1968,218. *O G. R De Mare M.C. Fontaine and P. Goldfinger J. Org. Chem 1968,33 2528. 41 (a) L. D. Weis T.R Evans and P. A. Leermakers J. Amer. Chem SOC. 1968,90 6109; (b) J. Grifiths and H. Hart ibfd. p. 5296. 42 J. L. R Williams and P. J. Grisdale Chem and Znd. 1968 1477. 43 J. A. Marshall and M. J. Wurth J. Amer. Chem SOC.,1967,89,6788. 44 J. A. Marshall and A R Hochstetler,Chem Comm. 1968 296. Photochemistry 191 Benzene-sensitised photolysis of cyclonona-1,2-diene (9) in the vapour phase yields one major product (11).The reaction probably proceeds by ring-closure to the cyclopropylidene (lo) followed by transannular insertion into a C-H bond.45 The cyclisation of a planar and therefore highly vibra- tionally excited ground state of allene to cyclopropylidene is symmetry- allowed as a non-rotatory process. The cyclisation of (9) to (10) in this way represents a formal reversal of the synthesis of the allene reported earlier.46 0-T'r> -CD W W (9) (10) (11) (12) (13) 8 -b+(15) hv 72'bR2 a R = H (17) b:R=D cis,cis-Cyclo-octa-1,4-dieneis converted quantitatively by light into the tricyclic isomer (12).47 trans,trans-Cyclo-octa-l,5-dieneis obtained in low yield by photolysis of a copper(1) complex of the cis,cis-isomer. It is not yet known which of the two possible conformations (13) and (14) is preferred by the trans,trans-diene ; though when irradiated it gives the tricyclo-octane (15) in good yield.48 Further analogues of the norbornadiene-to-quadricyclene conversion [(2) +(l)] have been reported by Prinzbach and his collabora- tor~.~~ Bicyclo[3,2,l]octadiene (16a) is converted by light into two tricyclic isomers (17a) and (lsa) the latter predominating on direct and the former on triplet-sensitised photolysis.The labelling pattern observed in the products (17b) and (18b) from the dideuterio-diene (16b) suggests that (17) and (18) 45 H. R Ward and E. Karafiath J. Amer. Chem SOC. 1968,90,2193. 46 W. R Moore and H. R Ward J. Org. Chem. 1962,27,4179. 47 Sung Moon and C. R Ganz Tetrahedron Letters 1968,6275.48 G. M. Whitesides G. L Goe and A. C. Cope,J. Amer. Chem SOC. 1967 fB, 7136. 49 H. Prinzbach P. Wursch P. Vogel W. Tochtermann and C. Franke Helv. Chim. Acta 1968 51,911 ;H.Prinzbach W. Eberbach and G. Philippossian Angew. Chem. 1968,80,910 (Angew. Chem Internat. Edn 1968,7 887); cf also J. R Edman and H. E. Simmons J. Org. Chem. 1968,33 3808. A. C.Day and E. J. Forbes are formed by the two possible divinylmethane rearrangements available to (16) viz. the [2~ + 2a] cycloadditions (0+ R) and (0’ + R’) in formula (16) respectively. Further examples of the divinylmethane rearrangement are discussed below. trans,trans-Hexa-2,4-diene(19) undergoes disrotatory cyclisation to cis-3,4- dimethylcyclobutene (20) on irradiation (2537 A),’ in harmony with the Woodward-Hoffmann rules.l7 This is apparently the first example in which photochemical disrotatory ring-closure has been conclusively demonstrated (19) (20) (21) t- 111 OMe (22) (23) (21) for a diene free from all steric constraints on its mode of closure. The stereo- chemistry of the thermal ring-opening of the cyclobutene (20) was some time ago shown to be conrotatory the product being the cis-trans-hexadiene (21).52 Srinivasan further presents evidence that unlike cyclobutene formation the photocyclisation of butadiene to bicyclobutane is a non-concerted process.’ Conjugated vinylacetylenic hydrocarbons dimerise on sensitised photolysis e.g. the enyne (22) gives the cyclobutane (23) and a small quantity of the corresponding cis-isomer.The methoxy-analogue (24) is however merely isomerised to the trans-isomer under the same conditions. 1,l -Diphenylindene (25) is isomerised by light to 1,2- and 2,3-diphenylindene (27) and (28) respectively via (26).54As a suprafacial [1,5]sigmatropic shift the migration of phenyl is allowedi7 in the first excited state provided that the migrating phenyl group maintains contact with the orbitals of the five- membered ring through a p-orbital [cf (29)] rather than its sp2-orbital The subsequent H-migrations leading from (26) to the products must from sym- metry be ground-state processes. In agreement with this view 1,1,3-trideuterio- indene does not undergo photochemical deuterium-migrati~n:’~ The corresponding symmetry-allowed migration in the ground-state has been known for some time.” 1,l-Dimethylindene gave only dimers on photolysis ;54 R R Sauers and A.Shurpik J. Org. Chem. 1968,33 799. 51 R Srinivasan .I.Amer. Chem SOC.,1968,90 4498. s2 R E. K. Winter Tetrahedron Letters 1965 1207. 53 G. T. Kwiatkowski and D. B. Selley Tetrahedron Letters 1968,3471. s4 J. J. McCullough Cad. J. Chem 1968,46,43. 5s W. R.Roth Tetrahedron Letters 1964 1009. Photochemistry H.Ph (30) (31) (32) the failure to observe methyl-migration suggests that the necessary [1,5]-sigmatropic shift with inversion of the migrating group is energetically un- favourable for an alkyl group in the indene system. Symmetry-allowed sigmatropic shifts have also been observed in the photochemistry of cyclo- heptatrieness6 and their ben~o-analogues.~~ Methyl groups at C-2 and C-3 exert a strong directive influence in the photochemistry of 7,7-dimethylcyclo- heptatrienes.56 For example 2,7,7-trimethylcycloheptatriene(30) gives only one (32) of the two possible bicyclic isomers and undergoes [1,7]sigmatropic shift of methyl in only one direction giving (31).On further photolysis the photo-product (31) is converted exclusively into (33).56b A similar high selec- tivity is exhibited by 3,7,7-trime t h ylcyclo heptatriene. A thermal equilibrium exists between the cyclo-octatetraene bond-shift 56 L. B. Jones and V. K. Jones J. Amer. Chem. SOC.,(a) 1967,89,1880; (b) 1968,90,1540. ’’ M. Pomerantz and G. W. Gruber J. Amer. Chem.SOC.,1967,89,6798; 6799. A. C. Day and E. J. Forbes isomers (34) and (35). Isomer (34) predominates but the equilibrium can be disturbed in favour of the less stable component by irradiation (3500 .$) at low temperatures.58 Below -60°,cyclo-octatetraene is photoisomerised to semibullvalene (36). The reaction (which is of preparative value) follows a triplet mechanism and may involve conversion of the tetraene into the valence tautomer (37) followed by the divinylmethane-vinylcyclopropane rearrangement mentioned earlier; however more direct routes via (38) or (39) can also be en~isaged.’~ ‘Me (34 (35) (36) (37) (38) (39) In previous studies of the photochemistry of the [CHI series semibullvalene (36) and barrelene (40) were converted into cyclo-octatetraene but only the bicyclic isomer (37) benzene and acetylene were obtained from cyclo-octa- tetraene.60 In the present a stationary state (88 % cyclo-octatetraene; 12 % semicullvalene) could be reached by extended photolysis.Semibullvalene (36) may also be implicated in the pyrolytic conversion of cyclo-octatetraene into the dihydropentalene (41).6’ Triplet-photosensitised isomerisation of benzobarrelene (42) gives benzo- semibullvalene (44) and benzocyclo-octatetraene (48) in the ratio cu. 10:1. Direct irradiation on the other hand gives by a singlet mechanism solely benzocyclo-octatetraene (48).62*63 Studies with deuteriated benzobarrelene (42; see note in formula chart) showed that the triplet pathway occurs by a 58 F.A. L. Anet and L. A. Bock J. Amer. Chem. SOC..1968.90. 7130. ” H.E.Zimmerman and H. Iwamura J. Amer. Chem. SOC.,1968,90,4763. 6o Refs. cited in ref. 59. 61 M. Jones jun.,and L. 0.Schwab J. Amer. Chem. SOC.,1968,90,6549. 62 H.E.Zimmerman R S. Giveps and R M. Pagni J. Amer. Chem. SOC.,1968,90 (a) p. 4191 ;(b) p. 6096. Photochemistry 195 t q& ’ -t (4 2) (43) (4 5) singlet t Formulae (42H48) has significance only for the. labelling studies discussed in the text where it indicates CH; elsewhere in the formulaes CD is then to be understood. divinylmethane rearrangement via (43) rather than by the analogous aryl- vinylmethane rearrangement via (45). Benzocyclo-octatetraene (48) was shown with labelled material to be formed by two pathways (a and b) both of which involve an initial [2 + 2]cycloaddition leading to either (46a) or (46b) followed by a [2 + 2 + 2lthermal retrogression.A marked preference for path a was noted (94% a versus 6% b).62 The (42) + (46a) -,(48a) pathway is mechanistically analogous to the photochemical rearrangement of the dihydro- naphthalene (49) to benz[floxepin (50).64The preferred courses of the singlet and triplet reactions of benzobarrelene (42) can be simply rationalised in energetic terms.62 The direct photoisomerisation of benzosemibullvalene (44) to benzocyclo-octatetraene (48b) from the observed labelling pattern involves initial fission of a cyclopropane bond to (47).62 Qualitatively similar photo- chemistry has been observed with diben~obarrelene.~~ The photoisomerisation of 7,8-diphenylbenzocyclo-octatetraeneand the related bicyclo[4,2,0]octa- triene is reported to involve ‘~aging’.~’ Cyclo-octatetraene oxide (51) is converted by light into the isomers (52)-(54) 63 P.W. Rabideau J. B. Hamilton and L,Friedman J. Amer. Chem. SOC.,1968,90,4465. 64 G. R. Ziegler and G. S. Hammond J. Amer. Chem. SOC. 1968,90,513. P. J. Collin and W. H. F. Sasse Tetrahedron Letters 1968 1689. I96 A. C.Day and E. J. Forbes and cycloheptatriene.66 Oxacyclononatetraene (55) is presumed to be a common intermediate in the formation of the photoisomers the processes being controlled by orbital symmetry throughout. A final [1,2]shift of hydrogen is required to give (53). The [CH180 system has interest as a hetero-analogue of the [CHI anion the chemistry of could well become as multi- farious as that of the [CHI8 and [CHIlo series.As noted last year,l the tetracyclic hydrocarbon (56) is considered to play an important r61e in the interconversions of the [CHI, valence isomers. This compound which previously eluded detection has now been obtained in 65-70% yield by photolysis of the isomers (57) and cis-and trans-(58) at -110". It isomerises quantitatively to the bicyclodecatetraene (59)on warming (k 1-25 x sec-' at -15°).68 Monodeuteriated tetraene (59) undergoes a thermal scrambling of the label at +170° indicative of degenerate valence (60) (61) 66 J. M. Holovka P. D. Gardner C. B. Strow M. L. Hill and T.V. Van Auken J. Amer. Chem. SOC.,1968,90 5041.'' T. J. Katz and P. J. Garratt J. Amer. Chem SOC. 1964,86 5194; E. k LaLancette and R E. Benson ibid. 1965,87 1941. 68 S. Masamune R T. Seidner H. Zenda M. Wiesel N. Nakatsuka and G. Bigam J. Amer. Chem. SOC.,1968,90,5286. Photochemistry I97 tautomerisation via the tetracycle (56).6g As a [4 + 2) process the thermal interconversion of (56) and (59) is symmetry-allowed. l7 The thermally for- bidden [4 + 4)retrogression of (56) to cis-(%)also occurred at 170°,but much more slowly than the deuterium scrambling in (59).69 Analogues of the tetra- cyclic diene (56) have been obtained in the photolysis of 9,lO-bridged dihydro- naphthalenes. The azabullvalene (60) a fluxional molecule prepared photochemically from (61),7 exhibits a diversity of photochemical behaviour comparable to that of bullvalene itself.72 Parallel and equally fascinating studies have been reported for the benzo-analogue of (60).It is of some interest to consider more closely the divinylmethane-vinyl- cyclopropane rearrangement (62) + (63). Many cases have been seen in the foregoing and Zimmerman62b has listed numerous examples in some of which the bond x’ in (62) is part of an aromatic ring (‘di-x-methane redrrange- ment’). The second double bond (n‘) appears essential to the process since whilst (64) rearranges to (65),74 the conversion of a simple cyclohexene into a bucyclo[3,l,0]hexane is not normally observed. (Contrast also the photo- chemistry of simple alkylated 3-methylenecyclohexa-1,4-dienes75 with that of 3,3-dimethyl-6-methylenecyclohex-l-ene.76) To implicate the sccond double bond one should regard the rearrangement as a [n2a+ u2s+ ,2,] processlg (66) (or equivalent representation) rather than [,2u + ,,2=].Both should be I (661 69 M. Jones jun.and B. Fairless Tetrahedron Letters 1968,4881. ’O E. Babad D. Ginsburg and M. B. Rubin Tetrahedron Letters 1968,2361. 71 L. A. Paquette and T. J. Barton J. Amer. Chem Soc. 1967 89 5480; L. A. Paquette T. J. Barton and E. B. Whipple ibid. p. 5481. 72 L. A. Paquette and G. R Krow J. Amer. Chem. SOC. 1968,90,7149. 73 L. A. Paquette and J. R. Malpass J. Amer. Chem. SOC.,1968,90,7151 74 W. Reusch and D. W. Frey Tetrahedron Letters 1967,5193. ” H. Hart J. D. DeVrieze R M. Lange and A. Sheller Chem.Comm. 1968,1650. 76 W. G. Dauben and W. A. Spitzer J. Amer. Chem. SOC.,1968,90 802. 198 A. C. Day and E. J. Forbes photochemically allowed but a zeroth-order Huckel calculation for the simple case of a basis set of pure p-orbitals (GCplus Huckel cyclobutadiene versus Mobius benzene77) favours the [2 + 2 + 21 process by 0.93p i.e. (4J(3) -6)P. The photosensitised addition of cis-and trans-dichloroethylene to indene gives four stereoisomeric adducts (67)in proportions dependent on the tem- perature rotational equilibration being favoured by increase of temperature. c1 (67) (68) The reaction therefore probably follows a non-concerted mechanism in which an intermediate triplet lP-biradical undergoes spin inversion and bond rotation at comparable rates.78 Ethylenic bonds are cleaved photochemically by nitrobenzene to give inter ah ketones and/or aldehydes and anilides.The original ~uggestion,~~ that adducts analogous to molozonides are intermediates in the reaction has recently been supported by the isolation of an unstable crystalline adduct (68) in the photolysis at -70" of cyclohexene and nitrobenzene.80 Similar adducts were obtained from other representative olefins. 8o Other aliphatic cycloadditions are described in the following section. Carbonyl Compounds.-Solvent effects in the photochemistry of dialky18 and alkyl ary182 ketones have received attention. For example polar solvents enhance the quantum yield of type I1 photoelimination and cyclobutanol formation from triplet-excited butyrophenones.82 For alkyl ketones which react by both singlet and triplet mechanisms only the triplet process is sensitive to solvent polarity. 81 Solvation probably stabilises an intermediate hydroxy- biradical cJ (70),in the triplet pathway. The insensitivity of the singlet com- ponent to solvent polarity suggests that this pathway is concerted. In a related study it was concluded that the photoreactivity of butyrophenone is maximised in solvents of moderately high polarity; in more highly polar solvents relatively inert n,n* character is introduced into the reactive n,n* state.83 " H. E. Zimmerman J. Amer. Chem. Soc. 1966,88 1564. W. Metzner Tetrahedron Letters 1968 1321. 79 G. Buchi and D. E. Ayer J. Amer. Chem. Soc. 1956,78,689.J. L. Charlton and P. de Mayo Cad. J. Chem. 1968,46,1041. (a)J. A. Barltrop and J. D. Coyle Tetrahedron Letters 1968 3235; (b) P. J. Wagner ibid. p. 5385. 82 (a) P. J. Wagner J. Amer. Chem. SOC., 1967,89 5898; (b)P. J. Wagner and A. E. Kemppainen, ibid. 1968 90 5896; J. N. Pitts jun. D. R Burley J. C. Man6 and A. D. Broadbent ibid. p. 5900; (c)J. A. Barltrop and J. D. Coyle ibid. p. 6584. R D. Raub and P. A. Leermakers J. Amer. Chem SOC.,1968 90 2246; cf also N. C. Yang and R. L. Dusenbery,ibid. p. 5899. Photochemistry The ease of intramolecular abstraction of y-hydrogen in the triplet states of ketones (69; R = Me)81a and (69; R = Ph)826pc increases markedly with the degree of substitution at the y-position (R’and R2 = H alkyl) and thus parallels the strength of the bond being broken.It has been stressed that the quantum efficiency of type I1 photo-elimination is not directly related to triplet-state reactivity when intramolecular hydrogen-abstraction e.g. (69) + (70) is reversible.82b The stability of the t-butyl radical causes type I fission to dominate the photochemispy of alkyl t-butyl ketones. 84 The epimeric ketones (71) show a striking difference in photochemical behaviour (71a) undergoes type I1 photoelimination to t-butylcyclohexanone through a singlet or very short-lived triplet state whereas (71b) is epimerised to (71a) through a longer- lived triplet. The six-membered transition state necessary for type I1 fission I b R’ = H:R2 = Pr“ is readily achieved by epimer (71a) cJ:(72) but is stereochemically unfavourable for (71b) which consequently reacts solely by reversible mcleavage.85 A second example where epimeric ketones show different photochemical be-haviour has been reported by Cookson et a1.86 The photolysis of camphor (73) has been reported to give as major products a-campholenic aldehyde (74) and 4-acetyl-3,3,4-trimethylcyclopentene(75).87 Three independent groups have now reinvestigated the photochemistry of camphor [for the formation of the ketone (75) poses mechanistic problems] and have been unable to detect any trace of (75) the major primary photo- lysis product being campholenic aldehyde (74). All of the products obtained can therefore be accounted for in terms of an initial a-cleavage of camphor 84 N.C. Yang and E. D. Feit J. Amer. Chem. SOC.,1968,90,504. ” N. J. Turro and D. S. Weiss J. Amer. Chem. SOC.,1968,90 2185. 86 R.C.Cookson,R P. Gandhi and R. M. Southam J. Chem SOC.(C) 1968,2494. ” G. Ciamician and P. Silber Ber. 1910,43 1340; R Srinivasan J. Amer. Chem. SOC.,1959,81 2604. A. C.Day and E. J. Forbes or subsequent photolysis of the aldehyde (74). 88 Photochemically camphor thus behaves as a typical bridged-ring ketone. 88a 89 A rationalisation of Cia- mician and Silber’s original observations has been offered.88a- Type I cleavage has been observed with benzyl methyl ketone which gives a low yield of 0-and p-methylacetophenonesgo (c$ the photo-Fries rearrange- ment) and for two bridged sulphides (76; R = H Ac) which cleave as in- di~ated.~’ In contrast to (76) a variety of cases has been described where 1,2-photoelimination of thiol occurs e.g.the 1-tetralone (77) gives 1-naphthol and dibenzyl di~ulphide.’~ Recent examples of the light-induced cleavage of a-sulphonyloxy-ketones provide strong evidence for a heterolytic mechanism. O.SO,Me I OT s (79) (80) Thus the simple methanesulphonate CH *CO*CH,*O*SO,*CH yields 0-and p-methoxybenzyl methyl ketone with anisole and the naphthalene (78) undergoes intramolecular photosubstitution to give the ketone (79).93 The skeletal rearrangement occurring in the photochemical conversion of the tosyloxy-ketone (80) into cyclohexenone (81) is likewise indicative of a cationic intermediate.94 The trans-isomer of (80) was photolysed only extremely slowly and it was therefore concluded that elimination of the sulphonyloxy- group requires a non-eclipsed conformation of the C=O and C,-0 bondsg4 Homolytic cleavage of the epoxide ring occurs in the photolysis of steroidal ap-epoxy-ketones and ctp-unsaturated y6-epo~y-ketones.~~ (a) J.Meinwald and R A. Chapman J. Amer. Chem SOC. 1968 90 3218; (b) W. C. Agosta and D. K. Herron ibid. p. 7025; (c) H. Takeshita and Y. Fukazawa Tetrahedron Letters 1968 3395. 89 C. D. Gutsche and J. W. Baum J. Amer. Chem. SOC.,1968,90,5862. Y. Ogata K. Takagi and Y. Izawa Tetrahedron 1968,24,1617. 91 C. Ganter and J.-F. Moser Helu. Chim. Acta 1968,51,300. 92 J. R Collier and J. Hill Chem. Comm. 1968 700. 93 A. Tuinman S. Iwasaki K. Schaffner and 0.Jeger Helu.Chim.Acta 1968,51,1778. 94 S. Iwasaki and K. Schaffner Helu. Chim Acta 1968,51 557. ” J. A. Saboz T. Iizuka H. Wehrli K. Schaffner and 0.Jeger Helu. Chim Acta 1968,51,1362; H. Wehrli C. Lehmann T. Iizuka K. Schaffner and 0.Jeger ibid. 1967,50,2403 and earlier references. Photochemistry 20 1 A characteristic photochemical reaction of cyclic &I-unsaturated ketones is a-cleavage followed by recyclisation at either end of the allylic system. A recent example is the conversion of cyclo-octen-3-one into 2-vinylcyclo- he~anone.~~ The octalone (82) shows entirely different behaviour rearranging photochemically to the tricyclic ketone (83).98 Direct photolysis of the cycloheptadienone (84; R = H) gives solely carbon monoxide and a mixture of cis- and trans-hexatrienes probably by symmetry- allowed concerted fragmentation of the excited singlet.Triplet-sensitised photolysis on the other hand leads exclusively to the bicyclic ketone (85) R" probably via the cispans-isomer of (84) which could cyclise conrotatorily to (85) in a thermal step.99 Steric crowding in the tetramethyl analogue (84; R = Me) prevents both decarbonylation and formation of a bicyclo[3,2,0]- heptenone and in this case the bicyclo[4,l,O]heptenone (86) is obtained on direct or sensitised photoly~is.~' in the vapour-phase photolysis of trans-crotonaldehyde'OO has bearing on the mechanism of photoisomerisation of afhnsaturated ketones and aldehydes to their &.-isomers. The analogous reaction of simple acyclic acrylic acids and esters has now been studied,'0',102 and shown to involve cis-trans- isomerisation through an excited triplet state followed by intramolecular photoenolisation of the singlet-excited cis-isomer e.g.(87) +(88). The enol then protonates at the a-carbon atom; in MeOD one deuterium atom is incorporated at this position. In the t-butyl system (89) intramolecular hydrogen transfer can occur in both isomers the cyclopropane (90) being formed by transfer of a t-butyl hydrogen in (89b).1°1b 96 L. A. Paquette and R F. Eizember J. Amer. Chem Soc. 1967,89 6205; J. K. Crandall J. P. Amngton and J. Hen ibid. p. 6208. For other examples see ref. 97 and M. Fischer and B. Zeeh Chem. Ber. 1968,101,2360. 97 L. A. Paquette R F. Eizember and 0.Cox J.Amer. Chem. Soc. 1968,90 5153. 98 J. R. Williams and H. Ziffer Tetrahedron 1968,24,6725. 99 D. L Schuster B. R Sckolnick and F.-T. H. Lee J. Amer. Chem Soc. 1968,90 1300. loo J. W. Coomber J. N. Pitts jun. and R R Schrock Chem. Comm. 1968 190. lo' (a) J. A. Barltrop and J. Wills Tetrahedron Letters 1968 4987; (b) M. J. Jorgenson and L. Gundel ibid. p. 4991. lo* R R Rando and W. von E. Doering J. Org. Chem. 1968,33 1671. A. C.Day and E. J. Forbes /H O \:,*OEt -% CH H-0 \ C*OEt CH \CH=CH b-d4-I (894 Wb) (90) Trends due to ring size in the photochemistry of conjugated cycloalkenones have been summarised by Eat~n.~ Recent additional examples include the following (i) solvent effects on the proportions of head-to-head and head-to- tail photodimers obtained from i~ophorone;'~~ (ii) a 'salt effect' in the ratio of analogous photodimers from coumarin ;Io4 (iii) the P-addition of water and alcohols to cycl~heptenone~~~~ and of alcohols to cyclo-octenone,'05b both of which involve conversion into the unstable trans-enone followed by non- photochemical addition of ROH; (iv) the conversion of cyclodecenone to the b-unsaturated isomer which undergoes a slower subsequent photoisomerisa- tion (a-cleavage and recyclisation) to 2-vinylcyclo-octanone.Io6 Zimmerman' O7 has criticised Taylor's notation '08 for the representation of n,n* excited states; the problem of notation has also been discussed by Tezuka log who stresses the dualistic nature of the reactive intermediates in enone and dienone photochemistry.As usual photochemical activity has been high over the whole field of cyclo hexenones cyclohexadienones and related systems and numerous important papers have appeared. In view of restrictions on space however we focus on a single aspect uiz. the detection of ketens as photochemical intermediates. Notwithstanding we would draw particular attention to papers by Zimmerman,' lo especially one,' Ioa a microcosm of modern photo- chemistry which well merits detailed study. We must also note that the lumi- rearrangement of alkylated cyclohex-2-enones e.g. (91) -+(92),has been found to be restricted to compounds fully alkylated at C-4; in other cases the main lo' 0.L. Chapman P. J. Nelson R W. King D. J. Trecker and A.k Griswold Rec. Chem Progr. 1967,28,167. Io4 H. Morrison and R. Hoffman Chem Comm. 1968,1453. lo5 (a) H. Nozako M. Kurita and R Noyori Tetrahedron Letters 1968 2025; (b) R Noyori A. Watanabe and M. Kata ibid. p. 5443. Io6 R G. Carlson and J. H. Bateman Tetrahedron Letters 1967,4151. lo' H. E. Zimmerman Chem. Comm. 1968,174. lo* G.A. Taylor Chem. Comm. 1967,896. T. Tezuka Tetrahedron Letters 1968 5677. If' H. E. Zimmerman K. G. Hancock and G. C. Licke J. Amer. Chem SOC. 1968 90 4892; (b) H. E. Zimmerman and R L. Morse ibid. p. 954; H. E. Zimmerman and K. G. Hancock ibid. p. 3749; H. E. Zimmerman and D. S. Crumrine ibid. p. 5612; cf also T. M. Brennan and R K Hill ibid. p. 5614. Photochemistry products are photodimers.Dauben has therefore suggested that positive charge is developed at C-4 during the lumi-rearrangement this being of course most favourable for a tertiary centre. '' Many photochemical reactions of unsaturated ketones have been considered to involve ketens as transient intermediates; but the evidence has generally been indirect being based on trapping experiments conducted in the presence of nucleophiles. The case of cyclohexa-2,4-dienones is typical. As noted last 0 Q (91) 6 (92) (95) /6HX 6 (98) (100) year,'*'13 these may undergo the usual Barton-Quinkert ring cleavage to ketens (trappable as- 3,5-dienoic acid derivatives in protic media) or isomerise to bicyclo[3,l,0]hexenones. The actual path followed by a given dienone is very sensitive to the degree and pattern of alkyl substitution as the following example shows.Photolysis of the dienone (93) in methanol gives solely the acyclic ester (94) whereas under identical conditions the isomeric dienone (95) gives solely the bicyclohexenone (96). The failure of methanol to trap a keten in cases such as (95) led Collins and Hart'13 to infer that diene-ketens W. G. Dauben G. W. Shaffer and N. D. Vietmeyer J. Org. Chem. 1968,33,4060. '" (a)J. S. Swenton E. Saurborn R Srinivasan and F. I. Sonntag J. Amer. Chem SOC. 1968 90 2990; (6) D. I. Schuster and D. J. Patel ibid. p. 5145 footnote 48a; cj also solvent effects W. V. Curran and D. I. Schuster Chem Comm. 1968,699. P. M.Collins and H. Hart J. Chem SOC.(C),1967 1197; H. Hart and R K. Murray jun.J. Org. Chem. 1967,32,2448. *The rearrangement of the dienone (91) to the lumi-ketone (92) has recently been observed in the vapour phase and it has therefore been concluded that zwitterionic species are unlikely to be involved in the reaction;' lzathis conclusion has however been cnticised.' lZb 204 A. C.Day and E. J. Forbes are not involved in the isomerisation to bicyclohexenones. However direct spectroscopic evidence has now been obtained from photolyses conducted at -100" that in all cases the primary photo-products following n,x* excita-tion41b are diene-ketens. l4 The behaviour of the cyclohexadienone system can therefore be represented schematically as in formulae (97H100) ; the [2 + 2 + 21 cyclisations of the diene-keten (98) to starting material (97) and bicyclohexenone (99) are both thermal (and therefore symmetry-allowed) processes.The ketens derived from certain dienones e.g. (95) apparently A R I R R (101) (102) (103) a R=H; b R=Me cyclise so rapidly that they cannot be trapped by such weak nucleophiles as alcohols.L14As more potent nucleophiles amines are better a3le to compete with cyclisation and give acyclic amides (100; X = NR,) even with such cases as (93.' 14* Diene-ketens (102) are also formed in the decomposition both thermal and photochemical of 2-azidotropones (101). The ketens undergo thermal cyclisation to the dienones (103) the rate for the trimethylketen (102b) being ca. lo3 times that for the unsubstituted compound (102a).'I6 The rate-enhancing effect of alkyl substituents in this case and in the dienone photolyses above has been attributed to steric factors.'16 In low-temperature photolyses Chapman and his co-workers' have obtained with a cyclohexadienone results analogous to Griffiths and and have also detected ketens as primary photoproducts in the ring-opening of tetrachlorocyclobutenone and in the photoisomerisation of l-methoxy-bicyclo[3,2,0]hepta-3,6-dien-2-one to the 7-methoxy-isomer.l7 [The latter example is of considerable significance in tropolone photochemistry (vide i@a).] Likewise ketens have been observed spectroscopically as intermediates in the photolysis (at -190") of umbellulone and lumisantonin."' For a reverse Diels-Alder reaction whicb gives dimethylketen (photochemically easy but thermally very difficult) see ref.119. Ketens have been invoked but not directly observed in several other photochemical reactions. 120 '14 J. Griffiths and H. Hart J. Amer. Chem. SOC. 1968,90 3297. 115 H. Perst and K. Dimroth Tetrahedron 1968,24,5385. 116 J. D. Hobson M. M. a1 Holly and J. R Malpass Chem. Comm. 1968,764. 11' 0.L. Chapman and J. D. Lassila J. Amer. Chem. SOC.,1968,90,2449. 11* L. Barber 0.L. Chapman and J. D. Lassila J. Amer. Chem. SOC.,1968,90 5933. R K. Murray jun.,and H. Hart Tetrahedron Letters 1968,4995. IZc H. A. Staab and J. Ipaktschi Chem. Eer. 1968,101 1457; K. F.Cohen J. T. Pinhey and R J. Smith Tetrahedron Letters 1968 4729; F.M. Beringer R E. K. Winter and J. A. Castellano ibid. p. 6183 and ref. 115.Photochemistry An intriguing stereospecificity has been observed in the ring-opening of the cyclobutenones (105; R = Me Cl) to vinylketens. The ketens (104) and (106) were captured with methanol as &-unsaturated esters the stereochemistry of which demonstrated that ring-opening gives one (106) of the two possible geometric isomers photochemically the other (104) thermally. 12' The reasons c1 c1 k0 A hv -l-i:R .-? Ph? Pt n 1 I R H ( 104) ( 1051 (106) underlying the specificity are unknown. Orbital symmetry considerations shed no light on the problem since both ketens (104) and (106) could in principle be produced by either conrotatory or distotatory processes. Con- and dis- rotation are in any case experimentally indistinguishable in the cyclobutenone system which necessarily lacks any stereochemical marker at the carbonyl centre.The photochemical generation of diene-ketens from cyclohexa-2,4- dienones poses an analogous stereochemical problem. ' '' The photochemistry of cyclo-octa-2,7-dienone (107) is explicable in terms of a preliminary cis-trans-isomerisation followed by thermal conrotatory cyclisation to the zwitterion (108). This may then capture a nucleophile to give a 2-substituted bicyclo[3,3,0]octan-3-one''2 or form the cycloadduct (109) with furan. ' On the photochemistry of cyclohepta-2,6-dienone see Nozaki et The photocycloaddition of the cyclopentenone (1 11) to the cyclohexene (110) shows a marked sensitivity to solvent the ratio of (1 12) to (113) changing from 98 :2 in hydrocarbon solvents to 45 :55 in methanol.124 As in the dimerisa- tion of i~ophorone,'~~ the effect is due to dipole interactions; the transition state leading to isomer (1 12) has the dipoles of (110) and excited (1 11) opposed and so will be favoured in a nonpolar medium.Solvent effects also influence the proportions of cyclobutane and oxetan adducts in the photocycloaddition of 1,l-dimethoxyethylene to cyclohexenones. 37 lZ1 J. E. Baldwin and M. C. McDaniel J. Amer. Chem SOC.,1968,90,6118. (a)J. K. Crandall and R P. Haseltine 1.Amer Chem. SOC. 1968,90,6251; (b)R Noyon and M. Katb Tetrahedron Letters 1968 5075. H. Nozaki M. Kurita and R Noyori Tetrahedron Letters 1968,3635. lZ4 B. D. Challand and P. de Mayo Chem Comm.1968,982. A. C. DayandE. J. Forbes (110) (113) A kinetic analysis of the photocycloaddition of acetone to fumaronitrile shows that oxetan formation proceeds via a singlet excited complex of acetone and the nitrile. 12’ The photocycloaddition of several saturated ketones to acrylonitrile methacrylonitrile and crotononitrile also follows a singlet mechanism; the products are exclusively 2-cyano-oxetans rather than the 3-cyano-oxetans which would have been expected from consideration of the most stable biradical intermediates. 126 cis-1 -Methoxybut-1-ene gives olgetans with (n,n*)-excited acetone. Singlet and triplet acetone are both effective and the isolation of pairs of geometric isomers (1 14) and (1 15) and the corresponding IIIIIIH fi”’ b4ZH Et 2-methoxyoxetan pair indicates a nonconcerted mechanism.The presumed precursor (116) of the oxetans (114) and (115) behaves like a typical 1,4-bi- radical in that the extent of rotational equilibration is greater for the triplet than for the singlet process [ratio of (114) to (115) triplet 1.1; singlet 4.1].’27 The situation in which biradicals or radical pairs of different multiplicity show different chemical behaviour has been termed by Bartlett’28 a ‘spin-correlation effect’. The effect has been observed recently with both 1,3- and 1,4-biradicals generated photolytically from cyclic azo-compounds. ’29 The photocycloaddition of thiobenzophenone to olefins and dienes has received further attention. ’30 Aromatic Compouuds.-The photochemistry of benzene and its simple derivatives continues to receive attention and from this some of the fine details 12s N.J. Turro P. A. Wriede and J. C. Dalton J. Amer. Chem. SOC.,1968,90,3274. 126 J. A. Barltrop and H. A. J. Carless Tetrahedron Letters 1968,3901. N. J. Turro and P. A. Wriede J. Amer. Chem. SOC., 1968,90,6863. P. D. Bartlett and P.S. Engel J. Amer. Chem SOC. 1968 90 2960; P. D. Bartlett and J. M. McBride Pure Appl. Chem. 1967,15,89; c$ M. C. R Symons Nature 1967,213 1226. (a) 1,3-Biradicals R Moore A. Mishra and R J. Crawford Cad. J. Chem. 1968 46 3305; S. D. Andrews and A. C. Day J. Chem SOC.(B) 1968 1271; P. Scheiner J. Amer. Chem SOC.,1968 90 988; (b) lP-biradicals P. D. Bartlett and N. A. Porter ibid. p. 5317. K.Yamada M. Yoshioka and N. Sugiyama J. Org. Chem. 1968 33 1240; A. Ohno Y. Ohnishi M. Fukuyama and G. Tsuchihashi J. Amer. Chem SOC.,1968,90,7038. Photochemistry are beginning to emerge. Thus the observation that irradiation of liquid benzene at 2537 A gives ben~valene'~' and that photolysis of benzvalene under the same conditions gives benzene suggested that carbon scrambling should occur. This has now been confirmed with [1,3,5-2H]benzene (117) D D D (117) [1 ,3,4-2H]benzene (1 18) being detected among the products. '32 The wavelength dependency of the quantum yield rules out the possibility that a benzvalene intermediate participates in olefin additions but is consistent with its involve- ment in the additions of alcohols to benzene.'32 Kaplan and Wilzbach'33 have nicely demonstrated a photochemical equilibrium between benzene and benzvalene the latter being formed in a 'hot' vibrational state and its re-con- version into benzene being sensitized by benzene itself.Photolysis of liquid benzene at 165-200 nm gives Dewar benzene benzvalene and fulvene in the ratio 1:5 :2. '34 It has been shown that vibrationally excited intermediates might be involved in the photoisomerisation of o-xylene. 13' In sharp contrast to the above results the photolysis at 2537 A in the vapour phase of hexa-fluorobenzene yields the Dewar isomer (75%) and no other product.'36 It has been shown that vibrationally excited states are involved here also.I3' Excited states of the aromatic nucleus might be involved in the formation of p-xylene from the sulphonhydrazone (1 19) and methyl-lithi~rn'~~ though it is more certain that they are involved in the photohydrolysis of m-hydroxy- and m-amino-benzotrifluorides (120; R = OH and NH2) to the corresponding benzoic acids in yields of 70-80 %.'39 As yet it has not proved possible to isolate a 1:1adduct from the photolysis of maleic anhydride in benzene. With the less reactive dienophile acrylonitrile however a small amount of the 1:1 adduct (121) has been isolated.14' Under more strenuous conditions a 2:l adduct analogous to that obtained with maleic anhydride is formed. Acrylonitrile also forms a 1:1 adduct with naph- thalene.14' The 'H n.m.r. spectrum of maleic anhydride shows the olefinic protons at higher fields in benzene than in carbon tetrachloride.This upfield K E. Wilzbach J. S. Ritscher and L. Kaplan J. Amer. Chem SOC. 1967 89 1031. K E. Wilzbach A. L. Harkness and L Kaplan J. Amer. Chem SOC. 1968,90,1116. L. Kaplan and K. E. Wilzbach J. Amer. Chem SOC. 1968,90,3291. H. R Ward and J. S. Wishnok J. Amer. Chem. SOC.,1968,90 1085. R B. Cundall and A. J. R Voss Chem. Comm. 1968,902. C. Camaggi F. Gozzo and G. Cevidalli Chem Comm. 1966 313; A. Bergomi and F. Gozzo Chimica e Industria 1968,50,745. 13' I. Haller J. Chem. Phys. 1967,47 1117. R K. Shapiro and K. Tomer Chem. Comm. 1968,460. R. Grinter E. Heilbronner,T. Petnilka and P. Seiler Tetrahedron Letters 1968 3845. B. E. Job and J. D. Littlehailes,J. Chem. SOC.(C) 1968,886. 14' J. J. McCullough C.Calvo and C. W. Huang Chem. Comm 1968,1176. A. C. Day and E. J. Forbes Me (120) (121) (119) hv - x shift has been attributed to exo-stereospecific (1 :1) association between solvent and solute molecules. 142Dimers have been obtained from the photolysis of 2-rnetho~ynaphthalene'~~ and biphenylene. 144 The structure of the latter has been shown to be (122). An interesting cyclisation is observed when the bianthranyls (123; X = bisoxycarbonyl amide or azo) are photolysed in benzene. The resulting compounds (124) are obtained in about 50% yields. The azo-compound is unaffected by light but is converted into its precursor (123) by heat.145 Interest in photocyclodehydrogenations continues. Timmons et al. have extended their work with styryl heterocyclic compounds146 and have also shown that cyclisation will proceed albeit in poor yield with concurrent displacement of a methyl group e.g.2,4,6-trimethylstilbene yields 1,3-di- rnethy1~hanthrene.l~' Similarly photolysis of (125) yields (126) in 13 % yield 142 D. Bryce-Smith and M. A. Hems J. Chem. SOC.(B),1968,812 143 P. Wilairat and B. Selinger Austral. J. Chem 1968,21 733; J. S. Bradshaw N. B. Nielsen and D. P. Rees J. Org. Chem. 1968,33,259. 14* N. L. Goldman and R. A. Ruden Tetrahedron Letters 1968 3951. 14' D. E. Applequist M. A. Linter and R. Searle J. Org. Chem 1968,33,254. 146 C. E. Loader and C. J. Timmons,J. Chem. SOC.(C), 1968,330. E. V. Blackburn C. E. Loader and C. J. Timmons,J. Chem SOC.(C), 1968,1576.Photochemistry S F &Je FF F (125) (126) (127) along with normal product (49 Quantum-yield studies'49 and HMO calculations150 on these systems give no clear-cut mechanistic conclusions. Photolysis of the episulphide (127) gives good yields of phenanthrene in both the presence and the absence of iodine.I5" In addition to the normal product photolysis of 4,4'-bisethoxycarbonylstilbene yields significant quantities of 2,6-bisethoxycarbonylanthracene.The triplet of the trans-stilbene is presumed to be an intermediate. '52 OCO-Me Me Me Me 1 OH MeCO' 0+ escape etc from cage Me Me ! SCHEME A The photo-Fries reaction and some closely related types continue to receive attention. Contrary to an earlier opinion'53 it would seem that the insensitivity of the quantum yield of the reaction to changes in viscosity of the medium is best interpreted in terms of an intramolecular rearrangement involving tightly bound moieties.154 In contrast the quantum yield of the concurrent production of phenols is viscosity dependent suggesting that a solvent cage is involved with the participation of free radicals as depicted in the Scheme (A).'54 A study of solvent effects on the proportions of rearrangement product and 14' K. L. Servis and K.-N. Fang Tetrahedron Letters 1968,967. 14' H. Jungmann G. Giisten and D. Schulte-Frohlinde Chem. Ber. 1968,101,2690. lSu G. Giisten and L. Klasine Tetrahedron 1968,24,5499; W. H. Laarhoven T. J. H. M. Cuppen and R J. F. Nivard Rec. Trm.chim 1968,87,687. 15' T. Sato Y. Goto T. Tohyama S. Hayashi and K.Hata Bull. Chem SOC. Japan 1967,40,2975. S. D. Cohen M. V. Mijovic and G. A. Newman Chem.Comm 1968,722. 153 A. C. Day Ann. Reports 1967,64 B 185. M. R. Sandner and D. J. Trecker J. Amer. Chem. SOC., 1967,89 5725. 210 A. C. Day and E. J. Forbes phenol has been made.'55 When ortho-and para-substituents are present in the benzene ring other reactions intervene.'56. lS7For example the ester (128) yields the ether (129) as the major pr~duct."~ The production inter alia of 2-methylbenzofuran from the photolysis of phenoxyacetone' 58 must involve OCO-Me Me "" 0 0 I I "'0"' (128) (129) the migration of substituted aikyl rather than the usual acyl group. Migration of the ethoxycarbonyl group occurs when N-phenylurethanes are photolysed.159 The product distribution is characteristic of that associated with photo- Fries rearrangements. y-Radiolysis of aryl esters produces qualitatively similar results to those produced by U.V. light.'60 A dichotomy of mechanism similar to that suggested for the photolysis of aryl esters may account for the formation of toluene and 2-phenylpropionaldehyde from the photolysis of benzyl vinyl ether. Quinones.-The reaction of olefins with quinones continues to excite interest. A contrast is displayed between the thermal and photo-reactions of cis- and trans-stilbene with tetrachloro-o-benzoquinone.The dark reactions at 128" proceed stereospecifically. Thus trans-stilbene affords (130) the photoreaction yields 88 % of the trans-isomer (130) and 12% of the cis-isomer.The cis-isomer does not arise from pre-formed ~is-stilbene,'~~ indicating that the addition is not a concerted process and that the stereospecificity probably arises from the marked dipolar character of the intermediate di- radical. A similar situation clearly does not pertain to the reactions of phen- anthraquinone with olefins. In the presence of an excess of cis- or trans-but-2-ene photolysis of phenanthraquinone yields an identical mixture of cis- and truns-ad duct^'^^ from lP-addition. When the olefm is more heavily substituted 15' R A. Finnegan and D. Knutson Tetrahedron Letters 1968,3429; D. A. Plank ibid. p. 5423. H. J. Hageman Chem Comm 1968,401. ''' J. S.Bradshaw E. L. Loveridge and L. White J. Org. Chem 1968,33,4127. M. K. M. Dirania and J. Hill J. Chem. SOC.(a, 1968.1311. lS9 D. J. Trecker,R. S. Foote and C. L. Osborn Chem. Comm.,1968,1034; D. BelluS and K. Schaff-ner Helv. Chim. Acta 1968,51 221. lB0 D. BelluS K. Schsffner and J. HoignQ Helv. Chim. Acta 1968,51,1980. 16' J. T.Pinhey and K. Schaffner Austral. J. Chem. 1968,21,2265. 16' D. Bryce-Smith and A. Gilbert,Chem. Comm. 1968 1701. 163 D. Bryce-Smith and A. Gilbert,Chem. Comm. 1968,1702. 164 Y. L. Chow and T. C. Joseph Chem. Corn 1968,604. Photochemistry 21 1 Ph & OH Ye \ / 0 OH (131) (132) (133) 1,Zadducts are also formed.165 Even at long wavelengths (e.g.4358 A) phen-anthraquinone166 and tetrachloro-o-benzoquinone'63 can abstract hydrogen from benzene to yield adducts of the type (131).1,4-Naphthaquinone reacts with diphenylacetylene to give products arising from addition to both carbonyl and olefinic group^.'^' The latter are similar to that (132) obtained from diphenylacetylene and anthraquinone.16* Anthraquinone reacts with cyclo- octene to form a mono- and a bis-adduct containing oxetan rings.' 68 Photolysis of t-butyl p-benzoquinones in the presence of alcohols gives hydroquinones (133) as major products. These arise by y-hydrogen abstraction followed by reaction of alcohol with a spiro-intermediate.'69 Troponoids-Irradiation of tropolone methyl ether gives ketone (1 34) via ketone (135).170 This interconversion has now been shown to involve a keten (stable at -180") which thermolyses mainly to the observed product of the room-temperature photolysis.l7 Photolysis of 2-chlorotropone gives products arising from [6 + 6)n-and [4 + 2]n-electron addition but not from [6 + 41n-or [6 + 2In-additi0n.'~~ As expected no valence tautomers were obtained from the photolysis of 2-methy1-4,5-benzotropone." Dimerisation is the feature of the photolysis of the dibenzo-compounds (136; X = CO CH2,and 165 S. Farid and K.-H. Scholz Chem. Comm. 1968 412; S. Farid D. Hess G. Pfundt K.-H. Scholz and G. Steffan ibid. p. 638. 166 M. B. Rubin and Z. Neuwirth-Weiss,Chem. Comm. 1968,1607. 167 S. Farid W. Kothe and G. Pfundt Tetrahedron Letters 1968,4147. 16* D. Bryce-Smith A. Gilbert and M. G. Johnson Tetrahedron Letters 1968,2863.16' C. M. Orlando jun.,H. Mark k K Bose and M. S. Manhas J. Amer. Chem. SOC.,1967,89 6527; J. Org. Chem. 1968,33,2512 170 W. G. Dauben K. Koch S. L. Smith and 0.L Chapman J. Amer. Chem SOC.,1963,852616. 17' 0.L. Chapman and J. D. Lassila J. Amer. Chem. SOC.,1968,90,2449. 17' T. Mukai H. Tsuruta A. Takeshita and H. Watanabe Tetrahedron Letters 1968,4065 173 T. Mukai T. Miyashi and Y.Tanaka Tetrahedron Letters 1968,2175. A. C.Day and E. J. Forbes (131) MeoMm MeoMa Me0 \ Me0 \ OMe (138) 6 H H (140) (139) NC CN HO (142) (143) C=CH,). 174 When tetra-0-methylpurpurogallin is photolysed in ethanol or in aprotic solvents the naphthol (137) and the ketone (138) are the sole products.'75 The reaction in ethanol is in sharp contrast to that in methanol where an adduct with solvent is virtually the sole product.'76 Full details have appeared of the photochemistry of 2,3-homotropone.177 In a variety of solvents the major product is (139).Photolysis of tropylium fluoborate in aqueous sulphuric acid yields mainly the alcohol (140).17' The cation (141) was presumed to be an intermediate. The photolysis of other stable carbonium ions triphenylcyclopropenylium (which gives hexaphenyl- benzene) and triphenylcarbonium is also reported.' 78 When guaiazulene-2- sulphonic acid is irradiated in 40 % sulphuric acid at 365 nm 2-hydroxyguaiazu- lene (142) is obtained. 17' In concentrated sulphuric acid photolysis yields 174 J. Kopecky and J. E. Shields Tetrahedron Letters 1968 2821.175 E. J. Forbes J. Grifiths and R A. Ripley J. Chem. SOC.(C), 1968,1149. 0.L. Chapman and T. J. Murphy J. Amer. Chem. SOC.,1967,89,3476. 17' L. A. Paquette and 0.Cox J. her. Chem. SOC. 1967,89,5633. 17* E. E van Tamelen T. M. Cole R Greely and H. Schumacher J. Amer. Chem Soc. 1968,90 1372 R Hagen E. Heilbronner and P. A. Straub Helo. Chim Act4 1967,50,2504. Photochemistry 213 2,2'-biguaiazulyl ; the 3-sulphonic acid behaves similarly. A stable tropy- lium ion is an intermediate. Photolysis of either 1,2- or 3,4-benzotropylidene gives the isomer (143). lS1 Deuteriation studies show that the tropylidenes are not photochemically interconvertible and that a hydrogen migration is involved. In contrast the photolysis of 7,7-dicyano-1,2-benzotropylidenegives cyclo butene (144).Heterocyclic Compounds.-The formation of the compound (145) from benzylidenecyclohexylamineis the first reported example of the photodimerisa- tion of a Schiff base.'83 Of greater synthetic importance is the production of trans-2,3-dihydroindoles(146; n = 4 or 5) when the N-aryl-enamines (147; R = H or Me) are irradiated in ether.ls4 When n = 3 irradiation gives the cis-isomer. In contrast the phenyl group of the quinone (148) photocyclises on to the carbonyl ~xygen.'~' Other oxygen heterocyclics are formed oia oxidative cyclisations of methyl groups on to carbonyl oxygen. Thus the phenol (149) is obtained from a bisdimethylamino-p-benzoquinone,'86 and the pyrone (1 50) arises from quercetin pentamethyl ether.'87 OMe @'\Me 0 Yh Me&MeO7 doMe II II \ Me,N \ Me \ n OH n (148) (149) (150) Photolysis of 1,2-bisethoxycarbonyl-3,6-diphenylpyridazinein ether gives a high yield of the dicarbamate (151).lS8 a-Pyrone undergoes ring cleavage (to 152) when photolysed directly in methanol although its sensitised photolysis results in dimerisation. The former reaction is not quenched by piperylene. 18' "'R Hagen E. Heilbronner and P. A. Straub Helu. Chim. Acta 1968,51 45. M. Pomerantz and G. W. Gruber J. Amer. Chem SOC. 1967,89,6798. ''' E. Ciganek J. Amer. Chem. SOC.,1967,89,1458. lE3 R 0.Kan and R L. Furey J. Amer. Chem. SOC.,1968,90,1666. lE40.L. Chapman and G. L. Eian J. Amer. Chem. SOC. 1968,90,5329. '13' M. Ogata and H. KanG Tetrahedron 1968,24,3725.ln6 D. W. Cameron and R G. F. Giles J. Chem. SOC. (C),1968,1461. l'' A. C. Waiss jun. R E. Lundin A. Lee and J. Corse J. Atner. Chem SOC. 1967,89,6213. "'J. Rigaudy and J.-C. Brelihe Bull. SOC.chim. France 1968,455. W. H. Pirkle and L. H. McKendry Tetrahedron Letters 1968,5279. 21.4 A. C.Day and E. J. Forbes The diazole (153; R = Ph) fragments as shown to benzonitrile and phenyl isocyanate when photolysed in ether. The precursor of the isocyanate can be trapped to give (154) when the analogue (153; R = Me) is photolysed in cyclopentene.190 Photolysis of heterocyclic compounds frequently results in the extrusion of small stable molecules such as carbon monoxide. To this number can now be added nitric oxide. Irradiation in benzene of the pyrroline (155) gives an 85% yield of the butadiene resulting from the loss of nitric oxide.Igl Loss of carbon dioxide occurs when compound (156) is photolysed.In cyclohexane the major product is NN'-diphenylurea. When the photolysis is conducted in dimethyl sulphide the unrearranged nitrene can be trapped.lgz Some of the complexities of the photochemistry of 3 H-pyrazoles (e.g 157) have been un- ravelled by Closs et ~1."~Irradiation at room temperature is known to give good yields of cyclopropenes with the loss of nitrogen. There is now evidence for the intermediacy of diazo-alkenes in this reaction. When the photolysis is conducted at -50° little nitrogen is evolved and an isomer to which structure (158) has been assigned is formed.Loss of nitrogen and the formation of acridone occurs when the cyclic azo-compound (159) is irradiated in acetone. The keten (160) is believed to be an intermediate in this rearrangernent.Ig4 It now seems that the intermediacy of unsaturated three-membered-ring compounds in the photoisomerisation of five-membered-ring aromatics is a Ph Ph (151) (152) (153) 0' Me (154) (155) (158) (159) lY" T. S. Cantrell and W. S. Haller Chem. Comm. 1968,977. Igl J. F. W. Keana and F. Baitis Tetrahedron Letters 1968 365. Ig2 J. Sauer and K. K. Mayer Tetrahedron Letters. 1968 319. G. L. Closs W. A. Boll H. Heyn and V. Dev J. Amer. Chem. SOC.,1960,90 173; cf A. C. Day and M. C. Whiting J. Chem. SOC.(C),1966 1719. G. Edge and F. Pasedach Chem.Ber. 1968,101,3079. Photochemistry fairly general phenomenon. The intermediate (161) in the isomerisation of 3,5-diphenylisoxazole to 2,4-diphenyloxazole' 95 has now been shown to photolyse to the starting isoxazole from a triplet state and to the product oxazole from a singlet state. lg6A similar intermediate (162) has been isolated from the photoisomerisation of 2,s-di-t-butyl- to 2,4-di-t-butyl-furan.l 97 It had already been suggested that a cyclopropene aldehyde was involved in the photo-isomerisation of 2-methyl- to 3-methyl-furan and this same group has now shown'98 that this is a one-photon process in contrast to the Singh- Ullman rearrangement. 196 A different type of rearrangement must be involved in the photo-isomerisation of 1,4,5-trimethylimidazoleto the 1,2,5-isomer.lg9 An extensive non-mechanistic study of the isomerisation of pyrazoles to imida- zoles and of indazoles to benzimidazoles has been reported.200 The photochemistry of N-oxides continues to receive attention. Although a variety of interesting reactions has come to light little can be said about their mechanisms. Loss of oxygen and ring-contraction are two well known processes for pyridine N-oxide. It has now been shown for the pyridazine N-oxide (163; R = H or Me) that this oxygen can be incorporated into hydro- carbons when these are used as solvents for the photolyses. Thus benzene is converted into phenol and cyclohexane gives cyclohexanol. 2o The pyridazine N-oxide (163; R = Ph) gives a high yield of the pyrazole (164) when irradiated CoPh 0N phw I? c"" N (162) R (161) (163) Ph Ph r(?coph Ph H (165) Ph H (167) (168) (169) 19' E.F. Ullman and B. Singh J. Amer. Chem. SOC.,1966,88 1844. 196 B. Singh and E. F. Ullman J. Amer. Chem. SOC.,1967,89,6911. E. E. van Tamelen and T. H. Whitesides J. Amer. Chem. SOC.,1968,90 3894. H. Hiraoka and R. Srinivasan,J. Amer. Chem. Soc. 1968,90,2720. P. Beak J. L. Miesel and W. R. Messer Tetrahedron Letters 1967,5315. H. Tiefenthaler W. Darscheln H. Gath and H. Schmid,Helv. Chim. Acra 1967,50,2244. '01 H. Igeta T. Tsuchiya M. Yamada and H. Arai Chem. and Pharm. Bull. (Japan),1968,16 767. A. C. Day and E. J. Forbes216 Dh Ph (170) (1711-CO Et (172) in acetone.202 The 4,5-benzo-analogue of (163; R = Ph) however rearranges to (165) under the same conditions.203 The initial reaction in both cases is probably the formation of an oxiran followed by ring-cleavage to a diazo- ketone.That from the 4,5-benzopyridazine N-oxide must necessarily lose nitrogen. The photo-transformation of 6-phenylphenanthridine N-oxide is solvent-dependent. In ethanol it gives a quantitative yield of N-phenylphenan- thridine while in benzene it gives the cyclic oxide (166) as the major The latter probably arises from an initially formed oxiran although the break- down of this oxiran must take a different course from those cited above. A closely similar ring expansion gives (167) when 4-phenylquinazoline-3-oxideis phot~lysed.~~’ Acridine N-oxide gives a high yield of (168) when irradiated in benzene or in methylene chloride.206 This reaction is also thought to involve a seven-membered-ring oxide as an intermediate.The oxiran (169) which has been obtained by photolysis of the corresponding isoindole N-oxide has now been obtained in a novel photo-isomerisation from (170).’07 Whereas photolysis of pyridine N-oxides can lead to products ‘of ring-contraction photolysis of the ester (171) leads to the diazepin (172) via a diaza-norcaradiene.208 A number of photoalkylations have been reported for nitrogen heterocyclics. These occur when photolysis is conducted in an alcohol usually in the presence of acid. Alkylation usually takes place on a carbon atom adjacent to nitrogen. Thus high yields of the pyrimidine (173; R = Me) and of the condensed pyrimidine (174; R = Me) are obtained when the analogues (173; R = H) and (174; R = H) are photolysed in methanol containing hydrogen ~hloride.~” Less efficient C-ethylations have been reported for quinoline,2 lnisoquino-lines,’ lo,” and phenanthridine.’ C- and N-Benzylations have been effected in the flavin series.212 The biological importance of pyrimidines and related compounds continues to prompt widespread photochemical investig?tions. Most of these pertain to the well known dimerisations which can be only briefly mentioned. Di-methylthymine (175) gives only two dimers when photolysed in ice but all four ’02 P. L. Kumler and 0.Buchardt J. Amer. Chem. SOC.,1968,90,5641. ’03 0. Buchardt Tetrahedron Letters 1968 1911.’04 E. C. Taylor and G. G. Spence Chem. Comm. 1968,1037. ’05 C. Kaneka and S. Yamada Tetrahedron Letters 1967,5233. ’06 M. Ishikawa C. Kaneko and S. Yamada Tetrahedron Letters 1968,4519. ’07 B. Singh J. Amer. Chem. SOC.,1968,90 3893. ‘08 J. Streith and J. M. Cassal Tetrahedron Letters 1968,4541; Angew. Chem 1968,80 117. 2c9 M. Ochiai E. Mizuta Y. Asahi and K. Morita Tetrahedron 1968,24,5861. ‘*O F. R Stermitz C. C. Wei and W. H. Huang Chem Comm. 1968,482. F. R Stermitz R. P. Seiber and D. E. Nicodem J. Org. Chem. 1968,33 1136. ‘12 W. H. Walker P. Hemmerich and V. Massey Helu. Chim. Acta 1967,50,2269. Photochemistry 0 I R I I I (178) R' (179) R (180) R possible dimers when photolysed in water. ' The photosensitised dimerisation of pyrimidines has been and the mechanism of thymine dimerisa- tion has been disc~ssed.~'~ When purines are photolysed in methanol the 6-hydroxymethyl-l,2-dihydro-derivative is formed When the irradiation is conducted in isopropyl alcohol an analogous addition compound is formed.2 l6 Of more general interest is the reported photolysis of dihydrothymidine (176; R = deoxyribosyl) which has some resemblance to the photolysis of cyclic ketones.Ring cleavage followed by hydrogen transfer leads to an iso-cyanate (177) which in methanol gives a urethane and in water gives a urea.217 An interesting reduction ensues when uridine (178; R' = ribosyl R2 = H) is photolysed in the presence of sodium borohydride. 4,5-Dihydrouridine is the majbr product photohydration occuring to only a minor extent21s Under similar conditions thymidine (178; R' = deoxyribosyl R2 = Me) gives (179; R = deoxyribosyl) by further reduction of its 4,5-dihydro-derivative and N-deoxyribosylurea by further reduction of the photohydrate (180; R = deo~yribosyl).~~~ Reduction of the 4,5-bond also occurs when uracil and 1,3-dimethyluracil are photolysed in isopropyl alcohol or in water-EDTA mixtures.220 '13 H.Morrison,A. Feeley and R Kleopfer Chem. Comm 1968,358. '14 A. Kornhauser J. N. Herak and N.TrinajstiC Chem Comm. 1968 1108; C. L. Greenstock and H. E. Johns Biochem Biophys. Res. Comm. 1968,30,21. '"A. A. Lamola and J. Eisinger Proc. Nut. Acad. Sci. U.S.A.,1968,59,46. 216 H. Linschitz and J. S. Connolly J.Am. Chem. SOC.,1968,90,2979. 217 Y. Kondo and B. Witkop J. Amer. Chem. SOC. 1968,90,3258. P. Cerutti Y. Kondo W. R Landis and B. Witkop J. Amer. Chem. SOC.,1968,90,771. '19 Y. Kondo and B. Witkop J. Amer. Chem. SOC.,1968,90,764. 220 D. Elad and I. Rosenthal Chem. Comm. 1968,879. 218 A. C.Day and E. J. Forbes Photo-oxidation.-There has been an increasing amount of chemistry based on the photosensitized generation of singlet oxygen. Apart from work directed towards an understanding of its reactions with simple olefins and dienes there has been an increasing emphasis on those reactions which could have a bearing on biological oxidations. Most dye-sensitised photo-oxidations are considered to involve the more stable form of singlet oxygen ‘Ag,although the triplet energy of some of the sensitizers used would make it seem likely that some of the less stable form ‘El,is also generated.Kearns et a1.221 have shown that the dependence of the ratio of the products of the sensitized oxidation of cholest-4-en-3B-01 (181) on the triplet energy of the sensitizer222 is due to the differing ratios of .‘L (181) the two forms of singlet oxygen produced. The production of epoxy-ketone (182) and enone (183) indicates that when the possibilities are present the two forms of singlet oxygen will behave differently. It has been shown that singlet oxygen can be generated in the gas phase from a chemical source uiz. the decomposition of the triethyl phosphite-ozone addu~t.~~~ E.s.r. measure-ments show that this oxygen contains the ‘Agform.224 One of the most intriguing reactions reported this year is the formation of allenes by the reaction of singlet oxygen with derivatives of ionone (184; R = CH or CH*C02Me).225*226 Both compounds give the expected pro- duct~~~~~~~~ and the allenes (185; R = CH or CH*CO,Me) by removal of a vinylic hydrogen the alcohols arise by reduction of the initially formed hydro peroxide.The presence of a similarly disposed allene in certain carote- noids such as fucoxanthin (186) suggests that this moiety arises in a similar manner in uivo. The structure and stereochemistry of the hydroperoxides obtained by photo-oxidation of ( -)-caryophyllene and ( -)-isocaryophyllene have been determined.228 &-Unsaturated carbonyl compounds normally form hydroperoxides with migration of the double bond towards the carbonyl The ester (187) however yields in addition to the normal endoperoxide 221 D.R Kearns R A. Hollins A. U. Khan R W. Chambers and P. Radlick J. Amer. Chem Six. 1967,89 5455; D. R Kearns R A. Hollins A U. Khan and P. Radlick ibid. p. 5456. ”’ A. Nickon and W. L. Mendelson J. Amer. Chem. SOC.,1963,85 1894; 1965,87 3921. 223 R. W. Murray and M. L. Kaplan J. Amer. Chem. SOC.,1968,90,4161. 224 E. Wasserman R W. Murray M. L. Kaplan and W. A Yager J. Amer. Chem SOC.,1%8 Po 4160. 225 C. S. Foote and M. Brenner Tetrahedron Letters 1968 6041. 226 M. Mousseron-Canet J.-P. Dalle and J.-C. Mani Tetrahedron Letters 1968 6037. ’” M. Mousseron-Canet J.-P. Dalle and J.-C.Mani Bull.SOC.chim France 1968 1561. ”* K. H. Schulte-Elte and G. Ohloff,Helu. Chim. Acta 1968,51,494; K. Gollnick and G. Schade Tetrahedron Letters 1968 689. 229 N. Furutachi Y. Nakadaira and K Nakanishi Chem. Comm. 1968 1625; A Nickon and J. F. Bagli J. Amer.Chem. SOC.,1961,83 1498. Photochemistry 219 a hydroperoxide and thence the spiro-peroxy-lactone (188) resulting from a migration of the double bond away from the carbonyl The sensitised photo-oxidation of enamines gives amides and carbonyl compounds as a result of the cleavage of the olefinic bond. When the oxidation of N-aryl-enamines is conducted in isopropyl alcohol only moderate yields of products are obtained.231 When benzene is used as a solvent however photo-oxidation of piperidino-enamines gives high yields of N-formyl-piperidine and the pertinent ketone.232 There is spectral evidence for an intermediate which is stable at low temperatures in these reactions but it is not in agreement with any obvious structure.232 1-Dimethylaminoprop-1-yne is smoothly oxidised to NN-diethylpyruvamide under the same condition^.^ 32 A morphilino-enamine has been satisfactorily photo-oxidised as a suspension in dirnethylf~rmamide.~~~ An interpretation of substituent effects in the addition of singlet oxygen to anthracenes has been made on the basis of MO calculations.234 The reaction of singlet oxygen with a photo-en01 is probably involved in the photo-oxidation of aromatic C-methyl groups.235 Aromatic a-diketones are converted into anhydrides in a self-sensitized oxidation in Crystalline endo- peroxides have been obtained from the low-temperature photo-oxidation of tetra-0-methylpurpurogallin (189)237"and tropolone methyl ether.237b The formation of singlet oxygen is the only photochemical process involved in their photo-oxidations to y-la~tones.~~~"~ 230 E.Demole and P. Enggist Helu. Chim. Am 1968,51,481. 231 K. Pfoertner and K. Bernauer Helv. Chim. Acta 1968,51 1787. "'C. S. Foote and J. Wei-Ping Lin Tetrahedron Letters 1968 3267. 233 J. E. Huber Tetrahedron Letters 1968 3271. 234 0.Chalvet R Daudel C. Ponce and J. Rigaudy Internat. J. Quantum Chem. 1968,2 521. 23' P. Yates A. C. Mackay and F. X Garnay Tetrahedron Letters 1968 5389. 236 C. W. Bird Chem. Comm.,1968 1537.237 E. J. Forbes and J. Grifiths J. Chem SOC.(C) 1968; (a)p. 572; (b)p. 575. A. C.Day and E. J. Forbes Numerous examples have appeared of the photo-oxidative cleavage of heterocyclic rings. The sensitized photo-oxidation of (190) results in the loss of C-3 in a reaction analogous to that achieved by enzymic oxidation of the des-0-methyl compound.238 Loss of C-2 occurs when the carbanions of ” (190) Ph[>’;h -I R (191) (192) 3-phenyloxindole and 3-phenylcoumaranone are ~xidised.~~~ Photo-oxidation of the imidazole (191; R = Ph) gives a high yield of (192).240 Unlike lophine (191; R = H) the imidazole (191; R = Ph) cannot form a hydroperoxide. It has been suggested that the formation of (192) might involve a zwitterionic peroxide which would be formed by the heterolytic cleavage of an initially formed endoper~xide.~~~ It seems more likely that the zwitterionic peroxide would be formed by a direct reaction between the highly substituted imidazole and singlet oxygen.Zwitterionic peroxides have also been considered as intermediate^^^' in the reactions of purines with singlet oxygen.241* 242 Good yields of o-cyano-acids can be obtained by the photo-oxidation of oxa~oles.~~~ Miwellmeom.-From material falling under t&s head and having some novelty we list the following items-an admittedly subjective selection photoisomerisation of stereoisomeric 1,4-dichlorospiropentanes;244 photo-vinylation with acetylenes ;245 identity of chemical behaviour of phenylcarbene generated from phenyldiazomethane and phenyl-substituted cyclopropanes and oxirans ;246 substituent effects in the fragmentation of 2,3-diphenyl-oxirans ;247 e.s.r.detection of phenylnitrene from 2-phenyloxaziridines ;248 238 T. Matsuura H. Matsushima and H. Sakamoto J. Amer. Chem. SOC. 1967,89,6370. ’”P. Aeberli and W. J. Houlihan J. Org. Chem. 1968,33 1640. 240 H. H. Wasserman K. Stiller anrl M. B. Floyd Tetrahedron Letters 1968,3277. 241 T. Matsuura and I. Saito Tetrahedron Letters 1968,3273. 242 T. Matsuura and I. Saito Tetrahedron 1968,24,6609. 243 H. H. Wasserman and E. Druckrey J. Amer. Chem. SOC. 1968,90,2440. 244 D. E. Applequist and E. G. Alley J. Org. Chern. 1968,33,2741. 245 R Srinivasan and K. H. Carlough Cad. J. Chem. 1967 45 3209; G. Friedman and A.Komem Tetrahedron Letters 1968 3357. 246 H. Dietrich G. V. Griffin and R C. Petterson Tetrahedron Letters 1968 153. 247 T. I. Temnikova I. P. Stepanov and L. k Dotsenko Zhur. org. Khim 1967 3 1707; T. I. Temnikova and I. P. Stepanov ibid. p. 2253 (J. Org. Chem U.S.S.R.,1967 3 pp. 1665 and 2203 respectively). 248 J. S. Splitter and M. Calvin Tetrahedron Letters 1968 1445. Photochemistry 22 1 hv - c.- Ph cN A Ph Me (193) (194) (195) photolysis of ozonides (ozonide of RCH-CHR + R,; e.g. cyclopentene ozonide to cyclopropane) ;249 photoisomerisation of 1-methyl-2-pyrazolines to cyclopropylazomethanes e.g. (193) + (194) photofragmentation of p-lactams ;251 synthesis of medium and large nitrogen-containing rings (196) from N-phenyl-lactams (195);252 formation of cyclopropyl ketones by triplet- sensitised photolysis of adiazo-ketones in the presence of olefins (only the Wolff rearrangement is observed on direct photolysis);25 carbon-nitrogen cleavage in the photolysis and pyrolysis of triphenylmethyl azide ;254 photolysis of cyclic dithioacetals ;255 cationic intermediates in the photolysis of arylsul- phenyl derivatives ;256 photolysis of dimethylthiocarbamates as a route to deoxy-sugars;2 57 photodesulphurisation of a sulphoxide ;25* formation of cyclopropanes by photolysis of a phosphorus ylid and a phosphazine (Ph3P=N-N=CPh,) in the presence of styrene;259 photolysis of a metal complex of trimethylenemethane.260 249 P.R. Story W. H. Morrison tert. T. K. Hall J.-C.Farine and C. E. Bishop Tetdedron Letters 1968 3291. 250 H. J. Rosenkranz and H. Schmid Helv. Chim. Acta 1968,51,1628. 251 M. Fischer Chem. Ber. 1968,101,2669. ”* M. Fischer Tetruhedron Letters 1968,4295. 253 M. Jones jun. and W. Ando J. Amer. Chem. SOC. 1968,90,2200. 254 F. D. Lewis and W. H. Saunders,jun. J. Amer. Chem. SOC. 1968,90,3828. 255 J. D. Willetf J. R GrunwelJ and G. k Berchtold J. Org. Chem. 1968,33,2297. 256 D. H. R. Barton T. Nakano and P. G. Sammes J. Chem.Soc. (C),1968,322 257 R. H. Bell D. Horton and D. M. Williams Chem Comm. 1968,323. 258 A. G. Schultz C. D. DeBoer and R H. Schlessinger J. Amer. Chem Soc. 1968,90,5314. ’” A. Ritter and B. Kim Tetrahedron Letters 1968 3449. z60 A. C. Day and J. T. Powell Chem. Comm. 1968 1241.

ISSN:0069-3030    

DOI:10.1039/OC9686500187

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

6 PULSE RADIOLYSIS STUDIES ON REACTIVE INTERMEDIATES IN ORGANIC CHEMICAL PROCESSES By G. E. Adam (British Empire Cancer Campaign for Research Research Unit in Radio- biology Mount Vernon Hospital Northwood Middx. England) DURING the current year pulse radiolysis has been applied to an increasing number of problems in organic chemistry including the study of general oxidation and reduction processes in aliphatic aromatic heterocyclic and polymeric molecules involving the formation of neutral anionic and cationic intermediates. Short-lived excited singlet and triplet states produced in a number of irradiated compounds have been observed directly under a variety of conditions and their role in energy-transfer processes investigated. Electron- hydrogen atom and proton-transfer reactions occurring in both polar and nonpolar media have already received considerable attention.Although space restrictions have necessitated the very brief treatment of some of the material an attempt has been made to classify the literature according to processes occurring in the gas phase aqueous solution nonaromatic and aromatic solvents and in systems relevant to some biochemical and biological processes. Gas-phase Processes.-Reactions of atomic oxygen produced by pulse radiolysis of oxygen mixtures have been reported.’-3 Transient spectra assigned to the free radicals C10 and CCl have been observed in irradiated mixtures of CO, O, and CCl,.l The mechanism was interpreted in terms of competition of CCl and O2 for an excited CO species a process which produces CC1 and singlet oxygen reactions of singlet oxygen with CCl led to the production of C10.Reactions of 3P ground-state oxygen atoms produced in N,O or C02 mixtures with benzene and related compounds have been reported,2 and the formation and decay kinetics of the transient spectra described. However absolute identification of the species responsible has not yet been accomplished. With a pulse sampling technique Wentworth and Steelhammer have surveyed some thermal dissociative and nondissociative electron-attachment proce~ses.~ From these studies rate data the electron affinities of some radicals and molecules bond-dissociation energies and some activation energies G. M. Meaburn D. Perner J. Le Calvt and M.Bourene J. Phys. Chem. 1968,72 13920. I. Mani and M. C. Sauer jun. in ‘Advances in Chemistry Series No. 82,’ed. R. F. Gould American Chemical Society 1968 142. ’J. F. Riley and R. W. Cahill in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 540. W. E. Wentworth and J. C. Steelhammer in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 75. 224 G.E. Adams were measured for a range of organic compounds including substituted hydrocarbons aromatic carbonyls and alkyl halides. Aqueous Solutions.-Studies on reactions of the fundamental species the hydrated electron and the OH radical have continued. Anbar and Hart’ measured the reactivity e; with many ketones and related substances.Rate constants varied from 3 x 10% ’sec.-’(for urea) to diffusion-limited values and for substituted aldehydes ketones and carbocyclic acids log k was linear with Taft’s c* function. Amides and esters deviated from this relationship due to the presence of mesomeric structures in which the electrophilic centre is localised on either the nitrogen or the alkoxy-oxygen. Transient spectra assigned to OH adducts of substituted benzenes have been observed previously by several groups.6-’ ’ Interest in the chemistry of cyclohexadienyl radicals produced by either OH or H atom addition to aromatic molecules has continued during the current year. Benzoate salicylate and various aromatic ethers ketones nitro-compounds halides amines acids esters and sulphur-containing compounds’ 2-’ ’have all been investigated in aqueous solution.In all cases OH addition to these compounds produces radicals which possess fairly intense absorption maxima in the region between 300 and 400 nm. Invariably the addition reactions proceed at rates which approach diffusion-controlled limits and the adducts decay usually by second-order radical-radical processes. However earlier work,*P has shown that unimolecular elimination reactions can occur from hydroxycyclohexa- dienyl radicals and may predominate under conditions where the maximum free-radical concentrations are very much lower than those usually obtained in pulse radiolysis. Cercek has measured the effect of electron-donating and electron-with- drawing substituents on the rate constants for both the second-order decay of hydroxycyclohexadienyl radicals and also for their reaction with oxygen.’’ In both reactions the effect of the substituent on the rate constants was cor- related with the Hammett substituent constants calculated from para-and meta-values for the electron-withdrawing groups. The trends were reflected also in the activation energies for the reactions. Despite the fact that the rate constants for the primary OH-addition reactions show much less variation E. J. Hart E. M. Fielden and M. Anbar J. Phys. Chem. 1967,71,3993. L. M. Dorfman I. A. Taub and R. E. Biihler J. Chem. Phys. 1962,36,3051. ’ L. M. Dorfman I. A. Taub and D. A. Harter J. Chem. Phys. 1964,41,2954. G. E. Adams E. J. Land and B. D.Michael Nature 1966,211,293. G. E. Adams J. H. Baxendale and J. W. Boag Proc. Roy. SOC.,1964 A 277,549. lo A. Wigger A. Henglein and K. D. Asmus Ber. Bunsengesellschaft Phys. Chem. 1967,71,513. ’’ B. Chutny Nature 1967,213,593. l2 P. Neta and L. M. Dorfman in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 222. l3 C. B. Amphlett G. E. Adams and B. D. Michael in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 231. l4 K. D. Asmus B. Cercek M. Ebert A. Henlgein and A. Wigger Trans. Faraday SOC. 1967 63 2435. l5 B. Cercek and M. Ebert in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 210. R.0.C. Norman Proc Roy. SOC.,1968 A 302 315. Pulse Radiolysis Studies on Reactive Intermediates 225 Neta and Dorfman have observed a similar correlation with the Hammett constants for the different substituents.12 Nitro-compounds have been studied by pulse radiolysis.In nitrobenzene solutions where both H-atom and OH-adduct radicals are formed l4 nitro-phenol isomers are produced by disproportionation of the hydroxy-adduct radicals although it is suggested that nitrocyclohexadienes are among the major products. In the pulse radiolysis of nitrophenol solutions however it is claimed that OH addition produces initially the radical HOC6H4N02H. This then undergoes intramolecular rearrangement to form the OH ring-adduct which in turn disproportionates to nitrophenol and dihydroxynitro- benzene.’ Nitroalkanes in both the nitro- and the aci-anion form are reduced rapidly by el although addition reactions of H and OH appear to occur exclusively with the aci-forms.’ Pulse radiolysis has been applied to the study of electron-transfer processes in aqueous solutions of organic compo~nds’~~~~ and also in organic solvents2l,22 (see later).In a general review of past and recent work,” processes occurring in irradiated solutions of aliphatic and aromatic organic compounds have been discussed with respect to general redox phenomena in radiation and free-radical chemistry. In aqueous solutions nondissociative electron trans- fers from protonated and anionic forms of radicals produced from alcohols ketones thymine formate ion and a polymer poly(ethy1ene oxide) have been followed by direct observation of the bleaching of ferricyanide solutions and of the formation of spectra of radical-anions of aceto- and benzo-phenones.The bleaching of ferricyanide has been used in a kinetic competition method for the measurement of rate constants for peroxy-radical formation. R’ + 02 + RO2 (1) As expected the k values approach diffusion-controlled limits. In alkaline aqueous mixtures of acetone acetophenone benzophenone and ferricyanide a process has been observed in which electron attachment to acetone is followed by sequential cascade electron transfer through acetophenone and benzophenone resulting finally in reduction of [Fe(CN),] -. In studies on alcohol radical oxidation by biacetyl it was found23 that the transient spectra of the biacetyl anion and the protonated radical could not be resolved.However the pK of the radical (4.4)was found by a pulse con- cuctivity method to be in agreement with a previous approximate value.16 ‘Repair’ of various organic radicals (X-) by hydrogen transfer from the SH l7 B. Cercek J. Phys. Chem. 1968,72 3832. l8 K-D. Asmus and I. A. Taub J. Phys. Chem. 1968,72 3382. l9 G. E. Adams B. D. Michael and R. L. Willson in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 289. 2o E. J. Land and A. J. Swallow Biochem. Biophys. Acta 1968 162 327. 21 L. M. Dorfman N. E. Shank and S. Arai in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 58. 22 S. Arai and L.M. Dorfman in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 378. ” J. Lilie G. Beck and A. Henglein Ber. Bunsengesellschuji Phys. Chem. 1968,72 529. 226 G. E. Adams group of cysteamine (RSH) has been followed24 by measurement of the forma- tion of the coloured radical complex RS SR- (ref. 25). X + RSH + XH + RS. (2) RS-+ RSH (or RS-) + [RS*SR-] (3) The repair reactions proceed via H atom and not by electron transfer. General aspects of the pulse radiolysis of monomers and polymers have been discussed.26-28 Primary radiolysis radicals react at the vinyl group to give radical ions or radicals although in styrenes ring addition also occurs. Mechanisms of polymer protection by sulphur-containing compounds are discussed and preliminary data on solid poly(methy1 methacrylate) are pre- sented.The pulse radiolysis of aqueous solutions of organic dyes has been re~iewed.~’Quinoid dyes react rapidly with e; and reductive bleaching occurs generally by semiquinone dismutation. Oxidative decolouration is complex involving several OH radicals. In an extension of early a combined pulse radiolysis and flash photolysis technique has been employed in the study of radiation-induced chemiluminescence produced by interaction of ea; with dye intermediates including OH adducts and excited triplet state^.^' Some inconsistencies in published OH reactivities are now resolved. Rate constants determined by the CNS-or I-method appear32* 33 to be somewhat low due to the use of incorrect values for the reference constants.34 Reactivities measured by use of hydroxycyclohexadienyl radicals as reference systems,I2 have shown that a correction factor of ca.1.7 should be applied. The radiation- induced bleaching of p-nitrosodimethylaniline has also been employed to measure OH rate constant^.^' However recent pulse radiolysis studies on this system36*37 have cast doubts on the general validity of the method. Nonaqueous Systems.-Interest in the pulse radiolysis of organic liquids is increasing and there is now available considerable additional data on the nature and reactivity of short-lived intermediates in a wide variety of organic liquid systems. Furthermore advances in technique3* have permitted the direct observation of transient species with life-times confined to the sub- microsecond range.24 G. E. Adams G. S. McNaughton and B. D. Michael Trans. Faraday SOC. 1968,64 1256. 25 G. E. Adams G. S. McNaughton and B. D. Michael in ‘The Chemistry of Ionisation & Excita-tion,’ ed. G. R. A. Johnson and G. Scholes Taylor and Francis 1967,283. 26 C. Schneider and A. J. Swallow Makromol. Chem. 1968 114 155. 27 C. Schneider and A. J. Swallow Makromol. Chern. 1968 114 172. 28 A. J. Swallow in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould American Chemical Society 1968 499. 29 L. I. Grossweiner in ‘Advances in Chemistry Series No. 82,’ ed. R. F. Gould 1968 309. 30 W. Priitz K. Sommermeyer and E. J. Land Nature 1966,212 1043. 31 L. I. Grossweiner and A. F.Rodde jun. J. Phys. Chem. 1968,72 756. ’’ G. E. Adams J. W. Boag and B. D. Michael Trans. Faraday SOC. 1965,61 1417. 33 J. K. Thomas Trans. Faraday SOC. 1965,61 702. 34 J. H. Baxendale P. L. T. Bevan and D. A. Stott Trans. Faraday SOC. 1968,64 2389. 35 I. Kralic and C. N. Trumbore J. Amer. Chem. SOC. 1965,87 2547. 36 F. S. Dainton and B. Wiseall. Trans. Faraday SOC. 1968 64 694. 3’ J. H. Baxendale and A. A. Khan Internat. J. Radiation Phvs. and Chem. in press. J. W. Hunt and J. K. Thomas J. Chrni. Phys.. 1967. 46. 2954. Pulse Radiolysis Studies on Reactive Intermediates 227 Several groups have studied energy-transfer phenomena in organic solvents and earlier work has been reviewed by C~ndall.~’ In irradiated cyclohexane containing napthalene anthracene pyrene and other aromatic compounds 43 fluorescence spectra are seen characteristic of the singlet excited The spectral profiles change and the fluorescence yields increase with solute concentration an effect partly attributable to excimer equilibria.Singlet fluorescence is quenched by N20 SF, nitromethane and other electron- scavengers40u indicating together with pulse experiments with an electric field40b that singlets arise by charge transfer to the solute followed by ion- neutralisation. The yields of triplet excited states of the solute also increase with solute concentration and it is proposed that these electronically excited states are derived also from ionic precursors some directly uia charge neutralisa- tion and some through the intermediate formation of the singlet.Studies in the nanosecond range have helped to clarify the mechanism of triplet formation.42a In cyclohexane solutions of naphthalene or anthracene 3N (or 3A) is produced in two time-resolved processes. One is present im- mediately after the three nanosecond pulse. the other developing over a period of several hundred nanoseconds. The growth kinetics of the latter are identical to the decay kinetics of the solute radical-ion spectrum and since N,O inhibited the process it was proposed that the triplet is derived from charge neutralisation. The fast component is formed too rapidly for inter- system crossing and thus direct excitation of the solute by sub-excitation electrons was suggested. Data have been obtained on the reactivity of hydrogen atoms and cyclo- hexyl and hexyl radicals in cyclohexane and hexane Excited states in benzene solutions have been studied in both the micro- ~ second4’.44 and nanosecond time range.45 Cundall et u E .~ observed phos- phorescence from dilute biacetyl in benzene and attributed the emission to triplet biacetyl produced from excited benzene. From experiments with cyclohexene as a selective triplet scavenger in this system a mechanism was proposed from which the yields of singlet and triplet excited benzene were calculated i.e. G(’B) = 1.43 and G(3B) = 1.24. Nanosecond studies4’ have revealed the absorption and fluorescence spectra of singlet benzene where the decay kinetics (tt = 18 nanosec.) agree 3y R. B. Cundall In ‘Energetics and Mechanisms in Radiobiology,’ ed.G. 0. Phillips Academic Press 1968. 40a F. S.Dainton G. A. Salmon T. Morrow and G. F. Thompson Chem Comm. 1968 326. 40b E. J. Frankevich T. Morrow and G. E. Salmon. Nurure 1968 219 481. 41 E. J. Land and A. J. Swallow Trans. Faraday SOC.. 1968 64,1247. 42a J. K. Thomas K. Johnson T. Klippert and R. Lowers J. Chem. Phys. 1968,48 1608. 42b M. C. Saver jun. and I. Mani J. Phys. Chem.. 1968,72,11. 43 F. S. Dainton C. T. Peng and G. A. Salmon J. Phys. Cheni. 1968 72 3801. 44 R. B. Cundall G. B. Evans P. A. Griffiths and J. P. Keene J. Phys. Chem. 1968,72 3871. 45 R. Cooper and J. K. Thomas J. Phys. Chem. 1968,48 5097. 228 G.E. Adarns well with previous e~timates.~~9 47 Naphthalene and anthracene quench the fluorescence to yield the singlet-fluorescence and triplet-absorption spectra of the solute.45 Addition of penta-l,3diene a selective triplet scavenger to naphthalene solutions led to the determination of the primary yields of the benzene excited states i.e.G(lB) = 1-62 and G(3B) = 1.85. This singlet value is in reasonable agreement with that quoted above. Temperature studies have revealed however that the benzene fluorescence should be assigned to a singlet excimer state.48 Similar processes are observed in pulse-irradiated aniline solutions.49 Triplet states of aromatic solutes (naphthalene anthracene etc.) are pro- duced in irradiated ethereal solutions (dioxan,” tetrahydrof~ran,~ ,and 1,2-diethoxybenzene’ ’). In dioxan solutions the spectrum of anthracene radical anion is produced.The absorption is increased and stabilised by the addition of LiAlH4 due to replacement of reactive cations by the inert Li’ ion which reduces the effect of charge-neutralisation processes. However the absorption of the anthracene triplet was unaffected and it was concluded that ions from neither the solute nor the solvent were involved in its formation. Similar conclusions were drawn for the formation of triplet states of some aromatic solutes present in irradiated solid polystyrene. 52 A pulse radiolysis of the radiation-induced geometrical isomerisa- tion of stilbene has revealed transient spectra of the stilbene anion and the solute triplet. In dilute solutions the isomerisation appears to proceed to a radiostationary state predominantly via the triplet whilst in concentrated solutions trans-stilbene is formed exclusively from processes involving mostly ions or radicals.The pulse radiolysis of amines of low-ionisation potential in various aro- matic and nonaromatic solvents produces intense transient absorption spectra which closely resemble those of the radical cations. However studies with ion-scavengers showed (except in dimethyl sulphoxide) that the species are triplet excited states rather than positive ions. Studies on electron- and proton-transfer reactions in organic liquids have been reported22 and earlier work has been reviewed.2’ Protonation rate constants for aromatic radical anions e.g. biphenyl anthracene p-terphenyl etc.are not large (k < 105~-sec. -l) and it appears that low preexponential factors rather than high activation energies are responsible. Electron-transfer processes of the type Arene + Arene 4Arene + Arene (4) 46 M. A. Dillon and M. Buxton in ‘Pulse Radiolysis,’ ed. M. Ebert J. P. Keene A. J. Swallow and J. H. Baxendale Academic Press London and New York 1965,259. 47 I. B. Berlman in ‘Handbook of Fluorescence Spectra of Aromatic Molecules,’ Academic Press New York 1965. 48 J. K. Thomas personal communication. 49 R. Cooper and J. K. Thomas J. Chem. Phys. 1968,48 5103. J. H. Baxendale and M. A. J. Rodgers J. Phys. Chem. 1968,72,3849. 51 T. J. Kemp and J. P. Roberts Trans. Faraday SOC. 1968,64,2106. 52 S. K. Ho S. Siegel and H. A. Schwarz J. Phys.Chem. 1967,71,4527. Pulse Radiolysis Studies on Reactive Intermediates 229 have been studied extensively. Rate constants are high and in some cases are diffusion limited. The data have been used with some success as a test of the Marcus theory of homogeneous electron-transfer.’ In two instances (pyrene- anthracene and pyrene-9,lO-dimethylanthracene)where equilibrium conditions can be studied the derived equilibrium constants are in excellent agreement with those measured potentiometrically. In the pulse radiolysis of carbon tetrachloride solutions radical cations and charge-transfer spectra are produced. In solutions containing benzene or other aromatic solutes a linear correlation was found between the ionisation potential of the solute and the energy of the absorption maxima of the charge- transfer complexes.’ Systems of Biochemical and Biological Interest.-Reviews include the application of pulse radiolysis to problems in radiobiology’’ and recent work on the radiation chemistry of aqueous DNA and constituent n~cleotides.’~ Pulse studies include measurement of the reactivities of both e& and OH with DNA pyrimidines purines and related nucleotides and nu~leosides.’~-~’ Analysis of transient spectra has confirmed that the site of OH attack on thymine and 5-methylcytosine normally at the 5,6 double-bond changes with pH in the alkaline regi~n.’~ In the corresponding nucleotides the site of attack is partially shifted with increasing pH from the base to the pentose residue.The reactivity of OH with the base uracil is however almost in- dependent of PH.~* In long-chain polynucleotides the reactivity decreases with chain length6’ and the influence of heterogeneity on the kinetics of such reactions have been discussed by Loman and EberL6’ The rate constant for reaction of e with uracil is reduced in alkaline solution an effect which is partly attributable to tautomeric structural changes.60 Although transient spectra from DNA indicate that OH attack occurs at sites on all four n~cleotides,’~ the radical complex Cl is more specific in its reactions.” In experiments with enzymes it has been shown that unlike the reactivity of OH radicals the reactivity of e& with ribonuclease increased threefold at the transition temperature.62 This was interpreted as evidence for direct reductive attack at the disulphide linkage which becomes accessible at this temperature due to the uncoiling of the molecule.53 R. A. J. Marcus J. Chem. Phys. 1956,24,966. s4 R. E. Biihler Helu. Chirn. Acta 1968,51 7. 55 G. E. Adams in ‘Radiation Chemistry of Aqueous Systems,’ ed. G. Stein Interscience 1968,241. 56 G. Scholes in ‘Radiation Chemistry of Aqueous Systems,’ ed. G. Stein Interscience 1968 259. 57 R. M. Danziger E. Hayon and M. E. Langmuir J. Phys. Chem. 1968,72 3842. 58 L. S. Myers jun. M. L. Hollis and L. M. Theard in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 345. 59 J. F. Ward and I. Kuo in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould Amcrican Chemical Society 1968 368.6o C. L. Greenstock M. Ng,and J. W. Hunt in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 397. 61 H. Loman and M. Ebert Internat. J. Radiation Biol. 1968 13 549. 62 R. Braams and M. Ebert in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 469. 230 G. E. Adams Direct evidence for electron attack at the -S-S-group in enzymes has been obtained from the pulse radiolysis of lys~zyme~~ where a transient spectrum is produced which is almost identical to that of the radical ion RS-SR-observed from ~ysteamine.~’ Attack on lysozyme by OH radicals produces a U.V. transient absorption spectrum reminiscent of hydroxycyclohexadienyl radicals.Similar spectra are formed from phenylalanine tyrosine and the polypeptide polytyrosine and there is evidence64 that these radicals also undergo unimolecular elimina- tion reactions analogous to those observed in phenols. An interesting new development has been the application of pulse radiolysis to the study of luminescence phenomena in microcrystalline adenine6’ and DNA.66 In preliminary studies emission spectra containing several temperature- dependent components have been observed in the time range 5 psec. to 5 min. and have provided information on the nature of energy-trapping sites in the irradiated solids. Future work in this field particularly in the natural nucleic acids is awaited with interest. Interesting examples of electron-transfer processes in the pulse radiolysis of some biochemical substances have been presented.These include the reduction of nicotinamideadenine dinucleotide (NAD +)and cytochrome c by formate ion and electron transfer from the radical ion AMP- to NAD’. Chemical mechanisms of radiosensitisation by organic substances of high electron affinity e.g. quinones and a-dioxo-compounds have been discussed’ ’ in relation to electron-transfer studies by pulse radiolysis and a pulse radiolysis of the radiosensitiser 2,2,6,6-tetramethylpiperidin-4-one N-oxide has shown that this stable free-radical reacts rapidly with radicals produced by OH attack on thymine (k =3 x 10’~~’ set.-I). Finally a series of papers has appeared in which pulse radiolysis has been used to study ion-binding between various polyanions and their associated counter ion^.^'-^^ By measurement of the changes in reactivity of e with dyestuffs following the addition of various mucopolysaccharides DNA and other polymeric substances the formation of ion complexes has been demon- strated.The rapid time resolution permits the study of such phenomena before any precipitation can occur. 63 J. W. Davies M. Ebert and R. J. Shalek Internat. J. Radiation Biol. 1968 14 19. 64 J. Chrysochoos Radiation Res. 1968,33,465. 65 E. M. Fielden and S. C. Lillicrap in ‘Advances in Chemistry Series No. 81,’ ed. R. F. Gould American Chemical Society 1968 444. 66 E. M. Fielden and S. C. Lillicrap personal communication. ‘’P. T. Emmerson and R. L. Willson J. Phys.Chem. 1968 72. 3669. E. A. Balazs J. V. Davies G. 0.Phillips and D. S. Scheufele,J. Chem. SOC.(C) 1968 1420. 69 E. A. Balazs J. V. Davies G. 0.Phillips and D. S. Scheufele J. Chern. Soc. (C) 1968 1424. 70 E. A. Balazs J. V. Davies G. 0.Phillips and D. S. Scheufele J. Chem. SOC.(C) 1968. 1429.

ISSN:0069-3030    

DOI:10.1039/OC9686500223

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

7 ELECTRO-ORGANIC CHEMISTRY By J. H. P. Utley (Chemistry Department Queen Mary College Mile End Road London E.l) THIStopic has not previously received separate attention in Annual Reports and accordingly key papers setting the background to the year’s work are included in the review. Division of the material into anodic and cathodic aspects is natural and under these headings it is organised according to the nature of the species believed to be discharged. The historical and theoretical significance of the Kolbe reaction justifies a further sub-division. Organic polarography has not been reviewed except where the results are clearly related to macro-scale processes. Anodic Processes.-Anodic Reactions of Organic Anions. Anodic oxidation of carboxylates to radicals (the Kolbe reaction).The scope and limitations of the Kolbe reaction are well summarised in early reviews.’ Recently much progress has been made in elucidating the mechanistic details of this reaction. The relevant results of physicochemical studies on simple organic systems have been critically reviewed by Conway2 and an account of product studies ‘the organic chemical approach,’ is a particularly useful part of a review by Eber~on.~ It has become clear that radical species are involved but it is doubtful that they can properly be called ‘free’ radicals. The involvement of the tri- deuteriomethyl radical in the electrolysis of trideuterioacetate ion in water was originally demonstrated by Clusius4 who found that a little perdeuterio- methane accompanied perdeuterioethane formation.This is consistent with a radical mechanism involving hydrogen-atom abstraction (Scheme 1). CD C02-CD3.CO; -CD; + CO CD; + CD,.C02-CD4 + ‘CD,*C02-(or CD 3C02H) (or ‘CD2.C02H) SCHEME 1 Nuclear methylation of trinitrotoluene was effected in 9 % yield by electrolysis of sodium acetate in the presence of trinitrotoluene at a platinum anode5 and trapping of anodically generated methyl radicals was demonstrated by several groups.6 In particular Smith and his co-workers6a electrolysed ’ (a) B. C. L. Weedon Quart. Rev. 1952,6 380; (b) B. C. L. Weedon Adv. Ory. Chem. 1960 1 1. ’ A. K. Vijh and B. E. Conway Chem. Rev. 1967,67,623. L. Eberson ‘Chemistry of the Carboxylic Acid Group,’ ed. S. Patai Interscience London in the press.K. Clusius and W. Schanzer 2.phys. Chem. (Leipzig) l943,192A 273. L. F. Fieser R C. Clapp and W. H. Daudt J. Amer. Chern. SOC. 194264 2052. (a) W. B. Smith and H. G. Gilde J. Amer. Chem. Soc.. 1959 81 5325; (b) W. B. Smith and J. L. Massingill J. Amer. Chem. SOC.,1961 83 4301; (c) R. V. Lindsey and M. L. Peterson J. Amer. Chem. SOC. 1959. 81. 2073. J. H. P.Utley acetate ion and butadiene in methanol. Prominent among a large number of products were pent-1-ene (l) 3-methylpent-1-ene (2) trans-hex-3-ene (77 % of the hexene fraction) (3) cis-hex-3-ene (2% of the hexene fraction) (4) deca-3,7-diene (5) and 3-ethylocta-1,5-diene (6). Et H \/ CH,-[CH,],.CH=CH EtCHMe.CH=CH H ,c=c \ Et (1) (2) (3) Et Et \c=c/ EtCH=CH .[CHJ2 CH==CHEt H / \H (4) (5) EtCH=CH*CH,*CHEt.CH=CH (6) Dimerisation coupling with methylradical and hydrogen abstraction involv- ing the radical intermediate (7) formed by addition of methyl to butadiene readily accounts for such products.CH3 + CH2=CH*CH=CH2-,[CH3*CH2*CHCH=CH2 c-r CH3 *CH2CH=CH *CH,] (7) It is probable that other products such as acetates alcohols and ethers arise from nucleophilic attack upon carbonium ions formed by anodic discharge of the alkene or diene or the first radical intermediate. The electrolysis of potassium acetate in the presence of 3,3-dimethylbut-l-ene illustrates this point.’ Twelve products are obtained the major one being of radical origin but with carbonium ion products well defined by virtue of the smooth re- arrangement of the first-formed cation (Scheme 2).Me,C.CH=CH M* Me,C.cH.CH,-CH Ace-/-.+ /y EtMeCH.CMe:CH i’ ii [Me,C.CHEt] Me,C-CHEtMe (4.4 %) (46 %) EtMeCH C(OMe)Me (8 %) Conditions:i rearrangement ;ii solvolysis or rearrangement SCHEME2 Differences in the reactivity of alkyl radicals generated anodically and chemic- ally have been sought. Products formed by thermal decomposition of propionyl ’ W. B. Smith and Y. H. Yuh Tetrahedron 1968,24 1163. Electro-organic Chemistry peroxide and electrolysis of potassium propionate have been compared8 and it is noteworthy that the production of ethane (by way of hydrogen abstraction) is less favoured in the anodic reaction whereas dimerisation to yield butane is favoured.This points to the electrochemically generated ethyl radicals reacting whilst adsorbed at the electrode surface although it is possible that such differences could arise from the inevitably steep concentra- tion gradient set up at the anode. The above and similar experiments strongly support the view that alkyl radicals are formed but give little information on the r61e of the electrode surface or on the electron transfer and subsequent reaction steps. General considerations have been deployed by C~nway,~ e.g. high yield of dimeric products little hydrogen abstraction from organic solvents necessity of noble-metal electrodes low energies of solvation but high energies of chemisorption and lack of e.s.r. evidence. He concludes that adsorption is of such importance in the Kolbe reaction that the radicals are qualitatively different from those familiar to the organic chemist.Eberson" summarised an alternative view pointing out that at least in aqueous solution strong adsorption is not likely on the oxide film known to cover the electrode.' ' In addition earlier stereochemical evidence concerning the production of optically inactive Kolbe dimer [meso or (+)I from (+) or (-) potassium a-methylbutyrate'2 is at variance with the projected dimerisation of tightly adsorbed radicals. An adsorbed radical would be expected to couple with complete retention (with an adjacent radical) or complete inversion (if ap- proached from the solution) to give in either case an optically active product.Attention is also drawn to the behaviour of cyanoalkyl radicals generated chemically and electrochemically. The ratio of C-C to C-N coupled products is comparable in the two cases13 in sharp contrast to the photolytic decompo- sition of Me,C(CN)N :N(CN)CMe adsorbed on silica gel which yields the C-C coupled product only.I4 It appears that clear cut e.s.r. evidence will be difficult to obtain.' ' A characteristic of the Kolbe reaction is the high anode-potential required (>2.0 v us. SCE) and partly related to this the limited range of electrode materials that can be used. In aqueous solution platinum and iridium are the only useful anode materials for the Kolbe reaction. In nonaqueous solvents such as glacial acetic acid gold palladium and lead (IV) oxide may be used.16 The typically high potential is also found for the Kolbe reaction of CF,CO,K in rigorously dried CF3C0,H.17 The coulombic yield is high (96%) and S.Goldschmidt W. Leicher and H Haas Annalen 1952 577 153. B. E. Conway and A. K. Vijh Electrochim. Acta 1967 12 102. lo L. Eberson Electrochim. Acta 1967 12 1473. (a) M. Fleischmann J. R Mansfield and W. F. K. Wynne-Jones J. Electroanalyt. Chem. 1965,10,511;(b)M. Fleischmann J. R Mansfield and W. F. K. Wynne-Jones J. Electroanalyt. Chem. 1965 10 522. l2 E. S. Wallis and F. H. Adams J. Amer. Chem SOC. 1933,55 3838. l3 L. Eberson and S. Nilsson unpublished results quoted in ref. 3. l4 P. k Leermakers L. D.Weis and H. T.Thomas J. Amer. Chem SOC. 1965 87 4403. l5 B. E. Conway Rev. Pure and Appl.Chem. 1968 18 109. l6 N. Sato T. Sekine and K. Sugino J. Ejectrochem SOC. 1968 115 242. l7 €3. E. Conway and M. Dzieciuck Canad. J. Chem. 1963,41,21 38,55. 234 J. H. P. Utley addition of water to the system lowers it considerably. It is suggested" that the high potentials are associated with the need to establish a carboxylate film on the electrode efficient Kolbe coupling depending on the then difficult discharge of carboxylate ions on top of the first layer. For the CF,C02K- CF,CO,H system film formation is not essential as some C2F6 is formed at potentials below 2.0 v. In aqueous systems however oxygen evolution pre- dominates at potentials below ca. 2.0 v (SCE) and the significance of this for electrolysis of aqueous solutions of acetate ions at platinum has been in- vestigated.Fleischmann et al." employed a repetitive potential-pulse tech- niqueIg that enabled them to examine the products of brief electrolyses (lo- sec. to 1 sec.) at various potentials above that required for normal Kolbe reaction in the steady state. At polarisation times of seconds acetate ions are oxidized completely to carbon dioxide and evolution of oxygen and ethane is completely inhibited. As the duration of electrolysis is increased the yield of oxygen goes through a maximum and production of ethane increases smoothly to its steady-state value at polarisation times of >lo- seconds. In this system it appears that the formation of an oxide film is essential to ethane formation. If the electrolysis time is too brief to allow oxide formation the acetate is completely oxidised at the bare metal surface.Interpretation of the kinetics of product formation with varying polarisation times suggests that the discharge of acetate ion is the slowest step. An attempt to calculate the standard potential of such a step has been made.20 Discharge of acetate ions to acetoxy-radicals and discharge of acetate with concerted decarboxylation were chosen as models the respective calculated standard potentials were 2-41 v and 1.55 v. Substitution of these values into the equation relating current standard potential and the rate constants for the forward and reverse processes indicated that the value of 1.55 v was com- patible with the known irreversibility of the Kolbe reaction whereas the 2.41 v value was not.Consequently it was suggested that electron transfer and decarboxylation were concerted. This calculation was severely criticised by Conway and Vijh2 on the grounds that probably high energies of chemisorp- tion were not taken into account. Their own calculations21 suggest that the standard reversible potential for the overall Kolbe reaction in aqueous acetate solutions is -0.396 v on the hydrogen scale. Empirical rejection of Eberson's view was sought by Skell., It was argued that the concerted process (8) would confer an incipient free radical nature to the alkyl group in the RC02-[R*...COi*..e(anode)] + R' + CO + e (8) transition state and that the rate of discharge should therefore be susceptible to normal substituent effects.Competitive electrolyses of a number of potassium T. Dickinson and W. F. K Wynne-Jones Trans. Faraday SOC. 1962,58,400. l9 M. Fleischmann J. R. Mansfield H. R Thirsk H. G. E. Wilson and Lord Wynne-Jones Electrochim Acta 1967 12 967. 2o L. Eberson Acta Chem. Scand. 1963,17 2004. B. E. Conway and A. K. Vijh Z. analyt. Chem. 1967 224 149. 22 P. H. Reichenbacher M. Yen-Chien Liu and P. S. Skell J. Amer. Chem. SOC. 1968.90 1816. Electro-organic Chemistry 235 carboxylates were run in water and in methanol the composition of starting and recovered acid mixtures being determined by V.P.C. The structure of R varied from primary to secondary to tertiary and the runs were conducted with large variations of current voltage anode potential concentration and extent of electrolysis.The reproducibility of results was admittedly poor but carboxylates of similar molecular weights were apparently consumed at comparable rates irrespective of the structure of R thereby suggesting rate limiting discharge to acetoxy-radical. However anodic chronopotentiometric oxidation of two of the acids included in the original study reveals that at low concentrations in unstirred acetonitrile solution the rate of one electron oxidation of carboxylate is diffusion ~ontrolled.~~ It is less likely that the rates of the competitive electrolyses are diffusion controlled and therefore similar but the possibility is not ruled out. In summary therefore the mechanism of the Kolbe reaction is at the moment probably best represented by Scheme (3).RC02 -(soln) + RC02 -(ads) '*RCO; (ads) fast R' I(ads) @!+ R-R -coz t slow? 1 abstraction -e disproportionation SCHEME 3 Among the applications to synthesis of the Kolbe reaction since the last review of that aspect' is the electrolysis of hydrogen methyl cis-cyclopropane- 1,2-dicarboxylate. At a platinum anode at uncontrolled potential and high current-density this carboxylate yields a mixture of stereoisomers of the coupled products in 25 %yield.24 Synthesis of dimethyl ( )-2-acetoxydodecane-dioate (9) a possible intermediate in the synthesis of ustilic acid B was ac- Me0,C- CH(0Ac)- [CHz]9.C02Me H02C.CH2. CH(0Ac). CO,Me Q) (10) complished in prohibitively low yield by the anodic cross-coupling of methyl hydrogen (f)-2-acetoxy-butanedioate(10)and methyl hydrogen decanedioate in methanol-sodium methoxide s~lution.~' Electrochemical oxidation of the cyclobutane-1,3-dicarboxylicacid (11) in dry methanol yields the bicyclo- butane (12).26 H02C C02Me D Me0,C (11) (12) 23 P.H. Reichenbacher M. D. Moms,and P. S. Skell J. Amer. Chem. SOC.,1968,90 3432 24 T. D. Binns R Brettle and G. B. Cox J. Chem SOC.(C) 1968 584. 25 R. Brettle and D. W. Latham J. Chem. SOC.(C) 1968,906. 26 A. F. Vellturo and G. W. Griffin J. Org. Chem. 1966,31 2241. J. H. P.Utley This is the only reported example of Kolbe coupling involving a dibasic acid and although coupling of the diradical (13) is an attractive rationalisation cyclisation and decarboxylation from the carbonium ion (14) is also possible.CO,Me C0,Me Me0,C0 Me02C (13) (14) The attempted anodic oxidation in methanol of another dicarboxylate disodium malonate in the presence of cyclohexene resulted not in the forma- tion and trapping of methylene to yield norcarane but in cyclohexenyl methyl ether.27 Benzene has been prepared in good yield by the electrolysis of trans-1,2-dihydrophthalic acid (15).28 More recently Dewar benzene has been prepared by a similar route (17).29A number of bicyclic olefins including homobarrelene derivatives (16) were synthesised by electrolytic decarboxylation of the corresponding vicinal dicarboxylic acids. 30 The method is often considerably more efficient than lead tetra-acetate bis- decarboxylation.The Kolbe electrolysis of a series of o-halogeno-alkanoic acids has been re- e~amined.~' It is confirmed that the chloro-acids give coupled products in good yield but the corresponding bromo- and iodo-acids liberate halogen and are converted only in low yield into dihalogenohydrocarbons. Inhibition of Kolbe coupling by halide ions has been reported,32 although this can be offset to some extent by the addition of cyclohexene to the reaction mixture. Anodic oxidation of carboxylates to carbonium ions. The limitations of the Kolbe reaction are well known.' The formation of major side-products such as esters alcohols olefins and ethers led to the suggestion by Walling33 that 27 E. K. Spicer and H. G. Gilde Chem. Comm. 1967,373.E. A. Pasquinelli Anules. Asoc. quim. argentinu 1943,31 181. 29 E.E.van Tamelen and D. Carty J. Amer. Chem. Soc. 1967,89 3922. 30 (a)P.Radlick R Klem S. Spurlock J. J. Sims E. E. van Tamelen and T. Whiteside Tetrahedron Letters 1968 5117; (b)H.H. Westberg and H. J. Dauben jun. Tetrahedron Letters 1968 5123. 31 K. Maruyama and K. Murakami J. Chem SOC.,Japan 1968,89 196. 32 M. Ya Fioshin and I. k Avrutskaya Elektrokhimiya 1967,3 1288. 33 C. Walling 'Free Radicals in Solution,' John Wiley New York 1958,p. 581. Electro-organic Chemistry further one-electron oxidation of the alkyl radical to a carbonium ion occurred when structural features could stabilise the cation. Such features e.g. sub- stitution at the a-position by electron-donating groups are common in cqrboxylic acids that fail to give Kolbe coupling.Early examples of anodically generated carbonium ions were provided by Corey and co-worker~’~ (Schemes 4 and 5). Pt anode /I% MeOH MeOH, 50 v 10” n -2e -COz pt anode MeCN -@H SCHEME 5 The electr~lyses~~ of cis-and trans-bicyclo[3,l,0]hexane-3-carboxylic acids (18) were particularly interesting as the anticipated carbonium ion is the trishomocyclopropyl cation (19).36 The electrolysis of either acid produces mainly the rearranged cis-and trans-bicyclo[3,1,03 hexan-2-ols good evidence of carbonium-ion intermediacy. However electrolysis of the deuterium- labelled acid (20) gave no product that suggested deuterium scrambling and it now seems unlikely that the trishomocyclopropyl cation is involved.Adjacent nitrogen provides good stabilisation of anodically formed car- bonium ions and can now be seen to account for the almost quantitative formation of methoxylated products in the electrolysis of N-acyl-a-alanines 34 E. J. Corey N. L. Bauld R T. LaLonde J. Casanova and E. T. Kaiser J. Amer. Chem SOC. 1960,82,2645. ” P. G. Gassman and F. V. Zalar J. Amer. Chem. SOC. 1966,88,2252. 36 S. Winstein Chem. SOC.Special Publ. 1967 No. 21 5. J. H. P.Utley (18) (19) (20) in rnethan~l.~’ A more recent example is the electrolytic decarboxylation of quinuclidine-2-carboxylic acid (2 1).38 electrolysis Pt electrodes MeOH-NaiMe li2o~~ The preseiice of products with rearranged carbon skeletons is probably the best evidence for carbonium-ion intermediacy such rearrangements of alkyl radicals being without precedent.This was the criterion adopted by Smith and Yuh7 and by Skell and Rei~henbacher.~’ The latter workers generated the + readily rearranged pinacolyl cation Me3C- CHMe by electrolysis of an aqueous solution of potassium 2,3,3-trimethylbutanoate at platinum electrodes. In addition to the Kolbe products all of the expected carbonium ion derived olefins esters and alcohols were obtained mostly with rearranged carbon skeletons. An exploration of the potential stereospecificity of the electrochemi- cal reaction (22)(cJ:ref. 34) and thus its synthetic usefulness revealed that both epimers gave similar mixtures of the two ketones Kolbe dimer and the expected ester.40 electrolysis DMF C anode + CQ H H (22) The electrolysis of p-anisylacetic acid in methanol results in formation of the corresponding benzyl methyl ether via the resonance-stabilised carbonium ion.41 The subsequent successive oxidation and methoxylation of the ether to give eventually methyl o-4-methoxybenzoate is clearly demonstrated.The striking utility of carbon anodes in promoting carbonium ion formation was established by K0eh1.~~ Acetate ion was oxidised to give methyl acetate 37 R. P. Linstead B. R. Shephard and B. C. L. Weedon J. Chem. SOC.,1951,2854. 38 P. G. tiassman and B. L. Fox J. Org. Chem. 1967,32,480. 39 P. S. Skell and P. H. Reichenbacher,J. Amer. Chem. SOC.,1968,90 3436. 40 L. Rand and C.Someswara Rao J. Org. Chem. 1968,33 2704. 41 B. Wladislaw and H. Viertler J. Chem. SOC.(B),1968 576. 42 W. J. Koehl J. Amer. Chem. SOC.,1964,86,4686. Electro-organic Chemistry in high yield thus implying the formation of methyl cation. Both straight and branched-chain alkanoic acids gave mixtures of esters olefins and cyclo- propyl compounds plausibly derived from the relevant carbonium ion. 3,3-Diphenylacrylic acid is oxidised in aqueous acetic acid at a carbon anode to yield 4-phenylcoumarin 2.2-diphenylvinyl acetate and diphenylacetaldehyde (all without rearrangement) and benzil and benzoin acetate (with rearrange- ment).43A similar mixture of rearranged benzil-derived products is obtained from electrochemical oxidation of diphenyl-acetylene indicating the similarity of the intermediates (23) and (24).-e- Ph,C==CH.CO; -co [Ph,C==CA] +,Pl$$=CHPh + products -e-Ph,C==CH PhCgPh -I%+ Ph&=cPh - products (24) Koehl's method of generation of what is probably a methyl cation42 has been further exploited.43 Electrolysis of acetate ion in acetic acid at a graphite anode apparently provides a source of methyl cations that will attack added aromatic substrates and result in nuclear methylation. With added benzene toluene is the major product and methylated products are likewise obtained from a series of aromatic hydrocarbons. With a platinum anode the corres- ponding acetoxylated products are formed. It is tantalising that no mention is made of overall yields and isomer distributions in the methylated products.Conclusive demonstration of the reactions in Scheme 6 implied in the above results has recently been provided. RC0,-(soln) RCO; (ads) -3R' (ads) A R+ (soln) 1 1 Kolbe olefins products alcohols esters rearrangement SCHEME 6 The chronopotentiograms of caesium 2,2-dimethylpropanoate and 2,2-dimethylpentanoate at a platinum anode in acetonitrile show successive one-electron oxidation^.^ The transition times for the separate discharges indicate that the first step is diffusion controlled whereas the second step gives rise to a much shorter transition time due to the competing consumption of adsorbed alkyl radicals by dimerisation. Anodic reactions of other organic anions. The electrolysis of ethyl sodio- acetate in ethanol with a platinum anode and mercury cathode at uncontrolled electrode potential resulted in a mixture of products best explained by the 43 W.J. Koehl J. Org. Chem. 1967,32 614. 44 V. D. Parker Chem. Comm. 1968,1164. 240 J. H. P. Utley condensation of ethyl acetoacetate and acetaldehyde produced during elec- troly~is.~'The results of similar electrolysis of dialkyl sodiomalonates are more clear cut although even here the expected products of initial discharge to radicals are supplemented with condensation products.46 However the presence of olefins during such oxidations at controlled potential allows the radicals to be trapped and the products of dimerisation abstraction and further oxidation to carbonium ions are i~olable.~' Electrochemical thiocyanation has been achieved.48 Ammonium thio- cyanate is oxidised at a platinum anode at controlled potential in dry aceto- nitrile to yield thiocyanogen via the formation of SCN radicals.Dimerisation is inhibited in the presence of phenol and p-thiocyanophenol is obtained in high yield. Anodic Reactions of Neutral Organic Molecules. Anodic oxidation of neutral organic molecules may be initiated by reaction with structurally simple radicals or cations generated from the solvent or supporting electrolyte or by direct discharge of the organic substrate. The terms indirect and direct oxidation are applied to the respective processes in this report although assignment is tentative in many cases. An exhaustive review of electro-organic oxidation has recently appeared complete with extremely useful tables of known reactions conditions and yields.49 Electrochemical fluorination has similarly been reviewed." Indirect oxidation.Acetoxylation and alkoxylation of NN-dimethylamide is readily achieved by electrolysis of solutions of the dimethylamide in acetic acid or alcohols provided that ammonium nitrate is pre~ent.~' It seems most likely that discharge of nitrate ion initiates the reaction according to Scheme 7. NO 2NO' NO' + R'COON(M~~)~ -HNO + R'CO-NMe-CH' R20H R'CO*Me*CH;~RCO*NMe*CH~-R'CO*NMe*CH2*OR2 SCHEME 7 Similar radical initiation was demonstrated in the electrolysis of methanolic solutions of alkylbenzenes in the presence of a variety of inorganic anions (X-).52 The anodic polarograms of solutions of sodium perchlorate in methanol and sodium perchlorate and ethylbenzene in acetonitrile show that methanol is oxidised at a significantly lower potential than the alkylbenzene.Scheme 8 has been proposed to explain the formation of a-methoxylated products and the dependence of current efficiency on the anion. 45 T. D. Binns and R. Brettle J. Chem. SOC.(C) 1968 336. 46 R. Brettle and J. G. Parkin J. Chem. SOC.(C) 1968 1352. 47 H. Schifer and A. Alazrak Angew. Chem. 1968,80,485. 48 G. Cauquis and G. Pierre Compt. rend. 1968,226 C 883. 49 N. L. Weinberg and H. R. Weinberg Chem. Rev. 1968,68,449. 50 S. Nagase Fluorine Chem. Rev. 1967 1 77. S. D. Ross M. Finkelstein and R. C. Petersen J. Amer. Chem.SOC. 1966,88,4657. 52 K. Sasaki. H. Urata. K. Uneyama. and S. Nagaura Electrochim. Am 1967 12 137. Electro-organicChemistry 241 RH + X'-R' + XH MeOHAMeO' + H+ R' + 'OMe-ROMe SCHEME 8 It is noteworthy however that other products include olefins and when halides are used as the supporting electrolyte nuclear-halogenated compounds. This with the significant absence of dimers of benzylic radicals suggests the pos- sibility of smooth anodic oxidation of the radical to a benzyl carbonium ion with subsequent solvolysis or elimination. Co-electrolysis at platinum electrodes of tetralin and sodium cyanide in a mixture of hydrogen cyanide and methanol gave a moderate yield of l-metho-xytetralin and a low yield of 6-c~anotetralin.~~ Repetition of the reaction at controlled potentials reveals that the side-chain methoxylation proceeds readily at <0-5 v (sce) at which potential the aromatic compound is not discharged.If however the reaction is carried out at 2.4 v (SCE) above the discharge potential or tetralin nuclear cyanation is observed.54 It is likely therefore that Scheme 9 best represents these and similar side-chain methoxyla- tion reactions.'' MeO-e -MeO-at > 0.5 v (sce) -I-*-03+MeO' -MeOH a* 03 OMe and SCHEME9 '' (a) K. Koyama T. Susuki and S. Tsutsumi Tetrahedron Letters 1965 627; (b) Tetrahedron 1967,23,2675. 54 V. D. Parker and B. E. Burgert Tetrahedron Letters 1968,2415. 55 S Tsutsumi and K. Koyama. Discuss. Faradav Soc.. 1968. No. 45.247. 242 J.H. P. Utley The electrolysis of a solution of anisole in methanol containing a little sulphuric acid results in the formation of methyl p-anisoate (45%) methyl o-anisoate (4.5 %) dimethyl maleate (15 %) dimethyl succinate (24%) and two uni$entified olefinic esters (11 %).56 This reaction is efficient at anode potentials as low as 1-25 v (sce) which precludes discharge of anisole. No mention of electrode material is made. The specific para-substitution suggests electrophilic attack and it is probable that methanol is oxidised to HOCH;. This species may attack anisole the p-anisyl alcohol thus formed being further oxidised and the acidic product being esterified. Production of methyl p- anisoate from electrolysis of p-anisyl alcohol in the same reaction conditions supports this hypothesis.The origin of the electrophile is not so clear. A mixture of formaldehyde and anisole in acidic methanol does not yield the required products thus eliminating the possibility of oxidation of methanol to formaldehyde with subsequent hydroxymethylation. Parker suggests Scheme 10 although the first step is at variance with predictions from bond- dissociation energies and known chemical behaviour. s ' MeOHaMeO. + Ht MeO' + MeOH-MeOH + *CH,*OH CH OHa'CH OH SCHEME10 Anodic side-chain substitution by cyanate has been reported,' triphenyl-methylisocyanate being obtained on electrolysis of mercuric cyanate in acetone containing triphenylmethane with sodium perchlorate as supporting elec- trolyte.No further experimental details are given. An attractive rationalisation involves production of an unspecified free-radical that abstracts hydrogen from triphenylmethane. Subsequent oxidation of the triphenylmethyl radical to the carbonium ion and reaction with the covalent cyanate is proposed. Formation of methoxy-radical by discharge of methoxide ion at a carbon anode appears necessary for the electrochemical methoxylation of cyclohexyl i~ocyanide.~~ Within a total yield of 20 % major products are (25) (26) and (27). NH-CO Me 0-CH2 cX"=T*OMe 0-CH OMe (25) (26) (27) Tertiary mines can be methoxylated electrochemically. NN-Dimethylani- line was electrolysed in methanolic potassium hydroxide at uncontrolled potential to give good yields of N-methoxymethyl-N-methylaniline and 56 V.D. Parker Chem. and Ind. 1968 1363. '' M. S. Kharasch J. L. Rowe and W. H. Urry J. Org. Chem. 1951 16,905. 58 V. D. Parker and B. E. Burgert Tetrahedron Letters 1968 3341. 59 T. Shono and Y. Matsumura J. Amer. Chem. SOC.. 1968,W.5937. Electro-organic Chemistry NN-bis(methoxymethy1)aniline in the ratio of 6 1. Similar reaction of NN-dimethylbenzylamine resulted in a mixture of a-methoxy-NN-dimethyl-benzylamine and N-methoxymethyl-N-methylbenzylamine, the latter product predominating.60 To explain the preferred substitution of the primary N-methyl protons Weinberg and Brown suggested that adsorption would facilitate concerted proton-loss from the N-methyl group rather than the benzylic carbon the resulting positive charge being further from the anode (Scheme 11)./ /+/+/+/+//+/ / /+/+/+// SCHEME 11 This interpretation has been questioned.61 The electrolysis of NN-dimethyl- benzylamine in the absence of methoxide ion and at controlled potential (+0.7 v vs. Ag/Ag+') resulted only in the production of protonated starting material with some dealkylation. Methoxylation was observed when methoxide ion was present. Weinberg and Brown's experiments were at potentials at which amine and methoxide ion were being discharged.62 Involvement of methoxy-radicals seems likely (Scheme 12) although the distribution of products commends part of the original suggestion. MeO' + PhCH,-NMe,-PhCH,-NMe*CH,* + PhtH-NMe + MeOH PhCH -NMe CH,-Z PhCH -NMe -CH; + products Ph& -NMe,a PheH.NMe + products SCHEME 12 Acetophenone is obtained in fair yield from the anodic methylation of benzaldehyde in the presence of potassium a~etate.~ Allylic acetoxylation and methoxylation has been described in which carbon electrodes and tetra- ethylammoniumtoluene-p-sulphonateas supporting electrolyte were used.64 Starting materials have included cyclohexene oct-1-ene and cyclo-octa-1'5-diene. There is little evidence concerning the mechanism of this reaction but an analogy is drawn with the allylic acetoxylation of olefins with t-butyl- peracetate in the presence of copper salts. Overall cis-addition of methoxy-groups is indicated by the ratios of meso-to racemic product obtained by the electrochemical dimethoxylation of 6o N.L. Weinberg and E. A. Brown J. Org. Chem. 1966,31,4058. 61 P. J. Smith and C. K. Mann J. Org. Chem. 1968,33,316. 62 N. L. Weinberg and T. B. Reddy J. Amer. Chem. SOC.,1968,90,91. 63 A. Takeda S. Torii and H. Oka Tetrahedron Letters 1968 1781. 64 T. Shono and T. Kosaka Tetrahedron Letters 1968,6207. 244 J. H. P. Utley cis-and tr~ns-stilbene.~~ Replacement of methoxide ion by perchlorate sup- presses methoxylation with trans-stilbene and use of ammonium bromide as supporting electrolyte results in erythro-stilbene dibromide as the major product.66 It is unlikely however that the reaction is entirely a free radical one as the rearranged product exo-syn-2,7-dimethoxybicyclo[2,2,l]heptane is produced by similar dimethoxylation of norbornene.This suggests the route shown-Scheme 13. SCHEME 13 Introduction of the methoxycarbonyl group has been achieved in an in- triguing electrochemical way.67 By using a cell embedded in an autoclave electrolysis of sodium methoxide in methanol at a platinum cathode and under a moderate pressure of carbon monoxide causes the platinum to dissolve and form a carbonyl complex of unknown composition. Subsequent introduction of an arene and electrolysis with a platinum anode results in the formation of the corresponding a,P-unsaturated carboxylate. For instance methyl trans-cinnamate is obtained from styrene. The participation of radicals generated from supporting electrolyte anions is strongly implied in the anodic reaction of toluene in wet acetonitrile.68 Major products are bibenzyl N-benzylacetamide and benzyl alcohol and its oxidation products.The reactions takes place at potentials below the polaro- graphic half-wave potential for toluene oxidation thus prompting a proposal of hydrogen abstraction from toluene to form a benzyl radical. This may couple or undergo further anodic oxidation to the benzyl cation which would react with the water present in the solvent. Direct oxidation. Anodic acetamidation was first reported by Eberson and N~berg.~'Durene and hexamethylbenzene were converted into the corres- ponding benzylacetamides by controlled potential electrolysis at platinum electrodes in dry acetonitrile in the presence of anhydrous sodium perchlorate. In wet acetonitrile the yield of benzylacetamide drops and for toluene at least a different reaction probably takes place.68 However the Eberson-Nyberg mechanism (Scheme 14) has received strong support from the results of a polarographic and voltammetric The two-electron transfer is con- 65 T.Inoue K. Koyama T. Matsuoka K. Matsuoka and s. Tsutsumi Tetrahedron Letters 1963,1409. 66 T. Inoue K. Koyama T. Matsuoka and S. Tsutsumi Bull. Chem SOC. Japan 1967,40 162. 67 (a)T. Inoue and S. Tsutsumi Bull. Chem. SOC. Japan 1965,38,2122; (b)J. Amer. Chem. SOC. 1965,87,3525. 68 V. D. Parker and B. E. Burgert Tetrahedron Letters 1968 2411. 69 L. Eberson and K. Nyberg Tetrahedron Letters 1966,2389. 'O A. E. Coleman H. H. Richtol and D. A. Aikens J. Electroanalyt. Chem Interfacial Electrochem.1968 18 165. Electro-organic Chemistry firmed and single sweep and cyclic voltammetry of the hexamethylbenzene- acetonitrile system indicates that reversible two-electron oxidation is followed by a fast irreversible chemical reaction. ' M Me e \ o z ~ -2e -+ 2e MeoMe Me I + Me -H+- Me G Me \ M e Me Me Me Me I MeCN + CH,NzCMe Me -,Me Me Me SCHEME 14 The anodic discharge of many aromatic hydrocarbons was studied in detail by L~nd.~~ showed by polaro- Subsequently Eberson and N~berg'~ graphy and controlled potential electrolysis that anodic acetoxylation of aromatic compounds generally involved initial discharge of the organic compound. Isomer distributions in nuclear acetoxylated compounds suggested significant similarities of mechanism with homogeneous electrophilic aromatic substitution and accordingly a Wheland intermediate was proposed (28) with acetate ion assisting the electron tran~fer.~ Side-chain acetoxylation does not require the presence of acetate ion and can be achieved in acetic acid solution in the presence of other anions.Further- more for the side-chain acetoxylation of ethylbenzene an isotope effect kdkD = 2-6 was found. Mechanisms involving rate-limiting proton loss from dications of the type [ArCH,CH3I2+ or from cation radicals of the type [ArCH,CH,] + have been proposed. The consequences of electrochemical oxidation of cyclo-octatetraene to the corresponding six n-electron dication 71 H. Lund. Acta Chem. Scad. 1957,11 1323. 72 L.Eberson and K. Nyberg J. her. Chem. Soc. 1966,88,1686. 73 L. Eberson J. Amer. Chem. SOC.,1967,89,4669. J. H. P.Utley have been explored. 74 Cyclo-octatetraene was electrolysed in acetic acid in the presence of acetate ion at both platinum and carbon anodes. Mixtures of mono- and di-acetates were obtained and in particular the production of the rearranged diacetates suggests the initial formation of the dication (29). - diacetates Cyclohexadiene has been methoxylated and acetoxylated an~dically.~ A further investigation of the products of electrochemical methoxylation of f~ran~~ has been described.77 The related methoxylation of nitrogen hetero- cycles is reported ;78 for instance 1-methylpyrrole affords l-methyl-2,2,5,5- tetramethoxypyrroline.Presumably the reaction proceeds via the dication to give a dimethoxypyrroline which unlike the corresponding dimethoxy- furans is further oxidised at the uncontrolled potential of the reaction. The anodic discharge of benzyl methyl ethers in methanol results in side-chain methoxylation with the consequent production of aromatic aldehyde^.^' Electrochemical cyanation involves initial discharge of the organic sub- strate.80 At controlled anode potentials there is no reaction at potentials above that required for cyanide-ion oxidation but below that for say anisole discharge. At 2.0 v (SCE) o-and p-cyanoanisoles are formed.80a For a number of monosubstituted benzenes naphthalene and biphenyl isomer distributions of nuclear-substituted products have been determined and the high positional selectivity of anodic cyanation demonstrated.80b It is also noteworthy that there is little side-chain attack in these reactions (cf.acetoxylation) and com- parison with the relatively random isomer-distribution following reaction with photolytically generated cyano-radicals firmly establishes the electrophilic nature of anodic cyanation.Related is the nuclear substitution by the cyanate ion of a number of aromatic corn pound^.^^ Isomer distributions and many 74 L. Eberson K. Nyberg M. Finkelstein R C. Petersen S. D. Ross and J. J. Uebel J. Org. Chem. 1967 32 16. 75 A. J. Baggaley and R. Brettle J. Chem SOC.(C) 1968 2055. 76 N. Clauson-Kass F. Limborg and K. Glens Acta Chem. Scad. 1952,6,531. 77 A. J.Baggaley and R. Brettle J. Chem. SOC.(C) 1968 969. 78 N. L. Weinberg and E. A. Brown J. Org. Chem. 1966,31,4054. 79 R. F. Garwood Naser-ud-din and B. C. L. Weedon Chem. Comm. 1968,923. (a) V. D. Parker and B. E. Burgert Tetrahedron Letters 1965 4065; (h) L. Eberson and S. Nilsson Discuss. Faraday SOC.,1968,45 242. Electro-organicChemistry 247 experimental details are not reported but the potential required suggests that the aromatic compound is discharged prior to reaction with the cyanate ion. The application of physical techniques such as cyclic voltammetry polaro- graphy and chronopotentiometry is now considered essential for the complete elucidation of electro-organic reaction mechanisms. There are many recent examples of the usefulness of these techniques.Cyclic voltammetry clearly shows the existence of five species a cation a free radical a monoanion a dianion and a trianion associated with the oxidation and reduction of di- phenylpicrylhydrazyl." The technique has also been used to demonstrate the oxidative coupling of vinylidenebisdimethylamine to the corresponding butadiene (30).' (Me,N),C=CH (Me,N),C=CH-CH=C(NMe,) (30) The anodic oxidation of diazodiphenylmethane in acetonitrile has been studied by polarography and controlled potential electrolysis.' The major product formed in 80 % yield is tetraphenylethylene. Benzophenone di- phenylmethanol and benzopinacolone are also formed in accordance with the reaction scheme put forward. Sweep voltammetry polarography and controlled potential electrolysis have been used to investigate the electro- oxidation of 1,5-di~hloroanthracene.~~ Electrolysis in acetonitrile at a potential slightly above that of the anodic wave produces a red solution which gives on addition of water moderate yields of 9-acetamido-1,5-dichloro-10-anthrone and 1,5-dichloroanthraquinone.Electrolysis in acetonitrile with added ethanol promotes coupling of the first-formed cation-radical to give a high yield of 1,1,5,5-tetrachlorobianthranyl.A very complete study of the electro-oxidation of anthracene in acetonitrile has been p~blished.'~ Bianthrone is the major product of electrolysis in 'dry' acetonitrile at 1.0 v (us.Ag/Agf) and coulo- metry showed that two electrons were lost electrochemically. Formally this is a three-electron oxidation and air oxidation of 9-anthranol during work-up is proposed to account for the extra step.Increasing formation of anthraquinone follows addition of water or oxidation at 1.4v (Scheme 15). Anodic hydroxylation of aromatic compounds has been achieved and its mechanism has been in~estigated.'~ 1,5-Dihydroxyanthracene,in aqueous perchloric acid at a carbon paste electrode yields 7-hydroxyanthraquinone which is readily electroreduced to the corresponding trihydroxy-compound. Cyclic voltammetry was used to establish the identity of the species formed and the sequence of the processes involved. For a series of 2-substituted hydro- quinones anodic hydroxylation at the 3-position was found to be dependent on the substituent.The probable mechanism involves electrochemical oxida- B. L. Funt and D. G. Gray Canad. J. Chem. 1968,46 1337. 82 J. M. Fritsch and H. Weingarten J. Amer. Chem. SOC.,1968,90 793. 83 W. Jugelt and F. Pragst Angew. Chem. 1968,80,280. 84 E. J. Majeski,J. D. Stuart and W. E. Ohnesorge,J. Amer. Chem. SOC.,1968,90,633. *' L. Papouchado G. Petrie J. H. Sharp and R. N. Adams J. Amer. Chem. SOC.,1968,90 5620. J. H. P.Utley + -2H+ I-e bianthrone [O’ anthraquinone-2e -3H’ hydrolysis SCHEME 15 tion of the hydroquinone to the quinone followed by 1,4-nucleophilic addition of water with subsequent further electro-oxidation to the substituted quinone. Related is the scheme proposed for the electro-oxidation of p-dimethylamino- phenol in aqueous solution.86 In acidic media p-benzoquinone and dimethyl- ammonium ion are formed irreversibly as demonstrated by cyclic voltammetry.At higher pH sufficient unprotonated dimethylamine is present to undergo Michael addition to the quinonoid species present. The effect of electrolyte on the electrochemical methoxylation and dimerisa- tion of NN-dimethylaniline is puzzling.62 Methoxylation of the N-methyl group is observed in methanolic potassium hydroxide (p. 242 ref. 60) whereas similar controlled potential electrolysis in methanol with ammonium nitrate as the electrolyte results in the formation of NNN’N’-tetramethylbenzidine. It is tempting to speculate that methoxide-ion discharge initiates the methoxyla- tion process whereas in the presence of nitrate ion the aromatic molecule is discharged and the resulting cation radical couples at the para-position.Adams and his co-workers have continued their work aimed at establishing a general pattern of behaviour for the electro-oxidation of aromatic amine~.~’ They have confirmed the suggestion8* that the oxidised forms of 4’-substituted- 4-aminodiphenylamines are formed during exhaustive electrolysis to azo-benzenes. For para-substituted anilines the formation of such diphenylamines necessitates the elimination of the substituent. The results of controlled potential coulometry and voltammetry are combined to provide a mechanistic hypo- thesis. The anodic oxidation in acetonitrile of partially substituted triphenyl- amines has been studied8’ as an extension of earlier work on triphenylamine which yields tetraphenylbenzidine and the fully para-substituted triarylamines “ M.F. Marcus and M. D. Hawley J. Electroanalyt. Chem. Interfacial Electrochem. 1968,18,175. ” R. N. Adams and J. Bacon J. Amer. Chem. SOC. 1968,90,6596. 88 S. Wawzonek and T. W. McIntyre J. Electrochem. SOC.,1967,114,1025. 89 R. F. Nelson and R. N. Adams J. Amer. Chem. Soc. 1968,90,3925. Electro-organic Chemistry 249 that yield stable cation-radical~.~~ The partially substituted compounds are converted into benzidines and the rates of coupling are markedly dependent on the substituent. The characterisation has been attempted of the electro- chemical processes associated with the anodic oxidation of carba~oles,~ pentaphenylpyrr~les,~~ and adenine.94 diphenylenedio~ide,~~ Cathodic Processes-Cathodic Reactions of Organic Cations.The decom- position at an aluminium cathode of quaternary ammonium salts containing groups capable of forming stabilised radicals e.g. benzyl leads to coupled products e.g. bibenzyl in aprotic solvents.95 Products involving hydrogen abstraction e.g. toluene are apparently formed in water. A polarographic and coulometric study of the reduction of one such compound benzyldimethyl- anilinium chloride revealed that two electrons are transferred in aqueous solution and one of the products is toluene whereas in acetonitrile 1-4electrons are transferred and both toluene and bibenzyl are dete~ted.’~ It was originally suggested that benzyl radicals formed according to Scheme 16 could either couple or abstract hydrogen the implication being that abstraction from water was preferred over abstraction from an organic solvent.This is unlikely and in the light of the coulometric results either Scheme 16 or at mercury Scheme 17 seems more plausible. PhCH2*NR3 LPhCH,*fiR -NR3 + PhCH; APhCH2- 1 1H2O dimer PhCH SCHEME16 2PhCH; + Hg-(PhCH2)2Hg A2PhCH2-+ Hg LH2O 2PhCH3 SCHEME17 A similar carbon-nitrogen bond cleavage is observed as a result of the electro- reduction of indolium and tetrahydroquinolium iodides in liquid ammonia. The hetero-ring is cleaved in such a way that only aliphatic amines result.97 Electrolytic reduction at a lead cathode of quinolinium ions in aqueous sulphuric acid gave good yields of mixtures of the corresponding piperidines 90 E.T. Seo R F. Nelson J. M. Fritsch L. S. Marcow D. W. Leedy and R N. Adams J. Amer. Chem. SOC. 1966,88 3498. 91 J. F. Ambrose and R F. Nelson J. Electrochem. SOC. 1968 115 1159. 92 G. Cauquis and M. Genies Bull. SOC.chim. France 1967 3220. 93 G.Cauquis and M. Maurey Compt. rend. 1968,266,C 1021. 94 G. Dryhurst and P. J. Elving J. Electrochem. SOC. 1968,115 1014. 95 (a) M. Finkelstein R C. Petersen and S. D. Ross J. Amer. Chem SOC. 1959 81 2361; (b)S. D. Ross M. Finkelstein and R C. Petersen J. Amer. Chem SOC. 1960,82 2361. 96 J. S.Mayell and A. J. Bard J. Amer. Chem SOC. 1963,85,421. ’’ J. T.Wrobel K. M. Pazdro and A. S. Bien Roczniki Chem. 1967,41 1279. 250 J.H. P. Utley and tetrahydropyridine~.'~ N-Methylquinolinium ions tend to be reduced more easily than protonated quinolines. A grey solid unstable to air heat and water forms at the surface of mercury cathodes during the electrolysis of solutions of tetramethylammonium salts." Many similar solids have been formed and identified as quaternary ammonium amalgams. Their reduction potentials are sufficient to enable transfer of electrons to aromatic species and they have been used to generate anion- radicals.' O0 For the tetramethylammonium amalgam the mercury-organic ratio seems to be ca. 12:1. The same procedures have been used for the forma- tion of quaternary phosphonium and tertiary sulphonium amalgams although these cannot always be isolated.'" Their properties are very similar to those of the quaternary ammonium amalgams.Polarography at the mercury electrode of cyanomethyldimethylsulphonium + ion Me's. CH 0 CN suggests a one-electron reduction. However macroscale reduction in the presence of styrene does not result in the expected production of y-phenylbutyronitrile from addition of cyanomethyl radicals. A probable explanation of this and an example of the care needed in using polarographic data is the consumption of two electrons by two moles of starting material according to Scheme 18. Me,i*CH,.CN Me,S + CH,.CN + ~ NC*CH,-+ Me,S-CH,*CN Me,i*CH;CN + MeCN SCHEME 18 Support for this mechanism includes the formation of large quantities of acetonitrile and the appearance of a two-electron polarographic wave when acetic acid is added and the process thereby halted at the first stage due to the removal of cyanomethyl anion."' Cathodic Reactions of Neutral Organic Molecules.Alkenes alkynes and aromatic compounds. Reduction of the benzene nucleus may be achieved by the electrolysis of lithium salts in a solution of methylamine containing the aromatic compound.103 Selection between di- and tetra-hydro-products is affected by the omission or insertion of a cell divider. This method does not involve the direct electroreduction of the organic substrate but the generation of lithium metal in methylamine. An interesting difference has been noted between the product of such reduction in an undivided cell of t-butylbenzene and trimethylsilylbenzene.' O4 Alkylbenzenes are mainly reduced to the 98 M.Ferles and A. Silhankova Z. Chem. 1968,8 175. 99 (a)J. D. Littlehailes and B. J. Woodhall Chem. Comm. 1967,665; (b)J. D. Littlehailes and B. J. Woodhall Discuss. Faraday SOC.,1968,45 187. loo J. Myatt and P. F. Todd Chem. Comm. 1967 1033. W. R T. Cottrell and R A. N. Morris Chem. Cornm. 1968 409. J. H. Wagenkneckt and M. M. Baizer J. Electrochem. SOC. 1967 114 1095. Io3 R k Benkeser E. M. Kaiser and R F. Lambert J. Amer. Chem. Soc. 1964,86 5272. Io4 R. A. Benkeser and C. A. Tincher J. Organometallic Chem.. 1968 13,139. Electro-organic Chemistry 25 1 corresponding 2,5-dihydro-compounds. Trimethylsilylbenzene is reduced to 1,4-dihydrotrimethylsilylbenzene. It is suggested that the intermediate radical-anion is stabilised by back bonding (31).SiMe SiMe, 0 0 (31) Coelectrolysis in an undivideb cell of lithium c-loride and dialkylacetylenes in methylamine produces the corresponding trans-olefins. Isomerisation of the first formed trans-olefin occurs if a divided cell is used presumably because of the more basic conditions in the cathode compartment. Nonconjugated arynes react similarly but conjugated arynes yield alkylbenzenes. O5 Electrolysis at an aluminium cathode of solutions of lithium chloride in ethanol and hexamethylphosphoramide results in the development of a deep blue colour in the cathode compartment. In the presence of tetralin the cathode compartment is colourless and tetralin is reduced to decalin in moderate yield implying the electrolytic generation of solvated electrons capable of addition to an aromatic nucleus."' Direct electroreduction of aromatic compounds was well characterized by Wawzonek and his co-workers by p~larography."~ Macroscale electrolysis of naphthalene in the presence of carbon dioxide gave a 30% yield of 1,4- dihydronaphthalene- 1,4-dicarboxylic acid.In the absence of carbon dioxide naphthalene was recovered unchanged. The high current efficiency formation of 1,4-dihydronaphthalene from naphthalene has however recently been described.lo8 A mercury cathode was employed with aqueous acetonitrile solvent and tetraethylammonium toluene-p-sulphonate as supporting elec- trolyte. Electroreduction of benzene is more difficult cathodic potentials of -3-3 v (SCE) being required.Most solvents decompose at this potential but the reduction has been achieved in aqueous diglyme with a mercury cathode and tetrabutylammonium bromide supporting electr~lyte.'~~ Both benzene and toluene have been converted into dihydro-compounds with reasonable current efficiency. Polarographic and e.s.r. studies of the cathodic reduction of anthracene benzophenone anthraquinone phenazines p-nitrobiphenyls p-nitrostilbenes and 4,5methylenephenanthrene confirm that where reduction is observed initial radical-anion formation is followed by rapid protonation further R A. Benkeser and C. A. Tincher J. Org. Chem. 1968,33 2727. lo6 H. W. Sternberg R E. Markby I. Wender and D. H. Mohilner J. Amer. Chem SOC.,1967 89,186.lo' S. Wawzonek and D. Wearring J. Amer. Chem SOC. 1959,81,2067. lo* A. Misono T. Osa and T. Yamagishi Bull. Chem. SOC.,Japan 1967,40,427. 109 A. Misono T. Osa T. Yamagishi and T. Kodama J. Elecrrochem. SOC.,1968 115 266. 1 J. H. P.Utley reduction to an anion and addition of the second proton.1'0-"2 A careful study of the electroreduction of ethylene at bright platinum suggests a scheme with rate-limiting transfer of an electron to ethylene to form the anion radical.' ' Protonation further electron transfer and reprotonation then proceed much as for aromatic reduction. The commercially important reductive-coupling reactions of activated olefins developed by Baizer,' l4 have been reviewed' ' and further mechanistic studies described.It seems clear that the reaction is an ionic one involving Michael addition of a cathodically generated anion to the activated olefin. The formation of dimeric product is greatly encouraged by the presence of quaternary ammonium ions and this has been attributed to the intermediacy of quaternary ammonium amalgamsg6 However a detailed voltammetric study of the hydrodimerisation of acrylonitrile in the presence of several cations shows that the quaternary ammonium ions are not reduced at a noticeable velocity under the conditions of the electro-organic reaction.' l6 Furthermore the distinction between quaternary ammonium ions and metal ions is unlikely to be due to secondary reactions involving discharged metal and the organic substrate.Hydrodimerisation of diethyl maleate is still favoured by the presence of quaternary ammonium cations rather than sodium ions in spite of the fact that the olefin discharges at a more positive potential than either of the supporting cations."7 Further examples of the cathodic hydrodimerisation of ap-unsaturated esters and mesityl oxide have been reported.' '* Carbonyl compounds. The scope and limitations of electroreduction of carbonyl compounds are well documented,' ' and much polarographic evidence concerning mechanism has been collected.'20 In acidic media the observation in many cases of two one-electron waves and the production of dimers (e.g. pinacols) as well as alcohols suggests Scheme 19. R,C :OH+& R,~-oH; R@OH-H+ R,CH-OH 1 dimerisation abstraction or reaction with Hg cathode.SCHEME19 'Io K. Umemoto Bull. Chem SOC.,Japan 1967,40 1058. ''I A. E. Brodsky L. L. Gordienko and L. S. Degtiarev Electrochim. Acta 1968 13 1095. '" J. Janata J. Gendell R G. Lawton and H. B. Mark J. Amer. Chem SOC.,1968,90 5226. '" A. T. Kuhn Electrochim Acta 1968 13,477. 'I4 (a) M. M. Baizer J. Electrochem. SOC.,1964 111 215; (b) M. M. Baizer and J. D. Anderson J. Electrochem Soc. 1964 111 223; (c) M. M. Baizer and J. D. Anderson J. Electrochem. SOC.,1964 111 226. (a) I. E. Gillet Chem.-1ng.-Tech. 1968,40,573; (b)R M. Hurd Hydrocarbon Processing 1964 43 154. 'I6 F. Beck Ber. Bunsengesellschaji Phys. Chem. 1968,72,379. M. M. Baizer and J. P. Petrovich J. Electrochem SOC.,1967 114 1023. 'I8 J.Wiemann and M. Larbi Bouguerra Compt. rend. 1967 265 C 751. 'I9 (a) F. D. Popp and H. P. Schultz Chem Rev. 1962 62 19; (b) 0.H. Wheeler 'Chemistry of the Carbonyl Group,' ed S. Patai Interscience London 1966 p. 523. (a) P. Zuman Coll. Czech. Chem Comm. 1968 33 2548; (b) P. Zuman D. Barnes and A. Ryvolova-Kejharova Discuss.Farday SOC. 1968,45,202. Electro-organic Chemistry In the presence of acrylonitrile cathodic-reduction of acetone in aqueous sulphuric acid yields 2-hydroxy-3-methylvaleronitrile and its hydrolysis product yy-dimethylbutyrolactone. Current us. potential plots show that acetone is discharged rather than acrylonitrile and Michael addition of the ketone-derived anion is suggested as a rationalisation.'2 la Similar electrolytic addition involving acetone and olefins such as styrene and cyclohexene produces tertiary alcohols although in this case it is probably the ketone-derived free-radical that reacts with the olefin.' 21b The electrochemical reduction of methyl vinyl ketones yields many of the products expected from the intermediacy of the radical (32) the major one being 0cta-2,7-dione.'~~ CH,=CH*t(OH)CHp+ eH2-CH2COCH3 (32) The product of controlled-potential electrolysis of cinnamaldehyde at the first half-wave value is phenylpropionaldehyde.This suggests that for clp-unsaturated aldehydes hydrogenation of the double-bond precedes subsequent reduction of the carbonyl group.'23 Reductive coupling of acetophenone produces a mixture of diastereo- isomeric 2,3-diphenylbutane-2,3-diols.In the electrochemical reaction a marked pH-dependent stereoselectivity has been found.'24 With isotopic- dilution techniques to analyse reaction products the ratio of (& ) :meso products is found to be ca.1:1 in acidic media but 2.5-3.2 :1 in basic media. This ratio is independent of cathode material and the separate pinacols are stable to the alkaline medium. Similar results are obtained for propiophenone where n.m.r. analysis supports the results of the isotopic-dilution method.'25 Equiva- lent amounts of the (+)-and meso-pinacol are obtained from benzaldehyde irrespective of the pH. An explanation put forward involves in basic solution hydrogen-bond assistance of the coupling process. This demands that the two oxygen atoms be adjacent which in turn demands for the transition state leading to the lesser meso product conformations in which the two phenyl groups and two alkyl groups are near-eclipsed.12' (a) K. Sugino and T. Nonaka Electrochim Acta 1968,13 613; (b)M. Nicolas and R Pallaud Compt. rend. 1967,265 C 1044. 12' J. Wiemann and M. L. Bouguerra Ann Chim (France) 1968,3 215. lZ3D. Barnes and P. Zuman,J. Electroanalyt. Chem Zntegacial Electrochem. 1968,16 575. lZ4 J. H. Stocker and R M. Jenevein J. Org. Chem. 1968,33,294. lZ5 J. H. Stocker and R M. Jenevein J. Org. Chem. 1968,33,2145. J. H. P.Utley 1-Acetylnaphthalene undergoes an interesting cathodic reaction in basic solution yielding tetrahydro- 1,4-methano-3 -benzoxepin (33).' The electrochemical reduction of benzophenone in aprotic media has been studied polarographically.27 Controlled potential coulometry and polaro- graphy have been used to elucidate the mechanism of reduction of the sym- metrical P-diketone 1,3-diphenylpropane-1,3-dione. 12* The products are dependent on cathode potential and pH and are best summarised in Scheme 20. PhC(OH)*CH,* COPh HO OH ph + PhCH(OH).CH,*CH(OH)Ph (oH).CH2.CoPh (+ 4e -2.0 v basic media.) + [(+e -1.15 v (SCE)] ph (+ 2e -1.35 v tentative identification) SCHEME 20 and phenylglyoxylhydroxamic acid PhCH(OH)CO*NH although in the fxst case some coupling product is reported which corresponds to pinacol formation from ketones.' 29 The many products of electroreduction of croconic acid (34) have been characterised.' 30 (34) 0Hvo 0 Miscellaneous unsaturated functional groups and nitro-compounds.The electrochemical reductions of camphor oxime and norcamphor oxime to exo-and endo-amines seem to be governed by steric-approach contr01'~' Scheme 21. The result of electrolysis at a mercury cathode are shown in the Table. SCHEME 21 126 J. Grimshaw and E. J. F. Rea J. Chem SOC.(C) 1967,2628. R F. Michielli and P. J. Elving J. Amer. Chem SOC.,1968,90 1989. 12* D. H. Evans and E. C. Woodbury J. Org. Chem. 1967,32 2158. lZ9 J. Armand P. Souchay and F. Valentini Compt. rend. 1967,265 C 1267. 130 M. B. Fleury P. Souchay and M. Gouzerh Bull. SOC.chim France 1968,2562. 13' A. J. Fry and J. H. Newberg J. Amer. Chem SOC.,1967,89,6374. Electro-organic Chemistry TABLE (ref.131) Reduction of camphor (R=Me) and norcamphor (R=H) oximes R Reagent exo-Amine (%) endo-Amine(%) Me Hg Cathode 99 1 Me LiAlH 99 1 Me Na-EtOH 4 96 H Hg Cathode 0 100 H LiAlH 0 100 H Na-Et OH 75 25 The exclusion of the possibility of conformationally mobile intermediates (e.g. radicals or carbanions) diffusing into solution suggests attack by the electrode from the least-hindered side of the molecule. It is possible that a carbon-mercury bond is formed which could subsequently be cleaved with retention of configuration. The electrolytic reduction in alkaline solution of benzaldehyde oximes and semicarbazones has been investigated by using controlled potential electrolysis and polarography.'32 The oxime is converted into benzylamine and benzyl- hydroxylamine whereas the semicarbazone yields the corresponding l-benzyl- semicarbazide. The reduction of the nitro-group is one of the oldest uses of the electro- organic method.' 33 At a lead cathode o-nitrobenzenearsonic acid is reduced in almost quantitative yield to o-arsanilic acid.' 34 Refinements of this method involve the control of potential and in a number of dinitrobenzoic acids amides and dinitrotoluenes some selectivity of reduction has been achieved.' 35 In organic solvents reduction is often dependent upon added proton donors and their r61e has recently been investigated.' 36 Controlled potential electrolysis of t-nitrobutane in acetonitrile results in the formation of t-butylhydroxylamine.' 37 Several aliphatic tertiary nitro- compounds were similarly reduced in a cell placed in the cavity of an e.s.r.spectrometer.' 38 Initially the spectrum of the anion-radical was observed but after switching off the electrolysis current it was replaced gradually by the spectrum of the di-t-alkylnitroxide radical. The mechanism following formation of the anion radical is presumably the same as for the preparation of di-t-butylnitroxide by sodium reduction of t-nitrobutane.' 39 Trifluoro-nitrosomethane may be reductively coupled in aprotic solvents and the e.s.r. spectrum of the resulting anion radical has been observed. A mechanism for the coupling has been proposed (Scheme 22). H. Lund Tetrahedron Letters 1968. 3651. C. J. Brockman 'Electro-organic Chemistry,' John Wiley New York 1926.134 K. Yasukouchi and H. Muto Denki Kagaku 1967,35,420. 13' (a)A. Tallec Compt. rend. 1966 263 C 722; (b)A. Tallec Compt. rend. 1966 262 C 1886. S. H. Cadle P. R Tice and J. Q. Chambers J. Phys. Chem. 1967,71,3517. 13' H. Sayo Y. Tsukitani and M. Masui Tetrahedron 1968,24 1717. 138 H. Say0 and M. Masui Tetrahedron 1968.24.5075. 139 A. K. Hoffman W. G. Hodgson and W. H. Jura J. Amer. Chem SOC.,1961,83,4675. J. L.Gerlock and E. G. Janzen J. Amer. Chem SOC.,1968,90 1652. 256 J. H. P.Utley CF,NO 4 CF,%-O' 0' 0-I CF3 SCHEME 22 The techniques of polarography cyclic voltammetry controlled potential coulometry e.s.r. and U.V. spectroscopy have been brought to bear on the problem of the mechanism of reduction of aromatic azocompounds in aprotic solvents.In dimethylformamide and in the presence of a proton donor such as hydroquinone (HQ) the stepwise mechanism (Scheme 23) has been estab- lished.' ArN=NAr & 1 ArN=NAr Is [ArN=NAr 1' + HQ + Ar-N-NHAr + Q-h Ar=N-NHAr s Arm. NHAr ArNaNHAr + HQ -+ ArNH-NHAr + Q-SCHEME 23 A similar study of 4,4'-azopyridine-l,1 '-dioxide in dimethylformamide is concerned with the six-electron reduction to the dianion of 4,4'-azopyridine.' 42 Polarographic data on the reduction in dimethyl formamide of many hetero- cyclic amine oxides has been re~0rted.I~~ The electrochemical deoxygenation on a preparative scale of pyridine N-oxides proceeds in good yield unless the preferentially reduced nitro-group is a s~bstituent.'~~ Reductive cleavage.Strained bicyclobutanes may be prepared in good yield by the electrochemical reduction of dihalogencyclobutanes in dimethyl- formamide at a mercury cathode (Scheme 24).14' The mechanism of electrolytic cleavage of carbon-halogen bonds is not entirely clear. It is likely that the partially positive carbon end of the bond accepts electrons from the electrode and polarographic studies of the electro- reduction of substituted halogenobenzenes and benzyl bromides supports this.'46 Positive slopes are found for plots of half-wave potential us.0 indicating a negatively charged transition state i.e. one in which the carbon-halogen 14' J. L. Sadler and A. J. Bard J. Amer. Chem SOC.,1968,90 1979.14' J. L. Sadler and k J. Bard J. Electrochem SOC.,1968 115 343. 143 T. Kubota K. Nishikida H. Miyazaki K. Iwatani and Y. Oishi J. Amer. Chem Soc. 1968 90,5080. 144 J. Hranilovic D. Koruncev and E. Gustak Electrochem Technol. 1968,6 62. 145 M. R Rifi J. Amer. Chem SOC.,1967,89,4442. 14' (a) J. W. Sease. F. G.Burton and S. L. Nichol J. Amer. Chem SOC.,1968,90,2595; (b)J. Grim-shaw and J. S. Ramsey J. Chem SOC.(B),1968,60. Electro-organic Chemistry Mee Br Me MeeMe SCHEME 24 bond is not completely broken. For benzylic halides considerable carbanionic nature is ascribed to the first-formed intermediate as electrolysis of benzyl chloride in the presence of carbon dioxide affords phenylacetic For alkyl halides analogies have been drawn with homogeneous nucleophilic substitution reactions although it is doubtful whether there is any significance in the small differences in reduction half-wave potential between alkyl bromides that undergo nucleophilic substitution at vastly different rates.' 48u-Bridge-head bromides do however reduce at significantly lower potentials (more In these systems it is likely that electron transfer is to the halogen atom despite the unfavourable ground state polarisation.SCHEME 25 Stereochemical evidence has been sought. Zinc metal and electrochemical reduction of optically active cyclopropyl bromides gave products with similar degrees of retention or inversion of config~ration.'~~ Assuming that zinc reduction involves attack upon the halogen atom this result implies the direct transfer of electrons to the halogen atom in the electrode reaction.This in- terpretation is supported by the stereoselectivity shown in the electrochemical reduction of a series of geminal dihalogenocyclopropanes (Scheme. 26).150 14' S. Wawzonek R C. Duty and J. H. Wagenknechf J. Electrochem. SOC. 1964,111 74. 14' (a) F. L. Lambert and K. Kobayashi J. Amer. Chem SOC. 1960 82 5324; (b) J. Zavada J. Krupicka and J. Sicher Coll. Czech. Chem. Comm. 1963 28 1644; (c)J. W. Sease P. Chang and J. L. Groth J. Amer. Chem SOC. 1964 86 3154; (d) F. L. Lambert A. H. Albert and J. P. Hardy J. Amer. Chem. SOC. 1964,86 3155. 14' R Annino R E. Erickson J. Michalovic and B. McKay J. Arner. Chem SOC. 1966,88,4424. A. J. Fry and R H. Moore J.Org. Chem. 1968,33 1283. 258 J. H. P.Utley H H H Hg cathode + 0" H H H X = C1 or Br cis trans SCHEME26 The less-stable cis-isomer is the major product and increasingly so in solvents of higher water content. This is rationalised in terms of electron transfer to the more accessible halogen atom with diffusion at the resulting carbanion into solution to be protonated before losing its configuration (Scheme 27). U H' U SCHEME 27 In the examples discussed above the carbon atom is not accessible and the results do not really test the strength of the analogy between electroreduction of alkyl and benzyl halides and the corresponding S,2 reactions. However electrochemical conversion of 2-chloro-2-phenylpropionic acid into 2-phenyl- propionic acid proceeds with inversion of configuration.'"This would seem to be powerful evidence in favour of electron transfer to the carbon atom with concerted loss of halide ion the resulting carbanion being of inverted con- figuration and rapidly protonated. Electrochemical cleavage of the carbon-fluorine bond is relatively smooth. An attempt to convert aaa-trifluoroacetophenone by electrolysis into the corresponding pinacol resulted in the formation of acetophenone.' 52 Similarly by the use of controlled potential electrolysis the para-fluorine atom of per- fluorobenzoic acid may be selectively replaced by hydrogen.' 53 In dimethyl- formamide solution 6-chloroquinoline has been reduced electrochemically to quinoline.'54 Cathodic reduction of alkyl toluene-p-sulphonates results in the formation of alcohols esters and toluene the latter product indicating cleavage of a carbon-sulphur bond in the first-formed anion-radical.' 55 Horner and his co-workers' 56 have shown by the electrolysis of toluene-p-sulphonates of optically active alcohols that this reaction allows the recovery of alcohols with 151 B.Czochralska Roczniki Chem. 1968,42 895. lS2 J. H. Stocker and R M. Jenevein Chem Comm. 1968,934. lS3 P. Carrahar and F. G. Drakesmith Chem Comm. 1968 1562 T. Fujinaga K. Takaoka T. Nomura and K. Yoshikawa J. Chem Soc. Japan 1968 W,185. lS5 P.Yousefzadeh and C. K. Mann J. Org. Chem. 1968,33 2716. lS6 L.Horner and R J. Singer Chem Ber. 1968 101 3329. Electro-organic Chemistry 259 retained configuration.Benzyl methyl ethers with electron-withdrawing substituents such as ortho- and para-methoxycarbonyl and para-cyano are cleaved electrolytically the corresponding substituted toluene being produced in excellent yield.79 The remarkably clear distinction between the oxidation (p. 238) and reduction of the benzyl ethers depends on the ring substituent and suggests that the initial step in both reactions is acceptance or withdrawal of electrons at the aromatic nucleus. The possible use of electrolytic-cleavage reactions to remove protecting groups has been explored.’ 57 0-Trityl and 0-cinnamyl ethers have been converted into alcohols by using controlled potential electrolysis in dimethylformamide. The removal of cyanide from 4-cyanopyridine has been achieved by electrolysis at potentials more negative than those required for the reduction of the nitrile gro~p.”~ Miscellaneous cathodic reactions.The (spark-free) electrochemical fixation of nitrogen has been rep~rted.”~ Nitrogen was bubbled through a solution of titanium tetraisopropoxide and aluminium chloride in 1,2-dimethoxyethane and the mixture was continuously electrolysed between platinum electrodes. After hydrolysis a low yield of ammonia was obtained. Studies involving controlled potential reduction of aquocobalamin (vitamin B 2,)160 and riboflavinI6 have been described. Similar reduction of a 1,4-benzodiazepine derivative resulted in the observation of an interesting ring contraction (35)’ 62 NHMe Ph (35) Practical IMovations.-CZectrode Materials.A mercury electrode suitable for use in thin-layer electrochemistry has been de~cribed.’~~ Tin oxide on quartz has been used as an optically transparent electrode in a study of the electrochemical oxidation of p-aminophenol. 164 The U.V. spectra of inter- mediates formed at the anode were measured. Other materials examined for possible use as electrodes include graphite impregnated silicone rubber,I6 lS7 (a)V. G. Mairanovskii A. Ya Veinberg and G.I. Samokhvalov Zhur. obschei Khim. 1968 38 666; (b) A. Ya Veinberg V. G.Mairanovskii and G. I. Samokhvalov Zhur obshchei Khim. 1968 38 667. J. Volke and A. M. Kardos Coll. Czech. Chem Comm. 1968,33 2560. lS9 E. E. van Tamelen and B. Akermark J. Amer. Chem.SOC. 1968,90,4492. 160 P. K. Das H. A. 0.Hill J. M. Pratt and R J. P. Williams J. Chem SOC. (A) 1968 1261. S. V. Tatwawadi K. S. V. Santhanam and A. J. Bard J. Electroanalyt. Chem Interfacial Electrochem. 1968,17,411. 16’ H. Oelschlager and H. Hoffman Arch. Pharm. 1967,34M 817. 163 A. T. Hubbard and F. C.Anson Analyt. Chem. 1968,40,615. 164 J. W. Strojek and T. Kuwana J. Electroanalyt. Chem Interfacial Electrochem. 1968 16 471. 165 E. Pungor E. Szepesvary and J. Havas Analyt. Letters 1968 1 213. J. H. P.Utley boron carbide,'66 and a paste of carbon and silver halide.'67 The characteris- tics of cells used for the electrolysis of flowing solutions have been described.'68 Soluents. A very useful review has appeared16' concerning the use in electro- chemistry and the relevant properties of solvents derived from formamide and acetamide.Tetrahexylammonium benzoate is liquid at room temperature and has been suggested as a useful solvent in electrochemistry.17' It is electro-chemically inert to -2.6 v (sce) and is highly polar. Another quaternary ammonium salt tetrabutylammonium bromide may be used as a solvent at its melting point (138") although thermal degradation appears troublesome in this sy~tern.'~' The electroactive region of sulpholane is reported to be +1.8 v to -3-2 v (us. Ag/Ag+)172 and the cathodic limit of highly purified acetonitrile is -2.84 v (us. Ag/Ag+).'73 Dimethylsulphoxide with lithium perchlorate supporting electrolyte is useful between + 1.7 v and -3.0 v (us. Ag/AgCli ).' Reference Electrodes.with the increasing use of organic solvents in electro- chemistry reported experience concerning the preparation and stability of reference electrodes is of great value. A Cd/CdC12 electrode is described as stable and useful in anhydrous dimethylformamide.' 75 The preparation and standard potentials of the following electrode-solvent systems have been reported Ag/AgCl/dimethylacetamide;'76 Ag/AgBr/formamide;' Ag/Ag tartrate/water ;' * hydrogen electrode/dimethyl sulphoxide.'79 166 A. M. Hartley and H. D. Axelrod J. Electroanalyt. Chem Interfacial Electrochem. 1968 18 115. 167 W. R Ruby and C. G. Tremmel J. Electroanalyt. Chem Interfacial Electrochem. 1968 18 231. 16' (a) R E. Sioda Electrochim Acta 1968 13 375; (b) R E.Sioda Electrochim Acta 1968 13 1559. D. S. Reid and C. A. Vincent J. Electroanalyt. Chem Interfacial Electrochem. 1968,18,427. 170 C. G. Swain A. Ohno D. K Roe R Brown and T. Maugh J. Amer. Chem SOC. 1967 89 2648. 171 P. Texier and J. Bafoz-Lambling Bull. SOC.ckim France 1968 1273. 17' J. Desbarres P. Ichet and R L. Benoit Electrochim. Acta 1968,13 1899. 173 E. 0. Sherman and D. C. Olson Analyt. Chem. 1968,40 1174. 174 J. Courtot-Coupez and M. Le Demezet Bull. SOC. chim France 1967,4744. 17' L. W. Marple Analyt. Chem. 1967,39 844. 17' B. Scrosati G. Pecci and G. Pistoia J. Electrochem SOC. 1968,115,506. 177 K. W. Morcom and N. L. Mujy Nature 1968,217 1046. 17' S. M. A. Naqvi and P. B. Mathur Electrochim Acta 1968 13 1569. M. Le Demezet Compt.rend. 1968,266 C 1438. 17'

ISSN:0069-3030    

DOI:10.1039/OC9686500231

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

8 GENERAL METHODS By R. Brettle (Department of Chemistry The University Sheffield S3 7HF) Reduction.-Catalytic hydrogenation. Homogeneous hydrogenation has con- tinued to be of use and intriguing prospects are afforded by the introduction of the first chiral homogeneous hydrogenation catalysts. One such catalyst was obtained by treating a solution of optically active trichlorotris(methy1- phenylpropy1phosphine)rhodium in benzene-ethanol containing an excess of triethylamine with hydrogen under pressure ;the hydrogenation of ( +)-a-phenylacrylic acid with catalyst prepared from ligand of 69% optical purity gave hydrotropic acid of 15% optical purity.' This Reporter expects that his successors will be able to report significant progress towards stereospecific homogeneous hydrogenation.Reports2 have appeared on the selective homogeneous reduction of conjugated dienes by means of chromium car-bonyls. For example methyl trans trans-hexa-2,4-dienoate is reduced in the presence of deuterium and (methyl benzoato)tricarbonyl chromium to give a nearly quantitative yield of methyl ~is-2,S-dideuteriohex-3-enoate.~' The relative rates of homogeneous hydrogenation with chlorotris(tripheny1-phosphine)rhodium of some steroidal olefins have been measured. Contrary to earlier reports4 3-0x0-4-enes are slowly reduced and although the di- substituted double-bond in (1) is selectively reduced5 reduction of (2) with an equimolar proportion of hydrogen gave a mixture of the 3-oxo-4-ene (2) and the saturated ketone.6 0f-J&, o&I1 i'0,Me (1) (2) W.S.Knowles and M. J. Sabacky Chem. Comm. 1968 1445. (a) M. Cais E. N. Frankel and A. Rejoan Tetrahedron Letters 1968 1919; (b)A. Miyake and H. Kondo Angew. Chem. Internat. Edn. 1968 7 631; (c) E. N. Frankel E. Selke and C. A. Glass J. Amer. Chem. SOC. 1968,!20,2447. W. Voelter and C. Djerassi Chem Ber. 1968 101 58. cf Ann. Reports 1966 337. E. Piers and K. F. Cheng Canad. J. Chem. 1968,46 377. P. Wieland and G. Anner Helv. Chim. Acta 1968 51 1968. 262 R. Brettle Details have appeared' of the palladium-induced hydrogenation of 3-acylpyridinium salts (3) in the presence of triethylamine to give vinylogous amides (4) a key step in several elegant alkaloid syntheses. The hydrogenolysis of geminally disubstituted cyclopropanes (5) provides a useful route to com- pounds containing quaternary carbon atomsY8 but unfortunately the method is not stereospecific.8b The reduction of substituted vinylcyclopropanes without hydrogenolysis to give the alkylcyclopropanes can be effected by catalyticg or ionic (trifluoracetic acid-triethy1silane)'O hydrogenation or with di-imide.' ' Aluminium Hydrides.Sodium dihydrobis(2-methoxyethoxy)aluminate,'2 which unlike other complex aluminium hydrides is soluble in aromatic hydrocarbons generally resembles lithium aluminium hydride in its reac- tions.' Other new reagents are the pure alkoxyaluminium hydride~,'~ which resemble the 'mixed hydride reagents' (hydridoaluminium chlorides)' but have a much lower electrophilicity.Further applications of aluminium hydride have been explored. It does not liberate hydrogen from acidic methylene groups or undergo conjugate addition and so is the preferred reagent for the reduction of phenylaceto- and cinnamo-nitriles to the corresponding amines. ' It reduces p-0x0-esters to the 0x0-alcohols easily reducible with sodium boro- hydride to the diols so that for example (8) is now available from (7) in 82% overall yield.16 It converts the pyrrolidine enamines of acyclic and cyclic ketones into olefins in good yield [e.g. (9)gives (lo)]." The reduction of cyclic acid anhydrides to lactones by lithium aluminium hydride has again attracted interest. In five-membered rings the y-lactone can be obtained at -55" and in several unsymmetrical cases the more hindered carbonyl group is reduced.'* However in one six-membered ring a &lactone ' E.Wenkert K. G. Dave F. Haglid R. G. Lewis T. Oishi R. V. Stevens and M. Terashima J. Org. Chem 1968,33 747. (a) C. W. Woodworth V. Buss and P. v. R. Schleyer Chem. Comm. 1968 569; (b)Z. Majerski and P. v. R. Schleyer Tetrahedron Letters 1968 6195. S. R. Poulter and C. H. Heathcock Tetrahedron Letters 1968 5339. lo Z. N. Pares G. A. Khotimskaya M. Yu. Lukina and D. N. Kursanov Proc. Acad. Sci.(U.S.S.R.) 1968 178 88. l1 J. B. Pierce and H. M. Walborsky J. Org. Chem. 1968,33 1962. J. Vit B. Clsenky and J. Machlkk F.P. 1,515,582/1967. l3 V. Baiant M. Capka M. Cerny V. Chvalovsky K. Kochloefl M. Kraus and J. MAlek Tetrahedron Letters 1968,3303 ;J.F. Corbett Chem. Comm. 1968,1257. l4 B. Cooke E. C. Ashby and J. Lott J. Org. Chem. 1968,33 1132. l5 c$ E. L. Eliel Rec. Chem. Progr. 1961,23 129. l6 N. M. Yoon and H. C. Brown J. Amer. Chem. Soc. 1968,90,2927. l' J. M. Coulter J. W. Lewis and P. P. Lynch Tetrahedron 1968,24,4489. J. J. Bloomfield and S. L. Lee. J. Orq. Chem. 1967. 32. 3919. 263 General Methods was formed in which reduction occurred at the less-hindered carbonyl group.' An excellent procedure for the reduction of small samples of volatile materials (here methyl acetate) by lithium aluminium hydride uses tetrahydrofuryl- oxytetrahydropyran as solvent.20 Boron hydrides. Several important new syntheses based on organoboranes are discussed in later sections of this Report.9-Borabicyclo[3,3,1]nonane (11),2 now conveniently available by the hydroboronation of cyclo-octa- 1,5-diene is a crystalline solid stable to air and to heat which is an ideal reagent for effecting selective hydroboronations.22 Most olefms react within 5 minutes and the selectivity is comparable with that found for other bulky dialkyl- boranes such as disiamylborane. It is much to be hoped that this useful reagent will become commercially available. The hydroboronation of hetero-sub-stituted but-1- but-2- and but-3-enyl cyclopent-3-enyl and cyclohex-3- and OOEt *.'OH -0°" cyclohex-4-enyl derivatives has been studied with respect to orientation stereochemistry and the extent to which the hetero-substituent is eliminated.23 Ethyl vinyl ethers give the P-boron derivatives essentially quantitatively and these can be oxidised readily to the corresponding glycol half-ethers [e.g.(12) gives (13)].23a The products from the hydroboronation of several allylic alcohols (derived from ap-olefinic ketones by reduction with lithium aluminium hydride) when heated with acetic acid-acetic anhydride gave substantial amounts of 1,2-diacetates although no specific oxidising agent is present [e.g.(14) gave (15) in 28 % yield24]. Hitherto no group was known which would l9 B. E. Cross and J. C. Stewart Tetrahedron Letters 1968 3589. 'O J. W. Taylor and J. C. Martin J. Amer. Chem. Soc. 1967,89,6904. 21 R. Koster Angew. Chem. Znternat. Edn. 1964,3 174. " E. F. Knights and H. C. Brown .I.Amer. Chem. SOC.,1968,90 5280; 5281.23 (a)H. C. Brown and R. L. Sharp J. Amer. Chem. SOC. 1968,90,2915;(b)H. C. Brown and R. M. Gallivan jun. ibid. p. 2906; (c)H. C. Brown and M. K. Unni ibid. p. 2902; (d)H. C. Brown and E. F. Knights ibid. p. 4439; (e)D. J. Pasto and J. Hickman ibid. p. 4445. 24 K. Bailey and T. G. Halsall J. Chem. SOC.(C),1968 679. 264 R. Brettle 0 AcOa protect a carbonyl group towards hydroboronation and yet keep the carbonyl carbon atom trigonally hybridised (a condition necessary to prevent double- bond isomerisation in certain 0x0-olefins). The 2,4-dinitrophenylhydrazone has now been shown to be satisfactory for this purpose; the carbonyl group can be regenerated by low temperature ozon~lysis.~~ Diborane unlike lithium aluminium hydride will reduce N-substituted pyrrole and indole ketones to the corresponding N-substituted alkyl-pyrroles and -indoles,26 and selectively reduces NN-disubstituted amides to the cor- responding amines in the presence of ester function^.^' Full details have appeared of the anomalous reduction of 14-alkenyl- codeinones by sodium borohydride in aqueous pyridine.28 In methanol chalcone (16) is reduced by sodium borohydride to the allylic alcohol (17) but in pyridine or in diglyme containing some pyridine the saturated alcohol (18) is obtained.PhCO-CHSHPh PhCH(OH)* CH=CHPh PhCH(OH)*CH2*CH2Ph (16) (17) (18) The allylic and benzylic alcohol (17) is also reduced to (18) by sodium boro- hydride in the presence of a Lewis base.29 Azides are reduced to amines by sodium borohydride in good yield despite earlier contraindications and selective reduction of an 0x0-azide to a hydrozy-azide was a~hieved.~’ The choice of solvent may be important in such reductions.Conjugated nitro- alkenes are reduced to nitroalkanes by sodium borohydride in acid conditions which largely suppress unwanted by-product-forming Michael reactions ; similar conditions had earlier been used for the reduction of certain hetero- cyclic systems containing carbon-nitrogen double-bonds. 31 The reduction of several cyclic imide~~~ and some useful selective reductions of acids as their mixed carbonic anhydrides in the presence of a variety of potentially reducible functional-groups with sodium borohydride have been reported. 33 Several ” J.E. McMurry Chem. Comm. 1968,433. 26 K. M. Biswas and A. H. Jackson Tetrahedron 1968,24,1145. ’’ M. J. Kornet P. A. Thio and S. I. Tan J. Org. Chem. 1968,33 3637. ‘’ J. W. Lewis and M. J. Readhead Tetrahedron 1968,24 1829; cf Ann. Reports 1967,202. 29 K. Iqbal and W. R. Jackson J. Chern. SOC.(C),1968,616. ” Y. Ali and A. C. Richardson Carbohydrate Res. 1967 5,441. ’’ A. I. Meyers and J. C. Sircar J. Org. Chem. 1967 32 4134; A. I. Meyers and J. M. Greene ibid. 1966 31 556. 32 Y. Kondo and B. Witkop J. Org. Chem. 1968,33 206; S. J. Huang Chem. Comm. 1968,245. ’’ K. Ishizumi K. Koga and S. Yamada Chem. and Pharm. Bull. Japan. 1968,16,492. General Methods 265 papers have dealt with the reductive opening of epoxides with alu~ninium'~ or boron hydride~.~~ Other methods.The reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents has been reviewed. 35 Trialkyl phosphites rapidly reduce hydroperoxides to the corresponding alcohols36 and use has now been made of their stability to molecular oxygen to develop a one-stage preparation of tertiary a-ketols involving the base-catalysed autoxidation of ketones at low temperatures in their presence.37 The photolysis of dimethylthiocarbamates (19) in methanol is a promising new method for deoxygenation which has been applied in the carbohydrate fieid.38 ROH -RO-CS-NMe --& RH (19) Further work on ionic hydrogenation involving the reduction of carbonium ions derived from olefins,'O ethers,39 and alcohols4o with organosilanes has been reported.Potassium hexacyanodinickelate reduces afl-olefinic acids (and hexa-2,4-dienoic acid) to saturated acids.41 The hydrogen comes from the aqueous solvent so that in deuterium oxide deuteriation occurs; the stereochemistry of the addition has not been established. Reductions with chromium(1r) salts have been reviewed.42 Nitriles are reduced efficiently either by solvated electrons or ~athodically.~~ 34 H. C. Brown and N. M. Yoon J. Amer. Chem. SOC.,1968,90,2686; Chem. Comm. 1968 1549. 35 J. I. G. Cadogan Quart. Rev. 1968 22 222. 36 R. Joly J. Warnant J. Joly and J. Mathieu Compt. rend. 1964,258 5669. 37 J. N. Gardner F. E. Carlon and 0.Gnoj J. Org. Chem. 1968,33 3294; J. N. Gardner T. L. Popper F. E. Carlon 0.Gnoj and H. L.Herzog ibid.,p. 3695. 38 R.H. Bell D. Horton and D. M. Williams Chem. Comm. 1968 323. 39 V. I. Zdanovich R. V. Kudriavtsev and D. N. Kursanov Doklady Akad. Nauk S.S.S.R. 1968,182 593. 40 F. A. Carey and H. S. Tremper J. Amer. Chem. SOC.,1968,90,2578. 41 W. H. Dennis jun. D. H. Rosenblatt R. R. Richmond G. A. Finseth and G. T. Davis Tetra-hedron Letters 1968 1821. 42 J. R.Hanson and E. Premuzic Angew. Chem. Znternat. Edn. 1968,7,247. 43 P. G. Arapakos J. Amer. Chem. SOC.,1967,89,6794; P. G. Arapakos and M. K. Scott Tetra-hedron Letters 1968 1975. 266 R. Brettle The nitrile group is replaced non-stereospecifically by hydrogen [e.g. (20) gives (21)]. Vicinal dicyanides give a mixture of olefinic and saturated hydro- carbons. Kinetic control can be achieved in Meerwein-Ponndorf reductions by conducting them for times shorter than is customary; this has been used to increase the proportion of the axial 3p-01 in the reduction products from 3-0x0-5 P-pregnane derivative^.^^ A review of the Wolf-Kishner reaction deals with many of the side reactions ;45 double-bond isomerisation must always be expected during the reduction of cyclic ap-olefinic Oxidation.-Dichloromethane has advantages over pyridine as a solvent for dipyridinechromium(vI) oxide in the oxidation of alcohols ; excellent yields of aldehydes including nonconjugated olefinic and phenolic aldehydes have been achieved and product isolation is ~implified.~~ The tertiary amine (22) is oxidised to the ketone (23) by sodium tungstate in aqueous methanolic hydrogen peroxide at room temperat~re,~~ conditions previously used to 6; Ph oxidise primary amines to oximes.Further examples of the oxidation of trisubstituted olefins to a-ketols [e.g. (24) gives (25)] by osmium tetroxide in the presence of N-methylmorpholino-N-oxide-hydrogenperoxide49 have been rep~rted.~’ Ethers can be directly converted into esters by trichloroisocyanuric acid at or below room temperature [e.g. (26)gives (27)].” Further results with this MeCH,-0-CH,Me -+ MeCO-O-CH,Me (26) (27) oxidant are awaited with interest. Aliphatic aromatic and heterocyclic sulphides are all smoothly oxidised to pure sulphoxides by iodobenzene dichloride in aqueous pyridine making this an attractive alternative route to ~ulphoxides.~~ 44 R.Bouchard and Ch. R. Engel Canud. J. Chew. 1968 46 2201; G. Bach J. Capitaine and Ch. R. Engel ibid. p. 733. 45 H. H. Szmant Angew. Chem. Znternat. Edn. 1968,7 120. 46 I. Elphinoff-Felkin and M. Verrier Tetrahedron Letters 1968 1515. 47 J. C. Collins W. W. Hess and F. J. Frank Tetrahedron Letters 1968 3363. 48 H. E. Zimmerman K. G. Hancock and G. C. Licke J. Amer. Chem. SOC.,1968,90,4892. 49 W. P. Schneider and A. R. Hanze U.S.P. 2,769,823/1956. ’’ B. Gadsby M. R. G. Leeming G. Greenspan and H. Smith J. Chem. SOC.(C),1968,2647. 51 E. C. Juenge and D. A. Beal Tetrahedron Letters 1968 5819. ’’ G. Barbieri M. Cinquini S. Colonna and F. Montanari. J. Chem. SOC.(C).1968 659. General Methods Saturated and nonconjugated olefinic aldehydes are oxidised to acids by silver(I1) oxide in aqueous tetrahydrofuran at 25" in neutral conditions ; in the presence of cyanide ions as catalyst conjugated olefinic aldehydes are similarly oxidised with retention of the olefinic ge~metry.'~ Aromatic and ap-olefinic aldehyde cyanohydrins are oxidised in methanol by manganese dioxide to give the corresponding methyl esters.53 A preliminary survey of other oxidations with silver@) oxide has appeared ;54 silver@) carbonate precipitated on Celite has been used for the oxidation of carbonyl group^.'^ Selective oxidation of allylic alcohols is possible [e.g.(28) gives (29)l. QH (28) (29) h""0 (30) (31) 2,3-Oxidosqualene was degraded to the corresponding C aldehyde [cfi (30)-,(31)] with periodic acid in ether.56 Attention is drawn to potential explosion hazards with concentrated solutions of periodic acid in dimethyl sulpho~ide'~ and with lead dioxide in the presence of dicarbonyl compounds.' Lead dioxide converts y-0x0-acids into ap-olefinic ketones.' A detailed account of the oxidation of aldoximes by lead tetra-acetate with a summary of recent work has a~peared.'~ Further examples of the formation of y-lactones by the reaction of lead tetra-acetate with olefins in acetic acid have been reported,60 but a better preparative method uses manganic acetate as the oxidant [cf:(32) 4 (33); R = H)-J6' 53 E.J. Corey N. W. Gilman and B. E. Ganem J. Amer. Chem. SOC. 1968,90 5617. 54 T. G. Clarke N. A. Hampson J. B. Lee J. R.Morley and B. Scanlon Tetrahedron Letters 1968,5685. " M. Fetizon and M. Golfier Compt. rend. 1968,267 C 900. 56 D. H. R. Barton A. F. Gosden G. Mellows and D. A. Widdowson Chem. Comm. 1968 1067; cf Fieser L. F. and Fieser M. 'Reagents for Organic Synthesis,' Wiley New York 1967 p. 817. '' J. J. M. Rowe K. B. Gibney M. T. Yang and G. G. S. Dutton J. Amer. Chem. SOC.,1968,90 1924. (a) D. V. Hertzler J. M. Berdahl and E. J. Eisenbraun J. Org. Chem. 1968 33 2008; (b)R. Brettle and D. Seddon Chem. Comm. 1968 1546. '' G. Just and K. Dahl Tetrahedron 1968,24 5251. 6o E. I. Heiba R. M. Dessau and W. J. Koehl jun. J. Amer. Chem. SOC.,1968,90,2706. (a) J. B. Bush jun. and H. Finkbeiner J. Amer. Chem. SOC. 1968 90 5903; (b) E. I. Heiba R.M. Dessau and W. J.Koehl jun. ibid. p. 5905. 268 R. Brettle PhCH=CH -PhCH-CH /\ ok’CHR (33) The use of propionic acid as solvent gives a-methyl-substituted lactones [e.g.(33; R = Me)].61b The palladium chloride 7c-ally1 complex of cholest-4-en-3-one decomposes on warming to give cholesta-4,6-diene-3-one;62 further examples of this mild dehydrogenation method may be expected. The first simple olefin 2,3-dimethyl-but-2-ene has been dehydrogenated by 2,3-dichloro-5,6-di-cyanoquinone (DDQ).63 Full details have appeared of Foote’s work on oxygenation with hypo- chlorite-hydrogen peroxide.64 Olefms-The (Q and (2) system for the unambiguous specification of stereoisomerism about an olefinic bond6’ began to come into use during the year. Full details have appeared66 of the new syntheses of olefins (and ketones) based on P-hydroxyphosphon- and p-hydroxysulphin-amides reported last year.The 1,2-elimination of P-hydroxysulphinamides to form olefins occurs via a stereospecifically ciselimination pathway.66b Syntheses of the Juvenile Hormone of the giant silkworm moth (34) have focussed attention on routes to trisubstituted olefins of known geometry. The stereoselectivity of the crucial first stage of the Cornforth ~ynthesis,~~ in which an alkylmagnesium halide reacts with an a-halogeno-ketone can be substantially improved by working at a lower temperature,68 and this approach was used for the elaboration of the terminal epoxide function in one synthesis of (34).69A method for the preparation of homoallylic bromides by the action of hydrobromic acid on cyclopropylcarbinols which showed inadequate stereoselectivity when the double-bond formed was trisubstituted has now been successfully adapted.The carbinol [e.g. (35)] is treated with phosphorus tribromide and the resultant bromides are rearranged with anhydrous zinc bromide in ether to give the homoallylic bromide [e.g. (36)].68969 In another approzch the Corey trisubstituted olefin synthesis from prop-2-ynyl-alcohols reported last year70 was extended by the use of lithium diethylcopper for the 62 R. W. Howsam and F. J. McQuillin Tetrahedron Letters 1968 3667. 63 A. E. Asato and E. F. Kiefer Chem. Comm. 1968,1684. 64 C. S. Foote S. Wexler W. Ando and R. Higgins J. Amer. Chem. SOC.,1968,90 975. 65 J.E. Blackwood C. L. Gladys K. L. Loening A. E. Petrorca and J. E. Rush J. Amer. Chem. SOC.,1968,90 509. 66 (a) E. J. Corey and G. T. Kwiatkowski J. Amer. Chem. SOC. 1968 90 6816; E. J. Corey and T. Durst ibid. p. 5548; (b)E. J. Corey and T. Durst ibid.,p. 5553; I.$ Ann. Reports 1967 64,206; 250. 67 J. W. Cornforth R. H. Cornforth and K. K. Mathew J. Chem. SOC.,1959 112. S. F. Brady M. A. Ilton and W. S. Johnson J. Amer. Chem. SOC.,1968,90,2882. 69 W. S. Johnson T. Li D. J. Faulkner and S. F. Campbell J. Amer. Chem. SOC.,1968,90,6225. ’O Ann. Reports 1967,64,209 ;268. General Methods alkylation step7' (see also under Alkylation). In a totally different approach two trisubstituted olefinic bonds were introduced by sequential fragmentation reactions.Control of the olefin geometry was thereby transposed to control of the relative stereochemistry in a bicyclic system and known synthetic methods allowed the desired relationships amongst the five centres in the precursor (37) to be established. NaH THF 20" TsO (37) 2 steps I (38) Me The steric requirements for concerted-fragmentation reactions then inexorably lead to the formation of the desired olefin (38).72 Yet another synthesis is based on the cleavage of p-lactones in refluxing collidine ;routes to the necessary diastereoisomerically pure p-lactones [e.g. (39)] have been establi~hed.~ (39) R= n-C4H '' E. J. Corey J. A. Katzenellenbogen N. W. Gilman S. A. Roman and B. W. Erickson,J. Amer. Chem. SOC.,1968,90 5618. '' R.Zurfliih E. N. Wall J. B. Siddall and J. A. Edwards J. Amer. Chem. SOC.,1968,90,6224. 73 M. U. S. Sultanbawa Tetrahedron Letters 1968,4569. 270 R.Brettle Several other new olefin syntheses have also been reported. Propylmagnesium bromide reacts with allylic alcohols in the presence of dichlorobis(tripheny1- phosphine)nickel to give propene and a mixture of ~lefins,~~ and Grignard reagents lacking a P-hydrogen atom give alkylated 01efins.~ The proportion of olefins from a range of alcohols has been determined. The general results are summarised in Scheme 1. Olefins can be obtained from a-halogeno- ketones by the oxidative decarboxylation with alkaline hydrogen peroxide of the Favorskii cyclopropanone intermediate^.^^ A method related to the Corey-Winter synthesis uses the fragmentation of the benzylidene derivatives R1CH=CH-CH,R2 PrMgBr R’CH=CH CH(OH)R -+ (Ph3P),NiC12 RICH CH=CHR2 R’CH==CH*CHRR2 RMgBr R~CH=CH-CH(OH)R -+ (Ph3),NiC12 R’RCH CH=CHR R = Me Ph or PhCH SCHEME 1 of 1,2-diols (3-phenyldioxolans) with alkyl-lithiums [e.g.(40) gives (41)] ; it is not applicable if the hydroxy-groups are ben~ylic.~’ A modified preparation of xanthates makes the Chugaev reaction more attra~tive.~~ The reaction of toluene-p-sulphonyl-hydrazones with methyl-lithium works for the synthesis of the highly strained olefin bicyclo[2,l,l]hex-2-ene when the Chugaev Hofmann amine oxide and acetate pyrolysis reactions all fail ;’’extension of this synthesis to ctp-olefinic hydrazones provides a useful route to con- jugated dienes.80 A satisfactory one-step method for the oxidative decarboxylation of acids has been found in the cupric acetate-catalysed action of lead tetra-acetate on 74 H.Felkin and G. Swierczewski Compt. rend. 1968,266 C 1611. 75 C. Chuit H. Felkin C. Frajerman G. Roussi and G. Swierczewski Chem. Comm. 1968 1604. 76 J. E. Baldwin and J. H. I. Cardellina Chem. Comm. 1968 558. ” J. N. Hines M. J. Peagram G. H. Whitham and M. Wright Chem. Comm. 1968,1593. 78 A. de Groot B. Evenhuis and H. Wynberg J. Org. Chem. 1968,33,2214. 79 J. Meinwald and F. Uno J. Amer. Chem. Soc. 1968,90,800. W. G. Dauben M. E. Larber N. D. Vietmeyer R. H. Shapiro J. H. Duncan and K. Tomer J. Amer. Chem. SOC. 1968,90,4762.General Methodi 271 primary and secondary carboxylic acids ;8 the reactions can be conducted thermally at 80" or photochemically at 30". Selective decarboxylation of diacids is possible [e.g. (42) gives (43)]. The bisdecarboxylation of 1,2-diacids with lead tetra-acetate frequently goes in much better yield if carried out in HO2C*[CHJ6*CO2H -r H02C* [CH2]4*CH=CH2 (42) (43) the presence of oxygen;82 full details have appeareds3 of the alternative electrochemical method which is particularly useful for small-ring olefins. A wide variety of P-substituted alkyl halides undergoes elimination to give olefins with the chromium(r1)ethylenediamine complex,84 but the reaction is only stereospecific for 1,2-dibromide~.~~ Phosphonium halides can be reduced to ylids electrochemically in the presence of carbonyl compounds so that a Wittig reaction can be performed in a non-basic system.86 a-Metallated isocyanides [e.g.(44)]are more reactive alternatives to phosphorus ylids for the preparation of olefins from carbonyl Stereospecific syntheses Ph,C(Li)NC + PhCHO + Ph,C--CHPh + LiOCN (44) of conjugated dieness based on vinyl-boranes and -alanes have developed out of earlier works9 on the synthesis of simple olefins. Syntheses from hex-3- yne are shown in Scheme 2. Additions to olefines leading to the formation of carbon-nitrogen bonds have continued to attract attention. Work on N-chloroamide~,~~ N-chloro-sulph~namides,~~ 00-diethyl NN-dichlorophosphoramidate,92 halogen azide~,~~ iodine nitrite,95 and iodine nitrateg6 has appeared.nitrosyl f~rmate,~~ Dinitrogen tetroxide in the presence of oxygen gives nitro-per~xynitrates~~ J. D. Bacha and J. K. Kochi Tetrahedron 1968,24,2215. 82 C. M. Cimarusti and J. Wolinsky J. Amer. Chem. SOC.,1968,90 113. 83 P. Radlick R. Klem S. Spurlock J. J. Sims E. E. van Tamelen and T. Whitesides Tetrahedron Letters 1968 5117; H. H. Westberg and H. J. Dauben jun. ibid. p. 5723; cf Ann. Reports 1967 64 207. 84 J. K. Kochi D. M. Singleton and L. J. Andrews Tetrahedron 1968,24 3503. 85 J. K. Kochi and D. M. Singleton J. Amer. Chem. SOC.,1968,90 1582. 86 T. Shono and M. Mitani J. Amer. Chem. SOC.,1968,90,2728. 87 U. Schollkopf and F. Gerhart Angew. Chem. Internat. Edn. 1968,7 805. 88 G. Zweifel N.L. Polston and C. C. Whitney J. Amer. Chem. Soc. 1968,90,6243. 89 c$ Ann. Reports 1967 64 209 232 250. R. S. Neale J. Org. Chem. 1967 32 3263; R. S. Neale and N. L. Marcus ibid. p. 3273; ibid. 1968,33 3457. F. A. Daniker M. T. Melchiar and P. E. Butler Chem. Comm. 1968 T. Ohashi M. Sugie M. Okahara and S. Komari Tetrahedron Letters 1968 4195; D. Greatbanks T. P. Seden and R. W. Turner ibid. p. 4863; T. P. Seden and R. W. Turner J. Chem. SOC.(C),1968 876. 92 A. Zwierzak and A. Zoziara Angew. Chem. Internat. Edn. 1968,7,292. 93 A. Hassner and F. Boerwinkle J. Amer. Chem. SOC.,1968 90 210; J. S. Brimacombe J. G. H. Bryan T. A. Hamor and L. C. N. Tucker Chem. Comm. 1968 1401. 94 H. C. Hamann and D. Swern J. Amer. Chem. SOC. 1968,90,6481. 95 W. A. Szarek D.G. Lance and R. L. Beach Chem. Comm. 1968,356. 96 J. E. Kropp A. Hassner and G.J. Kent Chem. Comm. 1968,906. 97 D. R. Lachowicz and K. L. Kreuz J. Org. Chem. 1967,32,3885. 272 R. Brettle EtC=CEt Et 'Cd/"' I H/\ AIR^ ii v vi I + R' = hexyl; RZ= isobutyl Reagents i R'BH,; ii Me,NO; iii NaOH-I,; iv R2,AlH; V EtCzCEt; vi H30+ SCHEME2 which can easily be converted into nitro-ketones and nitrosyl chloride provides a route to chloro-ket~nes.~~ Alkylfluoroxy-compounds [e.g. (46)]bring about the electrophilic fluorina- tion of some activated olefins e.g. steroidal enol-acetates [cf. (45)]; many + CF,OF CFCl, d} -'.d} -75" Ac 0 (46) 0 H H functional groups survive unchanged." Solvents other than carbon halides may lead to explosions.Carbonyl compounds.-A complete volume of Organic Reactions has been devoted to the aldol condensation,100 and 'directed aldol condensations' have 98 B. W. Ponder and D. R. Walker J. Org. Chem. 1967,32,4136. 99 D. H. R. Barton L. S. Godinho R. H. Hesse and M. M. Peshet Chem. Comm. 1968,804. loo A. T. Nielsen and W. J. Houlihan Org. Reactions 1968 16 1. General Methods been reviewed."' The latter reaction enables ketones to be converted into ap-olefinic aldehydes via condensation with a-metallated N-cyclohexylaldimines. Condensation with an a-formylalkylidenetriphenylphosphorane does not occur but condensation with the anion from a diethyl a-alkyl-P-(cyclohexyl- imin0)ethylphosphonate provides a good alternative synthesis.'02 Both routes are illustrated in Scheme 3.P-Hydroxycarbonyl compounds can be RiZCO (1) LiCHRL.CH=NR3+ R '2 C(0H)* CHR2* CH=NR (ii) H20 1I (EtO),P(0).CHRZ.CH=NR3 H~O+A NaH R ',C=CR2 CH=NR H30++ R',C=CR2-CH0 R3 = cyclohexyl SCHEME 3 dehydrated under very mild conditions with dicyclohexylcarbodi-imide in the presence of cupric ions.lo3 Cyclic ap-unsaturated ketones [e.g. (48)]can be prepared by the action of alkylidenetriphenylphosphoranesor the anions from dialkyl alkylphosphonates on enol lactones [e.g. (47)l; this is a useful alternative to the well known Grignard method.lo4 R' (48) Trialkylboranes react with carbon monoxide in the presence of lithium trimethoxyaluminohydride to give intermediates which on oxidative hydrolysis give aldehyde^.'^' The syntheses of aldehydes and ketones by the reaction of trialkylboranes with ap-olefinic carbonyl cornpoundslo6 have been extended by the use of a-bromo- and a-methyl-a~rolein'~~ and olefinic ketones formed in situ from the quaternary salts of Mannich bases derived from cyclic ketones.'08 The generality of this procedure has still not been comprehensively investigated ; lo' G.Wittig and H. Reiff Angew. Chem. Znternat. Edn. 1968,7 7. Io2 W. Nagata and Y. Hayase Tetrahedron Letters 1968,4359. E. J. Corey N. H. Andersen E. Vedejs I. Vlatters and R. E. K. Winter J. Amer. Chem. Soc. 1968,90,3245. C. A. Henrick E. Bohme J. A. Edwards and J. H. Fried J. Amer. Chem. Soc. 1968,90 5926. H. C. Brown R. A. Coleman and M.W. Rathke J. Amer. Chem. SOC.,1968,90 499. cf Ann. Reports 1967 64 210 264. H. C. Brown G. W. Kabalka M. W. Rathke and M. M. RogiC J. Amer. Chem. SOC.,1968 90,4165. H. C. Brown M. W. Rathke G. W. Kabalka and M. M. RogiC J. Amer. Chem. SOC. 1968,90 4 166. 274 R. Brettle it failed for croton- and ~innam-aldehydes.~~' Two other ketone syntheses depend on the reaction of trialkylboranes with diazoacet~ne'~~ and with a-bromo-ketones in the presence of potassium t-butoxide."O -*MeCO*CH2R1 MeCO. CHN R2CO-CH2Br * R',B R2CO*CH2R' Bu'OK In all of these methods only one of the alkyl groups in the trialkylborane is incorporated into the product. P-0x0-sulphones may be better than the more extensively used sulphoxides in the Corey-Chaykovsky ketone synthesis because of their greater stability to oxidative and reductive processes elsewhere within the molecule and their lack of asymmetry at sulphur.'" The adducts from Grignard reagents and aldehydes are oxidised to ketones in good yield by diethyl azodicarboxy- late.''* The conditions necessary for the preparation of ketones by the reaction of nitriles with alkylidenephosphoranes have been established." The genera- tion of a carbonyl group from a primary amine by N-bromination dehydro- bromination to the imine and hydrolysis has again proved useful in a synthetic sequence,lo3 and the fact that primary aliphatic azides on photolysis give imines has likewise been used as a route to aldehydes.'14 The Serini reaction,115 the formation of aldehydes or ketones by the action of zinc on glycol mono- acetates has been applied in acyclic the product from (49; R = OAc) differs from those formed in the acid-catalysed pinacol rearrangement of the parent glycol (49 ;R = H).PhCH(OR).CMe(OH)Me (49) PhCO- CHMe PhCMe,.CHO + PhCHMeaCOMe The sensitized photo-oxygenation of enamines causes fission of the carbon- carbon double-bond,"'" and synthetic use has been made of this new route to carbonyl compounds.117b R' = alk or Ph; RZ = alk Ph or H; R3 = 1-piperidyl log J. Hooz and S. Linke J. Amer. Chem. SOC., 1968,90 5936. 'lo H. C. Brown M. M. RogiC and M. W. Rathke J. Amer. Chem. SOC. 1968,90 6219. H. 0.House and J. K. Larson J. Org. Chem. 1968,33,61. T. Mukaiyama K. Takahashi and I.Kurvajima &ll. Chem. SOC.Japan 1968,41 1491. 'I3 R. G. Barnhardt jun. and W. E. McEwen J. Amer. Chem. SOC.,1967,89,7009. D. Horton A. E. Leutzow and J. C. Wease Carbohydrate Res. 1968,8,366. ''' cf N. L. Wendler in 'Molecular Rearrangements,' ed. P. de Mayo Interscience New York and London 1964 part 11 p. 1039. E. Ghera Chem. Comm. 1968 1639. ''' (a)C. S. Foote and J. W. Lin Tetrahedron Letters 1968 3267; (b)J. E. Huber ibid. p. 3271. General Methods Enamine oxidation is also involved in the conversion of a ketone [e.g. (SO)] into the a-acetoxy-ketone by oxidation of the derived enamine with thallium(m) acetate followed by hydrolysis which is frequently a better route than direct acetoxylation with thallium(II1) or lead(1v) acetates.' ' The autoxidation of the PO Reagents i TI(OAc),; ii water; iii air; iv H,O' enamines [cf (51)] or Schiffs bases [cf.(53)] of ap-olefinic ketones provides a route to y-0x0-ap-olefinic ketones [e.g. (52)I.l l9 NN-Dimethylformaldimonium trifluoracetate prepared from trimethyl-amine N-oxide and trifluoroacetic anhydride is an excellent Mannich re- agent ;120 bisdimethylaminomethane reacts in acetic anhydride to give not the Mannich bases but the derived methylene compounds.'2' The nitration of carbonyl-activated methylene groups can be performed with advantage in liquid ammonia.122 The reaction of the ketone (54) with isopropylmagnesium bromide at +35" gave the tertiary alcohol (55) in only 5% yield because of the enolisation of the carbonyl group but at -50" the yield was increased 09 09 0nciso -p3 OH (54) (55) to 50%; this sort of improvement is reported to be generally attainable.123 Full details have appeared of the decarbonylation of aldehydes (and acyl 11* M.E. Kuehne and T. J. Giacobbe J. Org. Chem. 1968,33,3359. '19 S. K. Malhotra J. J. Hostynek and A. F. Ludin J. Amer. Chem. SOC. 1968,90 6565. lZo A. Ahoud A. Cavt C. Kan-Fan H. Husson J. de Rostolan and P. Potier J. Amer. Chem. SOC.,1968,90 5622. lZ1 E. C. Taylor and Y. Shvo J. Org. Chem. 1968,33 1719. H. Feuer A. M. Hall S. Golden and R. L. Reitz J. Org. Chem. 1968,33,3622. 12' J. E. McMurry J. Amer. Chem. SOC.,1968,90,6821. 276 R. Brettle halides) with rhodium-catalysis attention has also been drawn to the generality of aldehyde decarbonylation with palladium charcoal as cataiyst.12 Carboxylic Acid Derivatives.-Several new syntheses of acid derivatives based on reactions of trialkylboranes have been reported.At present these are based on trialkylboranes containing three identical alkyl groups prepared by the action of diborane on an olefin so that only one third of the olefin can be incorporated into the product; it should be possible to overcome this disadvantage however by using suitable dialkylboranes in the hydroborona- tion step. The new reactions are those with diazoacetonitrile,'26 ethyl diazo- acetate,126 and ethyl bromoacetate in the presence of potassium t-b~toxide.'~' With ethyl dibromoacetate syntheses of either a-bromo-esters or secondary esters are possible.'28 Another generally less useful route involves reactions with sulphur ylids e.g.ethyl (dimethylsulphurany1idine)acetate. * These new syntheses and the initial reagents are summarised in Scheme 4. N,CH.CN 'RCH,*CN / N,CH*CO,Et or BrCH,-CO2Et R3B R-CH,-CO,Et Br,CH. C0,Et RCH(Br)*CO,Et or RCH.CO,Et Me,S=CH.CO- NEt L RCH CO .NEt SCHEME 4 The carbonylation of olefins has been reported as a route to acid derivatives before but the use of catalysts such as dichlorobis(tripheny1phosphine)-palladium now enables olefins to be converted into esters at temperatures below loo" with the minimum of side-reactions.' 30 Selective carbonylation is possible with polyolefins. ap-Olefinic aldehydes can be converted into the corresponding saturated esters by conversion into the cyano-amines [e.g.(56)] which on treatment with alcoholic alkali gives the saturated imino-ethers [e.g. (57)]readily hydrolysable to the esters.' 31 Several papers have been concerned with the preparation of the cis-forms of 60" co aCHMe-C02Me MeOH (85 %) (Ph,P),PdCl 124 K. Ohno and J. Tsuji J. Amer. Chem. SOC. 1968,90,99. 125 J. W. Wilt and V. P. Abegg J. Org. Chem. 1968,33,923. 126 J. Hooz and S. Linke J. Amer. Chem. SOC. 1968,90,6891. 127 H. C. Brown M. M. RogiC M. W. Rathke and G. W. Kabalka J. Amer. Chem. SOC.,1968,90 818. 128 H. C. Brown M. M. RogiC M. W. Rathke and G. W. Kabalka J. Amer. Chem. SOC.,1968,90 1911. 129 J. J. Tufariello L. T. C. Lee and P. Wojtkowski J. Amer. Chem. SOC.,1967,89,6804.K. Bittler N. v. Kutepow D. Neubauer and H. Reis Angew. Chem. Internat. Edn. 1968,7,329. 131 J. S. Walia P. S. Walia L. Heindl and H. Lader Chem. Comm. 1967 1290. General Methoh R2NH2 HOAc R'CH=CH. CHO R 'CH4HCH(CN)NHRZ (56) MeOH 1 0H;R'OH H30+ RCH2.CHz COzR3 R'CH * CH .C(OR3)===NRZ (57) 0" ap-olefinic acids or their derivatives from carbonyl compounds by means of phosphorus ylids or phosphonate anions.' 32 Attempts to extend the synthesis of acids from carbonium ions and dichloroethylene to trichlorethylene were not generally successful although some particularly stable carbonium ions did give a-chloro-acids.' 33 Aliphatic a-0x0-acetals which are available from P-0x0-sulphoxides,' 34 rearrange in the presence of water and tin@) chloride to give a-hydroxy-esters a route to aromatic a-hydroxy-esters had been reported earlier.' 34 The preparation of p-0x0-esters from alkan-Zones is much better with magnesium methyl carbonate than for example with dialkyl carbonates.' The perennial problem of esterification has received attention.Esterification with iodomethane in NN-dimethylacetamide in the presence of sodium hydrogen carbonate is very slow but q~antitative.'~' The use of molecular sieves able selectively to absorb water from aqueous methanol is the basis for a convenient large-scale preparation of methyl esters.' 38 Acid anhydrides including mixed anhydrides can be prepared virtually quantitatively by the action of acyl or aroyl halides on thallium(1) carboxylates.' 39 Alkylation and CouplingReactions.-Interest in reactions with organocopper reagents has continued.Their range has been extended by the preparation of higher homologues of lithium dimethyl~opper.'~~ Halogen-copper exchange is more apt to occur with these but this can be nullified by subsequent addition to the reaction mixture of an excess of the appropriate alkyl halide. Coupling has been accomplished with alkyl allyl vinyl (see under Olefins) and aryl halides ;free acid and amide groups do not interfere. Lithium di-(trans-prop-1- eny1)copper has been coupled with a vinyl halide.I4' The complex formed from copper(1) iodide methyl-lithium and trimethyl phosphite has certain advantages over lithium dimethylcopper ;it undergoes conjugate addition to cyclohex-2- enones in the presence of lithium iodide.142 Lithium dialkylcopper reagents react with ethynylcarbinyl acetates to give allene~.'~~ 132 G.Pattenden and B. C. L. Weedon J. Chem. SOC. (C) 1968 1894; 1997; G. Jones and R. F. Maisey Chem. Comm. 1968,543; R. K. Huff; C. E. Moppett and J. K. Sutherland J. Chem. SOC.(C) 1968,2725; T. H. Kinstle and B. Y. Mandanas Chem. Comm. 1968,1699. "' K. Bott Angew. Chem. Znternat. Edn. 1967,6,946. 134 cf Ann. Reports 1967,64,212. 13' J. E. Thompson J. Org. Chem. 1967,32,3947. L. Crombie P. Hemesley and G. Pattenden Tetrahedron Letters 1968 3021. 13' F. S. Alvarez and A. N. Watt J. Org. Chem. 1968,33,2143; cj A. J. Parker Adv. Org. Chem. 1965 5 1. H. R. Harrison W. M. Haynes P. Arthur and E. J. Eisenbraun Chem.and Znd. 1968,1568. ''' E. C. Taylor G. W. McLay and A. McKillop J. Amer. Chem. SOC.,1968,90,2422. 140 E. J. Corey and G. H. Posner J. Amer. Chem. SOC. 1968,90 5616. 14' G. Biichi and J. 'A. Carlson J. Amer. Chem. SOC.,1968,90,5336. 14' H. 0.House and W. F. Fischer jun. J. Org. Chem. 1968,33,949. 14' P. Rona and P. Crabbe J. Amer. Chem. SOC. 1968,90,4733. 278 R. Brettle Much attention has been directed towards the coupling of allylic systems. The reaction of a n-allylnickel halide with an ally1 halide leads to nonspecific ~oup1ing.l~~ However the reductive coupling of allylic alcohols with a reagent from titanium trichloride and methyl-lithium (a development of an earlier idea) is much more promi~ing.'~' Geraniol (58) gives mainly (59) and a little (60).Cross-coupling is also possible and as much as 70% conversion to the cross-coupled product has been obtained by using a tenfold excess of one of the allylic components. The conversion of (58) into (59) is of course an example of the tail-to-tail coupling of isoprenoid units a process of great interest in connection with the biosynthesis of the central bond in squalene and in phytoene and a further method for allylic coupling which involves sulphonium ylids represents an attempt to provide an in uitro model for these processes. The hitherto unknown vinylsulphonium ylids generated from sulphonium salts with undergo an electrocyclic rearrangement under very mild conditions to give sulphides. Reduction of (61; R = Me,CH=CHCH,) BF; it BF; 4 with lithium in liquid ammonia gives (60).14*When two allylic groups are involved the products depend on the alkylation patterns in these groups and on 144 E.J. Corey M. F. Semmelhack and L. S. Hegedus J. Amer. Chem. SOC. 1968,90,2416. 145 K. B. Sharpless R. P. Hanzlik and E. E. van Tamelen J. Amer. Chem. SOC.,1968,90 209. 146 R B. Bates and D. Feld Tetrahedron Letters 1968 417; B. M. Trost and R La Rochelle ibid. p. 3327; J. E. Baldwin R. E. Hackler and D. P. Kelly Chem. Comm. 1968 537. 14' G. M. Blackburn W. D. Ollis J. D. Plackett C. Smith and 1.0. Sutherland Chem. Comm. 1968,186. 148 J. E. Baldwin and D. P. Kelly Chem. Comm. 1968 899. General Methods the mode of operation. Three cases in which high yields of the tail-to-tail 14*9 coupled products were obtained are shown in Scheme 5.Reduction of (62; R = Me,C=CH*CH,) gives (S9).14ga i ii ;iii WSEt SPh iv Reagents I BuLi ; ii diazo-1,Cbicyclo [2,2,2]octane ; iii Me,CH :CH CH,Br ; iv benzyne SCHEME 5 A further approach to 1,5-di-unsaturated systems used the reaction of lithio-l-trimethylsilylprop-1-yne with an allylic halide ;Is0 this excellent route to 1,s-enynes has been used during a synthesis of (34)71(see Scheme 6). Reagents I LiCH,C rCSiMe ;11 Ag’ ; 111 CN -. SCHEME 6 Ketone enolates can be isolated and separated by g.l.c. as their trialkylsilyl ethers and in this way kinetically generated enolates of specific structure can be trapped.’”” Lithium or magnesium enolates can be prepared from these ethers with organometallic reagents”’” and these can be alkylated ;‘’Ib isomerisation to the more stable enol ether allows the isomeric alkylation product to be obtained.15’” These methods are illustrated in Scheme 7.Mono-C-alkylation of P-dicarbonyl compounds (e.g. ethyl acetoacetate) without any 0-alkylation or dialkylation can be achieved in very high’ yield 14’ (a)G. M. Blackburn and W. D. Ollis Chem. Comm. 1968,1261 ;(b)J. E. Baldwin R. E. Hackler and D. P. Kelly J. Amer. Chem. Soc. 1968,90 4758; J. F. Biellmann and J. B. Ducep Tetrahedron Letters 1968 5629. E. J. Corey and H. A. Kirst Tetrahedron Letters 1968 5041. ’” (a) G. Stork and P. F. Hudrlick J. Amer. Chem. Soc. 1968,90 4462; (b)G. Stork and P. F. Hudrlick ibid.p. 4464. 280 R. Brettle Reagents i MeMgBr; ii Cu' ;iii Me,SiCl; iv Et,N; v MeLi; vi MeI; vii (C02H),. SCHEME 7 by using the thallium(1) derivati~es.''~ Further work has appeared on the alkylation of NN-disubstituted amides ; this can be accomplished with ad- vantage in aprotic media through metallation with the lithium derivative of ~-trithian.'~~ The reactions of the monoanion of dimethyl sulphoxide (the dimsyl) ion are well known ;the dianion has now been prepared and its alkyla- tion and other reactions have been st~died.''~ Miscellaneous.-A book on the synthesis structure and reactions of enamines has a~peared.'~' Reviews have appeared on the chemistry of cyanic reactions with chlorosulphonyl isocyanate,' '' and the aprotic solvents hexamethylphosphoramide' ''and 1,2-dimethoxyethane.' 59 Several important new syntheses of iodides have appeared.Alcohols can be converted into iodides at room temperature by the route shown with o-phenylene phosphorochloridite (63).I6O (63) The reaction of trialkylboranes with iodine is catalysed by methanolic sodium hydroxide and this provides a route from terminal olefins to primary iodides.I6' The mixed organoborane resulting from the reaction of an olefin with disiamyl- E. C. Taylor G. H. Hawks and A. McKillop J. Amer. Chem. SOC. 1968,90,2421. D. N. Crouse and D. Seebach Chem. Ber. 1968,101,3113. E. M. Kaiser and R. D. Beard Tetrahedron Letters 1968,2583;M. Moskowitz J. Blanc-Guenee and M. Miocque Compt. rend. 1968,267 C 898. 'Enamines Their Synthesis Structure and Reactions,' ed.A. G. Cook Marcel Dekker New York 1968. M. Hedayatallah hll. SOC.chim. France 1968 1472. R. Graf Angew. Chem. Znternat. Edn. 1968,7,172. H. Normant Angew. Chem. Znternat. Edn. 1967,6,1046; Bull. SOC.chim. France 1968 791. C. Agami Bull. SOC. chim. France 1968 1205. 160 E. J. Corey and J. E. Anderson J. Org. Chem. 1967,32,4160. 161 H. C. Brown M. W. Rathke and M. M. RogiC J. Amer. Chem. SOC. 1968,90,5038. General Methods 28 1 borane permits nearly quantitative conversion of the olefin to the iodide. Selective reaction with dienes is possible [e.g. (64)to (65)].Primary bromides or iodides can be converted into the next higher iodide by the sequence shown in Scheme 8.162 A modified reagent for the first step is advantageous for allylic halides.CH; ii iii RCH,Hal 2RCH SPh -RCH CH I + PhSMe Reagents i Ph.SCH,Li+ ; ii Me1 (large excess); iii N-NaI in Me,NCHO SCHEME 8 Ethers are efficiently and selectively cleaved by mixed sulphonic-carboxylic anhydrides at room temperature.' TOS-OAC AcOYoToS Some work on sulphonium ylids has been reported above but there has been other work in this field as well. For example a new class of sulphur ylids [e.g. (68; Ar = p-MeC,H4)] has been prepared from sulphoximines (67) which are accessible from sulphoxides (66)by the route shown in Scheme 9.164 P i ii -Me -!!+Ar +-CH2 I NMe (66) (67) (68) Reagents i TosN, Cu; ii H,SO,; iii Me,O BF,- Na,CO,; iv NaH Me,SO; v NO+ PF,-SCHEME 9 By starting with optically active sulphoxides stable optically active ylids can be obtained which give optically active oxiranes and cyclopropanes with carbonyl compounds and electrophilic olefins respectively.l6 The optical purity of the products (e.g. 30 % for the cyclopropane from methyl cinnamate) is higher than in previous asymmetric syntheses of the same products. It is 16' E. J. Corey and M. Jautelat Tetrahedron Letters 1968 5787. 163 M. H. Karger and Y. Mazur J. Amer. Chem. SOC.,1968,90,3878. 164 C. R. Johnson E. R. Janiga and M. Haake J. Amer. Chem. SOC.,1968,90,3890. "' C. R. Johnson and C. W. Schroek,J. Amer. Chem. SOC.,1968,90,6852. 282 R.Brettle of interest that optically active sulphoximines (67) which can also be obtained by direct resolution can be converted stereospecifically into optically active sulphoxides (66) with nitrosyl hexafluorophosphate.'66 A review on sulphur ylids has appeared.167 166 D.R. Raper D. M. von Schriltz,J. Day and D. J. Cram J. Amer. Chem SOC. 1968,90,2721. 16' H.Konig Fortschr. Chem. Forsch. 1968,9,487.

ISSN:0069-3030    

DOI:10.1039/OC9686500261

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

9 ORGANOMETALLIC COMPOUNDS Part (i) The Main Group Elements By M. F. Lappert J. D. Smith and D. R. M. Walton (School of Molecular Sciences Universit-v of Sussex Brighton BN1 9Q.J) THEFIELD of organometallic chemistry continues to be well-served by reviews and monographs. We welcome a general undergraduate text-book.’ At the other extreme of comprehensiveness the Annual Surveys of Organometallic Chemistry have begun to appear in a new soft-cover format under the title ‘Organometallic Chemistry Reviews Section (B)’; individual contributions deal with lithium,’“ sodium and potassium,2b beryllium,’“ magnesium,2d zinc,2e cadmium,’f mercury,’g boron 2h aluminium,” gallium and indium,2j thallium,2k silicon,2z germanium,’“ tin,’” lead,’” arsenic,2p antimony,24 and bismuth.2r Section “A” coptinues to provide specialist reviews and these have been concerned with organoberyllium compounds ;3n the mercuration reac- tions ;3b oxymetallation ;3c the toxicology of organometallic compounds ;3d silicon-nitrogen polymers ;3e cleavage by main group halides of amides of organo-silicon -germanium and -tin ;3f trichlorogermane chemistry ;3g organo- metallic compounds with metal-metal bonds between different metals ;3h vibrational spectra of trimethylsilyl derivatives ;3i and organosilicon compounds containing pho~phorous.~’ Organometallic compounds having transition- metal-to-main-group metal bonds (especially Si Ge Sn and Pb) are the subject of several articles.3h* A historical essay with 957 references provides an account of developments in organometallic chemistry in the U.S.S.R.’ A comprehensive book on reaction mechanisms in organometallic chemistry6 has been published.The first part of a comprehensive account of the organo- G. E. Coates M. L. H. Green P. Powell and K Wade ‘Principles of Organometallic Chemistry,’ Methuen. London 1968. Organometallic Chemistry Reoiews (B),1968,4 (a)W. H. Glaze p. 161; (b) W. H. Glaze p. 189; (c) E. C. Ashby p. 194; (d) E. C. Ashby p. 198; (e) J. G. Noltes p. 233; (f)J. G. Noltes p. 239; (9) D. Seyferth p. 242; (h) D. S. Matteson p. 267; (i) J. J. Eisch p. 308; b’) J. J. Eisch p. 331; (+) J. J. Eisch p. 336; (I) R W. Bott p. 427; (m)E. J. Bulten p. 339; (n)J. G. A. Luijten p. 359; (0)L. C. Willem- sens p. 394; (p) G. 0.Do& and L.D. Freedman p. 405; (4)p. 421; (r) p. 426. Organometallic Chemistry Reviews (A) 1968 3 (a) N. R Fetter p. 1; (b) W. Kitching p. 35; (c) W. Kitching p. 61; (d)J. M. Barnes and L. Magos p. 17; (e) B. J. Aylett p. 151; (f)0.J. Scherer p. 281; (9) V. F. Mironov and T. K. Gar p. 311; (h) N. S. Vyazankin G. A. Razuvaev and 0. F. Kruglaya p. 323 ; (i) H. Burger p. 425 ; 0’) E. A. Chernyshev and E. F. Bugerenko p. 469. F. G. A. Stone in ‘New Pathways in Inorganic Chemistry,’ ed. E. A. V. Ebsworth A. G. Mad-dock and A. Sharpe Cambridge University Press 1968 p. 283; J. F. Young Ado. Znorg. Chern. Radiochem. 1968 11,92; M. C. Baird Prog. Inorg. Chem. 1968,9 1. N. A. Vol’kenau B. L. Dyatkin S. T. Ioffe V. I. Kuznetsov L. G. Makarova 0.Y.Okhlobystin, and R k Sokolik Ram Org.Khim SSSR. 1917-1967 Akad. Nauk SSSR. Inst. Istor. Estestuozn Tekh. 1967,100. 0.k Reutov and I. P.Beletskaya ‘Reaction Mechanisms of Organometallic Compounds’ North Holland Amsterdam 1968. K 284 M. F. Lappert J. D. Smith and D. R. M. Walton metallic chemistry of the Group IV elements7 has a 420 page authoritative review (with ca. 2500 references) of the synthesis and reactions of the Si-C bond,7a and a useful summary of the physical basis of the chemistry of the Group IV element^.^' A full treatment of theoretical and practical aspects of silylation*" has appeared and also an account covering literature from 1937 to 1964 on the organometallic compounds of arsenic antimony and bismuth.8b redistribu-Specialist reviews deal with the following topics :mass ~pectra,~ tion equilibria," n.m.r.studies of exchange processes," Group I1 alkyls,12 boron-halogen' and -sulphur14 compounds carboranes,' aspects of organo- aluminium chemistry,16 Group IVb radical reaction^,'^ the direct synthesis of organohalogenosilanes organotin chemis try,Ig organo tin hydrides and free radicals,20 and electrophilic substitution at phosphorus.21 We have attempted to read all publications on organometallic chemistry which appeared in 1968. Nevertheless the treatment which follows is selective. Attention has been restricted to the chemistry of processes involving M-C bond-making or -breaking. The largest numbers of papers deal with boron silicon and phosphorus. We depart from previous practice in attempting brief coverage of the Group V compounds ;however for space reasons we are unable to deal with phosphorus although this is clearly a topic which should receive attention in the future.Group I.-N.M.R. spectroscopy continues to play an important role in structural determination of species in solution and applications to exchange studies by using 7Li and 'H resonances have been reviewed." Ether or tetra- hydrofuran solutions of allyl-lithium exhibit a typical AB pattern at room temperature. Unlike ally1 derivatives of cadmium magnesium and zinc however the spectrum of the lithium compound is resolved into an AABB'C system at -87". The fast intermolecular exchange anticipated for the more ' A. G. McDiarmid (ed.) 'Organometallic Compounds of the Group IV Elements,' Marcel Dekker New York vol.1 Part I 1968; (a)C. Eaborn and R. W. Bott ch. 2; (b) E. A. V. Ebsworth ch. 1. (a) A. E. Pierce 'Silylation of Organic Compounds,' Pierce Chemical Company Rockford Illinois 1968; (b) M. Dub. (ed.) 'Organometallic Compounds Methods of Synthesis Physical Constants and Chemical Reactions,' VOL 3 Compounds of Arsenic Antimony and Bismuth. M. J. Bruce Ado. Organometallic Chem. 1968 6 273; D. B. Chambers F. Glockling and J. R C. Light Quart. Rev. 1968,22,317; J. Lewis and B. F. Johnson Accounts Chem Res. 1968,1,245. lo K. Moedritzer Adv. Organometallic Chem. 1968,6 171. T. L. Brown Accounts Chem. Res. 1968,1,23. I2 B. J. Wakefield Ado. Inorg. Chem. Radiochem. 1968,11,92. l3 A. G. Massey Adv.Inorg. Chem. Radiochem 1968,10,1. l4 R. H. Cragg Quart. Reports Sulphur Chem. 1968,3,1. V. I. Bregadze and 0. Y. Okhlobystin Uspekhi Khim. 1968 37 353; K V. Korshak I. G. Sarishvilli A F. Zhigach and M. V. Sobolevskii ibid. p. 2068; M. F. Hawthorne Accounts Chem. Research 1968 1 281. l6 K Wade Chem. in Britain 1968 4 503. R A. Jackson Adv. Free Radical Chem. 1968,3,231. V. Bafant J. Joklik and J. Rathousky Angew. Chem. Internat. Edn 1968,7,112. A. G. Davies Chem. in Britain 1968 4 403. 'O H. G. Kuivila Accounts Chem. Res. 1968,1,299. S. G. Warren Angew. Chem. Internat. Edn,1968,7,606. 22 P West J I Purmort and S V. McKinley J. Amer. Chem. SOC,1968,90,797 Part (i) The Main Group Elements ionic lithium derivative is presumably reduced by extensive association of species in solution.The analogy between pyridine and the isoelectronic phenyl carbanion is apparent in lithium magnesium and calcium aryls in that the ‘H n.m.r. parameters for these compounds are solvent- and concentration- independent and are much more similar to those of the corresponding hetero- cycles than to values for covalently substituted benzenes.23- 24 The magnitude of the downfield shift of ortho-hydrogens in phenyl-lithium and -magnesium bromide provides a qualitative measure of the carbon-metal bond ionic character. Further examples of stereochemical stability exhibited by carbanions include menthyl-lithium; ‘H n.m.r. data shows this compound to be confor- mationally rigid in hydrocarbon solvents at 50” and in ether solution below 00.25 Analogous examples for cyclopropyl systems are 7-exo-bromo-2-oxa- bicyclo[4,1,0]hept-7-yl-lithium26and the sodio-compound prepared from ( -)(R)-l-bromo-l-methyl-2,2-diphenylcyclopropane and n-pentylsodium :27 By far the greater part of organolithium chemistry is devoted to metallation studies particularly to promotion of exchange reactions in donor solvents.Toluene undergoes polylithiation when treated with an excess of butyl- lithium in the presence of NNN‘N’-tetramethylethylenediamine(TMEDA). Product analysis via alkylation (R,SO,) or silylation (Me3SiC1) reveals ring as well as side-chain metallation. 28 Dibenzenechromium is readily lithiated by the n-BuLi-TMEDA c~mplex.~’Metallation of t-butylbenzene with n-phenylsodium is much more efficient in the presence of t-butoxysodi~m.~~ A further example of intramolecular assistance by nitrogen is provided by 2-dimethylaminoethylferrocene which lithiates exclusively ortho to the Me2N* CH CH group.31 Likewise o-toluene sulphonamides are metallated in the methyl as well as in the arnido-gro~p.~~ Lithiations of acetonitrile and of NN-dimethyl-acetamide with butyl-lithium in tetrahydrofuran give LiCH,.CN and LiCH,.C0.NMe2,33 while treatment of dimethyl sulphoxide with two 23 G. Fraenkel D. G. Adams and R R. Dean J. Phys. Chem. 1968,72,944. 24 G. Fraenkel S. Dayagi and S. Kobayashi J. Phys. Chem 1968,72,953. ” W. H. Glaze and C. M. Selman J. Org. Chem. 1968,33,1987. 26 K. G.Taylor and W. E. Hobbs Tetrahedron Letters 1968 1221.” J. B.Pierce and H. M. Walborsky,J. Org. Chem. 1968,33 1962. 28 A. J. Chalk and T. J. Hoogeboom J. Organometallic Chem. 1968,H 615;R West and P. L. Jones J. Amer. Chem. SOC.,1968,90,2656. 29 C. Elschenbroich J. Organometallic Chem 1968,14 147. 30 R.A.Benkeser T. F. Crimmins and W.-H. Tong J. Amer. Chem. SOC.,1968,90,4366. 31 D. W. Slocum T. R Engelmann and C. A. Jennings Austral. J. Chem 1968,20 2319. ” H. Watanabe and C. R. Hauser J. Org. Chem 1968,33,4278. 33 E.M.Kaiser and C. R Hauser J. Org. Chem. 1968,33 3402; D. N.Crouse and D. Seebach Chem. Ber.. 1968,101 31 13. 286 M. F. Lappert J. D. Smith and D. R. M. Walton equivalents of butyl-lithium gives the potentially useful reagent (LiCH2)2S0.34 Low-temperature metallation and metal-halogen exchange reactions continue to provide novel halogenoalkyl-lithium reagents.Examples include exo-and endo-7-chloro-7-norcaryl-lithium35u Br,CLi,35b ClCH,Li and CHBrClLi.35' The compound CHC1,Li itself may be used to metallate bromoalkanes (CH,Br, CHBr, and PhCHBr,);35d whilst at low temperatures halogen- metal exchange between bptyl-lithium and the compounds R,C :C(C1)CN and R,C :C(Br)CO,R is rapid taking precedence over addition reactions to the cyano- and carboxyalkyl groups. 5e A kinetic study of the metallation of aromatic compounds with butyl- lithium confirms that the rate of reaction is controlled by the inductive effect of the ring s~bstituent.~~ Reactions with both n- and t-butyl-lithium are singularly free from steric effects and a multistep mechanism is proposed in which the orientation of substitution and rate-determining removal of a proton by the carbanion are separate processes.The ether-cleavage method has been used to prepare allyl-lithium com- pounds e.g. RCH=CH*CH,*OPh CH,=CH;CH(R)*OPh 1 I Li-THF Li-THF RCH=CH*CH,Li RCH(Li)*CH=CH (75-80%) The half-life of perchloronorbornadienyl-lithium is 3 hr. at 27-29' in diethyl ether and provides a further interesting example of a system in which 1,2 elimination of LiCl is precluded by ring strain in the ensuing products.38 Numerous perchloroaryl-lithium corn pound^,^^ including 2-tetrachloro-pyridyl-lithi~m,~~ have been reported. Organolithium compounds add readily to suitably substituted alkenes reactions which serve as a basis for anionic polymerisation.Reversal of the initial step has now been dem~nstrated.~~ Upon treatment of 2-iodoethyl- triphenylmethane with n-BuLi at -40" in THF the characteristic red colour of trityl-lithium immediately develops and ethylene is evolved. The reaction is so rapid that when butyl iodide is present initially 91 % of butyltriphenyl- methane is obtained. Intramolecular cyclisation of the carbanion-type intermediates may also 34 E. M. Kaiser and R D. Beard Tetrahedron Letters 1968 2583. 35 (a)G. Kobrich and W. Goyert Tetrahedron 1968,24 4327; (b) R H Fischer and G. Kobrich Chem Ber. 1968,101,3230; (c) G. Kobrich and W. Goyert Tetrahedron 1968,24,4343 ;(d)G. Kobrich G. Fischer and R Hartmuth Chem. Ber. 1968 101 3208; (e) G.Kobrich H. Trapp and A. Akhtar Chem Ber. 1968,101,2644. 36 D. A. Shirley J. R Johnson and J. P. Hendrix J. Organometallic Chem 1968 $1,209. 37 P. Miginiac and C. Bouchoule Bull. SOC.chim. France 1968 4156. '* D. Seyferth A. B. Evnin and D. R Blank J. Organometallic Chem. 1968 13 25. j9 1968 1278. I. Haiduc and H. Gilman J. Organometallic Chem. 1968,1479; Chem and Id. 40 J. D. Cox and B. J. Wakefield J. OrganornetallicChem. 1968 13 15. 41 H. P. Fischer E. Kaplan and P. N. Neuernschwander Chimia (Switz.),1968,22 338. Part (i) The Main Group Elements compete successfully with polymerisation as has now been demonstrated in the reaction between a-methylstyrene and ethyl-lithium :42 e.g. Optimum conditions for transition-metal salt dimerisation of phenyl- benzyl- and cyclohexyl-lithium have been reported.Anhydrous CoCl is the catalyst of choice and product yields are superior in benzene. A cage effect presumably operates efficiently in this solvent.43 The thermal decomposition of t-butyl-lithium in decalin has been studied.44 The reaction is approximately of one-half order. Triethylamine has no effect on the initial rate in agreement with the lack of donor interaction previously noted between this amine and t-butyl-lithium whereas naphthalene reduces the initial rate a fact which is consistent with complex formation. Group II.-Ber.j-//iurn. The sustained increase in all aspects of organo-beryllium chemistry is reflected in a recent review article. 3u Detailed structural data are reported for monomeric di-t-butylberyllium4su and for the complex Li,BeMe,.45b In the latter compound the tetrahedral disposition of methyl groups is distorted the C-Be-C angle varying between 102 and 113" with a Be-C bond length of 1.84 A.Although dicyclopentadienylberylliumforms nonisolable complexes with various aromatic solvents the donor-acceptor interactions in solution are sufficient to induce paramagnetism and hence large 'H n.m.r. shifts.46 Alkylberyllium alkoxides formed by addition of dialkylberyllium compounds to aldehydes or by alcoholysis of organoberyllium halides are tetrameric in benzene Lower degrees of association are encountered if alkyl or alkoxy-groups are bulky thus the compounds Bu'BeOBu' and MeBeOBu' are dimeric in benzene and in ether solution respectively whilst MeBeOBu' can be crystallised as an ether-free tetramer.Alkylthioberyllium compounds possess structures based on a B4S4 cube and show a marked tendency to disprop~rtionate.~~~ Magnesium analogues behave similarly R1MSR2+ R',M + M(SR2) M = Be,Mg Optimum conditions are described for synthesis of high purity beryllium- 42 D. Margerison and V. A. Nyss J. Chem. SOC. (C) 1968 3065. 43 R Pallaud and J.-M. Pleay Compt. rend. 1968 267 C 507. 44 R L. Eppley and J. A. Dixon J. Orgummetallic Chem. 1968 11 174. 45 (a)A Almenningen A Haaland and J. E. Nilsson Acta Chem. Scand. 1968,22,972; (b)E W. Eiss and R Wolfrum J. Orgummetallic Chem. 1968 12 257. 46 G. L. Morgan and G. B. McVicker J. Amer. Chem. SOC.,1968,90,2789.47 (a)G. E. Coates and A H. Fishwick J. Chem. SOC.(A),1968,477; (b)ibid. p. 635. 288 M. F. Lappert J. D. Smith and D. R. M. Walton alkyl~.~~ Diphenylberyllium is best prepared from phenyl-lithium (itself prepared in an hydrocarbon solvent) and BeCl in diethyl-ether. Lithium chloride is precipitated and pure Ph,Be remains in solution. Contrary to earlier reports dialkyl- and diaryl-beryllium compounds redistributed rapidly with BeCl and BeBr in ether; the reaction is best represented as,49 R,Be + BeX + 2RBeX. Acylberyllium halides react with ketones to give acceptable yields of 01-hydroxy-ketones. e.g. AcBeBr + Me,CO + H20-AcC(Me,)*OH Traces of halogenoketone in the products may be removed by careful treatment with potassium hydroxide; however this may lead to an unwanted crotonic- type condensation reaction.Magnesium. With few exceptions structural investigations continue to move away from the problem of Grignard reagents in solution. The 1 1 di-t-butylmagnesium-dioxan complex is insoluble in toluene whereas the bis- tetrahydrofuran complex is partially dissociated in benzene. The complexes (Pr'MgNMeC,H,NMe,) and (BufMgNMeC2H4NMe,) have been charac- terised and on pyrolysis eliminate alkanes as opposed to alkenes. Butyl- lithium and dimethylmagnesium form the complex LiMgMe,Bu-OEt in ether. Pyrolysis gives lithium hydride and dimethylmagnesium. In contrast to its beryllium analogue ethylmagnesium n-propoxide is oligomeric thus emphasising the greater co-ordinative tendency of magnesium as compared with beryllium.Chain-branching in the a-position reduces the degree of aggregation and the isopropoxy-compound is a tetramer. 52 Reaction of a dialkylmercurial with an excess of magnesium without catalyst is the best method for the preparing ultrapure magnesium alkyl~.~~ The yield from dimethylmercury by this route is quantitative. Pentafluorophenyl- magnesium bromide can be efficiently prepared by the metal-halogen exchange reaction :53 C,F,Br + EtMgBr EtBr + C,F,MgBr (98%) If a two-fold excess of Et,Mg is used decafluorobiphenyl is formed. The stoicheiometry of this reaction requires ethylmagnesium fluoride as an inter- mediate. The crystal structure of the addition complex between methylmagnesium bromide and acetone has been described :54 Mg(3d) orbital participation is 48 E.C. Ashby and R C. Amott J. Organometallic Chem 1968 14 1. 49 J. R Sanders jun.,E. C. Ashby and J. H. Carter 11 J. Amer. Chem. SOC. 1968,90,6385. so 1. I. Lapkin and T.N. Povamitsyna Zhur. Obshchei. Khim. 1968 99. st G. E. Coates and J. A Heslop J. Chem SOC.(A) 1968 514. 52 G. E Coates J. A. Heslop M. E. Redwood and D. Ridley J. Chem. SOC.(A) 1968 1118. 53 W. L. Respess and C. Tamborski J. Organometallic Chem. 1968 11,619. 54 P. T. Moseley and H. M. M. Shearer Chem Comm.,1968,279. Part (i) The Main Group Elements invoked to account for the particularly short Mg-0 bond of the ring (viz. 1-91A compared with the exocyclic Mg-0 bond 2.01 A) I BU‘ Products of enolisation and reduction as well as addition are often en- countered in reactions between sterically hindered Grignard reagents and ketones.Reactions of neopentylmagnesium chloride with benzophenone gives benzopinacol (2%) together with neopentane in addition to the anticipated tertiary carbinol. E.s.r. measurements confirm that a radical process is in- volved.s s There is continued interest in factors governing rearrangements observed in certain reactions of the benzyl Grignard reagent.56 Triene inter- mediates (1)and (2) aromatised during hydrolysis,’ are invoked to account for the ortho- and para-products formed in couplings with diethyl sulphate Detailed investigations of the CoC1,-catalysed decomposition of methyl- magnesium iodide (the Kharasch reaction) have been published.s8 A 2 1 ratio of Grignard to CoCl results in reduction of the CoCl and formation of a [Coo-MgX,] complex whilst a colloidal suspension of suggested composition Co[MeMgI], heavily solvated is formed when the Grignard is taken in excess.Reaction with 1-bromo-octane leads to an intermediate which can in turn break down to form alkyl free radicals or products of hydrogen- abstraction reactions possibly involving a cobalt hydride. The results are compatible with both free- and non-free-radical processes. Calcium. Previous work has stressed the necessity for spectroscopic quality calcium for efficient reaction with alkyl halides ; however reagent-grade metal apparently reacts readily with alkyl iodides in tetrahydrofuran to form Grignard type reagents.59 Amalgamated calcium and trityl chloride in tetrahydrofuran 55 C. Blomberg and H. S. Mosher J. Organometallic Chem. 1968,13 519. 56 I. Partchamazad A. Guillemonat and J. C. Traynard Compt. rend. 1968,266 C 717. 5’ R. A. Benkeser T. E. Johnston and W.-H. Tong J. Org. Chem. 1968,33,2203. ’* M. H. Abraham and M. J. Hogarth J. Organometallic Chem. 1968 12 1,497. 59 R Masthoff and C. Vieroth J. prakt. Chem. 1968.38. 182 290 M. F.Lappert J. D.Smith and D.R. M. Walton yield Ph3CCaC1(THF),(n = 1 or 2) both compounds decomposed under high vacuum to Ph,CCaCl and tetrahydrofuran.60 Bistritycalcium formed from hexaphenylethane and the amalgam in tetrahydrofuran initiates styrene polymerisation and reacts conventionally with benzaldehyde to give phenyl- tritylcarbinol.The organocalcium compound is postulated to contain solvated cations [Ca(THF),I2 + and free trityl anions.,' A potentially useful procedure for quantitative analysis of Group I and I1 (and some Group 111) compounds involves specific cleavage of dialkyl or diary1 sulphides by the organometallic compound followed by titration of the sulphide formed with silver ion., The method has its limitations and cannot for example be used for diethylzinc or dodecynyl-lithium which react sluggishly with disulphides. Zinc and Cadmium. 'The Organic Compounds of Zinc and Cadmium' Vol. 3 in the Russian series of 'Methods of Elemento-Organic Chemistry,' has been published in English translation. 63 Dialkylzinc compounds can be prepared in reasonable yield from solid aryl-lithiums or from Grignard reagents and ZnC1,.64 Magnesium salts are conveniently precipitated with dioxan to leave the required dialkylzinc in solution.Di-n-butyl ether is the recommended solvent for large-scale preparations. Formation of iodomethylzinc iodide from zinc and di-iodomethane is well known. The reaction has been extended to other am-di-iodoalkanes which interact readily with a zinc-copper couple at 100" to give the species I[CH,],ZnI (n = 4 5).65 Pure alkylcadmium halides have been synthesised apparently for the first time either by reaction of Grignard reagents with CdCl or by dispropor- tionation :66 R,Cd + CdX + 2RCdX. Infrared and Raman spectral comparisons between dialkyl-zinc and -cadmium compounds on the one hand and mixtures of these organometallics with anhydrous metal halides on the other in ethereal solvents favour RMX species as the major components in the Schlenk equilibrium for the system :,' R,M + MX + R,M MX + 2RMX.Magnesium halides often enhance the reactivity of cadmium and zinc alkyls; e.g. in additions to aryl-ketimines and -cyanides.68 Unwanted side-reactions 6o R Masthoff H. Schiiler and G. Krieg J. Organometallic Chem. 1968 13 37. 61 M. A. Coles and F. A. Hart Chem. and Ind. 1968,423. 62 C. A. Uraneck J. E. Burleigh and J. W. Cleary Amlyt. Chem. 1968,40 327. 63 N. I. Sheverdina and K. A. Kocheshkov 'The Organic Compounds of Zinc and Cadmium,' North Holland; Amsterdam 1967. 64 N. I. Sheverdina I. E. Paleeva and K.A. Kocheshkov Izvest. Akad. Nauk. S.S.S.R. 1967 582 587; K.-H. Thiele S. Wilcke and M. Ehrhardf J. Organornetallic Chem. 1968 14 13. " K.-H. Thiele and I. Benthin 2.Chem. 1968,8 344. 66 I. E. Paleeva N. I. Sheverdina and K A Kocheshkov Zzoest. Akad. Nauk S.S.S.R. Ser. Khim. 1967,1219. 67 D. F. Evans and I. Wharf J. Chem SOC.(A) 1968 783; A. N. Rodionov I. E. Paleeva D. N. Shigorin N. I. Sheverdina and K. A. Kocheshkov Izvest. Akad. Nauk S.S.S.R.,Ser. Khim. 1967,1031. 68 J. Thomas E. Henry-Basch and R. FrCon Compt. rend. 1968,267 C 176. Part (i) The Main Group Elements 29 1 may occur; thus the direction of epoxide ring-opening by pure diallylzinc is specific e.g. PhCHCH,O + (CH,=CHCH,)Zn -+ PhCH(OH)(CH2),CH=CH2 addition of magnesium bromide isomerises the styrene oxide to phenylacet-aldehyde and mixed addition products ensue.69 Mercury.The n.m. r. spectrum of dicyclopentadienylmercury exhibits a single line (z 4.2) at room temperature. Resolution at -70" is solvent-dependent and splitting occurs only in liquid sulphur dioxide; on the basis of comparison with (a-C5H5)(Et,P)C~I,70 this is said to indicate a series of metal atom 1:2 (as opposed to 1 :3) shifts. Methylmercuric halides and pseudohalides undergo smooth exchange in dimethylformamide viu four-centre bridged transition states commonly encountered in organomercury chemistry. 72 The exchanges are strongly catalysed by tetramethylammonium bromide and are considerably faster in co-ordinating solvents e.g. pyridine. Complexes actually containing bidentate ligands also disproportionate readily.' e.g.2C6F5HgXL2 + L2Hfl2 + (C6F5)2HgL2 X = Br C1; L = 2,2'-bipyridyl 1,lO-phenanthroline a-Substituted organomercurials provide a continued source of novel electron- deficient carbon species. Thus photolysis products from diethylmercurybis- diazoaceta! are wavelength-dependent. Below 280 nm. a novel carbene EtO *COO C HgC(N,)CO- OEt is generated ; whereas at longer wavelengths in cyclohexane products compatible with intermediate ethoxycarbonyl- methyne formation are obtained :74 EtO*COc + C6H12 + C,H,,C *CO*OEt+ dimer ?.. C,H ,CH -CO*OEt The rates of dichlorocarbene transfer from aryl(bromodichloromethy1)-mercurials to substituted alkenes have been measured. The narrow range of substituent effects in both carbene source and alkene substrate is consistent with an electrophilic addition with a slightly polar transition-state.Reversal of the carbene extrusion process (ie.,insertion) has been demonstrated :76 reflux PhH PhHgCC1,Br + PhHgCl -PhHgCCl + PhHgBr 69 D. Abebhaim E. Henry-Basch and P. Frkon Compt. rend. 1968,267,C 655. 70 G. M. Whitesides and J. S. Fleming J. Amer. Chem. SOC.,1967,89,2855. E. Maslowsky and K. Nakamoto Chem. Comm. 1968,257. 72 L.L.Murrell and T. L. Brown J. Organometallic Chem. 1968,.13,301. 73 A.J. Canty and G. B. Deacon Austral. J. Chem. 1968,21,1757. 74 0.P.Strausz T. Dominh and J. Font J. Amer. Chem. SOC.,1968,90 1930. 75 D. Seyferth J. Y.-P.Mui and R. Damrauer J. Amer. Chem. SOC.,1968,90,6182.76 D. Seyferth M. E. Gordon and K. V. Darragh J. Organometallic Chem. 1968,14,43. M. F. Lappert J. D. Smith and D. R. M. Walton The direct synthesis of a mercury-carbene complex (3) has been accomplished from 1,3-diphenylimidazolium perchlorate and mercuric acetate :77 Ph Ph Ph Ph (3) Mercuration of double-bonds in which a neutral or charged ancillary nucleophile participates in a second step of the reaction sequence is fast developing into a useful synthetic method.78 e.g. + a.::*coR aNcR HCl-‘*HgNO Similar additions may be achieved with Hg[C(N03)3]279 and mercuric nitrate in the presence of sodium The question of nucleophilic promotion in protonolysis of arylmercurials has been reviewed.81 Assistance is held to account for the enhanced cleavage rates for the heavier more electropositive metals (ie.,Hg Sn Pb) since electron- release by these metals is weak.Transition states of type (4) are envisaged for protodemercuration of arylmercuric halides in the presence of excess of halide ion. 82 (4) Group II1.-Boron. The Council of the American Chemical Society has laid down rules of nomenclature for boron corn pound^.^^ 77 H.-W. Wanzlick and H.-J. Schonherr Angew. Chem. 1968,80 154. V. 1. Sokolov and 0.A. Reutov Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1968,222. l9 V. k Tartakovskii,N. S. Zefirov and V. N. Chekulaeva,Izvest. Akad. Nauk S.S.S.R.,Ser. khim. 1968,1638. V. I. Sokolov and 0.A. Reutov Izvest. Akad. Nauk. S.S.S.R. Ser. Khim. 1967 1632. A. L. Kurts I.P. Beletskaya and 0.k Reutov Izvest. Akad. Nauk S.S.S.R. Ser. khim 1967 2121. 1. P. Beletskaya A. L. Kurts and 0.A. Reutov Zhur. org. Khim 1968 4 352 83 J. Carter Inorg. Chem 1968,7,1945. Part (i) The Main Group Elements 293 There continues to be a shortage of structural information (except for carboranes). In the cage compound (9,X-ray data show B-C as 1-56 An electron-diffraction study of gaseous tetramethyldiborane provides a B-C distance of 1.59 _+ 0.3 A.85The i.r. and Raman spectra of this and other methyl derivatives of diborane,86 as well as of X,B* CH CH BX (X = F or Cl),87 have been examined. ''B N.m.r. chemical-shifts cover a wide range and despite the rather broad signals (s = 8) which are observed useful structural correlations may often be made ;data on 71 derivatives R -"BX (n = 0 1,or 2 ; X = Hal OMe NMe, or SMe)*' and on some heteroaromaticsS9 have been obtained.'H N.m.r. variable temperature studies with allylboranes show them to be fluxional molecules with activation exchange barriers in the range 5-15 kcal./m~le.~~ CH, /,,;--*--3CH2=2CH-1CH,-B -CH :. '*) <+ 'CH2=2CH-3CH,-B / \-\'= ______.-,' CH2 MO Calculations of Q + n* and n + Q* transitions in vinylboranes have been described." The photochemical B-C cleavage reactions of Na' [BPh4] -and related derivatives shows that formation of biphenyl and phenylhexadienes proceeds by intramolecular reaction around the boron atom.92 Triethylborane is a strong phosphorescence quencher of biacetyl triplets.93 Among the more exotic compounds synthesized during 1968 are a cage (5),84,94 compounds having more than two boryl groups attached to a single carbon atom [such as C(B(OMe),) from CCl,/Li/B(OMe)3,95 and C(BCl,) from96 C vapour-B,Cl,] F,BCH :CHBF,," and (F,BCH :CH),BF [which in the vapour at room temperature eliminates BF and affords compound (6) isoelectronic with p-ben~oquinone],~~ Cl,BCH,CH(BCl,)(MX,) (MX = 84 P.C. Thomas and I. C. Paul Chem. Comm.,1968 1130. 85 B. L. Carroll and L. S. Bartell Imrg. Chem. 1968,7,219. 86 J. H. Carpenter W. J. Jones R W. Jotham and L. H. Long Chem Comm.,1968,881; M. J. D. Low R. Epstein and A. C. Bond J. Chem. Phys. 1968,48,2386. D. F. Shriver J. F. Jackovitz and M. J. Biallas Spectrochim Acta 1968,24,1469.88 H. Noth and H. Vahrenkamp J. Orgummetallic Chem. 1968,12,23. 89 F. A. Davis M. J. S. Dewar and R Jones J. Amer. Chem. SOC. 1968,90,706. 90 V. S. Bogdanov Y.N. Bubnov S. I. Frolov and B. M. Mikhailov Izvest. Akud. Nauk S.S.S.R. Ser. khim. 1968 307; Y. N. Bubnov S. I. Frolov and B. M. Mikhailov ibid. p. 824. 91 D. R. Armstrong and P. G. Perkins Theoret. Chim Acta 1968,9,412. 92 J. L. R Williams J. C. Doty P. J. Grisdale T. H. Regan G. P. Happ and D. P. Maier J. Amer. Chem SOC.,1968,90 53; J. L R Williams P. J. Grisdale J. C. Doty M. E. Glogowski B. E. Babb and D. P. Maier J. Orgummetallic Chem. 1968 14 53; P. J. Grisdale B. E Babb J. C. Doty T. H. Regan D. P. Maier and J. L. R Williams ibid. p. 63. 93 J. Grotewold and E A. Lissi Chem Comm,1968,1367; J.Chem SOC.(C) 1968,264; idem and A. E. Villa J. Polymer Sci. (A) 1968,6 3157. 94 J. J. Miller and F. A. Johnson J. Amer. Chem. Soc. 1968,90,218. " R. B. Castle and D. S. Matteson J. Amer. Chem. SOC.,1968,90,2194. 96 J. E. Dobson P. M. Tucher R. Schaeffer and F. G. A. Stone Chem. Comm. 1968,452 '' A. K. Holliday G. M. Jessop and R. P. Ottley J. Organometallic Chem. 1968 14 211. 9* P. L. Timms J. Amer. Chem. SOC. 1968,90,4585. M.F. Lappert J. D.Smith and D. R. M.Walton H (6) (5) Bu' SiCl, GeCl, or SnC1,),99 boryl ketones and thioketones (7),'0° hetero-aromatics and carboranes. The reactions of boron compounds with small gaseous carbon molecules,96. lo' the BF-C,H [+(6)] reaction,98 and the preparations'00 of the stable monomeric derivatives (7) and (8) are especially noteworthy.Bun2BClNa-K-Et 0*Bu",BK in Et,O (Y =0 or S) RC(Y)CI/CICOZEt Bun,B-C 0 NY \R (7) \/Bun,B-C OEt\ (8) \/ Since B- C N' and 0,' are isoelectronic so too are the units ,B-N, \/ \/ B-0 ,and ,C=C,. Hence the substitution of such boron-containing fragments for C=C in an aromatic system gives rise to the boron hetero- aromatics; early work has been reviewed.lo2 Principal interests centre on the characterisation of new ring systems,lo3* synthetic methods,los comparisons of electronic spectra with those of aromatic analogues,lo6 Lewis acid versus proton acid behaviour,'06* O7 ring-substitution reactions and orientation effects (also comparisons of experiment with MO calculations) including 99 T. D. Coyle and J.J. Ritter J. Organometallic Chem. 1968 12 269. loo G. Schmid and H. Noth Chem. Ber. 1968,101,2502. Io1 S. R Prince and R Schaeffer Chem. Comm. 1968,451. Io2 M. F. Lappert 'The Chemistry of Boron and its Compounds,' ed E. L. Muetterties Wiley New York 1967 ch. 8. lo3 M. J. S. Dewar and R. Jones Tetrahedron Letters 1968 2707. Io4 F. A. Davis and M. J. S. Dewar J. Amer. Chem SOC.,1968,90,3511; J. Namstvedt Acta Chem. Scad. 1968,22 1611; J. Namstvedt and S. Gronowitz ibid. p. 1373. P. I. Paetzold and H. Maisch Chem. Ber. 1968 101 2870; P. I. Paetzold and G. Stohr ibid. p. 2874; P. I. Paetzold G. Stohr H. Maisch and H. Lenz ibid. p. 2881. Io6 M. J. S. Dewar R Jones and R H. Logan J. Org. Chem. 1968,33,1353. lo' F. A. Davis and M. J. S.Dewar J. Org.Chem. 1968,33 3324. Part (i) The Main Group Elements determination of partial rate factors ;Io8 each of these features received atten- tion during 1968. The violet 12,ll-borazarophenaleniumion (9) is an interest- ing new species. An important development in organoboron chemistry is the synthesis of monomeric boron imides (10).Io5 which are isoelectronic with acetylenes ; they undergo 1,3-dipolar cycloadditions which offers an attractive route to heterocycles. Similar reactions but affording saturated analogues are observed with triphenylborane-phenyl isonitrile and related adducts. log 2Et N R2 R’NH + R2BHal -’+R’N Ph R2 + -PhNCO ph~0‘‘BR PhNd-BR + [PhN=CR-BR,j -\/ OC-NPh The carboranes represent a class of organometallic compound peculiar to boron ; recent reviews are available.15.The substitution of two carbon atoms for two boron atoms in the (BH),’-polyhedral ion series results in the isoelectronic neutral series the 2C-carboranes B,C2H,+ ’. These materials normally display extreme stability and indeed appear to be ‘aromatic’ in the sense that the majority of their reactions are substitutions and their skeletal integrity is maintained; it is of course especially interesting that this ‘aromatic’ lo* M. J. S. Dewar and R H. Logan J. Arner. Chem SOC.,1968,90 1924; M. J. S. Dewar and R. Jones ibid.,p. 2137. log G. Bittner H. Witte and G. Hesse Annulen 1968,713 1 ; H. Witte M. Gulden and G. Hesse ibid. 1965 687,1; ibid.,1968 716 1. M. F. Hawthorne ‘The Chemistry of Boron and its compounds,’ ed.E. L. Muetterties Wiley. New York,1967 ch. 5. M. F. Lappert J. D. Smith,and D. R. M. Walton electron-delocalisation is three-dimensional. In general compounds formally derived from a polyhedral boron hydride in which one or more boron atoms are replaced by carbon constitute as a class the carboranes. The majority of papers deal with the icosahedral 1,2-(known as ‘carborane’ o-carborane’. or by Russian workers ‘barene’) and 1’7- (also known as ‘neocarborane’ or ‘neobarene’) (or to a much lesser extent-the third isomer-the 1,3-or p-) dicarbaclovododecaborane Bl0C,H (11); these. are often represented trivally as (12). The species BgC,Hll2- derived from these is trivially known as the dicarbollyl ion which is capable of acting as a n-ligand [see (13) for the (3)-1,2-BgC,H 12-1 (similar to cyclopentadienyl) to transition metals ; such compounds are examples of metallocarboranes.During 1968 full details have been published relating to the preparation and characterisation of the (3)-1,2- and (3)-1,7-dicarbadodecahydrodecaborate-(-1)ions,’” their conversion112 to the dicarbollide ions from which by boron- insertion,’’ the icosahedron may be reconstituted or by reactions with transition-metal salts,1129 the x-dicarbollyl derivatives become accessible. The following species are known (R2-represents x-BgC2H,12-) [RM(C0),l2- and [RM2(C0),l2- (M = Moor W) [RMO(CO),-W(CO),]~- [RM(CO),]-(M = Mn or Re) [RMn(CO),]- ReRZ2- FeR,- [n-C,H,)FeR] - [n-C,H,)FeR] [CoR,] - (.nC,H,)CoR NiR,- NiR, (n-Ph4C4)PdR PdR, PdR,- PdR,,- and (M = Cu or Ag) MR,- and MR,,-;a feature of the dicarbollide ion is that it has the property unlike other digands such as (n-C5Hs)- of stabilising high oxidation states (e.g.Ni4+). MeO-2MeOH NaH-THF(-H 2) BlOC2H12 * B9C2H12-[-B(OMe), -H23 Transition-x-B9C2H11 complexes ‘11 M. F. Hawthorne D. C Young,P. M. Garrett D. A. Owen S. G.Schwerin F. N.Tebbe and P. A. Weger J. Amer. Chem. SOC.,1968,90,862 ‘Iz M. F. Hawthorne D. C. Young,T. D. Andrews D. V. Howe R L. Pilling A. D. Pitts M. Reintjes L. F. Warren and P. A. Wegner J. Amer. Chem. SOC.,1968,90 879. ’” M. F. Hawthorne and P. A. Wegner J. Amer. Chem. SOC. 1968,90,896. ‘14 L F. Warren and M. F. Hawthorne .I.Amer. Chem. SOC. 1968,90,4823. Part (i) The Main Group Elements While carborane is normally prepared from decaborane and acetylene in presence of a ligand benzocarbotane (14)'" must be regarded as derived from benzyne.Compound (14) is unattacked by Br,-CCl or concentrated sulphuric acid at loo" and only slowly oxidised by KMnO,. Li C-CLi (1) cis-ClCH,-CH:CH-CH,Cl -(2) NBS-trace azobisisobutyro-nitde \o/ 'BlO (14) There have been numerous publications dealing either with 'I7 or side-chain118* 119 reactions; these aspects are exemplified"6* 'I8 by the following equations. u-or rn-HC-\0/cT:3 Br2-HOAc 0-or m-HC- ,(Jc*yH~Br - BlOH,O An interesting development in metallocarborane chemistry is the bidentate x-bonding ligand (3,6)-1,2-dicarbacanastideion B8C2H 02-(15) as found in [(B,C2H,,)Co(B8C2Hl,)Co(B,C2H1 1]2-.120 The higherpolyhedralB,-,C,H series is now complete from n = 5 to 12.12' N.K. Hota and D. S. Matteson J. Amer. Chem. SOC. 1968,90 3570. L. I. Zakharkin and N. A. Ogorodnikova J. Organometallic Chem. 1968,12 13. 11' L. A. Leites N. A. Ogorodnikova and L. I. Zakharkin J. Organometallic Chern. 1968 15 287; V. I. Stanko and V. A. Brattsev Zhur. obschei Khim. 1968,38,662; V. I. Stanko and T. V. Klimova ibid. p. 1193; V. I. Stanko and A. I. Klimova ibid. p. 1194;V. I. Stanko V. kBrattsev,T. N. Vostrikova and G. N. Danilova ibid. p. 1348; L I. Zakharkin and L. S. Podvisotskaya Izvest. Akad. Nauk S.S.S.R. Ser. khim 1968 681; L. I. Zakharkin and V. N. Kalinin ibid. p. 685; ibid. p. 688. 118 L. I. Zakharkin A.1. L'vov S. A. Soshka and I. P. Shepilov Zhur. obshchei Khim. 1968,38 255. W. E. Hill Inorg. Chem. 1968,7,222; €€ D. Smith and L. F. Hohstedt ibid. p. 1061;V. Gregor S.Hefmhek and J. PleHek Coll. Czech. Chem Comm. 1968,33,980; L. I. Zakharkin G. G. Zhigareva and A. V. Kazantsev Zhur. obshchei khim. 1968,38,89; V. L Stanko V. A. Brattsev N. E. Al'perovich and N. S. Titova ibid. p. 1056; V. L Stanko G. k Anorova and T. V. Klimova ibid. p. 1193; L. I. Zakharkin V. N. Kalinin and L. S. Podvisotskaya Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1968 679; L. I. Zakharkin A. V. Grebennikov and L. A. Savina ibid. p. 1130; L. I. Zakharkin and V. N. Kalinin ibid. p. 423. lZo J. N. Francis and M. F. Hawthorne J. Amer. Chem. SOC..1968.90 1663. F. N Tebbe P. M.Garrett and M. F. Hawthorne J. Amer. Chem. SOC. 1968.90. 689. M. F. Lappert J. D. Smith and D. R. M. Walton (3)-1-Ph-1,2-BgC2H -H+(3)-1 -Ph-1,2-B9C,H, -1 100" -Ph-1,8-BgC,H,o - -HC (-HJ H+ (3)-1-Ph-l,7-BgC,H 1 -=(3)-1-Ph-1,7-BgC,H,, -H+ B7C2H13 system (see also ref. 123) Other 2C-carboranes to have received attention are B5C2H7,10 1*124 B4C2H6,124 B4C,H8,125* 126 and B3C,H,.124 Further variety is offered by studies in (a)the 1C-system BloH,,CNH3,127 B,CH, and B,CH,; and (b) the 3C-system B3C3H7.I2' B,CH is believed to be a member of the iso- steric series B6-nCnH10-n with a pentagonal-based pyramid (as in B6H,o).128 Another member is of type (16) an organocarborane. These (see ref. 102 for earlier work) unlike the others mentioned above are not prepared from boron hydrides.For example (16) is obtained as follows:129 R' R' R' =R2 =R3 =Et RIB R' 5 R4 =Me I RZ R4 150-180" BR',/\R3,B MeEo:e 'ec'Rz R3 =Et R' =R2=Me 1 2 Me B 0 1 A c. 0 10 o=B .=c 9 (15) I?' V. A. Brattsev and V. I. Stanko Zhur. obshchei Khim. 1968,38 1657. 123 G. B. Dunks and M. F. Hawthorne,Inorg. Chem. 1968,7,1038. lZ4 J. F. Ditter Inorg. Chem. 1968,7 1748. 12' R N. Grimes C. L. Bramlett and R L. Vance Inorg. Chem. 1968,7,1066. 126 J. R Spielman R. Warren G. B. Dunks J. E. Scott and T. Onak Inorg. Chem. 1968,7 216. 12' F.R. Scholer and L.J. Todd J. Organornetallic Chem. 1968,:14 261. 12* M. A. Grassberger E. G. Hoffmann G. Schomburg and R Koster J.Arner. Chem SOC. 1968 90.56. 12' P. Binger Angew. Chem. 1968,80,288. Part (i) The Main Group Elements 299 While thermal isomerisations are well-established with carboranes the first such rearrangements for the metallocarboranes [Co(B,C,H,),] -and (x-C,H,)Co(B,C,H,) have been reported. 130 Substantial advances have been made during 1968 to increase the scope of hydroboration as a synthetic tool. In principle the success of the technique (for a review of earlier work see ref. 102)depends on (a)the rapidity selectivity and substantial stereospecificity of B-C bond-making from B-H and an unsaturated compound and (b) the variety and stereospecificity of subsequent B-C bond-breaking procedures ; additionally processes (a) and (b) are essentially quantitative.New syntheses are exemplified below ; the symbol HB corresponds to the B-H addition step usually using diborane in tetra- hydrofuran or other solvents but also with the more hindered ‘disiamyl- borane’ ‘thexylborane’ or the new and unusually stable (b.p. 195”/12 mm.) 9-borabicyclo[ 3,3,l]nonane (‘9-BBN’),(17),13 which offer higher selectivity. HB (ref. 131) X2CH’C0zEt(ref. 133) = Clor Br) HB‘CH=CH -R’CH,*CH,B BuQK-Bu’OH Bu‘OH-Bu‘OK R’[CH,],*CORZ (1) CH,=C(Y).CHO (2)H,O (ref 136) R[CH,],*CHY*CHO (Y = Br Me) (1) ZCHN (ref. 137) -R[CH,],Z (Z = Ac CN or C0,Et) I,-NaOH (ref. 138) * RCCH21,I T. A. George and M. F. Hawthorne J. Amer. Chem SOC. 1968,90,1661. 131 E. F. Knights and H. C. Brown J. Amer. Chem. Soc. 1968,90,5280 5281 M.F. Lappert J. D. Smith and D. R. M. Walton (A general stereospecific (1) HB annelation; also for (2)CO-H,O-NaOAc (ref. 140) other fused 7,6- 7,5- 6,6- and 6,5-bicyclics) n I R2 R' ~ (1) HB (2)I,-NaOH \ /"='\ R'C= cR~ (ref. 142) /c=c\R2 H H It is particularly noteworthy that it is now possible to carry out not only 1C- but also 2C- and 3C-homologations. The reactions with potassium t-butoxide may proceed via a carbanion mechanism. '33 Bu'OK + BrCH2C02Et-xo3 K' [HCBrCO,Et] -1 R,B K+ [R,BrBCHR.CO,Et] -+ K+ [R,BCHBrCO,Et] -1-KBr R2B.CHR.C02EtBu'Os RCH,.CO,Et + Bu'OBR2 132 E. F. Knights and H. C. Brown J. Amer. Chem. SOC.,1968,90 5283. 133 H. C. Brown M. M. RogiC M. W. Rathke and G. W. Kabalka J. Amer.Chem SOC.,1968,90 1911. Part (i) The Main Group Elements 30 1 The rate of carbonylation (with CO) of BR (lC-homologation) is greatly enhanced by addition of Li[AlH(OEt),]. 143 Substituent effects have been further examined for C :CX,144* 145 MeCH :CH*CH,X,146 CH,:CH[CH2]2X,147 ~yclopent-3-enyl,'~* systems (also ArCH=CH,- and the ~yclohex-3-enyl'~~ ClBH,). 50 The problems considered have been those of relative reactivities specificity stereochemistry and coupling-elimination reactions. For example for vinylic derivatives the a-transfer reaction is intramolecular. 144 I I I I I I -c--c-x I I -C-C-H H B H B / \H / \x The mechanism of the Friedel-Crafts dichloroboronation of arenes has been further examined. ' l-Dichloroboryl-2,5-dimethylbenzene(ArBCl,) does not isomerise at 150°,except in the presence of excess hydrogen chloride; this is due to the reversibility of the reaction and the AlC1,-catalysed isomerisation of p-xylene (ArH).ArH + BCl A"-,ArBCl + HCl heat 134 H. C. Brown M. M. RogiC M. W. Rathke and G. W. Kabalka J. Amer. Chem SOC.,1968,90 818. 13' H. C. Brown M. W. RogiC and M. W. Rathke J. Amer. Chem SOC.,1968.90.6218. H. C. Brown G. W. Kabalka M. W. Rathke and M. M. RogiC J. Amer. Chem SOC. 1968,90 4165. 137 J. Hooz and S. Linke J. Amer. Chem SOC.,1968,90 5936,6891. 13' H. C. Brown M. W. Rathke and M. M. Rogik J. Amer. Chem SOC.,1968,90 5038. 13' H. C. Brown M. W. Rathke G. W. Kabalka and M. M. RogiC J. Amer. Chem SOC.,1968,90 4166. 140 H.C. Brown and E. Negishi Chem Comm 1968 594; J. Amer. Chem SOC.,1967,89 5477. 14' D. Sethi G. C. Joshi and D. Devaprabhakara Cad. J. Chem. 1968,42632 142 G. Zweifel N. L. Polston and C. C. Whitney J. Amer. Chem SOC. 1968,90,6243. 143 H. C. Brown R k Coleman and M. W. Rathke J. Amer. Chem SOC. 1968,90,499. 144 D. J. Pasto J. Hickman and T. C. Cheng J. Amer. Chem SOC.,1968,90 6259. 14' H. C. Brown and R L. Sharp J. Amer. Chem SOC.,1968,90 2915; k Suzuki K Ohmori H. Takenaka and M. Itoh Tetrahedron Letters 1968,4937. 146 H. C. Brown and R M. Gallivan J. Amer. Chem. SOC.,1968,90,2906. 147 H. C. Brown and M. K. Unni J. Amer. Chem. SOC., 1968,90,2902. 14' H. C. Brown and E. F. Knights J. Amer. Chem. SOC. 1968,90,4439. D. J. Pasto and J. Hickman J. Amer. Chem.SOC.,1968,90,4445. D. J. Pasto and S.Z. Kang,J. Amer. Chem SOC.,1968,90,3937. lS1 E. L. Muetterties and F. N. Tebbe Inorg. Chem. 1968,7,2663. 14' M. F. Lappert J. D. Smith and D. R. M. Wulton Further methods of B-C bond making are illustrated below. ref 152 (n-CSHs),Fe + B2C14 -(n-Cl,BCSH4)Fe(CSHs-x); BI + PhI ref.15?PhBI + I In general B-C bonds may be cleaved by reactions by unsaturated com- pounds protic species or oxidising agents. The allyl (and to a lesser extent aryl groups) are especially mobile. Reactions with nitriles have provided monomeric154 (see also ref. 155) and dimeri~~~~~ lS69 iminoboranes; it is likely that dehydroboration is the first step,IS4 and that dimerisation is sterically controlled. The allyl rearrangement is noteworthy ;I 56 other examples are with acetylenic insertion into B-Ally1 groups (All = allyl).’’* lS8 R2 \ /H C It H N Ph ’ All PhCAII, \/ N Y i”\I /-.\ Al12BydBAl12 /\ BAllJ + RC=CR + R’ = Allyl; R2 = Ph J.Kotz and E. W. Post,J. Amer. Chem. SOC. 1968,90,4503. Is3 M. Schmidt and W. Siebert Chem. Ber. 1968,101 281. V. A. Dorokhov and M. F. Lappert Chem. Comm 1968,250. l’’ J. R Jennings I. Pattison C. Summerford K. Wade and B. K. Wyatt Chem. Comm. 1968,250. Y. N. Bubnov V. S. Bogdanov and B. M. Mikhailov Zhur. obshchei Khim. 1968,38,260 Is’ B. I. Tikhomirov A. I. Yakubchik L. N. Mikhailova and Z A. Matveeva Zhur. obshchei. Khim. 1968,38 192 B. M. Mikhailov Y. N. Bubnov S. A. Korobeiinikova and S. I. Frolov Izvest.Akad Nauk S.S.S.R.,Ser.khim. 1968 1923. IS9 B. M. Mikhailov G. S. Ter-Sarkisyan and N. A. Niikolaeva Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1968 541 1655. Part (i) The Main Group Elements Insertion reactions of ketones or p-benzoquinone into B-C bonds have been observed.'59 The olefin displacement reaction previously established with BR,,'02 has now also been noted with Bu"BBr, Bu'BBr, Pr,BSEt (but not R',BOR2 or R,BN c)and olefins such as heptene and nonene (to afford for example C,H ,BBr,). '6o The stereochemistry of the oxidative and hydrolytic cleavage of the 1-phenylethyl (R in the scheme below) -carbon-boron bond has been investiga- ted.' 6' The Me,N-+O reaction is viewed as an intramolecular nucleophilic rearrangement the carboxylic acid cleavage as involving a six-centre transition state (18) the alkali cleavage as an S,1 process at C and the metal ion-OH reaction as taking a free radical path.( + )-RB(OBu) + Me,N+O +(+)-ROB(OBu) + Me,N or T13+ \-B(OD) + [(-) + (*)I-ROH (k)-RR + (-t)-ROH The 1,3-elimination of alkyl boronate in alkali proceeds with inversion at both carbon atoms. 162 The oxidation of trialkylboranes by alkaline hydrogen peroxide is a standard procedure ;lo it is now found that in neutral media radical-paths supervene. '63 160 B. M. Mikhailov M. E. Kuimova and E. k Shagova Zzvest. Akad. Nauk S.S.S.R. Ser.khim. 1968,548. 16' A. G. Davies and B. P. Roberts,J. Chem SOC.,(C) 1968,1474. J. k Marshall and J. H.Babler Chem Comm. 1968,993.D. B. Bigley and D. W. Payling Chem Comm. 1968 938. M. F. Lappert J. D. Smith and D. R. M. Walton R2 aq. NaOH B(OH) + ROH -H202 \ H,02-THF-CCI -R2 + RCl Aluminium Gallium Indium and Thallium. The compounds R,Al are usually dimeric in solution and in the solid states with electron-deficient bridges (19). (19) (20) Bridging and terminal groups exchange readily and are usually indistinguish- able at 20"by n.m.r. but at lower temperatures separate signals due to bridging and terminal groups can be seen. In this way relative bridging abilities have been found e.g. Bu' < Pf < Et < Me < Ph < RC=C.'64 In tricyclopropylalu- minium ' made from dicyclopropylmercury and trimethylaluminium co-alescence of bridging and terminal signals is observed only above 70".Cyclo- propyl bridges seem to be the strongest yet studied perhaps because of overlap between filled nonbonding p-orbitals on carbon and empty antibonding orbitals of the bridge. 1.r. and n.m.r. spectra provide evidence for intramolecular interaction between aluminium and olefinic n-electrons in trans-hex-4-enyldi- isobutylaluminium (20). Similar intermolecular interaction may be the first step in the addition of alkylaluminiums to olefins. Spectroscopic studies show that the symmetry of the compound (Et,AlN,) 67 is D,h and that of (R,AlF) (R = Me,Et)D4h.'68 The analogous gallium fluorides are trimeric but a tetrameric form of dimethylgallium fluoride has been isolated at low temperatures. '69 Mixed bridging compounds Me,AlXYAlMe have been detected by n.m.r.'70 and the first crystal structure of such a compound Me,A1(NPh2)MeA1Me2(21) has been determined. 164 0.Yamamoto and K Hayamizu J. Phys. Chem 1968,72,822; E k Jeffery T. Mole and J. K. Saunders Austral. J. Chem 1968,21 137. D. A. Saunders and J. P.Oliver J. Amer. Chem SOC. 1968,90 5910. 166 G. Hata Chem Comm 1968 7. J. Miiller and K. Dehnicke J. Organometallic Chem. 1968,12 37. J. Weidlein and V. Kriegl J. Organometallic Chem. 1968 11 9. 169 H. Schmidbaur J. Weidlein H-F. Klein and K Eiglmeier Chem Ber. 1968 101 2268; H. Schmidbaur and H-F. Klein ibid. p. 2278. E k Jeffery T. Mole and J. K Saunders Austral. J. Chem 1968,21,649. I" V. R Maginuson and G. D. Stucky J. Amer. Chem Soc. 1968,90 3269. Part (i) The Main Group Elements 305 NPh, PIMe2 (21) In organometallic compounds of Groups I1 and 111 angles at bridging atoms are almost independent of the metal e.g.MCM ca. 75"; MNM ca. 86"; MOM ca. 97" (M =Be Mg Al). The complex Mg[Al(OMe),Me,] C4H802 is based on an infinite polymer with an -AlOMg-frame~ork.'~~ Organoaluminium compounds form complexes with many common solvents and this allows a wide measure of control over their reactivity. So far accurate thermochemical data has been scarce but careful calorimetric measurement~~~ on the Lewis acidity of trimethylaluminium with respect to various amines ethers phosphines and sulphur derivatives have confirmed that the donor- acceptor interaction decreases in the series N >P >0 >s.Measurements in solution and in the vapour phase have been related. Another attempt has been made to correlate chemical shifts with base strengths.174 The exchange of R' and R2 between Ri3A1D and R2,A1D in the presence of electron donors (D)may proceed by a dissociative mechanism ;e.g. with weak bases like anisole ; or by an &2 mechanism without loss of base :e.g. for reactions in an excess ofpyridine.' 7s Exchange of organic groups between Me3Ga and (CH,=CH),Ga has also been 0b~erved.l~~ Simple preparations of a large number of new trialkynylaluminiums and bis(alkyl)alkynylaluminiums and also some analogous gallium compounds have been described. 177 The alkynylaluminium compounds react with ketones giving an easy route to alkynyl tertiary alcohols.R3 OH R2,A1Cl R3R4C0 H,O e.g.,NaC=CR' -R2,AlC=CR1 -\/ C /\ R4 C=CR' The versatility of organoaluminium reagents in organic syntheses is illustrated by hydroaluminations e.g. 1'12 J. L Atwood and G. D. Stucky J. Organometallic Chem. 1968,13 53. 173 C. H. Henrickson D. Day and D. P.Eyman Znorg. Chem. 1968,7,1047; C. H. Hendrickson K. N. Nykerk D. P. Eyman ibid. p. 1028. 1'14 K. Hatada and H. Yuki Tetrahedron Letters 1968,213. N. S. Ham E. A. Jeffery T. Mole and J. K. Saunders Austral. J. Chem. 1968,21 659; E. A. Jeffery and T. Mole ibid. p. 1497. H. D. Visser and J. P. Oliver J. Am. Chem SOC. 1968,90 3579. 1'17 H. Demarne and P. Cadiot Bull. SOC.chim France 1968 205 211 216; E. A. Jeffery and T. Mole J. Organometallic Chem.1968 11 393. 306 M.F. Lappert J. D. Smith and D. R. M. Walton R' R2 R' R2 HO \/ z\/ R'C=CR2 Bu'2AiF /""\ AlBu' H H (22) R' AlBu' R' R2 H CO,H Thus diphenylacetylene (R1 = RZ = Ph) yields the cis-compound(22) which can be carbonated and hydrolysed to the carboxylic acid (23).The cis-compound (22) is carbonated much faster than the trans-isomer (24).178The products from alkynes and di-isobutylaluminium hydride depend critically on solvents and temperature. For example when R' = Ph and R2 = SiMe, reaction in hydrocarbons gives a 95 % yield of the trans-derivative (24) whereas reaction in a donor solvent such as N-methylpyrrolidene gives a 96% yield of the cis-derivative(22). The intermediates (25) and (26) illustrate the mechanisms propo~ed"~ to account for the difference.RCEC-SiMe H-AIBU' 1 1 \IBui RCgC-SiMe -+ (24) -1 A1 (25) Bu''I'H Bu' When R1 = Ph and RZ = H cis-hydroalumination predominates; this re- action unlike trans-hydroalumination is catalysed by nickel salts. '8o In amines or ethers metallation [reaction (a)] or bis-addition [reaction (b)] is observed.I8 (a) PhHC=CHAIBu' + PhCECH PhCdAlBu' + PhHC=CH (b) PhHC==CHAIBu' + Bu',AlH -,PhH,C. CH(AlBu',) J. J. Eisch and M. W.Foxton J. Organometallic Chem 1968 11 P7. 179 J. J. Eisch and M. W. Foxton J. Organometallic Chem 1968 $1 P24. J. J. Eisch and M. W. Foxton J. Organometallic Chem. 1968,12 P33. V. V. Gavrilenko B. k Palei and L. I. Zakharkin Izvest. Akad. Nauk S.S.S.R.Ser. khim 1968,910. Part (i) The Main Group Elements The photochemical addition of triphenylaluminium to diphenylacetylene’82 yields after hydrolysis with D,O considerable amounts of the cis-C2H2J-stilbene-PhDC :CDPh and an Al(1) intermediate has been proposed to account for this observation. A kinetic study’ 83 of the addition of trimethylaluminium to benzophenone shows that the mechanism depends on the stoicheiometry because of complex formation between trimethylaluminium and the product Ph,MeCOAlMe,. A number of compounds with eight-membered rings derived from organoalu- minium compounds and isocyanates N-substituted acid amides or oximes have been described:84 and the crystal structure of one of them (Me,AlOCPh :NPh), (27) has been determined.18’ This can co-ordinate two molecules of acetalde-hyde and the resulting complex may be an intermediate in polymerisations.Ph I Ph The reactions between alkylaluminium compounds and nitriles vinyl com- pounds or cyclic ethers have been studied’86 and may give products from addition alkylation or reduction. A mixture of triethylaluminium and hydrogen cyanide in diethyl ether earlier used in steroid chemistry converts sugar epoxides to cyanohydrins. ’87 The heat of formation of trimethylindium has been foundlB8 and the mean InMe bond energy is 38.9 kcal./mole. The reactions of the compounds Me,InX and MeInX (X = C1 I) e.g. with nitrogen and phosphorous donors are like those of analogous gallium and aluminium derivatives. lB9 The exchange between complexes Me,InD (D = Me,N or Me,NH) and trimethylindium in dichloromethane like the reactions of the corresponding gallium compounds proceeds by a dissociative mechanism.When D = MeNH an S,2 mechanism is important.lgO J. J. Eisch and J. L. Considine J. Amer. Chem. SOC.,1968,90,6257. E. C. Ashley J. Laemmle and H. M. Neuman J. Amer. Chem. SOC. 1968,90,5179. lS4 J. R. Horder and M. F. Lappert J. Chem. SOC.(A) 1968 2004; J. R. Jennings K. Wade and B. K. Wyatt ibid. p. 2535; I. Pattison and K. Wade ibid. p. 2618. Y. Kai N. Yasuoka N. Kasai M. Kakudo H. Yasuda and H. Tani Chem. Comm. 1968,1332. la6 S. Pasynkiewicz and S. Maciaszek J. Organometallic. Chem 1968 15 301; S. Pasynkiewicz and W. Kuran ibid. p. 307; D. B. Miller ibid. 14,253; Y.Baba Bull. Chem SOC. Japan 1968,41. 928. ’” B. E. Davison R. D. Guthrie and A. T. McPhail Chem. Comm.,1968 1273. W. D. Clark and S. J. W. Price Canad. J. Chem 1968,46,1633. H. C. Clark and A. L. Pickard J. Organometallic Chem 1968,13 61. K. L. Henold and J. P. Oliver Inorg. Chem. 1968,7,950. M. F. Lappert J. D. Smith and D. R. M. Walton Group 1V.-Silicon and Germanium. Organometallic compounds of silicon and germanium are often compared with analogous compounds of carbon. Differences between the structures and reactions of carbon compounds and those of the heavier elements are attributed to the polarity of C-Si and C-Ge bonds and to the role of d-orbitals. Quantitative assessment of these effects is not easy. During 1968 vertical ionisation potentials (from mass spectra) charge-transfer transitions half-wave potentials electronic spectra and e.s.r.spectra of radical-ions have been given for a wide range of organo-silicon compounds (e.g. trimethylsilyl-benzenes alkenes alkynes ketones and bi- phenyls) and the results allow approximate energy-level diagrams to be con- structed. ''' Molecular orbital calculations confirm the greater inductive effect of SiMe compared with CMe, and Si + C x-interactions. The value of 67 kcal./mole has been found for the bond-dissociation energy D(Me,Si-SiMe,) from a kinetic study of the pyrolysis of hexamethyldi~ilane'~~ and some of the discrepancies in earlier values have been accounted for. By combination of this result with appearance potentials the following bond-dissociation energies D(Me,Si-X) are derived:IQ3 (X=)Me 76; H 81; C 88; Br 78-5; I 69 kcal./mole.Information about n-interactions from bond-energy data and from spectroscopic and kinetic studies on disilanes seems to be equivocal.194 Interaction involving d-orbitals may account for the fluxional properties of the derivatives C,H,SiMe (28) Me,M(C,H,Me),-, (M = Si Ge Sn; n = 0 1,2) in which the point of attachment of the silicon to the ring moves by an intra- molecular proce~s.'~' (cf p. 313). The crystal structure of a~etyltriphenylgermane'~~ has been found. The long Ge-C (acetyl) distance 2.011 A is explained by a contribution from the 19' H. Bock and H. Seidl J. Organometallic Chem. 1968 87; H. Bock and H. Alt ibid. p. 103; H. Bock Angew.Chem. 1968,80,368; H. Bock H. Alt and H. Seidl ibid. p. 906; F.Gerson J. Heinzer H. Bock HAlt and H. Seidl Helu. Chim Acta. 1968 51,707; H. Bock H. Seidl and M. Fochler Chem. BET.,1968,101,2815; H. Bock and H. Seidl J. Amer. Chem. SOC. 1968,90,5694;J. Chem. Soc. (B) 1968 1158; k L. Allred and L. W. Bush J. Amer. Chem SOC.,1968,W 3352 19' I. M. T. Davidson and I. L.Stephenson J. Chem SOC.(A) 1968,282 19' S. J. Band I. M. T. Davidson and C. A. Lambert J. Chem SOC.(A) 1968,2068. 194 S. J. Band I. M. T. Davidson and C. A. Lambert J. Organometallic Chem. 1968,12 P3; B. G. Gowenlock and J. Stevenson ibid 13 p. P13; F. K.Cartledge ibid. 13 516; A. G.Brook D. G. Anderson J. M. Duff P. F. Jones and D. M. MacRae J. Amer. Chem. SOC.,1968,90,1076 H.Sakuri H. Yamamori and M. Kumada Chem. Comm. 1968,198. 19' (a) Yu. A. Ustynyuk A. V. Kisin and 0.E. Oksinoid Zhur. obshchei. Khim. 1968 38 391 ; (b) A Davidson and P. E. Rakita J.Amer. Chem Soc. 1968,90,4479. R W. Harrison and J. Trotter J. Chem Soc. (A) 1968 258. 19' Part (i) The Main Group Elements /O-structure Ph,Ge+ :C but the Ge-C (phenyl) distance 1.945 A is normal. \ Me In the compound PhSiN(CH,CH,O), (29) the atoms round silicon form a distorted trigonal bipyramid. lg7 There is continued interest in reactions of optically active corn pound^,^^^^ with a view ultimately to establishing details of the mechanisms of substitu- tions at silicon and germanium. There is still no evidence of siliconium ions substitutions at silicon and germanium involve bimolecular processes which may lead either to inversion or to retention of configuration.Reactions at asymmetric germanium seem to follow a similar pattern to those at silicon :*O there is usually inversion with good leaving groups e.g. C1 SPh SGeR, and retention with poor leaving groups e.g. OMe OGeR,. Reactions at germanium are in general less stereospecific than at silicon but this may be because of racemisation of products after stereospecific substitution.202 A number of simple mechanisms have been proposed for reactions proceeding with retention and several anomalies have been accounted The kinetics and mechanisms of pyrolysis of hexamethyldisilane have been studied at a variety of temperatures and pressures192* '04 and the pyrolysis of cyclopropyltrimethylsilane at 450" has been shown to give almost exclusively Me,SiCH,CH :CH2.205 Kinetic mass spectral and chemical evidence has 19' J.W. Turley and F. P. Boer J. Amer. Chem SOC.,1968,!M 4026. 19* L.H. Sommer "Sterochemistry Mechanism and Silicon" McGraw Hill New York 1965. 199 L. Homer and M. Worm Tetrahedron Letters 1968,2447;A. Holt A. W. P. Jarvie and G. J. Jervis ibid. p. 4087;K-D. Kaufmann H. Bormann K. Riihlmann G.Englhardt and H. Kriegsmann Chem Ber. 1968 101 984; C. Eaborn R E. E. Hill P. Simpson A. G. Brook and D. MacRae J. Organometallic Chem. 1968 15 241; G. R Bull L. Spialter and J. D. Austin ibid. 14 309; G. J. D. Peddle J. M. Shafir and S. G. McGeachin ibid. 15 505; G. J. D Peddle and D. N. Roark Cad.J. Chem. 1968,46 2507. 'O0 R Corriu M. Leard and J. Masse Bull. SOC.chim France 1968,2555;M. W. Grant and R H. Prince Chem Comm. 1968 1076. '01 C. Eaborn R E. E Hill and P. Simpson Chem Comm. 1968 1077. 'O' E H. Carre R J. P. Corriu and R B. Thomassin Chem Comm 1968,560;R J. P.Corriu and M. Leard J. Organometallic Chem. 1968,15,25;R J. P. Corriu and G. Royo ibid. 14291. '03 L. H. Sommer and H. Fujimoto J. Amer. Chem SOC.,1968,W 982; L.H. Sommer L.A. Ulland and A. Ritter ibid. p. 4486;C. Eaborn R E. E. Hill and P. Simpson J. Organometallic Chern. 1968 15,P1;D. G. Anderson and D. E. Webster J. Chem. SOC.(B),1968,765,878,1008. '04 H. Sakurai A. Hosomi and M. Kumada Chem. Comm. 1968,930;C. Eaborn and J. M. Simmie ibid. p. 1426. 205 H.Sakurai A. Hosomi and M.Kumada Tetrahedron Letters 1968 2469. 3 10 M. F. Lappert J. D.Smith and D.R. M. Walton been found for silylene intermediates Me2Si in the pyrolysis of the methoxy- disilane MeO(SiMe,),OMe. These react with alkynes probably to give cyclo- propenes which dimerise in a specific way to the cyclic compounds (30).206 (30) (31) Silylenes have also been postulated in the photolysis of trimethyl~ilane.~~' A number of new ethynylsilanes have been characterised.208 They undergo Diels-Alder additions with cyclopentadienes to give bicyclo [2,2,1] hepta- diene~,~"e.g. (31) and can be used in oxidative couplings to give polyacety- lenes.2 O Et3SiC=CH + Et,Si[C=C],SiEt Ph[Cd],H + Br[C=C],SiEt + Ph[C=C],+,SiEt SiR groups can be readily removed by aqueous ethanolic alkali.More compli- cated reactions occur in cleavages of ethynyl groups in the presence of platinum salts.21' Hydrosilylations have been used for the preparation of a wide variety of silicon compounds2 ' Disilylation of conjugated dienes by trimethylchloro- silane and alkali-metal depends on the metal used sodium favours the cis-and lithium the trans-product ;*I3 e.g. R' \ /R2 Na-THF-Me,SiCl Li-THF-Me,SiCl t H,C :CR'.CR2 :CH2----+ /=\CH -Me,Si. CH SiMe R' \ /CH2.SiMe3 Me+ \RZ CH ' 206 W. H. Atwell and D. R Weyenberg J. Amer. Chem. SOC.,1968,90,3438. '07 0.P. Strausz K. Obi and W. K. Duholke J. Am. Chem. Soc. 1968,90,1359. '08 (a)0.A. Novikova V. P. Kuznetsova and K. A. Kornev Zhur. obshchei Khim. 1968,38,178; (b) N.P. Smetankina V. P. Kumetsova and S. D. Lyukas ibid. p. 171; (c)B. C. Pant and H. F. Reiff J. Organometallic Chem. 1968 15 65. 209 C. S. Kraihanzel and M. L. Losse J. Org. Chem. 1968,33 1983; J. Amer. Chem SOC. 1968,90 4701. 'lo R Eastmond and D. R M. Walton Chem Comm. 1968 204. '11 J. E. Poist and C. S. Kraihanzel Chem Comm. 1968,607. 212 A. G. Brook,K. H. Pannell and D. G. Anderson J. Amer. Chem SOC. 1968,90,4374; R A. Benkeser S. Dunny G. S. Li P. G. Nerlekar and S. D. Work ibid. p. 1871; K. Yamamoto and M. Kumada J. Organometallic Chem. 1968 13 131; T. Frainnet and J. Causse Bull. SOC.chim France 1968,3034; V. F. Mironov Y. P. Kozyukov and V. D. Sheludyakov Doklady Akad. Nauk S.S.S.R. 1968,178,358 (Chem. Sect.); I. I. Lapkin and T.N.Povarnitsyna Zhur. obshchei Khim. 1968,s 643; M. F. Shostakovskii N. V. Komarov and T. D. Burnashova Izuest. Akad. Nauk S.S.S.R. Ser. khim. 1968,629. D. R Weyenberg L.HToporcer and L. E. Nelson J. Org. Chem 1968,33 1975. Part (i) The Main Group Elements 31 1 Many papers have appeared on the preparation of trimethylsilyl derivatives of polychloro- and polyfluoro-organic corn pound^.^^^ l4 Lithiation of tri- methylchlorosilane at low temperature has been achieved.2 e.g. Me Sic1 Bu'Li + Me,SiC1-78" LiCH2*SiMe2C1A+ Me,Si*CH2*SiMe2C1 (33 %) Preparative routes for halogenomethyl derivatives of silicon and germanium include the reaction of dichloromethyl-lithium216 or diazomethane2I7 with chloro-compounds. The reactions of bis(trimethylsilyldichloromethyl)mercury have been described.218 Chloromethylsilyl compounds have been used both as a source of silylcarbenes and for the preparation of cyclic ~ilanes.~" A number of rearrangements have been studied.220 Cleavages of l-halogeno- methyl-1-phenyldisilanesby ethoxide are said to involve MePhSi :CH2 inter- mediates.221 There have been several further examples of the rearrangement of compounds with carbonyl functions p to silicon;222 e.g.R',SiCH2-COR2 -+ R' ,SiOCR2=CH2. Silylation of 1-methylcyclohexanones gives 0-silyl enols and silylacetic acids also rearrange with migration of silicon from carbon to oxygen.223 Trimethylsilyl ethers are used as volatile derivatives for separa- tions by gas chromatography and so the analysis and detection of small quantitities are important.Rearrangements occur in mass spectra ;224 e.g. trimethylsilyl glycols give a peak at m/e = 147 corresponding to Me,SiO+=SiMe,. Trisilylthiols rearrange by a radical mechanism :225 Me,Si*SiMe(SH)*SiMe,+ Me,Si*SiHMe*S*SiMe 214 F. G. Drakesmith 0.J. Stewart and P. Tarrant J. Org. Chem. 1968,33 472; I. Haiduc and H. Gilman J. Organometallic Chem. 1968 $1 55; 12 291; 13 257 P4; 14 73; T. Brennan and H. Gilman ibid. 11 185 625; 12 291; P. J. Morns and H. Gilman ibid. $1,463; K Shina T. Brennan and H. Gilman ibid. p. 471 ;S. S. Dua and H. Gilman ibid. 12 234 299; D. Ballard and H. Gilman ibid. 12 237; 14 87; 15 321; F. W. G. Fearon and H. Gilman ibid. 13 73. 215 G. A. Gomowin and R West J. Amer. Chem SOC. 1968,90,4478.216 H. Sakurai H. Yamamori and M. Kumada J. Org. Chem. 1968,33 1527. 217 D. Seyferth and J. HetflejS J. Organometallic Chem. 1968 11 253. 218 D. Seyferth and E. M. Hanson J. Amer. Chem Sac. 1968,90,2438. 219 D. Seyferth A. W. Dow H. Menzel and T. C. Flood J. Amer. Chem SOC. 1968 90 1080; R Corriu B. Henner and J. Masse Bull. SOC.chim France 1968 3013. 220 J. W. Connolly J. Organometallic Chem. 1968,:11 429; 0.W. Steward W. J. Uhl and B. W. Sands ibid. 15 329. 221 M. Kumada K. Tamao M. Ishikawa and M. Matsuno Chem Comm. 1968,614. 222 I. F. Lutsenko Yu I. Baukov 0.V.Dudukina and E. N. Kramarova J. Organometallic Chem. 1968 11 35; I. F. Lutsenko Yu. I. Baukov I. Yu. Belavin and A. N. Tvorogov ibid. 14,229; R. M. Ismail ibid. 11,49;R. M. Ismail and E. Bessler ibid.13,G.J. D. Peddle and J. E. H. Ward ibid. 13,269 ; 14 131. 223 G. Stork and P. F. Hudrlik J. Amer. Chem SOC. 1968,90,4462;A. G. Brook D. G. Anderson and J. M. Duff ibid. p. 3876. 224 J. Diebman J. B. Thomson C. Djerassi J. Org. Chem. 1968,33,2271; W. J. Richter and A. L. Burlingame Chem Comm. 1968 1158. 225 C. G. Pitt and M. S. Fowler J. Amer. Chem SOC. 1968,90 1928. 312 M. F. Lappert J. D. Smith and D. R. M. Walton Anionic rearrangements of silicon-nitrogen compounds have also been de- scribed.226 Difficulties associated with the preparation of perfluorophenyl compounds (C,F,),SiCI have been overcome either by starting from the compounds C1,SiX (X= H OEt NMe,) or by making the perfluorophenyl Grignard reagent by the addition of ether to a mixture of bromopentafluorobenzene magnesium and Six (X = OEt Cl).227 The Grignard reagent is thus consumed as soon as it forms.Compounds R~S~(C~FS)~ and RSi(C,Fs) (R = Me3Ph) have been described. The reactions between germanium halides and Grignard or alkylaluminium reagents have been studied and conditions have been found for the formation of good yields of polygermanes.228 Considerable progress has been made in the characteri~ation~~’ of germanes especially by mass and n.m.r. spectra at 220 MHz Me3Ge Me2Ge and MeGe species are differentiated. Much in- formation has been obtained with very small amounts of material. Selective dealkylation of germanes with tin tetrachloride2304 may be used for the prepara- tion of unsymmetrical compounds; e.g.SnCl PrMgX SnCl Me,BuGe -A Me2BuGeC1-Me,PrBuGe --A MePrBuGeCl4 etc. The redistribution of substituents X between various R,GeX,- has been investigated.230b Though distribution of halogens over various MeGeX compounds is not far from statistical there is a strong tendency for OR sub- stituents to appear on silicon in systems which contain both silicon and ger- manium. Tin.Crystal structures (X-ray) have been reported on the following com- pounds (PhCH,),SnOAc (linear polymer with trigonal bipyramidal Sn bridging OAc and equatorial PhCH groups;231 (C6H ,),S~OAC~~~ (CH2C02EtCHC02Et),SnBr2232,Me3SnMn(C0)5234 [n-C5H5)Fe(CO)21 2-226 R West M. Ishikawa and S. Murai J. Amer. Chem SOC.,1968,90,727;R P. Bush N. C. Lloyd and C. A.Pearce Chem Comm. 1968 1191. ”’ M. F.Lappert and J. Lynch Chem. Comm. 1968,750;A. Whittingham and A. W. P. Jarvie J. Organometallic Chem. 1968 13 125; M. Schmeisser N.Wessal and M. Weidenbruch Chem. Bm. 1968,101,1897. 228 J-C. Mendelsohn F. Metras J-C. Lahournbre and J. Valade J. Organometallic Chem. 1968 12 327; D.Quane and G. W. Hunt ibid. 13 P16; E.J. Bulten and J. G. Noltes J. Organometallic Chem. 1968,11 P19; V.F.Mironov L. M. Antipin and E. S. Sobolev Zhur. obdshchei Khim. 1968 38 251. 229 F.Glocking J. R C. Light and J. Walker Chem Comm. 1968,1052;F.Glockling and J. R C. Light J. Chem. SOC.(A),1968 717; K. Kiihlein and W. P. Neumann J. Organometallic Chem. 1968, 14317. 230 (a) E.J. Bulten and J. G. Noltes J. Organometallic Chem 1968,15,P18;(b) J.R van Wazer and K. Moedritzer J. Amer. Chem SOC. 1968,90,47;J. Organometallic Chem 1968,13 145; Znorg. Chim Acta 1968,2,111;K.Moedritzer,J. R van Wazer and R E. Miller Inorg. Chem 1968,7,1638. 231 N. W. Alcoclcand R E. Timms,J. Chem SOC.(A) 1968 1873. 232 N.W.Alcock and R. E. Timms J. Chem. SOC.(A) 1968 1876. 233 M. Yoshida I. Ueki N. Yasuoka N. Kasai M. Kakudo I. Omae S. Kikkawa and S. Matsuda Bull. Chem SOC.,Japan 1968,41 1113. 234 R F. Bryan J. Chem SOC.(A),1968 696. Part (i) The Main Group Elements 313 Sn(C5H5-~)2235, and [n-C,H,)Fe(CO),] ,Sn[OS(:O)Ph] 2236,all approx. tetra- hedral Sn); (MeSn),S (32)237; and [Me,SnCl,terpy] -t [Me,SnCl,]-(terpy = 2,2',2"-terpyridyl ; highly distorted octahedral Sn cation ; trigonal bipyra- midal Sn anion with Me’groups equatorial).238 The Sn-C distance is in the range 2.06-2.24 A.Me I Me Several papers dealt with ‘H n.m.r. spectra;’95b* 239-241 a detailed hetero- nuclear double-resonance experiment has been carried out with Me6Sn2.242 The temperature and concentration-dependent ‘H n.m.r. spectrum of Me(Ph)(Cl)SnCH,CPhMe in benzene suggested rapid inversion at the asym- metric tin atom;,,’ this may explain failures to resolve optical enantiomers. The temperature-dependent ‘H n.m.r. spectrum of various cyclopentadienyl- (a single proton signal except at low temperatures) or methylcyclopentadienyl- tin compounds was explained in dynamic terms (they are “fluxional” mole- cules);’”’ a limiting low-temperature spectrum need not refer to a molecule ‘locked’ in a given configuration and the characteristic 2 :2 :3-pattern for Sn(C,H,Me) need not imply a static configuration.Important mass-spectral studies have been reported.243* 244 The vertical ionisation potentials for the radicals Me,Sn (7.10 0.05 ev) Me,Sn (7.95 0.05 ev) and MeSn have been measured directly ;243 combined with appear- ance potential data Me,Sn-R bond dissociation energies (R = Me 2-62& 0.08 235 B. P. Biryukov U. T. Struchkov K. N. Anisimov and N. E. Kolobova Chem Comm. 1968 1193. 236 R F. Bryan and k R Manning Chem Comm. 1968,1220. 237 C. Diirfelt A. Janeck D. Kobelt E. F. Paulus and H. Scherer J. Organometallic Chem 1968 14 P22. 238 F. W. B. Einstein and B. R Penfold J. Chem (A) 1968 3019. 239 G.J. G. Peddle and G. Redl Chem Comm. 1968,626. 240 E.W.Randall and J. J. Zuckerman J. Amer. Chem. SOC. 1968,90,3167. 241 J. Lorbeth and H. Vahrenkamp J. Orgummetallic Chem. 1968,11,111;E. V.van den Berghe and G. P. van der Kelen ibid. p. 479;L. Vedonck G.P.van der Kelen and Z. Eeckhant ibid. p. 487; and G.P. van der Lelen ibid. p. 479;L. Verdonck G.P. van der Kelen and Z. Eeckhant ibid. L. Verdonck and G. P. van der Kelen ibid. p. 491;E. V. van der Berghe G. P. van der Kelen and J. Albrecht Znorg. Chim. Acta 1968,2 89. 242 W. McFarlane J. Chem. SOC.(A) 1968 1630. 243 F. W.Lampe and A. Niehaus J. Chem Phys. 1968,49,2949. 244 A.L.Yergey and F. L. Lampe J. Organometallic Chem. 15 339. 314 M. F. Lappert J. D. Smith,and D. R. M. Walton ev; R = Et 2-37 & 0.15 ev; R = Pr" 2-40 f0.10ev; R = SnMe, 2.74 &-010 ev) were derived.The Me,Sn-R bond dissociation energies vary from 55-70 & 6 kcal./mole for a wider range of alkyl groups while the thermochemical mean Sn-C bond energy term was calculated as 45 & 6 kcal./mole. Mossbauer spectral work has continued,245 with special emphasis on cor-relations for determining the co-ordination number of Sn and the stereo- chemistry. Similar problems have also been tackled by measurements of electric dipole and n.q.r. spectra.247 R,Sn radicals have been detected and characterised by their ex. spectra.248 1.r. and Raman sometimes with normal mode vibrational analy~es,~ of many simple organotin compounds have continued to be studied. Sn-C Compounds of special interest have been organotin-metal com-pounds related to metal-carbene complexes [such as (33)],251v 252 diazomethane derivatives R,SnC(N2)C02Et,253 organotin ketones Ph,SnCOR (related to (33) the better known analogues of Si and Ge),254 perhalogeno-derivatives [such as Me3SnC6F5 and 1,4-(Me,Sn)2C6Cl,],Z and alkynyl derivatives.Compound 245 B. W. Fitzsimmons N. J. Seeley and A. W. Smith Chem Comm. 1968,390; R V. Parrish and R H. Platt ibid. p. 1118; B. F. E. Ford B. V. Liengme,and J R Sams ibid.,p. 1333 B. W. Fitzsimmons ibid. p. 1485; N. W. G. Debye E. Rosenberg and J. J. Zuckerman J. Amer. Chem. SOC. 1968,90 3234; V. V. Khrapov V. I. Goldanskii A. K. Prokofiev V. Y. Rochev and R G. Kostyanovsky Izvest. Akad. Nauk S.S.S.R.,Ser. khim. 1968,1261;A.N. Nesmeyanov V. I. Goldanskii,V. V. Khrapov V. Y. Rochev D. N. Kravtsov and E. M. Rokhlina ibid.,p. 2938; A. U. Alexandrou,V. I. Goldanskii L. A. Korytko V. A. Maltsev and N. A. Plate Vysokomol Soedineniya. (B) 1968,10,209 ; M. A. Mullins and C. Curran Znorg. Chem. 1967,7 2584; A. A. Petrov B.I. Rogozev L. M. Krizhanskii and V. S. Zavgorodnii Zhur. obsckei Khim. 1968,38 1196. 246 H. H. Huang K. M. Hui and K. K. Chiu J. Organometallic Chem. 1968,:11,515;K. S. Minga-leva B. I. Ionin V. S. Zavgorodnii L. G. Sharanina and A. A. Petrov Zhur. obshchei Khim. 1968,38 606. 247 C. K. Semin T. A. Babushkina A. K. Prokofiev and R G. Konstyanovsky Izvest. Akad. Nauk S.S.S.R.,Ser. khim. 1968,1401 ; H. A. Stoeckler and H. Sano Trans. Faraday SOC. 1968,64,577. 248 G.Lassmann and K. Hoppner 2.Naturforsch. (B) 1968,23,622 249 R J. H. Clark A. G. Davies and R J. Puddephatt J. Chem SOC. (A),1968,1828; J. C. Maire and R Ouaki Helu. Chim. Acta 1968,51 1150; C. J. Cattanach and E. F. Mooney Spectrochim. Acta (A) 1968,24 407; H. Geissler and H. Kriegsmann J. Organometallic Chem 1968 11 85. H. Kimmer and C. R Dillard Spectrochim Acta (A) 1968,24 909;H. Kriegsmann,H. Hoffmann and H. Geissler 2.anorg. Chem. 1968,359,58; V. Galasso G. De-Alti and A. Bigotto Z. phys. Chem. (Frankfurt) 1968,57 132 251 H. M. J. C. Creemers J. G. Noltes and G. J. M. van der Kerk J. Organometallic Chem. 1968 14,217. 252 F. J. A. des Tombe G. J. M. van der Kerk and J. G. Noltes J. Organometallic Chem. 1968,13 P9. 253 J. Lorbeth J. Organometallic Chem.1968 15,251. 254 G. J. D. Peddle J. Organometallic Chem 1968 14,139. 25s K.Shiina J. Brennan and H. Gilman J. Organometallic Chem. 1968,:11,471. Part (i) The Main Group Elements (33) was made from Ph,SnH and EtMgBr in ether;251and an unstable zinc analogue from P~,S~C~-Z~-CU.~ 52 The stannyl diazoacetates were obtained from R13SnNR22/CH(N2)C02Et,253 the ketone from Ph,SnLi and RCOC1,254 and the perhalogenophenyl derivatives from the corresponding lithium The alkynyltins were obtained from acetylenesand (i)organotin halides (HX eliminati~n),~(ii) organotin amides (R2NH eliminati~n),~ 56 ’’ (iii) organotin hydroxides or oxides (H,O elimination);2ss or from alkynyl-Grignards or -lithium reagent^.^'^-^^'* 208c They were studied,as exemplified below either to observe side-chain reactions or to determine the relative ease of cleavage of M-CSR and M-R (or Ar) bonds.Me,SnCF, Me,Sn-C=CCF ____-+ Me,Sn-C--CCF \/ CF2 -,1,l-Diels-Alder adduct (R = H) \ Et,SnC==CH I1 N CH 2PhNCO \I 4 N OC-NPh II Et,SnC=C CO I \/ RN Ph R,Sn(CSPh) -,(4-n)PhzPC1-+(44) Ph,P-C=CPh +R,SnC14 - 256 W. R. Cullen and M.C. Waldman Inorg. Nuclear Chem. Letters 1968,4 205. 257 W. Siebert W. E. Davidsohn and M. C. Henry J. Organometallic Chem. 1968,15,69. F. G. Kleiner and W. P. Neumann Annulen 1968,316,19; M. F.Shostakovskii N. V. Komarov T.D.Burnashova and I. S. Akchurina Izuest. Akad. Nauk S.S.S.R., Ser. khim. 1968,625. 259 W. P. Neumann and F.G. Kleiner Annalen. 1968.316 29 H. Hartmann. ihid.. 714. 1. z60 N. V. Komarov M. F. Shostakovskii and T. D. Burnashova Zhur. obshchei. Khirn.. 1968,38 1398; V. S. Zavgorodnii L G. Sharanina and A. k Petrov ibid. p. 1146 1150. z61 J. C. Masson Le-Quanh-Minh. and P. Cadiot Bull. SOC.chim. France. 1968. 1085. 316 M. F. Lappert J. D. Smith and D. R. M. Walton Hydrostannation explored for addition to fluoro-olefils,262 01efrns,~~~* 264 alkyne~,~~' ap-unsaturated nitriles or esters,268 and some iridium (I) -267 complexe~,~is illustrated below. Further evidence has accumulated for a 69 free-radical mechanism. 26 3-266 Bromostannation has also been reported. 270 Amidotin (like Si and Ge analogues) compounds were used for Sn-C bond-making by insertion of acetylenes with electronegative substituents or alp-unsaturated carbonyl compounds ;27 ' steric effects could alter the course of reactions.The synthesis of compound (34) provides the first example of a 1,l-insertion outside transition-metal chemistry. Tetrafluoroethylene has been inserted into Sn-Co Additional examples of the 'direct' synthesis of Sn-C compound from elemental tin were published.273 Me,M*CF:CF A+Me,M*CFH*CF,*SnMe -,Me,SnF Me,SnH Me,M*CF(SnMe,)*CF,H (M = Si Ge or Sn) CH,=CH n H*COEt R,Sn.CH CH OEt R*Ph R,SnH 1 -Pr"CO,Et + Ph,Sn \\ MeCH :CHCO,Et R = Bun -MeCH.CH,-CO,Et + MeCH :CHCO,EE H trans(Ph,PMe),l(CO)Cl / -(Ph,PMe),Ir(CO)Cl \ SnR 262 M. Akhtar and H. C. Clark Canad. J. Chem. 1968,46,633,2165.R. Sommer and H. G. Kuivila J. Org. Chem. 1968,33,802. 264 D. Seyferth T. F. Jula D. Dertouzos and M. Pereyre J. Organometallic Chem. 1968;11,63. 265 A. J. Leusink and H. A. Buding J. Organometallic Chem. 1968;11,533. 266 A. J. Leusink H. A. Buding and W. Drenth J. Organometallic Chem. 1968;11 541. 267 M. A. Kazamkova N. P. Protsenko and I. F. Lutsenko Zhur. obshchei. Khim. 1968,38 106; E. N. Maltseva V.S. Zavgorodnii I. A. Maretina and A. A. Petrov ibid. p. 203. 268 M. Pereyre G. Colin and J. Valade Bull. SOC. chim. France 1968,3358. 269 M. F. Lappert and N. F. Travers Chem. Comm. 1968,1569. 270 H. Meyer J. Organometallic Chem. 1968,:€1 525. 271 T. A. George and M. F. Lappert J. Organometallic Chem. 1968,14 327. 272 A. D. Beveridge and H. C. Clark J.Organometallic Chem. 1968,:11,601. 273 K. Sisido T. Miyanisi K Nabika and S. Kozima J. Organornetallic Chem. 1968,:11 281; K. Sisido and S. Kozima ibid p. 503 ;T. Hayashi S. Kikkawa and S. Matsuda Kogyo Kagaku Zusshi 1968,71 710; M. Nomura N. Matsui and S. Matsuda ibid. p. 1526; M. Nomura S. Ando and S. Matsuda. ibid.. p. p. 394; T. Hayshi. S. Kikkawa and S. Matsuda ibid.. p. 710. Part (i) The Main Group Elements 317 ,-R PhC:CCl ,Sn(Ph) C:C(C1) NR2 Rl3SnNR' R,Sn*OMe+ CH,:C(Me).CO*NMe (R = Me) ,SnR' p-MeC6H4* N=C \ NR2 (34) The redistribution reaction involving exchange of ligands (one of which is R) has been extensively employed in the following systems Sn-Zn274, Sn-Hg264.275-277 Sn-B278 Sn-Al279 Sn-Tl277 Sn-Si280 Sn-Sn259* 281 Sn-pb272 Sn-P281 Sn-As281 Sn-Sb282 and Sn-MeC0.259 The objectives were either synthetic or mechanistic.A detailed examination of the Et,Sn-HgI system in 96% MeOH-4 % H20 led to the proposal of the following mechanism with AG*20-6kcal./mole AH* 11-7 kcal./mole and AS* -30 cal./deg./mole at 298°K.275 The transfer of cis-alkenyl groups in the Sn-Hg and Sn-T1 systems proceeds with retention of configuration. 77 Et,Sn + Hg12 rate-detg. -+EtHgI + Et,SnI + Et,Sn (solv.)' + Hg13-Organotin compounds having halogen especially F or C1 in a-,p- or y-positions readily undergo Sn-Hal elimination reactions. This principle has been used as a source of dihalogenocarbene~,~~~* 283 and hal~genodienes~~~ (e.g.,2,2-dichloro-l-trimethylstannylcyclopropane42-chlorobuta-1,3-diene); Me,SnC,Cl at 300" (4 hr.) afforded Me,SnCl but no evidence for perchloro- benzyne was Tin-carbon bond-breaking has been effected by insertion reaction of 274 D.Seyferth and T. F. Jula J. Amer. Chem SOC.,1968,90,2938. '"M. H. Abraham and T. R Spalding J. Chem SOC.(A) 1968,2530. 276 R M. G. Robertsand F. E. Kaissi J. Organometallic Chem. 1968,12 97. "'A. N. Nesmeyanov A. E. Borisov N. Y. Novkova and E. I. Fedin J. Organometallic Chem. 1968 15 279. 278 T. Chivers and B. David J. Organometallic Chem. 1968,13 177. 279 J. R.Horder and M. F. Lappert J. Chem. SOC. (A) 1968,1167. N. S. Vyazankin G. S. Kalinina and 0.A. Kruglaya Zhur. obshchei Khim. 1968,38,906. H. G. Kuivila R.Sommer and D. C. Greene J. Org. Chem. 1968,33,1119.E. A. Besolova V. L. Foss and I. F. Lutsenko Zhur. obshchei Khim. 1968,38,1574. '*' D. Seyferth B. Prokai and R.J. Cross J. Organometallic Chem.. 1968.13. 169. 318 M. F. Lappert J. D. Smith and D. R. M. Walton ketones284 (with R,SnCH,X; X = CN Ac CO,Et or CO,NEt,) S02,2369285 or S03.286Ozonolysis of Et,Sn gave acetaldehyde (Et,SnO) and a peroxidic tin compound.287 The relative ease of fission of various SnR bonds (aryl > alkyl > cycloalkyl) by reaction with halogens,288. 289 hydrogen halides,289 alcohols (C6C15 > Me),278 and aqueous alkali2'0 has been examined for mixed SnR'R' compounds. Lead. Crystal structures (X-ray crystallographic) on Ph,PbCl (polymeric) and Ph,Pb show a distorted tetrahedral arrangement around lead with a Pb-C bond of 2~15A.~~' The i.r.and Raman spectra and solution molecular weights of the halides Me,PbX and Me,PbX suggest that they are monomeric in a nonco-ordinating solvent but associated (and more so than Sn analogues) through halogen-bridges in the solid.292 The 'H n.m.r. spectra of a series of some neopentyl-lead compounds provide J[207Pb/'H(y_C)] .293 The com- pounds (Bu'C C),Pb are themally and hydrolytically more stable than many other lead alkynyls. Redistribution reactions (Ph-OAc but not R-OAc exchange) provide good methods for preparing PhPb(OAc) or Ph,Pb(OAc) (but not P~,P~OAC).~'~ Organolead alkoxides such as Ph,PbOMe have previously been shown to be susceptible to Pb-0 insertion reactions by unsaturated compounds; reactions of this type have been invoked to account for their catalysing the addition of methanol to Et0,CC i CC0,Et or CH :CHCN and the maleic + fumaric ester eq~ilibrium.~'~ The compounds Ph,M(C iC),Ph (n = 1 or 2)(M = Sn or Pb) preferentially cleave (Ph faster than Sn) their alkynyl groups in HClO,/EtOH (mono-acetylenes faster) and NaOH-EtOH (di-acetylenes faster).261 Pb-C fission has also been reported in the PbEt,- Et,MH (M = Si Ge or Sn)280 and the PbR,-I,296 systems.Arsenic Antimony and Bismuth. The crystal structure of triphenylbi~muth~'~ shows that the molecule is pyramidal with CBiC angles of 94"and a Bi-4 bond length of 2.24A. The compounds (MeO),SbPh and MeOSbPh have trigonal 284 J. G. Noltes H. M. J. C. Creemers and G. J. M. van der Kerk J. Organometallic Chem.1968 11,P21. 285 R C. Edmondson and M. J. Newlands Chem. Comm. 1968,1219. 286 H.Schmidbaur L. Sechser and M. Schmidt J. Organometallic Chem. 1968,15 77. 287 Y.A. Aleksandrov N. G. Sheyanov and V. A. Shushunov,Zhur. obshchei Khim. 1968,38,1352 288 L.N. Snegur and S. M. Manulkin Zhur. obshchei Khim. 1968,38 102; G. F.Rubinchik and Z M. Manulkin ibid. P841;Le-Quanh-Minh,Compt. rend (C) 1968,266,832;H. Zimmer A. Bayless and W. Christobel J. Organometallic Chem. 1968,14,222;H. Zimmer C. W. Blewett and A. Brakas Tetrahedron Letters 1968 1615; S. Boue M. Gielen and J. Nasielski ibid. p. 1047. 289 F. J. Bajer and H. W. Post J. Organometallic Chem. 1968;11 187. 290 R M. G. Roberts and F. E. Kaissi J. Organornetallic Chem. 1968,11 79. 291 M. Mammi V.Busetti and A. Del Praa Inorg. Chim Acta 1968,1,419; V.Busetti M. Mammi A. Signor and A. Del Pra ibid. p. 424. 292 R J. H. Clark A. G. Davies and R J. Puddephatt J. Amer. Chem SOC. 1968,90,6923. 293 G. Singh J. Organometallic Chem. 1968,11 133. 294 L.C. Willemsens and G. J. M. van der Kerk J. Organometallic Chem. 1968 13 357. 295 A G. Davies and R J. Puddephatt,J. Chem SOC.(C) 1968 317. 296 G. Pilloni and G. Tagliavini J. Organometallic Chem. 1968 11 557. 297 D. M. Hawley and G. Ferguson J. Chem Soc. (A) 1968,2059. Part (i) The Main Group Elements 319 bipyramidal molecules298 and a redetermination of the crystal structure of pentaphenylantim~ny~~~ confirms the square-pyramidal structure so far unique among the organometallic compounds of Group V.N.m.r. studies3" have shown the presence of mixed halides in mixtures of trimethylantimony halides Me,SbX, Me,SbY, and the two configurations of the diarsine MePhAsAsMePh (35) have been detected.,'l Among the more interesting organometallic compounds are an arsenic analogue (36) of a 1,3,5-substituted pyrrole made from phenylarsine and a dia~etylene,~" and trimethylarsinemethylene (37).303 This behaves like the phosphorus analogue (38) in its reactions with iodomethane and hydrogen halides. The Me,SiCH:AsMe + Me,SiOH + (Me,Si),O + Me,As:CH (37) // \e3p Me,AsEt+I-Me,As+I-Me,As + Me,P:CH (38) phosphorus compound (38) can be obtained from the arsinemethylene and trimethylposphine. The Group V elements are neatly distinguished by exchange reactions304 between the pentaphenyl derivatives and tritium-labelled phenyl- lithium.Withphosphorusandarseniccompounds,exchangeisslow and probably proceeds by a four-centre mechanism like other transmetallations.Penta- phenylantimony exchanges one group faster than the others showing the intermediate Li' [Ph*SbPh5]-. Pentaphenylbismuth with a larger central atom rapidly exchanges all five groups. Kei-Wei Shen W. E. McEwen S. J. LaPlaca W. C. Hamilton and A. P. Wolf J. Amer. Chem. SOC.,1968,90 1718; G. 0.Doak G. G. Long and L. D. Freedman J. Organometallic Chem. 1968 12 443. 299 A. L. Beauchamp M. J. Bennett and F. A. Cotton J. Amer. Chem SOC.,1968,90 6675. 300 C. G. Moreland M. H. O'Brien C. E. Douthit and G. G. Long Inorg. Chem. 1968,7 834.301 J. B. Lambert and G. F. Jackson J. Amer. Chem SOC. 1968,90 1350. 302 G. Markl and H. Hauptmann Tetrahedron Letters 1968,3257. 303 H. Schmidbaur and W. Tronich Inorg. Chem. 1968,7 168. 304 H. Daniel and J. Paetsch Chem. Ber. 1968,101,1451.

ISSN:0069-3030    

DOI:10.1039/OC9686500283

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

9 ORGANOMETALLIC COMPOUNDS. Part (ii) Transitional Elements By R.S. Coffey (Imperial Chemical Industries Limited Heavy Organic Chemicals Division Billingham Teesside) The preparation of novel complexes continues. Of particular interest is the first benzyne complex (1) obtained from Ni(C0)4 and o-di-iodobenzene’ ; reaction of the benzyne precursors benzenediazonium 2-carboxylate and 1,2,3- benzothiadiazole 1,l-dioxide with PtL,’ and PtL,(C2H4)3 gave the hetero- cycles (2) and (3) (L = PPh,) respectively. 0 II 0 II 0 (11 (2) (3) The remainder of this report is concerned with the reactions of the co- ordinated ligand,4 particularly when it has a bearing on the mechanism of homogeneous catalysis and the emphasis is on systems which contain some type of metakarbon bond.This precludes consideration of many ligands containing oxygen and nitrogen and only passing reference is given to poly- merisation. Numerous reviews on various aspects of homogeneous catalysis have and it is clear that several phenomena occur in all catalytic E. W. Gowling S.F. A. Kettle and G. M. Sharples Chem. Comm. 1968,21. T. L. Gilchrist F. J. Graveling and C. W. Rees,Chem. Comm. 1968 821. C. D. Cook and G. S.Jauhal J. her. Chem. SOC. 1968,90,1464. M. M. Jones ‘Ligand Reactivity and Catalysis’ Academic Press New York and London 1968. ‘Advances in Chemistry Series,’ vol. 70 American Chemical Society 1968. S. Card and R.Ugo Inorg. Chim. Acta 1967,1,49. ’ R. Ugo Co-ord. Chem. Rev. 1968,3 319. R.Mason Nature 1968,217,543.J. P. Collman Accounts Chem. Res. 1968,1 136. lo R. Cramer Accounts Chem. Res. 1968 1 186. L. Vaska Accounts Chem. Res. 1968,1,335. l2 G. N. Schrauzer Adu. Catalysis 1968,18,373. (a)J. Halpern Ref. 5 p. 1; (b) J. Halpern Chem. Eng. News,1966 Oct. 31,44,68. 322 R. S. Coffev reactions. The true catalyst must be co-ordinately unsaturated or substitution labile in order to combine with at least one of the substrates and the significance of the promotional energy of (n -1)d-electrons into np orbitals has been em- phasised.6* Co-ordination alters the properties of substrates and X-ray measurements show that the geometry of olefins dienes carbon disulphide oxygen etc. as ligands approaches that of their excited states.* Many sub- stances e.g.halogens hydrogen organic halides etc.,dissociate when they add to complexes and this results in a formal twoelectron oxidation of the metal; reversible two-electron oxidations are often key steps in catalysis.6* 9-11 It has '* been suggested that fractional oxidation between 0 and 2 occurs when sub- strates XY are added to IrClCO(PPh,) and this has been related to kinetic effects and the physical properties of the adduct IrClCO(PPh,)2XY.11 The activated ligands (if more than one the metal is regarded as a templatelo) must be reorganised and this takes place by either (a) a series of steps usually in- volving insertion reactions (see later) or (b) a concerted 'multicentre process' involving rearrangement of bonds of neutral ligands,' when the metal catalyst can remove symmetry restrictions as well as lower activation-energies.The relationship between chemisorption and low-valent co-ordination compounds which are virtually solvated metal-atoms has been emphasised.6 The zero-valent d'' complexes M(PPh,),.,, (M = Ni Pd and Pt) are the best models for this comparison. The platinum complexes form well-defined products Pt(PPh,),L from reaction with L (L = C2H4 02,CS, H,S and S). The sulphur adduct is very stable and gives rise to sulphur-type poisoning of catalyst s~rfaces.~ A comparison between organic reactions and reactions involving metal complexes is interesting.' Disproportionation pismutation or Metathesis) of 0lefm.-Although the heterogeneous reaction has been known for some years the new homogeneous analogue'4*l5 is remarkable.Addition of pent-2ene to a benzene solution of WC1,-AlC1,Et-EtOH (ratio 1:4 :1)at 20" gives a 1:2:1 mixture of but-2-ene pent-Zene and hex-3-ene in less than 1 min. The reaction is thermoneutral and entropy controlled and mixtures of olefins give the statistical distribution of products expected from a trans-alkylidenation the thermodynamic ratio of cis :trans isomers being formed. No double-bond migration occurs but traces of alkyl-benzenes are formed. The three-step mechanism which is suggested for the reactions involves (i) the formation of a cis-bisolefin tungsten complex (4) (valency and other ligands not specified) (ii) trans-akylidenation by a con- centrated process'2 via a quasicyclobutane intermediate (5) to give (6),followed by (iii) olefin exchange.Evidence for this mechanism comes from co-dispropor- tionation of but-2-ene and perde~teriobut-2-ene'~ and reaction of ethylene and [2-i4C]propene Over Re,07-A1,0 to give nonradioactive ethylene and radio- l4 (a) N. Calderon H. Yu Chen and K. W. Scott Tetrahedron Letters 1967 3327; (b)Chem. Eng. News 1967 Sept. 25,45 51. N. Calderon E. A. Ofstead J. P. Ward W. A. Judy and K. W. Scott J. Amer. Chem. SOC. 1968,90,4133. Part (ii) Transitional Elements 323 W (4) W W I W R'CH =HCR' ' active butene. A cyclobutadiene intermediate has been suggested for hetero- geneous catalysts'' and W0,-SO leads to disproportionation of acetylenes at 350".'* A less-active catalyst is obtained from WCl,-lithium alkyls and the inter- mediate it is suggested is ~is-bisolefin-WCl,.'~ Disproportionation and double- bond migration occurs with MC12(NO),L2 (M = W or Mo; L = Ph,P Ph,PO or pyridine) and various aluminium alkyls; thus hept-2-ene gives a mixture of C2-CI2 olefins.20 The WCI system promotes ring-opening polymerisation of cyclic olefins to stereoregular polymers2' (e.g.cyclo-octa-1,5-diene +poly-l,4-butadiene and 5-phenylcyclo-octene gives the terpolymer butadiene-ethylene-styrene). Ring-opening polymerisation by Ziegler-Natta catalysts less reactive tungsten- aluminium systems (see ref. 21) and WC16 MoCl and ReCl in CS2or CCl,, are well known and were thought to take place by scission of an allylic carbon- carbon bond. It now appears the WCI,-AlCl,Et/EtOH system goes by way of disproportionation and the products are large macro cycle^,^ (e.g.cyclo-octene gives inter aka a cyclic C3,HS4 hydrocarbon2,). Clearly some of the earlier conclusions in this field will have to be re-examined. Insertion Reactions.-These are important in some olefin isomerisations [equations (1) and (2)] polymerisations [equations (l) (6) and (2)] hydrogena- tion [equations (1) and (3)] and carbonylation [equations (l),(4) and (5)]. l6 J. C. Mol J. A. Moulijn and C. Boelhouwer Chem. Comm. 1968 633. l7 J. C. Mol J. A. Moulijn and C. Boelhouwer J. Catalysis 1968 11 87. F. Pennella R. L. Banks and G. C. Bailey Chem. Comm. 1968,1548. l9 J. Wang and M. R. Menapace J. Org. Chem. 1968,33,3794. *' E. A. Zuech Chem.Comm. 1968,1182. '' N. Calderon E. A. Ofstead and W. A. Judy J. Polymer Sci.,Part A-1 Polymer Chem. 1967 5 2209. 22 T. Oshika and H. Tabuchi Bull. Chem. SOC.Japan 1968,41,211. 23 Chem. Eng. News 1968 April 15,46 54. 24 E. Wasserman D. A. Ben-Efraim and R. Wolovsky J. Amer. Chem. SOC. 1968,90,3286. 324 R. S.Cofley LnMH + olefin $ LnMH (olefin) + LnMalkyl (1) LnMalkyl ... LnMH (olefin') ... LnMH + olefin' (2) LnMalkyl + H + LnMH + parafin (3) LnMalkyl + CO+ LnM alkyl(C0) +LnM acyl (4) LnMacyl + HX + LnMH + RCOX (X = H or RO etc.) (5) LnMalkyl + olefin + LnMalkyl (olefin) + LnM alkyl' (6) There are excellent reviews on the importance of these reactions in rhodium- catalysed isomerisation and dimerisation of olefins" and the catalytic reactions of cobalt carbonyl~.~~ (a) OEeJin isomerisation.It has proved difficult to detect the metal hydridein many isomerisations.'O The isomerisation of olefins and simultaneous con- version of RhDCO(PPh3)326 and RuDC~(PP~,),~' into the corresponding hydrides can be observed by n.m.r. spectroscopy. The deuterium-hydrogen exchange is more rapid than isomerisation due to preferential formation of n-alkyl-metal than isoalkyl-metal derivatives (see later). A similar but much slower exchange occurs betwen PtDCl(PEt,) and related platinum alkyk2* 1 / -DCl -'a J 2 2 (7) (81 'I 2 25 A. J. Chalk and J. F. Harrod Ad. Organometallic Chem. 1968,6 119. 26 D.Evans J. A. Osborne and G. Wilkinson J. Chem. SOC.(A),1968,3133.27 P.S.Hallman B. R. McGarvey and G. Wilkinson,J. Chem. SOC. (A),1968,3143. 28 J. Chatt R. S. Coffey,A. Gough and D. T. Thompson J. Chem. SOC.(A) 1968,190. Part (ii) Transitional Elements 325 Deuteriation of the organic ligand occurs when (7) is treated with DCl-EtOD to give (8) which is isolable. &-Addition to the ligand gives (9) and exchange occurs by loss of H to the ~olvent.’~ cis-Addition of HCO(CO)~ to 1,Zdiphenyl- cyclobutene occurs during its hydrogenation to cis-1,2-diphenylcyclobutane.30 A kinetic study of the isomerisation of 4-phenylbut-1-ene and its isomers by PdCl,(PhCN) shows that stepwise migration of the double-bond does not occur since the rate of rearrangement of the olefin ligand is faster than the rate of olefin exchange.,l The reverse is true for rhodium catalysis.” (b) Hydrogenation.Further studies on the catalyst RhClP, (lo) and its analogues support the accepted mechanism (Scheme 1; S is solvent).32 The phosphines are mutually cis in compounds(11H13) as are the olefin and hydride RhClP =RhClP,S =RhClH,P,S 4.fast (11) + paraffin RhClH,P,(olefin) -+ 113) SCHEME 1 ligands in compounds (12) and (13) formation of the latter being rate deter- mining. Formation of (1 1) in sitir from RhCl(cyc10-octene) and two equivalents of phosphine gives a more active ~atalyst.,~-~~. The order of activity is > PPh > P(p-XC,H,) > P(alkyl) (R is electron-donating and X is electron-attracting relative to H) which suggests that the aryl groups of the ligand are sinks which absorb positive or negative charge on the metal at key stages.’ An important suggestion based on n.m.r.studies is that complete dissociation of a phosphine [conversion of (10) into (1 l)] does not occur but one of the phosphine-rhodium bonds is longer than the other two and one phosphine can be regarded as being held in an outer co-ordination sphere;thismay be typical ofmany complexes whichact as catalystsin ~olution.~’ Although simultaneous addition of both hydrogens to the olefin in (13) has been proposed the transfer is stepwise viu a metal alkyl in sterically hindered olefins as extensive isomerisation and H-D exchange between olefin and deuterium occurs.36 Detailed studies on RhHCO(PPh,),37 and RuHCI(PP~,),~~ (the most 29 B.L. Shaw Chem. Comm. 1968,464. 30 W. L. Fichteman and M. Orchin J. Org. Chem. 1968.33 1281. 31 B. Cruickshank N. R Davies and A. D. DiMichiel Austral. J. Cheni. 1968,21 385. 32 S. Montelatici A. van der Ent J. A. Osborne and G. Wilkinson J. Chem. SOC.(A),1968 1054 and references therein. 33 R. Stern Y. Chevalier and L. Sajus Coniyt. rend. 1967,264 1740. 34 L. Homer M. Buche and H. Siegel Tetrahedron Letters 1968,4023. ’’ D. R. Eaton and S. R. Stuart J. Amer. Chem. SOC.,1968,90,4170. 36 J. F. Biellmann and J. M. Jung J. Amer. Chem. SOC.,1968,90,1673. 3’ C. O’Conner and G. Wilkinson. J. Chem. SOC.(A),1968. 2665. 326 R.S. Coffev active catalyst reported) show that these are specific catalysts for terminal ole-fins under ambient conditions.The rhodium complex dissociates to (14) in which the trans-phosphines ensure that the n-alkyl derivative (16) which has not been isolated but inferred from exchange reactions (see above) is formed via (15). Hydrogenolysis of (16)gives (14)and product. The catalyst RhCl(PMe- PhPr"), containing optically active phosphine hydrogenates some olefins to optically active products.38 Kinetic measurements show that initial dissociation of IrClCO(PPh,) occurs during the hydrogenation of maleic anhydride.39 The complexes Cr(arene) ( CO)340,[ Cr(n-C,H ,) (CO),]2,4 and cyclopentadiene complexes/ aluminium alkyls4 catalyse hydrogenation of dienes to monoenes but further studies on PtCl,(PR,),/SnCl show that it is specific for terminal olefins and not conjugated dienes as was first thought.43 Reduced systems of [Ni(CN),12- containing [Ni2(CN)6IL reduce dienes to monene~~~ and are similar to the Co(CN),-KCN-H system ; this latter system has been re~iewed.~' The inter- mediates proposed in homogeneous and heterogeneous catalysis are similar.46 (c) Carbonylation.A new hydroformylation process which uses phosphine- cobalt catalysts has been described.47 Key intermediates are [Co(CO),PBu,] [which has no bridging carbonyls but does contain a long Co-Co bond (2.66A)],48 HCo(CO),PBu, and the co-ordinately unsaturated HCo(CO),PBu,. The process operates at much lower pressures (ca. 30 atmos.) than the conventional nonliganded system (ca. 200 atmos.) due to stabilisation of the catalyst. In the presence of a ratio of H :CO of 2:1 alcohols instead of aldehydes are formed directly.The ratio of normal to branched-chain oxo-W. S. Knowles and M. J. Sabacky Chem. Cornrn. 1968,1445. 39 B. R. James and N. A. Menon Canad. J. Chem. 1968,46,217. 40 M. Cais E. N. Frankel and A. J. Rejoan Tetrahedon Letters 1968 1919; E. N. Frankel E. Slke and C. A. Glass J. Amer. Chem. SOC. 1968,90 2446. 41 A. Miyake and H. Konda Angew. Chem. Internat. Edn. 1968,7 663. 42 Y. Tajima and E. Kurioka J. Org. Chem. 1968,33 1689. 43 R. W. Adams G. E. Batley and J. C. Bailar jun. J. Amer. Chem. SOC. 1968,90 6051. 44 T. Mizuta H. Samejima and T. Kwan Bull. Chem.SOC.Japan 1968,41,727; W. H. Dennis jun. P. H. Rosenblatt R. R. Richmond G. A. Finseth and G. T. Davies Tetrahedron Letters 1968 1821. 45 M.G. Burnett P. J. Conolly and C. Kemball J. Chem. SOC.(A),1968,991 ; J. Kwiatek and J. K. Seyler ref. 5 p. 207. 46 G. C. Bond ref. 5 p. 25. 4' L. H. Slaugh and R. D. Mullineaux J. Organometallic Chem. 1968 13,469. 48 J. A. Ibers J. Organometallic Chem. 1968 14 423; R. F. Bryan and A. R. Hannay Chem. Comm. 1968 1316. Part (ii) Transitional Elements products is higher in the new system (e.g. 80-90 % n-aldehyde or alcohol from 1enes 2-ene~,4~ * 49-5 and even 4-enes51) than the nonligand one (60% n-aldehydes from lene). This is due to steric effects and the more hydridic nature of HCo(CO),PBu (PK cu. 7) than HCo(CO) (PK ca. 1)which favours the formation of n-alkyl and n-acyl cobalt intermediates [see equations (1j(6) p. 324].47 The effect of phosphines on rhodium hydroformylation which is much more rapid than the corresponding cobalt rea~tion,’~ is to prevent double-bond migration.Thus cis-but-2-ene gives a 1:1 ratio of n- :iso-valeraldehydes in the absence of phosphine but in its presence the ratio is 1 :99.” An interesting development which should have preparative uses is that RhHCO(PPh) is an active hydroformylation catalyst under ambient con- ditions whilst under carefully controlled conditions 95 % yields of n-aldehydes can be obtained.26 Extensive i.r. and n.m.r. studies on solutions of RhHCO(PPh) under atmospheres of H2-CO indicate that the active catalyst is (1 7). Reaction with olefin gives an acyl(l8) compound which reacts with H2-C0 to give (17) probably uia (19). The cis-arrangement of phosphines is favoured as RhHCO(PPh,) is more specific in its hydrogenation and H-D exchange reactions with olefins (see above).Carbonylation studies of (20) show that when L is 13C0 the MeMn(CO) + L + cis-MeCOMn(CO),L (20) (21) ‘insertion reaction’ is in fact a cis-migration of the methyl to a neighbouring carbonyl ligand.54 When L is PPh, the possibility of insertion rather than methyl migration cannot be excluded.55 Isomerisation of (21)to the trans-form goes readily and the isomers are difficult to ~haracterise.~~ It would seem that previous work in this field is very c0nfused.~’9 56 Palladium-catalysed carbonylation of olefins to saturated esters dienes to 49 (a)E. R. Tucci Ind.and Eng. Chem. (Product Res. and Development) 1968,7,32;(b)ibid.p. 125 ; (c)ibid. p. 227. ’O A. Hershman and J. H. Craddock Ind. and Eng. Chem. (Product Res. and Development) 1968 7,226. ” B. Fell W. Rupilius and F. Asinger Tetrahedron Letters 1968 3261. ” B. Heil and L. Markb Chem. Ber. 1968 101,2209. 53 D. Evans G. Yagupsky and G. Wilkinson J. Chem. SOC.(A),1968,2660. 54 K. Noack and F. Calderazzo J. Organometallic Chem. 1967,10 101. ’’ K. Noack and F. Calderazzo Inorg. Chem. 1968,7,345. 56 K. Noack J. Organometallic Chern. 1968. 12 181. 328 R. S. Coffey unsaturated esters and allyl compounds to unsaturated acid chlorides has been re~iewed.’~~ 58 A hydride mechanism has been proposed” [equations (l),(4) and (5); p. 3241. An alternative mechanism is nucleophilic attack of carbon monoxide on a polarised co-ordinated olefin as shown in Scheme 2.’ H R1 H ‘C’(6-) ‘c ’(&) R1 co RZoH Pd + R1CH2*CH,*C02R2 l- Pd ,-(6+) H”‘ H =O H SCHEME2.Carbonylation of mixturesof allyl halides and conjugated dienes give substituted C,-dienoic acids (See Scheme 3)’’ Reactions of allyl halides acetylenes carbon monoxide and various nickel catalysts containing thiourea ligands give R H C ‘pd /--\ /-c-\ CH :CR*CH,Cl- CH,’ Pd ‘CH,sCH2:CR-CH,CH,*CH’ Pd ’ CH 0‘ c1 H‘ C1 CO/HX CH :CR CH CH CH :CH -CH COX SCHEME 3 hexen-2,5-dienoic acid derivatives.60 (d) Decarbonylation. The decarbonylation of aldehydes and acid halides proceeds by the general mechanism shown in Scheme 4. For the catalytic decarbonylation of acid chlorides M + RCOX + RCOMX + RM(C0)X + M + CO + RX (22) (23) SCHDlE 4 by RhC1(PPh,),57-6’ or RhCl(CO)(PPh,) [M = RhCl(PPh,),; X = 57 J.Tsuji and K. Ohno ref. 5 155. 58 K. Bittler N. v. Kutepow D. Neubauer and H. Reis Angew. Chem. Znternat. Edn. 1968,7,329. 59 R. van Helden C. F. Kohll D. Medema G. Verberg and T. Jonkhoff Rec. Trav. chim. 1968 87,961. 6o G. P. Chiusoli M. Dubini M. Ferraris F. Guerrieri S.Merzoni and G. Mondelli J. Chem. SOC.(C),1968,2889;F. Guerrieri and G. P. Chiusoli J.Organornetallic Chem. 1968,15,209;F. Guemeri Chem. Comm. 1968,983. 61 J. Tsuji and K. Ohno J. Amer. Chem. SOC. 1968,90,99. Part (ii) Transitional Elements halogen]62 intermediates (22) and (23) have been isolated. The kinetics of the stoicheiometric decarbonylation of aldehydes by RhCl (PPh,) fits Scheme 4 [X = H; M = RhCl(PPh,),S; S = solvent or aldehyde] but no intermediates have been detected.63 For palladium metal-catalysed decarbonylations [Scheme 4 ;M = Pd ;X = H or halogen] (22) is thought to be an intermediate in the Rosenmund reduction.64 (e) Oligomerisation (i) Olefins.Many oligomerisations particularly of con-jugated dienes are more related to concerted reaction^.^^ Dimerisation of CD :CH CH :CD with FeEt bipyridyl gives mixture of [3,3,4,4,7,7,8,8-2H8]cyclo-octa- 1,5-diene and 4-[ PP-2H2]vinyl-[ 3,3,5,5,6,6-2H6]cyclohex-1-ene the formation of which favour a concerted mechanism.66 Dimerisation of buta-diene by Fe(CO),(NO) gives 4-vinylcyclohexene exclusively whilst isoprene gives 1,4-dimet h yl-4-vin ylc yclohexene and 1,5-dimeth yl-5-vin ylc yclo hexene.Co-oligomerisation of 1,3-dienes and acetylenes by Ni(PPh,) give 1,2,4,5- tetrasu bstituted benzene.6 Butadiene and its derivatives are dimerised to octa-1,3,7-trienes by PdC1269 and other complexes of palladium and In the presence of phenol alcohols primary and secondary amines and carboxylic acids the major products are 1- or 3-substituted octa-2,7-dienes together with octa- triene.69-73. In CH,OD deuterium is incorporated on carbon-6 (24) and the suggested intermediates (25) and (26) are identical to those in cyclodimerisa- tion of butadiene.71 Electrolysis of nickel salts in the presence of butadiene gives trans,trans,trans-n-hexadeca-l,6,10,14-tetraene.74 Co-dimerisation of conjugated dienes and olefins to give 1.4-dienes is 62 J. Blum H. Rosenman and E. D. Bergmann J. Org. Chem. 1968,33 1928. 63 M. C. Baird C. J. Nyman and G. Wilkinson J. Chem. SOC.(A),1968,348. 64 J. Tsuji and K. Ohno J. Amer. Chem. SOC. 1968,90,94. 65 G. Wilke Angew. Chem. Internat. Edn. 1966,5 151. A. Yamamoto K. Morifuji S.Ikeda T. Saito Y. Uchida and A. Misono J. Amer. Chem. SOC. 1968,90 1878. " J. P. Candlin and W. H. Janes J. Chem. SOC.(C) 1968 1856. P. Heimbach and R. Schimpf Angew. Chem. Internat. Edn. 1968,7 727. 69 E. J. Smutny J. Amer. Chem. SOC.,1967,89 6793. 'O S. Takahashi T. Shibano N. Hagihara Tetrahedron Letters 1967,2451. S. Takahashi,T. Shibano and N. Hagihara Bull. Chem. SOC.Japan 1968,41,254.'' S. Takahashi T. Shibano and N. Hagihara &ll. Chem. SOC.Japan 1968,41,254. 73 P. Heimbach Angew. Chem. Internat. Edn. 1968,7,882. 74 N. Yamazaki and S. Murai. Chem. Comm.. 1968 147. 3 30 R. S.Cofley catalysed by rhodium,1° iron,75 cobalt,76 and complexes and goes by way of insertion reactions (Scheme 5).’0977 Product analysis of the $4- dienes obtained from nickel-catalysed co-dimerisation of ethylene and various CH CH -MH 66 M-1 -M CH,:CH*CH; CH:CHCH \‘CH AH, I SCHJME 5. 1,3-dienes indicates that the Ni-H adds 1,2 to the diene and not 1,4 in the first stage of the reaction. This catalyst causes skeletal isomerisation uiu the cyclo- propane (27).78 Insertion into an allyl group may go uiu the o-ally1 intermediate as suggested for the polymerisation of methyl methacrylate by Cr(allyl),.The kinetics the incorporation of one allyl group into the p~lymer,’~ and the reaction of allyl compounds with esters and olefins” indicate Scheme 6. The interchange of o and n-ally1 groups has been extensively studied.81 SCHEME 6. ” M. Iwamoto Bull. Chem. SOC.Japan 1968,41,2188. G. Hata and D. Aoki J. Org. Chem. 1967,32 3754. 76 M. Iwamoto and S. Yuguchi Bull. Chem SOC.Japan 1968 41 150; M. Iwamoto K. Tani H. Igaki and S. Yuguchi J. Org. Chem. 1967,32,4148; M. Iwamoto and S. Yuguchi Chem. Comm. 1968 28. 77 R. G. Miller T. J. Kealy and A. L. Barney J. Amer. Chem. SOC. 1967,89,3756. 78 R. G. Miller J. Amer. Chem. SOC.,1967,89,2785 ;R. G. Miller and P. A. Pinke J. Amer. Chem.SOC. 1968,90,4500. 79 D. G.H. Ballard and T. Medinger J. Chem. SOC.(B) 1968,1176. D. G. H. Ballard W. H. Janes and T. Medinger. J. Chem. SOC.(4,1968 1168. J. Powell and B. L. Shaw J. Chem. SOC.(A) 1967 1839; 1968 583. Part (ii) Transitional Elements 33 1 The conversion of acrylonitrile into adiponitrile 1,4-dicyanobutene and propionitrile is catalysed by many ruthenium complexes in the presence of These hydr~gen.~~-~~ products are accounted for by a hydride-alkyl mechanism8 but the products obtained under deuterium are consistent with formation of the ruthenium vinylidene intermediates (28) and (29). Decomposi- tion of (29) gives 1,4dicyanobut-lene and deuteriolysis of (29) Ru(CH :CH*CN) +HRu*CH:cH.CN +CN*CH,*CH,Ru*CH:CH*CN (28) (29) gives both deuteriated propionitrile and a~rylonitrile.~~ (ii) Acetylenes.Cyclotrimerisation of CH C :C* CD by Cr(C,H,),(tetra- hydrofuran) gives a mixture of isomeric trimethyltris(trideuteriomethy1)-benzenes the composition of which indicates that a concerted mechanism (three bound acetylenes rearrange to benzene) occurs rather than formation of an intermediate cyclobutadiene chromium species.85 However PdCl trimerisation of but-2-yne goes oia addition of PdC12 across the triple-bond followed by insertion of two acetylenes to give (30) which has been isolated and which upon being heated gives hexamethylbenzene.86 As trimerisation of Me \ ,Me Me Me C "\ c1 -+ PdCl A. Misono M. Hidai Y. Uchida and I. Inomata Chem. Cornm. 1968 704.83 J. D. McClure R. Owyang and L. H. Slaugh J. Organometallic Chem. 1968 12 P8. 84 E. Billig C. B. Strow and R. L. Pruett Chem. Comm. 1968 1307. G. M. Whitesides and W. J. Ehmann J. Amer. Chem. SOC. 1968,90,804. 86 H. Reinheimer H. Dietl J. Moffat D. Wolff and P. M. Maitlis J. Amer. Chem. SOC. 1968,90 5321. 332 R. S. Coffey RCiC C1-M-111 -Ic PPh R PPh R (31) (32) phenylmethylacetylene by PdCl gives some 1,2,3-trimethyl-4,5,6-triphenyl-benzene an acetylene bond must be broken.87 The complexes (31) and (32) (M = Rh or Ir; R = carboxylic ester) are isolable intermediates in the tri- merisation of disubstituted acetylenesa8 The polymerisation of phenyl-acetylene by RhCl(PPh,) gives polymers and tr~ns-1,4-diphenylbutenyne,~’~ 90 while HCiC*CR,*OH gives similar dimer~.~~ These reactions go by way of oxidative addition of mono-substituted acetylenes to (11) to give RhH(C=CR)Cl(PPh,),(HC...CR) followed by insertion eka9 (0 Nitrogen. As interest in nitrogen fixation increases so does interest in nitrogen complexes many of which can be prepared from molecular nitrogen. The M-N=N group in the best known complexes IrClN,(PPh,), CoHN,(PPh,), and [M(NH3)5N,]2+ (M = Ru or 0s) has not yet been converted into ammonia in spite of a previous claim.” Treatment of many transition-metal compoun 4s with strong reducing agents and nitrogen give products which give ammonia on hydrolysi~.’~ Reaction of (7c-C,H,)2TiCl with phenyl-lithium under nitrogen. and subsequent hydrolysis gives ammonia and aniline the latter from a TiN :NPh group formed by nitrogen insertion into TiC6H5.93 (g) Carbon Dioxide.The reaction of C02 with HCoN,(PPh,) gives the formate HCO OCO(PP~,),.’~ (h) Sulphur dioxide. The insertion of SO2 into o-metal-carbon bonds to give MSO,R is well known but treatment of MnCH,*CiCH(CO) with SO gives MnSO 0 CH :C :CH2(CO)5 because the linearity of the acetylene makes it more susceptible to attack by oxygen than by sulphur in the inter- mediate MnCH * C i CH(CO),(S02).95 Ethylene and sulphur dioxide are catalytically converted to ethyl vinyl sulphone and butenyl ethyl sulphone by PdCl by way of a series of insertion reactions.96 ’’ H. Diet1 and P. Maitlis Chem. Comm. 1968 481. ’’ J. P.Collman J. W. Kang W.F. Little and M. F. Sullivan Inorg. Chem. 1968,7 1298. H. Singer and G.Wilkinson J. Chem. SOC.(A) 1968,849. R.J. Kern Chem. Comm. 1968 706. 91 J. Chatt. R. L. Richards J. E. Fergusson and J. L. Love Chern. Comm. 1968. 1522. 92 G. Henrici-Olive and S. Olive Angew. Chem. 1968,80 398;E. E. van Tamelen and B. Aber-mark J. Amer. Chem. SOC. 1968 90,4492; M. E.Vol’pin M. A. Ilatovskaya L. V.Kosyakova and V. B. Shur Chem. Comm. 1968. 1014 and Doklady AkaL Nauk. S.S.S.R. 1968,180 103. 93 M. E. Vol’pin V. B. Shur R. V. Kudryavtsev. and L. A. Prodayko Cheni. Conini. 1968 1038. 94 A. Yamamoto and S. Ikeda J. Amer. Chem. SOC.,1968,90 3896;A. Misono Y. Uchida M. Hidai and T. Kuse Chem. Cornm. 1968,981. 95 J. E. Thomasson and A. Wojcicki J. Amer. Chem. SOC.,1968,90,2709.96 H.S.Klein Chem. Comm. 1968,377. Part (ii) Transitional Elements 333 Some Aspects of Oxidation.-Known radical processes are in the main omitted here as they are dealt with elsewhere. (a) Activation of oxygen. Several low-valent complexes react with oxygen to give adducts e.g. (33) which gives the sulphate (34) carbonate nitrate and nitrite with SO, C02 NO, and NO respectively (see also p. 322)7 and the adduct (35) with acetone.” Compound (33) also catalyses the oxidation of isocyanides to isocyanates and phosphines to phosphine oxides. The kinetics of the latter reaction indicate that (36) is formed slowly and decomposes / PPh /< ,.o PPh + 0, 0-(33) -Ph3P-PtI.__ i -kh3P -Pt (OP Ph3) -(33) + ZPhSPO] \\ ‘0 (37) rapidly to (37).98 Suggestions that (33) and related peroxides derived from RhCl(PPh,), are important intermediates in hydrocarbon autoxidation appears wrong as autoxidation of cyclohexene catalysed by RhCl(PPh,), rhodium salts and cobalt salts give identical products by free-radical processes.99 (b) Epoxidation.A detailed kinetic study shows that epoxidation of cyclo- hexene with t-butyl hydroperoxide catalysed by VOacac goes via a five valent vanadium species which co-ordinates with the peroxide (38) and causes heterolytic splitting of the oxygen-oxygen bond. No radicals are involved and the metal does not co-ordinate with the olefin. t-Butyl alcohol formed in the reaction poisons it and catalysts for this oxidation must exchange ligands readily. O0 But But (-Vy-0 [HOQ1-.i.v Lo/But + I + / + o( ‘0-H ‘H 97 R. Ugo,F. Conti S. Celini R. Mason and G. B. Robertson Chem. Comm. 1968 1498. J. P. Birk J. Halpern and A. L. Pickard J. Amer. Chem. SOC.,1968,90,4491. 99 V. P. Kurkov J. Z. Parky and J. B. Lavigne J. Amer. Chem. Soc. 1968,90,4743. loo E. S. Gould R. R. Hiatt and K. C. Irwin J. Amer. Chem. SOC.,1968,90,4573. 334 R. S. Coffey as with Pb(Ac), and uia electrophilic species such as (39) and (40).'02 CH,=C /O\ Mn(OAc) or CH,=C ,OMn(OAc) \/0 'OMn(OAc) (39) (d) Palladium oxidations. The reactions of palladium with unsaturated compounds has been comprehensively revie~ed.'~ The mechanism of the Hoechst-Wacker oxidation of ethylene to acetaldehyde in aqueous media by Pd2+-Cu2+-02 is in the main well understood two key features are (i) formation of a -complex [H20C12Pd-CH2-CH,*OH] -,and (ii) dissociation of this to acetaldehyde during which process hydrogen is transferred from the C-H bond of the a-carbon (attached to the OH group) to the P-carbon.The mechanism of similar oxidations in nonaqueous solvents are less well understo~d.'~~-'~~ In acetic acid a bewildering number of products are formed according to the conditions. Ethylene can give high yields of vinyl acetate together with ethylidene diacetate and by analogy it is assumed o-complexes are present. There is evidence for them in the palladium-catalysed conversion of vinyl chloride into vinyl acetate,lo6 when trans-CHD :CHCl is converted to a 1:1 mixture of cis- and trans-CHD CH*O*COMe.107 This indicates formation of [Cl,Pd CHD * CHCl(0 OCMe)] -followed by elimina- tion of [PdClJ-.Vinyl acetate is formed by a similar nucleophilic displace- ment of hydrogen by acetate ion." A systematic variation of temperature and chloride acetate and copper(r1) concentrations in Pd(OAc) oxidation of hex-l-enelo4 shows that in the absence of copper hexenyl 1-and 2-acetates are the major products. The ratio of 1-acetate :2 acetate increases with an increase in acetate and chloride and in the presence of chloride increases further with temperature. The products are similar in the presence of copper@) if acetate:chloride > 1-25 but at the ratio 0.15 1,2-diacetoxyhexane comprises 80 % of the product. A similar oxidation of but-l-ene gives 1,2- 1-3- and 1,4dia~etoxybutanes.'~~ The simplest ex- planation is insertion of olefin into the palladium acetate bond (41) followed by either (i) elimination of PdH (ii) nucleophilic displacement of palladium by acetate or (iii) a similar displacement after isomerisation and consequent movement of palladium along the carbon chain (Scheme 7).*01 E. I. Heiba R. M. Dessau and W. J. Koehl J. Amer. Chem. SOC. 1968 90 5905. lo2 J. B. Bush jun. H. Finkbeiner J. Amer. Chem. SOC. 1968,90,5905. lo3 E. W. Stern Catalysis Rev. 1967,1 73-152. lo4 R. G. Schultz and D. E. Gross ref. 5 p. 97. P. M. Henry ref. 5 p. 126. lo6 C. F. Kohl1 and R. van Helden Rec. Trau. chim. 1968,87,481. lo' H. C. Volger Rec. Trav. chim. 1968,87 501. Part (ii) Transitional Elements AcOPd OAc R'CH CH :C(OAc)R2 R1CH2*CH*CHR27R'CH,*CH(OAc)* CH(OAc)R2 (iih (41) SCHEME 7 The gross differences in the copper systems is thought to be due to species such as (42) where the bridging ligand [CuC1,I2- affects the electronic proper- ties of the pal1adi~m.l~~ 1-2 Hydrogen shifts may also occur to give LxPd*CH(OAc)Me which gives vinyl acetate on elimination of LxPdH and ethylidene diacetate on nucleophilic displacement.Intramolecular nucleo- philic displacement of palladium in (41) may give (43) which would readily RCH = CH( ,Cl ,,Cl + Pd Cu Me CO 01\Cl/ \Cl give diacetates and finally it is possible that non-ligand acetate may attack the co-ordinated olefin directly (see below).lo4 Ethylene palladium complexes react with alcohols to give vinyl ethers and acetals probably by a similar mechanism.Higher olefins and alcohols are less reactive in this reaction. lo* Miscellaneous Palladium Complexes.-There are few stable compounds containing o-palladium-carbon bonds. However they can be prepared in situ from merc~ry,'~~~ 'lo tin and lead,' lo organometallic compounds [equation (7)] and used in various syntheses. Most of these go by way of insertion into RHgCl + PdCl -* HgCl + RPdCl (7) the Pd-R bond followed by elimination of PdClX where X can be hydrogen or a functional group. Some reactions for ArPdCl are shown in Scheme 8. Some reactions can be made catalytic on palladium with an oxidising agent e.g. Cu2+,Fe3+etc. but chlorinated products are often formed under these oxidising conditions.' lo Stable cyclic complexes of palladium are formed by substitution of hydrogen in aromatic rings when the aromatics contain benzylic nitrogen.Thus PdCl lo' A. D. Ketley and L. P. Fisher J. Organometallic Chem. 1968,13,243. lo9 P. M. Henry Tetrahedron Letters 1968,2285. R. F. Heck J. Amer. Chem. SOC.,1968,90,5518; ibid. pp. 5526,5531,5535,5538,5542. and 5546. 336 R. S. Coffev ArCH2CH2.CH0 I-\ CH :CH*CH,OH + /Ar-Ar ArCH :CH*CH,OH + ArCH,CH:CH ArPdCl 3-ArCOAr + ArCOCl SCHEME 8 and dimethylbenzylamines,' ' ' azobenzenes,' ' and 2-phenylpyridines' ' give (44),(43 and (46) respectively. Addition of PdCl across the acetylene group of RCi C*CR,* NMe gives (47).' l4 The reaction of palladium salts with aromatic hydrocarbons has been studied in detail and appears to go 2 R / apd N 'C1 2 / 2 by way of palladation (analogous to mercuration) the palladium aryl reacting further to give biaryls and an isolable monovalent palladium complex.' ' Evidence for direct attack of nucleophiles on co-ordinated olefin is the reaction of (48) with alkoxides,' l6 and sodio-derivatives of malonic esters,' '' and A.C. Cope and E. C. Friedrich J. Amer. Chem. SOC.,1968,90,909. R. F. Heck J. Amer. Chem. SOC.,1968,90,313. 'I3 A. Kasahara Bull. Chem. SOC.Japan 1968,41 1272. 'l4 T. Yukawa and S. Tsutsumi Znorg. Chem. 1968,7 1458. J. M. Davidson and C. Triggs J. Chem. SOC.(A) 1968 1324. '16 M. Green and R. I.Hancock J. Chem. SOC.(A),1967,2054. H. Takahashi and J. Tsuji J. Amer. Chem. SOC. 1968.90.2387. Part (ii) Transitional Elements 337 B-diketones118 to give (49) [B = OMe CH(CO,R), and CH(COR) re-2 MiscellaneousCompounds.-Intramolecular metallation of aryl rings occur in several aryl phosphine complexes of other metals. These may be regarded as oxidative additions of the C-H bond in the ortho-position in the aryl ring to the metal. Thus IrCl(PPh,) gives IrC1H(o-C6H4PPh,)PPh3,’ l9 Fe(C2H4)(Ph2PC2H4PPh,) gives F~H(O-C,H,P(P~)C~H~PP~~)(P~~PC~H~PP~,),~~~ RhCH,(PPh,) gives Rh(o-C,H,PPh,)(PPh,) ’ (methane is eliminated) and FeH,(N,)(PEt,Ph) gives F~H(o-C,H~PE~,)N~(PE~,P~),.~~~ In the complexes RuHC~(PP~,),,~’ RhHCO(PPh3)3,37 and COHN~(PP~,),”~ no metal-carbon complexes are isolated but the hydrogen at the ortho-position of the ligand exchanges with deuterium showing that transient hydrido-aryls are formed.Nucleophilic substitution into systems containing dienes can be achieved by the sequence complex formation with iron carbonyl (50) proton abstraction by Ph3CBF to (51) followed by reaction with nucleophiles (e.g. NaOMe NaCN H,O Pdiketones and lithium alkyls etc.) to give (52). A variety of products are formed with substituted cyclohexadienes. 124 B. F. G. Johnson J. Lewis and M. S. Subramaniam J. Chem. SOC.(A),1968 1993. ‘19 M.A. Bennett and D. L. Milner Chem. Comm. 1967,581. G. Hater H. Kondo and A. Miyake J. Amer. Chem. SOC. 1968,90,2278. lZ1 W. Keim J. Organometallic Chem.1968,14 179. lt2 A. Sacco and M. Aresta Chem. Comm. 1968,1223. 12’ G. Parshall J. Amer. Chem. SOC.,1968,90 1669. It4 A. 3. Binch P. E. Cross J. Lewis D. A. White and S. B. Wild J. Chem. SOC.(A). 1968. 332

ISSN:0069-3030    

DOI:10.1039/OC9686500321

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

10 AROMATIC COMPOUNDS By H. Heaney (Department of Chemistry University of Technology Loughborough Leicestershire) General.-Nuclear magnetic resonance spectroscopy continues to be used as a major probe in the chemistry of aromatic compounds. Hindered rotation has been reported for a number of compounds particularly where bulky substituents were present for example in the 2- and 6-positions in a benzene derivative with respect to the hindered function. The variable temperature n.m.r. spectrum of (1)is interesting since at -40" the aromatic protons resonate as an ABC system with additional coupling (IJI 0.5 Hz) between the proton A and the dichloromethyl proton D. This spectrum is consistent with a locked conformation as shown resulting in stereospecific five bond spin-spin coup-ling.2 The application of the intramolecular nuclear Overhauser effect can provide useful information in overcrowded molecules.Signal enhancement has been observed for the protons at positions 5 and 10 in 1,2,3,4-tetramethyl- phenanthrene. The first resolutions of ketones where the asymmetry is ascribed to restricted rotation about the carbonyl group have been described. The half-life of a chloroform solution of (2) was calculated4 as 6.2 min. at 20.5". Unfortunately no racemization data which would confirm the resolution of (3) were pre- ~ented.~ The energy barrier to rotation of (4) has been shown to be ca. 17.7 kcal. mol.-' by n.m.r.6 Several contradictory examples of n.m.r. and polari- metric rate constants have been pointed and comparison of results obtained should be made with caution.Disastereomeric platinum complexes of (5) have been isolated by use of optically active ct-methylbenzylamine.8 However decomposition of the partially resolved complexes at 0" gave inactive olefin. The low temperature n.m.r. spectrum of (5) has shown that the trans-olefinic bond can rotate easily with respect to the remainder of the molecule ' T. H. Siddall and W. E. Stewart Tetrahedron Letters 1968 5011; E. A. Chandross and C. F. Sheley J. Amer. Chem. SOC. 1968 90 4345; T.H. Siddall and W. E. Stewart Chem. Comm. 1968 1116; J. F. K. Wilshire Tetrahedron Letters 1968 475; A. Mannschreck and H. Muensch ibid. 1968 3227; H. P. Fischer ibid. 1968 4289; H. Kessler A. Rieker and W. Rundel Chem.Comm. 1968,475; H. Kessler and W. Rundel Chem. Ber. 1968,101 3350. T. Schaefer R. Schwenk C. J. Macdonald and W. F. Reynolds Canad. J. Chem. 1968,46,2187. R. H. Martin and J. C. Nouls Tetrahedron Letters 1968 2727. A. G. Pinkus. J. I. Riggs and S. M. Broughton J. Amer. Chem. SOC. 1968,90 5043. ' K. V. Narayanan R. Selvarajan and S. Swaminathan J. Chem SOC.(C) 1968 540. D. Lauer and H. A. Staab Tetrahedron Letters 1968 177. ' R. E. Carter and P. Berntsson Acta Chem. Scand. 1968,22,1047. A. C. Cope and B. A. Pawson. J. Amer. Chem. SOC.. 1968.90. 636. 340 H. Heaney C Me 02N Me Me COzH But But (4) at room temperature.' The first example of a planar chiral compound (6) with known absolute configuration has been reported. ' New evidence obtained from U.V.and n.m.r. spectra leads to the conclusion that the compounds previously supposed to be ethane derivatives which are obtained from alkyl substituted diarylmethyl radicals and from triaryl- methyl radicals are in fact two series of dimers." Those di- tetra- and penta- phenylethanes which do not readily afford radicals are true ethanes while those which in solution are in .equilibrium with their free radicals are 2-methylenecyclohexa- 1,4diene derivatives e.g. (7). Me Me -Me Me Me Me It has been found previously'2 that rneta-couplings in a series of unsym- metrical paradisubstituted benzenes follow an additive relation. Further G. M. Whitesides B. A. Pawson and A. C. Cope J. Amer. Chem. SOC.,1968,90 639. lo H. Falk and K.Schlogl Angew. Chem. Znternat. Edn. 1968,7 383. l1 H. Lankamp W. Th. Nauta and C. MacLean Tetrahedron Letters 1968 249. l2 B. Dischler Z. Narurforsch. 1965.20a. 888. Aromatic Compounds 34 1 results for disubstituted benzenes have been reported13 and the values if used with caution may prove useful in the interpretation and analysis of n.m.r. spectra. The protonation of halogenobenzenes has been studied by n.m.r. spectro- scopy in hydrofluoric acid and antimony pentafl~oride.'~ Homoaromaticity. Developments of the researches reported in the three previous years include evidence for the existence of the bishomocyclopenta- dienyl system15 (8) derived from (9); and for benzo- and dibenzo-homotropy- lium cations. N.m.r. evidence supports the MO and SCF-MO prediction that benzocyclo-octatetraene should exist as the benzohomotropylium ion in sulphuric acid.Experiments with the [2H,]-compound (10) show that the ion has structure (11) and that it is not deprotonated in 98 % deuteriosulphuric acid.16 Although the gain in delocalisation energy involved in forming the D D & \ OH /Br (13) (14) homotropylium species should be considerably reduced by the presence of benzo-groups evidence has been produced which supports the existence of dibenzohomotropylium ions. The alcohol (12) gives the ion (13) with fluoro- sulphonic acid," and the bromide (14) gives (15) with antimony pentafluoride in sulphur dioxide. * l3 J. M. Read R. W. Crecely R. S. Butler J. E. Loemker and J. H.Goldstein Tetrahedron Letters 1968 1215. l4 G. A. Olah and T. E. Kiovsky J. Amer. Chem. SOC. 1967,89 5692; D. M. Brouwer Rec. Trau. chim. 1968,87,335 ;342. J. B. Grutzner and S. Winstein J. Amer. Chem. SOC. 1968,90,6562. l6 W. Merk and R. Pettit J. Amer. Chem. SOC. 1968,90,814. l7 R. F. Childs and S. Winstein J. Amer. Chem. SOC. 1967,89,6348. G. D. Mateescu C. D. Nenitzescu and G. A. Olah J. Arner. Chern. Soc.. 1968.90.6235. 342 H. Heaney Antiaromaticity. The anti-aromatic character of small rings with 4n n-electrons (n = 1)has been re~iewed.’~ Proton exchange in the optically active cyclopropenes (16) (R= Bz or CN) is accompanied by more racemisation than in the cyclopropane series and this has been interpreted on the basis of the antiaromatic interaction between the carbanion and the double bond.’O The increased tendency for racemisation cannot be due to a stabilising inter- action in the flattened anion since the exchange rate is much decreased in the cyclopropenes.Attempts to generate diphenylcyclopropenylidene have met with limited success possibly for similar reasons. Base-induced decomposition of the NN-dimethyl carbamate (17) with potassium t-butoxide in the presence of dimethyl fumarate gave the adduct (18) with undefined stereochemistry.2’ Benzene and Derivatives.-A number of reviews have appeared. The topics covered include the electronic structure in aromatic compounds,22 the reverse Diels-Alder reaction,’ electrochemical oxidation^,'^ the reduction of nitro- and nitroso-compounds by tervalent phosphorus reagent,” the interaction of aromatic nitro-compounds with bases,26 the photolysis of iodoaromatic compound~,~’ cyclisation reactions of 2,2’-disubstituted biphenyls,2 * and classical and modern syntheses of polyphenyls.29 The mass spectrum of l4C-labe1led benzene has been studied by means of normal mass spectrometry and by use of an autoradiographic technique.30 This method should have several advantages over the 3C-labelling method and be generally applicable. New information has been presented on the electron-impact fragmentation of salicylic acid by use of [carboxy-14C]-material.31 Several new syntheses of arenes with organometallic catalysts have been l9 R. Breslow Angew. Chem. Internat. Edn. 1968,7,565. ’O R.Breslow and M. Douek J. Amer. Chem. SOC.,1968,90,2698. W. M. Jones M. E. Stowe E. E. Wells and E. W. Lester J. Amer. Chem. SOC. 1968,90 1849. ’’ E. Clementi Chem. Rev. 1968,68,341. ” H. Kwart and K. King Chem. Rev. 1968,68,415. 24 N. L. Weinberg and H. R. Weinberg Chem. Rev. 1968,68,449. ” J. I. G. Cadogan Quart. Rev. 1968,22,222. ” E. Buncel A. R. Norris and K. E. Russell Quart. Rev. 1968,22 123. ” R. K. Sharma and N. Kharasch Angew. Chem. Internat. Edn. 1968,7,36. ’’ R. E. Buntrock and E. C. Taylor Chem. Rev. 1968,68,209. 29 W. Ried and D. Freitag Angew. Chem. Internat. Edn. 1968,7,835. ’O H. Knoppel and W. Beyrich Tetrahedron Letters 1968,291. ’’ J.L. Occolowitz. Chem. Comm.. 1968. 1226. Aromatic Compounds 343 reported.The reaction of alkyl-or aryl-substituted acetylenes with norborna-diene gives benzene derivatives in which four ring atoms are derived from the acetylene. Phenylacetylene,for example gives m-terphenyl.32 1,2-Disubstituted cis-4,5-divinylcyclohexeneswhich are obtainable indirectly from reactions of acetylenes with buta-1,3-diene may be isomerised almost quantitatively to 1,2,4,5-tetrasubstitutedbenzenes by potassium t-butoxide in dimethyl sul-phoxide.33 A complex cyclotrimerisation of methylphenylacetylene occurs in benzene in the presence of dichlorobis(benzonitri1e)palladium at 25". As well as the expected 1,2,4-trimethyl-and 1,3,5-trimethyl-triphenylbenzenes a small amount of 1,2,3-trimethyltriphenylbenzene was obtained.34 Controversy centres around the possible importance of metal-cyclobutadiene complexes.However deuterium-labellingexperimentshave provided clear evidence which excludes a chromium-tetramethylcyclobutadiene complex in the cyclo-trimerisation of [1-'H3] but-2-yne by triphenyltris(tetrahydrofuran)chromium-(111). The base-catalysed trimerisation of methyl neopentyl ketone affords a useful method of synthesis of 1,3,5-trineopentylbenzene,by use of sodium hydride a 45 % yield was obtained.36 Similarly 1,3,5-tri-t-butylbenzenewas obtained in 30% yield. The pyrylium salt (19) gives the nitro-compound (20) R Ph Ph 0 O'C02H Me NO; Br CHiC02H (19) (20) (21) (22) in 75 % yield with nitromethane and base.37A new synthesis of diary1 ethers is exemplified by the reaction of 4-bromoisophorone with silver perchlorate in benzene 3,4,5-trimethylphenol (38 %) and di-3,4,5-trimethylphenyl ether (37%) were ~btained.~' It was suggested that the hemiacetal (21) is probably the key intermediate.Cyclohexenones of the general type (22) aromatise at about 200" in the presence of pyridine hydrochloride.39The key intermediate in the transformation is probably (23). N-Nitropyridinium tetrafluoroborate may be prepared from nitronium tetrafluor~borate,~~ and homologuesare effective as transfer nitrating agents4' 32 A. Carbonaro A. Greco and G. Dall'Asta Tetrahedron Letters 1968 5129. 33 P. Heimbach and R. Schimpf Angew. Chem. Internat. Edn. 1968,7,727. 34 H. Diet1 and P. M. Maitlis Chem. Comm. 1968,481. 35 G. M. Whitesides and W.J. Ehmann J. Amer. Chem. SOC.,1968,90 805. P. Martinson and J. Marton Acta Chem. Scand. 1968,22 2382. 37 K. Dimroth K. Vogel and W. Krafft Chem. Ber. 1968 101,2215. M. Meislich A. Speet and M. Blejwas Tetrahedron Letters 1968,409. 39 G.Palazzo and L. Baiocchi Tetrahedron Letters 1968,4739. 40 G. A. Olah J. A. Olah and N. A. Overchuk J. Org. Chem. 1965,30 3373. 41 C. A. Cupas and R. L. Pearson. J. Amer. Chem. Soc.. 1968,90,4742. 344 H. Heaney Thus although (24; R = H) does not nitrate toluene at 25" the salt derived from a-picoline (24; R = Me) nitrates toluene quantitatively at room tem- perature. Other salts were reported to be effective and the possibility exists 0 for the design of reagents of varied reactivity. Full details have been reported for aromatic nitrations which proceed with a rate-limiting proton-transfer step.42 The rates of nitrosation of phenol and [4-2H]phenol in aqueous perchloric acid are not inconsistent with reaction involving the nitrosonium ion.43 Reactions involving fluorine atoms and fluoride ions are well known.Reactions involving electrophilic fluorine have now been reported. Suitably activated aromatic rings have been fluorinated by use of fluoroxytrifluoro- methane at low temperature^.^^ Oestrone methyl ether affords lop-fluoro-19- norandrosta-l,4-diene-3,17-dione (25). Salicylic acid yields mainly 5-flUOrO- salicyclic acid together with the 3-isomer and N-acetyl-P-naphthylamine gives N-acetyl-a-fluoro-p-naphthylamine. The Friedel-Crafts acetylation of phenylcyclopropane has been re-investigated.Both the previously reported major product p-cyclopropylacetophenone,45and the minor product arise from electrophilic attack on the benzene ring.46 The Friedel-Crafts cyclo-alkylation of primary phenylalkyl chlorides has been studied by use of deuterium labelling. The results show that extensive deuterium scrambling between carbon atoms 1 2 and 3 accompanies cyclisation of for example 1-chloro-5- phenyl-[2-2H2]pentane but not l-chlor0-5-pheny1-[4-~H~]pentane.~~ Hydro-gen-bridged ions of the type (26) and (27) were suggested to explain the results. Another exception to the failure of nitrobenzene to undergo the Friedel- Crafts reaction has been reported ; N-(m-nitrobenzy1)-imidesare obtained from reactions of nitrobenzene with N-chloromethyl-imides in the presence of aluminium chloride.48 42 P.C. Myhre M. Ben& and L. L. James J. Amer. Chem. SOC. 1968 90 2105 see also Ann. Reports 1966 63 292 396. 43 B. C. Challis and A. J. Lawson Chem. Comm. 1968,818. 44 D. H. R. Barton A. K. Ganguly R. H. Hesse S. N. Loo and M. M.Pechet Chem. Comm. 1968 806. 45 H. Hart and G. Levitt J Org. Chem. 1959,24 1261. 46 H. Hart R. H. Schlosberg and R. K. Murray J. Org. Chem. 1968,33,3800. 47 L. R. C. Barclay and E. C. Sanford Canad. J. Chem. 1968,46,3315. 48 S. Sisido. H. Tnada. and T. Tsada. Tetrahedron Letters. 1968. 5267. Aromatic Compounds 0- +\ P-N-O The photolysis of 2,4-dinitrobenzenesulphenyl derivatives of carboxylic acids in benzene solutions has been previously shown to liberate the carboxylic acid and simultaneously form the 2,4-dinitrodiphenyl sulphide in good yield.49 The reaction may not necessarily involve a free sulphenium ion ;decomposition of (28) or analogue could explain the observed produ~ts.’~ A similar thermal rearrangement of aryl 2-nitrobenzenesulphenateshas also been reported for ROO-S& R\ 6‘6 R (29) (30) (31) example (29; R = Me) gives (30; R = Me).On the other hand (29; R = H) in anisole gives mainly (31; R =OMe). These results have been interpreted to indicate that heterolysis of the sulphur-oxygen bond takes place to give an ion pair and that the sulphenium ion acts as the electrophile.” A new method of introducing a t-butyl groups2 depends on the conversion of the cyanomethyl group uia the key intermediate (32).The alkylation of phenols including hindered compounds such as 2,6-di-t- butylphenol has been carried out in dimethyl sulphoxide. Transmethylation from the solvent was not detected in tride~teriomethylation.’~ The t-butyl group does however in many reactions sufficiently block reactive functions with the result that reaction courses change. The reaction of 4-nitro-2.,6di-t- butylphenol with diazomethane yields the nitronic ester exclusively whereas 2,6-di-isopropyl-4-nitrophenolgives a mixture of the nitronic ester and the isomeric anisole. 54 The reaction probably proceeds by proton abstraction by the diazomethane and in the ambident anion e.g. (33) the less hindered site displaces nitrogen from the methanediazonium ion.The bromination of 49 D. H. R. Barton Y. L. Chow A. Cox and G. W. Kirby J. Chem. SOC.,1965,3571. 50 D. H. R. Barton T. Nakano and P. G. Sammes J. Chem. SOC.(C) 1968,322. D. R.Hogg J. H. Smith and P. W. Vipond J. Chem. SOC.(C) 1968,2713. 52 N. M. Waldron J. Chem. SOC.(C) 1968 1914. 53 R. G. Gillis. Tetrahedron Letters 1968. 1413. 54 J. S. Meek. J. S. Fowler. P.A. Monroe. and T. J. Clark. J. Oig. Chern.. 1968. 33. 223. 346 H. Heaney 0 0-Me Arm(!-(: I 0 I ‘s/ Me Nt -0’ 0- (33) (34) 1,3,5-tri-t-butylbenzenein acetic acid with bromine and silver salts or with preformed acetyl hypobromite gives 2,4,6-tri-butylbromobenzene 3,5-di-t-butylbromobenzene and variable amounts of 3,5-di-t-butylphenyl acetate and 2,4,6-tri-t-butylphenyl acetate.” The acetoxylation reactions probably arise by an addition-elimination mechanism for example the loss of t-butyl bromide from (34) yielding (35).The methylation of N-methyl-2,4,6-tri-butyl-aniline with methyl iodide at 100”and 5000-5500 atmos. yields NN-dimethyl-2,4-di-t-butylaniline and i~obutene.’~ The intramolecular alkylation of an ortho-t-butyl group has been observed with the alcohol (36; R = H) and the ether (36; R = Me).57The hydrocarbon (37) was isolated from the alcohol by reaction with thionyl chloride,and from the ether in 85 % yield with hydrio-dic acid. When aryl azides are thermally decomposed under pressure of carbon monoxide aryl isocyanates are produced.’ This reaction may proceed oia the nitrene.A detailed investigation of the reaction of 2-nitrophenyl aryl sulphides with triethyl phosphite,” in which phenothiazine derivatives are produced has now revealed that a rearrangement is involved.60 Attack at the electron-rich 1’-positionto form a five-memberedintermediate is indicated and this may well involve a nitrene as shown (38). Large upfield shifts are observed in the ethylenic proton resonances of maleic anhydride and certain derivatives when the spectra are determined in benzene.61 These shifts were discussed in terms of a 1:1 em-stereospecific ” P. C. Myhre G. S. Owen and L. L. James J. Amer. Chem. SOC. 1968,90,2115. 56 Y. Okamoto and H. Shimizu Tetrahedron Letters 1968 2751. 57 L. R. C. Barclay and M. C.MacDonald Tetrahedron Letters 1968 881. R. P. Bennett and W. B. Hardy J. Amer. Chem. SOC. 1968,90,3296. 59 cf Ann. Reports 1966,63 467. 6o J. I. G. Cadogan S. Kulik and M. J. Todd Chem. Comm. 1968,736. 61 D. Bryce-Smith and M. A. Hems. J. Chem. SOC.(B). 1968. 812. Aromatic Compounds 347 association of the solute with the solvent and its relation to the corresponding photoaddition reaction. The 2 :1 photoadducts of dienophiles with benzene are well known and have been shown to proceed by successive 1,2-and 1,4-aCN PhNH -S0,Me (thermal) additions. The photochemical adduct (39) has now been isolated from a reaction at 0" and this gives 2 1 or 1 :1:1 adducts thermally.62 The thermal decomposition of methanesulphonyl azide in benzene was shown to give N-methylsulphonylaniline (40).63However when the reaction was repeated in the presence of tetracyanoethylene the adduct (41) was These results suggest an equilibrium between (42) (43) and (44).Although hexafluorobenzene has been used as an inert solvent for certain reactions of carbenes the isolation of adducts involving the solvent indicates that the general use of hexafluorobenzene should be avoided.6s A number of cyclo-pentadienylidene adducts with arenes have been reported e.g. (45H48).66 Ph''&c1 \ -Ph \ E E Me Ph a P h ' Ph Ph c1 Ph Me Ph (45) (46) (47) (48) Although polyfluorobenzenes are known to undergo nucleophilic substitu- tion with methoxide in methanol the exchange reaction for example with tritiated methanol is much faster.No change in base concentration was 62 B. E. Job and J. D. Littlehailes,J. Chem. SOC.(C) 1968 886. 63 R.A. Abramovitch J. Roy,and V. Uma Canad. J. Chem. 1965,43 3407. 64 R.A. Abramovitch and V. Uma Chem. Comm. 1968 797. 65 D.M.Gale J. Org. Chem. 1968 33 2536; M.Jones ibid. p. 2538. 66 H. Diirr and G. Scheppers Angew. Chem. Internat. Edn. 1968,7,731;Tetrahedron Letters 1968 6059. 348 H. Heaney detected even when complete isotopic equilibration has been reached.67 The von Richter reaction68 of p-chloronitrobenzene with potassium cyanide in dimethyl sulphoxide takes a novel course.69 Under anhydrous conditions three products were isolated (49). The question of aryl participation in solvolytic reactions has been the subject of recent contr~versy.~' The results obtained in the formolysis of a series of 2-arylethyl tosylates,' where nucleophilic assistance by the solvent would be expected to be less important than in acet~lyses,~~ have been interpreted as involving an ion formulated as (50) or (51).The acid-catalysed addition of nucleophiles to the spirocyclopropene (52) results in aromatisation for example trifluoroacetate yields (53),possibly involving (54)' H R' I c10"" II \ -(49) (50) a; R' = CN,R2= H b; R' = CO-NH,,R2= H C R' = R2= CO-NH 0 Base-catalysed isomerisations and disproportionations in oligohalogeno- arenes have been rationalised previously74 in terms of a sequence of nucleo-philic displacements on halogen in which an aryl anion displaces another aryl anion.Surprisingly 1,2,4-tri-iodobenzene did not isomerise to any marked extent. Results have now been rep~rted'~ which show that [2-1311]-l,2,4-tri- 67 A. Streitwieser J. A. Hudson and F. Mares J. Amer. Chem. SOC., 1968,90 648. '* J. F. Bunnett Quart. Rev. 1958,12 1. 69 G. T. Rogers and T. L. V. Ulbricht Tetrahedron Letters 1968 1029. 70 A. Streitwieser Solvolytic Displacement Reactions McGraw-Hill New York 1962; H.C. Brown K. J. Morgan and F. J. Chloupek J. Amer. Chem. SOC. 1965,87,2137. '' M. D. Bentley and M. J. S. Dewar J. Amer. Chem. SOC.,1968,90 1075. 72 L. Eberson J. P. Petrovich R. Baird D. Dyckes and S. Winstein J. Amer. Chem. SOC. 1965 87 3504. 73 W. H. Pirkle D. Chamot and W. A. Day J. Org. Chem. 1968,33 2152. "C.E. Moyer and J. F. Bunnett J. Amer. Chem. SOC.,1963,85 1891. '5 J. F. Bunnett and D. J. McLennan J. Amer. Chem. SOC. 1968.90. 2190. Aromatic Compounds 0 Ph (55) (56) iodobenzene undergoes iodine scrambling. The reaction of sodiodipivaloyl-methane with triphenylmethyl chloride demonstrates the reaction of an ambient electrophile with an ambient nucleophile. 76 The alkylated product (55) was isolated in almost 90% yield and 0-alkylation accounted for the majority of the product other than (55). With the less hindered sodioacetylace- tone the normal product was obtained. Internal nucleophilic participation has been reported in the solvolysis of o-nitrobenzhydryl bromide which occurs much more rapidly than with the para-isomer. The product in aqueous acetone is o-nitrosobenzophenone which probably arises by proton loss from (56).77 Meisenheimer complexes such as (57) have been postulated previously and have now been shown to be the exclusive product of the reactions of ethers such as (58) with methoxide ion.78 The formation of the kinetically controlled product (59) in the reaction of methoxide ion with 2-cyano-4,6-dinitroanisole has been observed by n.m.r.spectroscopy. The stable spectrum is due to the ion (60).79Meisenheimer-type complexes have also been observed between so; l6 H. E. Zaugg. R. J. Michaels. and E. J. Baker. J. Amrr. Chern. Soc.. 1968. 90.3800. 77 A. D. Mease M. J. Strauss I. Horrnan L. J. Andrews and R. M. Keefer. J. Amer. Cheni. SOC. 1968.90. 1797. 78 E.J. Fendler. J. H. Fendler. W. E. Bryne and C. E. Griffin. J. Org. Chem.. 1968 33. 4141. 79 E. J. Fendler and C. E. Griffin Tetrahedron Letters 1968 5631. 350 H. Heaney aromatic and aliphatic nitro-compounds in basic media. go A new rearrangement of the Smiles reaction type has been observed in which the sequence (61) - (64) was suggested. Several examples of cyclisations involving 2,2'-disub- stituted biphenyls have been reported g2 Thus (66; X = NMe,) is formed from (65; X = NMe, Y = NH,) on heating with acid and nitrite;g2d diazonium salts of the type involved have been isolatedg2" (65; e.g. X = I Y = N,). (65) (66) (67) The rearrangement of for example (67) to (68) proceeds through a ten- membered transition state and is the first example of a [5,5]sigmatropic rearrangement.g3 Also [1,2]- ]3,3]- and [3,4]-sigmatropic rearrangements have been described.g4 yy-Dimethylallyl phenyl ether gives a mixture of 4-(yy-dimethylallyl)phenol,2-(ap-dimethylally')phenol(69)+(73) and 2,2,3-trimethyl- coumaran thermall~.~' The ratio of products is solvent dependent.The thermal rearrangement of 2,6-dimethylphenyl prop-2-ynyl ether takes an extremely C. A. Fyfe Canad. J. Chem. 1968,46,3047. '' K. G. Kleb Angew. Chem. Internat. Edn. 1968,7,291. 82 (a)D. M. Collington D. H. Hey and C. W. Rees J. Chem. SOC.(C) 1968 1030; (b)D. W. Allen and I. T. Millar J. Chem. SOC.(C) 1968 2406; (c) B. G. Pring and N. E. Stjernstrom Acta Chem. Scand. 1968,22 538; (4D. Hellwinkel and H. Seifert Chem. Comm. 1968 1683; (e)H.Heaney and P. Lees Tetrahedron 1968,24 3717; (f) R. D. Chambers and D. J. Spring J. Chem. SOC. (C) 1968 2395; (9)R. Filler and A. E. Ferbig Chem. Comm. 1968 606. 83 Gy Frater and H. Schmid Helu. Chim. Acta 1968 51 190. 84 H.-J. Hansen B. Sutter and H. Schmid Helu. Chim. Acta 1968 51 828. 85 F. Scheinmann. R. Barner. and H. Schmid. Helt.. Chim. Acta. 1968,51. 1603. Aromatic Compounds Me interesting course to (74) via (75).86 An attempted para-thio-Claisen rearrange- ment of ally1 2,6-dimethylphenyl sulphide gives a mixture of cyclic products rationalised in terms of a series of migrations based on different conformational forms of (76).87 Rearrangements of dienones e.g. (77) have been shown to yield for example (78) and pentamethylphenol.The spontaneous dienone- phenol rearrangement of (79) to (80) occurs in 76% yield in dilute alkaline solution. *’ (77) m 0 0 OH (79) (80) Benzene isomers. Calculations have been made whch indicate the degree of strain in various benzene valence isomers.g0 The vacuum U.V. photolysis of liquid benzene yields only three volatile products Dewar benzene benzvalene and fulvene.” Although the photolytic conversion of substituted benzenes into the corresponding Dewar benzenes is well known thisg1 was the first report of the direct photoconversion of benzene into Dewar benzene. The ready availability of hexamethyl Dewar benzeneg2 has again led to a number of (81) R = H or COzMe (82) (83) 86 J. Zsindely and H. Schmid Helv. Chim. Acta 1968,51 1510.H. Kwart and M. H. Cohen Chem. Comm. 1968 1296. B. Miller Chem. Comm. 1968 1435. 89 K. H. Bell Tetrahedron Letters 1968 3979. M. RandiC and Z. Majerski J. Chem. SOC.(B),1968 1289. ’* H. R. Ward and J. S. Wishnok. J. Amer. Chem. Sac.. 1968.90. 1085. 352 H.Heaney papers describing its reactions. The copper-salt-catalysed reactions of methyl diaz~acetate~~ lead to the formation of tri- and tetra-cyclic and dia~omethane~~ systems (81) and (82). Some electrophilic additions to hexamethyl Dewar benzene result in rearrangements. Reaction with dry hydrogen chloride or hydrogen bromide in methylene chloride gave (85; X = C1 or Br) probably as indicated via (83) and (84).95The betaine (86) is implicated in the reaction of 0 0 chlorosulphonyl isocyanate which leads to (87).96The 1 :1 adduct of hexamethyl Dewar benzene and ethyl N-sulphonylcarbamate has the structure (88).g7The acid-catalysed rearrangements of hexamethylprismane gives the homofulvene (89) and then the cyclopentadiene (90).98The related homofulvenes (91) were reported to be thermally stable but they do aromatise to (92) on photolysis.IR CD3 (93) (94) 92 Ann. Reports 1967,64,281. 93 H. Prinzbach and E. Druckrey Tetrahedron Letters 1968,4285. 94 E. Muller and H. Kessler Tetrahedron Letters 1968 3037. 95 L. A. Paquette and G. R. Krow Tetrahedron Letters 1968,2139. 96 L. A. Paquette Tetrahedron Letters 1968,2133. '' G. M. Atkins and E. M.Burgess J. Amer. Chem. SOC.,1968,90,4744. 98 R. Criegee and H. Griiner Angew.Chem. Internat. Edn.. 1968.7. 1651. Aromatic Compounds However (93) which cannot aromatise rearranges thermally to (94) and the labelling experiment (indicated) shows that only the C(l)-C(6) bond breaks during the pyrolysis.99 Cyclophanes. New synthetic methods have been published which should make these interesting systems more readily available. The solvolysis of (95) with boiling pyridine lead to [2,2]paracyclonaphthane (96)as the only product yH,O T s . (95) (96) in 90 % yield.looThe n.m.r. and U.V. spectra were essentially identical with those of the known anti-isomer which has been prepared previously by a Hofmann reaction in 3 % yield."' [2,2]Paracyclophane was prepared in 40 % yield from the analogous ditosylate (mixed isomers).loO A new method applicable to the synthesis of metacyclophanes and metaparacyclophanes utilises the inter- mediate (97).lo2When (97) is treated with two equivalents of n-butyl-lithium and the dianion reacts with p-xylylene dibromide [2,2]metaparacyclophane (98) may be isolated in 36% yield.(97) Chemical and spectral evidence indicates the presence of strong trans- annular electronic effects in [2,2]paracyclophane and its derivatives. A study of the electrophilic substitution in monosubstituted [2,2]paracyclophanes has confirmed the presence of the transannular effect.lo3 The data for the bromina- tion of a number of substituted [2,2]paracyclophane derivatives (99) except (99; X = CN) indicates that the major product reflects attack pseudo-gem to the most basic position or substituent of the substituted ring.The suggests 99 H. Hart and J. D. De Vrieze Chem. Comm. 1968,1651. loo G.W. Brown and F. Sondheimer J. Amer. Chem. SOC.,1967,89 7116. lo' D. J. Cram C. K. Dalton and G. R. Know J. Amer. Chem. SOC. 1963,85 1088. lo2 T. Hylton and V. Boekelheide J. Amer. Chem. SOC.,1968,90 6887. '03 H. J. Reich and D. J. Cram. J. Amer. Cheni. Soc.. 1968.90. 1365. 354 H. Heaney that the productdetermining step is proton transfer to an acceptor site on the originally substituted ring as shown for (100) -,(103). (100) (101) (102) (103) Evidence for.the distortion of the benzene rings in [2,2]metacyclophane has been reported lo4 and high temperature n.m.r. measurements indicates that for example (104; R = Me) is conformationally rigid.lo5 Optical resolution @ \ Me / has been achieved for (104; R = Me or C0,H).The methyl groups (Me’) in (105) are as expected strongly shielded in the n.m.r. spectrum. Io6 The bridged metacyclophane (106) has been prepared but no evidence was found for its reorganisation to a [14Tjannulene valence isomer. lo7 Benzynes. No slackening of interest in aryne chemistry has been detected and only about a half of the papers published can be discussed. A transition metal complex (107) was obtained from the reaction of o-di-iodobenzene with tetracarbonylnickel. lo* No reactions of the complex were reported except that with water. Nickel@) and iodide ions were identified but the organic material was not.The n.m.r. spectrum of a solution in deuteriomethanol was R. Flammang H. P. Figeys and R. H. Martin Tetrahedron 1 68,24 1171. lo’ T. Sato S. Akabori M. Kaniosho and K. Hata Bull. Chem. SOC.Japan 1968,41 218. lo6 T.Sato S. Akabori S. Muto and K. Hata Tetrahedron 1968,24,5557. lo7 H.B.Renfroe J. Amer. Chem. SOC. 1968,90,2194. lo’ E. W.Gowling. S. F. A. Kettle and G. M. Sharples Chem. Comm. 1968,21. Aromatic Compounds 355 reported. It will be of interest to regenerate benzynes from complexes since the reactions may well be significantly different from those of the free benzynes. Other attempts to form benzyne-platinum complexes analogous to acetylene- platinum complexes have failed at the stage of generation of benzyne. Complexes of the type (108) and (109) were i~olated.''~ Platinum complexes of cyclo- octyne have been 'lo The complex (109) is a stable benzyne precursor which gives benzyne on irradiation.'Ogb Titanium aryne complexes may be involved in the formation of rn-and p-toluidine when molecular nitrogen reacts with o-tolyl-lithium and dicyclopentadienyltitanium dichloride.' ' ' Calculations on benzynes and other dehydroconjugated molecules have been reported,' l2 and they will undoubtedly stimulate further experimental work. o-Benzyne is a ground state singlet species. 'p:h3 ,PPh3 Go QJ&+Ph3 N+N co; 0" PPh, 0 (108) (109) (110) (1 11) (112) (113) The mechanism of the formation of benzyne from benzenediazonium-2- carboxylate has been studied '' in acetonitrile-water-furan mixtures and in acetonitrile-methanol-furan mixtures.The results indicate a stepwise forma- tion of benzyne from benzenediazonium-2-carboxylateand it was suggested that (110)-(113) are involved. The rate of dimerisation of gaseous benzyne has been measured by the flash photolysis of phthalic anhydride and the reported half-life' l4 is in accord with previous measurements. A reinvestigation of the pyrolysis of phthalic anhydride in the presence of benzene and hexa- deuteriobenzene has been published. 'l5 The major benzyne reaction was shown F Li -(114) '09 (a)C. D. Cook and G. S. Jauhal J. Anier. Chem. SOC. 1968 90,1464;(b)T. L. Gilchrist F. J. Graveling and C. W. Rees Chem. Comm. 1968 821. 'I0 G. Wittig and P. Fritze Annalen 1968,712 79.' '' M. E. Volpin V. B. Shur R. V. Kudryavtsev and L. A. Prodayko Chem. Cornm. 1968 1038. (a) R. Hoffmann A. Imamura and W. J. Hehre J. Amer. Chem. SOC 1968,90 1499;(b)T. Yonezawa H. Konishi and H. Kato Bull. Chem. SOC.Japan 1968,41 1031. '13 R. Gompper G. Seybold and B. Schmolke Angew. Chem Internat. Edn. 1968,7,389. 'I4 G.Porter and J. I. Steinfeld J. Chem SOC.(A),1968 877. L. Friedman and D. F. Lindow. J. Arner. Chem. SOC.,1968.90.2329. 356 H. Heaney to involve 1,4-cycloaddition. Full papers have appeared on the 1,4-cycloaddi- tion reactions of tetrahalogenobenzynes with arenes. 'l6 Surprisingly tetra- -FpJyJ; F/ \ \ chlorobenzyne shows a greater selectivity than tetrafluorobenzyne in reactions with arenes. As expected the isomeric trifluorobenzynes generated from organo- lithium compounds show a reduced electrophilicity in arene cycloadditions.' '' The preparation of (115) from 2,2'-dilithio-octafluorobiphenylhas been des- cribed.' This product (115)apparently arises by the route previously hinted ' ' involving (114).The reactions of benzyne with bicyclobutane,12' have now been studied by use of endo-2deuteriobicyclobutane,and the products have been shown to arise by attack on the underside of the molecule.'21 Several reports of studies of the non-concerted112a 1,2-cycloadditions of benzyne have been reported including reactions with cis-and trans-propenyl ethers and acetates'22 and cis-and trans-di~hloroethylene.'~~ No analogy between the lack of stereo- specificity in these reactions and the spin states of carbenes should be drawn.A synthesis of hemi-Dewar naphthalene (117)has been achieved by dechlorination of (116) obtained from the reaction of benzyne with cis-3,4-dichlorocyclobu-tene. '24 Reactions of benzyne with acetylenic ethers (e.g. 1-ethoxypropyne) give rise to products including those in whch allenes formed initially react J. P. N. Brewer I. F. Eckhard H. Heaney and B. A. Marples J. Chem. SOC. (C) 1968 664; H. Heaney and J. M. Jablonski J. Chem. SOC.(C) 1968 1895. '17 R. Harrison and H. Heaney J. Chem. SOC.(C) 1968 889. 11* S. C. Cohen D. Moore R. Price and A G. Massey J. Organometallic Chem. 1968 12 37. G. Wittig Suomen Kem. 1956,29A 283. lZo Ann. Reports 1966,63,403. M. Pomerantz G. W. Gruber and R.N. Wilke J. Amer. Chem. SOC. 1968,90,5040. lz2 I. Tabushi and R. Oda Tetrahedron Letters 1968 3743; H. H. Wasserman A. J. Solodar and L. S. Keller ibid. 1968 5597; L. Friedman R. J. Osiewicz and P. W. Rabideay ibid. 1968 5735. M. Jones and R. H. Levine Tetrahedron Letters 1968 5593. R. N. McDonald and D. G. Frickey J. Amer. Chem. SOC. 1968,90,5315. Aromatic Compounds further to give for example (118) and (119).'25 Reactions with ynamines have also been reported. '26 In the reaction of benzyne with cyclo-octatetraene it was suggested that a benzo[ lolannulene may be in~olved.'~' The major products isolated were 9-phenyl-9,lO-dihydrophenanthreneand (120). The aprotic diazotisation of 2,5-di-t-butylaniline in the presence of furan leads to the adduct (121) derived from 3,6-di-t-but~lbenzyne?~* A compression effect was detected in the acid- catalysed ring opening of (121) and a similar effect in the related naphthalene (122) was shown by the intramolecular nuclear Overhauser effect.+ + t (121) (1 13'1 Reactions of benzyene with diallyl sulphides have been studied in order to generate ylides as models for terpenoid biosynthe~is.'~~ Thus for example benzyne gives (124) and (125) in a ratio of 19 1 on reaction with (123). Tetra- fluorobenzyne cleaves tetrahydrothiophen to 2,3,4,5-tetrafluorophenylvinyl sulphide.130 Ethers such as diethyl ether are thought to give betaines of the type (126) with benzyne but no products derived from their breakdown have been reported.'31 Cleavage products have been isolated with tetrachloro- ben~yne.'~' In an attempt to form a spirocyclic dienone the aryne (127) was generated. However the only products obtained were derived from the formal transfer of hydride to the aryne as shown. 32 The reaction of benzyne with N-phenylsydnone affordes (128),' 33 and reac- tions of tetrachlorobenzyne with ap-unsaturated aldehydes give 2H-chromens probably involving o-quinone methides (129).' 34 (123) (124) H. H. Wasserman and J. M. Fernandez J. Amer. Chem. SOC.,1968,90,5322. lZ6 J. Ficini and A. Krief Tetrahedron Letters 1968,4143. E. Vedejs Tetrahedron Letters 1968 2633. R. W. Franck and K. Yanagi J. Org. Chem. 1968,33,811; J. Amer. Chem. SOC., 1968,90,5814. G. M. Blackburn W. D. Ollis J.D. Plackett C. Smith and I. 0.Sutherland Chem. Comm. 1968 186; G. M. Blackburn and W. D. Ollis ibid. p. 1261. 130 J. P. N. Brewer H. Heaney and J. M. Jablonski Tetrahedron Letters 1968 4455. G. Wittig Angew. Chem. 1957 69 245. 132 E. J. Forbes and C. J. Gray Tetrahedron 1968,24,6223. 133 H. Gotthardt R. Huisgen and R. Knorr Chem. Ber. 1968,101 1056. 13* H. Heaney and J. M. Jablonski. Cheni. Comm.. 1968. 1139. 358 H. Heaney c1 (1 27) The need for caution when interpreting reactions which may be thought to involve arynes is indicated by the careful study of the amination of 2-bromo-thiophen by amide ion in liquid ammonia.13’ This reaction turns out not to involve an aryne. Non-benzene Systems.-Three-and four-membered rings. In an attempt to prepare tetraphenyltriafulvene by the reaction of diphenylmethyl-lithium with 3,3-dichloro-1,2-diphenylcyclopropene,tetraphenylbutenyne (131) was iso-lated.136 The proposed mechanism involves attack of the organolithium Ph i PhCGC -C ph H Yh Ph *C-Ph I Ph Ph Ph (130) (131) (132) CH,CHO CH CHO c1c4 C1 CHO CHO 13’ M. G. Reinecke and H. W. Adickes J. Amer. Chem. SOC.,1968,90,511. 136 G. Melloni and J. Ciabattoni Chem. Comm. 1968 1505. Aromatic Compounds 359 compound on the carbon-carbon double bond of the diphenylcyclopropene followed by the spontaneous dehydrochlorination of the intermediate (130) as shown. The transition-metal-catalysed isomerisation of 1,2,3-triphenyl-cyclopropene to 1,2-diphenylindene (132) was achieved during an attempt to displace ethylene from di-p-chloro-dichlorobis(ethylene)diplatinum(rI).'37 The ozonolysis of 1,6,7,7-tetrachloro-cis-bicyclo[4,l,O]hept-3-enefollowed by an oxidative work up and removal of peroxides with sodium hydrogen sulphite gave some 2,3,4-trichlorobenzaldehyde.The dialdehyde (133) was found to give the benzaldehyde in good yield on treatment with sodium hydrogen sulphate probably involving (134) and (135).13* The photolysis of the tropylium ion in 5% sulphuric acid gives rise to the [3,2,0]-valence-bond isomer (Dewar tropylium ion) (136) isolated as (137) and (1 38).39 H H H Et 0,C clll+ph c1 -i--c1 Et,N ]i-NEt2 CO,Et (139) ( 140) A relatively stable cyclobutadiene (139) has been prepared14' as a result of theoretical predictions that suitably positioned electron-releasing and electron- withdrawing groups should increase the stability of the parent ring system.Tetrachlorocyclobutadiene is similarly stabilized,14' and in the absence of trapping agents a dimer is formed while in the presence of styrene the adduct (140) is formed. Cyclobutadiene-(.n-cyclopentadieny1)cobalt(141) has been prepared and undergoes electrophilic substitution in the four-membered ring.142 The expected stabilization of the .n-allylcobalt complex (142) can be 13' J. A. Walker and M. Orchin Chem. Comm. 1968 1239. 138 J. K. Hecht Tetrahedron Letters 1968 3505. E. E. van Tamelen T. M. Cole R. Greenley and H. Schumacher J. Amer. Chem. Soc. 1968,90 1372. R. Gompper and G.Seybold Angew. Chem. Znternat. Edn. 1968,7,824. 14' K. V. Scherer and T. J. Meyers J. Amer. Chem. Soc. 1968,90,6253. '42 R. G. Amiet and R. Pettit. J. .4iwr. Cheni. SOC..1968. 90.1060 360 H.Heaney Ph G P h ph@o co 0 PhCH / OR -0 OH Ph reasonably associated with the high reactivity of the cyclobutadiene ligand. Phenylcyclobutenedione reacts with dibenzyl ketone in the presence of sodium methoxide and potassium hydroxide to give the hydroquinones (144; R = H or Me) probably by way of (143).143 o-Phenylenediamines react with 1-bromo-2-phenylcyclobutene-3,4-dione(145)to form 1-phenylcyclobuta[b]-quinoxalin-2-(8H)-ones (146). 144 The reaction of o-phenylenediamine with 1-phenylcyclobutene-3,4-dionedoes not take the same course and leads to (147). 145 The formation of (146)undoubtedly involves displacement of bromide ion from the vinylogous acid bromide (148).14' Benzocyclobutadiene reacts with 1,2,3,4-tetrachlorocyclopentadieneto yield two adducts (149) and (150).147Remarkably (150) is the major product. Stable benzocyclobutenyl cations have been prepared by the dissolution of compounds for example (15 l) in sulphuric acid.148 The n.m.r. spectrum shows that a considerable delocalization of the positive charge over the benzo-ring occurs. The aromatic methyl resonances both occur at lower field in (152) than in (151). 143 W. Ried and W. Kunkel Annalen 1968,717,54. 144 W. Ried and W. Kunstmann Angew. Chem. Znternat. Edn. 1968,7 135. 14' E. J. Smutny M. C.Caserio and J. D. Roberts J. Amer. Chem. Soc. 1960,82 1793. See Ann. Reports 1956,53 198; and ref. 145. 14' A.-u-Rahman A. J. Boulton and J. Sandosham Tetrahedron Letters 1968 1163. 148 H. Hart and J. A. Hartlage J. Amer. Chem. Soc. 1967.89 6672. Aromatic Compounds 36 1 Five-and seven-membered rings. Fulvenes and substituted fulvenes have been reviewed.14' A simple preparation of 6-substituted fulvenes (155) has been described starting from sodium formylcyclopentadienide (153) which reacts with toluene-p-sulphonyl chloride in ether at -5" to form 6-fulvenyl toluene-p- sulphonate (154)in 60% ~ie1d.I~' Reactions of (154) with secondary amines alkoxides carbanions and sodium azide occur below 0" with C-0 bond gd0 K = PHX T (1 53) (154) (155) Ph Ph (156) (157) fission presumably involving an addition-elimination mechanism.Isobenzo- fulvenes with electron-releasing substituents on the exocyclic carbon atom should be stable. Dehydrochlorination of (156) with ethyldi-n-propylamine yields (157) in over 80 % yield.' The analogous compound lacking the phenyl substituents was not isolated but evidence for its preparation was obtained by the isolation of derivatives. The effects of 6-aryl and 6-alkyl substituents upon the rotational barriers in 6-dimethylaminofulvenes have been reported and provide a quantitative approach to the estimation of n-electron delocalisa- tion energies. ' 'The results are consistent with the aromatic character sug- '49 E. D. Bergmann Cheni. Rec.. 1968,68 41.150 K. Hafner. W. Bauer. and G. Schulz. rlnqeiv. Chew Infernof. Etln.. 1968. 7. 806. 15' A. P. Downing W. D. Ollis. and I. 0.Sutherland. Chcni.Comni.. 1968. 1053. 362 H.Heaney gested previously ;Is2 the delocalisation energy of 6-dimethylaminofulvene is greater than 25.5 kcal. Br Br A Br 0 Ph Full details of the synthesis of octabromofulvalene (158) have been re-ported. 's In studies of triafulvene and triapentafulvalene derivatives it has been assumed that dipolar structures contribute significantly to the ground state and hence the molecules display aromatic characteristics and high dipole moments. Thus electron-withdrawing substituents at the exocyclic carbon of or in the five-membered ring of (160) are common. The dipole moments of the functional groups in (161) and (162) have a direction opposite to that in the triafulvene system and yet the compoundsaremarkedly aromaticin character.154 The n.m.r. spectrum of tetrabenzoheptafulvalene (163) proves not only that the molecule is non-planar but also that it has the transoid conformation.155 Ph \-/ 8 (165) (166) K. Hafner K. H. Hafner C. Konig M. Kreuder G. Ploss G. Schulz E. Sturm and K. H. Vopel Angew. Chem. Znternat. Edn. 1963,2 123. lS3 R. West and P. T. Kwitowski J. Amer. Chem. SOC. 1968,90,4697; ct Ann. Reports 1966,63 408. lS4 I. Agranat R. M. J. Loewenstein and E. D. Bergrnann J. Amer. Chern. SOC.,1968 90,3278; and references cited therein. E. D. Bergmann. M. Rabinovitz. and I. Agranat. Chem. Comm..1968. 334. Aromatic Compounds Interest in cyclic cross-conjugated compounds containing two potentially aromatic partial systems continue^.'^' The reaction of 9-fluorenyl-lithium with (164) and dehydrogenation of the product was reported last year.156 The first report of a non-annelated 6 n,lh system has appeared in which (165) was prepared by an analogous route involving the reaction of (164) with tetra- phenylcyclopentadienyl-lithium.'57 In contrast to the product reported last year but like the analogous calicene and sesquifulvalene derivatives (165) is protonated in for example trifluoroacetic acid. N.m.r. and U.V. measurements indicate that electrophilic attack occurs largely at position 11 (166) with the charge delocalised over the aryl-substituted five-membered ring.The n.m.r. spectrum of (165) indicates a considerable degree of double-bond fixation. Azulenyltropylium ions are of interest in view of their possible formulation either as heptafulvenes or as azulenes. (3 -Ethoxycarbon ylazulene-1 -yl)tro pyli urn perchlorate is best formulated as (167).' 58 Diphenylcyclopropenone has been ,co*CCI, ON shown to react readily with a number of activated isocyanates to form imino- cyclopropenes and troponimines. Also (168) has been prepared in a similar way.159 When tropone was treated with slightly more than two equivalents of toluene-p-sulphonyl isocyanate the compound (170) was isolated presuambly formed by ring closure of the dipolar species (169). Tropone reacts with mesyl- sulphene generated by the action of triethylamine on mesyl chloride to give an adduct (172) for the formation of which an analogous mechanism (171) to that suggested (169) was proposed.16' Benzosuberanone can be brominated to the act-dibromo-derivative which is dehydrobrominated to benzo[2,3]tropone in high yield by lithium chloride in dimethylformamide.161 An analysis of the 156 Ann.Reports 1967,64 294. 15' H. Prinzbach and L. Knothe Angew. Chem. Internat. Edn. 1968,7,729. lS8 T. Nozoe T. Toda and A. Yamanouchi Chem. Comm. 1968 1675. lJ9 L. A. Paquette and N. Horton Tetrahedron Letters 1968 2289. 160 J. Ciabattoni and M. Cabell Tetrahedron Letters 1968 2693. E. W. Collington and G. Jones. Chem. Comm.. 1968.958. 364 H. Heaney mo2m \ \ \ RO S0,Me 0 spin-spin coupling constants indicates that while benzo[2,3]tropone is appa- rently planar 1,2-benzoheptafulvene is rapidly interconverting between two non-planar forms.162 Indanone reacts with tropylium fluoroborate to give two products including (1 73). However no firmconclusion was reached concerning the aromaticity of (173) or of its protonated form.163 Compounds of the type (174; R = H or Me) have been synthesised.164 Their physical properties are in accord with their formulation as aromatic compounds but in their chemical reactions they behave like quinone methides. A number of conversions of tro- pones into benzenoid derivatives have been reported. As an example deuteria- tion studies have shown16' that 2-chlor0-7-nitro[3,5-~H,]tropone (175) is converted into 4-chloro-3-hydroxy-2-nitro[5-2H]benzaldehyde(176) and therefore C-6 becomes the aldehyde carbon.Annulen-.-The structure of the carboxylation product obtained from the cyclo-octatetraene dianion has been corrected to (177).166Since cyclohex- 2-ene-1,4-dione readily isomerises to hydroquinone in the presence of dilute acid,167it was of interest to prepare cyclodeca-2,4,8-triene-1,6-dione, the diketo- tautomer of 1,6dihydroxy[ 10lannulene. In practice the enolisation step was not achieved.168 The magnetic c.d. spectra of a number of ann~lenes,'~~ in-''' D. J. Bertelli J. T. Gerig and J. M. Herbelin J. Amer. Chem. SOC. 1968,90 107. D. J. Bertelli P. 0.Crews and S. Griffin Tetrahedron 1968,24 1945. 164 G. R.Proctor and A.H. Renfrew J. Chem. SOC.(C),1968 1187. E.J. Forbes M. J. Gregory and D. C. Warrell J. Chem. SOC.(C),1968,1969. lci6 T.S.Cantrell Tetrahedron Letters 1968 5635. Ann. Reports 1965,62 307. ''* P. J. Mullingan and F. Sondheimer,J. Amer. Chem. SOC.,1967,89 7118. B. Briat D.A. Schooley R. Records E. Bunneberg and C. Djerassi J. Amer. Chem. SOC. 1967.89,7062. Aromatic Compounds cluding bridged [10lannulenes' 70 have been reported. Some interaction between the non-bonding electrons in hetero-bridging groups and the 10 n-electron system was suggested. 11,l l-Dihalogeno-1,6-methano[lO]anndenes have been prepared by the now well established route.17' These compounds are effective as dihalogenocarbene transfer agents. Thus on heating (178) in benzene naphthalene and (179) are formed.The presence of excess of cyclo- hexene completely suppresses the formation of (1 79). The dichlorocarbene transfer was shown to be stereospecific by reactions with cis-and trans-but-2- ene. The analogous 11,ll-difluoro-compound (180) forms the Diels-Alder adduct (1 81) with dicyanoacetylene and this forms 1,l difluorobenzocyclo- propene (182) in a retro-Diels-Alder reaction.172 An analysis of the chemical shift and coupling constant data for certain nitrogen- and ethylidene-bridged [lOlannulenes has been used as evidence for aromaticity in these systems,173 and the acidity of bridged [lO]annulenes has been studied.174 [16]Annulene has been prepared by two different routes but although eighteen different configurations are possible the same isomer was isolated in the two syntheses.Valence isomerisation has now been studied by low tempera- ture n.m.r. spectroscopy. 17' The initially observed spectrum is compatible with the structure in which the double bonds are alternately cis and trans (183). At temperatures below -90" the twelve exterior protons resonate at z 4-67 and the induced paramagnetic ring current results in the four interior protons resonating at very low field (z -0.56 at -130"). At about -60" all the protons are magnetically equivalent. Rapid isomerisation between eight equivalent structures (1 83) and thirty-two equivalent structures (184) accompanied by conformational changes account for the observed spectra The reaction of potassium with [16lannulene in tetrahydrofuran affords the aromatic [16)-annulene dianion.76 The diamagnetism associated with the 18 n-electron system is shown by the n.m.r. spectrum in which the inside protons resonate at z 18.17 and the outside protons at z 1.17 (8 protons) and 2.55 (4 protons). 170 B. Briat D. A. Schooley R. Records E. Bunneberg C. Djerassi and E. Vogel J. Amer. Chem. SOC.,1968,90,4691. 17' V. Rautenstrauch H.J. Scholl and E. Vogel Angew. Chem. Internat. Edn. 1968 7 288; see E. Vogel Chem. SOC.Special Publ. No. 21 1967 p. 113. E. Vogel S. Korte W. Grimme and H. Giinther Angew. Chem. Internat. Edn. 1968,7 289. 173 H. Giinther and H.-H. Hinrichs Tetrahedron 1968,24,7033. 174 W. A. Boll Tetrahedron Letters 1968,2595. "'J. F. M.0th and J.-M. Gilles Tetrahedron Letters 1968 6259."'J. F. M. 0th. G. Anthoine. and J.-M. Gilles. Tetrnhedron Letters. 1968. 6265. 366 H. Heaney (185a) (185b) (185c) The anion remained unchanged when heated for 2 days at 100". The details of the synthesis of several dehydro[ 16lannulenes have also appeared together with an analysis of variable temperature n.m.r. spectra. 77 Interconversion between non-equivalent conformers for example (1 85a-d) is indicated. Four isomeric 7,8 :17,18-dibenz0[20]annulene 1,4:11,14-bisepoxides (186-189) have 17' I. C. Calder Y.Gaoni and F. Sondheimer J. Amer. Chem. SOC. 1968,90 4946; I. C.Calder Y.Gaoni P. J. Garratt and F. Sondheimer ibid.. p. 4954. Aromatic Compounds have been prepared.178 The chemical shift data for the olefinic protons do not suggest the presence of a peripheral paramagnetic ring current.The synthesis of[18lannuleneand[24]annulenepolyoxidesfrom sucrose has been reported. The reactions presumably involve a phosphonium ylide derived from 5-chloro- methyl-2-furfural and triphenylphosphine in the presence of lithium ethoxide. The Wittig reaction of (190) with o-phthalaldehyde affords a route to the first [lolannulene (191) for which a KekulC structure incorporating the cyclo- decapentaene system can be written.leo The synthesis of two stable conforma- tional isomers of trans-dibenzo [a,c]fur0 [3,4-9 lUJannulenes (192) and (193) J have been reported.' The conformational isomerism must arise as a result of the steric interference of the internal hydrogen atoms.The tetradehydro[ 181-(194) '" J. A. Elix and M. V. Sargent J. Amer. Chem. SOC.,1968,90 1631. 179 J. A. Elix Chem. Comm. 1968 343. K. Grohmann and F. Sondheimer J. Amer. Chem. SOC. 1967,89 7119. '*'A. P. Bindra. J. A. Elix and M. V. Sargent Tetrahedron Letters 1968,4335. 368 H. Heaney annulene (194) is capable of sustaining a diamagnetic ring current as evidence by the signal of the inner protons at z 15.25 and that of the outer protons at z 034.' 82 Biphenyl-2,2'-dicarboxaldehydereacts with 1,2-bis(triphenylphos- phorany1)benzocyclobutene to give (195) the first known derivative of the (4n + 4n) n-electron system (196).183 However in spite of the fact that the system formally possesses (4n + 2) x-electrons in the bicyclo[6,2,0]decapen- taene part of the molecule these electrons are not delocalised.Transannular Reactions in Cyclic Systems and Related Cyclisations.- Studies of spatial effects in cyclic and acyclic acetylenes and olefins have revealed a number of interesting cyclisation reactions. A Diels-Alder adduct obtained from a mixture of (192) and (193) with dimethyl fumarate has been reported. 181 The product (197) slowly changed over a period of 2 weeks at 0" into the tri- phenylene (198). Rather more spectacular was the isolation of zethrine (200) (199) from the reactions of l-ethynyl-8-iodonaphthalenewith copper and from a mixture of 1,8-di-iodonaphthalene and 1,8di-ethynylnaphthalene with copper.185 This reaction must be presumed to involve the reductive cyclisation of the diacetylene (199).The diene (201) undergoes a similar cyclisation to 7,14- dihydrozethrine which in the presence of air is oxidised to zethrine.lg4 1,8- Diethynylnaphthalene is reduced by hydrogen in the presence of a Lindlar catalyst to 1,2-dirnethylacenaphthylene,and the intermediacy of the diene (202) was shown by the formation of an adduct with maleic anhydride.'85 2,2'-18' J. Ojima T. Katakami G. Makaminami and M. Nakagawa Tetrahedron Letters 1968 1115. P. J. Garratt and R. H. Mitchell Chem. Comm. 1968 719. R.H. Mitchell and F. Sondheimer. J. Amer. Chem. SOC.. 1968.90. 530. Aromatic Compounds $c (204) Diethynylbiphenyl(203) reacts with hydrogen bromide to form 9,lO-bisbromo- methylphenanthrene and 2-ethynyl-2-iodobiphenylforms a copper compound (204) which forms the cumulene (206) presumably by way of (205).186 The synthesis of (208) by the action of cuprous chloride on the dilithio-compound -CGCPh (210) H.A. Staab A. Nissen and J. Ipaktschi Angew. Chem. Internat. Edn. 1968,7 226 H. A. Staab. H. Mack. and E. Wehniger. Tetrohedron Letters. 1968. 1465. 370 H. Heaney (207) has been reported.' 87 1,8-Bis(phenylethynyl)naphthalene (209) gives 7-phenylbenzo[k]fluoranthene (210) when warmed to 1UOo,'88and (21 1) gives (212) and the cis-isomer thermall~.'~~ The bisphosphorane (213) reacts with phthalaldehyde to form (214) and (215),'899 and with o-xylylene dibromide in the presence of base to form (216).19' The electroreduction of for example 1,8-bis-4-methoxybenzoylnaphthalene gave (2 17).'92 '-CH=PPh3 8 \ CH=PPh3 \/; \/ Polycyclic Compounds.-The labelling of naphthalene with C does not'93 confirm the results obtained' on the scrambling of [l-14C]naphthalene which were reported last year.The remarkable basicity of 1,8-bis(dimethyl-amino)naphthalene (pK 12.34) is ascribed to the strain associated with the peri-interaction which is effectively relieved by protonation. Titanium tetraisopropoxide and sodium naphthalide produced a system which is capable Is' H. A. Staab and E. Wehniger Angew. Chem Internat. Edn. 1968,7,225. ls8 B. Bossenbroek and H. Shechter J. Amer. Chem. SOC.,1967,89 7111. IS9 R.H. Mitchell and F. Sondheimer Tetrahedron Letters 1968 2873. 190 P. R. Houlten and W.Kemp Tetrahedron Letters 1968,4093. 19' H. J. Bestmann and D. Ruppert Angew. Chem. Internat. Edn. 1968,7 637. 19' R. N. Gourley and J. Grimshaw J. Chem. SOC.(C) 1968,2388. 193 A. T. Balaban D. Farcasiu V. A. Koptyug I. S. Isaev M. I. Gorfinkel and A. I. Rezvukhin Tetrahedron Letters 1968,4757. 194 R. W. Alder P. S. Bowman. W. R. S. Steele. and D. R. Winterman Chem. Comm.. 1968. 723. Aromatic Compounds of fixing nitrogen in the presence of molecular oxygen.lg5 The system can be operated as a cyclic process and has been used to obtain a 340% yield of am- monia 1,2-Naphthalene oxide has been prepared and the n.m.r. spectrum excludes the isomeric benzo-oxepin. 96 The reaction of 4-bromo-1 -naphthol with aniline under nitrogen yields the expected product.However in the absence of nitrogen the isolated product was (218).lg7 Interesting products have been N/Ph NHPh c1 0 ,? 8-p \ / c1 '0 C1 / obtained from reactions of quinones with diazoalkanes. Thus 3,4-dichloro-1,2- naphthoquinone reacts with diazomethane in ether to yield (219) in the presence of methanol (220) and in the presence of lithium chloride (22l)."* 9,lO-Phenanthraquinone affords (223) presumably by way of (222).''' The first naturally occurring 2,2'-dinaphthyl derivative (224) showing optical activity due to restricted rotation has been reported.200 Evidence which supports the presence of an induced paramagnetic ring current in the four-membered ring in bipheny1ene2O1 has been adduced from 19' E. E.van Tamelen G. Boche and R. Greeley J. Amer. Chem. SOC.,1968,90 1671. 196 E. Vogel and F.-G. Klamer Angew. Chem Znternat. Edn. 1968 7 374. 19' V. Calb and P. E. Todesco Chem. Comm. 1968 571. 19* B. Eistert and L. Klein Chem. Ber. 1968 101 391. 199 B. Eistert R. Wollheim G. Fink H. Minas and L. Klein Chem. Ber.. 1968 101. 84. zoo T. J. King and L. B. de Silva Tetrahedron Letters 1968,261. Ann. Reports 1967,64,273. 372 H.Heaney an analysis of the ‘H n.m.r. spectra of 1- and 2-de~teriobiphenylene,~O~ and from the I3C n.m.r. spectrum of bi~henylene.~’~ Aryl positions adjacent to a fused strained ring have increased acidity and reduced reactivity towards electrophiles. The atomic orbitals involved in the formation of the strained ring have a higher than normal p character and hence the remaining orbital has a higher s character.Base-catalysed protodetritiations of tritiated biphenyl- enes and triptycenes have been discussed in these terms,204 and metallation of bip hen ylene with n-butyl-lithium gives predominantly 1 -1ithio biphen ylene. ’ The nitration of biphenylene with nitric acid in acetic anhydride gives (225) as the major product presumably by the &-addition of nitronium acetate at positions 4a and 8b.206 The non-catalysed reaction of bromine with biphenylene gives 3,8-dibromobiphenylene and two tetra bromide^.^'^ The probable con- formation of the major tetrabromide is as shown (226). 9,9-Di benzyl-9,lO-di hydro-1 0-hydroxyant hracene gives 9,lO-dibenzylan thra- cene when heated in acetic acid.208 The reaction may involve a series of consecutive 1,2-shifts.An almost quantitative yield of 9-cyano-2-methoxy- anthracen-10-01 was obtained by the action of sodium methoxide in dimethyl sulphoxide on 2-cyanomethyl-2’,4’-dimethoxybenzophenone followed ’02 H. P. Figeys Angew. Chem. Internat. Edn. 1968,7,642. 203 A. J. Jones and D. M. Grant Chem. Comm. 1968 1670. 204 A. Streitwieser G.R. Ziegler P. C. Mowery A. Lewis and R. G. Lawler J. Amer. Chem. Soc. 1968,90 1357. 205 W. Baker A. J. Boulton C. R. Harrison and J. F. W. McOmie Proc. Chem Soc. 1964 414; A. J. Boulton J. B. Chadwick C. R. Harrison and J. F. W. McOmie J. Chem. SOC.(C) 1968 328. 206 J. W. Barton and K. E. Whitaker J. Chem SOC. (C) 1968 1663. 207 J. W. Barton and K.E. Whitaker J. Chem. SOC.(C),1968,28. ’08 A. L. J. Beckwith W. B. Renfrow A. Renfrow and J. K. Teubner Tetrahedron Letters 1968 3463. Aromatic Compounds by a~idification.~'~ The reaction presumably involves (227). The transient existence of 9,9'-dehydrodianthracene(229) was first suggested in studies of 9-bromodianthracene with strong bases.210 Photolysis of the cyclopropanone (228) has now2" been shown to give (229) which has been epoxidised and re- duced catalytically. An interesting new synthesis of the triptycene system depends on the ring closure of compounds (231) to (232) by means of.poly- phosphoric acid.212 Compounds of the type (231) need not be isolated since they are formed from (230) under the reaction conditions. The measurement of apparent pK values for bridged ethanoanthracenes of the type (233; X = C1 Y = H ; and X = H Y = C1) has revealed a marked angular dependence of the CN Ph (235) (236) (237) substituent effect.21 Reactions of dicyanoacetylene with arenes were reported last year.214 A mono-adduct (234) and two di-adducts (235 ; R' = H R2 = CN and R' = CN R2 = H) have been isolated from reactions with 9,lO-diphenyl- anthracene ;' ' these reactions parallel the reaction of dicyanoacetylene with 1,4di~henylnaphthalene.~ l4 A large deuterium isotope effect was observed in the reaction of gdeuterioanthracene with tetracyanoethylene oxide.2 l6 This indicates that the first step involves attack at the electrophilic oxygen followed by the rapid cyclisation of the dipolar intermediate (236) to form (237).209 J. S. Davies V. H. Davies and C. H. Hassall Chem. Comm. 1968 1555. D. E. Applequist R. L. Litle E. C. Friedrich and R. E. Wall J. Amer. Chem. SOC. 1959 81 452; D. E. Applequist R. Searle M. Steinhardt E. C. Friedrich and R. L. Litle J. Org. Chem. 1965 30,2126. 211 N. M. Weinshenker and F. D. Greene J. Amer. Chem. SOC.,1968,90,506. 212 H. M. Walborsky and T.Bohnert J. Org. Chem. 1968,33 3934. 213 E. J. Grubbs and R. Fitzgerald Tetrahedron Letters 1968,4901. 214 Ann. Reports 1967,64,280. 21 J. Rigaudy and M. Ricard Tetrahedron 1968,24,3241. 216 P. Brown and R. C. Cookson Tetrahedron 1968,24,2551. 374 H. Heaney fp” The bridged ethananonaphthalene derivative (238) and its epimer both re- arrange on dehydration with strong acid to the benzodihydropentalene (239).2 l7 The mechanism of the reaction was studied by deuterium labelling and by the isolation of intermediates.The difference between the thermal and photo- chemical retro-Diels-Alder reaction in compounds of the type (240) has been pointed out.2 Dimethylketen is lost thermally while photochemically (242) is obtained in addition to (241). Activation parameters for conformational inversion in a number of 4,5-disubstituted phenanthrenes have been determined by n.m.r. spectroscopy. The acetoxymethyl group is more effective than the alkoxycarbonyl group in preventing planarity of the phenanthrene system.219 Phenanthrene-9,lO-diol when heated in chloroform with 2,3-dimethylbutadiene and hydrogen chloride forms (243).220 The same structural type is present220 in the product isolated from a reaction of 3-chloro-1,2-naphthoquinonewith 2,3dimethylbutadiene2* Previous attempts to prepare (244) have resulted in the formation of self-con- densation products.However during the dehydrobromination of (245) with ”’ A. C. G. Gray and H. Hart J. Amer. Chem. SOC. 1968,90,2569. ”* R. K. Murray and H. Hart Tetrahedron Letters 1968,4995. ’19 R. Munday and I. 0.Sutherland J. Chem. SOC.(B) 1968,80. ’’O M. F. Ansell and R. A. Murray Chem. Comm. 1968 1583. 221 L. F. Fieser and J. T. Dunn. J. Amer. Chem. Soc.. 1937,59 1016. 1021 Aromatic Compounds 3 75 pyridine a transient violet colour was observed. In the presence of furan the colour was absent and (246) was isolated.222 The n.m.r.spectrum of 4,7-dihydroxyphenalenoneconsists of a simple AB system and the i.r. carbonyl absorption occurs at 1630 cm.-1.223 These data are interpreted in terms of a rapid tautomeric exchange involving (247). In dilute aqueous sodium hydroxide the dianion (248) is formed. The synthesis of azupyrene (249) was undertaken in order to test the treatment of cyclocondensed aromatic hydrocarbons based on the free-electron model.224 Thus suggests that a stable peripheral x-electron system would be dominant and that the inner x-electrons would be separated by a circular node(s). Physical measurements indicate a degree of aromaticity for (249) although perhaps this is not marked as would be expected for a planar 14 x-electron Me system.Methyl derivatives of the unknown hydrocarbon (250; R = H) have been prepared.225 The compound (250; R = Me) is strongly basic and re- versible protonation occurs even in 2~-sulphuric acid to give (251). The nitra- tion of indeno[1,2,3-~d]fluoranthene(252) has been shown to occur almost exclusively at position 2.226 It was concluded that steric interference by the 12- hydrogen atom is probably responsible for the absence of 1-substitution even though this is predicted by MO calculations to be the most reactive position. Restricted conformational inversion in (253) has been studied by variable temperature n.m.r.227 Compounds in the cyclotriveratrylene series (254) have 222 E. Le Goff and R. B. Lacount J. Org. Chem. 1968,33,2529.223 M. Jarcho J. Amer. Chem. SOC.,1968,!MJ 4644. 224 A. G. Anderson A. A. MacDonald and A. F. Montana J. Amer. Chem. SOC.,1968,90,2993. 225 K. Hafner G. Hafner-Schneider and F. Bauer Angew. Chem. Znternat. Edn. 1968,7 808. 226 A. Davies and K. D. Warren J. Chem. SOC.(B) 1968 1337. 227 P. T. Lansbury and M. Klein. Tetrahedron Letters. 1968. 1981. 376 H. Heaney (253) Me0 OMe (254) 0 alo been studied by n.m.r. i.r. and U.V. spectroscopy.228 Although the parent compound (254; R' = R2 = H2)exists in a rigid crown conformation the monocarbonyl compound (254); R1 = 0,R2 = H2) shows spectral character- istics associated with a conjugated carbonyl group and hence is flexible. The oxidation of (254; R' = R2 = H2) with dichromate gives the monoketone (254; R' = 0,R2 = H2)and a lactone (255) which is presumed to arise from the triketone (254; R' = R2 = 0)by a transannular rearrangement.229 A 'chair- like' and a 'boat-like' conformer have been selected for the minimum energy states of (256).230 228 R.C. Cookson B. Halton and I. D. R. Stevens J. Chem. SOC.(B) 1968,767. 229 J. E. Baldwin and D. P. Kelly Chem. Comm. 1968 1664. 230 P.A. Temussi. A. Segre. and F. Bottino Chem. Comm.. 1968. 1645. Aromatic Compounds 377 Syntheses of hexa- hepta- octa- and nona-helicenes by the photo-induced cyclisation of annelated cis-stilbenes have been reported. Heptahelicene (256) has been obtained optically active by spontaneous resolution. Variable temperature n.m.r. spectroscopy and c.d.studies both in solution and in the solid state allow the determination of the solid-state conformation and the absolute configuration of tri-o-thymotide.2 32 NC 8' (259) OOMe P h L (264) R Quinodimethanes and quinone methides. A stable anthraquinodimethane which lacks substituents on the methylene groups (257)has been prepared233 in order to investigate its U.V. Other stable monocyclic and polycyclic quinodimethanes (258)-(261),have been prepared,234 and certain of their reactions described.235 The isolation and characterisation of a number of cis-and trans-cinnamyl- and obtu~aquinone~~~ (262),a new type of natural quinone methide have been reported. The p-benzoquinone methide (263) lacking the usual stabilising substituents at positions 2 and 7 has been prepared.238 It does however polymerise rapidly at room temperature.Heterocyclic compounds related to o-quinodimethane (264) have been reported.239 23' R. H. Martin M. Flammang-Barbieux J. P. Cosyn and M. Gelbcke Tetrahedron Letters 1968 3507. 232 A. P. Downing W. D. Ollis I. 0.Sutherland J. Mason and S. F. Mason Chem. Comm. 1968 329. 233 R. R. Hill and G. H. Mitchell Chem. Comm. 1968 1243. 234 R. Gompper H.-U. Wagner and E. Kutter Chem. Ber. 1968 101 4123; K. Eiglmeier and Th. Eicher Angew. Chem. Znternat. Edn. 1968,7 809. 23s R. Gompper H.-U.Wagner and E. Kutter Chem. Ber. 1968,101,4144. 236 M. Gregson K. Kurosawa W. D. Ollis B. T. Redman R. J. Roberts I. 0. Sutherland A. Braga de Oliveira W. B. Eyton 0.R. Gottlieb and H.H. Dietrichs Chem. Comm. 1968 1390. 237 M. Gregson W. D. Ollis B. T. Redman I. 0.Sutherland and H. H. Dietrichs Chem Comm. 1968 1395. 238 J. J. Murray J. Org. Chem. 1968,33 3306. 239 J. M. Holland and D. W. Jones Chem. and Ind. 1968 549.

ISSN:0069-3030    

DOI:10.1039/OC9686500339

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

11 ALICYCLIC COMPOUNDS By J. M. Brown (School of Molecular Sciences University of Warwick) Useful quantitative data on heats of hydrogenation and combustion have appeared ;this allows the estimation of strain energy effects for bicyclo[l,l,O]- butane,' bicyclo[ 2,1,O] pentane ' quadricyclene,l and a number of C4H6 cyclic hydrocarbons.2 The strain energy in the bridgehead olefin bicyclo[3,3,1]- non-1-ene is estimated3 from the heat of acetic acid addition to be 12 kcal. per mole. Quantitative assessment of strain and conformational effects has allowed estimation of activation energies for a variety of small ring thermolyse~.~ Assumption of a non-concerted diradical mechanism shows good fit with experiment in all cases studied except those of cyclobutene ring opening and rearrangement of cis-1 -methyl-2-vinylcyclopropanes,which are much more ready and therefore follow a concerted path.Differential thermal analysis may be used to measure activation energies and heats of reaction simultaneously as in the dimerisation of cyclopentadiene and the endo,exo-cyclopentadiene dimer isomerisati~n,~ and in thermolyses of hexamethyl dewar benzene and hexamethylprismane.6 Reviews of fulvenes,' cyclopropanols,8 metal-catalysed reactions of nor- bornadiene,' 1,2- and l,.l-addition to conjugated dienes," the steric course of cyclohexenone additions," and solvolysis of norbornen-7-yl systems' have been produced. An issue of Angewandte Chernie13 is devoted to small ring chemistry. Conformational Analysis.-The sophistication and accuracy in computer- simulation of energy and geometry in cyclic systems increases ; Hendrickson has presented more detailed analysis of medium rings and of methyl-substituted cycloalkenes coupled with a full topological analysis of conformer inter- conversion pathways.l4 Allinger's school has refined the Westheimer-Hen- ' R. B. Turner P. Goebel B. J. Mailon. W. von E. Doering J. F. Coburn Jun.. and M. Pomerantz. J. Anier. Chem. SOC..1968. 90,43 15. ' K. B. Wiberg and R. A. Fenoglio. J. Anier. Chem. So(.. 1968.90. 3395. P M. LesLo and R. B. Turner. J. Amrr. Chem. SOC..1968,90,6888. H. E. O'Neal and S. W. Benson. J. Phy.s. Chem.. 1968.72 1866. E. J. Barrett H. W. Hoyer and A. V. Santoro Terruhedron Lerters 1968,603. J. F. M. Oth Rec.Trau. chim. 1968,81 1185. ' E. D. Bergmann Chem. Rev. 1968,41. C. H. DePuy Accounts Chem. Res. 1968,1 33. G. N. Schrauzer Adu. Catalysis 1968,18 373. lo P. D. Bartlett Science 1968 159 833. E. Toromanoff Topics Stereocheni. 1967. 2. 157. l2 H. Tanida Accounts Chem. Res. 1968 1 239. l3 R. Criegee R. Breslow J. M. Conia S. Sarel U. Schollkopf Anyew. Chem. Internor. Edn. 1968 7 No. 8. l4 J. B. Hendrickson J. Amer. Chem. SOC. 1967,89 7036 7043 7047. N 380 J. M. Brown drickson-Wiberg approach to the point where accurate energies may be calculated for a number of saturated acyclic and cyclic hydrocarbons. ' Unsaturated hydrocarbons present a more difficult problem but calculations get the relative energies right for methylenecyclohexane vis-d-vis the methyl- cyclohexenes.' The ethyl 3-t-butylcyclobutanecarboxylateisomers equilibrate (7 :3) in favour of cis with substituents in the equatorial positions (l)." Cross-ring participation in a puckered cyclobutane give a stereoelectronic explanation of product distribution in a variety of cyclobutyl tosylate solvolyses,18 as was predicted in CNDO calculations.' Gauche interaction energies are shown to be similar in cyclohexanes and acyclic systems by dipole-moment studies.l9 At very low temperatures t-butyl- cycloalkanes show restricted rotation of the t-butyl group on an n.m.r. time scale. The rotational barriers of 6-8 kcal. mole- are much higher than would be predicted on simple torsional grounds.20 A note of caution has been introduced by Eliel into the n.m.r.shift method of determining A-values.2' The 19F chemical shifts of cis-and trans-4-t- butylfluorocyclohexane and 1,l-difluorocyclohexane were determined ; it is apparent that the t-butyl group affects the fluorine chemical shifts markedly thus invalidating conformational equilibrium conclusions based on such measurements. A Values have been determined by equilibration for formyl (0.74) hydroxymethylene (1.65 + 2-06) and acetyl (1*5),22 and for various cyclohexylmagnesium compo~nds,~ and a compendium of values has been tab~lated.'~ Equilibration in the 1-methyl-4-t-butylcyclohexanol series shows equatorial methyl to be favoured by only 0.24 kcal. mole- ' much less than would have been e~pected.~' Equilibration studies on (2) allow the determination of the degree of allylic 1,3-interaction; A'.(Me-H) is esti- mated26 at 1-4kcal. mole-' A'T~(M~-M~) at 6 kcal. mole-'. N.m.r. methods have been used to examine 4,4disubstituted cyclohexanols,27 1,4 dihydroxy- cyclohexanes,28 and 1,3-bridged cyclohexane ketones ;29 the interesting l5 N. L. Allinger J. A. Hirsch M. A. Miller I. J. Tyminski and F. A. Van-Catledge J. Amer. Chem. SOC., 1968,90 1199. l6 N. L. Allinger J. A. Hirsch M. A. Miller and 1. J. Tyminski J. Amer. Chem. SOC. 1968 90 5773. I' G. M. Lampman K. E. Apt E. J. Martin and L. E. Wangen J. Org. Chem. 1967,32 3950. '* K. B. Wiberg and J. G. Pfeiffer J. Amer. Chem. Soc. 1968,90 5324. I9 H. R. Buys and E. Havinga Tetrahedron Letters 1968 3759. 2o F.A. L. Anet M. St. Jacques and G. N. Chmurny J. Amer. Chem. SOC.,1968,90 5243. 21 E. L. Eliel and R. J. L. Martin J. Amer. Chem. Soc. 1968,90 682 689. 22 E. L. Eliel D. G. Neilson and E. C. Gilbert Chem. Comm. 1968 360. 23 F. R. Jensen and K. L. Nakamaye J. Amer. Chem. Soc. 1968,90 3248. 24 J. A. Hirsch Topics Stereochem 1967 1 199. 25 J. J. Uebel and H. W. Goodwin J. Org. Chem. 1968,33,3317; N. L. Allinger and C. D. Liang ibid. p. 3319. 26 S. K. Malhotra and F. Johnson Chem. Comm. 1968 1149. " R. D. Stolow T. Groom and P. D. McMaster Tetrahedron Letters 1968 5833; R. D. Stolow T. W. Giants and J. D. Roberts ibid. p. 5777. 28 R. D. Stolow and A. A. Gallo Tetrahedron Letters 1968 3331. 29 E. Dunkelblum and J. J. Klein Tetrahedron Letters 1968 55. Alicyclic Compounds 381 conclusion arises that 4,4-difluoro-substitutionstabilises an axial substituent with benzoyloxy being 74% axial and chloro ~60%.Combustion studies show cis-l,4-di-t-butylcyclohexaneto have 4-7 kcal.more energy than the trans-isomer and probably to have a twist-boat conformation. 30 Accurate coupling constants have been obtained for cyclohexane and cyclohexa-1,4- diene3' by use of partially deuteriated derivatives; they suggest that in the latter the ring is nearly but not quite planar.31 Rates of ring inversion have been measured for cyclohexanes with one sp2carbon atom; 32 cyclohexanone (which is in flattened-chair conf~rmation)~ inverts with AGS t5-1kcal. mole-' but methylenecyclohexane with AGt 7.7 kcal. mole- '.Cyclohexanesemidione (3) undergoes half-chair-half-chair interconversion with E ca. 4kcal. molew1 suggestive of the utility of variable temperature e.s.r. in conformational analysis.34 2,2,7,7-Tetramethoxy-cis-decalin inverts35 with AGt 12.4kcal. mole-' a similar value to that for cis-decalin. (2) R = H or Me (3) Cherest and Felkin have developed a theory to explain the course of addition to cyclohexanones. The major factor inhibiting equatorial approach is tor- sional strain in the transition state but an axial approach requires the reagent to overcome Pitzer strain by interaction with axial hydrogen or with sub- stituents at the 3-and 5-positions. Thus increasing bulk in the reagent directs the product towards axial alcohol as observed in addition of Grignard rea- gent~~~tri-t-butoxyaluminium hydride.37 Stereochemical aspects of and enolate anion reactions3* and cyclohexyl radicals3' are delineated. There are no steric or stereoelectronic effects by 4-substituents operating in the deprotona- tion of cyclohe~anones.~~ Conformational effects in Simmons-Smith reactions and hydroborations are e~aluated.~' 30 H. van Bekkum M. A. Hoefnagel L. de Lavieter A. van Veen P. E. Verkade A. Wemmers B. M. Wepster J. H. Palm L. Schafer H. Dekker C. Mosselman and G. Somsen Rec. Trau. chim. 1967 86 1363. 31 E. W. Garbisch and M. G. Griffth J. Amer. Chem. SOC.,1968,90 3590 6543. 32 J. T. Gerig J. Amer. Chem. SOC. 1968,90 1065; F. R. Jensen and B. H. Beck ibid. p. 1066. 33 J. B. Lambert R. E. Carhert P. W.R. Corfield and J. H. Enemark Chem. Comm. 1968,999. 34 G. A. Russell G. R. Underwood and D. C. Lini J. Amer. Chem. SOC. 1967,89 6636. 35 A. Geens D. Tavernier and M. Anteunis Chem. Comm 1967 1088. 36 M. Cherest and H. Felkin Tetrahedron Letters 1968,2205; G. D. Meakins R. K. Percy E. E. Richards and R. N. Young J. Chem. SOC.(C) 1968 1106. 37 J. Klein E. Dunkelblum E. L. Eliel and Y. Senda Tetrahedron Letters 1968 6127. 38 H. 0.House B. A. Tefertiller and H. D. Olmstead J. Org. Chem. 1968 33 935. jg F. R. Jensen L. H. Gale and J. E. Rodgers J. Amer. Chem SOC. 1968,90 5793. 40 F. G. Bordwell and R. G. Scamehorn J. Amer. Chem SOC.,1968,90,6748. 41 B. Rickborn and J. H. H. Chan J. Org. Chem. 1967,32 3576; D. J. Pasto and F. M. Klein ibid.. 1968 33 1468. 382 J.M. Brown Medium and bridged rings. Norbornane and 1,4-dichloronorbornane structures have been determined by electron diffraction and bond angles differ ~ignificantly.~~ 3-Methoxycarbonylbicyclo[3,3,l]nonane is shown to have a 3 kcal. preference for equatorial epimer (chair-chair with considerable 3H,’IH-interaction) over axial (~hair-boat)~~ epimer from equilibration studies. Variable temperature n.m.r. allows the deduction that 7-phenyl and 7-t- butyl-cycloheptatriene prefer the quasi-equatorial position (4)for the sub- ~tituent.~~ cis-2,7-Dichlorocycloheptanone also has a preference for the quasi-equatorial conformation but i.r. studies show that the preference is only a small The cyclo-octyne (5) shows variable temperature n.m.r.behaviour consistent with rapid enantiomerism by internal rotation around the 5,6-bond with AGS 8 kcal. mole-’ (cf trans-cy~lo-octene).~~ Cyclophane (6) could not be resolved by the Cope method but detailed n.m.r. analysis on deuteriated analogues showed rapid enantiomer interconversion by rotation of the methylene bridge through the ring with AGf ca. 14 kcal. mole’ ;47 (7) however has a temperature-invariant n.m.r. down to -107°.48 Tetramethylcyclo-decene4’ n.m.r. spectra and tetramethylcyclododecene equilibria” have been studied. H (4) (5) (g q3 \ (6) (7) Inspection of unit-cell dimensions and symmetry rule out the possibility of a boat conformation for cis,cis-cyclodeca-l,6diene.In Variable tempera- ture n.m.r. studies have been carried out on the ketone (8) its 1,3-dioxolan derivative and the corresponding tetramethylol.Ib The conformation was 42 J. F. Chiang C. F. Wilcox jun. and S. H. Bauer J. Amer. Chem. SOC.,1968,90 3149. 43 R.A. Appleton C. Egan J. M. Evans S. H. Graham and J. R. Dixon J. Chem. SOC.(C) 1968 1110. 44 H. Gunther M. Gorlitz and H. H. Hinrichs Tetrahedron 1968,24 5665. ” R.Borsdorf W. Flamme H. Kumpfert and M. Muhlstadt Tetrahedron 1968,24 65. 46 A. Krebs Tetrahedron Letters 1968,4511. 47 A. C.Cope and B. A. Pawson J. Amer. Chem. SOC.,1968,90,636. 48 A. C.Cope and M. W. Fordice J. Amer. Chem. SOC.,1967,89,6187. 49 C. Cordes V. Prelog E. Troxler and H. H. Westen Helo. Chim. Acta 1968,51 1663. 50 J. Sicher M. Svoboda B. J. Mallon and R. B. Turner J. Chem.SOC.(B) 1968 441. 51 (a) H.L. Carrell B. W. Roberts J. Donohue and J. J. Vollmer J. Amer. Chern. SOC.,1968.90. 5263;(b)B. W. Roberts J. J. Vollmer. and K. L. Servis. ibid. p. 5264. Alicyclic Compounds thought to be chair with rapid equilibration in (8) by a chair-boat-chair route which is much slower in the acetal owing to non-bonded interactions. A full report has appeared on the isolation of conformational isomers in the cyclotriveratrylene series by the Southampton Thus the equa- torial alcohol (9) exists as the crown (9a) and flexible (rapid pseudorotation between three equivalent conformers) forms (9b); the latter is converted into (9a) on heating. Cy~lotetraveratrylene~ may exist in chair or saddle structure e.g. (10). OMe A most elegant study has been made on conformational equilibria in [16]-annuleneS4 (see Aromatic section).Detailed line-shape analysis of the variable- temperature n.m.r. spectrum shows that rotation and tautomerism in the 85-isomer (11) cannot be the only processes involved and the 91-isomer (12) is suggested to occur to the extent of about 25% with relatively slow (ca. 17 sec-I) interconversion of the two species. Rate constants for rotation tau- tomerism and isomerisation are all presented. Ring Synthesis.-(A) Cyclopropanes. ‘Carbene’ routes. A development of 52 R. C. Cookson B. Halton and 1. D. R. Stevens J. Chem. SOC.(B),1968,767. 53 J. D. White and B. D. Gesner Tetrahedron Letters 1968 1591. 54 J. F. M. 0th and J. M. Gilles Tetruhedron Letters. 1968 6259.384 J. M. Brown &3 -/ (11) the Simmons-Smith procedure using diethylzinc in place of zinc-copper couple gives higher yields particularly with polymerisation-sensitive olefins ;5 unlike the Simmons-Smith procedure this route allows the use of alkyl- and aryl-substituted methylene iodides without impairing yields. 56 An alternative method of methylene transfer uses the cuprous chloride catalysed decom- position of diazomethane and has been applied to hexamethyl dewar benzene,57 giving (13) and (14) and to the preparation of [2H,]cycloheptatriene.58 The dewar benzene derivative (15) was prepared from ethyl dia~oacetate,~' which reacts6' with acetylenes to give bicyclobutanes ; all the possible geometrical isomers of (16)were characterised.Competition experiments establish that addition of dichlorocarbene to acyclic olefins is strongly affected by steric hindrance.61 The major product in chlorocarbene addition to norbornene derivatives is the exo anti-isomer.62 In hydrocarbon solution photolysis of diazocyclopentadiene gives stereo- specific addition of cyclopentadienylidene to olefins e.g. (17).63 This carbene adds to hexafluorobenzene as does bistriflu~romethylcarbene.~~ Photolysis of 55 J. Furukawa N. Kawabata and J. Nishimura Tetrahedron 1968,24,53. 56 J. Furukawa N. Kawabata and J. Nishimura Tetrahedron Letters 1968,3495. " E. Muller and H. Kessler Tetrahedron Letters 1968 3037. P. Muller and J. Rocek J. Org. Chem. 1968,33 3001. 59 H. Prinzbach and E. Druckery Tetrahedron Letters 1968,4285.6o J. H. Leftin E. Gil-Av and A. Pines Chem. Comm. 1968,396. 61 R. A. Moss and A. Mamantov Tetrahedron Letters 1968 3425. 62 C. W. Jefford and W. Wojnarowski Tetrahedron Letters 1968 193. '' R. A. Moss and J. R. Przybyla J. Org. Chem. 1968,33 3816. 6* D. M. Gale J. Org. Chem. 1968.33 2536. M. Jones jun. ibid.. p. 2538. Alicyclic Compounds 385 the diazo-quinone (18) in acetylenes gives acid-sensitive spirocy~lopropenes.~ 1,2-Dimethylcyclobutene reacts with iodocarbene (from iodoform photolysis) with rearrangement to iodovinylcyclopropane (19)? Pyrazolines. A note of caution is introduced into the interpretation of the course of photolysis of diazo-compounds in the presence of olefins. Thus diazofluorene reacts with bicycloheptadiene photochemically to give the pyrazoline (20) which rearranges to (21) on heating in digl~rne.~~ The authors propose that pyrazoline formation in these reactions previously assumed to occur by a carbene route may in fact be quite common.Details have appeared on Closs's preparation of cyclopropenes6* by Pyrex photolysis of 3H-pyrazoles. Pyrazoline formation by dirnethyldia~omethane~~ in its addition to cis-3,4-dichlorocyclobutenewas observed ;photolysis of the products gave corresponding bicyclo[2,1,0]pentanes. 70 The diazacyclo-pentadiene dimer7 (22) is photolysed in ether non-stereospecifically to (23) and (24); the latter on photolysis in pentane gave tricyclic (25) which iso- merises to cyclohexa-1,4-diene at 160".(22) (23) (24) (25) General. A stereospecific route to trans-disubstituted cyclopropenes involves the reaction of stable carbanions with vinyl dimethyl sulphonium salts.72 Dimethylsulphoxonium methylide reacts with a-halogenocarbonyl compounds 65 W. H. Pirkle D. Chamot and W. A. Day J. Org. Chem. 1968,33 2152. 66 N. C. Yang and T. A. Marolewski J. Amer. Chem. Soc.. 1968 90 5644. 67 N. Filipesen and J. R. DeMember Tetrahedon. 1968,24. 5181. 6* G. L. Closs W. A. Boll H. Heyn and V. Dev J. Amer. Chem. Soc. 1968,90 173. M. Franck-Neumann Angew. Chem. Internat. Edn. 1968,7,65. 70 M. Franck-Neumann Tetrahedron Letters 1968,2979. 71 E. L. Allred and J. C. Hinshaw J. Amer. Chem. Soc. 1968,90,6885. '' J. Gosselck H. Ahlbrecht F. Dost H. Schenk and G. Schmidt Tetrahedron Letters 1968,995; J.Gosselck and G. Schmidt. Angew. Chem.. Internat. Edn.. 1968.7. 456. 386 J. M. Brown to give acylcyclopropanes. 73 P-Bromo-aldehydes reacts with lithium in ether to form homoenolate anions which may be trapped as cyclopropyl acetates by acetic anhydride. 74 The highly strained ring systems trans-bicyclo[ 5,1,0]-octane7’ and trans-bicyclo[6,1,0]octane76 and their derivatives have been prepared. (B) Cyclobutanes. Cycloaddition routes. Benzyne reacts with enol-ethers and chloro-olefins with a lack of stereospecificity.’ Dimerisations of transient perchlorocyclobutadiene7 and dimethylcyclo- butadiene7’ have been studied; it is of interest that the same mixture of tetra-methyltricyclo-octadienes (26)-(28),is obtained irrespective of the isomer of dichlorodimethylcyclobutenedechlorinated.This suggests that the dimerisation actually involves free dimethylcyclobutadiene. For the first time a stable cyclobutadiene has been characterised. The ‘push-pull’ ynamine (29)is dimerised under Lewis-acid catalysis to cyclobutenium cation (30) which is Me %Me ge Me3 Me Me Me Me Me COzR NRZ NR COzR D COZR NRz COzR (29) (30) (31 converted into (31)with sodium hydride. Ethyl8’ and methyl8’ derivatives have been prepared independently. The compounds are yellow rapidly liquefy in air and show i.r. stretching vibrations at 1660and 1610cm-l. Other cyclobutane-forming cycloadditions include the reactions between dichloroketen and cis- or trans-cyclo-octenes which are stereospecific,82 73 P.Bravo G. Gaudiano C. Ticozzi and A. Umani-Ronchi Tetrahedron Letters 1968,4481. 74 D. P. G. Hamon and R. W. Sinclair Chem. Comm. 1968 890. 7s P. G. Gassman F. J. Williams and F. Seter J. Amer. Chem. Soc. 1968 90 6893. 76 E. J. Corey and J. I. Shulman Tetrahedron Letters 1968,3655;C. H. Depuy and J. L. Marshall J. Org. Chem. 1968 33 3326. 77 1. Tabushi,R. Oda and K. Okazaki Tetrahedron Letters 1968,3743; M. Jones jun. and R. H. Levin ibid. 5593; H. H. Wasserman J. Solodar and L. S. Keller ibid. p. 5597. 78 K. V. Scherer jun. and T. J. Meyers J. Amer. Chem. Soc. 1968,90,6253. 79 R. Criegee W. Eberius and H. A. Brune Chem. Ber. 1968,101,94. R. Gommper and G. Seybold Angew. Chem. Internat. Edn. 1968,7 824. M. Neuenscbwander and A.Niederhauser Chimia (Switz.) 1968,25 491. R. Montainge and L. Ghosez Angew. Chem. Internat. Edn. 1968 7 221. Alicyclic Compounds 387 the dimerisation of dichloromethylenecyclopropane,83and the 1,2-addition reactions of l,ldichloro-2,2-difluoroethylene. 84 Reaction of trichloroallenyl- lithium with methyl chloroformate results in formation of the cyclobutene (32) presumably via cycloaddition of methyl trichlorobuta-2,3-dienoate to the organolithium reagent followed by elimination of lithium chloride. *’ Acyloin routes. The acyloin reaction has not previously been very useful for the preparation of smaller rings because traces of alkoxide formed promote Dieckmann and other condensation reactions. However in the presence of chlorotrimethylsilane which removes alkoxides high yields of acyloins may be produced from succinate esters via isolation as enol-bistrimethylsilyl 87 the protecting groups may be removed by heating under reflux in methanol.trans-Fused systems such as (33) which undergoes rapid con- rotatory ring-opening to (34)may be produced. Semidiones may be detected by e.s.r. in these reactions.88 ]rc2a c1 Me OSiMe &SiMe3 ,c1 OSiMe I H OSiMe c1 H C0,Me (33) (34) C1 H (32) (35) (C) Other cycloaddition routes. Diels-Alder reactions. With cis-planar con- jugated dienes 1,1,2,2-dichlorodifluoroethylenereacts by both 1,2- and 1,4- addition and the amount of 1,Zaddition increases with increasing distance between the diene termini. Tetrachloro- and trichlorofluoro-cyclopropene are good dienophiles and subsequent cyclopropyl-ally1 transformations (Scheme 1) make the products synthetically useful.g0 cis-3,4-Dichlorocyclo- butene reacts with dienes apparently giving products having the stereo- chemistry as shown in for example (35).91 Unusual dienes and dienophiles include norbornadiene-2,3-dicarboxylicanh~dride,’~ isophorone enami~~e,~~ and dimethylfulvene in its addition to dimethyl acetylenedicarboxylate.94 Others. [4 + 21 Addition of allylic cation to diene has been observed and 83 W. R. Dolbier jun. D. Lomas and P. Tarrant J. Amer. Chem. SOC. 1968.90 3594. 84 P. D. Bartlett G. E. H. Wallbillich A. S. Wingrove J. S. Swenton,L. K. Montgomery and B. D. Kramer. J. Amer. Chem. SOC.,1968,90,2049;P. D. Bartlett and J.S. Swenton ibid. p. 2056. 8s G. Kobrich and E. Wagner Angew. Chem. Internat. Edn. 1968,7 470. 86 J. 3. Bloomfield Tetrahedron Letters 1968 587. G. E. Gream and S. Worthley. Tetrahedron Letters 1968 3319. 88 G. A. Russell and P. R. Whittle J. Amer. Chem. SOC. 1967,89,6781. 89 P. D. Bartlett A. S. Wingrove and R. Owyang J. Amer. Chem. SOC.,1968,90,6067. 90 D. C. F. Law and S. W. Tobey J. Amer. Chem. SOC.,1968,90,2376. 91 M. Avram I. G. Dinulescu and G. D. Mateescu Rev. Roumaine Chim. 1968,13 505. 92 J. R. Edman and H. E. Simmons,J. Org. Chem. 1968.33,3808. 93 H. Nozaki T. Yamaguti S. Ueda and K. Kondo Tetrahedron 1968,24 1445. 94 H. Prinzbach and J. Rivier. Angew. Cheni. Internat. Edn.. 1967. 7 1068. 388 J. M. Brown c1 F pc1 \/ + 0-c1 CI SCHEME1 a SCHEME2 promises to be a useful preparative rea~tion.~’ Thus 2-methallyl iodide reacts with silver trichloroacetate in liquid sulphur dioxide in the presence of furan or cyclopentadiene to produce the bicyclo[3,2,1]octene derivatives shown (Scheme 2).Addition probably occurs from the e~o-side,’~ since in acetonitrile the product from cyclopentadiene and methallyl iodide has the stereochemistry shown. Trimethylenemethyl produced by photolysis of its iron tricarbonyl ’’ H. M. R.Hoffmann D. R.Joy and A. K. Suter J. Chem. SOC.(B) 1968,57. 96 H. M. R. Hoffmann and D. R. Joy,J. Chem. SOC.(B). 1968 1182. Alicyclic Compounds derivativeg7 undergoes 1,4-addition to cyclopentadiene giving (36) in a complex mixture of products.2,2-Dimethylcyclopropanone reacts with N-methyl- pyrrole to give the 1,4-addition product (37);’* with furfural both (38) and the product of a 1,2-addition (39) are formed; this latter course of reaction dominates with chloral. A [I6 + 41 addition is implicated in the triethylamine- catalysed dimerisation of the maleic anhydride derivatives (Scheme 3).9 This reaction forms a model for a proposed nonadride biosynthetic pathway; the unnatural trans Et-Et isomer is produced. This is attributed to an iso-merisation in the in vitro system rather than to a differing stereochemical course. SCHEME 3 Tetramers have been formed from allene under the influence of tnstri- pehnylphosphmerhodium chloride (40) ;l O0 from dimethylacetylenedicarboxy-late thermallylol (41); and from ethoxyacetylene under the influence of EtO‘ (40) (41) R = C0,Me (42) 91 A.C. Day and J. T. Powell Chem. Comm. 1968,1241. 98 N. J. Turro. S. S. Edelson J. R. Williams and T. R. Darling J. Amer. Chem. SOC..1968.90. 1926. 99 R. K. Huff C. E. Moppett and J. K. Sutherland Chem. Comm. 1968 1192. loo F. N. Jones and R. V. Lindsey jun. J. Org. Chem. 1968,33 3838. *01 J. C. Kauer and H. E. Simmons J. Org. Chem. 1968,33 2720; E. Winterfeldt and G. Giesler Chem. Ber. 1968 101 4022. 390 J. M. Brown X 3 h e 11111 x 73 .... Ill11x u h Alicyclic Compounds 39 1 sodamide in ammonia at O",in which case a 30 % yield of the cyclo-octatetraene (42) is obtained. lo2 (D)Cyclisation routes.Pride of place must go to the work of W. S. Johnson and his collaborators and developments in their cationic cyclisations. The acetal cyclisation (Scheme 4a) takes place in 30 % yield with stereospecificity ;Io3 in an attempt to study the extent of asymmetric induction the optically active acetal (Scheme 4b) derived from (R,R)butane-2,3-diol was cyclised to the mixture of epimers each of which contained 92% of one enantiomeric form. This was demonstrated by conversion into a hydrindanone of known specific rotation. The acid-catalysed cyclisation of ally1 alcohols was developed and applied to synthesis of the tricyclic compound related to fichtelite(Scheme 4c). lo4 A hydroboration route has been used to convert 1,3-dienes to cyclopenta- nones via carbonylation of cyclic organoboranes.With endo em-cyclic dienes the reaction is highly stereospecific ; thus 1 -vinylcyclohexene gives exclusively (43). Hept-5-ynylmagnesium chloride and related Grignard reagents cyclise on heating to 100" in tetrahydrofuran,lo6 and 1,5-dienes may be con- verted into cyclic ketoneslo7 by a nickel carbonyl insertion route hexa-1,5- diene giving a 70% yield of a 65:35 mixture of 2-methylcyclohexanone and trans- 1,3-dimet hylcyclopentanone and cyclo-octa- 1,5-diene being converted into bicyclo[3,3,l]nonan-9-one. Substituted bicyclo[3,3,0]octanediones may be formed in the reaction between a-diketones and glutaric esters.lo8 Conialog has reviewed aspects of his work on cyclisation by intramolecular en-reactions of enols e.g.Scheme 5. A phosphonium-salt route to benzocycloalkanes is reported.' lo 0 SCHEME 5 The 'twistane' system which has been obtained optically active,"' may be prepared in one step through the cyclisation (44) -.) (45)'12 Reaction of octatrienes with sodium piperidide causes a carbanion cyclisation to isomers '02 J. F. H. Braams H. J. T. Bos. and J. F. Arens. Rec. 'I).au. chim.. 1968.87. 193. lo3 W. S. Johnson K. Wiedhaup S. F. Brady and G. L. Olson J. Amer. Chem. SOC. 1968 90 5277; W. S. Johnson C. A. Herbert and R. D. Stipanovic ibid. p. 5279. lo4 W. S. Johnson N. P. Jensen J. Hooz and E. J. Leopold J. Amer. Chem. SOC. 1968,90 5872. lo5 H. C. Brown and Ei-Ichi Negishi Chem. Comm. 1968,594. lo6 H. G. Richey and A. M. Rothman Tetrahedron Letters 1968,1457.lo' B. Fell W. Seide and F. Asinger Tetrahedron Letters 1968 1003. lo' U. Weiss and J. M. Edwards Tetrahedron Letters 1968,4885. lo9 J. M. Conia Bull. SOC.chim.France 1968 3057. 'lo H. J. Bestmann and D. Ruppert Angew. Chem. Internat. Edn. 1968,7,637. '11 K. Adachi K. Naemura and M. Nakazaki Tetrahedron Letters 1968,5467. A. Belanger J. Poupart. and P. Deslongchamps. Tetrahedron Letters 1968. 2127. 392 J. M. Brown of methylcycloheptadiene ; potassium piperidide merely causes isomerisa- tion.'' Internal cyclisations of cyclododecanones have been studied. 'l4 H (43) (E) Routes to macrocyclic and bridged compounds. A simple synthesis of large-ring ketones by photolysis or pyrolysis of ketone peroxides has been presented ; cyclohexanone diperoxide gives cyclodecane and 11-undecano-lactone and cyclohexanone triperoxide gave pentadecane and 16-hexadecano- lactone related to the perfume ingredient ambrettolide.''' Yields were low but no attempt was made to achieve optimum conditions. Cyclo-octyne''6 reacts under olefin metathesis conditions (tungsten hexachloride and di- ethylaluminium ethoxide in methanol) to give macrocyclic olefins up to C12, ring-size. Bicyclo[2,l,l]hex-2-ene and some derivatives have been produced by a variety of routes. ' ' The bridgehead olefin bicyclo[3,3,l]non-l-ene has been synthesised and defies Bredts rule in its stabi1ity.'l8 A full paper on a homo- cubane synthesis has appeared. ''' There has been great interest in adamantane chemistry encouraged by the antiviral acticity of 1 -aminoadamantane.A simple synthesis12' of the skeleton is outlined (Scheme 6).Polycyclic compounds (46),12' (47),'22 (48),12 and (49)'24 have been prepared. The last-named 'bastardane' was an artefact in an attempted synthesis of a diamantane. Solvolysis of the epimeric bicyclo- [3,2,2]nonan-6-y1 p-nitrobenzene sylphonates provides entry into the bicyclo- [3,3,l]nonane and bicyclo[4,2,l]nonane series.'25 Reactions.-(A) Thermolysis. Cyclopropanes and fused cyclopropanes. E. A. Zuech D. L. Crain and R. F. Kleinschmidt J. Org. Chem. 1968,33,771. L. A. Paquette and M. L. Wise Tetrahedron 1968,24,2937. P. R. Storey D. D. Denson C. E. Bishop B. C. Clark jun. and J. C. Farine J. Amer. Chem. SOC. 1968,90,817.'I6 E. Wasserman D. A. Ben-Efraim and R. Wolovsky J. Amer. Chem. SOC. 1968,90,3286. J. Meinwald and F. Uno J. Amer. Chem. SOC.,1968,90 800; F. T. Bond and L. Scerbo Tetra-hedron Letters 1968 2789; K. B. Wiberg and R. W. Ubersax ibid. p. 3063. ''*J. R. Wiseman J. Amer. Chem. SOC.,1967,89,5966; J. A. Marshall and H. Faube ibid. p. 5964. '19 G. L. Dunn V. J. DiPasquo and J. R. E. Hoover J. Org. Chem. 1968,33 1454. ''O H. Stetter and H. G. Thomas Chem. Ber. 1968,101 1115. ''I B. R. Vogt S. R. Suter and J. R. E. Hoover Tetrahedron Letters 1968 1609. B. R. Vogt Tetrahedron Letters 1968 1579. M. Takahashi Y. Kitahara I. Murata T. Nitta and M. C. Woods Tetrahedron Letters 1968 3387. P. von R. Schleyer E. Osawa and M. G. B. Drew J. Amer. Chem. SOC. 1968,90,5034.''' J. P. Schaefer L. S. Endres and M. D. Moran J. Org. Chem. 1967,32 3963. 393 Alicyclic Compounds n 0 CH,Br Et02C4 Et0,C ee,COzEt CO,Et 'OZEt 70,Et i SCHEME 6 Me0,C C0,Me c3 (48) (47) Racemisation of optically active trans-1,2-diphenylcyclopropaneis 1.5 times faster than its coversion into cis-l,2-diphenylcyclopropane ;this supports the intervention of a diradical intermediate. lZ6cis-2-Alkylcyclopropanecarboxylic esters are converted into 76-unsaturated esters at 260" by a 1,5-hydrogen shift mechanism. 12' Vinylcyclopropanes. The novel rearrangement of (50) to (51) and related lZ6 R. J. Crawford and T. R. Lynch Canad. J. Chem. 1968,46 1457. 12' D. E. McGreer and N. W. K. Chu Canad. J. Chem. 1968,46,2217.394 J. M. Brown transformations have been observed at 210".lZ8 Heating the aldehyde (52) above 100" causes a rearrangement'" to (53) with activation energy 25.5 kcal. mole- '. The vinylmethylene-cyclopropane(54) undergoes ready Cope rearrangement at 80" to give 5-methylenecyclopentene (55). 30 Pyrolysis of the homofulvene (65)gives (57). Since a suprafacial hydrogen shift is required the reaction is probably non-concerted. '31 Simonetta and his co-workers have combined a x-electron calculation with strain energy minimisation procedures to give an estimate of activation parameters for Cope rearrangements in cis-divinylcycloalkanes.'32 Thermolysis of vinylcycloheptatriene gives (58) the product of Cope rearrangement in vinylnorcaradiene albeit in low yield.' 33 In bullvalene chemistry the interesting observation has been made that fluorobullvalene exists almost entirely with fluorine at the methine position in accordance with the expectation that fluorine being highly electron-withdrawing will prefer substituent positions with hybrid orbitals of low s character.'34 Treatment of bromobullvalene with strong base gives the corresponding cycloalkyne which forms a dimer and a trimer. The dimer must initially exist as a cyclobutadiene but n.m.r. studies show that (59) predominates among the seventeen possible fluxional tautomers.' 35 Tetracyclo[4,4,0,02~lo 059 ']deca-3,8-diene (65) has previously evaded detection; now it is shown that photolysis of CloHlo isomers (60) (61) and (62) all give this species which rearranges at room temperature to M.S. Baird D. G. Lindsay and C. B. Reese Chem. Comm. 1968 784. F. Bickelhaupt W. L. De Graff and G. W. Klumpp Chem. Comm. 1968 53. 130 T. C. Shields W. E. Billups and A. R. Lepley J. Amer. Chem. SOC.,1968,90,4749. 131 H. Hart and J. D. DeVrieze Chem. Comm. 1968 1651. M. Simonetta G. Favini C. Mariani and P. Grarnaccioni J. Amer. Chem. SOC.,1968,90 1280. J. Daub and P. von R. Schleyer Angew. Chem. Znternat. Edn. 1968,7 468. J. F. M. Oth R. Merenyi H. Rottele and G. Schroeder Tetrahedron Letters 1968 3941. 13' R. J. Bottcher H. Rottele. G. Schroeder and J. F. M. 0th Tetrahedron Letters 1968 3925. Alicyclic Compounds (59) bicyclo[4,2,2]deca-2,4,7,9-tetraene (63).'36a A related observation has been made on an anhydride derivative,' 36b and intramolecular Diels-Alder reactions in (64)followed by reconversion of (65)account for the observation of deuterium scrambling between the vinyl and diene bridges' 36c and similar chemical transformations.'36d H H H (60) (61) R44 R / (63) R = H (64) R = D (65) Bicyclo-butanes and -pentanes. Pyrolysis of bicyclobutanes with one or two methyl groups is >90% stereospecific with dimethyl compounds giving the product anticipated from a reverse [02s + o'a] cycloaddition (Scheme 7). '37 The monosubstituted compounds where ring-opening is not governed by detectable orbital symmetry factors give >90 % trans-pentadiene. The highly strained tricyclo[4,1,0,02* 7]-heptane (66) pyrolyses by a pathway consistent with the involvement of a cis,trans-cycloheptadiene.138 Ethoxy-carbonyl migration probably via a zwitterion occurs139 in the thermolysis shown in Scheme 8.136 (a)S. Masamune R. T. Seidner H. Zenda M. Wiesel N. Nakatsuka and G. Bigam J. Amer. Chem. SOC. 1968 90 5286; (b) E. Babad D. Ginsburg and M. B. Rubin Tetrahedron Letters 1968 2361; (c) M. Jones jun. and B. Fairless ibid. p. 4881; (d) W. Grimme H. J. Riebel and E. Vogel Angew. Chem. 1968,80,803. 13' G. L. Closs and P. E. Pfeffer J. Amer. Chem. SOC. 1968,90,2452. K. B. Wiberg and G. Szeimies Tetrahedron Letters 1968 1235. '" M. J. Jorgenson and T. J. Clark J. Amer. Chem. SOC. 1968,90,2188. 396 J. M. Brown RidMe R2eMe R2 R' R' and R2 = H or Me SCHEME 7 SCHEME 8 Meo2c&C02Me A Me02C Meo2c&H MeO& C02Me co2Meh + co2M€€ C0,Me C0,Me C02Me C0,Me SCHEME 9 Prinzbach has observed a number of thermal rearrangements of exo-tricyclo[3,2,1,02*4]oct-6-enes which are rationalised in accordance with Scheme 9,l4O endo-Compounds in this class are normally more inert but the endo-Diels-Alder adduct of triphenylcyclopropene and cyclopentadiene 0'9 6,triphenyltetracyclo[3,3,0,04*to arearranges at 190" 8]octane.141 Cyclobutanes and cyclobutenes.The liklihood of a diradical intermediate in cyclobutane ring opening is ~ossidered.~ Pyrolysis of (67)to 9-methyl-6-octalone occurs at 360" and involves trans-ring hydrogen transfer to an 140 H. Prinzbach W.Eberbach M. Klaus and G. von Veh Chem. Ber. 1968,101,4066. H. Prinzbach and H.-D. Martin. Helv. Chim. Acta 1968,51 438. Alicyclic Compounds enolate radi~a1.I~~ Bicycle[l,l,l]pentane pyrolysis (E 49 kcal. mole- ') has been studied. 143 It is significant that cyclobutanones (68) and (69)interconvert in boiling water or xylene; this implies a concerted path for keten cycloaddi- tions by analogy with the Woodward-Katz arguments. 144 Many examples of pyrolysis of cyclobutenes to butadienes have accumulated and in the simplest case the enthalpy of reaction2* is about -10kcal. mole-'. In one case however steric hindrance so changes the relative stabilities of diene and cyclobutene that (70) cyclises stereospecifically to (7l) which sub- sequently epimerises by an ionic me~hanism.'~' A failure to interconvert cis,trans-1,4-dimethyl-l,2,3,4-tetraphenylbutadieneand the cis,cis-isomer dur- ing 51 days at 125" suggested a lower energy limit for the orbital-symmetry forbidden disrotatory opening of a cyclobutene since both diene isomers were in reversible thermal equilibrium with cyclobutenes.Activation energies have been obtained for pyrolysis of hexamethyl dewar benzene,6 hexafluoro dewar benzene,I4' 3-phenyl~yclobutene,'~~ hexamethylbicyclo[2,2,0]hexa- 2,5-dieneI4' and cis-or trans-fused tricyclic cyclobutenes (72).15* In accord with orbital symmetry expectation trans-fused (72) pyrolysed with E ca. 28 kcal. mole- ';that for the cis-fused compound (72) was >40kcal mole-'. Thermal or photochemical ring-opening of cyclobutenones is not governed by orbital symmetry criteria but is nevertheless quite stereoselective in ex- (7 0) (71) (72) n = 4,5 or 6 142 C.H. Heathcock and B. E. Ratcliffe J. Amer. Chem. SOC. 1968,33,3650. 143 R. Srinivasan J. Amer. Chem. SOC. 1968,90,2752. 14* P. Yates and A. G. Fallis Tetrahedron Letters 1968,2493. 145 G.A. Doorakian and H. H. Freedman J. Amer. Chem. SOC. 1968,90,3582. 146 G. A. Doorakian and H. H. Freedman J. Amer. Chem. SOC.,1968,90 5310. 14' I. Haller J. Pbys. Chem 1968,12,2882; E. Ratajczak and A. F. Trotman-Dickenson J. Chem. SOC. (A) 1968,509. 14* M. Pomerantz and P. H. Hartman Tetrahedron Letters 1968,991. 149 H. C. Volger and H. Hogeveen Rec. Trav. chim. 1967,86 1356. lSo R. Criegee and H.G. Reinhardt Chem. Ber. 1968,101 102. 398 J. M. Brown amples studied. An explanation based on minimising non-bonded interactions in the transition state was applied (Scheme 10). Cope rearrangements. Three papers'52 have appeared on the 'Oxy-Cope' rearrangement. Thermolysis according to Scheme 11 allowed estimation of the relative rates of rotation and ring-closure in the allylic diradical intermediates since the reaction must occur in a stepwise manner. Without rotation (A) would give exclusively (A) and (B) (B). The results show that closure competes successfully with rotation. c1 MeOH CI C0,Me\/CH 100" Ph Cl Cl SCHEME 10 9- 250" \ OR 5-SCHEME 11 Retro-Claisen rearrangement in the Diels-Alder endo-adduct of dimethyl- fulvene and cis-diacetylethylene has been 0b~erved.l~~ Cope reactions and electrocyclic reactions account for the product nature in thermolysis of cis-bicyclo[6,2,0]deca-x,9dienes.'54 All-trans-bicyclo[8,4,0]tetradeca-2,8-diene racemises with a half-life of 1day at 50°.155 Hydrogen and alkyl trunsfer.Equilibria and activation parameters are recorded for the isomerisation of methylcycloheptatrienes by a 1,5-hydrogen shift mechanism.156 Methyl transfer have been observed in the thermal isomerisation of 5,5dimethylcyclopentadienes.lS7 The thermolysis of methyl- 151 J. E. Baldwin and M. C. McDaniel J. Amer. Chem. SOC.,1968,90,6118. 15' J. A. Berson and E. J. Walsh jun. J. Amer. Chem. Soc. 1968,90,4729,4730,4732. 15' M. T. Hughes and R. 0.Williams Chem.Comm. 1968,587. 154 P. Radlick and W. Fenical Tetrahedron Letters 1967,4901. 15' P. S. Wharton and R. A. Kretchmer J. Org. Chem. 1968,33,4258. K. W. Egger J. Amer. Chem. SOC.,1968,90,6. lS7 J. W. De Haan and H. Kloosterziel Rec. Trap. chim. 1968 87 289; V. A. Mironov V. S. Pashegorova. T. M. Fadeeva and A. A. Akhrem Tetrahedron Letters 1968,3397. Alicyclic Compounds enehexamethylcyclohexadiene gives ethylpentamethylbenzene by an inter-molecular mechanism at 170° as shown by deuterium labelling.'58 Tosylhydrazone salt pyrolysis. A further example of the cyclopropanecarbo- aldehyde -,cyclobutene transformation has appeared allowing the preparation of exo-tricyc1o[4,2,1,O2* 5]non-3-ene.'59 In the case of (73) however pyrolysis of the dry sodium salt gave a pyrazoline (74) which gives bullvalene and (63) on further heating.16' Photolysis of (74) gave (63) and (75) as primary photo- products.Homofulvenes (77) were produced from the tosylhydrazones161 (76)and mechanisms of the reaction are discussed. In dioxan solution pyrolysis of the sodium salt of a-phenyl-a-tropylacetaldehydetosylhydrazone'62 gave hydrocarbon products and a pyrazole (78) and pyrazoline (79) and pyrolysis and thermolysis of the latter were studied. -R '@R1 130" IIIII U Ph (79) (B) Reactions involving cleavage of single bonds. Hydrogenation of cyclo-propanes has been applied to the introduction of quaternary carbon centres in adamantane'63 and ring-fused cyclohexanes. Reduction of (80) over platinum gave 9-methyldecalins without stereo~pecificity,'~~ but (81) was H.Hart and J. D. DeVrieze Tetrahedron Letters 1968,4257. 159 R. R. Sauers S. B. Schlosberg and P. E. Pfeffer J. Org. Chem. 1968,33,2175. 160 S. Masamune H. Zenda M. Wiesel N. Natatsuka and G. Bigam J. Amer. Chem. SOC. 1968 90,2727. 16' M. Rey U. A. Huber and A. S. Dreiding Tetrahedron Letters 1968 3583. 162 H. Tsuruta K. Kurabayashi and T. Mukai J. Amer. Chem. SOC., 1968,90,7167. 163 C. W. Woodworth V. Buss and P. von R. Schleyer Chem. Corn. 1968,569. 164 Z. Majerski and P. von R. Schleyer. Tetrahedron Letters 1968 6195. 400 J. M. Brown reduced by lithium in ammonia-t-butyl alcohol to P-(trans-hydrindanyl)-ethanol stereoselectively.165 Vinylcyclopropanes are catalytically isomerised to pentadienes by a catalyst formed through reduction of bis(tributy1phosphine)- nickel (ZI) chloride with di-isobutylaluminium hydride in the presence of ethylene.166 Spiropentane reacts with 167 ethylenepalladium(I1) chloride dimer to give a ring-opened n-ally1 complex.Peracid oxidation of the alkenylidene- cyclopropane (82) gave inter aliu the ring-opened product (83).16' Much work has been presented on the reaction between bicyclobutanes or bicyclopentanes and dienophiles and mechanisms involving diradical intermediates formed by cleavage of the most strained bond are invoked. This explains the preponderance of en-type products over 1,2-addends and the observed steric course of reaction. Deuterium-labelled bicyclobutane reacts with benzyne; isolation of the minor product (Scheme 12) shows reaction to have occurred at the 'inside' face of bicyc10butane.l~~ Tricyclo[4,1,0,02*7]-heptane reacts with benzyne uiu (84) to give endo-3-phenyltricyclo[3,2,0,0z~ '1-heptane (85a).170 Bicyclopentane reacts at room temperature with dicyano- acetylene and at 100" with dicarbomethoxyacetylene to give mainly en-product with dimethyl propiolate three products are formed corres- SCHEME 12 ponding to l,Zaddition en-reaction and a different kind of hydrogen-shift in the intermediate diradical (86) giving (87).17' Solvent effects show that a zwitterion is not involved.16' A steric course similar to Scheme 12 was demon- strated in the 1,2-addition of maleic anhydride to a specifically labelled bicyclo- lfi5 H.0.House and C. J. Blankley J. Org. Chem. 1968,33,47. 166 R. G. Miller and P. A. Pinke J. Amer. Chem. SOC.,1968,90,4500. lfi7A. D. Ketley and J. A. Braatz Chem. Comm. 1968,959. lfiE J. X.Crandall D. R. Paulson and C. A. Bunnell Tetrnhedron Letters 1968 5063. M. Pomerantz G. W. Gruber and R. N. Wilke J. Amer. Chem. SOC.,1968,90,5040. 170 P. G. Gassman and G. D. Richmond,J. Amer. Chem. SOC.,1968,90,5637. 17' P. G. Gassmann and K. T. Mansfield J. Amer. Chem. SOC.,1968,90,1517. 172 P. G. Gassman and K. T. Mansfield J. Amer. Chem. Soc. 1968,90 1524. Alicyclic Compounds 401 pentane (45% yield).173 The opposite steric course is involved in hydrogenation of bicyclopentanes;thus (88) is reduced to (89) stereoselectively. 174 Bicyclo[3,l,0]hexanes generally react much less readily because of a much lesser degree of strain; however (85b) reacts with dicyanoacetylene to give the en-product (8%).17' Hexamethyl dewar benzene very readily rearranges under acid catalysis giving (90a) with sulphuric acid in methan01,'~~ and (90b) with hydrochloric acid in methylene chloride. 177 Toluene-p-sulphonic acid rearranges hexa- methylprismane to (91),l 76 also obtained from (90b) and morpholine.' These observations help to rationalise the formation of (pentamethylcyclo- pentadiene)platinum dichloride and (pentamethylcyclopentadieny1)rhodum chloride' 79 in treatment of hexamethyl dewar benzene with appropriate metal salts and the formation of (90a) in reaction between but-2-yne and bis(benzonitri1e)palladiumdichloride.8o R NC-c \ 'C -H (85) a; R = Ph (Sf-' I 14) b; R=H CN Me (88) (90) a; R = CH:CH (911 b; R = CHClMe '13 P. G. Gassman K. T. Mansfield and T. J. Murphy J. Amer. Chem. SOC. 1968,90,4746. 174 M. J. Jorgenson Tetrahedron Letters 1968,4577. 175 P. G. Gassman and G. D. Richmond Chem. Comm. 1968,1630. 176 R. Criegee and H. Gruner Angew. Chem. 1968,80,447. 177 L. A. Paquette and G. R. Krow Tetrahedron Letters 1968,2139. P. V. Balakrishran and P. M. Maitlis Chem Comm. 1968 1303; J. W. Kang K. Mosley and P. M. Maitlis ibid. p. 1304. 179 J. W. Kang and P. M. Maitlis J. Amer. Chem. SOC. 1968,90 3259. lSo H. Reinheimer H. Dielt J. Moffat D. Wolff and P. M. Maitlis J. Amer. Chem. SOC.,1968,90 5321. 402 J.M. Brown Fragmentations. Lithium aluminium hydride reduction of P-keto-tosylates in bridged-ring systems occurs with fragmentation in certain bicyclic systems ; several examples are quoted"' in the work of Kraus and Rothenwohrer. p-Hydroxy-tosylates fragment in basic media typical examples are (92) -+ (93),'82b and (94) -+ (95);182c under solvolytic conditions (94) follows a different reaction course. OH (94) (95) (C) General chemistry. Small rings. Full details have appeared on earlier work relating to the preparation and properties of cyclopropanones. 83 The (presumed) cyclopropanone (96) previously postulated as an intermediate in Zimmerman's photochemical studies on bicyclo[3,l,0]hexenes has been generated chemically by two different routes with different substituents and observed in both cases to rearrange to (97) with inversion as predicted by orbital symmetry consideration^.'^^ Dichlorocyclopropenone has been pre- pared.'85 Selective deuteriation of the vinylic protons in cyclopropene in basic media is observed' 86 and the tri-t-butylcyclopropenylcation has been prepared.187 In the latter study it was observed that di-t-butylcyclopropenone was stable to sodium hydroxide solution at 100".Hydrolysis of dichlorocyclo- propenes gives mainly cyclopropenones but the corresponding ethynyl ketone is an important by-product. '''P-Arylcyclopropyl radicals are observed to lie between Ph. and Me-in selectivity towards an RH-CCl mixture.189 IE1 W. Kraus and W. Rothenwohrer Tetrahedron Letters 1968 1013.lE2 (a) W. D. I(.Macrosson J. Martin W. Parker and A. B. Penrose J. Chem. SOC.(C) 1968 2323; (b)C. H. Heathcock and R. A. Badger Chem. Comm. 1968 1510; (c)J. A. Marshall C. J. V. Scanio and W. J. Iburg J. Org. Chem. 1967,32 3750. IE3 N. J. Turro and W. B. Hammond Tetrahedron 1968,24,6017,6029. IE4 H. E. Zimmerman and D. S. Crumrine J. Amer. Chem. SOC.,1968,90,5612; T. M. Brennan and R. K. Hill J. Amer. Chem. SOC. 1968,90 5614. lSs R. West J. Chickos and E. Osawa J. Amer. Chem. SOC. 1968,90,3885. lE6 E. A. Dokro and R. W. Mitchell Tetrahedron Letters 1968,341. lE7 J. Ciabattoni and E. C. Nathan tert. J. Amer. Chem. SOC.,1968,90,4495. 188 E. V. Dehmlow Chern. Ber. 1968,101,410,427. lE9 T. Shono M. Akashi and R. Oda Tetrahedron Letters.1968 1507. Alicyclic Compounds Isomerisation of a-cyclopropylethanol in acidic media has been studied ;lgo of interest here is the use of a topological graph to interrelate all possible carbonium ion interconversions in the system and thereby to rationalise the product distribution. The lithium salts (98)and (99),are prepared by deprotonation of 1,2-dimethyl- 3,4-dimethylenecyclobuteneand 2,4-dimethyl-1,3-dimethylenecyclobutanere-~pectively.'~' The former formally has four sp2 carbon atoms in a four- membered ring and proton chemical shifts suggest that negative charge resides largely on the exo-methylene groups. Bromomethylenecyclobuteneis rearranged by strong bases to 1-substituted pentene derivatives apparently most rapidly in non-polar media.192 R'-R' R' 0cp :.,+'. (99) (97) (98) The epimeric l,ldichloro-2-hydroxycyclobutanes(110) rearranges stereo- specifically in basic media (101a) being in equilibrium with a 4-oxabicyclo- [3,2,l]octadiene by a fast reversible Claisen reaction. lg3The fused cyclobutene (102) can be epoxidised and the epoxide is of particular interest in being a derivative of tetrahedral carbon with all valence bonds on one side of a plane; despite this the product is reasonably stable. lg4 Monocyclic systems. Dehydrobromination of (103) follows a different course with different bases ; with sterically hindered dicyclohexylethylamine the product is (104) and with potassium t-butoxide (105); both are formed in high yields. Bisthiocarbanion reactions have been studied in cyclohexenone systems and in a new route to C3+ cyclic systems.'96 Highly strained trans,trans-cyclo-octa-l,5dienehas been prepared in low yield by photolysis of the &,cis-isomer in the presence of cuprous chloride and has A,, 246 nm.(E 1500).lg7A detailed investigation of cs+8cycloalkynes and cyclic allenes is reported;lg8 the best method or preparing cycloalkynes involved the analogues of Rees's benzyne precursor. 19' The major product M. Julia and Y. Noel Bull. SOC. chim. France 1968,3749. lgl D. Seebach and B. Graf Angew. Chem. Internat. Edn. 1968,7,538. lg2 K. L. Erickson B. E. Vanderwaart and J. Wolinsky Chem. Comm. 1968 1031. lg3 P. R. Brook Chem. Comm. 1968,565. lg4 K. B. Wiberg J. E. Hiatt and G. Burgmaier Tetrahedron Letters 1968 5855.lg5 H. Stetter and H. J. Sandhagen,Annalen 1968,712 67. lg6 E. J. Corey and D. Crouse J. Org. Chem. 1968 33 298; D. Seebach N. R. Jones and E. J. Corey ibid. p. 300. lg7 G. M. Whitesides G. L. Goe and A. C. Cope J. Amer. Chem. SOC. 1967,89 7136. G. Wittig and H. L. Dorsch Annalen 1968 711 46; G. Wittig H. L. Dorsch and J. Meske-Schuller. ibid.. p. 55 G.Wittig and J. Meske-Schuller ibid. p. 65; G.Wittig and P. Fritze ibid.. p. 82. 404 J. M. Brown from vinyl bromides (106) on reaction with potassium t-butoxide in dimethyl sulphoxide appears to be the cyclic allene which may be trapped with phen- cyclone or diphenylisobenzofuran ; in the absence of trapping agents the dimer (107) is produced. The steric course of Favorskii reaction in a pair of medium-ring old-dibromo-ketone diastereomers is consistent with a con-certed cis-elimination (108) with disrotatory ring-closure.''' C1 (100a) (101a) (102) (100b) C(6)-epimer (101b) C(6)-epimer Br (103) (104) (7JBr (106) Bridged compounds. Electrophilic addition of deuterium chloride of acetic C2H]acid to bicyclo[2,1,1] hex-2-ene occurred with cis-specificity and ruled out the possibility of a bridged-ion intermediate.200 Norbomene-7-one reacts with deuterium chloride by a &,em-mechanism again without the incursion of a bridged ion. 2o 1-Chloronorbornene is extremely unreactive towards hydrogen chloride eventually giving the 1,3-dichloro-isomer ;however 2-chloronorbornene which reacts rapidly gives the 2,2-dichloro-isomer.Thus bridged ions are not important in these systems.202 The effects of 7-substituents on additions to norbomene2O3 and free radical reactions of norb~rnenes~~~. 205 lg9 E. W. Garbisch jun. and J. Wohllebe Chem. Comm. 1968 306. 'O0 F. T. Bond J. Amer. Chem. Soc. 1968,!MJ 5326. R. Caple H. W. Tan and F. M. Hsu J. Org. Chem. 1968,33 1542. '02 A. F. Fry and W. B. Farnham Tetrahedron Letters 1968. 3345. 203 W. C. Baird and M. Buxa J. Org. Chem. 1968,33,4105;T. T. Tidwell and T. G. Traylor ibid. p. 2614. '04 T. V. Auken and E. A. Rick Tetrahedron Letters 1968 2709. 'OS C. L. Osborn T. V. Van Auken and D. J. Trecker J. Amer. Chem. SOC. 1968,90 5806; L. E. Barstow and G. A Wiley Tetrahedron Letters 1968 865; C. W. Jefford and W. Wojnarowski ibid.p. 3763. Alicyclic Compounds are described including a case where such addition is substantially endo. Reduction of epimeric norbomen-7-yl bromides with tributyltin deuteride gave anti-7deuterionorbomene with some stereoselectivity a result claimed to be consistent with a nonclassical radical.206 The possibility of steric and inductive effects causing this stereoselectivity was not considered. Pre-reduced palladium catalysts selectively catalyse conversion of norbornadiene into exo-5,6-dideuterionorborneneby deuterium. O7 Many papers have appeared on reactions and interconversions of adaman-tane derivatives,208 including the interesting ring expansion (109) -+ (110). Further work on bridgehead free radicals strongly indicates the desire for coplanarity at a radical centre.209 Non-classical carbanions (1 1 1)2 and (112)2l1 have been prepared ; the former by ether cleavage and the latter via the reaction between dimsyl sodium and methanonaphthalene.In both cases the n.mr. spectra support the occur- rence of a high degree of charge delocalisation and in (111)both vinyl bridges appear to be equivalent consistent with a symmetrical structure or perhaps more reasonably with rapid bridge-flipping on an n.m.r. time scale. Prostaglandins. Many efforts have been made to synthesise members of the important family of prostaglandin hormones produced in uiuo by oxidative GJ: then EtOH D (coc''z H -R R,q 0 (109) (111) '06 J. Warkentin and E. Sanford J. Amer. Chem.SOC.,1968,90 1667. 'O' B. Franzus W. C. Baird jun. and J. H. Surridge J. Org. Chem. 1968,33,1288. '08 A. C. Udding J. Strating and H. Wynberg Tetrahedron Letters 1968 1345; T. Sasaki S. Eguchi and T. Tory Tetrahedron Letters 1968 4135; W. H. W. Lunn W. D. Podmore and S. S. Szinai J. Chem. SOC.(C) 1968 1657; K. Bott Chem. Eer 1968,101,564. R. C. Fort and R. E. Franklin J. Amer. Chem. SOC. 1968,90,5267; J. P. Lorand S. D. Chodroff and R. W. Wallace ibid. p. 5266. 'lo J. B. Grutzner and S. Winstein J. Amer. Chem SOC. 1968,90,6562. '11 W. A. Boll Tetrahedron Letters 1968 5531. 406 J. M. Brown cyclisation of CZOunsaturated fatty acids. This enzymic process may be mimicked in one case by autoxidation of all-cis-8,11,14-eicosatrienoicacid followed by stannous chloride reduction where a yield of 0.1 % of prostaglandin El(113) is obtained.212 More rational approaches have been used to produce 7-0xaprostaglandins,~~~ in which the key reaction is conversion of the epoxide (1 14) into (1 15) with diethyl(oct-1-yny1)aluminium.The main dificulty in most synthetic routes has been the production of a cyclopentane with correct stereochemistry; this has been circumvented by use of non-stereospecific reactions and complex isolation techniques,214 or by aiming at the production of prostaglandins with few epimeric centres. 215 This difficulty proved no deterrent to Corey's group and an elegant and widely applicable total synthesis is presented.216 Readers are referred to the original communications for full details.The key step is formation of the Diels-Alder adduct (116) and its transformation after appropriate derivitization glycol formation and lead tetra-acetate cleavage into (117),which cyclised to (118) when treated with oediger base. This last compound was converted into (&)-prostaglandin El in 20% yield and the product was transformed into other members of the prostaglandin series. A second non-stereoselective synthesis from the same laboratory is presented cyclisation of (119) leads to all the possible C-9 and C-11 epimers. 0 iI OCH,Ph OH OH (115) (116) 'I2 D. H. Nugteren H. Vonkeman and D. A. Van Dorf Rec. Trav. chim. 1967,86,1237. '13 J. Fried S. Heim S. J. Etheredge P. Sunder-Plassmann T. S. Santhanakrishran,J. I. Himizu and C.H. Lin Chem. Comm. 1968,634. *I4 K. G. Holden B. Huang K. R. Williams J. Weinstock M. Harman and J. A. Weisbach Tetrahedron Letters 1968,1569; W. P. Schmieder U. Axen F. H. Lincoln J. E. Pike and J. L. Thomp- son J. Amer. Chem. SOC. 1968,90 5895. 215 R. B. Morin D. 0.Spry K. L. Hauser and R. A. Mueller Tetrahedron Letters 1968 6023; R. Klok H. J. J. Pabon and D. A. Van Dorp Rec. Trav. chim. 1968,87 813. '16 E. J. Corey N. H. Andersen R. M. Carlson J. Paust E. Vedejs I. Vlattas and R. E. K. Winter J. Amer. Chem. SOC. 1968 90 3245; E. J. Corey I. Vlattas N. H. Andersen and K. Harding ibid. p. 3247. 407 Alicyclic Compounds

ISSN:0069-3030    

DOI:10.1039/OC9686500379

出版商:RSC    

年代:1968

数据来源: RSC

摘要:

12 TERPENOIDS AND STEROIDS By A. B. Turner (Department of Chemistry University of Aberdeen Aberdeen AB9 2UE) Biogenetic-type Syntheses. -Chemical simulations of further stages in the build-up and cyclisation of acyclic polyenes are reported this year. The total synthesis of a steroid has been achieved by conversion of the tetraenol (1) into the tetracyclic diene (2) in the presence of trifluoroacetic acid. This transformation which involves the stereospecific formation of five asymmetric centres provides the closest non-enzymic analogy so far for the biological formation of the steroid nucleus. The stereoselectivity of the process is confirmed by oxidative ring-opening of the crude olefin (2) to the triketo-aldehyde (3) followed by double ring closure to ( +)-16-dehydro-progesterone (4) in an overall yield of 29%.[The trisubstituted double bond of the tetraene (1)was introduced by means of a new stereoselective synthesis exemplified by conversion of the alcohol (5) into the trans-olefin (6),in 85% yield using anhydrous zinc bromide.21 In an extension of this type of process cyclisation of the tetraenic acetal (7) with tin (IV) chloride gives the tetracyclic alcohol (8) in 30% yield.3" Both C4 isomers are obtained. A high degree of asymmetric induction attends the cyclisation of the related acetal(9; R = Me) with the same ~atalyst,~' while its optically inactive derivative (9; R = H) is known to give racemic mixtures. Studies in this area of bio-organic chemistry have been re~iewed,~" as have those on the cyclisation of terminal ep~xides.~' Details of the olefinic cyclisations promoted by the generation of allylic carbonium ions have appeared.' ' W.S. Johnson M. F. Semmelhack M. U. S. Sultanbawa and L. A. Dolak J. Amer. Chem. SOC. 1968,90,2994. S. F. Brady M. A. Ilton and W. S. Johnson J. Amer. Chem. SOC. 1968,90,2882. (a)W. S. Johnson K. Wiedhaup S. F. Brady and G. L. Olson J. Amer. Chem. SOC.,1968.90 5277; (b)W. S. Johnson C. A. Harbert and R. D. Stipanovic ihid.. p. 5279. '(a)W. S. Johnson Accounts Chem. Res. 1968 1. 1;(b) E. E. van Tamelen. ibid. p. 111. W. S. Johnson N. P.Jensen J. Hooz and E. J. Leopold J. Amer. Chem. SOC. 1968,90 5872. 0'& A. B. Turner HH IH OCH;CH,OH (9) (7) (8 ) Stereospecific cyclisation of methyl trans,trans-farnesate (10) on cation-exchange resins is reported to give methyl monocyclofarnesate (12; R = Me) and the bicyclic products (13; R = Me) in better yields than those obtained using Lewis acid catalysts.ti Addition of benzoyloxy-radicals to the corres- ponding acetate (11)gives the bicyclic product ( 14).7 This free-radical pathway is also remarkably specific. Monocyclofarnesic acid (12; R = H) gives the rearranged bicyclic acid (15) along with the normal products (13; R = H) on treatment with boron trifluoride.* I (14) (15) The possibility that the interfarnesyl bond of squalene is formed with the aid of a thiol grouping of the enzyme has led to a new understanding of the mechanism and scope of the sigmatropic rearrangements of allylic sulphonium H.Moriyama Y. Sugihara and K. Nakanishi Tetrahedron Letters 1968,2851. ' R. Breslow S. S. Oh and J. T. Groves Tetrahedron Letters 1968 1837. * Y. Kitahara T. Kato and S. Kanno J. Chem. SOC.(C),1968,2397. Terpenoids and Steroids 411 ylides and related compounds. Studies in a number of laboratories have shown that carbon-carbon bond formation occurs readily in these systems under mild condition^.^ The process involves concerted rearrangement of six electrons in a five-membered transition state by virtue of the capacity of the sulphur atom to undergo conversion from the formally quadrivalent into the divalent state The reaction is illustrated by the conversion of the dimethylallylsulphonium fluoroborate (16) into the rearranged sulphide (17) which occurs almost quantitatively at room temperature with a variety of ba~es.'~?~ Conversion of the unsymmetrical diallylsulphonium salt (18) into the rearranged sulphide (19) provides chemical analogy for the enzymic coupling of farnesol as well as a simple procedure for tail-to-tail coupling of allyl units.'O In an alternative method for the coupling reaction benzyne-promoted rearrangement of digeranyl sulphide (20) gives the phenyl sulphides (21) and (23) while geranyl linalyl sulphide (22) gives mainly (21).la Similarly unsymmetrical coupling of geraniol can be achieved by conversion of the sulphide (20) into its ethyl- sulphonium salt which rearranges in the presence of potassium t-butoxide to the ethyl analogue of (23).'lb The transformation (20) -+(23) provides analogy for the biogenesis of skeletal features of artemesia alcohol and bakuchiol.Another convenient method for tail-to-tail coupling of allyl units involves (a)G. M. Blackburn W. D. Ollis J. D. Plackett C. Smith and I. 0.Sutherland Chem. Comm. 1968 186; (6) J. E. Baldwin R. E. Hackler and D. P. Kelly ibid..pp. 537 538; (c)R. B. Bates and D. Feld Tetrahedron Letters 1968 417; (d)B. M. Trost and R. La Rochelle ibid ,p 3327 lo J. E. Baldwin R. E. Hackler and D. P. Kelly J. Amer. Chem. Soc. 1968,90 4758; cf ref. 12. l1 (a) G. M. Blackburn and W. D. Ollis Chem. Comm. 1968 1261; (b)J. E. Baldwin and D. P. Kelly ibid.,p. 899. 412 A. B. Turner the combined action of titanium trichloride and alkyl- or aryl-lithium on ally1 alcohols.This procedure allows reductive coupling of geraniol to the 1,5-diene in good yield.12 Monoterpenoids.-Conflicting results for the addition of maleic anhydride to the allo-ocimenes (24) and (25) are explained by a competing cis-trans- isomerisation which is catalysed by the anhydride. This isomerisation of (24) to (25) occurs with a variety of x-acids. Steric inhibition of the s-cis-form of the triene (24) accounts for the concerted addition-abstraction reaction with azodicarboxylates while addition to the 2,4diene system readily occurs to the s-cis-form of the isomer (25) both with azodicarboxylates and maleic anhydride. In the latter reactions the cis-relationship of the two methyl groups rules out the competing addition-abstraction process.Syntheses of the alcohols (26 and its 2,3-dihydro-derivative) and (_+)-cis-verbenol are reported. l4 These are the principal components of the sex attract- ant produced by the male bark beetle Ips confusus boring in ponderosa pine. The alcohol (27) of a new skeletal type has been isolated from the oil of Arternisiafeddei growing in the suburbs of Hiroshima.' Sensitized photo-oxygenation of methyl homosafranate (28) gives the spiroperoxy-lactone (29) as well as the expected endo-peroxide (30). Further l2 K. B. Sharpless R. P. Hanzlik and E. E. van Tamelen J. Amer. Chem. Soc. 1968,90,209. l3 E. Koerner von Gustorff and J. Leitich Tetrahedron Letters 1968,4689; 4693. l4 C. A. Reece J. 0.Rodin R. G. Brownlee W. G. Duncan and R. M. Silverstein Tetrahedron 1968,24,4249.S. Hayashi. K. Yano. and T. Matsuura. Tetrahedron Letters 1968 6241. Terpenoih and Steroids 41 3 transformations of these intermediates lead to (4)-loliolide and related compounds. Transformations of several types of epoxide have been studied. The ct-hydroxy-epoxide (31) is converted into pinocarvone (32) in basic solution :I7 The a-keto-epoxide pulegone oxide (33) gives the products of ring opening (34) methyl migration (35) and ring enlargement (36) upon treatment with electrophilic catalysts or pyrolysis in the liquid or gas phase.18 The behaviour of a-cyclopropyl epoxides in the presence of acids has been examined using various epoxy~aranes.'~ Opening of the epoxide ring is controlled by the adjacent cyclopropane ring giving the pseudoallylic carbonium ion and the cyclopropane ring itself is then cleaved to give the tertiary carbonium ion e.g.A similar mechanism is probable for the deamination of caran-2- and -5-amines leading to menthadienes and menthenols.20 Concerted processes may operate in these cases as the geometry is favourable. Details of thermal and acid- catalysed rearrangements of (+)-carene,2'" and further work on the stereo- l6 E. Demole and P. Enggist Helv. Chim. Acta 1968 51,481. l7 J. M. Coxon E. Danstead M. P. Hartshorn and K. E. Richards Chem. Comm. 1968 1076. '* W. Reusch D. F. Anderson and C. K. Johnson J. Amer. Chem. SOC.,1968,90,4988. l9 G. Ohloff and W. Giersch Helv. Chim. Acta 1968,51 1328. 2o W. Cocker D. P. Hanna and P.V. R. Shannon Tetrahedron Letters 1968,4217; W. Cocker A. C. Pratt and P. V. R. Shannon J. Chem. SOC.(C),1968,484. (a)W. Cocker D. P. Hanna and P. V. R. Shannon,J. Chem.SOC.(C),1968,489; (b)P. J. Kropp D. C. Heckert and T. J. Flautt Tetrahedron 1968 24 1385; (c) M. S. Carson W. Cocker D. H. Grayson A. C. Pratt and P. V. R. Shannon J. Chem. SOC.(B) 1968 1136. 414 A. B. Turner chemistry of its electrophilic substitution,2 lb have been published The con- formational preferences of the cis-caranols have been deduced using a combina- tion of n.m.r. data relative rates of oxidation and esterification and hydrolysis of their dinitrobenzoates.21c The effect of primary amines on cis-and trans-carbone tribromides shows that a-axial halogeno-ketones can in fact undergo Favorskii rearrangement.22 The importance of solvent polarity in controlling the course of such reactions is again apparent.The natural tropone nezukone (37),has been synthesised by the action of silver fluoroborate on the adduct (38) of dichlorocarbene and the Birch reduc- tion product of 4-i~opropylanisole.~~ The co-occurrence of this tropone with pinenes and thujanes in a Thuja spp. reinforces the suggestion that tropones arise from the latter by ring expansion. The glucoside loganin which is of some importance in the biogenesis of indole alkaloids is present in plants of the Rubiaceae family.24a Loganic acid co-occurs with gentiopicroside in Gentianuceue spp.24b Sesquiterpen0ids.-Identification of substances active in the communicative systems of insects continues.2,3-Dihydro-6-trans-farnesolis the main com- ponent of the marking perfume of male bumble bees.25 The acid (39),containing a degraded sesquiterpene skeleton is secreted during courtship by the male monarch butterfly.26 Its structure has been confirmed by synthesis. 22 J. Wolinsky R. 0.Hutchins and T. W. Gibson J. Org. Chem. 1968,33,407. 23 A. J. Birch and R. Keeton J. Chem. SOC.(C),1968 109. 24 (a)D. S. Bhakuni and R. S. Kapil Experientia 1968,24,1185 ;(b)C. J. Coscia and R. Guarnaccia Chem. Comm. 1968 138. 25 G. Bergstrom B. Kullenberg S. Stallberg-Stenhagen and E. Stenhagen Arkiu Kemi 1968,28 543. J. Meinwald,A. M. Chalmers T. E. Pliske and T. Eisner Tetrahedron Letters 1968,4893. Terpenozds and Steroids 415 Total syntheses are announced of the racemic juvenile hormone (40) responsible for arresting development of Hyalaphora cecropia at the pupa The high yields and stereoselectivity of these make the hormone readily accessible.One ~ynthesis,’~“ which starts from p-methoxytoluene makes use of a number of novel procedures. These include the stereospecific conversion of prop-2-ynyl alcohols into trisubstituted olefinic alcohols using organometallic reagents selective propynylation and a novel conversion of ally1 alcohols into esters under mild conditions by the following sequence :2 -(!d-CO-CN A&-CO,R + HCN The reactions are carried through without isolation of intermediates. The a-and fJ-sinensals which form the flavour constituents of the Chinese orange have been prepared from myrcene.” They exist in the all-trans- configuration being aldehydes derived from D-and a-farnesene respectively.A new three-stage route to abscisic acid (abscissin) is de~cribed.~’ The first stage makes use of t-butyl chromate to oxidise both allylic positions of a-ionone. A stereospecific synthesis of natural (+)-juvabione (41) is reported and the absolute configurations of the two asymmetric centres are established by chemical correlation. 31 The allene (42) forms part of the repellent secretion emitted by grasshoppers when di~turbed.~’ Its structure resembles the end-groups of the recently isolated carotenoid pigments fucoxanthin and neoxanthin. 27 (a)E. J. Corey J. A. Katzenellenbogen N. W.Gilman S. Roman and B. W. Erickson J. Amer. Chem. SOC. 1968,90,5618;(b)W. S. Johnson T. Li D. J. Faulkner and S. F. Campbell ibid. p. 6225; cf R. Zurfliih E. N. Wall J. B. Siddall and J. A. Edwards ibid. p. 6224. ’* E. J. Corey N. W. Gilman and B. E. Ganem J. Amer. Chem. SOC.,1968,90 5616. 29 G. Buchi and H. Wuest Helv. Chim. Acta 1967 50 2440; E. Bertele and P. Schudel ibid. p. 2445; CJ R. Teranishi A. F. Thomson,P. Schudel and G. Buchi Chem. Comm. 1968,928. 30 D. L. Roberts R. A. Heckman B. P. Hege and S. A. Bellin J. Org. Chem. 1968,33 3566. 31 B. A. Pawson H.-C. Cheung S. Gurbaxam and G. Saucy Chem. Comm. 1968 1057. 32 J. Meinwald K. Erikson M. Hartshorn Y. C. Meinwald and T. Eisner Tetrahedron Letters 1968 2959. 416 A. B. Turner 0 GQo (43) (44) (4 7) (48) AcO oc-0 oc- 0 (49) (50) (51) .=\ HO 02) (5 3) (5 4) Chemical support is provided for hypotheses concerning the biogenesis of various classes of bicyclic and tricyclic sesquiterpenes from cyclodecadienes.Photocyclisation of the dienone (43) yields ketones (44) and (45) containing the copaene and bourbonene ring system.33 Whereas cyclisation of the 1,2- epoxide of the germacrane (46) with electrophilic catalysts gives the expected trans-decalin the 5,6-epoxide yields the guaiane (47). 34 The only analogous cyclisation is that of the germacranolide parthenolide which is also a 5,6-epoxide. It is therefore possible that biochemical control of cyclisation to the guaiane ring system is achieved via monoepoxide formation.It is suggested3’ that cis-fused eudesmanes could arise from cyclodeca-l,5-dienes by dehydro- 33 C. H. Heathcock and R. A. Badger Chem. Cornm. 1968 1510. 34 E. D. Brown and J. K. Sutherland Chem. Comm. 1968 1060. ” A. G. Hortmann Tetrahedron Letters 1968 5785. Terpenoich and Steroich 417 genation to the triene (48).Thermal (disrotatory) cyclisation would then lead to the cis-ring-fusion. Another explanation involves cyclisation of alternative conformers of the 1,5-dienes. Furanodiene (49) the likely precursor of many mono- and bi-cyclic sesquiterpenes has been isolated. 36 N.m.r. studies show that the germacranolide dilactone (50) exists in two distinct conformers at room temperat~re.~~ The conformations of the related lactone (51) have been investigated using nuclear Overhauser effects,38 and the absolute configura- tions of derived epoxides have been e~tablished.~~ The essential oil of Hedycarya augustifolia is a rich source of elemol(52) but this compound has now been shown to be an artifact.When the leaves are extracted at room temperature the major product is its precursor hedycaryol (53) and only a trace of elemol is ~btained.~' This confirms earlier doubts about the natural occurrence of some elemanes. By contrast the dilactone vernolepin (54) can be isolated along with the isomeric lactone by hot or cold extraction of Vernonia hymen ole pi^.^' A germacranolid. occurs in the same species.42 More furanoid elemanes are reported,43 as well as their thermal rearrangement through germacranes to cad inane^.^^ The alternative mode of cyclisation to those described above reflects the influence of the carbonyl group in the ten-membered ring and leads to the aromatic structure (55) The structure of eremophilene has been revised to (56) following synthetic in which the stereospecific introduction of the angular methyl group is achieved by the action of lithium dimethyl copper on an ap-unsaturated ketone.This method of reductive methylation originally used by House provides an efficient entry into the vicinal dimethyl system of the eremophilanes 36 H. Hikino K. Agatsuma and T. Takemoto Tetrahedron Letters 1968,931. 37 H. Yoshioka T. J. Mabry and H. E. Miller Chem. Comm. 1968 1679. 38 K. Takeda I. Horibe M. Teraoka and H.Minato Chem. Comm. 1968,940. 39 K. Takeda I. Horibe and H. Minato Chem. Comm. 1968. 1168. 40 R. V. H. Jones and M. D. Sutherland Chem Comm. 1968 1229. 41 S. M. Kupchan R. J. Hemingway D. Werner A. Karim A. T. McPhail and G. A. Sim J. Amer. Chem. SOC.,1968,!MJ 3596. 42 R. Toubiana and A. Gaudemer Tetrahedron Letters 1967 1333. 43 H. Hikino K. Agatsuma and T. Takemoto Tetrahedron Letters 1968 2855; S. Hayashi N. Hayashi and T. Matsuura ibid. p. 2647. 44 H. Hikino K. Agatsuma C. Konno and T. Takemoto Tetrahedron Letters 1968 4417. 45 E. Piers and R. J. Keziere Tetrahedron Letters 1968 583 ;R. M. Coates and J. E. Shaw ibid. p. 405 cf J. Krtpinsky' 0.Mtol L. DolejS L. Novotnf V. Herout and R. B. Bates ibid. p. 3315. 418 A. B. Turner and has also been used in the total synthesis of (+)-nootkatone from 4-acetyl-l- e thox ycycl ohexene.46 P-Gorgonene (57) from Pseudopterogorgia ~rnericana,~~ is an analogue of the intriguing monoterpene sylvestrene.The cyclopropane (58) is a minor component from the same species Sirenin (59) the sperm attractant of the water mould AIlomyces is an isoprene homologue of ~arane.~’ Syntheses of a-and P-agarofurans (a),“’ and routes to various aristolones (e.g. 61),” are reported. Several spirolactones having the bicyclononane structure (62) have been isolated.51 This new skeletal type appears to be biogenetically related to the eremophilanes. Details of the extensive chemical work on anisatin (63)52 and coriamyrtin (64),’ and their hydroxy-derivatives have appeared.Microbiological oxidation has been useds4‘ in the structural elucidation of guaioxide (65) which can be prepared from guaiol by lead tetra-acetate oxidation followed by catalytic hydrogenation. 54b Liguloxide has finally been shown to be 4-epi-g~aioxide.’~ Further natural examples of the rare fulvene system have been found in the guaiane class (e.g. 66).” The total 46 M. Pesaro G. Bozzato and P. Schudel Chem. Comm. 1968 1152. 47 A. J. Weinheimer P. H. Washecheck D. van der Helm and M. B. Hossain Chem. Comm. 1968 1070. 48 W. H. Nutting H. Rapoport and L. Machlis J. Amer. Chem. SOC. 1968,90,6434. 49 H. C. Barrett and G. Buchi J. Amer. Chem. SOC. 1967,89 5665; J. A. Marshall and M. T. Pike J. Org. Chem. 1968 33 435; A. Asselin M.Mongrain and P. Deslongshamps Canud. J. Chem. 1968,46,2817. 50 E. Piers W. de Waal and R. W. Britton Chem Comm. 1968 188; R. M. Coates and J. E. Shaw ibid. p. 515 ; C. Berger M. Franck-Neumann and G. Ourisson Tetrahedron Letters 1968,3451. N. Abe R. Onoda K. Shirahata T. Kato M. C. Woods and Y. Kitahara Tetrahedron Letters 1968,369; N. Abe R. Onoda K. Ro and T. Kurihara ibid. p. 1993; K. Naya I. Takagi M. Hayashi S. Nakamura M. Kobayashi and S. Katsumura Chem. and Ind. 1968 3 18. 52 L. K. Yamada S. Takada S. Nakamura and Y. Hirata Tetrahedron 1968,24,199. 53 T. Okuda and T. Yoshida Chem. and Pharm. Bull. (Japan),1967,15 1687 1697 1955. ’‘ (a) H. Ishii T. Tozyo and H. Minato Chem. Comm. 1968 649; (b)C. Ehret and G. Ourisson Bull. SOC.chim. France 1968,2629 ;(c)H.Ishii,T. Tozyo and H. Minato Chem. Comm. 1968,106,1534. 55 D. J. Bertelli and J. H. Crabtree Tetrahedron 1968 24 2079. Terpenoids and Steroids 419 synthesis of racemic bulnesol is reported. 56 Pseudoguaianolides have been reviewed,57aand syntheticapproaches to the ring system made from ~antonin.~~' Full papers are now available on the structural elucidation of the himacha- lenes and related products.58 P-Himachalene (67) can be converted directly into cuparene (68) in 40% yield by pyroly~is.'~ The high yield is apparently due to the ready formation of a biallylic diradical which recyclises to the easily oxidised dihydro-derivative of (68). However incomplete racemization suggests the partial operation of a concerted mechanism which the authors claim conflicts with theoretical predictions concerning [1,3]-sigmatropic rearrangements in this type of system.A total synthesis of sativene (69) is reported.60a This makes use of a general route to the tricyclic carbon skeleton involving intramolecular alkylation in a cisdecalone a process which is similar to that used to prepare copaene.60b Cyclosativine (70) is related to sativine (69) as longicyclene is to longifolene. Prolonged treatment with copper(@ acetate in refluxing acetic acid gave the same equilibrium mixture from both compounds (69 and 70) indicative of a common carbonium-ion intermediate. 61 The mould metabolite culrnorin (71) is the first longifolene-based product isolated from a micro-organism. 62 Its J. A. Marshall and J.J. Partridge J. Amer. Chem SOC. 1968,90 1090. 57 (a)J. Romo and A. Romo de Vivar Fortschr. Chem. org. Naturstoffe 1967 25 90; (6) J. B. Hendnckson C. Ganter D. Dorman and H. Link Tetrahedron Letters 1968 2235. T. C. Joseph and S. Dev Tetrahedron 1968,24 3809 et seq. '' H. N. Subba Rao N. P. Damodaran and S. Dev Tetrahedron Letters 1968,2213. 6o (a) J. E. McMurry J. Amer. Chem. SOC. 1968 90 6821; (6) cf Ann. Reports 1966 63 447; C. H. Heathcock R. A. Badger and J. W. Patterson J. Amer. Chem. SOC.,1967,89 4133. 61 L. Smedman and E. Zavarin Tetrahedron Letters 1968 3833; J. E. McMurry ibid. 1969 55. 62 D. H. R. Barton and N. H. Werstiuk J. Chem. SOC.(C),1968 148. 420 A. B. Turner structure was established by degradation to tetrahydroeucarvone and con- version into enantio-longiborneol.Its absolute configuration is thus the inverse of that of the plant metabolite longifolene (72). Studies on stable carbonium ions obtained by dissolving longifolene in fluorosulphonic acid reveal that the normal acid-catalysed rearrangement to iso-longifolene (73) is by-passed in favour of other useful intermediate^.^^ At temperatures below -3O" n.m.r. data indicate that the cation (74) is formed both from (72) and (73); quenching with sodium carbonate at this stage gives the diene (75) in high yield. Similar combination of physical and chemical evidence demon- strates the further rearrangement of the carbonium ion (74) into the ions (76) and (77) as the temperature of the fluorosulphonic acid solution rises.The transformation (76) +(77) probably involves spiro-intermediates analogous to those in the dienone-phenol and anthrasteroid rearrangements. I (71) (72) (73) Hydroboration of longifolene (69) followed by oxidation with hydrogen peroxide gives the expected longifolol (78) whereas the isomeric alcohol (80) formed by transannular transfer is also obtained when silver oxide is used as the oxidising agent. 64a Lead tetra-acetate oxidation of longifolol (78) gives the tetrahydrofuran (81) and the longicyclenes (82; R = H and CHO).64b The epimeric alcohol (79) also gives the cyclopropane hydrocarbon but not its aldehyde derivative. Tricyclenes are not produced by oxidation of endo- and em-camphanols with lead tetra-acetate cyclic ethers only being formed.64c 63 D.G. Farnum and G. Mehta Chem. Comm. 1968 1643. 64 J. Lhomme and G. Ourisson (a) Tetrahedron 1968,24 3167; (b) ibid. p. 3177; (c)ibid. p. 3201. Terpenoids and Steroids 421 Details of the structual elucidation of longicyclene (82; R = Me) have been p~blished,~~" and \Ir-longifolic acid is shown to have structure (82; R = C0,H). 65b (78;R = CH,OH R1= H) (80) (81) (82) (79; R = H R1= CH,OH) Tricyclovetivene (83; R = Me) and various oxidised forms (e.g. 83; R = CH,OH and C02H)occur in vetiver oils.66 The revised structure of p-vetivone has been confirmed by synthesis of its ra~emate.~~ A total synthesis of illudin-M (84) is remarkable for the sequence used to protect and expose particular carbonyl groups.68 Michael addition of the fl-keto-sulphoxide (85) to the cyclopentenone (86) gives exclusively the product (87).This adduct undergoes Pummerer rearrangement to the ketone (88) which on heating in ethanol yields a single methyl ketone (89). Base treatment of (89) affords the enedione (go) the new keto-group of which is selectively attacked by methylmagnesium iodide to give an intermediate readily con- vertible to (+)-illudin-M (84). Details of the photochemical syntheses of a-and P-bourbonenes are avail- able,69 and a new route to the a-isomer involves photolysis of 1,6-dienes of type (91)'' Syntheses of isoiresin (92) and dihydroiresin (93) from the ketone (94) have been reported.71 0 + 0 (83) (84) (85 1 (86) 65 (a) V. R. Nayak and S. Dev Tetrahedron 1968 24 4099; (b) G.Mahta V. R. Nayak and S. Dev ibid. p. 4105 c$ ref. 64b. 66 R. Sakuma and A. Yoshikoshi Chem Comm. 1968,41; H. Komae and I. C. Nigam J. Org. Chem. 1968,33 1771; F. Kido H. Uda and A. Yoshikoshi Tetrahedron Letters. 1968 1247; 6099; I. C. Nigam H. Komae G. A. Neville C. Radecka and S. K. Paknikar ibid. p. 2497. 67 J. A. Marshall and P. C. Johnson Chem Comm. 1968,391. 68 T. Matsumoto H. Shirabama A. Ichihara H. Shin S. Kagawa F. Sakan S. Matsumoto and S. Nishida J. Amer. Chem. Soc. 1968 90 3280. 69 J. D. White and D. N. Gupta J. Amer. Chem SOC.,1968,90,6171. 70 M. Brown J. Org. Chem. 1968,33,162. 71 S. W. Pelletier and S. Prabhakar J. Amer. Chem. SOC.,1968,90 5318. 422 A. B. Turner HO HO (90) (91) (92) 0 Ac 0 (93) (94) Diterpenoids.-Re-examination of the acid-catalysed dehydration of various labdadienol progenitors of the mesomeric cation (95)has shown that rosadienes (96) are formed as well as pimaradienes by a process akin to the biogenetic one.72 Hibane derivatives such as the tetracyclic alcohol (97) can also be isolated.This alcohol is thought to arise by coupling at the terminal methylene groups to give the cyclo-octene cation (98) which rearranges uia the cyclo- butane (99) to the bridged alcohol (97). This suggested mechanism is sub- stantiated by solvolysis of a model cyclo-octene tosylate. 72b Ruzicka's hydro- carbon now shown to have structure (lOO) is also obtained in good yield from manool and related compounds under more vigorous conditions.Ozonolysis of cis-abienol(101) gives the alkoxy-hydroperoxide (102'), the cleavage probably being initiated by intramolecular attack of the hydroxy-group on the ozonide linkage.74 Opening of ring B or of rings A and B occurs with concurrent aromatization of ring c when methyl levopimarate is treated with base at 72 (a)0. E. Edwards and R. S. Rosich Canad. J. Chem. 1968,46 1113; (b)E. Wenkert and Z. Kumazawa Chem. Comm. 1968 140; (c)T. McCreadie and K. H. Overton ibid. p. 288. 73 R. M. Carman and N. Dennis Tetrahedron Letters 1968,4127. 7* R. M. Carman and D. E. Cowley Tetrahedron Letters 1968,2723. Terpenoih and Steroih 200°.75 The first synthesis of a member of the trachylobane class (+)-methyl- trachylobanate (103) is described.76 The pentacyclic system is constructed from the major adduct of methyl levopimarate and n-butyl crotonate by successive oxidations and reductions leading to the bridged alcohol (104; R = H). The final stage involves borohydride reduction of the bridged cation from (104),to give directly the enantiomer of the natural acid. Details of the studies on hibaene and related compounds from the trunkwood of E. monogynum have been published. (98) do-oH@‘ HO (102) k7\/ C0,Me (101) (103) Two synthetic approaches to the tanshinones (e.g. 105) have been made. One7 involves condensation of a 2-hydroxy-lY4-naphthaquinonewith a P-chloropropionyl peroxide to give a dihydrofurano-o-quinone which is 75 H. Takeda W. H. Schuller and R. V. Lawrence J. Org.Chem. 1968,33,3718. 76 W. Herz R. N. Mirrington H. Young and Y. Y. Lin J. Org. Chem. 1968,33,4210. 77 R.McCrindle A. Martin and R. D. H. Murray J. Chem. SOC.(C),1968 2349; cf A. Martin and R. D. H. Murray ibid. p. 2529. 78 (a)A. C. Baillie and R. H. Thomson,J. Chem. SOC. (C) 1968,48;(b)H. Kakisawa M. Tateishi and T. Kusumi Tetrahedron Letters 1968 3783. 424 A. B. Turner dehydrogenated with dichlorodicyanobenzoquinone. The other starts from a trimethoxy-a-tetralone. 8b Various new syntheses of ring-c-aromatic diterpenes are reported. Isoxazole annelation has been employed to prepare ferruginol.79 The fact that quaternary salts of isoxazole derivatives are readily converted into acyl phenols makes this a convenient route. A method for constructing the 11,12-dihydroxylated ring c of carnosic acid derivatives makes effective use of P-keto-sulphoxides.80 The sequence can be adapted to produce ferruginol derivatives.In work on podocarpic acid and methyl dehydroabietate,81 the 1,3-diaxial shielding effect of a carbonyl group was useful for distinguishing stereoisomers at (2-4. Taxodione (106) and taxodone (107) are tumour inhibitors isolated from Taxodium distichurn.82a The anti-tumour activity of quinone methides has been noted previously. The extended quinone chromophore of taxodione is similar to that of fuscin but the stability of the quinone methide (107) is remarkable in a fused-ring system. (On treatment with acid it isomerises to dihydrotaxo- dione.) Both molecules possess the stabilizing feature of a hydroxyl group ortho to the 0x0-group and their structures have been confirmed by conversion into 11-methoxyferruginol methyl ether.The natural occurrence of these compounds is of interest in connection with the formation of artifacts derived from carnosic acid.82b Quinone methides analogous to taxodone (107) are likely intermediates in the formation of carnosol rosmaricine and other ferruginoid products isolated from plants of the Labiatae family and the extended quinone methide fuerstion also occurs in this group. 82c (106; X = 0) (105) (107; X = AH --OH) A novel fragmentation occurs on treatment of the totarol derivative (108) with sodium hydrogen carbonate. 83 Elimination of hydrogen bromide involves 79 M. Ohashi T. Maruishi and H.Kakisawa Tetrahedron Letters 1968 719. D. C. Shew and W. L. Meyer Tetrahedron Letters 1968,2693; W. L.Meyer R. W. Huffman I$ and P. G. Schroeder Tetrahedron 1968,24,5959. T. A. Spencer T. D. Weaver R. M. Villafica R. J. Friary J. Posler and M. A. Schwartq J. Org. Chem. 1968,33,712;T.A.Spencer R. J. Friary W. W. Schmiegel J. F. Simeone and D. S. Watt ibid. p. 719. 82 (a) S. M. Kupchan A. Karim and C. Marcks J. Amer. Chem. SOC.,1968,90 5923;c$ Ann. Reports 1965,62,336;(c)D.Karanatsios J. S. Scarpa and C. H. Eugster Helv. Chim. Act6 1966,49 1151. 83 R. C. Cambie and R. T. Gallagher Tetrahedron 1968,24,4631. Terpenoids and Steroids cleavage of the bond common to rings A and B and leads via the quinone methide (109) to the cyclodecanone (110) in 80% yield.This reaction is a modification of the classical method for the preparation of quinone methides and the fragmentation is favoured by the antiperiplanar configuration of the ring-a bonds involved. Inumakilactone (1 11) and related a-pyrones have been isolated from Podocarpus rnacropyllus. 84 Hydroxytotarol which occurs in the same plant may be their biogenetic precursor as catechols are known to yield a-pyrones by oxidative cleavage. The cyclopropanes (112) and (113) are formed along with the ether (114; R = Et) when abietic acid is photolysed in ethan01.’~ When methanol is the solvent only products of type (114; R = Me) are formed. The cyclopropanes could be formed from a bicyclobutane intermediate but the reason for the change to a carbonium-ion mechanism leading to the ether (1 14) in methanol is not clear.Br (10 8) (109) (1 10) 0 H COzH COzH (114) 84 S. Ito M. Kodama M. Sunagawa T. Takahashi H. Imamura and 0.Honda Tetrahedron Letters 1968,2065; Y. Hayashi S. Takahashi H. On& and T. Sakan ibid. p. 2071. 85 J. C. Sircar and G. S. Fisher Tefrahedron Letters 1968,5811. 426 A. B. Turner Total syntheses of (+)-gibberellins A, A, A, and A, have been achieved.86 Conclusion proof of the stereochemistry of atractylogenin has been provided by correlation with kaurenolides of proven stereochemistry at C-9. 87 Details of the chemical work on phorbol are reported.88 Its highly reactive hydroxy- cyclopropyl carbinol system leads to various reactions resulting from cation formation c1 to the cyclopropane ring Several interesting intramolecular oxidations with lead tetra-acetate are also encountered.Sesterterpen0ids.-Acyclic members of this group are reported this year. The alcohols moenocinol (115) and its allylic isomer (116) form the lipid portion of the antibiotic moenomycin. 89a The all-trans-alcohol geranyl nerolidol has been isolated from the fungus responsible for leaf spot disease in maize.89b The same organism also produces the ophiobolin (117) which is of biogenetic interest. 89b Ceroplasteric acid (1 18) and the corresponding alcohol are present in the waxy coating secreted by Ceroplastes alboline~tus.~~ Their structures were determined by X-ray analysis. These are the first of the ophiobolins to be isolated from an insect source.The fungal metabolite (117) F:Hrco2H H... (118) (119) OH 86 K. Mori M. Shiozaki N. Itaya T. Ogawa M. Matsui and Y. Sumiki Tetrahedron Letters 1968 2183; T. Ogawa K. Mori M. Matsui and Y. Sumiki ibid. pp. 125 2551. 87 E. L. Ghisalberti P. R. Jefferies S. Passannanti and F. Piozzi Austral. J. Chem. 1968 21 459; J. R. Hanson and A. F. White Tetrahedron 1968,24,2533. L. Crombie M. L. Games and D. J. Pointer J. Chem. SOC.(C) 1968 1347. 89 (a) R. Tschesche F.-X. Brock and I. Duphorn Tetrahedron Letters 1968 2905; (b)S. Nozoe M. Morisaki K. Fukushima and S. Okuda ibid. p. 4457. 90 Y. Iitaka. I. Watanabe I. T. Harrison and S. Harrison J. Amer. Chem. SOC.,1968 90 1092. Terpenoids and Steroids 427 fusicoccin (119)’ has the ophiobolin ring system.” An intriguing feature of this diterpene is the presence of a fifth isoprene unit attached to the glucose moiety.Triterpenoids.-The stereochemistry of the carbocyclic nucleus of ( +)-malabaricol has been established by correlation with ( +)-ambrein~lide.~~ The related tricyclic alcohol (120) is produced by enzymic cyclisation of 18J9-dihydrosqualene 2,3-0xide.’~” Analogous products were reported last year from the non-enzymic cyclisation of squalene 2,3-oxide but this is the first demonstration that the enzyme can produce structures different from lanosterol. The possibility that lanosterol biosynthesis could proceed via this tricyclic intermediate is discussed. The squalene derivative was obtained by reductive coupling of appropriate ally1 alcohols using a combination of titanium trichloride and alkyl- or aryl-lithi~m.’~~ This is a general synthesis of 1,Sdienes which can be carried out without isolation of intermediates.Limonoid bitter principles have been re~iewed.’~ Details of structural work on the tetranortriterpenoids mexicanolideg5” and grandif~lione’~~ have been published. The parent alcohol of a series of limonoids from the timber of Trichilia heudelotii is shown96a to be 14P,lSP-epoxy-7a,ll p,12a- trihydroxymeliac-1 -en-3-one (121)’ related to hirtin. The nomenclature is based on the 17-furylgonane system (122) of these limonoids for which the name ‘meliacan’ is proposed. Odoratol (123) and related tirucallols are ob- tained from various Cedrela specie^.'^ (120) (121) 91 A.Ballio M. Brufani C. G. Casinovi S. Cerrini W. Fedeli R. Pellicciani B. Santurbano and A. Vaciago Experientia 1968 24 631; K. D. Barrow D. H. R. Barton E. B. Chain C. Conlay T. V. Smale R. Thomas and E. S. Waight Chem. Comm. 1968 1195; K. D. Barrow D. H. R. Barton E. B. Chain U. F. W. Ohnsorge and R. Thomas ibid. p. 1198; E. Hough M. B. Hursthouse S. Neidle and D. Rogers ibid. p. 1197. 92 R. R. Sobti and S. Dev Tetrahedron Letters 1968,2215. 93 (a) E. E. van Tamelen K. B. Sharpless R. Hamlik R. B. Clayton A. L. Burlinghame and P. C. Wszolek J. Amer. Chem. Soc. 1967,89 7150; (b)cf ref. 12. 94 D. L. Dreyer Fortschr. Chem. org. NatwstojJie 1968,26 190. 9s (a) J. D. Connolly R. McCrindle and K. H.Overton Tetrahedron 1968 24 1489 et seq.; (b)J. D. Connolly K. L. Handa R. McCrindle and K. H. Overton J. Chem. SOC.(C) 1968 2227. 96 D. A. Okorie and D. A. H. Taylor J. Chem. SOC.(C) 1968 1828; cf E. K. Adesogan and D. A. H. Taylor ibid. p. 1974. 97 J. D. Connolly K. L. Handa R. McCrindle and K. H. Overton J. Chem. SOC. (C) 1968,2230; W. R. Chan N. L. Holder D. R. Taylor G. Snatzke and H.-W. Fehlhaber ibid. p. 2485. 428 A. B. Turner (122) (123) (124) (12 5) Fusidic and helvolic acids have been correlated using a combination of chemical and microbiological transformation^.^^ Autoxidation of lanostenyl acetate has been re-e~amined.~' The major products are the 7p-and 1lP-hydroperoxides and several minor peroxides are also formed. Further oxida- tion of these primary products has been studied.Cycloartenol has been synthesised from lanosterol. loo Functionalization at C-19 was achieved by photolysis of the 11 p-nitrite in the presence of iodine. The resulting 19-iodo- 11-alcohol was oxidised to the 11-ketone and treatment of this y-iodo-ketone with base gave the 9~,1Oj3-cyclopropyl-l1 -ketone which was finally reduced with lithium aluminium hydride. Actein (124)"' and ambonic acid (125)'02 are related compounds with differing degrees of side-chain oxidation. The (R)-configuration at C-25 of the latter is established by Baeyer-Villiger oxida- tion of the derived 24-ketone and 0.r.d. determinations on the resulting propane-l,2-diol. '02 Factors affecting the reactivity of olefins towards epoxidation are dis-cussed.'03 Euphol or euphorbol can be selectively attacked at the side-chain double bond particularly in ether.A synthesis of P-amyrin has been rep~rted."~ Olean-l2-ene is converted 98 W. von Daehne H. Lorch and W. 0. Godtfredsen Tetrahedron Letters 1968 4843; cf s. Okuda,Y. Sato T. Hattori and M. Wakabayashi ibid. p. 4841. 99 J. Scotney and E. V. Truter J. Chem. SOC.(C) 1968 1911; 2184; 2516. loo D. H. R. Barton D. Kumari P. Welzel L. J. Danks and J. F. McGhie Chem. Comm. 1968 643; J. Chem. SOC.(C) 1969,332. lo' H. Linde Arch. Pharm. 1967,300,982; 1968,301 120. lo' S. Corsano and E. Mincione Chem. Comm. 1968 738. G. Possinet G. Ourisson and G. Charles Bull. SOC.chim. France 1967,4453. Io4 D. H. R. Barton E. F. Lier and J.F. McGhie J. Chem. SOC.(C) 1968 1031. Terpenoids and Steroidr into olean-12-en-1-one by a sequence involving photolysis of its 1la-nitrite and several methods are developed for transformation of the 1 -oxo-compound into the 3-ketone. Acetyl migration between the 16a-and 28-hydroxy-groups of primulagenin A (126) is catalysed by acids and bases.lo5" The rearrangement occurs only when ring D is in the twist-boat form. Similar acetyl migrations reported earlier105b for compounds having an additional 22a-hydroxy-group may not therefore involve prior acetyl migration from the 16a- to the 22a-hydroxy- group. (126) Photo-oxidation of oleanic acid in acid medialo6" leads to the lla,l2a- epoxy-13,28-lactone system of eupteleogenin (127) (127) A similar reaction occurs with the derived alcohol (126; 16deoxy) and the competing reaction in which epoxide formation is accompanied by methyl migration is also observed.106b This latter reaction predominates in the absence of an oxygen function at C-28.'06' Serratenediol derivatives are present as methyl ethers in Sitka spruce.lo' Apetalactone (128) co-occurs in Calophyllurn spp. with canophyllol (129) from which it can be obtained by Baeyer-Villiger oxidation.lo8 It is the first lactone of the friedelin group to be found in Nature. lo' (a)0.D. Hensens and K. G. Lewis Tetrahedron Letters 1968 3213; (b) R. Tschesche B. T. Tjoa and G. Wulff ibid. p. 183; I. Yosioka T. Nishimura N. Watani and I. Kitagawa ibid. 1967 5343. lo6 (a)I. Kitagawa K.Kitazawa and I. Yosioka Tetrahedron Letters 1968 509; (b) ibid. p. 2643; (c) cf Ann. Reports 1965 62 342. lo' J. P. Kutney and I. H. Rogers Tetrahedron Letters 1968,761. T. R. Govindachari. D. Prakash. and N. Viswanathan. J. Chem. SOC.(C). 1968. 1323. 430 A. B. Turner H (128) (129) An intramolecular hydride transfer accompanies base-catalysed epimerisa- tion at C-19 in certain norlupanes:loga A related redox reaction between C-19 and C-3 occurs in the steroid series under ketalization conditions. '09* Rearrangement of 7P-hydroxyhopanes on dehydration with phosphorus pentachloride gives a number of B-nor-c-homohopenes. 'lo In the 7a-isomers dehydration is accompanied by migration of the C-8 methyl group. A series of isoprenoid ketones has been isolated from lipid extracts of human tubercle bacilli,'" The major components have structure (130 n = 6 or 7) with one of the double bonds reduced The minor constituents have n = 2 to 5.Steroids.-Oestrogen synthesis and biosynthesis '' and recent advances in the synthesis of heterocyclic steroids' 12' 'l3 have been reviewed. Synthesis of oestrones by coupling of vinyl carbinols with 2-methyl-cyclopentane-1,3-Io9 (a)A. VystrEil and M. Budesinsky Tetrahedron Letters 1968,4173;(b)J. Wicha and E. Caspi J.Chem. SOC.(C) 1968 1740. R. E. Corbett R. A. J. Smith and H. Young J.Chem. SOC.(C) 1968 1823. L. Coles and N. Polgar J. Chem. SOC.(C) 1968 2376. (a) P. Morand and J. Lyall Chem. Rev. 1968,68 85;(b)L. Starka Chem. listy 1968,62 1220. H.0.Huisman Bull. Soc. chirn. France 1968 13. Terpenoids and Steroids 431 dione is facilitated by use of the intermediate crystalline isothiourorlium salt (131).l14 The resulting dione (132) undergoes selective as well as stereo- specific reduction of the 17-keto-group with lithium tri-t-butoxyaluminium hydride. Bis-annelation of cyclohexanone enamines with 6-vinyl-2-picoline promises to be a useful method for steroid total synthesis."' Further routes to anthrasteroids have been described. l6 18-Hydroxyoestrone has been preparedl l7 from oestrone by a sequence involving photolytic oxidation of the amide (133) with iodine and lead tetra- acetate to the 18,20-anhydride (134). A re-examinationllEa of chromium trioxide oxidations of oestrone methyl ethers shows that 9P-hydroxy-11- ketones are formed as well as the 6-oxoderivatives the latter being the normal oxidation products of 3-acetoxyoestrones.Aerial oxidation of 3-methyl ethers also occurs at C-9 in the presence of soluble catalysts.'lEb Oestrone methyl ether can be obtained by removing the methyl group from its 1-methyl deriva- tive,' l9 thereby providing a new route from 1,4,6-triene-3-ones to 19-nor- steroids. The aromatic methyl group is first oxidised with ceric ammonium nitrate to an aldehyde which is then decarbonylated with tristriphenyl- phosphinerhodium chloride. (131) (132) (13 3) 'I4 C. H. Kuo D. Taub and N. L. Wendler J. Org. Chem. 1968,33,3126. 115 S. Danishefsky and R. Cavanaugh J. Amer. Chem. SOC.,1968,90 520.116 K. Wiedhaup A. J. H. Nollet,J. G. Korsloof and H. 0. Huisman Tetrahedron 1968 24 771; K. Wiedhaup F. H. Kesselaar and H. 0.Huisman ibid. p. 779. J. E. Baldwin D. H. R. Barton I. Dainis and J. L. C. Pereira J. ChPm. SOC.(0,1968,2283. (a) R. C. Cambie and T. D. R. Manning J. Chem. SOC.(C),1968,2603; (b)A. J. Birch and G. S. R. Subba Rao Tetrahedron Letters 1968 2917. '19 S. B. Laing and P. J. Sykes J. Chem. SOC.(C) 1968,2915. 432 A. B. Turner A remarkable dehydrogenation of oestrone to its A9(l')-derivative occurs with the adamantyl carbonium ion.12oo A similar oxidation takes place with dichlorodicyanobenzoquinone.'20b It is likely that both of these reactions involve the quinonoid intermediate (135),l2OC although the mechanism of the two oxidations may differ.In contrast dehydrogenation of the D-seco- compound (136) with the high-potential quinone does not give the 9,ll- dehydroderivative.' 21 Instead the A8-isomer with the tetrasubstituted double bond is formed. This transformation has been used in the synthesis of 8a,9P-oestrone methyl ether (137) which has ring C in the boat form. Details of the dehydrogenation of steroidal 6-lactones to ap-unsaturated lactones with dichlorodicyanobenzoquinonehave appeared. '22 Ring A a-pyrones have been prepared using the same reagent. Studies on the in vitro methylation of 2-hydroxyoestrone suggest that selective formation of the 2-methyl ether is achiev'ed by protection of the 3-hydroxy-group through sulphate conjugation. 123 It appears that both a methyl transferase and a sulphatase act in concert in the transformation of these catechol substrates.0 ~CH,CH~CO~H Me0 (135) (138; R = H or CH,OH) (139) (137) Treatment of the 4,6dien-3-ones (138) with sodium methoxide in dimethyl sulphoxide followed by acidification with aqueous acetic acid gives pre- cipitates of the free enols (139).124 Addition of acetic anhydride to the basic (a)W. H. W. Lunn and E. Farkas Tetrahedron 1968,24,6773 ;(b)W. Brown J. W. A. Findlay and A. B. Turner Chem. Comm. 1968 10; (c) CJ Ann. Reports 1963,60,418. ''I W. S. Johnson. S. G. Boots and E. R. Habicht. J. Org. Chem. 1968.33 1754. 12' B. Berkoq L. Cuellar R. Grezemkovsky N. V. Avila J. A. Edwards and A. D. Cross Tetra-hedron 1968,24,2851.J. Fishman M. Miyazaki and J. Yoshizawa J. Amer. Chem. SOC.,1967,89,7147; M. Miyazaki and J. Fishman J. Org. Chem. 1968,33 662. G. Kruger J. Org. Chem. 1968,33 1750. Terpenoids and Steroids mixtures produces the corresponding trienol acetates. Transformation pro- ducts of these intermediates include various ring B aromatic steroids. Autoxida- tion of enamines of ap-unsaturated ketones occurs at the y-po~ition,'~' leading to the corresponding 1,4diones on hydrolysis (i)air (ii) H,O@ 0 This contrasts with autoxidation of the enolate anions which gives a-keto- derivatives. The corresponding Schiff bases are also oxidised at the y-position probably by radical attack upon their enamine tautomers. Sensitized photo- oxygenation of the enamine (140) which is stable to ground-state oxygen gives progesterone.126 The mechanism is thought to involve 1,2-addition of singlet oxygen to the double bond with subsequent decomposition of the adduct to two carbonyl fragments. Eniminium salt formation is useful for protection of ap-unsaturated ketones during electrophilic reactions. 12' Thus the eniminium perchlorate (141) from progesterone can be converted into 17-hydroxyprogesterone in 48 % overall yield by epoxidation of its 17,20-enol acetate. Final mild alkali treatment regenerates both the keto- groups. The preferred conformation (142) of the side-chain in cortisol has been revealed by i.r. and n.m.r. studies'28o and by molecular orbital calculations.'28b These agree that the 20-keto-group projects towards the p-face of the molecule and the C-2042-21 bond eclipses the C-17-0(17a) bond.A weak intramolecular hydrogen bond is formed between the 20-carbonyl group and the 21-hydroxy- group but no such bond is formed between the 20-carbonyl and a 17a-hydroxy- group in the presence of a substituent at C-21. H 125 S. K. Malhotra J. J. Hostynek and A. F. Lundin J. Amer. Cheni. SOC.,1968,90 6565. J. E. Huber Tetrahedron Letters 1968 3271. 12' B. Gadsby and M. R. G. Leeming Chem. Comm. 1968,596. 128 W. G. Cole and D. H. Williams J. Chem. SOC.(C) 1968 1849; L. B. Kier J. Medicin. Chem. 1968 11 915. 434 A. B. Turner Details are available of the synthesis and conformational analysis of A-homo- B-nor- and A-nor-B-homo-steroids prepared by the stereospecific rearrangement of 1,2-cisdiol monotosylates.12' New syntheses of ~-homo-19-norsteroids involve incorporation or elimination of the C-19.In the former 19-mesyloxy-1,4-diene-3-onesand their 19-chloroanalogues are reduced with lithium and biphenyl in tetrahydrofuran and the latter' 30b involves decarbonylation or decarboxylation of the cyclopropanes (143) to the triene (144). The fragmentations occur with base and acid respectively with con- current cleavage of the cyclopropane ring. The synthesis of la,2a-methylene 19-nor pregnanes is best achieved by ring opening of (143) to the la-chloro- methyl derivative followed by acid-catalysed elimination of the C-10 carboxyl group and final regeneration of the cyclopropane ring.'30b A long-range directing effect which manifests itself in terms of changes in product ratio rather than in rate differences is exerted by the cholestane side-chain in the formation of A-homosteroid ketones.' 3'0 The direction of the ring enlargement of 3-ketones having a variety of substituents at C-17 is markedly influenced by the C8Hl side-chain. The effect may be due to micelle formation. Other abnormal effects have been observed in reactions of choles- tanes in aqueous solution. 31 Photochemical ring-contraction of A-homosteroids of type (145) gives the A-nor-derivatives (146).' 32 An efficient synthesis of A-nortestosterones involves photolysis of the epoxy-ketone (147) to give the hydroxymethylene compound (148).133 (146) (147) (148) lZ9 M.Nussim and Y. Mazur Tetrahedron 1968,24,5337; cf Ann. Reports 1961,58 328. lJo (a)P. Wieland and G. Anner Helu. Chim. Acta 1968 51 1932; (b)R. Wiechert Chem. Ber. 1968,101,2388. lJ1 (a) J. B.Jones and J. M. Zander Canad. J. Chem. 1968 46 1913; (b) J. B. Jones and D. C. Wigfield ibid. p. 1459. lJ2 M. Fischer and B. Zeeh Chem Ber. 1968,101,2360. 133 J. Pfister C.Lehmann and H. Wehrli Helu. Chim. Acta 1968,51 1505. Terpenoidrs and Steroids 435 12~-Mesyloxycholane is now found' 34 to rearrange in refluxing collidine to the c-nor-D-homoderivative (149) as do the epimeric 12P-mesylate and derivatives of the 12-oxo-compounds. The name 'cholajervine' is proposed for the parent hydrocarbon of these C, products to accompany the trivial names jervane and etiojervane commonly associated with the C27 and C, series.A13-Isomers of A12-5P-cholajervene (149) are also formed in the above rearrangement ;these have the usual C/D-C~Sring-junction. When 14P-hydroxy- 12P-tosyloxy-steroids are solvolysed the resulting c-nor-D-homo alcohols have the c/~-truns ring-junction.' 35 c-Nor-D-homo-analogues of oestrone and progesterone have been prepared from jervine. '36 Deamination of the amine (150; R = NH,) and attempted tosylation of the alcohol (150; R = OH) give c-homo-D-bisnor-steroidssuch as (151). 137 These results confirm the known tendency of some bicyclohexanes to rearrange with participation of a cyclobutyl C-C bond. Boron trifluoride-catalysed cleavage of 5P,6-epoxy-4-ketones and 4P,5- epoxy-6-ketones of the cholestane series leads to the corresponding fluoro- hydrins without skeletal rearrangement.13* The nucleophile attacks P to the carbonyl group and the C-5-0 bond remains intact; development of carbonium-ion character at C-5 in the co-ordinated complex is inhibited by the adjacent carbonyl group and there are no rearrangements of the type commonly observed with similar epoxides lacking a-keto-groups. A novel fragmentation occurs when the epoxy-cholestane (152) is kept for a short time in refluxing ~ollidine.'~~ The ether (153) is obtained along with the aromatic product (154). 18 I I H HA :Base (153) (154) (152) 134 F. C. Chang and R. C. Ebersole Tetrahedron Letters 1968 1985; 3521. 135 Y. Shimizu and T. Mitsuhashi Tetrahedron 1968,24,4207.136 S. M. Kupchan and M. J. A. El-Haj J. Org.Chem. 1968 33 647; S. M. Kupchan A. W. By and M. S. Flow ibid. p. 911. 13' J. Meinwald and J.-L. Ripoll J. Amer. Chem. Soc. 1967,89 7075. J. R. Bull Tetrahedron Letters 1968 5959. 13' J. M. Coxon R. P. Garland M. P. Hartshorn and G. A. Lane Chem. Comm. 1968 1506. 436 A. B. Turner Epoxidation of 19-hydroxy-As-androstenes and -cholestenes gives exclusively the Sp-epoxides in spite of p-face steric hindrance.14' The rate of the reaction is much reduced with the 19-acetoxy- and 10-methyl derivatives. The specificity of epoxidation is explained by formation of the complex (155) which is stabilized by hydrogen bonding. The orienting effect of an llp-hydroxy-group in the epoxidation of a 7,8-double bond is less marked although still sufficient to partially counteract the steric effects of the angular methyl groups on the p-face.The product mixture contains 25 % of the 7P,8P-epoxide and 70 % of the 7a,8a-isomer. Acid-catalysed rearrangement of the B-nor-lactone (156) gives the acid (157).I4l Lead tetra-acetate oxidation of a derivative (158) of Westphalen's diol gives the ether (159) thereby providing direct chemical evidence for the p-configuration at C-5 in the di01.l~~ Tracer studies'43 establish the following mechanism for the oxidative cyanohydrin<yanoketone rearrange-ment Details of the Baeyer-Villiger oxidation of A4-3-ketones and derived epoxides leading to A-norsteroids have appeared. 144 M. Mousseron-Canet B.Labeeuw and J.-C. Lanet Bull. SOC. chim. France 1968 2125; M. Mousseron-Canet and B. Labeeuw ibid. pp. 4165,4171. 141 M. S. Ahmad R P. Sharma H. Siddiqui and Shafiullah Austral. J. Chem. 1968 21 1867. 14* M. J. Harrington and B. A. Marples Tetrahedron Letters 1968,484. 14' J. Kalvoda Helv.Chim. Ada 1968,51,267. Terpenoids and Steroids Reduction of cholesteryl chloride with triphenyltin hydride or the sodium biphenyl radical anion gives only cholest-5-ene whereas 3,5-cyclocholestan-6-yl chloride gives mixtures of 3,5-cyclocholestane and the cholestene. 145a These and other results can be interpreted in terms of the formation of intermediate radicals which can rearrange or capture hydrogen. They show that the choles- teryl radical is significantly more stable than the cyclocholestanyl radical.Members of a new class of cyclo-steroids 5P,7P-cyclocholestanes have been obtained by addition of methylene to B-norcholesterol acetate or by irradiation of cholesta4,6diene. 145b The photochemical reaction could involve a bicyclo- butane intermediate. A transannular hydrogen transfer from C-5a to C-3a is shown by deuterium labelling to be involved in the formation of la,5-cyclo- k-cholest-2-ene (160)during attempted tosylation of the ally1 alcohol (161).145c The mechanism of photolysis of the keto-aldehyde (162) which shows strong conjugation between its aldehyde and enone systems has been discussed. 146 The products include the ketones (163) and (164) and the photochemistry differs markedly from that of the 19-deoxy-analogue.H (160) OAc 0& o@ CHO CHOH (1 6 2) (16 3) (164) 144 J. T. Pinhey and K. Schaffner,Austral. J. Chem. 1968,21 1873. 14’ (a) S. J. Cristol and R. V. Barbour J. Amer. Chem. SOC.,1968,90,2832; (b)P. G. Gassman and W. E. Hymans Tetrahedron 1968 24 4437 cf ref. 85; (c)S. B. Laing and P. J. Sykes J. Chem. SOC. (0,1968,421; 653; 937. 146 E. Pfenninger D. E. Poel C. Berse H. Wehrli K. Schaffner and 0.Jeger Helu. Chim. Act4 1968,51 772. 438 A. B. Turner The isomeric cyclobutenes (165) undergo thermal rearrangement to the bicyclohexene (166) possibly via a diradical intermediate. 14' The recent clarification of the role of steroid hormones in the metamorphosis of insects has been followed by recognition of the wide distribution of active substances in the plant kingdom.Among many reports this year there are further examples (167; hydroxylated at C-25 26 or 29) of moulting hormones based on the p-sitosterol ~ke1eton.l~~ Another C,,-hormone the lactone (168) is the probable precursor of cyasterone. Whereas all previous steroids exhibiting moulting-hormone activity have been 2P,3pdiols ponasterones B and c (171; R = H and OH respectively) have 2a,3a-hydroxy-group~.~~~~ Their companion in the leaves of Podocarpus Nakaii,ponasterone A (169) has the usual 2P,3Pdiol system. Further evidence for the structure of the latter has been obtained by degradati~n"'~ and synthesis. It occurs as the 3P-glucoside in Pteridiurn aquilinurn.150dThe isolation of rubrosterone 26 I II OH HOW H 11 0 0 147 P.H. Nelson J. W. Murphy J. A. Edwards and J. H. Fried J. Amer. Chem. SOC. 1968 90 1307; 5572. 148 M. N. Galbraith D. H. S. Horn Q. N. Porter and R. J. Hackney Chem. Comm. 1968 971; T. Takemoto K. Nomoto and H. Hikino Tetrahedron Letters 1968,4953. 149 T. Takemoto K. Nomoto Y. Hikino and H. Hikino Tetrahedron Letters 1968 4929; cf Ann. Reports 1967,64 371. 50 (a)K. Nakanishi M. Koreeda M. L. Chang and H. Y. Hsy Tetrahedron Letters 1968 1105 ; (b) H. Moriyama and K. Nakanishi ibid. p. 1111; (c) G. Huppi and J. B. Siddall ibid p. 1113; (d) T. Takemoto S. Arihara and H. Hikino ibid. p. 4199. Terpenoids and Steroids 439 (170) from Achyranthes rubrofuscasuggests that a metabolic pathway analogous to that from cholesterol to dehydroepiandrosterone exists for moulting hormones in plants.51 Several syntheses of rubrosterone are reported,'52"* as well as new routes to e~dysone'~~' and 20-hydroxyecdysone. 152d In two of these' 52a*d the 14a-hydroxy-group is introduced via epoxidation of 6- acetoxy-6,8( 14)-dienes. An alternative method of generating the 14a-hydroxy-7- en-6-one system which is a feature of all moulting hormones so far isolated involves photo-oxygenation of Py-unsaturated ketones :' {Q-# 0-OH @L pLg! 0 0 0 0 Deuterium work indicates a concerted cycloaddition mechanism for this reaction. A trimethylsilyl ether is used to protect a hydroxy-group in the synthesis of an isomer of ecdysone.'54 This may presage wide application of silyl ethers as protective groups in chemical synthesis. The fungal metabolite wortmannin (172) is related to viridin. lS5Cleavage of ring A does not appear to have involved p-elimination. The spirostan ring system can be readily constructed by Michael addition of l-acetoxy-5-nitro-2-methylpentane The adducts to A' 7(20)-pregnene-16-ones. cyclise directly to sapogenins on reduction and the method has been used to prepare kryptogenin diosgenin and yamogenin. ' Treatment of 21-acyloxy- 20-ketones of type (173) with alkali produces cardenolides (1 74). ' A survey of the Zimmermann reaction of various steroidal ketones has allowed correlations to be drawn between their structure and reactivity.' l8 Interactions between ketone functions at C-1 and C-11 in androstanes and pregnanes are responsible for various anomalous reactions.lS9 The degree of enolization of 1,3-diketones is markedly inhibited by an 11-0x0-substituent while 1,ll-diketones are resistant to metal hydride or catalytic reduction. The presence of an 11-keto-group inhibits oxidation of 1P-hydroxy-groups by chromium trioxide. lS1 T. Takemoto Y. Hikino H. Hikino S. Ogawa and N. Nishimoto Tetrahedron Letters 1968 3053. (a)K. Shibata and H. Mori Chem.and Pharm. Bull. (Japan),1968,16,1404;H. Mori K. Shibata K. Tsuneda and M. Sawai ibid. p. 1593; (b) H. Mori K. Shibata K. Tsuneda and M. Sawai ibid. p. 563; (c) P. Hocks U. Kerb R Wiechert A. Furlenmeier and A. Fiirst Tetrahedron Letters 1968 4281 ;H.Hikino Y. Hikino and T. Takemoto ibid. p. 4255 ;(d)U. Kerb R. Wiechert A. Furlenmeier and A. Fiirst ibid. p. 4277. 153 N. Furutachi Y. Nakadaira and K. Nakanishi Chem. Comm. 1968 1625. 154 M. N. Galbraith D. H. S. Horn E. J. Middleton and R. J. Hackney Chem. Comm. 1968,466. 155 J. MacMillan A. E. Vanstone and S. K. Yeboah Chem. Comm. 1968,613. lS6 S. V. Kessar and A. L. Rampal Tetrahedron 1968 24 887 et seq. 15' H.-G. Lehmann and R. Wiechert Angew. Chem. Internat. Edn. 1968,7,300. D. N. Kirk W. Klyne and A. Mudd J. Chem. SOC.(C),1968,2269. lS9 J. J. Schneider P. Crabbe and N. S. BhaccaJ. Org. Chem. 1968,33 3118. 440 A. B. Turner HO. HO" (171) O-0 0& COCH-1R R' [173; R' = PPh,P or PO(OEt),] (174)

ISSN:0069-3030    

DOI:10.1039/OC9686500409

出版商:RSC    

年代:1968

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13 HETEROCYCLIC CHEMISTRY By R. J. Stoodley (Department of Organic Chemistry University of Newcastle upon Tyne) Three-membered Rings.-The existence of a high barrier to nitrogen inversion in aziridines,which contain a hetero-atom attached to the ring nitrogen may be inferred from n.m.r. spectroscopy.l An elegant demonstration of this effect is provided by 1-chloro-2-methyla~iridine~ and 7-chloroazabicyclo[4,1,0] he~tane,~ where both invertomers may be isolated. Even more dramatic is the case of 2-methyl-3,3-diphenyloxaziridine, which may be isolated in optically active form by (1s)-peroxycamphoric acid oxidation of N-diphenylmethylene methylamine; this is the first example of asymmetric induction at trivalent nitrogen? The nomenclature of azirines has been confusing and to remedy this it is suggested that 2H-azirine should be called 1-azirine.’ Thus (1) should be named 2-methyl-3-phenyl-1-azirineand not 3-methyl-2-phenyl-2H-azirine as recommended by the Ring Index.Details of a general synthesis of 1-azirines involving photolysis of vinyl azides which was noted last year,6 are presented ph)=N .;Me H11C6 1- R in the same paper. The base-catalysed elimination of trimethylammonium iodide from quaternary salts [e.g. (2)] provides a route to azirines which is particularly useful when 3,3-disubstituted 2-phenyl-1-azirines are required.’ Thermal rearrangement of (1)to 2-methylindoleprobably involves the dipolar intermediate (3;R = Me). However,both indole and benzyl cyanide are formed in equal amounts from the pyrolysis of 3-phenyl-l-azirine implying that hydrogen transfer within the intermediate (3; R = H) can compete with cyclisation.*Nucleophilic addition to the azirine double-bond occurs readily J.M. Lehn and J. Wagner Chem Comm. 1968 148 1298; R S. Atkinson ibid. 1968 676; S. J. Brois J. Am. Chem SOC.,1968 90 506; S. J. Brois Tetrahedron Letters 1968 5997. S. J. Brois J. Amer. Chem SOC.,1968,90 508. D. Felix and A. Eschenmoser Angew. Chem. Internat. Edn. 1968,7,224. F. Montanari I. Moretti and G. Torre Chem. Comm. 1968,1694. A. Hassner and F. W. Fowler J. Amer. Chem. SOC.,1968,90,2868. Ann. Reports (B) 1967,376. S. Sato Bull. Chem. SOC.Japan 1968,41,1440. * K. Isomura S. Kobayashi and H. Taniguchi Tetrahedron Letters 1968 3499.442 R.J. Stoodley although the adducts generally undergo rearrangement. For example 2,4- diphenylpyrrole is formed from 2-phenyl-1-azirine and acetophenone in the presence of a strong base,g diazomethane and 3-methyl-2-phenyl-1-azirine react to give a mixture of cis-and trans-l-azido-2-phenyl-but-2-ene together with 3-azido-2-phenyl-but-1 -ene lo while a-ammoniumisobutyrophenone anil perchlorate is obtained from 3,3-dimethyl-2-phenyl-l-azirine and anilinium perchlorate." Even acid chlorides and acid anhydrides will add to the azirine double-bond :' the N-acylchloroaziridines derived from benzoyl chloride and 3-methyl-2-phenyl-1-azirine are stable in benzene although they readily rearrange to 4-methyl-2,5-diphenyloxazoleand the dichloro-amide (4) in acetone.PhCCl,*CHMe* NHBz (4) (5) The reaction of olefins with organic azides at elevated temperature may give aziridines and/or Schiff bases via intermediate 1,2,3-triazolines. Thus when cyclopentene cycloheptene and cis-cyclo-octene are heated with phenyl azide the Schiff bases are the major products while the bicyclic aziridines are formed from cyclohexene and trans-cyclo-octene.' Aziridines are the main products when 1,2,3-triazolines are decomposed photochemically and this process constitutes an excellent versatile approach to their ~ynthesis.'~ A number of examples in which nitrene intermediates are trapped by olefins were reported last year :15 the isolation of 3-phenyl-l-azabicyclo[l,l,0]butane from the photo- lysis of 3-azid0-2-phenyl-prop-l-ene'~ provides another remarkable illustration of the same process.Irradiation of 1-azido- 1-phenylethylene in benzene gives mainly 2-phenyl-1-azirine and also a small amount of the azabicyclopentane (5),which may arise by photocycloaddition of keten phenylimine and 2-phenyl- 1-azirine.' CO-NRPh + Ph\rC\l;fiPh Me,N.NH(CH,),-CO.NRPh It -v 1- (6) N H 0 Bu (8) (7) S. Sato H. Kato and M. Ohta Bull. Chem. SOC.Japan 1967,40,2936. lo V. Nair J. Org. Chem. 1968,33,2121. N. J. Leonard E. F. Muth and V. Nair J. Org. Chem. 1968,33,827. l2 S. Sato H. Kato and M. Ohta Bull. Chem. SOC.Japan 1967,40 2938; F. W. Fowler and A. Hassner J. Amer. Chem. SOC.,1968,90,2%75. l3 K. R. Henery-Logan and R. A. Clark Tetrahedron Letters 1968 801.l4 P. Scheiner Tetrahedron 1968,24 2757. l5 Ann. Reports (B) 1967 377. l6 A. G. Hortman and J. E. Martinelli Tetrahedron Letters 1968 6205. l7 F. P. Woerner H. Reimlinger and D. R. Arnold. Angew Chem Internar. Edn. 1968.7,130. Heterocyclic Chemistry The lithium aluminium hydride reduction of oximes to aziridines is well established:2-isoxazolines which are available from 1,3-dipolar cycloaddition of nitrile oxides and olefins are reduced in a similar manner." The quaternary salts [e.g. (6)] undergo base-catalysed elimination of trimethylammonium iodide and are therefore useful precursors of aziridine 2-carboxyanilides (7). The salts may be prepared by methyl iodide alkylation of the adducts formed from afhnsaturated carboxyanilides and hydrazine.Further examples of aziridine reactions in which C-C bond cleavage occurs2o have been reported :they may be induced by thermal or photochemical excitation and they probably proceed via azomethine ylide intermediates. Thus (8) which may be formed when trans-2-benzoyl- 1-t-butyl-3-phenylaziri-dine is heated at 220° cyclises to 2,5-diphenylo~azole.~' However added dipolarophiles may compete with this cyclisation since the 4-oxazoline (9) C6H4-OMe-p o I ~ ~ N.CH2Ph~ oMe02C@ Me02C N -C,H,.OMe-p CH2Ph 0 (12) (13) may be isolated when cis-or trans-2-benzoyl-1 -cyclohexyl-3-phenylaziridine and diphenylcyclopropenone are heated together in benzene.22 Similarly the diazabicyclohexene (10) and diethyl acetylenedicarboxylate afford the pyrrolo- imidazole (11) in refluxing xylene solution,23 while the cyclo-adduct (13) is formed by irradiation of (12) in the presence of dimethyl acetylenedicarboxy- late.24 An example which may involve an azomethine ylide rearrangement is furnished by cis-and trans- methyl 2-aryl-1 -cyclohexylaziridine-3-carboxylate which cleave to the arylcarbaldehyde and methyl a-cyclohexylglycinate in boiling benzene.However no rearrangement occurs in boiling methanol when the arylcarbaldehyde and methyl N-cyclohexylglycinate are formed.' K. Kotera Y. Takano A. Matsuura and K. Kitahonoki Tetrahedron Letters 1968 5759. G. R Harvey J. Org. Chem. 1968,33,887. 'O Ann Reports (B),1967,379. A Padwa and W. Eisenhardt Chem. Comm. 1968 380. 22 J. W. Lown R.K. Smalley and G. Dallas Chem. Comm. 1968 1543. 23 H. W.Heine A. B. Smith 111 and J. D. Bower J. Org. Chem. 1968,33 1097. 24 S.Oida and E. Ohki Chem. and Pharm. Bull. (Japan),1968,16 764. 25 P.B.Woller and N. H. Cromwell J. HeterocyCIic Chem. 1968,5 579. P 444 R. J. Stoodley 1,2,3-Triphenylaziridine,when irradiated in alcohol yields mainly ben- zaldehyde acetal N-benzylaniline and N-benzylideneaniline together with a small amount of alkyl benzyl ether.26 The ether may arise uia phenylcarbene or by solvent displacement of benzylideneaniline from the protonated ylide. However when cis-l-(2,4,6-trinitrophenyl)-2,3-diphenylaziridine is irradiated in a similar manner the benzimidazole (15) and benzaldehyde are produced (14) 1 PhFH-OMe SCHEME 1 suggesting that the ylide (14) undergoes intramolecular cyclisation prior to solvent attack.” The route suggested in Scheme 1 provides an interpretation which is an alternative to that proposed by the authors.2,CDiphenyloxazole and t-butylbenzylideneimine are obtained when cis-2-benzoyl-l-t-butyl-3-phenylaziridine is photolysed although with the trans-isomer 2,4-diphenyl- oxazole and the ketone (16) are formed. The ketone (16) must arise by C-N bond cleavage although deuterium studies reveal that the hydrogen at position 3 of the aziridine is transferred to the excited carbonyl group before this fission occurs.’* cis- and trans-2,3-Dibenzoylairidinesmay be equilibrated under basic conditions and the equilibrium constant depends markedly upon the dielectric constant of the solvent.For example the cis-form is preferred in dimethyl sulphoxide (cis/trans = 5.25)while in t-butyl alcohol the trans-isomer pre- dominates (cisltrans = 0.32).29In contrast trans-1,2-dibenzoylcyclopropane is favoured over the cis-isomer irrespective of solvent. However when the equilibration is carried out in isopropyl alcohol containing sodium isoprop- 26 H. Nozaki S. Fujita and R. Noyori Tetrahedron 1968,24,2193. ’’ H. W. Heine G. J. Blosich and G. B. Lowrie 111 Tetrahedron Letters 1968,4801. ” A. Padwa and W. Eisenhardt J. Amer. Chem. SOC.,1968,90 2442. 29 R. E. Lutz and A. B. Turner J. Org. Chem. 1968.33 516. Heterocyclic Chemistry 445 oxide the aziridines undergo an interesting aldol condensation with acetone (formed from the solvent) to furnish the tricyclononanone (17).30 Bz NHBU \=(Ph (16) (17) (18) (19) The first example of an azetidine -+ aziridine ring-contraction has been reported thus 1-t-butyl-3-chloroazetidineis converted into l-t-butyl-3-cyanoazetidine with methanolic potassium cyanide but gives l-t-butyl-2- chloromethylaziridine when heated.31 However no rearrangement is observed when 1-t-butyl-2-tolylsulp hon yloxymethylaziridine is treated with et hoxide although the thermal rearrangement of aziridinyl formates (18) to N-alkyl-2- oxazolidones (19) illustrates that the aziridine nitrogen may act as a neighbour- ing group.33 Hydrazones (20) prepared from 1-aminoaziridines and ap-epoxy-ketones undergo an intriguing thermally induced fragmentation to olefin acetylene ketone and nitrogen.34 An unusual reductive ring-opening occurs when methyl 1-t-butylaziridine-2-carboxylate is treated with hydrazine the evolution of nitrogen and the formation of 3-t-butylaminopropionic acid hydrazide suggest that di-imide may be formed as an intermediate.35 The chemistry of a-lactams has been reviewed36 and new examples of these fascinating compounds include l-(l-adamantyl)-3-t-butylaziridinone,37 3-(1-adamantyl)-1-t-butylaziridinone,38 and the spiro-a-lactam (2 1).The former derivative displays remarkable thermal and chemical stability requiring 90 hr. for decomposition in refluxing toluene and 48 hr. for solvolysis in boiling methanol. -0 I N Bt'N=NBu ' t + Bu 0eR4 R' /O\ Bu"-NBu' (20) (21) 30 A.B. Turner and R. E. Lutz J. Heterocyclic Chem. 1968,5,437. 31 V. R. Gaertner Tetrahedron Letters 1968 5919. 32 J. A. Deyrup and C. L. Moyer Tetrahedron Letters 1968,6179. " B. M. Culbertson and S. Dietz Cad. J. Chem. 1968,46 3399. 34 D. Felix J. Schreiber,K. Piers U. Horn,and A. Eschenmoser Helo. Chim. Acta 1968,51 1461. 35 J. A. Deyrup and S. C. Clough J. Amer. Chem. SOC. 1968,90,3592. 36 I. Lengyel and J. C. Sheehan Angew. Chem. Internat. Edn. 1968,7,25. 37 I. Lengyel and D. B. Uliss Chem. Comm 1968 1621. '' K. Bott Tetrahedron Letters 1968 3323; K. Bott Angew. Chem. Intermt. Edn. 1968,7,894. 39 E. R. Talaty and A. E. Dupuy jun. Chem. Comm.,1968 790. 446 R.J. Stoodley The cycloelimination of sulphur dioxide from thiiran 1 ,1-dioxides should be non-concerted according to orbital symmetry predictions although as noted last year,40 cis- and trans-stilbene episulphones undergo stereospecific decomposition in methanol.Evidence which supports a stepwise process is now available and of particular interest is the observation that both isomers yield only trans-stilbene in the presence of methoxide. Deuterium studies reveal that the cis-episulphone exchanges deuterium with retention of con- figuration considerably faster than its epimerises to the tr~ns-isomer.~~ The oxadiaziridine (23) which may be obtained by irradiating a pentane solution of the azoxy-derivative (22) at O" is the first example of a three- membered heterocycle which does not contain carbon in the ring.It is un-reactive to water presumably because the ring is sterically shielded by the t-butyl groups although at 20" it reverts to (22).42 Four-membered Rings.-A new route to N-arylazetidines involves cyclisation of 1-arylamino-3-phenoxypropaneswith aluminium chloride in benzene.43 Silver perchlorate is particularly effective in ring-closing 1 -alkylamino-3- chloropropanes to give tertiary azetidinium perch lor ate^.^^ The reaction of 1,1,2,2-tetrasubstituted aziridinium salts with weak nucleophiles generally results in 1-2 bond cleavage while the 1-3 bond is ruptured with strong nucleophiles In contrast the corresponding azetidinium salts are far less reactive thus 1,1,2,2-tetramethylazetidinium perchlorate is resistant to solvolysis in methanol while methoxide catalyses an eliminative ring-opening to afford N-(3-methyl-but-3-enyl)dimethylamine.However methanolysis does occur when the 1-2 bond is activated as with l-benzyl-2,3,4-trimethylaze-tidinium perchlorate although an eliminative ring-opening again results with meth~xide.~~ common with aziridinium saltsY4' azetidinium salts In undergo dipolar cycloaddition with nitrones to yield charged heterocycles [e.g. (24)].44 The stereospecific [2 + 2lcycloaddition of isocyanates and enol-ethers to give azetidin-2-ones was reported last year.46 A further example involves Ph,CH .CO* NPh. CH(0Me)Ph Me/\CH,Ph (25) (24) 40 Ann. Reports (B),1967,380. F. G. Bordwell J. M. Williams jun. E. B. Hoyt and B. I% Jarvis J.Amer. Chem Soc. 1968, 42 90,429; F. G. Bordwell and J. M. Williams jun. ibid.,1968 90 435. 42 S. S. Hecht and F. D. Greene J. Amer. Chem. Soc. 1967,89,6762. '' L. W. Deady R. D. Topson R. E. J. Hutchinson J. Vaughan and G. J. Wright Tetrahedron Letters 1968 1773. 44 N. J. Leonard and D. A. Durand J. Org. Chem. 1968,33 1322. 45 Ann. Reports (B) 1967,378. 46 Ann Reports (B) 1967 382. Heterocyclic Chemistry 447 the reaction of chlorosulphonyl isocyanate with cis-and trans-methylstyrene to give cis-and trans-1-chlorosulphonyl-3-methyl-4-phenylazetidin-2-one re~pectively.~~ Although a concerted pathway should be forbidden by orbital symmetry considerations the application of these rules to systems involving cumulative double-bonds has been q~estioned.~~ However there is evidence to support a two-step sequence in a number of cases.For example when the cycloaddition of diphenylketen and benzylideneaniline is quenched with methanol a low yield of the amide (25) may be i~olated.~’ Similarly N-diphenyl- acetyl-NN’-di-isopropylureais formed when the reaction between diphenylketen and di-isopropylcarbodi-imideis quenched with water. Dipolar intermediates may also be involved in the cycloaddition of tetramethylallene and the inner salt (26) since both (27) and (28) are f~rmed,~’ and also in the rearrangement of the p-lactam (29) prepared from chlorosulphonyl isocyanate and 2-methyl- butadiene at -lo”,to the &lactam (30) at 40°.52 Me Me Me + EtO,C-NSO,.&Et -I Mevj (26) Some control Et 0 ,+ N Me ,N/ OEt over P-lactam stereochemistry in the reaction involving azidoacetyl chloride amines and triethylamine may be realised by varying the sequence of addition of the reagent^.'^ Unfortunately a similar reaction in which the imine is replaced by the thiazoline (31) leads only to the penicil- lanic acid ester (32) with the trans-stereochemistry at position 5 and 6.Catalytic reduction of (32) followed by phenoxyacetylation yields the methyl ester of penicillin V which differs from the natural isomer only in the C-6 configura- ti~n.~~ The epimerisation of penicillanic acid derivatives at position 6 has been achieved for the first time and the 6-a isomers are favoured thermodynamic- ally.5 While methyl 6p-phthalimidopenicillanateis epimerised to the a-isomer with incorporation of deuterium in ['Hit-butyl alcohol containing potassium t-butoxide the a-isomer does not undergo isotopic exchange under similar conditions.It appears that relief of compressional interaction between the 2-P-methyl-group and the 6-substituent provides a considerable driving force 47 E. J. Moriconi and J. F. Kelly Tetrahedron Letters 1968 1435. 48 R. Huisgen and P. Otto J. Amer. Chem. SOC.,1968,90 5342. 49 H. B. Kagan and J. L. Luche Tetrahedron Letters 1968,3093. 50 W. T. Brady and E. D. Dorsey Chem. Comm. 1968 1638. 51 G. M. Atkins jun.and E. M. Burgess J. Amer. Chem. SOC.,1968,90,4744. ’* E. J. Moriconi and W. C. Meyer Tetrahedron Letters 1968,3823. 53 A. K. Bose B. Anjaneyulu S.K. Bhattacharya and M. S. Manhas Tetrahedron 1967,23,4769. s4 A. K. Bose G. Spiegelman and M. S:Manhas J. Amer. Chem. SOC.,1968,90,4506. ” S. Wolfe and W. S. Lee Chem. Comm 1968,242; D. A. Johnson D. Mania C. A Panetta and N. H. Silvestri Tetrahedron Letters 1968 1903. 448 R. J. Stoodley S0,CI ‘C0,Me (29) (30) R H Me Ph(OH)CH-tv yMe &-A. 0 kO,H (33) (32) for enolisation. particularly when a bulky base is employed this is also re- flected in a kinetic preference for endo-protonation. An interesting a-hydroxy- benzylation of 6-p-aminopenicillanic acid occurs in the presence of benzalde- hyde when 33; R = PhCHN) is formed. The imino-linkage of (33; R = PhCHN) may be cleaved by mild acid hydrolysis and after phenylacetylation the penicillin Ganalogue(33 ;R = PRCH,CONH)may be isolated.56 Solvolysis of (34) in methanol containing sodium hydrogen carbonate does not result in ring enlargement but yields the bicyclic aziridine (39 which may isomerise further to (36).57 PhCH * CO * NH CH,*OTs (34) (35 1 Me Me (36) The carbonyl oxygen of p-lactams may be replaced by sulphur with phos- phorus pentasulphide to give the first examples of P-thiolactams [e.g.(37)].58 The novel azetine (38) which may be reduced to the corresponding azetidine with lithium aluminium hydride is formed when N-unsubstituted p-lactams react with triethoxonium tetrafl~oroborate.~’ Cleavage of l-phenylazetidin- 2-ones occurs upon U.V. irradiation and the course of the reaction is sensitive to the nature of the 4-substituent.Thus an electron-withdrawing group promotes 1-4 and 2-3 bond rupture to give olefin and phenyl isocyanate while an electron-releasing substituent favours 1-2 and 34 bond fission to 56 R. Reiner and P. Zeller Helo. Chim. Acta 1968,51 1905. ’’ M. R. Bell and R Oesterlin Tetrahedron Letters 1968,4975. K. R Henery-Logan,H. P. Knoepfel and J. V. Rodricks J. Heterocyclic Chem. 1968 5,433. ” G. Pifferi P. Consonni G. Pelizza and E. Testa J. HeterocycZic Chem. 1967,4,619. Heterocyclic Chemistry afford imine and keten.60 The photoinduced ring-expansion of cis-3-benzoyl- 1 -t -butyl-2-phenylazetidine to 1-t-butyl-2,4-diphenylpyrrole was noted last year.61 Although the same material is also formed from the trans-isomer the major product is then l-t-butyl-2,3-diphenylpyrr0le.~~ Since the pyrroles are not interconverted the primary reaction probably involves transfer of the hydrogen at position 2 or 4 to the excited carbonyl group as with trans-3-ben~oyl-l-t-butyl-2-phenylaziridine.~~ The disubstituted pyrroles may arise via azabicyclo[2,1,0]pentanes formed by ring closure of the diradical (or dipolar) intermediates.The claim that [2 + 2lcycloaddition of benzaldehyde N-cyclohexylimine occurs upon irradiation in ethanol to give the 1,3-diazeti- dine63 has been shown to be erroneous. In fact a photoreduction takes place to afford rneso-NN’-dicyclohexyl- 1,2-di~henylethane-l,2-diamine.~~ However the 1,3-diazetidine structure (39) has been confirmed for the cyclo-adduct formed from phenyl isocyanate and NN-diphenylcarbodi-imide since it yields (40) with sodium n-b~toxide.~’ An intriguing cyclisation occurs when the a-chloroacylhydrazone (41) is treated with sodium hydride the cyclic azomethine imide (42) which is formed may be reduced to the diazetidinone (43) with sodium borohydride although in the presence of deactivated Raney nickel (45) is obtained providing evidence for the valence tautomer (44).66 N-NH.CO-CH,Cl-p-BrH,C6’i- J-!’+JC6H4Br-p -jI:Ph* C,H,Br-p 0 (41) (42) (43) (46) 6o M.Fischer Chem. Ber. 1968,101,2669. 61 Ann. Reports (B),1967,381. A. Padwa and R. Gruber J. Amer. Chem. SOC.,1968,90,4456. 63 R 0.Kan and R L. Furey J. Amer. Chem. SOC.,1968,90 1666. 64 A. Padwa W. Bergmark,and D.Pashayan J. Amer. Chem. SOC.,1968,90 4458. 65 W. J. Farrisey,jun.,R J. Ricciardi and A. A. R Sayigh J. Org. Chem. 1968,33 1913. 66 R. B. Greenward and E. C. Taylor J. Amer. Chem. SOC., 1968,90 5272. R.J. Stoodley X-Ray crystallography has not only confirmed the dipolar structure (42) but has provided evidence for an intramolecular hydrogen-bond between the ortho-hydrogen of the para-bromophenyl group and the anionic nitr~gen.~’ The reaction of sulphenes with ‘electron-rich’ olefins is well documented ynamines will also undergo this cycloaddition since the thiet (46) and its double-bond isomer are produced from para-substituted phenylmethane- sulphonyl chloride diethylaminomethylacetylene and triethylamine.68 The highly strained thiet 1,l-dioxide (47) is available from 2-dimethylaminomethyl- 3-dimethylamino-4-phenylthietan1,l-dioxide.The exocyclic double-bond 0 0 (49) of (47) is the more reactive since it undergoes a thermal [4 + 2)cycloaddition with 1,3-diphenylisobenzofuranto yield (48) and a photochemically induced dimerisation to (49); it may also be selectively hydr~genated.~’ While cis-and trans-2,4-diphenylthietan 1,l-dioxides give trans-12-diphenylcyclopropane-sulphinic acid with ethylmagnesium bromide a different route is followed in the presence of potassium t-butoxide. Thus the cis-and trans-isomers undergo stereospecific ring-expansion to cis-and trans-3,5-diphenyl-l,2-oxathiolan 2-oxide (50). The reaction of cis-and trans-2,4-diphenylthietan1-oxide with potassium t-butoxide also leads to ring contraction although a mixture of cis-1,2-diphenylcyclopropanesulphinicacid and cis-1,2-diphenylcyclopropane-thiol are obtained.” However different products result from the reaction of the cis-and trans-monoxides with methylmagnesium iodide.For example cis-2,4-diphenyltetrahydrothiophenand the benzothiepin (51) may be isolated from the cis-isomer while some deoxygenation occurs with the trans-monoxide to give trans-2,4diphenylthietan cis-and trans-diphenylcyclopropanesare formed from both isomer^.^' 67 C. J. Fritchie jun. and J. L. Wells Chem. Comm.,1968,917. 68 W. E. Truce R H. Bavry and P. S. Bailey jun. Tetrahedron Letters 1968 5651. 6q L. A. Paquette M. Rosen and H. Stucki J. Org. Chem. 1968,33 3020. 70 R M.Dodson P. D. Hammen and R. A. Davis Chem. Comm. 1968,9. ’I1 R. M. Dodson and P. D. Hammen Chem. Comm. 1968,1294. Heterocyclic Chemistry 45 1 Ph PhQBz Bz Ph 0-so Ph (52) (53) (54) R' Ph Me R'HN C0.C CR3 (57) \ The adducts (53) and (54) which are obtained when azobenzil and sulphur dioxide are heated together in benzene may arise by cycloaddition of di-phenylketen (formed by Wolf rearrangement) and the sulphen (52).72 A similar rearrangement occurs when azobenzil is decomposed in carbon disulphide although a five-membered ring (55) is formed in the cycloaddition step.73 In contrast the dithietan (56) is obtained when 3-diazobutan-2-one is heated in carbon disulphide. Its structure has been confirmed by X-ray crystallography and in common with de~aurins,~~ it possesses a short S-0 distance of 2.63 A.75 Five-membered Rings with One Hetero-atom.-The thermal rearrangement of cyclopropylimines to A' pyrrolines may be induced by a trace of acid catalyst.76 A novel synthesis of 0x0-A2-pyrrolines [e.g.(58)] is provided by the cyclisation of the amine adducts of pentadiyn-3-ones [e.g. (57)] in refluxing xylene although pyridones are also formed.77 cis-a-Formyl-P-chloro-olefins which may be prepared from ketones and phosphorus oxychloride in the presence of NN-dimethylformamide will react with ethyl N-methylamino- NTs NTs NTs C0,Me &C02Me -&C0,Me-Meo2Clt13C02Me C0,Me C0,Me ')'T. Nagai M. Tanaka and N. Tokura Tetrahedron Letters 1968,6293. 73 P.Yates and L. L. Williams Tetrahedron Letters 1968,6293. 74 Ann. Reports (B),1967,383. 'Is J. A. Kapecki J. E. Baldwin and I. C. Paul Tetrahedron Letters 1967 5307. 'I6 R. V. Stevens and M. C. Ellis Tetrahedron Letters 1967 5185. 'I' T. Metler A. Uchida and S. I. Miller Tetrahedron 1968,24,4285. 452 R.J. Stoodley acetate to give substituted pyrr~les.~~ Diels-Alder cycloaddition of l-p-tolylsulphonylpyrrole and dimethyl acetylenedicarboxylate gives the 7-azanorbornadiene (59) which in analogy with norbornadiene undergoes photochemical ring-closure to the 3-azaquadricyclane (60).79However in contrast to dimethyl quadricyclane-1,5-dicarboxylate,which undergoes photo- addition with methyl propiolate at the 6-7 bond (60) is attacked at position 2 and 4 to afford (61).80Pyrroles which are substituted at position 1 by alkyl- aryl- or benzyl-groups are thermally rearranged to a mixture of the 2- and 3-substituted isomers.2H-Pyrrole intermediates are probably involved since 2,5 -dimethyl- 1-p hen ylpyrrole gives 2,4-dimethyl-3 -phen yl- and 3,4-dimethyl-2- phenylpyrrole.8 Me Me Me -1 r\ 19 -N Me Me Me (62) Me Me Me Me-R- Me S. Hauptmann M. Weissenfels M. Scholz E. M. Werner H. J. Kohler and J. Weisflog Tetrahedron Letters 1968 1317. 79 H. Prinzbach R Fuchs and R Kitzing Angew. Chem. Internat. Edn 1968,7 67. 8o H.Prinzbach R Fuchs R Kitzing and H. Achenbach Angew. Chem. Internat. Edn. 1968,7 727. J. M.Patterson and S. Soedigdo J. Org. Chem. 1968 33 2057; J.M.Patterson L. T. Burka and M.R.Boyd ibid. 1968.33,4033. Heterocyclic Chemistry 453 Methylation of the nickel@) corrole (62) occurs at the metal to give (63) which is reversibly protonated at position 1in trifluoroacetic acid. An intriguing methyl shift results when (63) is heated in refluxing chlorobenzene and the nickel 3,3-dimethylcorrole (64) is the major product ;protonation of (64) now takes place at position 17.82 While salts of 1-methyltetrahydrocorrins(65 ; R = H) readily exchange their meso-protons 1,19-dimethyltetrahydrocorrin salts [e.g. (65; R = Me)] undergo electrophilic substituion only at the 5me~o-position.~~ The thermal behaviour of these salts (65; R = alkyl) is also of considerable interest and depends upon the nucleophilicity of the anion.Thus the halides and the nitrate are converted into the rneso-unsub-stituted porphin (66; R = H) while the meso-monoalkylporphin (66; R = alkyl) is obtained from the perchl~rate.~~ Corphins are of interest since they Me Me Me Me Me Me ";f\46 -1 -N -11 MeraMe LP(=JMe Me Me Me Me Me Me Me (67) (68) +-+ Reagents :i NaOMe-Pd2 ;ii Et,O,BF,; iii di-isopropylethylamine SCHEME 2 could be biogenetic precursors of corrins. A total synthesis of the corphin palladium complex (68) has been described by the Swiss School and is sum-marised in Scheme 2. The failure to form corphin complexes from the corres-ponding nickel (11) or cobalt (11) complexes of (67) forcibly demonstrates the importance of the metal in the cyclisation pro~ess.'~ The nonoxidative photocyclisation of N-aryl enamines which may be prepared from N-alkylanilines and ketones constitutes a useful two-step synthesis of 2,3-dihydroindoles.Although a mixture of stereoisomers result the trans-isomer is the major product.86 Indole rearrangements have been revie~ed.~' The course of cyclisation of the enolate (70) which is formed from the diester (69) and an equivalent of methoxide depends upon the reaction conditions. Thus the oxindole (71) is produced in the presence of excess of base while the indole (72) is obtained upon acidification.88 3-Vinylindoles 82 R. Grigg A. W. Johnson and G. Shelton Chem. Comm. 1968,1151. 83 R. Grigg A. W. Johnson and K. Richardson Chem. Comm.1968,896. 84 R. Grigg A. W. Johnson K. Richardson and K. W. Shelton Chem. Comm 1968,896. 85 A. P.Johnson P. Wehrli R Fletcher and A. Echenmoser Angew. Chem Znternut. Edn 1968 7,623. 86 0.L. Chapman and G. L. Eian J. Amer. Chem Soc. 1968,90,5329. 87 E. Giovannini and F.Karrer Chimia (Switz.) 1967,21,517. 88 J. W. Schulenberg,J. Amer. Chem. SOC.,1968,90 1367 7008. R. J. Stoodley 454 \CO,Me / \CO,Me C H C1-p l6 4 4 C6H4’ C0,Me-o OCH,*CO,Me cwc6H4c1-p C0,Me [e.g. (73)] lose their vinyl group as acetaldehyde in the presence of hydr~xide,~’ implying that the nucleophile adds to the 3H-indole form (74). The formation of (76) (and not the indolazanonane as suggested earlierg0) from the reaction of (75) with cyanide involves a similar nucleophilic addition to an intermediate 3-vinylindole [derived by Hofmann elimination of (75)].” 2-(2-Indolyl)ethyl toluene-p-sulphonate reacts with ethyl cyanoacetate and base in an uneventful manner to give (77).In contrast the benzazepine (78) is formed in the corres- ponding reaction involving the 5-methoxy-deri~ative.~~ +/Me RCO CHS \Me -R HoI MeoQcN I Me coLJ C0,Et 89 L. J. Dolby and G. W. Gribble Tetrahedron 1968,24,6377. 90 Ann. Reports (B),1967,422. G. H. Foster and J. Harley-Mason Chem. Comm. 1968 1440. 92 T. Sakan S. Matsubara H. Takagi,Y. Tokunaga and T. Miwa Tetrahedron Letters 1968,4925. Heterocyclic Chemistry The addition of dimethyl sulphonium methylide to enol-ethers of P-dicar- bony1 compounds provides a new route to furans for example 2,4-dimethyl- furan is formed from the sulphur ylide and 4-methoxy-pent-3-en-2-0ne.~~ In a closely related reaction (79) and keten dimer afford the 3-hydroxfuran A novel and general synthesis of 2,5-dihydrofurans which may be substituted at position 2 and 3 is available from the reaction of vinyltriphenylphosphonium bromide with a-hydroxy-ketone~.~~ Annulene polyoxides are formed in low yield from the phosphonium salt (81),which is available in two steps from sucrose.The ylide formed in the presence of ethoxide undergoes a self-Wittig reaction to afford the trioxide (82) together with tetroxides and pent oxide^.^^ The ability of 2-amino-furans -thiophens and -pyrroles to behave as enamines has been noted thus they are protonated at position 3 or 5.This observation is also of synthetic potential since 2-amino-3-cyano-4,5-dimethyl-furan reacts with bromoacetyl bromide to provide a simple route to the furopyrrole (83).97 Further investigation into furan photochemistry reveals that 2-methylfurans are converted into their 3-methyl-i~omers,~~ in analogy with thi~phens.~~ These reactions are considered to proceed via cyclopropene intermediate^^^ and indeed in the photoisomerisation of 2,5-di-t-butylfuran (84) may be isolated.loo In contrast to N-aryl-enamines,86 phenylsulphides [e.g. (85; R = Me)] undergo a low-yield reductive photocyclisation to afford CN Me&-Jo Me (81) (83) (82) benzothiophens [e.g. (86; Rf = R2= Me)]. However a mixture of (86; R' = Ph R2= Me) and (86; R1= Me R2= Ph) is formed from (85;R = Ph) although this is not due to photoisomerisation of the products.'o' Substitution reactions of furan and thiophen derivatives have been re-viewed.' O2 The isolation of 3-amino- and 3-bromo-thiophen from 2-bromo- thiophen and potassium amide may suggest that an aryne mechanism is involved.However the product ratio is not influenced by the addition of 93 T. M. Harris C. M. Harris and J. C. Cleary Tetrahedron Letterg 1968 1427. 94 H. Takei M. Higo K. Saito and T. Mukaiyama Bull. Chem. SOC.Japan 1968,41,1738. 95 E. E. Schweizer and J. G. Liehr J. Org. Chem. 1968,33 583. 96 J. A. Elix Chem. Comm. 1968,343. " C. T. Wie S. Sunder and C. D. Blanton jun. Tetrahedron Letters 1968,4605. '13 H.Hiraoka and R. Srinivasan J. Amer. Chem. SOC.,1968,90,2720. 99 Ann. Reports (B),1967,390. loo E. E. van Tamalen and T. H. Whitesides J. Amer. Chem. Soc. 1968,90,3895. S. H. Groen R. M. Kellogg J. Buter and H. Wynberg J. Org. Chem. 1968,33,2218. 456 R. J. Stoodley (84) (85) (86) potassium halides while at lower temperature 2,3-dibromothiophen may be isolated which is converted into 3-aminothiophen with an excess of amide. The results argue against the aryne mechanism and imply that transbromina- tion occurs via intermediate carbanions.' O2 A similar mechanism is probably involved in the amide-catalysed rearrangement of 3-bromo-5-methylisothiazole to the 4-brorno-i~omer.'~~ Bromination and deuteriation of substituted thiophens may be accomplished under very mild conditions thus 2-and 3-methylthiophen brominate at position 5 and 2 respectively while deuterium exchange occurs at the same positions in the presence of deuterioacetic acid.lo4 ,3TjNc4H8R' Me -rnCHCIMe0TR2 02 \ Me (87) 6OR1\ R2 Ph wcsmPh Ph Ph (89) (90) (91) Pyrrolidine induces an interesting rearrangement of the benzothiophen (87) to (88).The reaction probably involves an SN2' displacement of the halide by the base followed by ring opening and ring closure.'05 A remarkable sequence of reactions takes place when ally1 2,6-dimethylphenyl sulphide is heated in quinoline. The primary cyclic products which may arise by rearrangement of the initially formed o-thio-Claisen intermediate include (89; R' = R2= Me) (89; R' = H R2= Et),(90;R' = H R2= Me) and (90;R' = Me R2= H).'06 Methylene transfer from dimethyl sulphonium methylide is well known.However a unique transfer of the thiomethylene group occurs in its reaction with triphenylcyclopropenium bromide when 2,3,4-triphenylthiophen and (91) are formed.'" The thioketone (91) is of interest since it resembles the M. G. Reinecke and H. W. Adickes J. Amer. Chem. SOC.,1968,90,511. lo' D. A. de Bie and H. C. van der Plas Tetrahedron Letters 1968 3905. R M. Kellogg A. P. Schaap E. T. Harper and H. Wynberg J. Org. Chem. 1968,33 2902. lo' F. G. Bordwell R W. Hemwall and D. k Schexnayder,J. Org. Chem. 1968,33 3226 3233. H. Kwart and M. H. Cohen Chem. Comm. 1968 1298. lo' B. M. Trost and R. Atkins Tetrahedron Letters 1968 1225.Heterocyclic Chemistry intermediates proposed in the photoisomerisation of thi~phens.'~ However when irradiated it is converted into hexaphenylbenzene and carbon mono- sulphide. FivememberedRings with Twoor More Hetero-atoms.-It has been suggested that 1,3-dipolar cycloadditions involve spin-paired diradical intermediates although Huisgen has argued against this interpretation and favours a con- certed mechanism.'08 The principle continues to be of immense value in heterocyclic synthesis. Thus cycloaddition of nitrones to alkenes vinyl ethers ap-unsaturated esters or ap-unsaturated nitriles provides a versatile (94) Ph + ,CR'R~ rJ 1 Me \ route to isoxa~olidines.'~~ For instance cis-2,3,5-triphenylisoxazolidine(93) is the major product in the reaction involving styrene and (92).Adducts (94) and (95) are formed stereospecifically from nitrile oxides and ap-un- saturated esters.'" The concept may be applied also to the construction of fused aromatic heterocycles. Thus (96; R1= H R2 = Bz) and ethyl propiolate cyclise to the diazapentalene (97) indicating the ease with which the dihydro- intermediate autoxidises. However when oxidation is precluded as with (96; R1= R2 = CN) and dimethyl acetylenedicarboxylate ring expansion to (98) occurs."' lo* R. A. Firestone J. Org. Chem. 1968,33 2285; R Huisgen ibid. 1968,33 2291. R Huisgen R Grashey H. Hauck and H. Seidi Chem. Ber. 1968 101,2043 2548,2568. 'lo M. Christ1 and R. Huisgen Tetrahedron Letters 1968 5209.V.Boekelheide and N. A. Fedoruk J. Amer. Chem. Soc. 1968,90,3830. R.J. Stoodley A number of more conventional but interesting syntheses may be noted. Thus phenylhydrazine and hydroxylamine react with 1-cyanomethylacetylene to give 5-amino-3-methyl-1-phenylpyrazole and 3-amino-5-methylisoxazole respectively. However a different cyclisation occurs with hydroxylamine and 1-cyanomethyl-2-methylacetylene since 5-amino-3-ethylisoxazoleis formed.' The base-catalysed formation of (99) from phenylnitromethane and cis-a-nitrostilbene provides the first example of a nitrite displacement by nitronate anion.'' A novel synthesis of isothiazoles involves cyclisation of a-amino-ketones with thionyl chloride or sulphur dichloride for instance 4-hydroxy- 5-phenylisothiazole-3-carbaldehyde is obtained from l-amino-l-phenylbutan- 2-one and sulphur dich10ride.l'~ A general unambiguous route to 1,4-di-substituted imidazoles is available in the acid-catalysed cyclisation of a-amino-P-methylaminopropionic acid with triethyl orthoformate followed by manganese dioxide dehydrogenation.' Hydrazine reduction of (101) which may be prepared from an aromatic a-halogeno-ketone and (loo),affords 1,2-diarninoimida~oles.~When a-acylamino-acids are dissolved in acetic l6 anhydride and perchloric acid they are converted in high yield into 5-oxa- zolium perchlorates [e.g.(102; R1= H)] which deprotonate with triethylamine.The N-substituted salts [e.g. (102; R1= Me)] available from N-alkyl- or N-aryl-a-acylamino-acids are of special interest because on deprotonation they yield mesoionic oxazolones.Although these 'munchnones' are too unstable to be isolated they may be trapped by suitable dipolarophiles.'17 Further examples of stable molecules which incorporate the azomethine imine grouping have been prepared. These compounds [e.g. (103)],in which 'I2 E. Haruki Y. Hirai and E. Imoto Bull. Chem. SOC.Japan 1968,41 267. 'l3 A. T. Nielsen and T. G.Archibald Tetrahedron Letters 1968 3375. T. Naito S. Nakagawa J. Okumura K. Takahashi and K. Kasai Bull. Chem. SOC.Japan 1968,41 959. 'I5 P. K. Martin H. R Mathews H. Rapoport and G.Thyagarajan J.Org. Chem. 1968,33,3758. 'I6 H. Beyer A. Hetzheim H. Honeck D. Ling and T. Pyl Chem Ber. 1968,101 3151. 11' G. V. Boyd Chem.Comm.,1968 1410. Heterocyclic Chemistry the anionic charge is stabilised by an adjacent carbonyl- or imino-function may be obtained from pyrazolid-3-ones and carbonyl compounds or by dehydrogenation of 1-alkylpyrazolid-3-ones with mercuric oxide.' Interest in mesoionic compounds continues and the Munich School have exploited the reactions of sydnones [e.g. (la)] which are also examples of azomethine imines with dipolarophiles. Thus cycloaddition of sydnones and acetylenes constitutes a versatile approach to pyrazoles.' ' 1-Substituted pyrazoles result from 3-substituted sydnones and cyanoethylene although in the presence of tetrachlorobenzoquinone the reaction takes a different course to yield 1-substituted 3-cyanopyrazoles. 120 Sydnones will also react with alkenes enabling A*-pyrazolines to be prepared,' 21 while with arylcarbaldehydes they afford ring-opened products [e.g.(105)].'22 Loss of carbonyl sulphide occurs Me (108) (107) MeN-NH .CSPh (110) (109) in the cycloaddition of (106) to dimethyl acetylenedicarboxylate and dimethyl 2,5-diphenylthiophen-3,4-dicarboxylateis formed.'23 A similar reaction involving (107) also leads to elimination of carbonyl sulphide and to the formation of dimethyl 1-methyl-2-phenylpyrrole-3,4dicarboxylate. However in the presence of phenyl isothiocyanate (107) is converted into the new mesoionic derivative (108).'24 Another interconversion of mesoionic com- pounds is illustrated by the reaction of hydrogen sulphide with the isosydnone (109; X = 0)to yield (IlO) which cyclises to (109; X = S) in the presence of phosgene and acid.'25 The photochemical behaviour of five-membered heterocycles continues to attract considerable attention. An interesting photoinduced ring-contrac- tion of tetrahydro-3H-pyrazolones [e.g. (111; R = H)] provides the first '" H. Dom and A. Otto Angew. Chem Internut. Edn. 1968,7,214,888; Z. Chem. 1968,8,217,273; Chem. Ber. 1968,101,3287; Tetrahedron 1968,24,6809. 'I9 R. Huisgen H. Gotthardt and R. Grashey Chem. Ber. 1968,101,536. 120 R Huisgen R. Grashey and H. Gotthardt Chem. Ber. 1968,101,829. H. Gotthardt and R. Huisgen Chem. Ber. 1968,101,552. lZ2 H. Gotthardt R. Huisgen and R. Knorr,Chem. Ber. 1968 101 1056. lZ3H. Gotthardt and B. Christl Tetrahedron Letters 1968,4743.lZ4 K.T. Potts and D. N. Roy Chem. Comm. 1968 1061. A. R. McCarthy W. D. OIIis and C. A. Ramsden Chem. Comm. 1968,499. 460 R.J. Stoodley Me Me examples of 1-amino-substituted azetidin-2-ones (1 12). However if position 1 is substituted as with (111; R = Me) no reaction occurs.126 Ring contraction also takes place when 1-methyl-5-phenyl-A2-pyrazoline is irradiated in benzene although the derived cis-and trans-cyclopropanes (113) revert to the parent heterocycle upon being warmed. 12' The adduct (1 15) derived from 4-phenyl lH-l,2,4-triazole-3(2H),5(4H)-dioneand (114) loses nitrogen on photolysis to give (116) which rearranges to (117) upon being heated.128 In contrast the oxadiazoline (1 18) does not eliminate nitrogen when irradiated in alcohol but yields (1 19) possibly uia an intermediate a-lactam.12' Many heteroaromatic five-membered rings undergo interchange of adjacent atoms %' Me -N (115) / upon photochemical excitation. 30 Another example is furnished by 1,4,5- trimethylimidazole which rearranges to the 1,2,5-isomer.' 31 Such isomerisa- tions may display dramatic wavelength dependence thus with 3340 A light 3,5-diphenyloxazole is equilibrated with 3-benzoyl-2-phenyl-l-azirine, which rearranges only to 2,5-diphenyloxazole with 3 130 A light.'32 The behaviour of the azlactone (120) in isopropyl alcohol is also sensitive to the wavelength lZ6 S. N. Ege Chem. Comm. 1968,759. lZ7 H. J. Rosenkranz and H. Schmid Helu. Chim. Acta 1968,51,1628. lZ8 A.B. Evnin and D. R. Arnold J. Amer. Chem. SOC.,1968,90,5330. lZ9 C. J. Michejda Tetrahedron Letters 1968 2281. 130 Ann. Reports (B),1967,392. 131 P. Beak J. L. Miesel and W. R. Messer Tetrahedron Letters 1967 5315. 132 B. Singh and E. F. Ullman J. Amer. Chem. SOC. 1967,89 6911. Heterocyclic Chemistry 46 1 Ph NHBz Ph Ph Ph I )=N*NH*CO=CORPh (120) (121) Ph (119) of the light; it equilibrates with its double-bond isomer with 3650 A light and is converted into (121) with 2537 A light.'33 An instance in which adjacent atoms of a five-membered ring interchange which does not involve a hetero- aromatic system and which is thermally induced is exemplified by the re- arrangement of 4-isoxazolines into 4-oxazolines ; in appropriate cases the intermediate 2-acylaziridines may be isolated.' 34 Loss of nitrogen occurs when lH-l,2,3-triazoles are irradiated ;for example triphenylketenimine and 2,3-diphenylpyrrole are formed in equal amount from 1,4,5-triphenyl-lH- 1,2,3-triazole although only carbazole is obtained from l-phenylbenzotria- ~ole.'~' Photolysis of an ethereal solution of the oxadiazole (122) affords (123) and (124) indicating that N-0 bond cleavage occurs and that the solvent acts as a hydrogen donor.'36 A double fragmentation occurs in the irradiation of 1,2,5-oxadiazoles 1,2,5-thiadiazoles and 2H-1,2,3-triazoles :thus acetonitrile and (125) are formed from 3,4-dimethyl-l,2,5-oxadiazole in the presence of cyclopentene.' 37 The chemistry of 1,2.5-thiadiazoles and 1,3,4- thiadiazoles has been reviewed.' 38 133 N.Baumann M. Sung and E. F. Ullman J. Amer. Chem. Soc. 1968,90,4157. 134 J. E. Baldwin R G. Pudussery A. K Qureshi and B. Sklarz J. Amer. Chem. Soc. 1968 90 5325. 13' E. M. Burgess R Carithers and L. McCullagh J. Amer. Chem Soc. 1968,90 1923. '36 H. Newman Tetrahedron Letters 1968,2417 2421. 13' T. S. Cantrell and W. S. Haller Chem. Comm. 1968 977. L. M. Weinstock and P. I. Pollak Adv. Heterocyclic Chem. 1968 9 107; J. Sandstrom ibid. 1968 9 165. 462 R. J. Stoodley Heterolysis of the disulphide bond of (126) occurs in the presence of malonic acid and pyridine resulting in the formation of (127).13' Although dealkylation is the main reaction route when a dithiolium salt [e.g. (128; R = SEt)] is treated with aqueous pyridine some 6a-thiathiophthen (129; R' = SEt R2 = Bz R3 = Ph) is also obtained.This suggests that (128; R = SEt) reacts with ethyl benzoyldithioacetate which may arise by cleavage of (128; R = SEt).14' Indeed reaction of the dithiolium salt (128; R = SMe) with methyl benzoyldithioacetate or methyl ethoxycarbonyldithioacetateprovides a general synthesis of 6a-thiathio~hthens.'~' The reactivity of the methyl group of (128; R = Me) is illustrated by the formation of (128; R = CH:CHNMe,) in the presence of NN-dimethylthioformamide and acetic anhydride ; (128; R = CH:CHNMe,) will cyclise to (129; R' = Ph R2 = R3 = H) with s-s+ + X-s-s PhVR I (128) s-s-s R' R3 R2 sodium hydrogen ~ulphide.'~~ In agreement with charge-density calculations 6a-thiathiophthens undergo electrophilic attack at position 3 and nucleophilic attack at position 2.For instance (129; R' = Ph R2 = H R3 = SMe) may be nitrated to (129; R' = SMe R2 = NO2 R3 = Ph) and its 2-methylthio-group is displaced in the presence of eth0~ide.l~~ However addition of thiolate to position 2 of 6a-thiathiophthens triggers an interesting rearrangement to 4-thiapyran-4-thiones which constitutes a novel synthesis of these hetero- cycles.' 44 Six-memberedRings with One Hetero-atom.-Nitrogen Derivatives. The controversy over the relative size of a lone pair of electrons and a proton continues. On the basis of chemical-shift comparisons between the axial and equatorial protons of piperidine and N-methyl- and N-t-butyl-piperidine it has been concluded that the preferred conformation of piperidine is the one with an axial hydrogen on nitrogen.'45 However the assumptions in this approach have been questioned,' 46 while the slight preference for axial D.B. J. Easton D. Leaver and D. M. McKinnon J. Chem. SOC. (C) 1968,642. 140 C. Bouillon and J. Vialle Bull. SOC. chim. France 1968,4560. 14' R J. S. Beer R P. Carr D. Cartwright D. Harris and R A. Slater J. Chem. SOC. (C) 1968,2490. 142 J. G. Dingwall S. McKenzie and D. H. Reid J. Chem. SOC. (C) 1968,2543. 143 R J. S. Beer D. Cartwright R J. Gait R A. W. Johnstone and S. D. Ward Chem.Comm 1968,688. 144 J. D. Dingwall and D. H. Reid Chem. Comm. 1968,863. 14' Ann. Reports (B) 1967,399. M. J. T. Robinson Tetrahedron Letters 1968 1153.Heterocyclic Chemistry OMe (130) deuteriation of cis-3,5-dimethylpiperidine in deuteriotrifluoroacetic acid suggests the opposite conclusion. 14’ Evidence from i.r. and microwave spectroscopy supports the latter view.14* The intramolecular addition of carbonium ions to double-bonds is well known the extension of this principle to electrophilic nitrogen offers an attractive synthetic route to a wide range of azabicyclic compounds. For instance (131) may be obtained from (130) in the presence of methanolic silver nitrate,149 although with 5-(N-chloro)methylaminocyclohepteneand silver perchlorate (132) is f~rrned.”~ A similar retention of chlorine is observed in the silver-ion-catalysed rearrangement of (133) to mainly (134).’” The reaction of a-nitro-a-halogenoallcanes with imines provides a novel method for heterocyclic synthesis thus 1-cyclohexyl-2-methyl-3-nitropiperidineis formed from N-ethylidenecyclohexylamine and 4-bromo- 1-nitrobut ane.The salts (135) derived from glutaronitriles in the presence of ethereal c1fi -” NH2 MN f e Et c1 ‘*:-*. NH2 H c1-(136) C02Et 14’ H. Booth Chem. Comm. 1968,802. 14* R D. Baldock and A. R Katritzky Tetrahedron Letters 1968,1159; M. Tsuda and V. Kawazoe Chem. and Pharm. Bull. (Japan) 1968 16 702; P. J. Buckley C. Costain and J. E. Parkin Chem Comm. 1968,668. ’*’ P. G. Gassman F. Hoyda and J. Dygos J. Amer. Chem. SOC., 1968,90,2716. J. D. Hobson and W. D. Riddell Chem. Comm. 1968,1178. Is’ P. G. Gassman and R.L. Cryberg J. Amer. Chem. SOC. 1968,90 1355. lS2 J. E. Dolfini and E. J. Swain J. Org. Chem. 1968,33 2079. 464 R. J. Stoodley hydrogen chloride are converted into 3,4-dihydropyridines (136) after treat- ment with acetic anhydride followed by sodium hydrogen carbonate. 153 An interesting route to pyridoxal analogues involves Diels-Alder cyclo-addition of 5-ethoxy-4-methyloxazole and cis-2,5-dimethoxy-2,5-dihydro-furan. The exo-and endo-cyclo-adducts are transformed into 3-hydroxy-2- methylpyridine-4,5-dicarbaldehyde after reaction with alkali followed by In a closely related procedure the cyclo-adduct (137) derived from 5-ethoxycarbonyl-4-methyloxazoleand diethyl fumarate is converted into a mixture of diethyl 3-hydroxy-2-methylpyridine-4,5-dicarboxylate and diethyl 2-acetylpyrrole-3,4-dicarboxylate.1 5s This represents the first case of pyrrole formation via a Diels-Alder reaction and suggests that the imino-group of / CO*cH*HOMe NHMe 0 Me I -0Ts Me (138) (139) (140) Me (141) (137) is readily hydrolysed.The stable ylide (138) prepared from N-methyl- isatoic anhydride and dimethyloxosulphonium methylide is of value in heterocyclic synthesis since it undergoes an acid-catalysed cyclisation with triethyl orthoformate to give (139).lS6 A variety of heterocycles may be synthesised from a-and p-aminonitriles and carbonyl compounds. For example. 3-(N-methylamino)propriononitrileand cyclohexanone in refluxing toluene containing toluene-p-sulphonic acid yields the salt (140) which is transformed into (141) with alkali.lS7 The cycloaddition of ynamines and aryl isocyanates provides a new route to 2-quinolones,l while N-methylanilines and N-phenylmaleimide yield tetrahydroquinolines [e.g.(142)] in the presence of benzoyl peroxide.lS9 Suitably activated pyridines will undergo intramolecular electrophilic attack under mild conditions since (143) is formed from 2-(6-ethoxy-2-pyridy1)-2-ethylcyclohexanone and toluene-p-sulphonic acid.160 A number of new isoquinoline syntheses have been reported. For instance (144) yields (145) with formamide and phosphorus oxychloride 61 while 2-cyano- 1,3 -dimethyl- benzene dimerises to (146) in the presence of lithium di-isopropylamide.162 153 L. G. Duquette and F. Johnson Tetrahedron 1967 23,4531.lS4 T. Naito K. Ueno M. Sano Y. Oktura I. Itoh and F. Ishikawa Tetrahedron Letters 1968 5767. lS5 M. Murakami K. Takahashi J. Matsumoto K. Tamazawa K. Murase H. Iwamoto and M. Iwanami Bull. Chem. Soc. Japan 1968,41,628. lS6 A. M. Van Leusen and E. C. Taylor J. Org. Chem. 1968,33,66. S. Singh and A. I. Myers J. Heterocyclic Chem. 1968,5 737. lS8 J. Ficini and A. Krief Tetrahedron Letters 1968,947. ls9 R.B. Roy and G. A. Swan Chem. Comm. 1968,1446. 160 S. Danishefsky and M. Feldman Tetrahedron 1968,24,4083. 161 T. Koyama T. Hirota I. Ito M. Toda and M. Yamato Tetrahedron Letters 1968,4631. 162 H. van der Goot T. Bultsma and W. Th. Nauta Rec. Trau. chim. 1968.87 126. Heterocyclic Chemistry Ph (145) (142) Benzylethylenes react with nitriles when a halogen and a Lewis acid are present furnishing an interesting approach to 3,3-disubstituted 3,4-dihydro- isoquinolines [e.g.(147)l.163 A simple high-yield route to 5,6,7,8-tetrahydro- isoquinolines [e.g. (148)] is available from the reaction of 2-acylcyclohexanones with malononitrile or cyanoacetamide.' 64 Two new methods of side-chain functionalisation have been described. Thus peracid oxidation of 2-benzyloxy-6-methylpyridineyields 1 -benzyloxy- 6-methyl 2-pyridone which possessing an active methyl group can undergo condensation with carbonyl derivatives. The protecting group may then be removed by reduction to afford 6-substituted 2-pyridones. ' N-Allyl-anhydro-bases [e.g. (149)] derived from 1-allyl-2-methylquinolinium salts undergo Claisen rearrangement to 2-homoallylquinolines providing a new method for the introduction of an ally1 substituent onto a carbon attached to the C=N bond of unsaturated heterocycles.'66 The chemistry of Reissert compounds'67 and pyrrolopyridines' 68 has been reviewed.The cyclo-adduct (150) derived from 2-styrylpyridine and dimethyl acetylenedicarboxylate is cyclised thermally to the cyclazine (151 ; R = H) which undergoes an interesting stereospecific exchange of the a-hydrogen in deuterioacetic acid to give (151; R = D).16' There has been further interest in hydrogen-deuterium exchange in pyridine derivatives. 70 A convenient route to 2-deuteriopyridines involves decarboxylation of pyridine-2-[2H]carboxylic acids although the method is not so successful 163 A.Hassner R. A. Arnold R. Gault and k Terada Tetrahedron Letters 1968 1241. 164 F. Freeman D. K. Farquhar and R. L. Walker J. Org. Chem. 1968,33,3648. 165 R. B. Greenwald and C. L. Zirkle J. Org. Chem. 1968,33 2118. 166 R. K. Hill and G. R. Newkome Tetrahedron Letters 1968,5059. F. D. Popp Adv. Heterocyclic Chem. 1968,9 1. 16' R. E. Willette Adv. Heterocyclic Chem. 1968,9 27. 169 R. M. Acheson and R. S. Feinberg J. Chem. SOC. (C) 1968,351. 170 Ann. Reports (B),1967,402. 466 R.J. Stoodley C0,Me L-R f (149) Ph (150) D I C0,Me (152) with the isomeric acid~.'~' Exchange of the a-protons in 4-amino- and 4- dimethylamino-pyridine strongly predominates over P-exchange in deuterium oxide. Although the rate of exchange of the a-protons is not influenced by basicity the rate of P-exchange increases and eventually overtakes that of a-exchange suggesting that a-exchange proceeds via the ylide while replacement of the P-protons involves the carbanion.In contrast only the P-protons are exchanged in deuterioperchloric acid,' 72 providing evidence for the inter- mediacy of the dication (152) and illustrating the enamine character of the 4-amino-group. Further evidence that the substitution of hydride ion by organolithium compounds in pyridines proceeds via an addition-elimination ~equence'~ has been obtained thus the intermediate formed from n-butyl-lithium and pyridine may be observed by n.m.r. spectroscopy and it is converted into 2-n-b~tyl-[~H)-1,2-dihydropyridine with deuterium oxide at 0°.174Previous attempts to trap 2,3-pyridynes have been unsuccessful ; however 2,3,5,6- tetrachloro-4-piperidinylpyridineyields (153; R' = R2= C1 R3= C5HIoN) in the presence of n-butyl-lithium and furan whle (153; R' = R2= H R3= EtO) is formed from 2,3-dibromo-4-ethoxypyridine furan and lithium amalgam.17 2-Bromo-6-ethoxypyridine and sodium amide give a mixture of 2-amino- and 4-amino-6-ethoxypyridine.Although analogous products are formed from 2-bromo-6-phenoxypyridine under similar conditions the major J.A. Zoltewicz C. L. Smith and J. D. Meyer Tetrahedron 1968,24 2269. J. A.Zoltewicz and J. D. Meyer Tetrahedron Letters 1968,421. 173 Ann. Reports (B) 1967,403. G. Fraenkel and J. C. Cooper Tetrahedron Letters 1968,1825.17' J. D.Cook and B. J. Wakefield Chem. Comm 1968,297;H.N.M.van der Lans and H.J. den Hertog Rec. Trau. chim. 1968,87,549. Heterocyclic Chemistry derivative is 2-methyl-4-phenoxypyrimidine,which emphasises the subtle balance between displacement and ring opening. l-Hydroxy-2-phenyl-l,2-dihydropyridine, which may be prepared from p yridine N-oxide and phenylmagnesium bromide undergoes a ring-opening reaction upon benzoylation to furnish (154) which yields 5-phenylpenta-2,4- dienenitrile. 77 Evidence that the acetic anhydride-induced rearrangement of 2-alkylpyridine 1-oxides proceed via electrophilic intermediates is obtained from the rearrangement of 2-cyclopentylmethylpyridine 1-oxide which yields some l-(2-pyridyl)cyclohexene.'78 Peracid oxidation of NN-disubstituted 2-aminotetrachloropyridines affords hydroxylamines [e.g.(1591 derived by rearrangement of the expected N-oxides while thermal or photochemical excitation of (155) results in the formation of tetrachloro-2-pyridone.' Several papers have appeared dealing with the photochemistry of aromatic N-oxides which was noted last year.18' It is generally assumed that the first OHPh Ph G \ lh I OBz (154) + @Ph/ + 8:5 \/ step involves the formation of an unstable oxaziridine which undergoes subsequent rearrangement. In accord with this postulate oxidation of solvent occurs when pyridine N-oxide is photolysed in ethanol pyridine pyrrole-2- carbaldehyde and the acetal of pyrrole-1-carbaldehyde are also formed.' ' Irradiation of 2,4,6-triphenylpyridine N-oxide in acetone yields 2,4,6-triphenyl- pyridine and its 3-hydroxy-derivative the oxazepine (156) and 2-benzoyl-3,5- diphenylpyrrole while only deoxygenation is observed in ethanol.' 82 The J.W. Streef and H. J. den Hertog Tetrahedron Letters 1968,5945. "'T. Kato H. Yamanaka T. Adachi and H. Hiranuma J. Org. Chern. 1967,32,3788. R Bodalski and A. R Katritzky Tetrahedron Letters 1968 257; J. Chem. SOC.(B) 1968 831. S. M. Roberts and H. Suschitzky J. Chem. SOC.(C),1968,1537. Ann Reports (B) 1967,401. A. Alkaitis and M. Calvin Chem Comm. 1968,292. ''* P. L. Kumler and 0.Buchardt Chem. Cornm.,1968 1321. 468 R.J. Stoodley (1 62) (163) 0 PhN4 R photochemical behaviour of (157) is solvent dependent thus in ethanol (158) is formed while in benzene deoxygenation ring expansion to (159) and ring contraction to (160) are also 0b~erved.I~~ The oxazepine (161) formed by irradiation of 1-substituted isoquinoline N-oxides cyclises to the azetine (162) on further excitation which reverts to (161) upon being heated.184 Photolysis of acridone N-oxide affords (163).18' When (164) and diphenyl- acetylene are irradiated in methanol with 3500 8 light an interesting photo- cycloaddition occurs to yield (169 which presumably arises by isomerisation of the [4 + 2ladduct.The [2 + 2ladduct (166) is not an intermediate in this reaction although it is obtained from (165) upon irradiation in benzene with 2537 8 light.186 SCHEME 3 There is evidence for the interconversion of phenylnitrene and 2-pyridyl- carbene possibly by the route suggested in Scheme 3.Thus triethyl phosphite deoxygenation of o-nitrotoluene affords (167),187 while the azepine (168) is obtained in the corresponding reaction involving o-ethylnitrobenzene.' 88 Anthranil photolysis may also involve initial formation of a phenylnitrene ; for example irradiation of (169) in methanol furnishes a good yield of the azepine (170).' 89 Furthermore 2-methylcarbazole is produced from the ther- E. C. Taylor and G. G. Spence Chem. Comm. 1968,1037. C. Lohse Tetrahedron Letters 1968 5625. M. Ishikawa C. Kaneko and S. Yamada Tetrahedron Letters 1968,4519. A. I. Myers and P. Singh Tetrahedron Letters 1968,4073. R J. Sundberg Tetrahedron Letters 1968,777.J. I. G. Cadogan R. K. Mackie and M. J. Todd Chem. Comm. 1968,736. M. Ogata H. Kano and M. Matsumoto Chem. Comm. 1968,397. Heterocyclic Chemistry qN Me (169) (167) Ph Me\ N-N rT (171) molysis of the triazolopyridine (1 71).19* Nitrene intermediates may also intervene in the triethyl phosphite induced cyclisation of 1-(2-nitrobenzyl)- isoquinolines to benzo[a]carbazoles. Oxygen Derivatives. The relative free-energies of aldopyranoses in aqueous solution have been calculated taking nonbonded interactions and the anomeric effect into consideration ; the estimates correctly predict the predominant conformation and the a :p ratio."' Similar calculations are also shccessful in predicting the equilibria between aldohexoses and their 1,6-anhydrides and between heptuloses and their 2,7-anhydride~.'~~ The anomeric equilibrium mixture of 2-methoxy-trans-5,6-dimethyltetrahydropyran possesses more of the axial methoxy-anomer compared to either 2-methoxy-4- or 2-methoxy- 6-triethyltetrahydropyran which may be attributed to a compressional repulsion between the trans-methyl groups Ig4 A general method for the determination of the stereochemistry and absolute configuration at C-4 and C-5 of a 4-amino- 1,4-dideoxyhexose involves con- version into the 4-acetamidoglycoside which is then degraded by sequential periodate oxidation sodium borohydride reduction and acidic hydrolysis to the aminotetritol.The latter derivative may be characterised as its hydrogen oxalate salt.195 Circular dichroism measurements of cuprammonium com- plexes of 4,6-O-benzylidene hexosides indicate that the sign of the 580 mp band is opposite to that of the dihedral angle between the 2- and 3-substituents.Thus the diequatorial diol (172; R' = R3= OH R2= R4 = H) with a dihedral angle of -60" displays a positive band while the axial equatorial diol (172; R' = R4= OH R2= R3= H) with a dihedral angle of +60" 19* W. D. Crow and C. Wentrup Tetrahedron Letters 1968,6149. 191 T. Kametani T. Yamamaka and K. Ogasawara Chem. Comm. 1968 786. 192 S. J. Angyal Austral. J. Chem. 1968,21,2737. 193 S. J. Angyal and K. Dawes Austral. J. Chem. 1968,21,2747. 194 D. T. Sepp and C. B. Anderson Tetrahedron 1968,24,6873. 19' C. L. Stevens S. K. Gupta R P.Glinski G. E. Gutowski and C. P. Bryant Tetrahedron Letters 1968 1817. 470 R. J. Stoodley "-':ti. -OMe CN OH OMe (1 74) (172) exhibits a negative band. Furthermore the failure of the diaxial diol (172; R2= R4= OH,R' = 33 = H)to show circular dichroism confirms that no complex formation occurs. The method is preferable to the single wavelength measurement' 96 for the assignment of configuration and conformation and may be extended to acyclic ligand~.'~' An interesting approach to the sequential analysis of di- and tri-saccharides involves mass-spectral examination of their fully acetylated 3-methyl-1-naphthylglycosides. For example the maltose derivative expels 1-hydroxy-3-methylnaphthaleneand shows a strong peak characteristic of the disaccharide oxonium ion.98 Valuable information mziy also be obtained from the mass spectra of di- and tri-saccharide trimethylsilyl ethers and of methylated uronic aldobiouronic and aldotriouronic acids. 199 Alkylation of 0x0-sugars or their enamine derivatives provides a new route to branched-chain sugars. For instance methyl 4,6-0-benzylidene-3-deoxy-a-D-erythro-hexopyranosidulose or its pyrrolidine enamine alkylates from the a-face with methyl iodide to afford (173) which epimerises at position 3 in the presence of triethylamine.200 Triethylaluminium and hydrogen cyanide are useful for opening sugar epoxides thus (174) may be obtained from methyl 2,3-anhydro-4,6-0-benzylidene-a-~-mannopyranoside.~~' The latter epoxide undergoes oxidative ring-opening and loss of the benzylidene and methoxy- groups with dimethyl sulphoxide and boron trifluoride ;however the formula- tion of the product as (175) appears to be unreasonable.202 The outcome of the s -0 HO 196 Ann.Reports (B) 1967,405. 19' S. T. K. Bukhari R. D. Guthrie A. I. Scott and A. D. Wrixon Chem. Comm. 1968 1580. 198 J. Karliner Tetrahedron Letters 1968 3545. 19' N. K. Kochetkov 0. S. Chizhov and M. V. Molodtsov Tetrahedron 1968 24 5587; V. Kovacik S. Bauer J. Rosik P. Kovac Carbohydrate Res. 1968,8,282; V. Kovacik S. Bauer and J. Rosik ibid. 1968,8 291. R F. Butterworth W. G. Overend and N. R. Williams Tetrahedron Letters 1968 3239. '01 B. E. Davidson R. D. Guthrie and A. T.McPhail Chem. Comm.,1968 1273. 'O' G.Hanisch and G. Henseke Chem. Ber. 1968,101,4170. Heterocyclic Chemistry 471 reaction of methyl 2,3-anhydro-4,6-0-benzylidene-a-~-allopyranoside with an excess of methyl-lithium depends upon the source of the metal alkyl. Thus (176;R = Me) is formed if the metal alkyl is prepared from methyl chloride and lithium while (176; R = H) is obtained if methyl iodide and lithium are employed.203 The branched-chain derivative (177) is the first naturally occur- ring nitro-sugar to be isolated.204 A potentially important new approach to glycoside synthesis involves the boron trifluoride-catalysed reaction of alcohols and glycals to yield the 2,3- unsaturated glycosides which may be subsequently hydroxylated. For in- stance 1,2 :3,4-di-O-isopropylidene-a-~-galactopyranose and tri-0-acetyl-D- glucal afford the disaccharide (178).205 The thermal rearrangement of (179) to (180) is stereospecific and provides a useful preparative route to the thermo- dynamically unfavoured 3-deoxyald-2-enopyranoseesters.206 CH,*OAc CH,*OAc ML0 b,,JMe Aco-o-a, Me Me (178) u OAc (180) The chemistry of fructose and its derivatives has been re~iewed.~" Alkaline degradation of D-glucose 6-phosphate to lactic acid and inorganic phosphate proceeds uia retro-aldolisation of fructose 6-phosphate and provides a striking similarity to the enzymically controlled anaerobic pathway.208 The claim that 3-deoxyglucosulose is formed from D-glucose 3-phosphate in deuterium oxide without isotopic incorporation at position 3209 has been shown to be erroneous.2lo Neighbouring-group effects in carbohydrates2' and the chemistry of halogenated sugars2 has been reviewed.N.m.r. analysis of the acetoxonium ions derived from 1-ch~oro-2,3,4-tri-~-acetyl-~-~-xy~opyranose in the presence of antimony pentachloride indicates equilibration between the 1,2- 2,3- and '03 M. Sharma and R. K. Brown Canud. J. Chem. 1968,46,757. '04 A. K. Ganguly 0.Z. Sarre and H. Reimann J. Amer. Chem. Soc. 1968,90,7129. 'OS R. J. Ferrier and N. Prasad Chem. Comm 1968,476. '06 R. J. Ferrier N. Prasad and G. H. Sankey J. Chem. SOC.(C) 1968,974. '07 L. M. J. Verstraeten Adv. Carbohydrate Chem. 1967,22,230. '08 C. Degani and M. Halmann J. Amer. Chem. SOC.,1968,90,1313. '09 G. Fodor and J. P. Sachetto Tetrahedron Letters 1968,401.'lo E. F. L. J. Anet Tetrahedron Letters 1968 3525. '11 L. Goodman Adv. Carbohydrate Chem. 1967,22 109. J. E. G. Barnett Adv. Carbohydrate Chem. 1967,22 177. 472 R. J. Stoodley 3,4-derivatives.’13 However only the 1,2- and 2,3-acetoxonium ions are present for 1-chloro-2,3,4,6-tetra-O-acetyl-~-~-galactopyranoside.~~~ When the tetra-0-acetyl derivatives of D-arabino- and D-ribo-pyranoses are dissolved in hydrogen fluoride they are rapidly converted into their lP-fluor0-2,3,4-tri-O- acetyl derivatives which slowly rearrange to the common acetoxonium ion (181). The same ion is also formed from 1,2,3,5-tetra-O-acetyl-P-~-ribofuranose and methyl 2,3,5-tri-0-acetyl-c-~-arabinofuranose under similar conditiom2 Solvolysis of benzyl 5-O-p-bromophenylsulphonyl-2,3-O-isopropylidene-~-D-ribofuranose in aqueous methanol containing sodium acetate furnishes 5-0-benzyl-2,3-0-isopropylidene-~-ribofuranose.~~~ A 1,4-participation in- volving the methoxy-group is illustrated in the solvolysis of methyl 2,3-di-0- methyl-6-0-methylsulphonyl-~-~-galactopyranoside, when both methyl 2,3,6- tri-0-methyl-p-D-galactopyranosideand methyl 3,6-anhydro-2-0-methyl-P-D-galactopyranoside are obtained.l7 Methanolyis of methyl l-thio-6-O-p- tolylsulphonyl-P-D-glucopyranoside proceeds with migration of the methyl- thio-group to C-6.218 A novel route to carbohydrates in which the ring oxygen is replaced by sulphur is provided by the reaction of 1,2,3,4-tetra-O-acetyl- P-D-ribopyranose with methanethiol and zinc chloride.After deacetylation methyl 1,5-dithio-P-~-ribopyranoside is obtained in addition to the expected methyl 1-thio-P-D-ribopyranoside. The reaction proceeds via 2,3,4-tri-0-acetyl-D-ribose dimethyl dithioacetal and probably involves the intermediacy of (182) which may also account for the formation of 5-S-methyl-5-thio-~- ribose dimethyl dithioacetal. However the isolation of 4-S-methyl-4-thio-~- lyxose dimethyl dithioacetal suggests that (183) is also involved.219 Methyl 5-O-trityl-~-~-ribofuranosyl2,3-episulphide is obtained from methyl 5-0- trityl-2-thiobenzoyl-3-O-p-tolylsulphonyl-~-~-ribofuranoside and sodium benzoate demonstrating the preference for sulphur over oxygen participa- tion.220 The isolation of (184) from 2,3,4,6-tetra-O-benzyl-NN-dirnethyl-5-0-methylsulphonyl-D-gluconamideand sodium acetate provides an example of a carbonyl-assisted displacement of the methylsulphonyl-group.221 The reaction of hexopyranosides with methanesulphonylchloride in N N-dimethylformamide results in the selective replacement of the 6-hydroxy- group with chlorine and constitutes an improved synthesis of 6-chloro-6- deoxyhexopyranosides.222Selective hydrolysis of a primary sulphonic ester 213 H.Paulsen F. G. Espinosa W. P. Trautwein and K. Heyns Chem Ber. 1968,101 179. 214 H. Paulsen F. G. Espinosa and W. P. Trautwein Chem. Ber. 1968,101,186. ’15 C. Pedersen Acra Chem. Scad. 1968 22 1888; N. Gregersen and C. Pedersen ibid. 1968 22 1307. 216 J. S. Brimacombe and 0.A.Ching,carbohydrate Res. 1968,8,374. 217 J. S. Brimacombe and 0.A. Ching Chem. Comm 1968,781. 218 E. V. E. Roberts J. C. P. Schwarz and C. A. McNab Carbohydrate Res. 1968,7,311. 219 N. k Hughes R Robson and S. A. Saeed Chem. Comm. 1968,1381;N. Hughes and R Robson ibid. 1968 1383. 220 K. J. Ryan E. M. Acton and L. Goodman J. Org. Chem. 1968,33,3727. 221 H. Kuzuhara and H. G. Fletcher,jun. J. Org. Chem. 1968,33 1815. 222 M. E. Evans L. Long,jun. and F. W. Parrish J. Org. Chem. 1968,33 1074. 473 Heterocyclic Chemistry Me Me <H,.OBz BzO-OO -OBz (184) (185) (186) occurs on basic or neutral alumina.223 The difficulty of displacing the p- bromop hen ylsulphonyl-group of 3-p-bromophen ylsulp honyl- 1,2 :5,6-di-O-iso-propylidene-a-D-glucofuranose is dramatically illustrated in its reaction with dimethylamine at 160” when displacement of the halogen results to give 3-p -dimeth ylaminophenylsulp hon yl -1,2 :5,6-di-0-isopropylidene -a-D-gluco-f~ranose.~~~ Reduction of (185) provides a simple route to 5-amino-5-deoxy- sugars while in the presence of acetic anhydride (186) is produced allowing the ready preparation of 6-deoxy-5-ulose derivative^.''^ A number of photoinduced reactions of carbohydrates have been described.For example methyl 2,3,4-tri-0-acetyl-6-azido-a-~-glucopyranoside is oxi- dised to (187) when irradiated in cyclohexane.226 Photolysis of carbohydrate dimethyldathiocarbamate esters constitutes a useful preparative route to deoxy -sugars thus 6-deoxy- 1,2 :3,4-di-O-isopropylidene-a-~-galactopyranose is obtained when the 6-dimethyldithiocarbamate of 1,2 :3,4-di-O-isopropylidene-a-D-galactopyranose is irradiated in methan01.~” The photolysis of 6-deoxy-6-iodo-l,2 :3,4-di-O-isopropylidene-a-~-galactopyranose is determined by the solvent photoreduction occurs in alkaline methanol while in alkaline t- butyl alcohol the elimination product is also formed.228 The irradiation of (188) to yield the ap-unsaturated ketone (189) is of interest since it resembles the Nef reaction.’’’Photoinduced decarbonylation of ketones is well documented and is usually most successful in the gas phase. However (190)undergoes ring- contraction to give mainly (191) when irradiated in pentane; this possibly reflects the stabilisation of the diradical intermediate.’ 30 223 F.W. Parrish R. C. Chalk and L. Long jun. J. Org. Chem. 1968,33,3165. 224 D.Horton J. S. Jewell and H. S. Prihar Canad. J. Chem. 1968,46,1580. 22s H. Paulsen and D. Stoye Angew. Chem. Internat. Edn. 1968,7,134. 226 D.Horton A. E. Luetzow and J. C. Wease Carbohydrate Res. 1968,8,366. 227 R H. Bell D. Horton and D. M. Williams Chem. Corn 1968,323. 228 W. W. Binkley and R W. Binkley Carbohydrate Res. 1968,8,370. 229 G. B. Haworth D. G. Lance W. A. Szarek and J. K. N. Jones Chem. Comm. 1968,1349. ‘’O P.M.Collins Chem. Comm. 1968,403. 474 R. J. Stoodley O,N Me CHAc AcO--D-OMe Ob MeLo 8\to-$Me OAc bAc -MeLQ-1.e Me Me Me Me (187 ) (188) (189) Me (190) (191) An interesting approach to furanoside synthesis involves periodate oxidation of methyl 6-nitro-a-~-glucopyranoside.The intermediate readily cyclises to give mainly (192) which after sodium borohydride and catalytic reduction affords methyl-3-amino-3-deoxy-~-~-arabinofuranoside.~ H C,H2*OH HO-00 HO HOWOH OH ArNHoN NONHAr (193) (194) c1 231 H.H.Baer and I. Furic J. Org. Chem. 1968,33,3731. Heterocyclic Chemistry 475 A chemical model for inositol biosynthesis is illustrated by the acidic hydro- lysis of 1,2-O-isopropylidene-a-~-xylo-hexos-5-ulose which furnishes (193). Base-catalysed aldolisation of (193) yields (194) which after sodium boro- hydride reduction affords myo-and scyllo-ino~itol.~ Arylosazones of de- 32 hydro-L-ascorbic acid have been previously formulated as y-lactones.However the 6-lactone structure (195) is suggested by oxidation to (196) which is con-verted into (197) by base and acid treatment.233 A novel approach to chromen synthesis involves trapping of benzynes with ap-unsaturated carbonyl compounds. For example tetrachlorobenzyne generated by aprotic diazotisation of tetrachloroanthranilic acid affords (199) in the presence of crotonaldehyde. The unexpected formation of the 2H-chromen rather than the 4H-chromen suggests the intermediacy of (19~~~~ Benzoylacetonitrile and acetoacetonitrile undergo C-alkylation with phenols to provide a new route to coumarins [e.g. (200)].235 A novel dimeric coumarin named thamnosin (201),has been encountered in nature for the first Isoflavanoid biosynthesis from chalcone precursors involves an oxidation and a 1,2-aryl shift.The first chemical analogy for this process is illustrated by OMe Reagents i Th(OAc),-MeOH ;ii H,-Pd ;iii H+ (201) SCHEME 4 232 D. E. Kiely and H. G. Fletcher,jun.,J. Amer. Chem. SOC.,1968,90 3289. 233 H. El Khadem and S. H. El Ashry J. Chem. SOC.(C),1968,2247; ibid. 1968,2251. 234 H. Heaney and J. M. Jablonski Chem. Corn 1968,1139. z3s K. Sato and T. Amakasu J. Org. Chem. 1968,33,2446. 236 J. P. Kutney T. Inaba and D. L. Dreyer J. Amer. Chem. SOC.,1968,90,813. Q 476 R. J. Stoodley the conversion of 2’-benzyloxy-4,4’dimethoxychalcone (202) to 7,4’-dimethoxy- isoflavone as shown in Scheme 4.237 Although isoflavans had not previously been found in plants three examples [e.g.(203)] and an isoflavanquinone (204) have now been isolated.238 Further novelties include the isolation of scaposin (205),239 which is the first flavone with the 3’,4’,5’-oxygenation pattern in ring B HO “OH and of the first optically active biflavone (206).204 Silybin (207) is the first mem- ber of a new class of compound termed a flavanolignan which incorporates a 1,4-dioxan-unit and may be considered to arise by the oxidative combination of a flavanoid and coniferyl Sulphur Derivatives. Further evidence for the intermediacy of tetravalent sulphur species242 has been obtained. Thus (209) may be isolated when (208) 237 W. D. Ollis K. L. Ormand and I. 0.Sutherland Chem. Comm 1968,1237. 238 K. Kurosawa W.D. Ollis B. T.Redman L 0.Sutherland A. B. de Oliveira 0. R Gottlieb and H. M. Alves Chem. Comm. 1968,1263; ibid. 1968,1265. 239 M. B. Thomas and T. J. Mabry Tetrahedron 1968,243675. 240 M. Ilyas J. N. Usmani S. P. Bhatnagar M. Ilyas W. Rabiman and A. Pelter Tetrahedron Letters 1968,5515. 241 A. Pelter and R Hansel Tetrahedron Letters 1968,2911. 242 Ann. Reports (B) 1967,391. Heterocyclic Chemistry 411 S Ph gh -0 (209) I -0 (208) (210) 1 OH &-9 I Ph 0Ph -0 is heated in acetic anhydride containing N-~henylmaleimide.~~~ even An more dramatic example is the pyrolysis of (210) in acetic anhydride which affords (211); it is suggested that the stability of the latter compound may be attributed to the peripheral 14.n-ele~trons.~~~ Borohydride reduction of thio- xanthone sulphoxide yields the cis-derivative (2 12) which undergoes an unusual base-catalysed dehydration and reduction to (213) in the presence of alkaline b~rohydride.~~’ 4-Pyrone is known to undergo deuterium exchange at position 3 and 5 in the presence of D,O.In contrast the 2- and 6-protons are exchanged in 4H-thiopyran-4-one l,l-dio~ide.~~~ When heated 2,6-dimethythio-3,5-diphenyl-4H-thiopyran-4-thioneundergoes an intriguing rearrangement to give 4,6-dimethylthio-3,5-diphenyl-2H-thiopyran-2-thioney while at higher temperatures (214) is formed.247 The usefulness of 2,3-dichloropropene in cycloalkanone synthesis has been extended to the preparation of hetero-cycloalkanones. For instance 2-chloroprop-2-enethi01 available from 2,3- dichloropropene and thiourea reacts with epoxides to give adducts which undergo acid-catalysed cyclisation to yield thian-3-0nes.~~~ 243 R.H. Schlessinger and A. G. Schultz,J. Amer. Chem. SOC.,1968,90,1676. 244 I. S.Ponticello and R. H. Schlessinger J. Amer. Chem. SOC.,1968,90 4190. 24s A. L.Ternat,jun. and D. N. Chasar J. Org. Chem. 1967,32,3814. 246 L. A. Paquette and L. D. Wise J. Amer. Chem. SOC.,1968,90 807. 24’ A.Schonberg and R von Ardenne Chem. Ber. 1968,101,356. 248 P. T.Landsbury and D. J. Scharf,J. Amer. Chem. SOC.,1968.90 536. R. J. Stoodley Me I Me Ph Me MeS Me Six-membered Rings containing Two or More Hetera-atoms.-It has been suggested that there is an appreciable interaction between syn-axial electron pairs.This effect may explain why hexahydro-1,2,3-trimethyl-l,3-diazine exists as conformation (215),249 and why hexahydro-1,2,4,5-tetramethyl-1,2,4,5-tetrazine adopts conformation (216).250 The preference of duplodithio-acetone to exist as a twist boat form was noted last year.251 When the crystalline material is dissolved in carbon disulphide at -80° the n.m.r. spectrum reveals the presence of only the twist boat form indicating that the material exists as this conformation in the solid state.252 An intriguing difference is observed with (217) which prefers the chair conf~rmation.~ The chemistry of pyrimidines which contain pyrida~ines,’~ tetrahydr~pterins,~ have been reviewed. A number of 56 and phen~thiazines~~’ new synthetic procedures have been developed.For example while malonyl chloride is known to give 3-substituted 2-chloro-4,6-dihydroxy-pyridineswith monosubstituted acetonitriles its reaction with halogenoacetonitriles takes an unexpected course to afford 2-substituted 4-chloropyrimid-6-0nes.~~~ A general route to 6-substituted pteridine 8-oxides is summarised in Scheme 5. I I 0-0 Reagents i HOAc ; ii (NH,),C=NH-NaOMe ; iii NN-dimethylformamide SCHEME 5 249 R. 0.Hutchins L. D. Kopp and E. L. Eliel J. Amer. Chem. SOC. 1968,90 7174. 250 J. E. Anderson and J. D. Roberts J. Amer. Chem. SOC. 1968,90,4186. 251 Ann. Reports (B) 1967,417. 252 C. H. Bushweller J. Amer. Chem. SOC. 1968,90,2450. 253 C. H. Bushweller Tetrahedron Letters 1968,2785.254 E. Ronwin Quart. Reports Sulphur Chem. 1967,2,97. ”’ M. Tisler and B. Stanovnik Adv. Heterocyclic Chem. 1968,9 211. 256 M. Viscontini Fortschr. chem. Forsch. 1968,9,605. 25’ C. Bodea and I. Silberg Adv. Heterocyclic Chem. 1968 9 322. 258 J. A. Elvidge and N. A. Zaidi J. Chem. SOC. (C) 1968 2188. Heterocyclic Chemistry Since the latter derivatives may be readily reduced to 7,8-dihydropteridines which in turn yield 6-substituted pteridines on mild oxidation the method is of wide appeal and permits considerable variation in the substitution pattern of both the pyrimidine and pyrazine rings.259 The reaction of 6-amino- and 6-hydrazino-pyrimidines with diethyl azodicarboxylate provides a useful method for introducing nitrogen at position 5 and the adducts are versatile intermediates for making 6- and 7-azapteridines and purines.For example (218) cyclises in the presence of ethoxide to give (219; R = OH) which may be converted into the antibiotic fervenulin (219; R = H) by sequential treatment with phosphorus oxychloride hydrazine and mercuric oxide.260 The reaction of 5-nitrouracil with sodium azide in NN-dimethylformamide affords 8-azaxan- thine in good yield and constitutes a novel one-step procedure for preparing 2-oxo-8-azapurines which is easily adaptable to nucleoside synthesis.261 A remarkable single-step synthesis of adenine (43 % yield) may be achieved by heating phosphorus oxychloride with formamide.262 Several reactions which result in opening of the pyrimidine ring have been described.For example hydrazine reacts with uracil 5-carbaldehyde to give (22O),’ and 1,2-dihydr0-2-imino-1-me th ylp yrimidine undergoes Dimroth rearrangement in diethylamine to 2-(N-methylamino)pyrimidine,while in n-butylamine 1,3-dibutyliminopropane is formed.264 A unique ring-contraction ’’’ E. C. Taylor and K. Lenard J. Amer. Chem. SOC.,1968,90,2424. 260 E. C. Taylor and F. Sowinski J. Amer. Chem. Soc. 1968,90,1374. 261 H. U. Blank and J. J. Fox J. Amer. Chem. SOC.,1968,90,7175. 262 M. Ochiai R. Marumoto S. Kobayashi H. Shimazu and K. Morita Tetrahedron 1968 24 5731. 263 K. V. Zee-Cheng and C. C. Cheng J. Org. Chem. 1968,33,892 264 D. J. Brown P. W. Ford and M. N. Paddon-Row J. Chem. Soc. (C),1968,1452. 480 R. J. Stoodley of 6-chloro-4-methyl-2-phenylpyrimidine occurs upon reduction with zinc in acetic acid when a mixture of 2-methyl-5-phenyl- and 4-methyl-5-phenyl- pyrroles is formed via 4-methyl-2-phenylpyrimidine.265The reduction of (221; R = H) with iron in acetic acid also results in ring cleavage and the benzothiazole (223) is obtained presumably by hydrolytic cleavage of the 1,2- bond of (222).In contrast (221; R = Me) does not undergo ring opening but affords the thiazepine (224).266 On the basis of deuterium experiments the amide displacement of 6-bromo- 4-t-butylpyrimidine to give the 6-amino-derivative involves a 5,6-heteroaryne intermediate. 26 The reactivity of tetrafluoropyridazine with nucleop hiles shows an interesting dependence upon the reaction conditions ;thus displace- ment of the 4-fluorine or the 4- and 5-fluorines occurs under basic conditions while attack at position 3 results in the presence of acid.Support for the involve- ment of a pyridazinium cation under the latter conditions comes from the reac- tion of l-ethy1-3,4,5,6-tetrafluoropyridaziniumfluoroborate with water which furnishes (225).268 The valence tautomerism of diazonorcaradienes was mentioned last year.26 The simplest member (226 R1= R2= R3= H) which may in principle be prepared from cyclopropanedicarbaldehyde and hydrazine exists as the trimer (227) while the tetramethyl derivative (226; R1= R2 = Me R3= H) dimerises to (228). However both (226; R1= But R2= H R3= COBu') F (226) N-N C0,Me Me &CHyuMe .I Me Me02CfiC02Me9N I]C02Me Ph (229) 265 T.W. Thompson Chem. Comm. 1968,532. 266 K. J. M. Andrews and B. P. Tong J. Chem. SOC.(C),1968,1753. 267 H. C. van der Plas P. Smit and A. Koudijs Tetrahedron Letters 1968,9. 268 R D. Chambers J. A. H. MacBride and W. K. R Musgrave J. Chem SOC.(C) 1968,2989. 269 Ann. Reports (B) 1967,421. Heterocyclic Chemistry 481 and (226; R1= Ph R2 = R3 = H) exist as the monomers. The latter does not undergo Diels-Alder cycloaddition with dimethyl acetylenedicarboxylate but yields the adduct (229).270 Loss of nitrogen occurs in the photolysis of (230) and 1,3-diphenylisobenzo- furan is formed.27 Evidence that a diazo-intermediate may be involved comes comes from a study of 3,6-diphenylpyridazine N-oxide which does not lose nitrogen when irradiated in acetone but yields 3-ben~oyl-5-phenylpyrazole.~~~ Both reactions presumably involve an initial formation of the oxaziridine as -0 Ph QJJ+ '0-Ph H (233) does the photorearrangement of (231) to (232),273 while with 1,1,4-triphenyl- 2,3-benzoxazine the reaction stops at this stage to give (233).274 Irradiation of dimethyl 1,2-dihydropyridazine-1,2-dicarboxylate leads mainly to intramole- cular cycloaddition to afford (234).However the small amount of methyl 2-methoxycarbonylaminopyrrole-1-carboxylate which is formed suggests that ring opening also "N-Labelling experiments support the interme- diacy of the p-lactam (235) in the photorearrangement of 3,4-dihydro-4-oxo- 1,2,3-benzotriazine to a~ridone.,~~ An oxidation analogous to that encountered in the formation of sugar osazones occurs in the reaction of methylhydrazine and dimethylhydrazine with (236; R = CH2Cl) when (236; R = CH:NNHMe) and (236; R = CH "Me,) are formed.However when hydrazine is employed ring expansion to the benzodiazepine (237) The lactone (240) may be isolated from (238) after treatment with alkali and acidification. This remarkable transfor- "O G. Maier and T. Sayrac Chem Ber. 1968 101 1354; G. Maier and U. Heep ibid. 1968 101 1371. 271 0.Buchardt Tetrahedron Letters 1968 1911. 272 P. L. Kumler and 0.Buchardt J. Amer. Chem SOC.,1968,90 5640. 273 T. Sasaki and M. Takahashi,Bull. Chem. SOC.Japan 1968,41 1967. 274 B. Singh J. Amer. Chem. SOC.,1968,90,3893. 275 L.J. Altman M. F. Semmelhack,R B. Homby and J. C. Vederas Chem. Comm. 1968,686. 267 G. Ege Chem. Ber. 1968,101,3079,3089. '"M. E. Derieg R. I. Fryer and L. H. Sternbach J. Chem. SOC.(0,1968,1103. 482 R. J. Stoodley mation is probably the result of an intramolecular Cannizzaro reaction of (239) which may arise from (238) by tautomerism and dehydration.”* Seven-membered and Larger Rings.-1 H-Azepines are of interest because they are isoelectronic with the cycloheptatrientyl anion and if planar they could exhibit antiaromatic behaviour. However an X-ray crystallographic study of the iron carbonyl complex of 1-ethoxycarbonylazepine reveals typical polyene character with no tendency to exist as the azanorcaradiene or as the azahomo- aromatic rn~eity.’~~ Two new approaches to their synthesis have been des- cribed.Thus azaquadricyclanes thermally rearrange to a~epines,~’ while @j-a 0 Ts Ts C0,Me (241) \&r (242) (244) \N Ts (243) the adduct derived from the addition of dibromocarbene to (241) ring expands to (242) in refluxing pyridine and to the azepinone (243) with silver nitrate.280 An unusual [6 + 2lcycloaddition (presumably nonconcerted) occurs when N-ethoxycarbonyl-1H-azepineis heated with nitrosobenzene.28 Thermolysis of dimethyl azodicarboxylate in the presence of cycloheptatriene affords a 278 M. J. Haddadin and C. H. Issidorides Tetrahedron Letters 1968,4609. 279 I. C. Paul S. M. Johnson L. A. Paquette J. H. Barrett and R J. Haluska J. Amer. Chem SOC. 1968,90,5023.A. Cromarty and G. R. Proctor Chem. Comm. 1968,842. W. S. Murphy and J. P. McCarthy Chem. Comm. 1968 1155. Heterocyclic Chemistry mixture of the 2,3-(244) and the 4,5-homoazepines presumably as a result of valence tautomerism of the initially formed methoxycarbonylaziridine.282 The first fluxional heterocycle methoxyazabullvalene (245) was reported last year.283 It exhibits remarkable thermal stability although it decomposes at 600" to give mainly quinoline with smaller amounts of 2-methoxyquinoline and l-metho~yisoquinoline.~~~ The photochemistry of (245) is complex equilibration with (246) may occur initially which may undergo [2 + 2]cyclo-addition to give (247) and its em-isomer. However further rearrangements must occur since (248) and (249) may be isolated.285 Benzazabullvalene (250) has also been prepared and it undergoes an interesting thermal rearrangement to (251).286 Me0 4e (245) (246 1 (247) Me0 OMe Me0 & (248) (249) Irradiation of (252) gives l-ethoxycarbonyl-l,2-diazepine,which may be isolated as its iron carbonyl complex.The formation of 2-aminopyridine by alkaline hydrolysis of the diazepine provides evidence for the intervention of the dia~iridine.~~~ Further evidence for a similar valence tautomerism comes from the chemistry of diazepinones.288 Thus although (253) and (254) may be equilibrated under basic conditions they react differently with alkali. Equal 282 W. H. Okamura and W. H. Snider Tetrahedron Letters 1968 3367; L. A. Paquette and R J.Haluska Chem. Comm 1968,1370. 283 Ann. Reports (B),1967,420. 284 L. A. Paquette G. R Krow J. R Malpass and T. J. Barton,J. Amer. Chem SOC. 1968,90,3600. 285 L. A. Paquette and G. R Krow J. Amer. Chem. SOC.,1968,90,7149. 286 L. A. Paquette and J. R. Malpam J. Amer. Chem. SOC. 1968,90 7151. 287 J. Streith and J. M. Cassal Angew. Chem. Internat. Edn. 1968,7,129; Tetrahedron Letters 1968 4541. 288 M. G. Pleiss and J. A. Moore J. Amer. Chem. SOC.,1968,90 1369; ibid. 1968 90,4738. 484 R. J. Stoodley 0 -N*CO,Et (2 5 2) Ph Me H Ph Me -Ph Me I Phfi amounts of 2-amino-3-hydroxy-4-methyl-5-phenyl-and 2-amino-4-hydroxy- 3-phenyl-pyridine are obtained from (254) while (253) affords mainly the latter derivative.Furthermore photolysis of the betaine (255) furnishes the diaziri- dine (256). No deuterium is incorporated in the room-temperature rearrange- ments of (255) to (257) suggesting that a [1,5]sigmatropic hydrogen-shift is involved. Compound (258) undergoes an intriguing ring contraction when heated to give (259) which may represent the first example of a [1,3]sigmatropic shift from carbon to nitrogen.289 Allylic bromination of the azetine (260) followed by base-catalysed elimina- tion of hydrogen bromide furnishes (261) the first heterocyclic analogue of cyclo-octatetraene. Although the valence tautomer (262) cannot be detected at equilibrium its presence may be inferred by the isolation of (263) from the reaction with N-phenylmaleimide. However by incorporating tri- and tetra- methylene bridges [e.g.(264)] the equilibrium is completely displaced in favour of the azabicyclo-octatriene although witha pentamethylene bridge the azacyclo- 289 M. Israel L. C. Jones and E. J. Modest Tetrahedron Letters 1968 481 1 Heterocyclic Chemistry octatetraene may be observed at higher temperature^.^" An interesting cyclisation of a-diamines [e.g. (265)] occurs in the presence of a weak organic acid to yield phenhomazines [e.g. (266)].291 In analogy with azaq~adricyclanes,~~ oxaquadricyclanes thermally re-arrange to oxepin~.~ Evidence for benzene oxide-oxepin tautomerism in 92 Nature comes from the isolation of two metabolites (267) and (268) which HO‘ OH (269) (270) (271) (273) 290 L. A. Paquette and T.Kakihana J. Amer. Chem SOC.,1968,90 3897; L.A. Paquette and J. C.Philips ibid. 1968,90 3898. 291 G. A. Swan Chem. Comm. 1968 1376. 486 R. J. Stoodley incorporate a dihydro-oxepin entity.293 According to orbital symmetry pre- dictions valence tautomerism of cyclo-octatetraene oxide to oxacyclonona- tetraene should only occur in the first excited state. Indeed the photorearrange- ment of cyclo-octatetraene oxide to (269) (270) and (271) is consonant with symmetry-allowed ring closures of oxacy~lononatetraene.~~~ A novel approach to medium-ring oxygen hetereocycles from carbohydrates is illustrated by the formation of (273) from the dianhydride of D-ribose (272) by sequential perio- date oxidation sodium borohydride reduction and a~etylation.~~’ Sterically hindered thiacycloheptane derivatives show some interesting properties.296 For instance acid-catalysed dehydration of (274; R = H OH) leads to (275) in which the sulphur is involved in a transannular ring-closure.With thionyl chloride (274; R = 0)affords (276) the first example of a stable ene-diol sulphite. Little is known about the possible aromatic character of thiepins since even the highly substituted derivatives readily undergo thermal rearrangement withloss ofsulphur. In contrast (277 ;X = S)displays remarkable thermal stability and reacts with N-phenylmaleimide to give mainly (278). Furthermore the greater reactivity of (277 X = SO) and (277; X = SO,) in the Diels-Alder cycloaddition provides some evidence for aromatic-like charac- ter of(277 ;X= S),whichrnay bedue to participation ofthe sulphur d-0rbitals.2~~ Meo::~~q~~ R OH Me,MeAc-o>Me Me Me Me - Me 0 0- /SO (2 74) (276) 292 H.Prinzbach M. Arguelles P. Vogel and W. Eberbach Angew. Chem Internat. Edn. 1967 6 1070; G. R Ziegler and G. S. Hammond J. Amer. Chem. SOC.,1968,90,513. 293 N. Neuss R Nagarajan B. B. Molloy and L. L. Huckstep Tetrahedron Letters 1968,4467. 294 J. M. Holovka P. D. Gardner C. B. Strow M. L. Hill and T. V. van Auken J. Amer. Chem SOC. 1968,90 5041. 295 J. F. Stoddart and W. A. Szarek Canad. J. Chem. 1968,46,3061. 296 A. de Groot J. A. Boerma and €€ Wynberg Tetrahedron Letters 1968 2365; Chem Comm. 1968,347; A. de Groot J. A. Boerma J. de Valk and H. Wynberg J. Org. Chem.1968,33,#25. 297 R H. Schlessinger and G. S. Ponticello J. Amer. Chem SOC. 1967 89 7138; Tetrahedron Letters 1968 3017. Heterocyclic Chemistry The thermal decomposition of benzenediazonium o-carboxylate in acetone yields (279) and (280) which provides the first example of a 1,6-dipolar addition of dehydrobenzene and of a cycloaddition involving benzenediazonium o-car~oxylate.~~~ The photolysis of N-chloroacetyl derivatives of aromatic amino-acids was noted last year.299 Two new photocyclisation pathways are observed with N-chloroacetyl-3,4-dimethoxyphenethy1amine. Thus irradiation in aqueous ethanol affords (281) in addition to the expected azepines reflecting the electrophilic nature of the primary intermediate. In contrast the ten- membered lactam (282) is obtained in tetrahydrofuran in accord with a homo- lytic mechanism.300 298 T.Miwa M. Kato and T. Tamano Tetrahedron Letters 1968,2143. 299 Ann. Reports (B) 1967,421. 300 0.Yonemitsu Y. Okuno Y. Kanaoka I. L. Karle and B. Witkop J. Amer. Chem. SOC.,1968 90,6522.

ISSN:0069-3030    

DOI:10.1039/OC9686500441

出版商:RSC    

年代:1968

数据来源: RSC