摘要:

3 Reaction Mechanisms Part (iv) Polar Reactions By N. S. ISAACS Dept. of Chemistry The University Whiteknights Park Reading RG6 2AD 1 Nucleophilic Substitution Evidence against a unified mechanism for nucleophilic substitution at saturated carbon (Scheme 1) has been presented.’ The arguments in favour of discrete SN1and SN2 processes in the aqueous acetone hydrolysis of benzyl and benzhydryl .A. bR-S ‘ ;,”-R-NU Scheme 1 halides are based on detailed kinetic analysis and salt-effect studies.2 Activation volume studies have been shown to be inadequate to differentiate between solvo- lytic and ion-pair return reactions. Differences in AV* are less than the spread of values for each type and presumably emphasize similarities in the transition Secondary I4cand 37c1 isotope effects for SN2 transition states of each type are predicted to vary with the bond orders of the forming and breaking bonds ;4 the former should show a maximum value when the transition state is symmetrical as previously found for deuterium isotope effect^.^ This effect has not so far been demonstrated but will no doubt be sought.Recent studies show that neopentyl esters and related compounds undergo S,2 reactions by strong nucleophiles at appreciable rates6,’ and even in strong acidic solution may under- go substitution without rearrangement.8 Bimolecular displacements have also been carried out on tertiary halides.’ Displacements on ethynyl halides are believed to occur by a combination of addition-elimination and elimination- addition pathways (Scheme 2),’’as indicated by a kinetic analysis.Dibenzofuran J. W. Larsen and R. A. Sneen J. Amer. Chem. SOC.,1969,91 362,6031. B. J. Gregory G. Kohnstam A. Queen and D. J. Reich Chem. Comm. 1971,797. K. R. Brower J. Amer. Chem. SOC., 1972 94 5747. L. B. Sims A. Fry L. T. Netherton J. C. Wilson K. D. Reppaud and S. W. Crook J. Amer. Chem. SOC.,1972,94 1364. J. Bigeleisen Pure Appf. Chern. 1964 8 217. ti Y. Okamoto and T. Yano Tetrahedron Letters 1971 4285. ’ B. Stevenson G. Solladie and H. S. Mosher J. Amer. Chem. SOC.,1972 94 4184. * J. H. Exner L. D. Kershner and E. R. Larsen Chem. Comm. 1971 1174. F. G. Bordwell and T. G. Mecca J. Amer. Chem. Soc. 1972,94 21 19. lo J. I. Dickstein and S. I. Miller J. Org.Chem. 1972 37 2168. 160 Reaction Mechanisms-Part (iv) Polar Reactions + Scheme 2 is an efficient leaving group as judged by the powerful methylating properties of (1).' ' Trimethylaluminium is capable of exhaustive methylation of alcohols ketones and carboxylic acids e.g. (2)+(3).12 There are indications that modifi- cations to rates and product ratios may be effected by carrying out substitution reactions in ionic surfactant micelles.' 3-1 2 Solvolytic Reactions A comprehensive survey of cyclopropylsulphonate solvolyses has been pub- lished.'6,' ' The application of standard mechanistic criteria (solvent sensitivity leaving-group effects substituent effects) indicate that these slow reactions are nonetheless strongly assisted.Rate enhancements by anchimeric assistance are estimated by the above data and by comparison with rates predicted on the basis of cyclopropanone carbonyl-stretching frequencies to be of the order 104-105. The products of the reaction are derived from the allyl cation formed by the dis- rotatory opening of the three-membered ring. The recapture of leaving group by the allyl cation occurs asymmetrically in the ion-pair (Scheme 3).18 The Me0 -MeORHc' + c1 H c1 CI 91 :9 Scheme 3 l1 A. J. Copson H. Heaney A. A. Logun and R. P. Sharma J.C.S. Chem. Comm. 1972 315. l2 A. Neisters and T. Mole J.C.S. Chem. Comm. 1972 595. l3 C. A. Bunton and S. K. Huong J. Org. Chem. 1972,37 1790. l4 J. Albrizzio J. Archila T. Rudolfo and E. H. Cordes J. Org. Chem.1972 37 871. C. Lapointe and P. Viout Tetrahedron Letters 1972 4221. W. F. Sliwinski T. M. Su and P. von R. Schleyer J. Amer. Chem. SOC.,1972.94 133. " P. von R. Schleyer W. F. Sliwinski G. W. Van Dine U. Schoilkopf J. Pousi and K. Fellenberger J. Amer. Chem. Soc. 1972,94 125. * I. Fleming and E. J. Thomas Tetrahedron Letters 197I 2485. 162 N. S. Isaacs well-known Winstein-Grunwald equation’ is usually employed with the tacit implication that solvents thus compared are of comparable nucleophilic character so that differences in the nucleophilicity terms in the equation may be ignored. In an attempt to evaluate the solvent nucleophilicity term N methyl tosylate (rn = 0.3) has been allowed to solvolyse in a wide range of solvents of known Y.By assigning the factor 1 = 1 (ie.unit susceptibility to nucleophilic solvent participation) the remaining unknown N was found. Values range from ethanol (N = +0.09) to trifluoroacetic acid (N = -5.5).” The same authors propose a revised measure of substrate sensitivity to N in a solvolytic reaction (Q) by means of equation (1); solvolytic rates of the substrate (k)in a given solvent relative to 80 % aqueous ethanol (subscript 0)are related to the same ratio for two standard substrates of widely differing Q namely methyl tosylate (superscript A Q = 0) and 2-adamantyl tosylate (superscript B Q = 1). The values computed for a series of alkyl and cycloalkyl systems show differences from the solvent ionizing power constant rn . Solvolysis rates in the mixed system hexafluoropropanol- water show a minimum at the composition 1 :2 indicating the formation of a dihydrate and warning of possible complications which may occur when mixed solvents are used in solvolysis.2 The 2-adamantyl system is important because it is known to solvolyse without nucleophilic solvent assistance and so can be used as a reference to estimate the extent of this effect in other systems.” It has been concluded from isotope effects (k,&, = 1.22) and other results that whereas the rate-determining step is ionization to a tight ion-pair the product-determining step is the conversion of the latter to a solvent-separated ion-pair.This results in solvent selectivity for reactions of para-substituted adamantyl benzenesulphonates in ethanol-water the ratios of ether and alcohol products varying systematically with the para ~ubstituent.~~ A further probe for nucleophilic solvent assistance has been devized by Schleyer and co-workers in which by the introduction of azide ion into a solvolysis the separate rates of capture of the carbonium ion intermediate by solvent and by azide ion are obtained by partition of the total rates.A system such as 2-ada- mantyl shows no correlation between product and rate. If it is reasonably assumed that the probability of capture of the carbonium ion depends upon 19 E. Grunwald and S. Winstein J. Amer. Chetii. Soc. 1948 70 846. 20 T. W. Bentley F. L. Schadt and P. von R. Schleyer J. Amer. Chem. SOC.,1972,94,992. 21 D. E. Sunko and I. Szele Tetrahedron Letters 1972 3617.22 P. von R. Schleyer J. L. Fry L. K. M. Lam and C. J. Lancelot J. Amer. Chem. SOC. 1970,92,2542. 23 J. M. Harris F. J. Fagan F. A. Walden and D. C. Clark Tetrahedron Letters 1972 3023. 24 D. J. Raber J. M. Harris R. E. Hall and P. von R. Schleyer J. Amer. Chem. SOC. 1971,93,4821. Reaction Mechan isms-Par t (iv) Polar Reactions 163 nucleophilicity it follows that ionization does not. In contrast simple un- hindered secondary substrates do exhibit a rate-product correlation and are inferred to undergo nucleophilic solvent-assisted ionization as indicated above.24 Solvolyses of norbornane derivatives are still commanding attention some tetracyclic analogues (6H9) have now been studied [cf. (4)and (5)].25-2" The syn-and anti-isomers (10) and (1 I) behave very differently towards added acetate ion in acetolysis ;27 the former reaction is dramatically accelerated (possibly a combination of special salt effect and nucleophilic attack) whereas the latter is not affected.A" ANB (6) X = H Y = OPNB; 10" (8) X = H Y = OPNB; lo3 (7) X = OPNB Y = H; 10' (9) X = OPNB,Y = H; 10' XY v (10) X = Bros Y = H k, = 103 (11) X = H Y = Bros k, The benzotricyclononanes (12H15) solvolyse at rates shown;28 compound (14)may owe its reactivity to aryl participation while the lack of reactivity of the corresponding endo-isomer (15) is no doubt due to steric hindrance to ioniza- tion. 2- and 3-exocyclic double bonds can accelerate 7-norbornyl solvolysis in (16) [cf.(17) and (18)] but the activation parameters indicate that this is due to a marked change in both free-energy and entropies of activation the interpreta- tion of which is not attempted29 but probably points to significant differences in the transition states. 25 T. Svensson and S. Winstein J. Amer. Chem. SOC.,1972 94 2336. 26 P. Carter and S. Winstein J. Amer. Chem. SOC.,1972,94 2171. 27 S. J. Cristol A. L. Noreen and G. W. Nachtigall J. Amer. Chem. SOC.,1972,94,2187. 28 R. Baker and T. J. Mason J.C.S.Perkin ZZ 1972 18. 29 T. Tsuji H. Ishitobi and H. Tanida Tetrahedron Letters 1972 3083. 164 N. S. Isaacs krel &,el (12) X = OTOS,Y= H; 80 (14) X = OTOS,Y = H; 2675 (13) X = H,Y = OTOS; 46 (15) X = H,Y = OTOS; 1.2 (16) (17) (18) kreK 8.77 x lo-'' 6.16 x 10-l4 2.6 10-3 AH*/J mol -I 94 145 97 AS' /J deg-I -100 -12 +24 The effects of an a-methyl group are known to increase with the electronic demands of the solvolysis.The ratio kaMe/kaH for the series (19j(21) shows that the 7r-assisted compounds require little additional stabilization compared with the unassisted 7-norbornyl compounds ( 19).30 This effect has been justified the~retically.~ The direct comparison under the same solvolytic conditions of solvolytic rates of exo- and endo-2-norbornyl compounds is difficult on account of the magnitude of their difference. This has been achieved using '*O exchange of the alcohols (22H25) which ionize to a common intermediate (26).32 The exo-isomers are 103-105 times more reactive than their respective endo- analogues and are assumed to be a-assisted.30 R. K. Lustgarten J. Lhomme and S. Winstein J. Org. Chern. 1972 37 1075. '' S. Yoneda Z. Yoshida and S. Winstein Tetrahedron Letters 1972 2395. 32 C. A. Bunton K. Khaleeluddin and D. Whittaker J.C.S. Perkin 11 1972 1154. Reaction Mechanisms-Part (iv)PoIar Reactions Me Me V krel (22) x = OH,Y = H; 2.3 x 105 (23) X = H Y = OH; 1 Me (26) (24) X = OH Y = Me; 4 x 10’ (25) X = Me,Y =OH 3 x lo5 The 6-oxa-analogue (27) of 2-norbornyl brosylate behaves solvolytically like its carbon prototype but the exolendo rate ratio (7 x 10’) is much larger than the latter (3.5 x lo2) because oxygen participation is much more efficient.33 The trend towards a more carbonium ion-like transition state is evident in 0 aOBs reactions of benzyl chloride and p-methylbenzyl chloride with cyanide ion.The secondary a-deuterium isotope effect has the normal S,2 value (ca. 1.0) for the former but is 1.25-1.31 in the latter case.34 The solvolysis of P-phenylethyl tosylate a borderline phenyl-assisted reaction has been shown to be subject to a special salt effect indicating ion-pair return to be ~ignificant.~~ Volumes of activation have been cited as evidence for aryl participation in the solvolyses of 2-(p-hydroxyphenyl)ethyl and 4-(phydroxyphenyl)butyl brosylates but not in the corresponding 3-arylpropyl esters.36 cis-2-Phenylcyclopentyl tosylate (28) provides a model for a P-phenylethyl system for which aryl participation is sterically impo~sible.~’ A modest negative p-value (-1.5) is reported and the products are derived from the rearranged cation (29).The substituent effect however is not large enough for a process in which hydride migration occurred 33 L. A. Spurlock and R. G. Fayter J. Amer. Chem. SOC.,1972,94,2707. 34 A. V. Willi C. K. Ho and A. Ghanborpour J. Org. Chem. 1972,37 1185. 35 I. L. Raich A. F. Diaz and S. Winstein J. Amer. Chem. SOC.,1972 94 2256. 36 W. LeNoble and B. Gabrielson Tetrahedron Letters 1971 3417. 37 C. J. Kim and H. C. Brown J. Amer. Chem. SOC.,1972,94 5043. 166 N. S.Isaacs concurrent with ionization and it is concluded that the first intermediate is the ion-pair. It seems now widely accepted that solvolytic reactions may partake of two independent reaction pathways the direct displacement by solvent k, and the anchimerically assisted k .Frequently 2-phenylethyl systems exhibit both routes to products and these have been dissected into their components as previously reported.38 This treatment has been performed on several l-aryl-2- propyl and 2-aryl-1-propyl systems and additional evidence for the validity of the treatment obtained from the observation that added azide ion (a strong nucleophile) greatly accelerates reactions with a large k component but has a much smaller effect on those whose assisted pathway is dominant.39 The silver- ion-assisted solvolysis (in propionic acid) of 5-iodocyclopentadiene (30) is very slow particularly for a doubly allylic compound.The reluctance to form the cyclopentadienyl cation (31) is consistent with its expected anti-aromatic and hence high-energy ~haracter.~’ Solvolysis of bicyclo[5,l,0]octane derivatives (32) leads to trans-cyclo-octenyl derivatives (33).4 Br (33) 3 Carbonium Ions A situation of potential confusion in nomenclature has arisen with the proposals by Olah for the use of the term ‘carbocation’ generically for all species containing positive carbon ‘carbenium ion’ for the normal tervalent positive species and ‘carbonium ion’ reserved for ‘five-co-ordinate carbon’ compounds.42 This 38 N. S. Isaacs Ann. Reports (B) 1970 67 101. 39 D. J. Raber J. M. Harris and P. von R. Schleyer J. Amer. Chem. SOC. 1971 93,4829. 40 R. Breslow and J.M. Hoffmann J. Amer. Chem. SOC., 1972 94 21 10. 41 M. S. Baird and C. B. Reese Tetrahedron Letters 1971 4637. 42 G. A. Olah J. Amer. Chem. SOC.,1972 94 808. Reaction Mechanisms-Part (iv) Polar Reactions 167 consistent system has the disadvantage of changing the meaning of a well-established term. Consequently for the present the term ‘carbonium ion’ will be used in its originally proposed sense. There is nonetheless a need for a new name for quinquevalent carbon compounds whose existence as reactive inter- mediates cannot be do~bted.~~,~~ A great variety of new carbonium ions have been generated as stable species in superacid solution and their spectra and stabilities studied. These include the methyl and ethyl cations,45 cyclic allylic halogenocarbonium ions,47 benzylic cations:* protonated carboxylic anhydrides (which cleave to acyl cations and protonated carboxylic acids),49 protonated polyhydric phenols,” and aldehydes and ketone^.^ Benzenonium ions (called by Olah ‘benzenium’ ions) including the l-nitro- and l-chloro-species (34) (which must be intermediates in aromatic nitration and chlorination) have also been observed and their n.m.r.spectra recorded.52 The parent species (34c) has been shown to undergo rapid proton migration and to be non-planar since the methylene protons are non- eq~ivalent.~~ 13CN.m.r. is a valuable probe for the measurement of electron densities; this technique has been applied to allylic cations e.g. (35).54 The H +51.6 (34) a; X = NO (35) b;X=C1 c;X = H cyclopropylmethyl cation results from the ionization in superacid of cyclo- propylmethyl and cyclobutyl precursers.The proton and I3C n.m.r. spectra indicate two sets of three equivalent protons and a single proton and three equivalent and one non-equivalent carbon atoms. The species considered best to accord with this data is the rapidly equilibrating ‘five-co-ordinate’ cation (36) whose structure may be considered analogous to the non-classical 2-norbornyl 43 G. A. Olah. Chem. in Britain 1972 8 282. 44 D. M. Brouwer and H. Hogeveen Progr. Phys. Org. Chem. 1972,9 179. 45 G. A. Olah J. R. de Member R. H. Schlosberg and Y. Halpern J. Amer. Chem. SOC. 1972,94 156. 46 G. A. Olah G. Liang and Y. K. Mo J. Amer. Chem. SOC.,1972,94 3544.47 G. A. Olah Y. K. Mo and Y. Halpern J. Amer. Chem. SOC.,1972 94 3551. 48 G. A. Olah R. D. Porter C. Wenell and A. M. White J. Amer. Chem. Soc. 1972,94 2044. 49 G. A. Olah K. Dunne Y. K. Mo and P. Szilagyi J. Arner. Chem. Soc. 1972,94,4200. 50 G. A. Olah and Y. K. Mo J. Amer. Chem. Soc. 1972,94 5341. G. A. Olah Y. Halpern Y. K. Mo and G. Liang J. Amer. Chem. SOC.,1972,94,3554. 52 G. A. Olah H. C. Lin and Y. K. Mo J. Amer. Chem. SOC.,1972,94 3667. 53 G. A. Olah R. H. Schlosberg R. D. Porter Y. K. Mo D. P. Kelly and G. D. Mateescu J. Amer. Chem. SOC.,1972 94 2034. 54 G. A. Olah P. R. Clifford Y. Halpern and R. G. Johanson J. Amer. Chem. SOC. 1971,93,4219. 168 N. S. Isaacs cation (37).” Stable bicyclic cations have been generated; it appears that the bridgehead isomers (38) and (39) may be most stable,56 which seems to imply that the non-reactivity of bridgehead halides in solvolysis must be due to diffi- culties of solvent-stabilization during ionization.cH2 / \ It has been found possible to generate alkyl cations from olefins such that polymerization does not occur ;” methylacetylene allene and 2-fluoropropene are all converted into the fluorodimethylcarbonium ion (40)in superacid,’* and the cyclopentenyl cation (41) was found to be stable (though with rapid hydrogen migration) up to 150OC.” 7-Substituted norbornadienyl cations (42) undergo ‘bridge-flipping’ [(42a) *(42c)J via the bicyclo[3,2,0]heptenyl cations (42b) ; F H H (40) R (424 E (R = Me) = 12.4 kcal mol-’ 55 G.A. Olah C. L. Jewell D. P. Kelly and R.D. Porter J. Amer. Chem. Sac. 1972,94 146. 56 G. A. Olah G. Liang J. R. Wiseman and J. A. Chong J. Amer. Chem. Sac. 1972,94 4927. ” G. A. Olah and Y. Halpern J. Org. Chem. 1971,36 2354. 58 G.A. Olah Y. K. Mo and Y.Halpern J. Org. Chem. 1972,37 1169. 59 M. Saunders and R. Berger J. Amer. Chem. Sac. 1972,94,4049. Reaction Mechanisms-Part (iv)Polar Reactions activation energies for several such processes are now known;60 at high tempera- tures isomerization to the tropylium ion occurs. Photoelectron spectroscopy has proved a useful tool for investigating car- bonium ion structure. Since ionization is very rapid the method may resolve equilibrating species and gives significantly different spectra for 2s ionization potentials (IP) of carbons bearing different amounts of positive charge.Thus the t-butyl cation shows two carbon Is IPS in the ratio 3 1 due to methyl and central carbons. The norbornyl cation shows only one poorly resolved into a main peak and a shoulder. The implication is that there is no carbon in this ion which bears a large positive charge thus strengthening the argument for the non- classical representation (43).'j Relative stabilities of trityl cations have been measured by a stepwise series of equilibria R+Y-+ R'X * RX + R'+Y-where R R' are differently substituted trityl groups and the equilibrium constant is measured by n.m.r. spectro~copy.~~ Rates of hydrolysis of trityl cations66 and tropylium ions67 have been measured by appropriate techniques for fast reactions and equilibria such'as Ar,C+ + N3-Ar,CN3 have been studied with various nucleophiles the object being to use the equilibrium constant as a measure of the carbon-nucleophilicity of the ani~n.~**~' Rates of nucleophilic attack on arenediazonium ions have been used to obtain similar inf~rmation.~' Predictions of preferred conformations of carbonium ions (and carbanions) and rotation barriers due to hyperconjugation have been estimated theoretically.'O R. K. Lustgarten M. Brookhart and S. Winstein J. Amer. Chem. SOC.,1972 94 2347. 61 G. A. Olah G. D. Mateescu and J. L. Riemenschneider J. Amer. Chem. SOC.,1972 94 2529. 62 G. D. Mateescu and G. A. Olah 1st I.U.P.A.C. International Symposium on Physical Organic Chemistry Crans-sur-Sierre 1972.63 G. A. Olah G. D. Mateescu L. A. Wilson and M. A. Gross J. Amer. Chem. SOC. 1970,92,723 1. " G. D. Mateescu J. L. Riemenschneider J. J. Svoboda and G. A. Olah J. Amer. Chem. SOC.,1972 94 7192. '' S. V. McKinley J. W. Rakshys A. E. Young and H. H. Freedman J. Amer. Chem. SOC.,1971,93,4715. 66 C. A. Bunton and S. K. Huong J. Amer. Chem. SOC.,1972,94 3536. " C. D. Richie and H. Fleischhauer J. Amer. Chem. SOC.,1972 94 3481. 68 C. D. Richie and P.-0.Virtanen J. Amer. Chem. SOC.,1972 94 1589. C. D. Richie J. Amer. Chem. SOC.,1972 94 3275. 70 C. D. Richie and D. J. Wright J. Amer. Chem. SOC.,1971,93 6574. 170 N. S. Isaacs Conformer (44)is preferred over (45)in carbonium ions in which X is more electro- negative than H and conversely if X is less electronegative than H.The reverse is predicted for the corresponding car bani on^.^ Dioxacarbonium ions (46) are formed by protonation of vinyl esters; their stabilities (R = Me > cyclo-propyl > Pr' > Et > Pr" > Ph > H) have been estimated from heats of formation.73 A deuterium isotope effect on the equilibrium (47)-(48)has been observed from which it appears that (47) is the more stable.74 X x& H-H H -' H \ R 0 Me H' R C'{0LCHMe \c/ \/ + I1 II \'. + I 0 CH 0-CH Reactions of carbonium ions recently investigated include the photocycliza- tion (49)h (50),75976and the ally1 cation cycloaddition (51)+ (52).77The con- trolled rearrangements of the fenchyl cation have been studied in some The capacity for hydride abstraction makes the trityl cation an attractive reagent "20 -b (-J OH (49) 'I R.Hoffmann L. Radom J. A. Pople P. von R. Schleyer W. J. Heyre and L. Salem J. Amer. Chem. SOC.,1972,94 6221. 72 T. G. Traylor W. Hanstein. H. J. Berwin. N. A. Clinton and R. S. Brown. J. Amer. Chem. SOC.,1971,93,5715. l3 J. W. Larsen and S. Ewing J. Amer. Chem. SOC.,1971 93 5107. l4 M. Saunders M. H. Jaffe and P. Vogel J.Amer. Chem. SOC.,1971,93,2558,2559,2561. 75 E. E. van Tamelen R. H. Greeling and H. Schumacher J. Amer. Chem. SOC.,1971 93 6151 6158. 76 W. M. Horspool in 'Photochemistry' ed. D. Bryce-Smith (Specialist Periodical Reports) The Chemical Society London 1972 vol. 3 p.535. l7 H. M. R. Hoffmann K. E. Clemens and R. H. Smithers J. Arner. Chem. SOC.,1972 94 3940. 7g E. Huang K. Ranganayakulu and T. S. Sorensen J. Amer. Chem. SOC.,1972,94 1779. Reaction Mechanisms-Part (iu) Polar Reactions for ketal cleavage (Scheme 4).79 Vinylic cations prepared by protonation of alkynes absorb carbon monoxide to give oxycations (54) together with rearrange- ment.80 Conformational isomerization in homotropylium ions (55) occurs by Scheme 4 MeC-CH HSO F-SbF 5 co \ oc+ Me' /Me \H \Me Me + MeCH,CMeFO' II 'OH 1 7 4 (53) (54) -cr Q L 7 (554 A CI addition-elimination of a suitable anion.8 Some bishomotropylium ions (56) have been prepared and characterized in solution,82 and also some novel structures (57) in which positive charge is delocalized in two aromatic rings.83 Cyclopropenium ions can decompose to alkynes ;the mechanism and fate of the one-carbon fragment is so far unknown.84 No especial stability is found to be l9 D.H. R. Barton P. D. Magnus G. Smith and D. Zurr Chem. Comm. 1971,861. H. Hogeveen and C. F. Rovbeek Tetrahedron Letters 1971 3343. R. Huisgen and J. Gasteiger Tetrahedron Letters 1972 3661 3665. 8z P. Ahlberg D. L. Harris M. Roberts P. Warner P. Seidl M. Sakai D. Cook A. Diaz J. P. Dirlam H. Hamberger and S. Winstein J. Amer. Chem. SOC.,1972 94 7063. 83 C. Kabuto M. Oda and Y. Kitahara Tetrahedron Letters 1972,4851. 84 W. J. Gensler and J. J. Langone Tetrahedron Letters 1972 3765. 172 N. S. Isaacs 3.3 H H 6.63 H\H 8.87 (56) CN CN Ph (58) associated with the spiro-cation (58) predicted to partake of ‘spiroaroma- ti~ity’.~ ’9 86 4 Substitutibn at Vinylic Carbon The characteristics of vinylic nucleophilic substitutions continue to be delineated.Although not entirely stereospecific both vinylic substitution and concurrent elimination product ratios are affected by substrate ge~metry.~’ An ion-pair intermediate partly blocking the approach to the front side is postulated. A similar conclusion may be drawn from the trifluoroethanolysis of (59) which leads to inverted and retained stereochemistry of the product in the ratio 4 1.88 The solvolyses of allenyl halides (60) have been measured. Rates are low but the solvent sensitivity (m,= 0.9-1.13) and leaving group ratio (Br/Cl = 56) indicate an ionization me~hanism.~~,~~ Similar arguments and conclusions have been applied to vinyl sulphonate trifluor~acetolysis,~ but vinylic displacements of P-halogenoketen acetals (61) by aniline show very different characteristics (e.g.Br/Cl = 1) and are assigned an addition4imination mechanism (A~,-IY),~~ The same mechanism is inferred to take place at analogous compounds in the B5 M.F. Semmelhack R. J. DeFranco Z. Margolin and J. Stock J. Amer. Chem. Soc. 1972,94,2115,2116. 86 M. J. Goldstein and R. Hoffrnann J. Amer. Chem. SOC.,1971 93. 6193. ” R. H. Sumrnerville and P. von R. Schleyer J. Amer. Chem. SOC.,1972 94 3629. 88 T. C. Clarke D. R. Kelsey and R. G. Bergman J. Amer. Chem. SOC.,1972,94 3626.89 M. D. Schiavelli R. P. Gilbert W. A. Baynton and C. J. Barwell J. Amer. Chem. SOC. 1972,94. 5061. 90 C. V.Lee R. J. Hargrave T. E. Dulber and P. J. Stang Tetrahedron Letters 1971,2523. 91 Z. Rappoport and U. Kaspi Tetrahedron Letters 1971 4039. q2 Z. Rappoport and A. Topal J.C.S. Perkin II 1972 1823. Reaction Mechanisms-Parl (iu) Polar Reactions 173 fluoenylidene series e.g.(62).93 Vinyl cations are presumed intermediates in the acid-catalysed decomposition of vinyltriazenes (63),94though displacements by copper alkyl~~~ and alkylmercuric cations on vinyl halides96 are likely to occur by addition-elimination. A vinylic electrophilic displacement has been identified in the reaction of (64) with br~mine.~’ R R OCH,CF, MoTf -+ )=(Me + R+( Me Me Me OCH2CF Me Me (59) 4:1 Me0 R,C=C=C /ha’ \ HH R Me0 ha1 (40) (61) (62) R R R2C=C / R,C=C / + N + PhNH, \ N=N-NHPh (63) H H / Br / (p-MeOC,H,),C=C 4(p-MeOC,H,),C=C + Ph,C+ \ \ CPh Br (64) 5 Them-Effect It is now ten years since Edwards and Pear~on~~ drew attention to the enhanced nucleophilic reactivity of bases which have an unshared pair on the atom adjacent to the reactive centre a phenomenon termed the ‘a-effect’.The origin of this effect is still under active investigation. Ions such as peroxy (HOO-and ROO-) and hypochlorite (C10-) and molecules such as hydrazine and hydroxylamine are found to be more reactive as carbon nucleophiles than their basicity would suggest. Hence the a-effect is revealed through anomalies in Brernsted plots.The normal Bransted relation logk/k = PpK + c or the two-parameter ‘oxybase scale’ log k/k = crE + PH may be used as a basis for 93 Z. Rappoport and A. Gal J. Org. Chem. 1972,37 1174. 94 W. N. Y.Jones and F. W. Miller J. Amer. Chem. SOC.,1967,89 1960. 95 J. Klein and R. Levene J. Amer. Chem. SOC.,1972 94 2520. 96 P. Abley J. E. Byrd and J. Halpern J. Amer. Chem. SOC.,1972 94 1985. 97 Z. Rappoport and I. Schnabel J.C.S. Perkin If 1972 146. 98 J. 0.Edwards and R. G. Pearson J. Amer. Chem. SOC. 1962,84 16. 174 N. S. Isaacs this test,99 and the rate ratio kHoo-/kHo-is often indicative. In the aromatic nucleophilic displacement of chloride ion from 2,4-dinitrochlorobenzenethis ratio is as high as 300 despite the fact that HO -is the weaker base (to the proton).The magnitude of the a-effect has also been measured as the relative rate (in displacement base catalysis etc.) of hydrazine and glycylglycine (a base with similar pK but not a-unshared pair). The value of k(hydrazine)/k(glycylglycine) has been found to depend in a consistent manner upon the Brsnsted p coefficient of the reaction concerned,"' and this relationship holds for reactions of Mala- chite Green with nucleophiles."' The magnitude of the effect depends upon the leaving group in displacement reactions. Further examples include an unusual four-membered cyclization of (65) to the isolable chemiluminescent peroxide (66),lo3 and displacement reactions of diazines among which the 1,2-isomers pyridazine cinnoline and phthalazine are exceptionally reactive.' O4 0-0 The origin of the a-effect has stimulated much speculation ;electron repulsion between unshared pairs has attracted many authors but is not entirely consistent with the observations.'O5 A more deep-rooted explanation has recently appeared based on MO perturbation theory. This has the advantage of predicting an increase in the effect with increasing Brmsted coefficient and with the inverse of the ionization potential of the nucleophile. lo6 6 Participation by Neighbouring Groups The linear Hammett plot for solvolyses of 2-(3-p-X-phenyl)butyl brosylates (67) when X is a substituent with a positive o-value contrasts with the steeply curving X H*-? \ Me OBros b) 99 J.E. McIsaac L. R. Subbaraman J. C. Subbaraman H. A. Mulhausen and E. I. Behrman J. Org. Chem. 1972 37 1037 and references therein. loo J. E. Dixon and T. C. Bruice J. Amer. Chem. Sac. 1972 94 2052. lo' J. E. Dixon and T. C. Bruice J. Amer. Chem. SOC.,1971,93,6592. lo' M. Dessolin Tetrahedron Letters 1972 4585. Io3 W. H. Richardson and V. F. Hodge J. Amer. Chem. Sac. 1971 93 3996. Io4 J. A. Zoltewicz and L. W. Deady J. Amer. Chem. SOC.,1972,94,2765. 'O5 J. Hine and P.J. Dalsin J. Amer. Chem. Sac. 1972,94 6998. Io6 F. Filipini and R. F. Hudson J.C.S. Chem. Comm. 1972 522. Reaction Mechanisms-Part (iv)Polar Reactions plot for substituents with a negative a-value. Clearly the donor groups are causing the superposition of ever-increasing degrees of phenyl participation.'O7 Studies in the bicyclo[2,2,2]octene series suggests that n-participation is also possible here.'08 The bis-tosylate (68) is very inert compared with the mono- ester (69) whereas the unsaturated analogues are much more reactive. Four-membered participation is not very efficient and is not usually observed. Four-membered chloronium ions (70)do not form under conditions which readily generate the three- and five-membered analogue^.'^' That the latter are more stable than the former emerges from a thermochemical study,' lo yet until recently (halogen-5) participation had not been observed whereas (halogen-3) participa- tion was fairly common. This no doubt arises on account of the relative weakness of the effect by the electronegative halogens and the less-favourable entropy of formation of the larger ring.(Chlorine-5) participation has now been observed in the trifluoroacetolysis of (71) the products of which contain over 40% chlorine-rearranged material. '9 'l2 An example of four-membered-ring forma- tion is the bromolactonization (72)-+ (73),though it is unlikely that the reaction derives any anchimeric assistance from the carboxylate ion.' l3 Me I CH,Br Me (72) (73) 107 H. C. Brown and C. J. Kim J. Amer. Chem. SOC. 1971,93 5765. 108 J. B. Lambert and A. G. Holcomb J. Amer. Chem. SOC.,1971,93 3952. 109 G. A. Olah J. M. Bollinger Y. K. Mo and J. M. Brinich J. Amer. Chem. SOC.,1972 94 1164. 110 J. W. Larsen and A. V. Metzner J. Amer. Chem. SOC.1972 94 1614. 111 P. E. Peterson Accounts Chem. Res. 1971 4 407. I I2 P. E. Peterson and W. F. Boron. J. Amer. Chem. SOC.. 1971. 93. 4076. I I3 W. E. Barnett and J. C. McKenna Tetrahedron Letters 197 I 2595. 176 N. S. Isaacs The solvolysis (with Wagner-Meerwein rearrangement) of deuteriated neo- pentyl esters (74) shows a negligible isotope effect on the rate though there is a distinct preference for CH rather than CD migration. It is inferred that methyl migration occurs after the rate-determining step.' l4 In contrast the pinacol rearrangement of (75) shows a 7 % isotopic rate effect (per CD group) which is parallelled in the product composition. It appears that rate-determining and product-determining steps are identical and possibly that the rearrangement derives some anchimeric assistance.' R' ,R1 \ .C-CH,OSO,Ar Ph,C-C'-RZ I l\R2 OHOH R3 (75) R' RZ= CH or CD (74) R' RZ,R3 = CH or CD Neighbouring carboxy-groups may have either a nucleophilic function (espe- cially as the carboxylate ion) or an electrophilic one and act as an internal acid catalyst. The effects frequently are revealed as differences in rate between ortho-and para-substituted substrates. Thus the aspirin anion hydrolyses by a general base-catalysed mechanism (76) +(77),' l6 but the apparently structurally similar 0 0 II It slow fast Products enol ester (78) at alkaline pH undergoes a completely different solvolytic route uia an acyl transfer to the neighbouring carboxylate (Scheme 5).' '' Maleamic acids (79) undergo assisted hydrolysis by carboxylate ion.Rates vary over a range of 10" with alkyl substitution presumably on account of steric factors which affect the ease of participation. ' ' * Salicylyl phosphates (80) are hydrolysed far faster than p-hydroxybenzoic acid phosphates by either acid- or base-catalysed mechanisms.' 19,120Similar affects are found for the formaldehyde acetal (81)12' 'I4 W. M. Schubert and W. L. Henson J. Amer. Chem. SOC..1971 93 6299. 115 W. M. Schubert and P. H. LeFevre J. Amer. Chem. SOC.,1972,94 1639. '16 A. R. Fersht and A. J. Kirby J. Amer. Chem. SOC.,1967,89,4853,4857. 'I7 A. J. Kirby and G. Meyer J.C.S. Perkin 11 1972 1446. I '*A. J. Kirby and P. W. Lancaster J.C.S. Perkin ZI 1972 1206. R. H. Bromilow S.A. Khan and A. J. Kirby J.C.S. Perkin II 1972 91 1. '** R. H. Bromilow and A. J. Kirby J.C.S. Perkin 11 1972 149. B. M. Dunn and T. C. Bruice J. Amer. Chem. SOC.,1971,93 5725. Reaction Mechanisms-Part (iu) Polar Reactions 177 + + AcO-/Me *O II I1 $0 0 0 I c0,-C02H Scheme 5 and the acetal(82) which hydrolyses 3000 times faster than the para-isomer.'22 An even more dramatic difference is reported (ortholpara -lo5-lo6) for the hydrolysis of (83).'23 40 R' IC\ISHR3 R2 C0,-(79) 0 OEt / acH\OEt c-o-I1 0 7 Carbanions and Carbon Acidity Further measurements of carbon acidity by equilibria with cyclohexylamide have been reported; a series of 9-alkylfluorenes have acidities which fit'24 the linear free-energy equation log(k/k,) = pa* p = 4.55.Stabilization of the 9-alkylfluorenyl anions is considered to arise from a change in the cr-bond strength of the C-9-R bond upon ionization. The effect is greater upon carbon- carbon bonds than upon carbon-hydrogen thus explaining the greater acidity of 9-methylfluorene than the parent compound. The acidities of a-alkyl substi- tuted toluenes is similarly proportional to o*(p = 2.37).'25 A new H-scale 122 E. Anderson and B. Capon J.C.S. Perkin II 1972 515. T. H. Fife and E. Anderson J. Amer. Chem. SOC.,1971,93,6610. 124 A. Streitwieser C. J. Chang and D. M. E. Reuben J.Amer. Chem. SOC.,1972,94,5730. IZ5 A. Streitwieser P. C. Mowery and W. R. Young Tetrahedron Letters 1971 3931. 178 N. S. Isaacs based on the ionization of 1,3-diphenylindene (pK = 19.8) and fluoradene (pK = 18.2) in methanol-methoxide has been set up; the scale has been shown to correlate well with hydrogen exchange rates of 9-substituted fluorenes.'26 A critical examination of hydrocarbon acidities in cyclohexylamine has shown that electrostatic interactions between carbanion and metal cation affect measured pK values to a small extent. The temperature effects point to caesium salts existing as contact ion-pairs and lithium salts as a mixture of contact and solvent-separated pairs.12' Similar conclusions have been reached on the basis of n.m.r. studies of carbanion solutions. 1287129 A number of 1,3-diphenylallyl anions (84) have been studied by n.m.r. and their kindred rotation about the 4.1 I 3.46 H 5.37 (84) C-l-C-2 bond and the C-1-Ph bond ob~erved.'~' The problem has been studied by a kinetic approach observing rates of cis-trans isomerization of the diphenylpropenes.' N.m.r. studies have permitted measurement of rates of the rearrangement sequence shown in Scheme Scheme 6 Many new studies of kinetic acidity (proton exchange rates) have been reported. Methyl proton exchange rates in all possible methylazulenes (AzCH,) are found to correlate with acidities of the corresponding ammonium ions (AzNH3+) 126 A. Streitweiser C. J. Chang and A T. Young J. Amer. Chem. SOC., 1972 94 4888. A. Streitweiser C. J. Chang and W. B. Hollyhead J. Amer. Chem. SOC.,1972 94 5292,5288. Iz8 J. B. Grutzner J. M. Lawlor and L.M. Jackman J. Amer. Chem. Soc. 1972,94,2306. '29 J. W. Burley and R. N. Young J.C.S. Perkin ZZ 1972 1006. IJ0 J. W. Burley and R. N. Young J.C.S. Perkin ZZ 1972 1843. J. M. Figuera J. M. Gamboa and J. Santos J.C.S. Perkin IZ 1972 1434. 132 R. B. Bates S. Brenner and C. M. Cole J. Amer. Chein. SOC.,1972,94 2131. 133 C. Weiss Tetrahedron 1972 28 2599. Reaction Mechanisms-Part (iu)Polar Reactions the correlation may be summarized thus pK(AzCH,) = 14.73 + 6.23 pK- (AzNH,' ). Bridgehead acidities of some bicyclic compounds have been shown to follow a trend which depends largely upon the increasing s-character of the C-H bond.'34 Within a series of related compounds kinetic and thermo- dynamic acidities frequently give a satisfactory Brransted plot e.g.the di- and tri-arylmethanes' 35 and fluorene derivative^.'^^ Among factors affecting carbanion stability the inductive effect of electro- negative atoms and the juxtaposition of unshared pairs tend to stabilize and destabilize respectively. These factors are considered to be the most important features in a study of acidity of hexafluoropropyl compounds [(CF,),-CH-XI 137,138 and fluorine hyperconjugation appears not to play a significant role. The rates of ionization of strong carbon acids require techniques for fast reactions; bromomalononitrile for example has k = 8.1 x lo6 and k-= 4.3 x lo71 mol-s-' fast but not approaching diffusion rates. 139 Studies have also been reported on the perylene radical-anion protonation reaction. 140 An ingenious method for estimating isotope effects for protonation of a car- banion (9-methoxyfluorene anion) by methanol and methan[2H]ol uses a competitive reaction in the presence of a one-electron acceptor.From the rela- tive amount of hydrocarbon (RH) to the dimer (R-R) it was concluded that the lifetime of the carbanion in methan[2H]ol is greater than in methanol k,/k = 5.3.14' The iron-containing group in (85)activates the benzylic proton exchange moderately 14' but a cyclopropane ring does not impart significant stability to an adjacent carbanionic centre. 143 Allyl-and propargyl-benzenes have provided estimates of acidity (pK M 33) via proton exchange rates'44 and shown to exchange with retention of configuration. 145 The cyclopropyl-ally1 anion 134 A.Streitweiser M. J. McKornick and G. R. Ziegler Tetrahedron Letters 1971 3927. 135 A. Streitweiser W. B. Hollyhead G. Sonnichsen A. H. Pudjaatmaka C. J. Chang and T. L. Kruger J. Amer. Chem. SOC.,1971,93 5096. 136 A. Streitweiser W. B. Hollyhead. A. H. Pudjaatmaka P. H. Owens T. L. Kruger P. A. Ruberstein R. A. McQuarrie M. L. Brokow W. K. C. Chu and H. N. Niemayer J. Amer. Chem. SOC.,1971,93 5088. 13' K. J. Klabunde and D. J. Burton J. Amer. Chem. SOC.,1972,94 5985. 13* K. J. Klabunde and D. J. Burton J. Amer. Chem. SOC.,1972,94 820. 139 F. Hibbert and F. A. Long J. Amer. Chem. SOC.,1972,94 2647. 140 G. Levin C. Sutphen and M. Szwac J. Amer. Chem. SOC.,1972,94 2652. 14' R. D. Guthrie A. T. Young and G. W. Pendygraft J. Amer. Chem. SOC.,1971 93 4947.14' S. N. Anderson D. H. Ballard and M. D. Johnson Chem. Comm. 1971,779. 143 M. J. Perkins and P. Ward Chem. Comm. 1971 1134. 144 K. Bowden and R. S. Cook J.C.S. Perkin II 1972 1407. 14' H. M. Walborsky and L. M. Turner J. Amer. Chem. SOC.,1972,94 2273. 180 N. S. Isaacs rearrangement [(86)-+ (87)] has been studied but it was not possible to identify unambiguously the predicted conrotatory mode.146 A potentially useful olefin synthesis (Scheme 7) begins with an cr-nitro-carbanion (88).14’ Some dianions Scheme 7 e.g. (89) have been prepared and their chemistry examined. 148,149 Dynamic nuclear polarization has been observed in the products from the Stevens re- arrangement long described as a carbanionic 1,2-migration. It thus appears that radical intermediates are involved.R,C-CO-(89) 8 Elimination Reactions The subject of polar eliminations has in recent years become somewhat blurred and fraught with controversy. It appears now that the anti-E2 type of mechanism described by Ingoldlsl is only one extreme in a continuum of possible transi- tion states. The detailed description of these transition states has invited rival theories. Further evidence has been presented in support of the spectrum of mechanisms designated E,C-E,H (90) and (91) which has been backed by an 6- 6-‘46 R. Huisgen and P. Eberhard J. Amer. Chem. Soc. 1972,94 1346. 14’ N. Kornblum S. D. Boyd H. W. Pinnick and R. G. Smith J. Amer. Chem. SOC. 1971 93,4316. 14* Y.-N. Kuo J. A. Yahner and C.Ainsworth J. Amer. Chem. SOC.,1971 93. 6321. 149 D. R. Dimmel and S. B. Gharpure J. Amer. Chem. SOC.,1971,93 3991. I5O H. P. Benecke and J. H. Wikel Tetrahedron Letters 1971 3479. lS1 C. K. Ingold ‘Structure and Mechanism in Organic Chemistry’ Bell London 2nd edn. 1969. Reaction Mechanisms-Part (iu) Polar Reactions enormous output of kinetic data.'52 The basic difference between the two ex- tremes in this manifold lies in the partial bonding of base to a-C and the large development of double-bond character in the E2C transition state (90). This merges with the E,H situation (91) which comprises bonding of base only to P-H less double-bond character and more carbanion character at P-C. Thus the E2C and E2H transition states are envisaged as product-like and carbanion- like respectively.On account of the interaction with a-C E2C mechanisms should show similarities with the S,2 process as regards the effect of the nucleo- phile leaving group etc. and this supposition is supported by good Brmsted correlations for the two types of reaction on cyclohexyl tosylate,153 though not for reactions of t-butyl bromide ;'54 a poor correlation with H-basicity is also expected and found. The E2C extreme is promoted by nucleophiles which are relatively strong C-bases and weak H-bases. The series Br- C1- N3-, OAc- OEt- 0Bu'- is one of increasing tendency to promote the E2Hmechanism at the expense of the E2C. Halide ion-catalysed and t-butoxide ion-catalysed p-eliminations may be taken as prototype E,C and E2H processes respectively.The former is mildly accelerated by electron-withdrawal at p-C but the latter up to 1O6-fold even by a p-br~rnine.'~~ Substituents at a-C affect E2H reactions very little and appear to accelerate E2C types slightly. This latter observation has been put forward in criticism of the whole E2C-E2H ~oncept'~~.'~' on the grounds that a-substitu- tion should show a 'neopentyl' effect strongly retarding reaction by steric hind- rance. It has been pointed out in reply that rates of the compounds quoted (92) and (93)'would only show a neopentyl effect in a tight transition state. The effect in substitution reactions (kethyi/kneopenty,) varies indeed from Id" to only 4 152 P. Beltrame G. Biale D. J. Lloyd A. J. Parker M. Ruane and S.Winstein J. Amer. Chem. Sac. 1972,94,2240. 153 A. J. Parker. M. Ruane. G. Biale. and S. Winstein Tetrahedron Letters 1968 21 13. I54 A. J. Parker M. Ruane D. A. Palmer and S. Winstein J. Amer. Chem. SOC.,1972,94 2228. 155 G. Biale D. Cook D. J. Lloyd A. J. Parker I. D. R. Stevens J. Takahashi and S. Winstein J. Amer. Chem. SOC.,1971 93 4735. 156 D. Eck and J. F. Bunnett J. Amer. Chem. SOC.,1969 91 3099. 157 J. F. Bunnett and E. Baciocci J. Org. Chem 1970 35,76. 182 N. S. Isaacs with a less-tightly-bound transition state. The E,C transition state only implies weak long-range interactions between base and a-C so that these rates are not incompatible with an E,C mechanism. Reactions designated E,C tend to give more of the thermodynamic (Saytzeff) product than do E,H a property useful in preparative ~hemistry.'~~,~ 58 Thus bromide-ion-promoted elimination of 2-bromobutane gives but-2-ene but-1-ene in the ratio 24 1 whereas ethoxide and t-butoxide give the same products in 4 1 and 1 1 ratios respectively an observation previously attributed to steric effects.' 59 The stereochemistry of elimination is strictly anti for the E,C mechan-ism but the requirements are less stringent for the E,H which may show a con- siderable proportion of syn-product.153*'60 Different degrees of p-C-H bonding in the transition state for the two extreme types are reflected in differences for primary isotope effects.The values of k,/k, are small (2.3-3.2) for E2C re-actions but near a maximum (6-7) for E2H.16' Solvent effects upon E,C reactions are similar to those upon S,2 but E2H processes are dissimilar.De-halogenation reactions are of similar characteristics to the E,C and are postu- lated to partake of a transition state such as (94). b-Br ...Br .. 1. b-An alternative interpretation of p-eliminations emphasizes carbonium ion character at a-C and carbanion character at p-C in the two extremes (corre- sponding to E,H and E2C types respectively). 162-164 Many of the above observations may be explained on this model ; perhaps some elimination- substitution Brransted correlations are less easy to accommodate in this way. The extreme form of E,H transition state merges with the E,cb having a discrete carbanion (95) as intermediate.Three variants on this mechanism have been R,CH-CR,Y 2R,C-CR,Y !% R,C=CR + Y-k-I distinguished :165 (i) the irreversible k >> k-' (ii) the pre-equilibrium k > k, and more recently (iii) the situation k > k and k >> k-The latter typically occurring in compounds with a highly acidic /?-proton and poor leaving group IS' D. J. Lloyd D. M. Muir and A. J. Parker Tetrahedron Letters 1971 3015. IS9 H. C. Brown and I. Moritani J. Amer. Chem. SOC.,1956,78 2203. P. Beltrame A. Ceccon and S. Winstein J. Amer. Chem. SOC.,1972,94 2315. "I G. Biale A. J. Parker I. D. R. Stevens J. Takahashi and S. Winstein J. Amer. Chem. SOC., 1972 94 2235. Ib2 J. F. Bunnett Angew. Chem. Internat. Edn. 1962 1 228. J. F. Bunnett Surv. Prog. Chem. 1969 5 53.W. H. Saunders 'Chemistry of the Alkenes' ed. S. Patai Interscience 1964 Chap. 2. M. Albek S. Hoz and Z. Rappoport J.C.S. Perkin II 1972 1248. Reaction Mechanisms-Part (iv) Polar Reactions leads to the formation of carbanion in a non-steady-state concentration. The reaction is zeroth-order in base and independent of base strength as is proposed for the elimination of HCN by triethylamine from (96). An E,cb mechanism has been proposed for HCl elimination from DDT,'66and syn stereospecificity observed in E,cb elimination from (97).'67 A nitrogen analogue of the E,cb mechanism is proposed for hydrolysis of the carbamate (98).16* CH-C / \;I.CN NC CN (96) A second major area of interest has centred around the factors which influence the relative ease of syn and anti elimination.It has previously been shown'69 that certain eliminations (particularly in the medium- and large-ring series) partake of an anti geometry leading to cis-olefin and a syn geometry leading to trans-olefin. An explanation of this dichotomy has been proposed in terms of the steric effect of the leaving group (e.g. quaternary ammonium) which by forcing y-and B'-substituents into conformations such that an approach to the P-H from the anti side is hindered.17' Under these conditions syn elimination be- comes ~ompetitive.'~'-~~~ It is likely that two maxima for elimination rates are I" D. J. McLennon and R. J. Wong Tetrahedron Letters 1972 2887 2891. 16' V. Fiandanese G. Marchese and F. Naso J.C.S. Chem. Comm. 1972 250.16' A. F. Hegarty and L. N. Frost J.C.S. Chem. Comm. 1972 500. '69 N. S. Isaacs Ann. Reporfs(B) 1970,67 101. 170 J. K. Barchardt R. Hargreaves and W. H. Saunders Tetrahedron Letters 1972 2307. 17' D. S. Bailey and W. H. Saunders J. Amer. Chem. Sac. 1970,92 6904. D. S. Bailey F. C. Montgomery G. W. Chodak and W. H. Saunders J. Amer. Chem. Soc. 1970 92 691 1. I. N. Feit F. Schadt J. Lubinkowski and W. H. Saunders J. Amer. Chem. Soc. 197 I 93 6606. 184 N. S. Isaacs observed when the dihedral angles between P-H and leaving group are 180" and 0". Although these factors may be important it has now been shown that ion- pairing of the base greatly favours syn elimination.' 74-176 One technique of potentially wide application has been the introduction of 'crown ethers' to co-ordinate counterions from the sphere of influence.The proportion of syn product can change from 7.3 to 38 % with ion pairing and this factor may be the most important in the solvent control of elimination products. In a study of arenesulphonyl (E,cb) eliminations the similarity of leaving-group effects activation parameters and p-values for both syn and anti modes of elimina- tion has led to the conclusion that both originate from the same type of mechan-ism;'77 this contrasts with other work tending to suggest anti product arises from an E (E,C)transition state and svn from an E,cb (E,H). The same group have challenged the view that a significant 'driving force' to fi-elimination is imparted by the synchronous nature of the E process.'" Thus (99)eliminates only twice as fast as (loo),though the former gains at least five times as much stabilization (through aromatization) as the latter.' 79 M. Pankova M. Svoboda and J. Zavada Tetrahcdron Letters 1972 2465. J. K. Barchardt and W. H. Saunders Tetrahedron Letters 1972 3439. '76 (a)R. A. Bartsch and K. E. Wiegers Tetrahedron Letters 1972,3819; (6)R. A. Bartsch C. F. Kelly and G. M. Pruss J. Org. Chem. 1971 36 662; (c) J. Avraaoides and "' A. J. Parker Tetrahedron Letters 1971 4043. F. G. Bordwell J. Weinstock and T. F. Sullivan J. Amer. Chem. SOC.,1971,93 4728. 17' M. G. Evans and M. Polanyi Trans. Faraday SOC.,1938 34 11. F. G. Bordwell D. A. R. Happer and G. D. Cooper Tetrahedron Letters 1972,2759. Reaction Mechanisms-Part (iu) Polar Reactions Some novel eliminations have been reported ;boranes undergo smooth forma- tion of olefin under the action of a weak base;18' the nickel complex (Ph,P),- Ni(C0)2 brings about decarbonylation of the anhydride ( 101):'81elimination of the nitrosoamide (102)presumably occurs by an a-process and leads to much loss of deuterium;'82 and the 1,3-debromination of (103) is stereo~pecific.'~~ Y (102) X = -N(NO)CO,Et Y = D X = D Y = -N(NO)CO,Et 24 71 14 4 % loss of D f;.H Br VPh 02 (103) 9 Polar Additions The emphasis in many recent studies of addition reactions has been on attempt- ing to resolve the question of open or cyclic transition states.Activation energies for bromine addition to cis and trans pairs of 1,2-disubstituted ethylenes parallel the ground-state energies of the alkenes.184 This suggests that substitueiit repulsions are still present in the transition state as expected for a cyclic transi- tion state (104)but not for an open one. Relative rates of addition to norbornene and to 7,7-dimethylnorbornene have been proposed as a probe for cyclic or non-cyclic processes. For the former the ratio is large (500-1000) while for the latter it is much smaller (2-60).'85 Reagents which add by a cyclic mechanism include phenylsulphenyl chloride la* H. C. Brown E. Negishi and J.-J. Katz J. Amer. Chem. Soc. 1972 94 5893. B. M. Trost and F. Chen Tetrahedron Letters 1971 2603. T. Cohen A. R. Danewski G. M. Deeb and C. K. Shaw J. Amer. Chem. Soc.1972 94 1786. B. B. Jarvis S. D. Dutkey and H. L. Ammon J. Amer. Chem. SOC.,1972,94 2136. K. Yates and R. A. McDonald J. Amer. Chem. Soc. 1971,93 6297. la5 H. C. Brown and K.-T. Lin J. Amer. Chem. Soc. 1971 93 7335. 186 N. S. Isaacs .. Br .C-CH2 D-i Ar percarboxylic acids and di-imine whereas stepwise additions occur with acetic acid HCl etc. Rates of addition of borane and its deuterium and tritium ana- logues to some olefins have been measured. 186,187 Rates are high (E = 9.2 kcal mol- I AS* = -27 e.u.) and isotope effects diminish drastically with increas- ing alkyl substitution on the ethylene component. This presumably affects the degree of hydride transfer from boron to carbon by steric effects. Isotope effects on rates of bromination of substituted u-deuteriostyrenes are variable depending upon the substituent and suggest different degrees of asymmetry in the supposed bridged bromonium intermediate (104).188 A num-ber of addition studies of halogen to allenes and acetylenes have been published ; trans addition is usual and cyclic bromonium ions are inferred,' 89-192 a1though the strained olefin trans-cyclo-octene undergoes mercuration to cis-products.193 Optically active penta-2,3-diene and cyclonona-l,2-diene give optically active addition products e.g. (105).194*195 cis-Chlorination of olefins can be achieved using antimony pentachloride as catalyst whereas for example cupric chloride leads predominantly to the trans-dihalogenoalkane. 96 Bromination of alkenes in methanol shows a very large susceptibility to solvent polarity (m,= 1.17) and a large solvent deuterium isotope effect kMUleOH/kMeOD = 1.40.This is attributed to the importance of solvent hydrogen-bonding in the transition state (106). 197 f -c\. . H (105) (106) The addition of iodine thiocyanate to alkenes provides a useful route to thi- irans (107).'98 The bromolactonization of but-3-enoic acids leads in the first lg6 D. J. Pasto B. Lepeska and T.-C. Cheng J. Amer. Chem. Soc. 1972,94 6083. lg7 D. J. Pasto B. Lepeska and V. Balasubramaniyan J. Amer. Chem. Soc. 1972 94 6090. lgB C. L. Wilkins and T. W. Regulski J. Amer. Chem. Soc. 1972,94 6016. "' H. Khalaf. Tetrahedron Letters 1971 4233. E. Manger and E. Berliner J. Amer. Chem.SOC.,1972,94 194. ''' E. K. Raunio and T. G. Frey J. Org. Chem. 1971 36 345. 192 C. B. Reese and A. Shaw Tetrahedron Letters 1971 4641. lg3 R. D. Bach and R. F. Richter Tetrahedron Letters 1971 3915. '94 M. C. Findlay W. L. Waters and M. C. Caserio J. Org. Chem. 1971 36 275. lY5L. R. Byrd and M. C Cakerio J. Amer. Chem. Sue. 1971,93 5758. ''15 S. Uemura 0.Sasaki and M. Okana Chem. Comm. 1971 1064. 19' F. Garnier R. H. Downay and J. E. Dubois Chem. Comm. 1971 829. ''' J. C. Hinshaw Tetrahedron Letters 1972 3567. Reaction Mechanisms-Part (iv) Polar Reactions instance to an iodo-B-lactone (108)as kinetic product which rearranges to the more stable y-lactone (109) on standing. 199 W. E. Barnall and W. H. Cohn J.C.S. Chem. Comm.,1972,472.

ISSN:0069-3030    

DOI:10.1039/OC9726900160

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

Free-radical Reactions and Electron Spin 4 Resonance Spectroscopy By W. T. DlXON Dept. of Chemistry Bedford College Regents Park London NW 1 1 Introduction The main difficulty confronting a Reporter in the field of free radicals is that of classifying the work which has been done. In a way the first obvious choice is to divide the material into work involving (i) direct observation of free radicals which mainly involves the use of e.s.r. spectroscopy although visible and U.V. spectra are also employed for this purpose and (ii) other work which affords indirect evidence of free-radical reactions. In this second approach the analysis of the products or in kinetic runs the analysis of starting material at different times plays a major role. Since the Reporter has been concerned with e.s.r.this article is naturally biased in that direction but even a casual glance at the papers which have appeared over the past twelve months shows that investigators in the free-radical field are turn- ing more and more towards e.s.r. spectroscopy for inspiration or for proving a point. The introduction of this convenient technique into the subject has necessarily brought with it a different experimental approach and has polarized interest towards radicals which can be observed Previously in what one might broadly call the ‘kinetic’ approach experimental conditions were arranged so that re- actions were relatively controlled and as ‘clean’ i.e. as simple as possible. One aimed at obtaining’ analysable solutions. In contrast to some extent at least the principle aim in the e.s.r.field is that of obtaining high concentrations of a single type of radical so that by analysis of its spectrum various things about it including what it is can be deduced. Generally speaking although the radical observed does give rise to products which depend on its environment it is not usually the most important radical or species in the reaction in the sense that in the majority of interesting radical reactions a highly reactive species is produced and actually starts the reaction which then gives rise to secondary tertiary etc. stages. Somewhere along the line the balance of production and destruction of a radical leads to its having a high concentration and hence that species is the one observed in the e.s.r.spectro- meter. Since e.s.r. spectrometers have been widely available the emphasis in free- radical chemistry has swung more and more over to these ‘accessible’ radicals 189 190 W. T. Dixon and possibly their reactions. The means of generating them are still compara- tively few stretching chemical and mechanical ingenuity to the full. Most radicals are reactive and have to be generated continuously by one or other well-tried techniques e.g. by means of irradiation (radioactive electron or u.v.) flow systems electrolysis auto-oxidation and so on. Alternatively one might artifi- cially prolong the lifetime of a radical by trapping it in an unreactive solid. Although one could use the types of experiment as a basis for classification the Reporter prefers instead to use the types of radicals themselves to classify the rather heterogeneous material which appeared during 1972.2 Simple Alkyl and Aryl Radicals There has been little evidence this year of a great deal of work on the reactions of simple alkyl or aryl radicals although it seems that reactivity patterns of the methyl radical’ and the phenylation of heterocyclic systems such as thiophen or the pyridiumm cation3 still evince some interest. The way towards some further possibilities in this direction has been pointed out in the study of the decay of methyl radicals formed in y-irradiated methyl isocyanide between 77 and 125K4 As the well-known 1 :3:3:1 quartet ascribed to methyl diminishes a 1. 1 1 triplet increases in amplitude.This new signal is attributed to the radical CH,NC and strongly suggests that a hydrogen abstraction has taken place + CH,NC + ~H~NC CH~ + CH~ The absence of any splitting from the methylene protons is attributed to the large anisotropic coupling constants of the a-protons in a polar molecule of this sort so that the outer lines of the 1 :2 1 triplet (due to the rotating CH group) are broadened but the centre line (q= 0) remains sharp. A remarkable thing about this experiment is that if the deuteriated isocyanide is used cD3 decays much more slowly than CH3 and does not give the expected CD,NC. One cannot help feeling that this system needs further investigation. Other ‘simple’ aliphatic radicals have been investigated.For example the e.s.r. spectrum of an ‘old’ radical vinyl has been reported5y6 and some ‘new’ ones propargyl and butatrienyl,6 have been made in an argon matrix by photolysis of HI in the presence of the appropriate poly-yne e.g. (1). Unlike vinyl these radicals appear to be axially symmetric. Some new phenyl-type radicals have also been observed,’ i.e. 2- 3-,and 4-pyridyl using the technique’ of condensing the vapours of sodium metal mixed with a chloropyridine diluted with argon on ’ W. A. Pryor D. L. Fuller and J. P. Stanley J. Amer. Chem. SOC. 1972 94 1632. C. M. Carmaggi G. De Luca and A. Tundo J.C.S. Perkin II 1972,412. ’ J. M. Bonnier and J. Court Bull. SOC. chim. France 1972 1834. J. T. Wang and F. Williams J. Amer. Chem. SOC.,1972 94 2930. S.Nagai S. Ohnishi and I. Nitta J.C.S. Perkin II 1972 379. P. H. Kasai J. Amer. Chem. SOC. 1972 94 5950. ’ P. H. Kasai and D. McLeod jun. J. Amer. Chem. SOC. 1972 94 720. J. E. Bennett and A. Thomas Nature 1962 195 995. Free-radical Reactions and Electron Spin Resonance Spectroscopy to a cold finger thus trapping these reactive radicals in an inert matrix. Like phenyl the pyridyls appear to be a-radicals. H \ HCZC-C=CH % C=C=C=C-H / H (1) The usefulness of freezing-out radicals in an inert matrix was again demons- trated in the pyrolysisg of iodoacetic acid. According to the conditions one might see methyl and/or formyl which are secondary radicals the primary radical (2) being seen only after initial pyrolysis at a relatively low temperature.9" eH3 + co2 ICH2C02H % dH,C02H (2) 2eHO + H. The 'universal chlorine or bromine abstractor' triethylsilyl formed by the action of t-butyl radicals on triethylsilane has been used with good effect to obtain many halogenated alkyl radicals.' As regards thc a-halogenated methyl radicals almost every combination from CH to CF ,to CCl, and to kMe has been achieved."" Of particular interest are the 13C coupling constants in the fluorinated methyl radicals which vary from 272 G in CF to 38 G in CH3 indicating a much higher s character in the odd-electron orbital in the former case and suggesting that the geometry of these radicals changes from pyramidal in the case of CF to planar in the case of CH,. In an analogous investigation of radicals of the type (SiMe,)$iMe,-, the 29Si splitting changed from 183 G for x = 0 to 65 G for x = 3 presumably show- ing that the steric hindrance of the larger groups pushes the molecule closer to a planar configuration.In a somewhat similar investigation of fl-fluorinated ethyl radicals,lob the "F coupling constant was much smaller in the case of CH,CF (29.8 G) than with CH,CHF (49.5G) or with CH2CH,F (45.4G) and the spectra all showed temperature dependence and the broadening of lines arising most probably from restricted rotation about the carbon-carbon bond. Similar effects are observed when larger groups (e.g. trialkyl-silyl or -germanyl) are attached to the P-carbon atom.' 3 Neutral Radicals of Group IV Elements We have already seen that hydrogen is easily abstracted by t-butoxyl radicals from the trialkylsilanes and the resulting radical whose spin density is localized P.H. Kasai and D. McLeod jun. J. Amer. Chem. SOC.,1972,94 7975. lo (a)J. Cooper A. Hudson and R. A. Jackson Mol. Phys. 1972,23 209; (b)D. J. Edge and J. K. Kochi J. Amer. Chem. SOC.,1972 94 6485. IT. Kawamura P. Meakin and J. K. Kochi J. Amer. Chem. SOC.,1972 94 8065. 192 W. T. Dixon mainly on a relatively electropositive atom is attracted in a sense to electro- negative atoms such as C1 or Br rather than to hydrogen for example. Thus in contrast to methyl which tends if anything to abstract hydrogen the metalloid nature of silicon makes attack on halogen or oxygen more likely. In fact under favourable conditions chain reactions with di-t-butyl peroxide may take place l2 as is the case with a variety of diarysilanes (3).A rather similar type of chain Ph,SiH + BuO-+ Ph,SiH Buzoz) BuO. (3) process may also occur with the analogous tin compound and certain alkyl halides e.g. (4),once the reaction has been ~tarted.'~ Since the main organic product of this reaction is l,l,l-triphenylethane it seems that the chain involves ~~ o Ph,CCH,CI ~~ Ph,CHCH,Ph :+ Ph,CCH,~~ (4) 0.23 19 halogen abstraction by the stannyl radical and hydrogen abstraction by the alkyl radical (5). This is one example of homolytic substitution and it seems that the process is accelerated by electronegative ligands on the tin according to results Ph,CCH,Cl Bu3Sn+ Ph,CkH + Bu,SnCl (5) LBu SnH Ph,CCH + Bu,Sh of photolysis of di-t-butyl peroxide and chlorinated stannane derivatives l4 (which is in fact an S,2 process).These substitution reactions on tin have been used to investigate the inter- conversions between isomeric radicals,' 5316 e.g. the conversion of (6) into (7) and hex-1-ene (8) methylcyclopentane l2 S. L. Sosin V. P. Alekseeva and V. V. Korshak Doklady Akad. Nauk S.S.S.R.,1972 203 129. " M. L. Poutsma and P. A. Ibarbia Tetrahedron Letters 1972 3309. l4 A. G. Davies B. P. Roberts and J. C. Sciano J. Organometallic Chern. 1972 39 C55. l5 C. Walling and A. Cioffari J. Amer. Chem. SOC.,1972,94 6059. l6 C. Walling and A. Cioffari J. Arner. Chem. SOC.,1972 94 6064. Free-radical Reactions and Electron Spin Resonance Spectroscopy (8).Evidently when the competition between cyclization and further hydrogen abstraction favours the latter process (because the concentration of.the hydrogen source R3SnH is large) there is not enough time to form the cyclopentylmethyl radical which eventually would give rise to the cyclic product. Cyclization to the six-membered ring is apparently unfavourable here. More general studies of radicals of the type &lR have been ac~ornplished”~’~ and various kinetic experiments have been attempted using these systems. 18*19 For example relative rates of hydrogen abstraction from alkylated silanes (9) by CC13 have been found R’,SiH initiator) R’,Si. S-CCI (9) R’,SiV y,SiH R’,Si- R2,Si. cc\ JCCI .CCl by analysis of the products obtained say in carbon tetrachloride.Since all of the silicon radicals attack the solvent there is a continuous supply of tCl and so there is always a straight competition taking place. The rates of dimerization (or self-reaction) of the radicals of the type MMe (M = C Si Ge or Sn) and more complicated types in a system in which a chain reaction does not take place” have been measured using e.s.r. The chemical system is the familiar But202 + HMMe and the radical (10)production is kept But202!% 2Bu‘O. HMMe P -MMe (observed) (10) 1 (MMed2 going by U.V. irradiation. The irradiation is continually ‘chopped’ and the light- chopping is synchronized with the corresponding signal amplitude and ‘collected’ by a computer which makes it possible to measure the absolute decay rate.This is the technique developed by Weiner and Hammond.” The results are that the larger the radicals are the slower they decay suggesting that the reaction involved is largely diffusion-controlled. The kinetics are as expected second order. Another example of the tendency of the radicals of type &lR to attack electro- negative sites is their reaction with oxygen leading to the formation of peroxyl radicals (11).21 The order of stability of these peroxyl radicals as observed by e.s.r. is Pb > Sn > Si with Ge somewhere in between. But202 Me3M % Me,MOO. (11) M = Pb Sn Si or Ge J. E. Bennett and J. A. Howard J.C.S. Perkin II 1972 322. ’* G. B. Watts and K. U. Ingold J. Amer. Chem. SOC.,1972 94 491.l9 L. H. Sommer and L. A. Ulland J. Amer. Chem. SOC.,1972,94 3803. ’O S. A. Weiner and G. S. Hammond J. Amer. Chem. SOC.,1969 91 968. ’‘ J. E. Bennett and J. A. Howard J. Amer. Chem. SOC.,1972,94 8244. I94 W. T. Dixon 4 Decomposition of Organic Peroxides Some interesting work on cage effects has appeared showing that the course of the radical decomposition of acyl peroxides depends on the viscosity of the solu- tion and hence on whether the radical pair initially formed can diffuse easily out of the solvent cage. This has been investigated in solution22 by comparing the products when the solution contains a mixture of benzoyl peroxide and acetyl peroxide with those obtained when it contains only the mixed peroxide MeCO- O,.OCPh.In a parallel type of experiment in the solid phase,23 the mixed per- oxide gave toluene and methyl benzoate but none of the products found in the liquid phase at 60 "C. Labelling with '*O showed that the peroxy oxygen was favoured in coupling with methyl. Presumably the radicals once formed are kept together in a cage so we would expect Scheme 1 to apply to the main course Ac'~O-'~OBZ % MeCO"0-+ PhCO"0. 1 CH,+ CO PhCO"OCH3 L Ph. + COz FH PhCH Scheme 1 of the reaction. However it cannot be taken for granted that decompositions of this class of compound are always radical processes for the decompositions of cumyl peracetateZ4 and of cumyl peroxide or its potassium salt,25 are mixed radical-ionic reactions depending on the solvent.So too is the reaction between benzoyl peroxide and triethylamine26 in the absence of oxygen which has been shown to involve radicals in part by the use of the nitroxide radical (12). An I 0' analogous reaction with dimethyl ~ulphide,~' which is first order with respect to both the sulphide and benzoyl peroxide and which can become explosive is apparently a purely ionic process. 22 W. A. Pryor E. H. Morkved and H. T. Bickley J. Org. Chem. 1972 37 1999. 23 N. J. Karch and J. M. McBride J. Amer. Chem. SOC.,1972 94 5092. 24 J. E. Leffler and F. E. Schrivener jun. J. Org. Chem. 1972 37 1794. z5 N. A. Sokolov L. G. Usova and V. A. Shushunov Zhur. org. Khim. 1972 8 751. I6 B. M. Sogomonya and N. M. Beileryan Uch. Zap. Erevan. Unitl. Estestv. Nauki 1970 3 148." W. A. Pryor and H. T. Bickley J. Org. Chem. 1972,37 2885. Free-radical Reactions and Electron Spin Resonance Spectroscopy 195 5 Radicals Containing Quadrivalent Phosphorus The chemistry of alkylated phosphorus compounds parallels that of the organic chemistry of the silicon group of elements rather closely. Developments in tech- niques have now made it possible to observe the intermediate radicals in the reactions of the t-butoxyl radical with alkylated phosphines2* or phosphites.28- 30 The products and radicals formed can be explained by Schemes 2 or 3. In a num- ber of cases both the intermediate phosphoranyl radicals and the alkyl radicals R,POBu' + R. (displacement i.e. substitution) R3P + -0Bu' -+ R,POBu' radical R3P0 + Bu'.(oxidation) Scheme 2 (RO),POBu' + .OR (substitution) (RO)3P+ -OBI? -+ (RO),kOBu' /-(RO),PO + But. (oxidation) (RO),PO(OBu') + R. (oxidation + substitution) Scheme 3 formed from their decomposition have been observed by e.s.r. and as one might expect bulky groups like t-butyl make the best leaving groups.29 It is interesting that it is not apparently necessary to postulate phosphoranyl radical intermediates to explain the kinetics of the autoxidation of trimethyl phosphite triphenylphosphine or methyl diphenyl ph~sphinite,~ ' although steps where they could be formed are written as in Scheme 4. In fact one would have thought that phosphoranyl radicals would be important in autoxidations RO,. + R3P -+ R,PO + RO.RO. + R3P --+ R3P0 + R-Scheme 4 P. J. Krusic W. Mahler and J. K. Kochi J. Amer. Chem. Soc. 1972 94 6033. 29 A. G. Davies D. Griller and B. P. Roberts J.C.S. Perkin II 1972,993. 30 W. G. Bentrude E. R. Hansen W. A. Khan and P. A. Rogers J. Amer. Chem. SOC. 1972,94,2867. 31 Y. Ogata and M. Yamashita J.C.S. Perkin II 1972 730. 196 W. T. Dixon when they are formed because they react easily with molecular oxygen to give peroxyl radicals (13) which can be observed by means of their e.s.r. ~pectra.~',~~ X,P* + 0 -+ X,POO. (13) The structure of these phosphoranyl radicals is itself of great interest.28 According to theory33 the ligands are at four corners of a trigonal bipyramid and in the case of PF (14) the large "F coupling comes from the two fluorines in the axial positions.This helps the assignments in the alkylated radicals e.g. (15). up = 1330 G 'F ap = 631.5 G I I 'CH3 4.05G large 19F -+ F H 140G splitting The other striking thing about these radicals is the very large phosphorus hyper- fine splitting. The corresponding radicals of arsenic3 also have large splittings from the 'central' nucleus (1900G) showing that 20% of the odd-electron density is associated with the arsenic 4s atomic orbital. The chemical behaviour of these compounds is also similar to that of phosphoranyl radicals except that from the nature of the products it seems that the displacement is favoured e.g. Scheme 5. Ph,As + Bu'O. + Ph3AsOBu' 3 Ph,AsOBu' + Ph. Scheme 5 Similar results have been found with other simple phosphorus radicals P020H7 HP02' and PhP02-) in aqueous s~lution.~' These radicals are formed from the corresponding anions by hydrogen abstraction by hydroxyl in the titanium(111kH~0 system.They cannot really decompose in the manner of the phosphoranyl radicals just discussed but can add to double bonds and nitroalkane mi-anions to give appropriate radicals. The large 31Psplittings in these phosphorus(1v) radicals (-500 G) show that the general structure must be similar to the trigonal bipyramid with the odd electron confined mainly to the equatorial plane. The absence of splitting from the phenyl protons in PhPO,; indicates that the phenyl group is also probably joined to the phosphorus by a bond in the equatorial plane (in view of the other results just discussed) though 32 G.B. Watts and K. U. Ingold J. Amer. Chem. Soc. 1972 94 2528. 33 J. Higuchi J. Chem. Phys. 1969 50 1001. 34 E. Furimimsky J. A. Howard and J. R. Morton J. Amer. Chem. SOC., 1972 94 5932. j5 B. C. Gilbert J. P. Larkin R. 0.C. Norman and P. M. Storey J.C.S. Perkin IZ 1972 1508. Free-radical Reactions and Electron Spin Resonance Spectroscopy this is not exactly the view put forward.35 There are evidently some interesting stereochemical problems to be solved in these phosphorus radicals. 6 Phenoxyl and Semiquinone Radicals Various new ways of generating phenoxyl radicals have been found. In the first place since hindered phenols give stable radicals very easily they may be used as spin traps in any reactions in which there are more active radicals.36 As one might expect phenoxyl or related radicals may be obtained from derivatives of the keto-forms of phenol by appropriate choice of reagent,37*38 e.g.(16). \, (or ortho-isomer) The excited states of diketones have also been used not only to produce the phenoxyl radical from phenol itself but in such concentrations that the rate of dimerization can be found.39 In these experiments solutions of the diketone and phenol were photolysed and steady concentrations of radicals were observed by e.s.r. (Scheme 6). Again the rotating-sector method already mentioned was used to look at the relative rates of decay of these radicals so that termination constants could be measured. MeCO I +PhOH 3 PhO.+MeC(0H)COMe MeCO \ * / e.s.r. spectra observed Scheme 6 Irradiation of phenol in alkaline solution can give appreciable concentrations of phenoxyl dire~tly.~' This involves excitation of the phenoxy-anion most probably so that the first part of the reaction appears to be as shown in Scheme 7. Scheme 7 36 C. M. Camaggi and M. J. Perkins J.C.S. Perkin II 1972 507. 37 D. K. Rasuleva A. A. Volod'kin V. V. Ershov N. N. Bubov S. P. Solodovnikov A. P. Prokof'ev and S. G. Kukes Izuesr. Akud. Nuuk S.S.S.R.,Ser khim. 1972 1446. 38 A. P. Prokofev S. P. Solodnikov A. A. Volod'kin and V. V. Ershov Doklady Akad. Nauk S.S.S.R. 1972 204 1114. 39 L. R. Mahoney and S. A. Weiner J. Amer. Chem. Sac. 1972 94 585. 40 M. Coccivera M. Tomkiewicz and A.Green J. Amer. Chem. Soc. 1972 94 6598. 198 W. T. Dixon Now the e.s.r. spectra of semiquinone and even hydroxylated semiquinones are also observed in this system (depending on the pH) so their formation is ration- alized in Scheme 8 (in D,O for clarity). Once the polyhydroxylated benzenes H OD OD Scheme 8 are formed they would quickly react with the relatively reactive phenoxyl radicals to give corresponding semiquinones (Scheme 9). Presumably by the same token quinones could easily arise in a similar way since the steps leading to the other 4+/JJ+$+b \ \ \ \ 0-0-Scheme 9 radicals have been confirmed by e.s.r. work on the autoxidation of substituted hydroquinones (Scheme in the presence of oxygen these last steps take place with 0 as the oxidizing agent.Similar reactions take place in alcoholic 0' 0' 0 0-0-0-etc. t 0-Scheme 10 P. Ashworth and W. T. Dixon J.C.S. Perkin II 1972 1130. Free-radical Reactions and Electron Spin Resonance Spectroscopy sol~tions,~~,~~ the main difference being that (a) alkyl groups cause greater complexity of spectra which can increase the difficulty in analysing overlapping spectra though the ENDOR technique can be of help,43 and (b) the alkoxy- groups seem to add on in pairs so that the radicals observed starting with hydro- quinone are (17) (18) and (19). In these various reactions it is only the step (A) of Scheme 8 which is not imme- diately obvious as a possibility. It was introduced tentatively to account for the fact that during irradiation of phenolic solutions at pH 11 in DzO,both the aryl protons and those of the HDO formed gave emission n.m.r.spectra.40 The forma- tion of a radical pair of admittedly short lifetime of the type implied by step (A) could account for the spin polarization of both types of proton. This work shows how potentially powerful a combination of CIDNP and e.s.r. could be in probing the details of radical reactions. 7 Semidiones and Related Radicals More development has been taking place in the study or semidiones especially those in which the two oxygen atoms are attached to a ring system.44 For example it has been shown that the cyclobutene ring is a favourable system for delocalization of the spin density to remote nuclei starting from the parent compound (20) further rings have been built on so that in (21) the remote couplings even four carbon atoms away from the oxygen are still 0.4G.The assignments have been confirmed by deuterium substitution. In the remarkable radical (22) couplings with protons five carbon atoms away from the oxygen are H 40H%oo "'4G 0.4 G H H 11.1G 11.5 G (21) (22) 42 D. C. Reitz J. R. Holkihar F. Dravnieks and J. E. Wertz J. Chem. Phys. 1961 34 1457. 43 N. M. Atherton and A. J. Blackhurst J.C.S. Furaduy II 1972 68 470. 44 G. A. Russell P.A. Whittle R. G. Keske G. Holland and C. Aubrichon J. Amer. Chem. SOC.,1972,94 1693. 200 W. T. Dixon still observable (0.25G). These radicals were made by the autoxidation of the bis(trimethylsi1oxy)alkenein strong alkali.Another feature of semidiones which has been investigated as the formation of gegenions with metal ~ations.~’,~~ When acetol for example is treated with base (MOBu‘; M = alkali metal) the semidiones generated half life -0.5 min are in the cis-and tr~ns-forrns.~~ The larger the metal ion the greater is the proportion of the cis-form owing to chelate formation e.g. (23). This is favoured by bulky substituents. In the case of the -0.. -0’ H -M“ Group 11 metals these chelates can be quite stable and have been observed as a third type of radical when diketones are reduced by CH,OH or C02after these have been generated by the Ti”’-H,O,-substrate system in the first mixing chamber of a multi-flow system (Scheme 1 l).46 \/ + /c=c\ -0\\ 0-‘‘\,~,’/‘ chelate observed by e.s.r.Scheme 11 A number of perfluorinated semidiones and related radicals have been produced by electrolytic red~ction,~’ e.g. (24)-(29). For some reason these radicals are much more stable than their hydrogen analogues. (CF,),CO (CF,CO) (CF,),CS; (24) (25) (26) 45 G. A. Russell and D. F. Lawson J. Amer. Chem. SOC.,1972,94 1699. 46 A. J. Dodds B. C. Gilbert and R. 0.C. Norman J.C.S. Perkin II 1972 2053. 47 G. A. Russell G. L. Gerlock and G. R. Underwood J. Amer. Chem. SOC.,1972 94 5209. Free-radical Reactions and Electron Spin Resonance Spectroscopy 201 8 Sulphur Radicals Not every one-electron reducing agent gives rise to semidione-type radicals with diketones.Thus whereas the tetraketone (30),derived from ninhydrin reduced by dithionite with sulphuric acid gives the radical (31) biacetyl instead gives a t A. 0.. ‘H,o sulphur radical via the shown in Scheme 12. The big difference between the chemistry of sulphur and that of oxygen becomes apparent here for rather than form multiple bonds sulphur atoms tend to combine with each other. Thus S20,2-% H2S (MeCo)z’ MeCOCH(0H)Me so Me-C-SH H,SO MeC-SH II Me ‘oxiiati& -sH] ‘oxidation MeC-SH [“en] Scheme 12 whereas simple reduction of diketones gives semidiones (negative ions) under the above conditions 1,2-dithiole radicals (32),which are positive ions are produced. a-Hydroxy-ketones Na,S or Na,SO a-Diketones or Na,S,O ’ HW (32) (overall oxidation) The coupling constants of the nearest (p-)methylene protons in the bicyclic radicals (33) are only about a third of those in the semidiones (34),suggesting that the spin density is more on the sulphur in the former than it is on the oxygen in the latter.48 G. A. Russell R. Tanikega and E. R. Jalaty J. Amer. Chem. SOC.,1972 94 6125. 202 W. T. Dixon Rather similar types of radical (35) may be obtained by electrolytic reduction of dithi~lylium~~ ions in acetonitrile. When the temperature is lowered in this system the e.s.r. signal diminishes but presumably without appreciable broaden- ing. On reheating it reappears and this effect is ascribed to a dimerization- dissociation equilibrium. The thianthrene molecule ion (36) reacts with ammonia to give a cation” in which two thianthrene moieties are connected via sulphur to a nitrogen atom.N II QSD ‘s \ (210,-When there is a substituent in the 3-position in thiophen the 2-position is activated towards attack by phenyl radicals (relative to the 4-position) (Scheme 13).51 This tendency appears to hold whatever the nature of substituent X. It would appear from this that polar effects are not the most important ones. main product Scheme 13 Another rather unexpected result is that thiazol-2-yl radicals,’ formed by pyrolysis or photolysis of 2-iodothiazole attack aromatic compounds to yield ortho meta para ratios of products in the opposite sense to those accustomed in electrophilic substitution i.e. anisole 70 15 13 ; nitrobenzene 60 12 27.Thus para-substitution is relatively favoured by what are usually called ‘de- activating’ groups. 49 C. T. Pedersen K. Bechgaard and V. 0.Parker J.C.S. Chem. Comm. 1972 430. 50 H. J. Shine and J. J. Shine J. Amer. Chem. SOC., 1972 94 1026. ’’ C. M. Camaggi G. DeLuca and A. Tundo J.C.S. Perkin II 1972 1594. 52 G. Vernin R. Jauffred C. Ricard H. M. J. Dou and J. Metzger J.C.S. Perkin I[ 1972 1145. Free-radical Reactions and Electron Spin Resonance Spectroscopy Diphenyl thioketone reacts with Grignard reagent^'^ to give a thioether via a long-lived free radical (37) whose e.s.r. spectrum shows that it is of the benz- hydryl type. This implies attack by R-(alkyl) on the sulphur atom and other Ph,C=S RSCPh -+ RSCHPh (37) studies have further confirmed that sulphur is in general open to attack by radicals.For example the high yields of alkene which are found when episulphides (38) react with methyl indicate that the main process is abstraction of sulphur by the alkyl radical. product The products of photolysis of methyl ethyl sulphideS5 in order of their yields include MeH > MeSSEt > EtSSEt > EtH together with all the other com- binations showing the presence of methyl ethyl and the two sulphide radicals. Some interesting effects are observed when the various isomeric dithienyl- ethylenes (39) are reduced in alkali.56 Mixtures of the e.s.r. spectra of rotational isomers can be resolved with the aid of a computer. When anions of thiophen in which there are one or two CHO groups in the 2-or 5-positions are generated by photolysis of the parent molecule in alkaline solu- ti~n,'~ the aldehyde group does not rotate freely according to the e.s.r.evidence but is effectively locked. 9 Aromatic Radical Ions As with other well-worn fields it becomes increasingly difficult to say anything new or unexpected about aromatic radical ions. There still remain however many radicals which have incompletely explored structures and which are intrinsically interesting. Some of these are the annulene radical ions for they give information about the structure of the parent molecule. One example is 53 M. Dagonneau J. F. Hemidy D. Cornet and J. Vialle Tetrahedron Letters 1972,3003. 54 E. Jakubowski M.G. Ahmed E. M. Lown H. S. Sandhu R. K. Gosavi and 0. P. Strausz J. Amer. Chem. SOC.,1972 94 4094. 55 D. R. Tycholitz and A. R. Knight Canad. J. Chem. 1972,50 1734. '' L. Lunazzi A. Mangini G. Placucci P. Spagnolo and M. Tiecco J.C.S.Perkin ZZ 1972 172. 57 L. Lunazzi G. F. Peduli M. Tiecco C. Vincenzi and C. A. Veracini J.C.S. Perkin ZZ 1972 751. 204 W. T. Dixon [16]ann~lene,~* which has several possible structures having different combina- tions of cis-and trans-bonds. The anion can be generated electrolytically and in the e.s.r. spectrum the proton hyperfine splittings are approximately 4.0 G (8 protons) 1.0 G (4protons) and 0.75 G (4protons). This is consistent with an ‘85’ structure (40). The convention is to designate alternate bonds as either (40)[16]-85-Annulene cis (0),e.g.bond ‘a’ or as trans (l) e.g.bond ‘b’. Starting with cis the collection of zeros and units are then taken as a binary number and this is converted into decimals. In this case the binary number starting at ‘a’ is 01010101 = 26 + 24 + 22 + 1 = 85. The smaller coupling constants are assumed to be negative if we are to take the molecule as being planar and the Q-value is not to be too different from those usually applied to aromatic radical ions. The effect of bridging forces the annulenes out of the planar configuration and this is reflected in the coupling constants for example in the two bridged [14]annulenes (41) and (42).59 The .&.8G 4.5 G 3.5G (42) cis bridges (41) trans bridges different patterns of coupling constant can be ascribed to different degrees of puckering in the rings.Similarly in 7,12-dihydropleiadene anion (43) the coup- ling constants of the methylene protons give a clue about the conformation of the seven-membered ring.60 Here ENDOR has been used to help in the analysis of the e.s.r. spectrum. 58 J. F. M. Oth H. Baumann J. M. Giller and G. Schroder J. Amer. Chem. SOC.,1972 94 3498. 59 F. Gerson K. Mullen and E. Vogel J. Amer. Chem. SOC.,1972 94 2924. 6o R. D. Allandoerfer P. E. Gallagher and P. T. Lansbury J. Amer. Chem. Soc. 1972 94 7702. Free-radical React ions and Electron Spin Resonance Spectroscopy Kinetic studies of reactions of radical ions can be made using the linewidths in e.s.r.spectra as in the electrochemical oxidation/reduction of aromatic species where the broadening of lines arises from exchange between the radical ion and the parent molecule.6 For fast reactions the stopped-flow technique in conjunc- tion with the electronic spectra can be applied e.g. as in protonation of perylene radical ions.62 Slower reactions such as the reaction between the anthracene negative ion and can be followed in a more leisurely fashion. Linewidth alternation in the e.s.r. spectrum of a calicene derivative (44) has been used to arrive at a value of the barrier against rotation of the isopropyl groups of about 1.5 kcal mol- 1.64 In this and related radicals the rings are not coplanar. 10 Nitro Radical Anions The main interest in nitro radical ions is now as an aid in exploring different types of chemical structure.In fact the existence of the easily reduced nitro-group in a molecule enables a ‘spin probe’ to be introduced. Good examples of this are investigations of the cyclopropyl groups :65,66 when it is attached to the 4-position of the nitrobenzene negative ion (45) the p-and y-coupling constants are 1.39 G and 0.27 G re~pectively.~~ On introduction of two methyl groups into the 3-and 5-positions (46),the 8-splitting rises to 5.68 G showing that steric hindrance has pushed the cyclopropyl group as a whole into a more favourable position. Insertion of an acetylenic linkage between the aromatic ring and the cyclopropyl 61 B. A. Kowert L. Marcoux and A. J. Bard J. Amer. Chem.SOC.,1972 94 5538. 62 G. Levin C. Satphen and M. Swarc J. Amer. Chem. SOC.,1972 94 2652. 63 S. Bank and B. Bockrath J. Amer. Chem. SOC.,1972 94 6076. 64 S. Niizuma S. Konishi H. Kokubun and M. Konizumi Chem. Letters 1972 643. 65 L. M. Stock and P. E. Young J. Amer. Chem. SOC.,1972 94 7686. 66 C. E. Hudson and N. L. Bauld J. Amer. Chem. SOC.,1972,94 1158. 206 W. T. Dixon NO,: i Me Me H 1.39G 5.68G AH (45) (46) group (47) effectively removes strong steric interactions and the whole group rotates freely down to quite low temperatures.66 (47) An attempted study of the effects of conformation on 31Pcoupling constants6’ in radical ions of the type (48) where X = 0or S was not completely convincing (48) in view of the large number of factors involved.A good way of producing ali- phatic nitro-anions is to employ the aci-anion of nitromethane (49) as a spin trap.68*69 The resulting radicals have been examined by e.s.r. for a very wide selection of radicals X. produced by Ti3+-H,0 and other systems and again X. + CH,NO,-D XCH2N02; (49) the nitro-group can be regarded as a useful spin probe. Other mi-anions of nitroalkanes or oximes have been allowed to react with NO or NO to give radical anions (Scheme 14).69 0-NO +/ / NO+>C=N + >C \ \ 0-NO aci-anion nitronitroso radical ion Scheme 14 67 W. M. Gulick jun. J. Amer. Chem. SOC.,1972 94 29. 68 B. C. Gilbert J. P. Larkin and R. 0.C. Norman J.C.S. Perkin II 1972 1272. 69 G. A. Russell R. K. Norris and A.R. Metcalf J. Amer. Chem. SOC.,1972 94 4959. Free-radical Reactions and Electron Spin Resonance Spectroscopy 11 Nitroxide Radicals One might almost say that these have become the most popular class of radicals. This is largely because many of them are easily formed are quite stable and are not charged and so can be used in neutral solutions or in non-aqueous solvents. Thus nitroso-compounds (50)can be used as spin traps.70 The resulting nitroxide R \ R-N=O + X. N-0 / (50) X radicals may well have structures of interest and this is another useful role of the nitroxide grouping as a spin probe. Thus di-t-butyl nitroxide has been used to study micelles in detergent solutions' and various nitroxides have been used to study the pulse radiolysis of cytosine thymine guanine and adenine.72 In this case the fact that the nitroxide (51)does not react with the radical formed by adenine shows that the site of the odd electron must be relatively hindered.I 0' Being stable magnetic species nitroxides may be used to quench excited states c.g. of stilbene or na~hthalene.~~ They have also been used to produce 13C contact chemical shifts in the n.m.r. spectra of aromatic molecules.74 As with the nitro-group the main interest in nitroxides will probably be that they will introduce unpaired spin into structures of interest e.g. attached to a seven- membered ring as in (52).7 Similarly in azabicycloheptyl N-oxyl derivatives (53) 0' N\ Bu' (52) 2-t-butylaminotropone N-oxyl radical; aN= 12.7 G,a4 = a6 = 0.93 G,and a3 = a5 = a = 1.86G 'O C.M. Camaggi R. J. Holman and M. J. Perkins J.C.S. Perkin II 1972 501; A. L. Bluhen and J. Winstein J. Org. Chem. 1972 37 1748. N. M. Atherton and S. J. Strach J.C.S. Furaduy II 1972 68 374. '2 T. Brusted H. Bugge W. B. G.Jones and E. Wold Internat. J. Radiat. Biol. 1972 22 115. l3 R. A. Caldwell and R. E. Schwerzel J. Amer. Chem. SOC.,1972 94 1035. I. Morishima K. Kawakami T. Yonezawa K. Goto and M. Imanari J. Amer. Chem. SOC.,1972 94 6555. -'T. Toda E. Mori and K. Murayama Bull. Chem. SOC.Japan 1972,45 1852. 208 W. T.Dixon 0’ (53) the coupling constants have yet to be explained sati~factorily.~~ In this case zero coupling with bridging hydrogens shows that they are in the nodal plane.12 Radicals related to Amines Photolysis of halogenoamines gives rise to the corresponding amino-radicals. For example if conducted in sulphuric acid solution dialkylaminium cations (54) which are isoelectronic with the corresponding simple alkyls are obtained and can be detected by means of their e.s.r. ~pectra.~’ The interesting thing about the e.s.r. parameters of these radicals is that they are nearly the same as in their carbon analogues i.e. am= 23 G aB= 34 G and aN= 19 G. The simple amido-radicals (55) can also be made from the photolysis of the ~hloramide.~~ The coupling constants indicate that these radicals are of a n-type as are the simple amino-radicals. R / o=c \ NCH (55) UN = 15 G a,+= 29 G It is interesting to compare these results with those from the y-irradiation of N-halogenosuccinimide single crystals.79 The trapped radicals have the odd electron in the g* orbital of the N-halogen bond (56).It may be that in the solid dissociation is prevented and that some similar type of radical is a precursor to CH,-C (56) 76 D. J. Kosman Tetrahedron Letters 1972 3317. 77 W. C. Danen and R. C. Rickard J. Amer. Chem. Sac. 1972,94 3254. 78 W. C. Danen and R. W. Gillert J. Amer. Chem. Soc. 1972,94 6853. ’9 G. W. Nielson and M. C. R. Symons J.C.S. Faraday II 1972 68 1582. Free-radical Reactions and Electron Spin Resonance Spectroscopy 209 amino- or amido-radicals in solution. The interaction of solvated electrons (sodium in a large excess of liquid ammonia) with amides or thioamides leads also to mono- and di-negative ions (57) and (58) observable in a flow system by e.s.r.'O In the dianions the e.s.r.spectra show that there is restricted rotation about the C-Ar bond perhaps owing to repulsion of the two negative charges (one of which is associated with the ring approximately). ArCONH 5 [ArCONH,]-or [ArCONH]'-(57) (58) A kinetic study of the reaction between chlorine dioxide and triethylene- diamine" by means of the stopped-flow method shows that the red intermediate must be an aminium cation radical. The mechanism is as shown in Scheme 15. n n H I I +2HCHO H Scheme 15 In the photolysis of hexafluoroacetone imine8' the products analysed by mass spectrometry are in order of abundance CF,CH > CF,H > H, and this is explained by the sequence shown in Scheme 16.A somewhat related reaction [(CF,),C=NH]* --* CF + CF,k=NH JcF~c=NH ~w,~=NH CF,H + CF3CN 2CF,CN + H Scheme 16 the isomerization of oximethi~nocarbamate,~ yields a high enough concentra- tion of the iminyl radical intermediate (59) for it to be detected by e.s.r. Where there is an ortho-fluorine substituent the "F splitting is relatively large (2.3G) and this is rationalized by a through-space interaction (60). I. H. Elson T. J. Kemp and T. J. Stone J.C.S. Furuduy If 1972 68 1452. G. T. Davis M. M. Demet and D. H. Rosenblatt J. Amer. Chem. SOC., 1972,94,3321. 82 F. S. Toby S. Toby and G. 0.Pritchard J. Amer. Chem. SOC.,1972,94,4441. 83 R. F.Hudson A. J. Lawson and E. A. C. Lucken J.C.S. Chem. Comm. 1972 721. 2 10 W. T,Dixon NMe, / 2 Ar,C=N-OC -P Ar,C=N'+ '0-C \\ \ "S II (59) NMe 1 I '0 NMe (60) In the e.s.r. spectra of N-t-butylanilino-radicals,84the ring-proton splittings are relatively insensitive to substituents. In the case of the corresponding N-methylanilino-radicals (61) it is found that loss of nitrogen from the starting azo-compound is much greater when there is an ortho-methyl group on the aromatic ring. Also with the ortho-methyl compound the reaction is virtually independent of the para-substituent X implying that efficient overlap between the half-filled orbital on the nitrogen and the aromatic n-system is important in determining the rate of decomposition.13 Delocalization of Odd Electrons through o-Bond Frameworks Although already implied and accepted as the explanation of fl coupling constants in ethylvinyl etc. a-spin delocalization as it has been called,85 is more important than has previously been thought. Its presence has been deduced in nitro- aromatic radicals,86 nitro~ides,~~ or in or-substituted benzyl radical^,^' when steric hindrance pushes the plane of the ring away from that of the p-orbital of the extra-nuclear atom which has the largest spin density [see (62)]. This is shown S. F. Nielsen R. T. Landis L. H. Kiehle and T. H. Leung J. Amer. Chem. SOC.,1972 94 1610. W. J. VanderHoek B. A. C. Rousseluw J. Schmidt W. G. B. Huysmans and W. J. Mijs Chem. Phys. Letters 1972 13,429.D. R. Geske J. L. Ragle M. A. Bambenek and A. Balch J. Amer. Chem. Soc. 1964 86 987. A. Calder A. R. Forrester J. W. Emsley G. R. Luckhurst and R. A. Storey Mol. Phys. 1970 18 481. Free-radical Reactions and Electron Spin Resonance Spectroscopy 211 plane for example in the e.s.r. spectra of substituted methoxycarbonylcyanobenzyl radicals made by the thermolysis of 1 2 diarylsuccinonitriles (63). There is an increase in the coupling constants of the rn-protons relative to the opprotons CO,R I c Ar-C-CN I Ar -C -CN b I C0,R R' RbR2 (63) and an overall decrease in spin density on the ring. Whereas the spin densities on the 0-and p-positions decrease rapidly with the dihedral angle the m-proton splittings do not change much because even in the perpendicular conformation they are of the order of 2G.This has already been confirmed" by the e.s.r. spectrum of the benzoyl radical (64) which shows a major splitting from the rn-protons (1.16 G),which are equivalent indicating that this is a a-radical but that a rapid flip-over is occurring. The assignments here have been made relative to the P-splittings in the vinyl radical. H. '-2G' After a number of vain attempts it seems that the e.s.r. spectra of 1-and 2-adamantyl radicals (65) and (66) have been ob~erved.~~.~' As expected the patterns of coupling constants in these radicals are very different from each other H3.6G 6.6 G (6protons) H 0.8G (3 protons) H 3.1 G (3 protons) 20.5 G (1 proton) (65) 1-adamantyl (66) 2-adamantyl 88 P.J. Krusic and T.A. Rettig J. Amer. Chem. SOC.,1972 94 722. '' P. J. Krusic T. A. Rettig and P. von R.Schleyer J. Amer. Chem. SOC.,1972 94 995. 90 R. V. Lloyd and M.T.Rogers Chem. Phys. Lefters 1972 17 428. 212 W. T. Dixon but both show splittings remote from the site of the odd orbital. In the case of l-adamant~l~~ the low P-coupling constant (6.6 G) is attributed to the pyramidal conformation at the 1-position. The spectrum of 2-adamantylgO was badly resolved so the smaller coupling constants were obtained using the ENDOR technique. The assignments agreed with appropriate INDO calculations. An interesting point here is that 1-adamantyl is longer lived in an oxidizing atmo- sphere than its 2-isomer,” in spite of its being a a-radical.This perhaps shows that o-radicals are not necessarily more reactive than similar n-radicals. In fact there is evidence to show that negative groups attached to the carbon of a methyl radical tend to push it out of planarity.92 This is shown up by the way Q-and B-coupling constants do not run parallel to each other. Many simple radicals will have appreciable a-character according to this. As a final example the anion radical from a spirotetraenedione has been obtained93 and the coupling constants have been assigned as shown in (67). The L (67) splittings were assigned with reference to related radicals and also confirmed by calculation. Evidently the radical is not symmetrical as one might have expected by comparison with the semiquinone type radical from 4,4‘-dihydroxybiphenyl but then in this case the two rings are mutually perpendicular.One consequence of this is that if it were symmetrical there would be two degenerate orbitals into which the odd electron might go ;hence according to the Jahn-Teller theorem the molecule would distort to remove this degeneracy. The radical therefore resembles a cyclohexadienyl type with the spin localized on one ring [see (68)J. The 6-position in the other ring is effectively in the rn-position and has a larger spin density than the proton attached to the 5-position. This is thus another case of a-spin delocalization. 91 I. Tabushi Y. Aoyama S. Kojo J. Hamuro and 2.Yoshida J. Amer. Chem. SOC. 1972,94 11 77. 92 A. J. Dobbs B.C. Gilbert and R. 0.C. Norman J.C.S. Perkin 11 1972 786. 93 F. Gerson R. Gleiter G. Moshuk and A. S. Dreiding J. Amer. Chem. SOC.,1972 94 29 19.

ISSN:0069-3030    

DOI:10.1039/OC9726900189

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

5 Arynes Carbenes Nitrenes and Related Species By J. T. SHARP Chemistry Department University of Edinburgh Edinburgh 1 Arynes Generation.-A new low-temperature route to benzyne is provided by the decomposition of the highly unstable lithium salt (1)' When added to polar aprotic solvents this salt decomposes rapidly at or below room temperature to give benzyne which dimerizes to biphenylene in the absence of a trap or gives up to 65% of the tetracyclone adduct. This route to benzyne has the advantages of low temperature and absence of other reactants but these are somewhat offset by the extreme instability of (1). A wide range of anilines and anilides have been converted into arynes by in situ nitrosation with pentyl nitrite-acetic anhydride or p-chlorobenzoyl nitrite.2 This method provides a simple and convenient route to some arynes from readily available precursors but the yields of aryne adducts obtained vary widely depending on the substitution pattern of the amine or amide I \ Li' -N-N=N-NHTs (2) (1) precursor.Benzyne has also been generated from benzenediazonium hexa- chl~roantimonate~ by deprotonation with a variety of phosphyl compounds (2). The full report on the decomposition of N-nitrosobenzanilides (3) in carbon NO N-COT R2 I -R,h6 R$J (3) M. Keating M. E. Peek C. W. Rees and R. C. Storr J.C.S. Perkin I 1972 1315. B. Baigrie J. I. G. Cadogan J. R. Mitchell A. K. Robertson and J. T. Sharp J.C.S. Perkin I 1972 2563. Von H. Teichmann M. Jatkowski and G. Hilgetag J.prakt. Chem. 1972,314 129. 213 214 J. T. Sharp tetrachloride and benzene has now a~peared.~ In carbon tetrachloride in the absence of an aryne trap the reaction produced the corresponding benzoic acid benzoic anhydride and aryl chloride as major products. The proton of the benzoic acid originated in the aryl-diazonium system and the intermediacy of arynes was demonstrated by trapping experiments with tetracyclone. The mechanism of the aryl halide formation is not fully understood but may involve the intermediacy of the diazonium chloride. In benzene the results parallel those reported earlier for substituted N-nitrosoacetanilides. The photolysis of phthaloyl peroxide was recently reported to be a convenient route to benzyne; it has now been shown’ that its irradiation in a matrix at low temperatures also gives some benzyne but mainly the keten (4),possibly arising via (5).These species have previously been suggested as intermediates in various systems involving benzyne formation. 9,iO-Phenanthryne has been generated by the oxidation of l-amino-iH-phenanthro[9,10-d]triazoleand trapped in high yield with tetracyclone.6 However unlike benzyne generated in the same way it failed to dimerize to a biphenylene in the absence of a trap perhaps because of the instability of the tetrabenzobiphenylene. ‘Cross’-coupling reactions with benzyne and 2,3-naphthalyne were successful. The various reactions of aryl halides with nucleophilic reagents including aryne formation and particularly the non-aryne ‘halogen dance’ have been reviewed.’ cis-Hexa-i,5-diyn-3-ene (6) has been shown to undergo a thermal degenerate rearrangement involving the intermediacy of p-benzyne which is best represented as the benzene- 1,4-diyl diradical(7).* The intermediate showed typical diradical (7) rather than dipolar (8) reactivity and it was suggested that the former is the symmetry-allowed product whereas conversion into (9) is predicted to be for- bidden. D J. I. G. Cadogan D. M.Smith and J. B. Thomson J.C.S. Perkin I 1972 1296. V. Dvoiak J. Kolc and J. Michl Tetrahedron Letters 1972 3443. J. W. Barton and A. R. Grinham J.C.S. Perkin I 1972 634. ’ J. F. Bunnett Accounts Chem. Res. 1972 139. R. R. Jones and R. G. Bergman J. Amer. Chem. Soc. 1972,94,660. Arynes Curbenes Nitrenes and Related Species In an intensive investigation into the reaction of potassium t-butoxide with l-halogenocyclohexenes,9it has been shown that dehydrohalogenation to give cyclohexyne cyclohexa-l,2-diene and cyclohexa- 1,3-diene takes place.The first two intermediates react with the base to give 1-t-butoxycyclohexene and the last two undergo dimerization and cycloaddition reactions. However the reaction of (10) with potassium t-butoxide does not involve formation of a cyclohexyne or a cyclohexa-1,2-diene in the major reaction path.'O The reaction of l-lithio-2-bromocyclopentene with nucleophilic reagents such as phenyl-lithium gave products e.g. 1-phenylcyclopentene formally derived from a direct substitution but probably formed via a cyclopentyne intermediate.' ' The latter is also impli- cated as intermediate in the formation of cyclopentene by hydride transfer (1 1) from lithium piperidide and related reagents. Cycloalkynes have also been produced in the pyrolysis of cycloalkeno-1,2,3-selenadiazoles (12) via loss of selenium from the intermediate diradical (13) which also dimerizes.' The formation of approximately equal quantities of 2-methyl- and 3-methyl-butyl- ferrocene from the reaction of 2-methylchloroferrocene with butyl-lithium provides the first strong evidence for a ferrocyne intermediate.' The full report on the formation of 3,4-pyridyne by the flash photolysis of pyridine-3-diazonium-4-carboxylatehas now a~peared.'~ Alkyl-substituted 3,4-dehydropyridines have been generated by the action of potassium amide on variously substituted halogenopyridines.' The influence of the heteroatom and substituents on the orientation of the aminations is discussed. 2-Amino-3-bromo- and 2-amino-4-bromo-quinoline react with potassium amide to give 2,3- and 2,4-diaminoquinoline via 3,4-dehydroquinoline.' However the reaction of 2-bromo-4-aminoquinoline differed considerably from that of the analogous pyridine and gave no 2,4-diaminoquinoline but it was converted nearly exclusively ' A. T. Bottini F. P. Corson K.Fitzgerald and K. A. Frost Tetrahedron 1972,28,4883. lo '' A. T. Bottini F. P. Corson K. A. Frost and W. Schear Tetrahedron 1972 28 4701. G. Wittig and J. Heyn Annafen 1972 756 1. '' H. Meier and E. Voigt Tetrahedron 1972 28 187.l3 J. W. Huffman and J. F. Cope J. Org. Chem. 1971 36 4068. l4 J. Kramer and R. S. Berry J. Amer. Chem. Soc. 1972 94 8336. l5 L. Van der Does and H. J. den Hertog Rec. Trac. chim. 1972 91 1403. l6 H. J. den Hertog and D. J. Buurman Rec. Trac. chim. 1972,91 841. 216 J. T. Sharp into (14). The amination of 4-bromo- 4-chloro- and 4-fluoro-6-phenylpyrimi-dines proceeds very largely by the ANRORC mechanism which involves ring open- ing by amide ion attack at the 2-position and subsequent ring closure; however the 4-iodo-compound less activated at the 2-position reacts mainly by an EA mechanism involving a ‘2,3-pyridyne7 type intermediate. f-+q Reactions.-The facile cycloaddition reaction of benzyne with alkenes has been much studied experimentally and found to be non-stereospecific but somewhat stereoselective.Hoffmann has now calculated a potentiql energy surface for the addition of benzyne to ethylene.’* In consequence of the primarily S2 ground-state configuration of benzyne the least-motion concerted 2s + 2s approach is symmetry forbidden and the symmetry-allowed 2s + 2a approach was calculated to be at high energy due to the energetic cost of twisting the ethylene. The poten- tial energy surface was found to be complex with three distinct valleys one of which was a cul-de-sac interpreted as representing an intermediate. In the formation of such an intermediate it was suggested that the approach of the ethylene polarizes the weak third bond of the benzyne to give (15) which interacts with the ethylene to produce the effective intermediate (16).The reaction con- tinues by a complex rotation and relaxation to give the benzocyclobutene. The full report on the formation of 2H-chromens e.g. (19) from the reaction of @-unsaturated aldehydes e.g. (17) with arynes has now appeared.” The earlier suggestion that reaction takes place initially by a 1,2-cycloaddition to give a benzoxeten (18) rather than by a 1,4-cycloaddition has been confirmed by labelling experiments and the mechanism of the formation of (18) is discussed. The addi- tion of benzyne to (20)did not yield the expected adduct (21); this is apparently the primary product but goes on to react further with benzenediazonium-2- carboxylate to give (22).20 The great strength of the Si-0 bond is thought to l7 J.De Valk and H. C. van der Plas Rec. Trati. chim. 1972 91 1414. l8 D. M. Hayes and R. Hoffmann J. Phys. Chem. 1972,76 656. l9 H. Heaney J. M. Jablonski and C. T. McCarty J.C.S. Perkin I 1972 2903. 2o T. J. Barton A. J. Nelson and J. Clardy J. Org. Chem. 1972 37 895. Arynes Carbenes Nitrenes and Related Species Ph Cl provide the driving force for the reaction. The primary products (23) from the 1,3- dipolar addition of benzyne to some a-diazoketones undergo spontaneous ring expansion to N-acylindazoles e.g. (24).’ The cycloaddition of benzyne with 2,5-disubstituted-3,4-diazanorcaradienes and subsequent spontaneous nitrogen extrusion provides a new route to benzocycloheptatrienes.22 5-Phenyl-1,2-dithiole-fthione (25) reacts with benzyne to give the adduct (26) and 1,3-di- thiolan-2-thione (27) gives (28) with the extrusion of ethylene probably via a concerted cis-elimination.’ Tetrafluorobenzyne reacts with benzene and alkyl- benzenes only uia 4+ 2 cycloaddition; however it has now been shown that (25) (26) (27) (28) ’’ T.Yamazaki and H. Shechter Tetruhedron Letters 1972 4533. 22 R. E. Moerck and M. A. Battiste J.C.S. Chem. Comm. 1972 1171. 23 D. B. J. Easton D. Leaver and T. J. Rawlings J.C.S. Perkin I 1972,41. 218 J. T. Sharp benzyne itself is less selective and reacts with for example toluene to give the 4 + 2 cycloadducts plus (29) and di~henylmethane.~~ Compound (29) is thought to be formed by two consecutive ene reactions and diphenylmethane by an unusual insertion of benzyne into a benzylic C-H bond.The cycloaddition of dinitrophenylpyridinium betaine (30) to benzyne is accompanied by the novel displacement of the N-dinitrophenyl group to give (31); this is the first reported example of a displacement reaction at nitrogen by ben~yne.'~ Ph Continuing studies into the reactions of tetrafluorobenzyne with efhers have shown that it reacts with diethyl ether in the presence of iodine to give (32) via the betaine (33).26 The reaction with dioxan in the presence of bromine to give (34)parallels the reported cleavage of tetrahydrofuran by benzyne in the presence of water. Benzyne also reacts with N-alkylmorpholines to give products in which the morpholine ring is cleaved e.g.N-methylmorpholine gave N-methylaniline and 2-(N-methyl-N-phenyl)aminoethanol,thought to occur via cleavage of (35).27 N-Benzylaziridine adds to benzyne to give (36) which undergoes an unusual elimination of acetylene to give N-benzylaniline.28 The potassamide- induced cyclization of N-methyl-2-chlorobenzylaminoacetonitrilefollowed by 1 @OC2 4OC2 4Br F CH Br 0p" 1$61;H F F F (32) (33) (34) J. M. Brinkley and L. Friedman Tetrahedron Letters 1972 4141. 24 z5 N. Dennis A. R. Katritzky S. K. Parton and Y. Takeuchi J.C.S. Chem. Comm. 1972 707. 26 S. Hayashi and N. Ishikawa Bull. Chem. SOC.Japan 1972 45 642. 27 T. Kametani K. Kigasawa M. Hiiragi and T. Aoyama J. Org. Chem. 1972,37 1450. 28 A. G. Giumanini J.Org. Chem. 1972 37 513. 219 Arynes Carbenes Nitrenes and Related Species \ +A \ a Q ,T7 Me CH,Ph elimination of hydrogen cyanide gave 2-methylisoindole in high yield.29 Hydride transfer from lithium diethylamide to benzyne to give eventually a-phenyl- diethylamine is markedly increased at low temperatures relative to nucleophilic sub~titution.~’No analogous reaction was observed in the reaction of o-halo-genoanisoles with lithium di-n-propylamide ; in addition to the typical aryne addition reaction the former underwent reductive dehalogenation to ani~ole.~ The reduction occurred by two mechanisms ; direct halogen displacement and hydride transfer to the aryne at high and low amine to amide ratios respectively. 2 Nitrenes The latest volume in the ‘Chemistry of Functional Groups’ series draws together much valuable information on the formation and thermal and photochemical decomposition of a~ides.~ N-Nitrenes have been reviewed33 with literature coverage to mid-1971.The ‘nitrenoid’ (37) precursor to phenyl nitrene has been prepared by treating N-chloroaniline with butyl-lithium at -100 0C34 and detected by trapping experiments with trimethylchlorosilane. The equilibrium between (38) and the vinyl nitrene has been detected by trapping the latter with CN / -Ar -C=C N I\ (37) triphenylph~sphine.~ The intermediacy of nitrenes has also been established in the photodecomposition of benzoquinone imine N-~xides,~~ and in the LTA oxidation of aromatic anil~.~~ Some aliphatic nitroso-compounds react with trialkyl phosphites to give imines (39) and it has been suggested that these are formed either by 1,2-alkyl shifts to the nitrene or in its precursor (40).38In a related it has been demonstrated that the free nitrene is not an interme- diate and that the fast formation of (40) is followed by a slow anchimerically 29 B.Jaques and R. G. Wallace J.C.S. Chem. Comm. 1972 397. 30 G. Wittig and I. Stober Annalen 1972 758 84. 31 E. R. Biehl S. M. Smith S. Lapis and P. C. Reeves J. Org. Chem. 1972,37 3529. 32 ‘The Chemistry of the Azido Group’ ed. S. Patai Wiley-Interscience New’York 1971. 33 B. V. Ioffe and M. A. Kuznetsov Russ. Chem. Rev. 1972,41 131. 34 C. A. Wilkie and D. R. Dimmel J. Amer. Chem. SOC.,1972 94 8600.35 T. Nishiwaki J.C.S. Chem. Comm. 1972 565. 3h A. R. Forrester M. M. Ogilvy and R. H. Thomson J.C.S. Chem. Comm. 1972 483. 37 B. Rindone E. Santaniello and C. Scolastico Tetrahedron Letters 1972 19. 38 B. Sklarz and M. K. Sultan Tetrahedron Letters 1972 1319. 39 R. A. Abramovitch J. Court and E. P. Kyba Tetrahedron Letters 1972 4059. 220 J. T. Sharp R,C-N=O P(OEthk R3C-N-O-6(OEt) --* (EtO),P=O + R,CN (40) 1 R2C=N-R (39) assisted loss of phosphate and migration e.g. (41). Further evidence for metal nitrene complexes is provided by the observation that ferrocenyl and cymantrenyl isocyanates unlike most aryl isocyanates decomposed in boiling cyclohexane to give the mines and symmetrical ureas. However both these and aryl isocyanates decomposed at 80 "C in dimethyl sulphoxide to give similar products probably via a route involving cycloaddition (42) with the solvent.40 r7 R-NFC=O R-N-C=O lar I -+ R-N + Me,S (or Me,SO) Me,S-0 '-Me2S-0 + + C02(or CO) (42) Nitrenes generated initially in a singlet state can either undergo specific reactions in that state or decay to the triplet state which also undergoes charac- teristic reactions.Acceleration of singlet +triplet intersystem crossing by heavy-atom solvents has been reported for some nitrenes but it has also been predicted that this process will be retarded in solvents providing suitable overlap with unshared electron pairs. It has now been shown that dichloromethane has this effect on photochemically generated alkanoyl nitrene~.~' The yields of intra- molecular insertion products e.g.(43),formed from the singlet n-pentanoylnitrene Me H (43) were much higher in dichloromethane than in cyclohexane.It was therefore deduced that the singlet species is stabilized by solvent interaction without its insertion reactivity being impaired. A similar stabilization effect was observed for singlet pivaloyl nitrene in its addition to olefins. In the singlet insertion reaction of alkoxycarbonylnitrenes into the C-H bonds of cyclohexane it was expected that increasing dilution with hexafluorobenzene would lead to an 40 R. A. Abramovitcfi R. G. Sutherland and A. K. V. Unni Tetrahedron Letters 1972 1065. 41 C. R. Felt S. Linke and W. Lwowski Tetrahedron Letters 1972 2037.Arynes Carbenes Nitrenes and Related Species 221 increase in singlet +triplet crossing and a corresponding decrease in the yield of the insertion product. Surprisingly the reverse was found.42 This could be explained by the diluent functioning as a radical scavenger or more likely by some complexation of the singlet nitrene with the solvent to give a 'nitrenoid' species with a lower rate of intersystem crossing but as in the previous case unimpaired insertion reactivity. Surprisingly dichloromethane does not have a similar effect in this case. The photolysis of N-ethoxycarbonyliminodimethylsulphurane Me,S=NCO,Et produces a greater proportion of triplet ethoxycarbonyl- nitrene than does photolysis of ethyl azid~formate,~~ e.g. in cyclohexane the former gives 21 % of the singlet C-H insertion product ethyl cyclohexylcarbam- ate and 57% of the triplet abstraction product ethyl carbamate whereas the latter gives 54% and 4% respectively.Although many examples of intramolecular aromatic substitution by aryl nitrenes have been observed the analogous intermolecular reactions are relatively unknown. It has been suggested that this may be due to inefficient competition between substitution by the singlet nitrene and its decay to the triplet state and/or because of the low electrophilicity of aryl nitrenes. By using electron- withdrawing groups to increase the electrophilicity of the nitrene and by using more nucleophilic aromatic substrates intermolecular substitutions have now been achieved,44 e.g.decomposition of p-cyanophenyl azide in NN-dimethyl- aniline give (44) (25%) and the p-isomer (3%). The electrophilic aryl nitrene obtained by the phosphite deoxygenation of pentafluoronitrosobenzene not only reacts with aromatic compounds but also adds to olefins e.g. to tetramethyl- ethylene to give the aziridine in 30 % yield.45 Aromatic substitution has also been achieved in very low yield by reaction of p-tolyl- and p-anisyl-nitrenes with perfl~oronaphthalene.~~ Sulphonyl azides react thermally with pyridine to give both sulphonylaminopyridines and N-sulphonyliminopyridinium ylide~.~~ Phthalimidonitrene adds to benzo[b]furans to give unstable adducts e.g. (45; R' = H) which undergoes an interesting thermal rearrangement to (46).48 The analogue (45 ;R1= Ac) however thermolyses at 80 "Cmainly to regenerate its phthalimidonitrene and furan precursors and only gives a low yield of the 42 D.S. Breslow and E. I. Edwards Tetrahedron Letters 1972 2041. 43 Y.Hayashi and D. Swern Tetrahedron Letters 1972 1921. 44 R. A. Abramovitch S. R. Challand and E. F. V. Scriven J. Amer. Chem. Sac. 1972 94 1374; J. Org. Chem. 1972,37,2705. 45 R. A. Abramovitch and S. R. Challand J.C.S. Chem. Comm. 1972 1160. 46 J. Ashby E. F. V. Scriven and H. Suschitzky J.C.S. Chem. Comm. 1972 366. 47 R. A. Abramovitch and T. Takaya J. Org. Chem. 1972,37,2022. 48 D. W. Jones J.C.S. Perkin I 1972 225. 222 J. T. Sharp MeC\\o xCMe I NPhth rearrangement product.49 Some 2- and 2,Ssubstituted furans react differently e.g.2,Sdimethylfuran gives the hydrazone (47) via C-0 or C-N bond cleavage in an initial 1,2-add~ct.~' Photolysis of aryl azides in the presence of oxygen gives aromatic nitro- compounds via reaction of the triplet nitrene with triplet oxygen.51 p-Substituted azides also give azoxy-compounds probably via the nitroso-compound formed by reaction of the excited azide with oxygen. It has also been shown52 that nitrene (48)reacts with oxygen at 77 K to give (49)and (50),which photo-rearrange to p-nitrophenyl azide. The full report on the preparation of sulphoximides by the oxidation of a variety of N-aminolactams in the presence of sulphoxides has now appeared and includes the photochemical regeneration of nitrenes from these products and their therm~lysis.~~ NN-Dibenzylaminonitrene reacts with 4- phenyl-1,2,4-triazoline-3,5-dione to give the azimine (51)54but ethoxycarbonyl- nitrene does not react with azobenzene to give any isolable azimine although it may be intermediate in the formation of (52).55 Similarly (53) gave only 3% of (54) and the major product was (55).49 D. W. Jones J.C.S. Chem. Cornm. 1972 884. 50 D. W. Jones J.C.S. Perkin 1 1972 2728. R. A. Abramovitch and S. R. Challand J.C.S. Chem. Cornm. 1972 964. J. S. Brinen and B. Singh J. Amer. Chem. SOC.,1971 93 6623. 53 D. J. Anderson D. C. Horwell E. Stanton T. L. Gilchrist and C. W. Rees J.C.S. Perkin I 1972 1317. 54 R. Ahmed and J.-P. Anseime Canad. J. Chern. 1972 50 1778. 55 R. C. Kerber and P. J. Heffron J.Org. Chem. 1972 37 1592. Arynes Carbenes Nitrenes and Related Species 0 0 (53) (54) (55) The photolysis of o-alkylaryl azides in diethylamine does not lead directly to 3-alkyl-2-diethylamino-3H-azepines, as might be expected by analogy with phenyl azide but rather to unstable oxygen-sensitive intermediates thought to be 3-alkyl-2-diethylamino-1H-azepines. 'These intermediates are precursors to the 3H-azepines but are also readily oxidized to mixtures of pyridines. Analogous intermediates have been suggested but not detected in the phenyl azide decompo- sition. Photolysis of various 5-azidobenzo[b]thiophensin secondary amines to give 4-amino-5-NN-dialkylaminobenzo[b]thiophensis thought to proceed via (56),which ring-opens to the diamine rather than expanding to a thieno-azepine which would involve loss of resonance energy.5 Rate studies on the conversion of a range of 4'-substituted-2-azidobenzo-phenones into the corresponding 3-phenylanthranils indicate that the reaction does not proceed via a nitrene but rather by a concerted mechanism involving (57)and (58),which undergoes rapid loss of nitr~gen.~' Spirodienes e.g.(59) have previously been suggested as intermediates in the rearrangement reactions following deoxygenation of various aromatic nitro-compounds with trialkyl phosphites. Further support for such structures is provided by the formation of (60)in the presence of excess trimethyl ph~sphite.~~ The range of novel rearrange- ments resulting from such spirodienyl intermediates has been reviewed.60 Full papers have now appeared on the preparation of 2-substituted indoles from Ar (60) 5h R.J. Sundberg S. R. Suter and M. Brenner J. Amer. Chem. Soc. 1972 94 513. '' B. Iddon H. Suschitzky and D. S. Taylor J.C.S. Chem. Comm. 1972 879. 58 J. H. Hall F. E. Behr and R.L. Reed J. Org. Chem. 1972 37 4952. 59 J. I. G. Cadogan D. S. B. Grace P. K. K. Lim and B. S. Tait J.C.S. Chem. Comm. 1972 520. J. I. G. Cadogan Accounts Chem. Res. 1972 5 303. 224 J. T. Sharp :N (61) o-azidostyrenes6’ and on the ring expansion and subsequent decarbonylation of (61) to give tetraphenylpyridazine62 The rearrangement reactions of nitrenes are discussed together with those of carbenes in the last section of this chapter.3 Carbenes Structure and Reactivity.-The electronic properties of diradicals have been reviewed.63 Recent calculation^^^^^^ have put the (3B,*‘A1) singlet-triplet energy separation for methylene at ca. 1 1 kcal mol- which is not very different from Frey’s estimate66 of ca. 8 kcal mol- based on kinetic evidence. It has been suggested that methylene formed in photolysis of diazomethane is initially generated in the ‘Al* state (with two electrons in the orbital perpendicular to the plane of the molecule) and that this state is responsible for its stereospecific addi- tion to alkene~.~~ Hoffmann has carried out further calculations of the potential energy surfaces for the addition of methylene and difluoromethylene to ethylene confirming that the preferred approach is like (62).The modifications to the (62) reaction path imposed by steric considerations in the additions to isobutene are also discussed.68 In other calculations on the methylene-ethylene reaction the importance of HOMO-LUMO interactions in determining the reaction path has been empha~ized.~’ The related system involving methylene nitrogen and diazirine has also been investigated the~retically.’~ The most favoured approach in the 61 R. J. Sundberg H. F. Russell W. V. Ligon jun. and Long-Su Lin J. Org. Chem. 1972 37 719. 62 C. W. Rees and M. Yelland J.C.S. Perkin I 1972 77. 63 L. Salem and C. Rowland Angew. Chem. Internat. Edn. 1972 11 92. 64 C. F. Bender H. F. Schaefer D. R. Franceschetti and L. C. Allen J. Amer. Chem.Soc. 1972,94 6888. 65 P. J. Hay W. J. Hunt and W. A. Goddard Chem. Phys. Letters 1972 13 30. 66 H. M. Frey J.C.S. Chem. Comm. 1972 1024. 6’ S. Y. Chu A. K. Q. Siu and E. F. Hayes J. Amer. Chem. SOC., 1972 94 2969. 68 R. Hoffmann D. M. Hayes and P. S. Skell J. Phys. Chem. 1972,76 664. 69 H. Fujimoto S. Yamabe and K. Fukui Buff. Chem. SOC. Japan 1972 45 2424. ’* J. P. Snyder R.J. Boyd and M. A. Whitehead Tetrahedron Letters 1972 4347. Arynes Carbenes Nitrenes and Related Species dimerization of singlet methylene is such as to minimize the overlapping of occupied MO’s and maximize the HOMO-LUMO interaction between the two methylene~.~’The full details of Jones’ work on the generation and study of singlet and triplet bismethoxycarbonylcarbene by the direct and sensitized irradiation of methyl diazomalonate are now available.72 The singlet carbene gave stereospecific cyclopropanation whereas the triplet did not.In reactions with various dienes no 1,4-addition was detected so dicyanocarbene remains the only carbene shown to react in this way. Both this work and a CIDNP study of the reaction of methylene with toluene73 demonstrate the fundamental difference in mechanisms between C-H insertion reactions of singlet and triplet carbenes the former reacting via a one-step insertion whereas the latter uia abstraction to give radical pairs which can combine or disproportionate. It has also been shown that the intramolecular reactions of alkylmethoxycarbonylcarbenes can be reduced by using a triplet sensitizer allowing increased cyclopropanation of added alkene~.~~ p-Nitrophenylcarbenes show greater non-stereospecificity in their addition to olefins than do the m-isomers; this is attributed at least in part to the stabilization of the triplet state by the nitro-gr~up.~’ The predicted nucleo- philicity of singlet carbenes such as cycloheptatrienylidene in which the vacant p-orbital is part of a ‘4n+ 2’ n-electron system has been confirmed by a study of its reactions with substituted styrenes.76 The p-value was found to be + 1.05 cf negative values for dichloro- and ethoxycarbonyl-carbene requiring a transi- tion state like (63) with opposite charge distribution to that for normal carbenes.It has also been shown recently that di- and tri-benzocycloheptenylidenes which have triplet ground states give stereospecific reactions with alkenes via the triplet state.77 This was explained by the suggestion that a carbene of this type would form a stable charge-transfer complex with the alkene e.g.(64) which would undergo intersystem crossing fast enough to result in stereospecific addition. It has been proposed that addition to 1,l-dicyclopropylethyleneprovides an “ H. Fujimoto S. Yamabe and K. Fukui Bull. Chem. SOC.Japan 1972 45 1566. l2 M. Jones jun. W. Ando M. E. Hendrick A. Kulezycki jun. P. M. Howley K. F. Hummel and D. S. Malament J. Amer. Chem. SOC.,1972 94 7469. l3 H. D. Roth J. Amer. Chem. SOC.,1972 94 1761. l4 M. B. Sohn and M. Jones jun. J. Amer. Chem. SOC.,1972,94,8281. 75 S.H.Goh,J.C.S. Chem. Comm. 1972,512;G.L.ClossandS.H.Goh J.C.S. PerkinI 1972,2103. ’‘ L. W. Christensen E. E. Waali and W. M. Jones J. Amer. Chem. SOC.,1972,94,2118. ” S.-I. Murahashi I. Moritani and M. Nishino Tetrahedron 1971 27 5131. 226 J. T. Sharp effective method for investigating singlet-triplet reactivity of carbene~.~~ For example singlet fluorenylidene reacted with this alkene to give (65) directly whereas the triplet carbene gave the intermediate (66) which in large part re- arranged to give (67). The utility of this method depends of course on the relative rates of ring closure and rearrangement of species such as (66). CHEt P CH Generation and Reactions.-A new book on carbene chemistry has been pub- li~hed.~~ Synthetic methods in diazo chemistry," carbenoids,' ' transition-metal-carbene complexes,' the use of carbenes and nitrenes in biological receptor-site labelling,83 and the use of phenyl(trihalogenomethy1)mercury com-pounds as dihalogenocarbene precursor^,'^ have been reviewed.There have been two reports of convenient syntheses of dia~o[~H,]methane from non-deuteriated precursor^.^^-^^ The carbene or carbenoid (68) produced by reaction of 2,2-dichloro-3,3-dimethylbutane with sodium naphthalene either undergoes unexceptional intramolecular insertion reactions or is reduced by sodium naph- thalene to the carbene radical-anion (69) a new species of reactive intermediate CH,C(CH,),?CH CH,C(CH,),CClCH (68) (69) which interacts with solvent to give 2,2-dimethylb~tane.~' Some strained cyclic am-compounds e.g.(70) decompose in a major reaction path via electrocyclic ring-opening and elimination of nitrogen to give carbenes e.g. (71) rather than '' N. Shimizu and S. Nishida J.C.S. Chem. Comm. 1972 389. -'R. A. Moss in 'Carbenes' ed. M. Jones jun. and R. A. Moss Wiley-Interscience, New York 1972 vol. 1. M. Regitz Synthesis 1972 351. *' G. Kobrkh Angew. Chem. Internat. Edn. 1972 11 473. 82 D. J. Cardin B. Cetinkaya and M. F. Lappert Chem. Rec. 1972 72 545. 83 J. R. Knowles Accounts Chem. Res. 1972 5 155. 84 D. Seyferth Accounts Chern. Res. 1972 5 65. * J. R. Campbell Chem. and fnd. 1972 540. 86 S. M. Hecht and J. W. Kozarich Tetrahedron Letters 1972 1501. '' G. D. Sargent C. M. Tatum jun. and S. M. Kastner J.Amer. Chem. Soc. 1972 94 7174. Arynes Carbenes Nitrenes and Related Species solely by direct extrusion of nitrogen.88 In the generation of dichlorocarbene by the reaction of chloroform with aqueous base it has now been shown that the detergent cetrimide and cetyltrimethylammonium chloride are as effective (70) (71) cationic micellar agents as triethylbenzylammonium chloride and they work at lower concentration^.^' Benzyl(iodomethyl)mercury,90bis(iodomethy1)mercury-dibenzylmercury and iodomethylmercuric iodide-dibenzylmer~ury~~ have been found to be efficient methylene-transfer reagents of particular value in cases where an a1 ternative to iodomethylzinc iodide may be required. Phenyl(diha1ogeno- methoxycarbonylmethy1)mercury compounds e.g.(72) transfer halogenometh- oxycarbonylcarbenes to olefins but require rather high (120-140 “C) reaction PhHgCCI,CO Me (72) temperature^.^^ Trimethylgermyl- trimethylstannyl- and trimethylplumbyl- ethoxycarbonylcarbenes have been prepared by photolysis of the corresponding diazo-compounds and react in their singlet states.’ The unexplained catalytic effect of oxygen on the cyclopropanation of olefins with chloroiodomethane and diethylzinc is now thought to be due to a free-radical process;94 and the activat- ing effect of trace amounts of peroxides on trimethyl phosphite copper(1) catalysts used in diazo-compound decompositions is thought to be due to oxidation to give unspecified copper(1r) salts which form the actual catalyst.” The full paper on the generation of arylcarbenes by photocycloelimination reactions of cyclic sulphites has now been published.96 An aziridine ester has been prepared via the Simmons-Smith reaction on the imino-ester (73) but no analogous reaction was observed with simple imines,” although such reactions are known for halogenocarbenes.Dichlorocarbene generated from PhHgCC1,Br and from sodium trichloroacetate reacted with azo- dicarboxylate esters to give (74) rather than a diaziridine adduct.’* Palladium D. H. White P. B. Condit and R. G. Bergman J. Amer. Chem. Soc. 1972,94 1348; R. A. Keppel and R. G. Bergman ibid. p. 1350; D. F. Eaton R. G. Bergman and G. S. Hammond ibid. p. 1351. ” G. C. Joshi N. Singh and L. M. Pande Tetrahedron Letters 1972 1461. ’O R.Scheffold and U. Michel Angew. Chem. Infernat. Edn. 1972 11 231. ’’ D. Seyferth and C. K. Haas J. Organometallic Chem. 1972 39 C41. 92 D. Seyferth R. A. Woodruff D. C. Mueller and R. L. Lambert jun. J. Organo-metallic Chem. 1972 43 55. 93 U. Schollkopf B. Banhidai and H.-U. Scholz Annalen 1972 761 137. 94 S. Miyano J. Yamashita and H. Hashimoto Bull. Chem. Soi. Japan 1972 45 1946. 95 D. S. Wulfman and B. W. Pearce Tetrahedron Letters 1972 3903. 96 G. W. Griffin and A. Manmade J. Org. Chem. 1972 37,2589. ’’ P. Baret H. Buffet and J.-L. Pierre BUN. SOC.chim. France 1972 825. 98 D. Seyferth and H. Shih J. Amer. Chem. Soc. 1972 94 2508. 228 J. T. Sharp Bu'-N=CH -CO,Et (RO,C),NN=CCl (73) (74) acetate has been found to be an effective catalyst for the cyclopropanation of olefins with diazo-compounds under mild The carbene produced by the thermolysis of biphenyl-2-sulphonyldiazomethanereacts via intra-molecular cyclization to give (75)and (76); the effect of solvent polarity is con- sistent with transition state (77).loo The preference for the syn cyclopropanation of various oxygen-functionalized cycloalkenes in the Simmons-Smith reaction has been interpreted as due to primary complexation of the carbenoid with the oxygen atom and subsequent delivery to the nearer face of the double bond.It has now been shown that the methoxy-group also has a mild syn-directing effect on monochlorocarbenoid ;'O' however further evidence has been presented against any such synergism in the additions of dichlorocarbene.' O2 Stable sulphonium ylides result from the reaction of a range of acyl- and sulphonyl-carbenes with dimethyl ~ulphide"~ and from bismethoxycarbonyl- carbene and various alkyl and aryl ~ulphides."~ In the latter case it was un- expectedly found that the ylide with dimethyl sulphide was also formed in high yield in the presence of a triplet sensitizer and it was suggested that the presence of the sulphide may be in some way assisting triplet +singlet intersystem crossing.Acyl- but not aryl-carbenes also react with dimethyl sulphoxide to give ylides,'05 and the first example of a stable thiocarbonyl ylide (78) has been isolated from the catalysed decomposition of bis(to1uene-p-sulphony1)diazo-methane in the presence of 1,2-benzodithiole-3-thi0ne.~~~ Carbenes with sulphide groups in the p- y- and &positions react in part via intramolecular ylide forma- tion e.g.(79) and subsequent rearrangement. l O7 Bismethoxycarbonylcarbene reacts at the heteroatom of allyl sulphides ethers amines and chlorides and the intermediate ylides e.g.(go) rearrange to give allyl carbon-heteroatom insertion y9 R. Paulissen A. J. Hubert and Ph. Teyssie Tetrahedron Lelters 1972 1465. loo R. A. Abramovitch V. Alexanian and E. M. Smith J.C.S. Chem. Comm. 1972 893. I. Fleming and E. J. Thomas Tetrahedron 1972 28 5003. '02 R. A. Moss J. Amer. Chem. SOC.,1972,94 6004. Io3 W. Illger A. Leidhegener and M.Regitz Annalen 1972 760 1. Io4 W. Ando T. Yagihara S. Tozune I. Imai J. Suzuki T. Toyama S. Nakaido and T.Migita J. Org. Chem. 1972 37 1721. F. Dost and J. Gosselck Chem. Ber. 1972 105 948. lo6 S. Tamagaki and S. Oae Tetrahedron Letters 1972 1159. 'O7 K. Kondo and I. Ojima J.C.S. Chem. Comm. 1972,62,860; Chem. Letters 1972 119. Arynes Carbenes Nitrenes and Related Species (78) (79) C0,Me I + &C-0-CO-C(CO,Me) I / C0,Me products. lo' In reaction with allylic alcohols however the same carbene gave products uia insertion into the 0-H bond and by ring closure of the initially formed cyclopropane adduct to give a bicyclic la~tone."~ Aziridinium ylides e.g. (8l) formed from ethoxycarbonylcarbene and N-phenethylaziridine frag- ment with elimination of ethylene and no detectable Steven's rearrangement. lo CH,CH2Ph CHZ -+ 11 + PhCH,CH,N=CHCO,Et CHC0,Et CH2 (81) Rearrangements involvingCarbenes and Nitrenes.-There has been much interest recently in high-temperature carbene<arbene and carbene-nitrene rearrange-ments.Three mechanisms have been discussed for interconversions between (82) and (84) and (85) (X = N or CH) (i) uia bicyclic intermediates e.g. (83) whose formation from (82) most likely requires an orientation of the carbene in which its vacant p-orbital is conjugated with the 7r-system; (ii) via a single-step Wolff-type rearrangement e.g. (82) (86) (84) requiring that the vacant p-orbital be orthogonal to the n-system and so oriented for migration of a a-bond ; (iii) via a diradical ring-opening. Evidence has been presented this year supporting both (i) and (ii).The rearrangement of a series of substituted '08 W. Ando S. Kondo K. Nakayama K. Ichibori H. Kohoda H. Yamato I. Imai S. Nakaido and T. Migita J. Amer. Chem. SOC.,1972 94 3870. Io9 W. Ando I. Imai andT. Migita J.C.S. Chem. Comm. 1972,822;J. Org. Chem. 1972 37 3596. Y. Hata and M. Watanabe Tetrahedron Letters 1972 3827 4659. 230 J. T. Sharp (86) (83b) (83c) naphthylcarbenes has been examined to determine the importance of bond order. It was found that scrambling of the carbene only occurred between posi- tions 2 and 3 e.g.(87) to (89),and not between 1 and 2. The observed rearrange- ment results therefore from net cleavage of the bond of highest bond order the 1,2-bond with no detectable cleavage of the 1,9-or 2,3-bonds. It is suggested that these results are best explained by mechanism (i) i.e.tlia (88) which would be Me Me CMe (87) (88) (89) favoured by high double-bond character whereas for (ii) the reverse would apply. This result contrasts with an earlier observation of the 2-quinolylnitrene + isoquinolylnitrene conversion which requires 2,3-bond cleavage. In another mechanistic study calculations have been done on the total and binding energies of reactants and primary products for a variety of rearrangements including that of (90).'12 In this reaction the rearrangement takes place entirely uia 2-aza-tropylidene with no detectable expansion of the benzene ring. It was concluded uoMe .. (90) that this specificity was due to the operation of a Wolff-type mechanism (ii) in which the occupied a-orbital of the carbene was conjugated with the n-system of the more electrophilic pyridine ring leaving the vacant p-orbital conjugated with the benzene ring and so favourably oriented for a Wolff-type expansion of the pyridine ring.In general these workers concluded that ring expansions and contractions of this type could be one-step processes governed by the stabilities of the primary products and the energies of the reactants. Most examples of carben-arbene rearrangements have been limited to gas-phase reactions at high temperatures ;Jones has now provided the first low-temperature example in that benzocycloheptatrienylidene is converted into P-naphthylcarbene in solu- tion at room temperature. 111 T. M'itsuhashi and W.M. Jones J. Amer. Chem. Sac. 1972 94 677. C. Wentrup C. Mayor and R. Gleiter Helv. Chim. Acta 1972 55 2628 2633. l3 K. E. Krajca T. Mitsuhashi and W. M. Jones J. Amer. Chem. SOC., 1972 94 3661. Arynes Carbenes Nitrenes and Related Species 23 1 In addition to ring-expansion-ring-interconversion phenyl-carbenes and -nitrenes also ring-contract usually at higher temperatures e.g. phenyl carbene gives (91 ; X = CH) and (92) and phenyl nitrene gives (91 ; X = N) most likely cia (93). 2- 3- and 4-pyridyl carbenes generated from diazo or tetrazole precursors CH2 II c=x CH 6 (91) (92) (93) all rearrange to singlet phenyl nitrene (calculated112 to be most stable) which ring-contracts to (91; X = N); however pyrolysis of (94) produced only the triplet nitrene which dimerized to azobenzene.' l4 It was suggested that stepwise loss of nitrogen from (94) gives rise directly to triplet pyridylcarbene which iso- merizes to the triplet nitrene.The necessity to invoke hydrogen shifts as part of (94) the mechanism of these reactions is revealed by Crow's work on ring-contraction of labelled phenyl-carbene' and -nitrene.' In the pyrolysis of (85 ;X = 3CH) to give (91; X = CH) and (92) it was demonstrated that complete carbon scrambling occurs before ring-contraction probably via a mechanism involving intermediates (83) and (84) with concomitant hydrogen shifts e.g. (83b) (83c).' ' It had previously been suggested that ring-expansion and ring-contrac- tion were two separate and competing pathways.The related study on 14C-labelled phenyl nitrene also showed considerable randomization of the label before ring-contraction to (91; X = N).'16 This could have occurred by ring- expansion-contraction cycles coupled with hydrogen shifts as discussed above or via a cycloperambulatory walk of the nitrene also coupled with hydrogen shifts. In the pyrolysis of indazole to give fulvenallene (92)there are two possible modes of decomposition ;alternative hydrogen shifts and loss of nitrogen can lead to the intermediates (95) and/or (96). Decomposition of the analogous + 0'"' UGHm~ mp and/or \ \ N H NE (95) (96) (97) 'I4 W. D. Crow M. N. Paddon-Row and D. S. Sutherland Tetrahedron Letters 1972 2239. 'I5 W. D. Crow and M. N. Paddon-Row J.Amer. Chem. SOC.,1972,94,4746. W. D. Crow and M. N. Paddon-Row Tetrahedron Letters 1972 223 1. 232 J. T. Sharp compound (97) apparently proceeds by both routes to give (91 ; X = N) and a mixture of compounds considered to be azafulvenallenes and an ethynyl-pyrrole.' In an elegant experiment designed to determine the relative contribu- tions of the two modes of ring opening N-deuterio-3-methylindazole was shown to ring-open mainly via an intermediate analogous to (99''' The pyrolysis of oxindole gives a complex mixture of products arising from arylcarbene and nitrene intermediates.' A variety of hetarylnitrenes generated from pyrolysis of oxadiazolones underwent ring-contraction or ring-fission reactions. '2o The course of these reactions is discussed in terms of the effect on the whole n-system of the localization of 7c-electrons in the electrocyclic transition states for ring- contraction.The existence of the oxocarbene-oxiren equilibrium (98) has been demons- trated in the thermal and photochemical decomposition of a variety of diazoketones.'21 Cycloheptatrienylcarbene (99) reacts via three pathways (i) extrusion of acetylene from the norcaradiene valence tautomer to give benzene ; (ii) hydrogen migration to give heptafulvene ;(iii) ring-expansion to cyclo-octa- tetraene. '22 A new treatment of forbiddenness-allowedness based on nodal properties and called 'MO Following' has been developed and applied to these reactions. The nucleophilic carbene (100) undergoes a [2,3]sigqatropic re- arrangement to give (101).'23 The pyrolysis of (102) gave PhC-CH and W.D. Crow A. R. Lea and M. N. Paddon-Row Tetrahedron Letters 1972,2235. W. D. Crow and M. N. Paddon-Row Tetrahedron Letters 1972 3207. R. F. C. Brown and M. Butcher Austral. J. Chem. 1972 25 149. lZo R. F. C. Brown and R. J. Smith Austral. J. Chem. 1972,25 607. S. A. Math and P. G. Sammes J.C.S. Chem. Comm. 1972 11 ;J.C.S. Perkin I 1972 2623. H. E. Zimmerman and L. R. Sousa J. Amer. Chem. SOC.,1972,94 834. J. E. Baldwin and J. A. Walker J.C.S. Chem. Comm. 1972 354. Arynes Carbenes Nitrenes and Related Species PhCdH presumably uia phenyl and hydrogen migration in a benzylidene- carbene intermediate. 24 Diallyloxycarbene fragments into allyl radicals and carbon dioxide at 250 "C and methoxy(ally1oxy)carbene (103) additionally undergoes rearrangements via methyl and allyl shifts.'" A rare example of the migration of an alkoxy-group to a thermally generated carbene has been reported.'26 Full details are now available on the generation and ring-contrac- tion of cyclic oxycarbenes.'27 0 I1 H,C=CH-CH,O CH =CHCH,O-CMe \ 0 Me0 / II CH,=CHCH,-COMe (103) R.F. C. Brown and K. J. Harrington J.C.S. Chem. Comm. 1972 1175. lZ5 R. W. Hoffmann R. Hirsch R. Fleming and M. T. Reetz Chem. Ber. 1972,105,3532. P. G. Gassman and X. Creary Tetrahedron Letters 1972,4407. '27 A. M. Foster and W. C. Agosta J. Amer. Chem. SOC.,1972 94 5777.

ISSN:0069-3030    

DOI:10.1039/OC9726900213

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

6 Molecular Rearrangements By G. TENNANT Department of Chemistry University of Edinburgh West Mains Road Edinburgh EH9 3JJ 1 Introduction As in last year's Report an attempt has been made to highlight the major areas of current activity in the widespread field of molecular rearrangements. The format of this year's Report remains the same as in 1971 except for the inclusion of nitrenium ion and oxonium ion processes in the section on cationic rearrange- ments. The enlarged size of the section dealing with thermal photochemical and metal-catalysed rearrangements reflects the ever-increasing number and importance of such transformations. The high standard set by previous volumes is maintained in Volume four of the series' dealing with molecular rearrangements.2 Aliphatic Rearrangements Anionic Rearrangements.-Rearrangements of dienyl anions have been reviewed. [ 1,2]-Anionic (Wittig) shifts predominate in the allyl-lithium-catalysed re- arrangements of alkyl and aryl ally1 ethers. Only in the case of n-butyl cinnamyl ether is the [ 1,4]-shift to rearranged aldehyde ~bserved.~ In contrast dihydro- pyranyllithium (1) undergoes exclusive [ 1,4]-sigmatropic rearrangement to afford the cyclopropyl enolate (2).4 The timing of the bond-making and -breaking pro- cesses in this interesting transformation has yet to be elucidated. Anions derived from benzyl cyanoalkyl ethers rearrange by competing [ 1,2]- and [2,3]-sigmatropic shift^.^ The base-catalysed phosphonate to phosphate rearrangement [(3)-P (4)] has been shown to be isokinetic with the silylmethanol to silyl ether (Brook) (1) (2) 'Mechanisms of Molecular Migrations' ed.B. S. Thyagarajan Interscience New York 1971 vol. 4. M. J. Perkins and P. Ward in ref. 1 p. 55. G. Courtois and L. Miginiac Tetrahedron Letters 1972 241 1. V. Rautenstrauch Helu. Chim. Acta 1972 55 594. S. Julia B. Cazes and C. Huynh Compt. rend. 1972 274 C 2019. 235 236 G.Tennant transformation. The low activation energy and large negative entropy of activa-tion observed for this process are consistent with a three-membered cyclic transition state derived by rate-limiting nucleophilic attack at phosphorus by oxy- anion.6 The clean base-catalysed rearrangement of benzylthiotrimethylsilane to a-trimethylsilylbenzylthiol is claimed as the first example of a Wittig-type rearrangement involving migration from sulphur to ~arbon.~ The reverse process (unlike the anti-Wittig silylmethanol to silyl ether rearrangement) can be induced thermally or in the presence of radical catalysts the latter process constituting the first example of a radical-initiated [1,2]-shift of a trimethylsilyl group from carbon to s~lphur.~ Further examples of base-catalysed Sommelet-Hauser rearrangements in ylides derived from azasulphonium salts have been rep~rted,"~ and a compre- hensive kinetic study of the rearrangement of N-aryl-SS-dimethylsulphimides to o-thiomethoxymethylanilines has been carried out.' Ammonium and sul-phonium ylides (generated in situ by treatment of benzyldimethylamine or benzyl phenyl sulphide with phenylthiomethyl chloride in the presence of potassium t-butoxide) undergo preferential [2,3]-sigmatropic (Sommelet-Hauser) rearrange- ment." In contrast the azetidine ylide (5) (generated in situ from N-benzylaze-tidine and ethoxycarbonylmethylcarbene) ring-expands by exclusive [1,2]-shift to give the pyrrole (6).'* The demonstration that the corresponding three- and five-membered ammonium ylides undergo cleavage rather than rearrangement indicates the importance of ring-size effects. Predominant Stevens rearrange- ment is also observed with the thiolanonium ylide (7).13 [1,2]- (Stevens) and @i+fiPh -OC0,Et cfo &O2Et N 6) (5) 'Ph 'Ph (6) (7) [2,3]-shifts are now accepted pathways for the competing rearrangements exhibited by allylammonium and allylsulphonium ylides the [2,3]-processes being concerted and favoured at low temperatures.Thus rearrangement of the ylide (9a) derived from diallyldimethylammonium chloride occurs at -33 to A. F. Janzen and T. G. Smyrl Canad. J. Chem. 1972,50 1205. ' A. Wright D. Ling P. Boudjouk and R. West J. Amer. Chem. SOC.,1972,94,4784. P. G. Gassman G. Gruetzmacher and R. H. Smith Tetrahedron Letters 1972 497. U. Lerch and J. G. Moffatt J. Org. Chem. 1971 36 3686. lo P. Claus and W. Rieder Monatsh. 1972 103 1163. l1 S. Julia C. Huynh and D. Michelot Tetrahedron Letters 1972 3587. l2 Y. Hata and M. Watanabe Tetrahedron Letters 1972,4659. l3 K. Kondo and I. Ojima J.C.S. Chem. Comm. 1972 860. Molecular Rearrangements -73 "Cto give @a) the product of a [2,3]-shift and at higher temperatures (lOa) the product of a [1,2]-shift is also f~rrned.'~ Interestingly the analogous re- arrangements of allylpropargylammonium salts originate from only one [i.e.(9b)l of the two conceivable ylide intermediates.' Base-catalysed rearrangement Me Me Me LN -7"--(9) (10) /Me a; R = -CH=C \ Me b; R = -C-C-Ph of allylbenzyldimethylammonium chloride on the other hand results in products which stem from competing [1,2]- and [2,3]-shifts in both possible ylide interme- diates. l4 Allylpropargyl-sulphonium' and -thi~lanonium'~ ylides appear to rearrange exclusively by [2,3]-shifts whereas three- and four-membered cyclic sulphonium ylides rearrange by competing [1,2]- and [2,3]-pathways.' A sulphonium ylide rearrangement has been used to achieve the stereospecific ring-contraction of a cephalosporin to a penicillin derivative.l7 In accord with kinetic control in a product-like transition state [2,3]-sigma- tropic rearrangement of the sulphoxide (1 1) gives predominantly the less-stable H I (equatorial) sulphenate. * The allyl sulphoxide-ally1 sulphonate rearrangement has been elegantly adapted to the stereospecific synthesis of trisubstituted olefins.' Rearrangements of phosphinite esters of allyl alcohols are stereoselective giving exclusively the trans-dienylphosphine oxides.20 [2,3]-Sigmatropic shift in enynol l4 V. Rautenstrauch Helv. Chim. Acta 1972 55 2233. '' R. W. Jemison T.Laird and W. D. Ollis J.C.S. Chem. Comm. 1972 556. l6 K. Kondo and I. Ojima J.C.S. Chem. Comm. 1972 62. M. Yoshimoto S. Ishihara E. Nakayama and N. Soma Tetrahedron Letters 1972 2923. D. A. Evans and G. C. Andrews J. Amer. Chem. Soc. 1972,94 3672. l9 D. A. Evans G. C. Andrews and C. L. Sims J. Amer. Chem. Soc. 1971 93 4956; P. Grieco J.C.S. Chem. Comm. 1972 702. 2o M. Huche and P. Cresson Tetrahedron Letters 1972 4933. 238 G. Tennant phosphinites occurs exclusively to the triple bond to give allenylphosphine oxides.20 The observation of a large kinetic deuterium isotope effect (k,/k = 9) sup- ports an E2 mechanism (as opposed to an ylide pathway) for rate-determining breakdown of the sulphonium salt in the acetic anhydride-induced Pummerer rearrangement of dibenzyl sulphoxide.21 On the other hand the low deuterium isotope effect (kH/kD = 1.1-1.6) and fast hydrogendeuterium exchange demons- trated22 for Pummerer rearrangements of methylphenylsulphonium bis(methoxy- carbony1)methylide accord with an El cb (ylide) process.An ylide pathway is also consistent with the predominant rearrangement to the least-substituted carbon (formation of the most stable carbanion) observed in the Pummerer rearrangements of steroidal sulphoxide~.~~ The unprecedented attack at the most substituted a-position observed for axial 6P-sulphoxides is attributed to a steric effect.23 Iminosulphuranes undergo Pummerer-type rearrangements on reaction with acylating agents [e.g. (12)-(13) + (14)12“ or on thermolysi~.~~ Me \+ -S-NAc RCoX) MeSCH,X + MeCO.NHR / Me (12) (13) (14) X = C1or Ac R = Me or Ph Two examples of Pummerer rearrangements involving ring formation have been reported.26 The conversion of methoxymethyl phenyl sulphoxide into methoxymethyl phenylsulphenate is an example of a non-equilibrium-controlled sulphoxide to sulphenate transformation.A polar transition state for this rearrangement is consistent with its inhibition by protic media and its facilitation by the electron-donating methoxy-group.27 Base-induced rearrangements of cyclobutanones2’ and epoxide~~~ have been reviewed. Decafluorobenzil undergoes benzilic acid rearrangement ca. 10l5 times faster than benzil indicating substantial ‘benzenide’ character in the transi- tion state.30 The ready benzilic acid rearrangement of cyclobutanediones to hydroxycyclopropanecarboxylic acids reported last year [cf.Ann. Reports (B) 1971 68 2451 is reversed in the corresponding unsaturated ring systems [cf. (15)+(16)J31 The intermediate formation and acyloin rearrangement of a ’‘ G. E. Wilson and C. J. Strong J. Org. Chem. 1972 37 2376. ’’ T. Yagihara and S. Oae Tetrahedron 1972 28 2759. 23 D. N. Jones E.*Helmy and R. D. Whitehouse J.C.S. Perkin I 1972 1329. 24 H. Kise G. F. Whitfield and D. Swern Tetrahedron Letters 1971 4839. 25 H. Kise G. F. Whitfield and D. Swern J. Org. Chem. 1972,37 1125. ’‘ J. Kitchin and R. J. Stoodley J.C.S. Chem. Comm. 1972,959; T. Numata and S. Oae Chem. and Ind. 1972 726. ’’ T. J. Maricich and C.K. Harrington J. Amer. Chem. Soc. 1972,94 51 15. J. M. Conia and J. R. Salaun Accounts Chem. Res. 1972 5 33. 29 V. N. Yandovskii and B. A. Ershov Russ. Chem. Rev. 1972 41 403. 30 R. D. Chambers M. Clark and D J. Spring J.C.S. Perkin I 1972,2464. 31 C. D. De Boer J.C.S. Chem. Comm. 1972 377. 239 Molecular Rearrangements (17) (18) (19) hydroxycyclopropyl ketone (1 8) rationalizes the acid- or base-catalysed re-arrangement of the dicyclopropyl diol (17) to the hydroxycyclobutanone (19).32 The outcome of the base-induced rearrangements of 7-alkyl-7-halogenobicyclo-[3,2,0]hept-2-en-6-ones (20) depends on the subtle interplay between the nature of the alkyl substituent the stereochemistry of the halogen atom and the nature of the basic catalyst.A variety of bases (lithium meth~xide,~~ aqueous lithium hydr~xide,~or sodium carbonate34) convert the exo-chloro-epimer (20a) by stereospecific semi-benzilic ring-contraction into the bicyclo[3,1,0]hex-2-ene exo-carboxylic acids (21a) (or the derived methyl ester~).~~.~~ In the case of the endo-epimer (20b) the stereochemistry ofthe halogen substituent is not conducive to ring contraction which is only predominant in aqueous lithium hydroxide.33 In other basic media alternative rearrangement processes (S,2' displacement ring-opening to tropones or cyclohexene carboxylic acids) inter~ene.~~-~' ,HH &CR1 R2 (20) (21) a; R' = C1 R2 = alkyl a; R' = C02H R2 = alkyl b; R' = alkyl R2 = C1 b; R' = alkyl R2 = C02H Barbaralone (23) and bicyclo[4,2,1]nonatrienone (26) have been synthesized by the novel enolate-induced rearrangements shown (Scheme 1).36737 The processes [(22) +(23)] and [(24) +(25)] are formal vinylogous intramolecular SN2' displacements analogous to the bishomoconjugative Ramberg-Backlund rearrangements described last year [cJ:Ann.Reports (B),1971,68,247]. However the detailed courses of these interesting transformations await the outcome of 32 J. M. Denis and J. M. Conia Tetrahedron Letters 1972 4593. 33 D. L. Garin and K. L. Cammack J.C.S. Chem. Comm. 1972 333. 34 W. T. Brady and J. P. Hieble J. Amer. Chem. SOC.,1972,94,4278. 35 P. R. Brook and J. M. Harrison J.C.S. Chem. Comm. 1972,997. 36 L. A. Paquette R. H. Meisinger and R. E. Wingard J. Amer. Chem. Sac.1972 94 2155. 37 E. Vedejs R. A. Gabel and P. D. Weeks J. Amer. Chem. SOC.,1972,94 5847. 240 G. Tennant 0 0 Q (23) Br 0 Br u Scheme 1 further experimental work. Reaction of hydroxy-ketones (27) with diethyl carbonate in the presence of sodium hydride to give esters (29) is rati~nalized~~ by a novel Favorskii-type rearrangement in an intermediate carbonate (28). A cyclopropanone mechanism (as opposed to semi-benzilic or epoxide mechan- isms) for these reactions is consistent with the structure of the products and the results of 180-labelling experiment^.^^ ctcc-Dichloro-alkoxides undergo base-catalysed rearrangement to cc-chloro- ketones by processes whose precise mechanism has yet to be e~tablished.~~ The first examples of base-catalysed skeletal rearrangement and epimerization by 38 J.Cymerman-Craig A. Dinner and P. J. Mulligan J. Org. Chem. 1972,37 3539. 39 J. Villieras C. Becquet and J. F. Normant J. Organometaffic Chem. 1972 40,C1; G. Kobrich and J. Grosser Tetrahedron Letters 1972 41 16. Molecular Rearrangements 241 homoenolization have been demonstrated in bicyclo[2,2 llheptan~nes?~ The conversion of di(primary alkylj sulphones into cis-alkenesulphonic acid salts by reaction with potassium hydroxide-carbon tetrachloride in t-butyl alcohol is believed to involve Ram berg-Bac klund rearrangement in intermediate dichloro- s~lphones.~' A shortened sequence for the introduction of double bonds into molecules by means of the Ramberg-Backlund rearrangement has been de- scribed.42 The concomitant carbon-halogen bond homolysis observed42 under the conditions used is intriguing.Oxyphosphorane intermediates are invoked to account for the base-catalysed conversion of a-halogenophosphinates into rearranged pho~phates.~~ Cationic Rearrangements.-Acid-catalysed ketone rearrangernent~~~ and re-arrangements involving dienyl cations2 and halogenonium ions45 have been reviewed. Rearrangements of carbocations are surveyed in an authoritative article by Olah.46 Theoretical suggest that the 1,2-methyl shift in the 1-propyl cation involves a distorted corner-protonated cyclopropane intermediate and contrary to experimental findings should occur much more readily than the correspond- ing 1,3-hydride shift which is constrained to an edge-protonated cyclopropane pathway ca.10 kcal mol- higher in energy. The formation of equal amounts of 2- and 3-butyl trifluoroacetates in the trifluoroacetolysis of 2-(1,1,1,2-tetra- deuterio)butyl tosylate unaccompanied by label scrambling is adduced as evidence for a hydrogen-bridged butyl cation intermediate.48 Relief of ring strain is invoked to account for a cyclization reaction49 which involves the conversion by ring-expansion of a linear vinyl cation (30) into a predictably5' less stable bent structure. I (30) (31) 40 D. H. Hunter A. L. Johnson J. B. Stothers A. Nickon J. L. Lambert and D. F. Covey J. Amer. Chem. Soc. 1972 94 8582. 41 C. Y. Meyers and L. L. Ho Tetrahedron Letters 1972,4319. 42 J. C.Philips and M. Oku J. Amer. Chem. Soc. 1972 94 1012. 43 P. Burns G. Capozzi and P. Haake Tetrahedron Letters 1972 925. 44 A. Fry in ref. 1 p. 113. 45 P. E. Peterson Accounts Chem. Res. 1971 4 407. 46 G. A. Olah J. Amer. Chem. Soc. 1972,94 808. 47 L. Radom J. A. Pople V. Buss and P. von R. Schleyer J.Amer. Chem. SOC.,1972,94 31 1. 48 J. J. Dannenberg D. H. Weinwurtzel K. Dill and B. J. Goldberg Tetrahedron Letters 1972 1241. 49 W. D. Pfeifer C. A. Bahn P. von R. Schleyer S. Bocher C. E. Harding K. Hummel M. Hanack and P. J. Stang J. Amer. Chem. SOC.,1971 93 1513. 50 W. S. Johnson M. B. Gravestock R. .I.Parry and D. A. Okorie J. Amer. Chem. SOC. 1972,94,8604. 242 G.Tennant The migratory aptitude of the ethoxycarboxyl group in a pinacol rearrange- ment has been found to be akin to that of ethyl greater than that of methyl or hydrogen but less than that of ~henyl.~' The migratory aptitude of a cyclo- propyl group on the other hand is much greater than that of alkyl and as might be expected is enhanced by methyl substitution in the ring.52 The observation that a trans-2-methylcyclopropyl group retains its stereochemistry in the course of pinacol rearrangement is intriguing.52 1,2-Dialkylcyclobutane- 1,2-diols readily undergo both acid-catalysed and thermal pinacol-type rearrangements.In the thermal processes predominant aikyl-group (and in one case vinyl-group) migration occurs whereas acidic conditions promote exclusive ring-contraction to cyclopropyl ketones or aldehydes.53 The transformation [(32)+ (33) + (34)] exemplifies the hitherto unknown rearrangement of an a-to a /I-keto-ester.Sequential methyl or phenyl and ethoxycarbonyl shifts in carbonium ion inter- mediates provide a simple rationale for these interesting rearrangement^.^^ Ph I I Ph Me-C-COC0,Et H' --+ Ph I Me-C-COPh I C0,Et Ph I I C0,Et + Ph-C-COMe (32) (33) ( 34) Steric constraints on the energetically favoured disrotatory opening of cyclo- propyl cations results in otherwise unobserved hydride and carbon [l,Z]-~hifts.~~ On the other hand the remarkable transformation [(35) + (37)] requires cyclopropyl-ally1 rearrangement to the Bredt violator par excellence(36) which can be trapped in the form of its adduct with f~ran.~~ Failure to observe bi- molecular substitution with rearrangement in suitably constituted ally1 systems is cited as further evidence against the concerted nature of the S,2' process and for the involvement of tight i~n-pairs.~' A series of papers sets down the case '' J.Kagan D. A. Agdeppa and S. P. Singh Helu. Chim. Acra 1972 55 2252. 52 T. Shono. K. Fuiita S. Kumai T. Watanabe and I. Mishiguchi Tetrahedron Letters. 1972 3249. 53 J. M. Conia and J. P. Barnier Tetrahedron Letters 1971 4981. 54 J. Kagan and D. A. Agdeppa Helo. Chim. Acta 1972,55,2255. 55 D. B. Ledlie J. Org. Chem. 1972 37 1439; C. B. Reese and M. R.D. Stebles Tetra-hedron Letters 1972 4427. 56 P. Warner R. La Rose Chee-Man Lee and J. C. Clardy J. Amer. Chem. SOC.,1972 94 7607. 57 F. G. Bordwell and T.G. Mecca J. Amer. Chem. Soc. 1972,94 5825 5829. Molecular Rearrangements for tight ion-pair intermediates in allylic substitution reaction^.^ The syn-faciality observed in the hydride-induced S,2 dehalogenation of allylic halides is also interpreted in terms of tight ion-pair intermediate^.^^ Conversely S,2 substitutions of P-benzoyl-y-phenylallyl halides are formulated as concerted processes involving a polar transition state.60 The remarkable rearrangement [(38)-+ (39)] exemplifies intramolecular S,2' substitution by a cyclopropane ring in which ion-pair return occurs at a site four bonds (ca.4 A) removed from the centre of initial ionization!61 n CH-CH2 CH=CH The results of a number of elegant studies published in the past year enhance the status of the cyclobutyl-cyclopropylmethyl-homoallyl rearrangement sequence as a model for the in uivo construction of head-to-head linkages in terpenoid structures.The scope of this model has been considerably extended by the isolation and characterization6* of a biogenetic cyclopropylmethyl precursor (prephytoene pyrophosphate) of the C, carotenoid phytoene and by the in vitro of the conversion of a C ,cyclopropylmethanol (artemisyl) structure into the head-to-head linked chrysanthemyl skeleton. Moreover in uitro ~tudies~~,~~ under carefully controlled conditions have now successfuly demonstrated that cyclopropylmethyl-homoallyl transformations can provide the requisite stereoselectivity (scission of the correct cyclopropyl bond) and stereospecificity (net inversion of configuration at the trialkylmethyl carbon atom) observed in vivo.Two reports of the hitherto unknown ring-contraction of cyclobutenes to cyclopropenes have been p~blished.~',~~ Solvolysis of the dichlorocyclobutene (40a) affords65 the cyclopropenyl t-butyl ketone (41a) which is also formed together with the isomeric compound (41b) by forcing electrolysis66 of the di- carboxy-dianion (40b). Cyclobutenyl cation intermediates are implicated in both of these ring-contractions which appear to derive their stimulus from relief of 58 R. A. Sneen and J. V. Carter J. Amer. Chem. SOC.,1972,94,6990 and preceding papers in the series. 59 C. W. Jefford A. Sweeney and F. Delay Helv. Chim. Acta 1972 55 2214. A. D. George E.Doomes and N. H. Cromwell J. Org. Chem. 1971,36 3918. 61 G. Dann Sargent and M. A. Herkenham J. Amer. Chem. SOC.,1972,94,2892. 62 C. D. Poulter 0.J. Muscio C. J. Spillner and R. G. Goodfellow J. Amer. Chem. SOC. 1972 94 5921 and other recent papers cited therein. 63 B. M. Trost P. Conway and J. Stanton Chem. Comm. 1971 1639. "'R. M. Coates and W. H. Robinson J. Amer. Chem. SOC.,1972 94 5921. h5 J. Ciabattoni and A. E. Feiring J. Amer. Chem. SOC.,1972 94 51 13. 66 G. Maier and F. Bosslet Tetrahedron Letters 1972. 4483. 244 G.Tennant steric crowding in the t-butyl groups. Curiously ring-expansion [(41a) +(40a)l occurs when the ketone (41a) is treated with phosphorus pentachloride imply- ing some controlling effect by the carbonyl group.65 (40) a R = CI (41) a; R' = Bu' R2 = H b; R = C02-b; R' = H R2 = Bu' I3C N.m.r.spectroscopy in conjunction with deuterium labelling provides a valuable new method for the direct determination of label sites and hence for the elucidation of label scrambling in polycyclic carbocation rearrangement^.^ 'H and 13C n.m.r. spectroscopy continue to be exploited in general as powerful probes for the detection and study of fast degenerate rearrangement processes in carbocations. Line-broadening in the n.m.r. spectrum of the cyclopentenyl cation over the temperature range 85-112°C is interpreted in terms of a degenerate rearrangement involving a fast (E = 18.0 & 0.9 kcal mol- *)reversible [1,2]-hydride shift. The operation of an alternative thermally forbidden [1,4]-hydride shift is excluded by double-resonance experiments.68 The temperature dependence of the signals due to the annular methyl groups in the 'H n.m.r.spectra of 5-acyl-1,2,3,4,5-pentamethylcyclopentadiene-aluminium trichloride complexes (42) is attributed to a novel five-fold degenerate rearrangement [(42) (44F etc.] in which C-6 and its attached substituents migrate round the periphery of the five-membered ring.69 The marked decrease in the rate of rearrangement produced by increased electron donation in the C-6 substituent [i.e.(42;R = OEt or C,H,-p)] is considered to exclude a process involving direct [1,5]-sigmatropic shift of C-6. A bicyclic high-energy intermediate or transition state (43) and close analogy with the degenerate rearrangements of bicyclo- [3,1,0]hexenyl cations are p~stulated.,~ 'H N.m.r.studies in super-acid media (FSO,H-SbF,-SOCIF ; SbF,-SOCl) at low temperatures (i.e.'stable-ion' conditions) demonstrate the exclusive ring- '' J. B. Stothers C. T. Tan A. Nickon F. Huang R. Sridhar and R. Weglein J. Amer. Chem. SOC.,1972,94 8581. '8 M. Saunders and R. Berger J. Amer. Chem. Soc. 1972 94 4049. by M. Zeya and R. F. Childs J. Amer. Chem. SOC.,1972,94 289. Molecular Rearrangements 245 contraction of the cyclohexen-3-yl cation to the 3-methylcyclopenten-2-yl ~ation.~’ The [1,2]-hydride shift to the cyclohexen-2-yl cation (well known under solvolysis conditions) is not observed. Failure to observe the trishomocyclo- propenyl cation (the presumed intermediate in the cyclohexenyl to methylcyclo- pentenyl ring-contraction) under ‘stable-ion’ conditions which are conducive to the detection of other unstable carbonium ions is considered7’ to render the existence of this elusive species doubtful.Multistep rearrangements in cyclo- propylmethyl carbonium ion intermediates are invoked to account for the degenerate methyl shifts observed in polymethylcyclohexenyl cations.’ The intriguing s~ggestion,~~ based on theoretical considerations that (CH), + cations should be most stable in the square-pyramidal configuration (45) should stimulate the quest for the implied ‘polytopal’ rearrangements in cyclopentadien- 5-yl bicyclo[2,1,0]pent-2-en-5-yl carbocations. and tricycl0[2,1,0,0~~~]pent-2-yl Variable-temperature n.m.r.provide unequivocal support for the incidence in super-acid media of degenerate Wagner-Meenvein rearrangements in methylnorbornyl cations and activation parameters for these and related non-degenerate processes have been dete~mined.~~ H N.m.r. studies have also revealed an interesting relationship in the skeletal interconversions of a series of bicyclic C7-C9 carbonium ions under ‘stable-ion’ conditions. The 2-methyl- bicyclo[3,2,l]oct-2-yl cation (46b) is the stable end-product of the ring-contrac- tion of a variety of bicyclononyl derivatives or of the 2-ethylnorborn-2-yl cation (47b) in super-acid media at temperatures below -30 “C. At higher temperatures (46b) rearranges to the bicyclo[4,3,0]non-l -yl (8-hydrindyl) cation (48).7s On the other hand the observed76 rearrangement of a variety of bicyclo-octyl (46) a; R = H (47) a; R = Me b;R=Me b R = Et ’’ G.A. Olah G. Liang and Y. K. Mo J. Amer. Chem. Soc. 1972,94 3544. ” K. Rajeswari and T. S. Sorensen Cunad. J. Chem. 1972 50 2939. l2 W.-D. Stohrer and R. Hoffmann J. Amer. Chem. SOC.,1972 94 1661. l3 T. S. Sorensen and K. Ranganayakulu Tetrahedron Letters 1972 2447. 74 E. Huang K. Ranganayakulu and T. S. Sorensen J. Amer. Chem. SOC.,1972 94 1779 1780. ’’ G. A. Olah G. Liang J. R. Wiseman and J. A. Chong J. Amer. Chem. SOC.,1972 94 4927. ” G. A. Olah and G. Liang J. Amer. Chem. SOC.,1971 93 6873. 246 G. Tennant derivatives at < -30 "C to the bicyclo[3,3,0]oct-2-yl cation (49) and the trans- formation of the latter at > -30 "Cto the 2-methylbicyclo[2,2,l]hept-2-ylcation (47a) are contrary to the results of solvolysis studies which suggest precisely the reverse order of stability [i.e.(47a) < (46a) > (49)].The implied greater stability of (48) relative to (49) is consistent with the anticipated lower stability of a bridge- head carbenium ion in a 5 5 fused-ring system.75 Relief of bond-angle strain is believed to provide the driving force for the unique contraction of a five- to a four-membered ring observed in the acetolysis of tricyclo[3,3,0,03~7]oct-2-yl brosylate to 2-acetoxytricyclo[3,2,1,03~6]octane.77 Evidence for direct as opposed to sequential bond shift in this novel rearrange- ment is provided by deuterium-labelling studies.77 Several rearrangement processes leading to the adamantane skeleton have been described.The hitherto unknown di-adaman tane spiro[adamantane-2,2-adamantane](5 1) is obtained directly by ring-expansion [( 50)-+(51)]'* or in several steps after benzilic acid T T rearrangement [(52)+(53)+ + (51)].79 Ring- contraction of homoadaman- tene occurs on treatment with aluminium bromide to afford 2-methyladamantane in high yield (76-84%).80 The unexpected retention of configuration observed in the acetolysis [(54p (57)] can be explained by a course (Scheme 2) involving a novel cationic Cope-type rearrangement [(55)-(56)].8' The rearrangement [(%)-+ (59)] of a homobullvalenyl cation is implicated in the solvolytic conversion of a dibromo- homotropilidene derivative into tricyclo[5,4,0,04.' ']undeca-2,6,9-trien-8-0ne.82 " R. R. Sauers K. W. Kelly and B. R. Sickles J. Org. Chern. 1972 37 537. 78 W. D. Graham and P. von R. Schleyer Tetrahedron Letters 1972 1179. '' E. Boelema J. Strating and H. Wynberg Tetrahedron Letters 1972 1174. 8o 2. Majerski and K. Mlinaric J.C.S. Chem. Comm. 1972 1030. R. Breslow and J. M. Hoffman J. Amer. Chem. SOC.,1972 94 21 11. 82 J. T. Groves and B. S. Packard J. Amer. Chem. SOC.,1972,944 3252. Molecular Rearrangements 4 4 '4 AcOH, AcOH + (-HI) A% H- H- H- I OAc H OAc (54) (55) (56) (57) Scheme 2 Exclusive phenyl shift in competition with cyclopropyl migration is observed in the acetolysis of a dibenzosemibullvalenyl methylt~sylate.'~ Br The outcome of the deaminative rearrangement in trans-2-aminomethylcyclo-hexan-1-01 is consistent with the hypothesis of ground-state control for such processes rearrangement occurring faster than ring-in~ersion.'~ Deuterium- labelling indicates that contrary to previous belief the formation of 2-methyl- cyclohexanone in this rearrangement is the result of two [1,2]-hydride shifts rather than a single [1,3]-hydride shifts4 The influence of steric effects on the course of Tiffeneau-Demjanow ring-expansions in fused-ring systems has been in~estigated.'~ Deaminative rearrangement of camphor benzenesulphonyl- hydrazone occurs readily on photolysis in alkaline solution to yield pinane derivatives.86 In contrast similar rearrangement of the norbornane ring-system is unsuccessful unless promoted by electron-donating substituents at C-l.87 Deaminative rearrangement has been employed in ring-expansions of adaman- tanes88 and h~moadamantanes'~ to bishomoadamantanes.A considerable body of evidence has now been accumulated in support of the existence of nitrenium cation intermediates in the rearrangements of N-halo-genoalkylamines and similar substrates. Perhaps the simplest examples of such 83 L. A. Paquette and G. H. Birnberg J. Amer. Chem. SOC.,1972,94 164. 84 D. Farcasiu C. Kascheres and L. H. Schwartz J. Amer. Chem. SOC.,1972,94 180. W. E. Parham and C. S. Roosevelt J. Org. Chem. 1972 37 1975. O6 W. Kirmse and G. Arend Chem. Ber. 1972,105,2738. 87 W. Kirmse and R. Siegfried Chem. Ber. 1972 105 2754; W. Kirmse and G.Arend ibid. p. 2747. 88 H. Gerlach Helv. Chim. Acta 1972 55 2962; D. Skare and 2. Majerski Tetra-hedron Letters 1972 4887. 89 T. Sasaki S. Eguchi T. Toru and K. Itoh J. Amer. Chem. SOC.,1972,94 1357. 248 G. Tennant transformations are the C-+N alkyl shifts which occur when NN-dichloro-t- alkylamines are treated with aluminium ~hloride.’~ As yet the timing of these processes is unknown but the almost statistical value of 2.2 observed” for the relative migratory aptitudes of s-and n-butyl groups implies stepwise courses involving cationic species. Interestingly the same migratory preference is demonstrated in the preferential shift of the gern-dimethyl bridge which occurs in the aluminium chloride-catalysed rearrangement of NN-dichloroapocamphyl- ami~~e.~’ The formation of bridgehead halogen derivatives in these and the closely related rearrangements of N-chlor~adamantylamines~~ may have syn- thetic potential.Initial ionization to a nitrenium cation is also consistent with the formation of products of internal return in the silver perchlorate-induced skeletal reorganizations of N-bromoa~abornanes.~~ Full details of the silver perchlorate-catalysed rearrangements of N-chloroanilines have been published.94 Substituent effects on rate and the negative p value (-6.35) observed in these transformations again implicate nitrenium (anilenium) cation intermediate^.^^ The kinetics and mechanism of Baeyer-Villiger rearrangements of benzalde- hydes have been investigated. The rate-determining step (carbonyl addition or migration) in these reactions is governed by the nature of the aryl ~ubstituent.~~ The exclusive migration of a cyclobutane nucleus in the Baeyer-Villiger oxida-tion of a tricyclic ketone has been reported.96 Evidence has been presented for and against the mechanistic rationale for ozonolysis proposed by Story last year [cf Ann.Reports (B) 1972,68,257]. The postulated peroxy-epoxide intermediate receives support from the demonstration (by deuterium labelling) that the anoma- lous oxidative rearrangement [(60)+(61)] is most readily explained in terms of an epoxide intermediate.97 The isolation of dioxetans under carefully controlled ozonolysis conditions is likewise adduced as evidence for the intermediacy of the Staudinger mol~zonide.~~ However a discordant note is introduced by spectroscopic evidence that the Criegee ozonide is the first demonstrable inter- mediate in o~onolysis.~~ Carbene Nitrene and Related Rearrangements.-An excellent review’ O0 includes carbene-induced rearrangements in (CH) hydrocarbons.Evidence has been presented for and against oxiren participation in the photochemical and thermal Wolff rearrangements of oxocarbenes. A pre-dominant oxiren pathway for the photolytic rearrangements of unsymmetrically 90 T. A. Kling R. E. White and P. Kovacic J. Amer. Chem. Soc. 1972,94 7416. 91 R. T. Fisher T. D. Bogard and P. Kovacic J. Amer. Chem. Soc. 1972,94,7599. 92 S. J. Padegimas and P. Kovacic J. Org. Chem. 1972,37 3672. 93 P. G. Gassman K.Shudo R. L. Cryberg and A. Battisti Tetrahedron Letters 1972 875. 94 P. G. Gassman G. A. Campbell and R. C. Frederick J. Amer. Chem. Soc. 1972,94 3884; P. G. Gassman and G. A. Campbell ibid. p. 3891. 95 Y. Ogata and Y. Sawaki J. Amer. Chem. Soc. 1972,94 4189. 96 S. A. Monti and S. S. Yuan J. Org. Chem. 1971 36 3350. 97 P. G. Gassman and X. Creary Tetrahedron Letters 1972 441 I. 98 P. R. Story E. A. Whited and J. A. Alford J. Amer. Chem. SOC.,1972,94 2143. 99 L. A. Hull I. C. Hisatsune and J. Hcicklen J. Amer. Chem. Soc. 1972 94 4856. loo L. T. Scott and M. Jones Chem. Rev. 1972,72 181. Molecular Rearrangements substituted diazo-ketones is indicated by the formation of both possible I$-unsaturated ketone products. '' ' Oxiren participation is not observed in triplet- sensitized photolysis implying its exclusive origin in a singlet carbene precursor.In thermally induced rearrangements the oxiren pathway is followed only at elevated temperatures the predominant process observed at lower temperatures being hydrogen shift concerted with nitrogen loss. lo Essentially similar conclu- sions have been arrived at from 13C-labelling studies.lo2 In contrast absence of oxiren participation is indicated by the lack of label migration to the carboxy- group in adamantane-2-carboxylic acid produced by photolysis of 5-diazo[5- ''C]homoadamantan-4-one in 75 % aqueous dioxan. lo3 However the expecta- tionlo3 that oxiren participation will be generally absent in strained ring systems must be tempered in the light of the observation'02 that oxiren formation declines sharply with increase in the water content of the photolysis medium.The rearrangement [(62) +(60)] is an example of rare alkoxy-group migration (in h competition with alkyl migration) in a thermally generated carbene.lo4 That ring strain by inhibiting ring-bond shift and insertion plays a crucial role in this anomalous rearrangement is indicated by the exclusive insertion which occurs in the thermally generated carbene (63). lo4 Acylamino-shifts have been observed as minor pathways in the photochemical and thermal Wolff rearrangements of diazo-amides. lo In contrast to their carbocyclic counterparts cyclic oxy-carbenes undergo predominant ring-bond (alkyl) migration to afford moderate yields of ring-contracted ketones.lo6 An elegant study'" has provided the first authentic example of a [2,3]-sigma- tropic rearrangement in an allylcarbene. The rearrangement sequence [(64) + lo' S. A. Math and P. G. Sammes J.C.S. Perkin I 1972 2623. lo' K-P. Zeller H. Meier H. Kolshorn and E. Muller Chem. Ber. 1972 105 1875. Io3 Z. Majerski and C. S. Redvanly J.C.S. Chem. Comm. 1972 694. 'O4 P. G. Gassman and X. Creary Tetrahedron Letters 1972 4407. lo5 M. L. Graziano R. Scarpati and D. Tafuri Tetrahedron Letters 1972 2469; N. Buu and J. T. Edward Cunad. J. Chem. 1972,50 3719. '06 A. M. Foster and W. C. Agosta J. Amer. Chem. Soc. 1972,94 5777. lo' J. E. Baldwin and J. A. Walker J.C.S. Chem. Comm. 1972. 354. 250 G. Tennant (65)+(66)] is the carbene analogue of the nitrene-type allylic diazene rearrange- ment described last year [cf.Ann.Reports (B) 1972 68,2441. This novel trans- formation provides a valuable new method for stereospecific carbon-carbon bond formation as well as a synthetic route to rearranged carboxylic acids from allylic halides [the precursors of (64)~"~ An equally elegant study log reports the thermal generation and subsequent rearrangement of allyloxycarbenes. Un-happily the elevated temperatures imposed by the mode of carbene generation precluded the observation of concerted sigmatropic processes and the ally1 shifts which occur are shown by deuterium labelling to involve radical fragmenta- tion-recombination pathways."* l 3C-Labelling indicates that the high-yield rearrangement of thermally generated benzylidenecarbene to phenylacetylene involves predominant (75 %) hydrogen migration and only 25 % phenyl migra- tion."' A number of papers dealing with the deep-seated rearrangements of phenylcarbene and related species have been published.* The 'abnormal' Beckmann rearrangement,' and rearrangements involving azido-groups l2 isocyanates,' l3 and N-nitrenes,' l4 have been reviewed. The dichotomy towards competing ring-bond migration observed in the Beckmann and Schmidt ring-expansions of hydrindane derivatives is due to the greater susceptibility of the latter process to steric effects.' Evidence for oxa- ziran intermediates in photo-Beckmann rearrangements has been reported. l6 Beckmann rearrangement of benzotropone oximes has been used as a synthetic entry to the benzoazocine ring system.' l7 Contrary to a previous suggestion,' l8 alkyl shifts which occur in the course of the phosphite reduction of t-nitrosoalkanes do not emanate from nitrene inter- mediates.The enhanced ratio of aryl to alkyl migration observed in these "* R. W. Hoffmann R. Hirsch R. Fleming and M. T. Reetz Chern. Ber. 1972 105,3533. '09 R. F. C. Brown and K. J. Harrington J.C.S. Chem. Comm. 1972 1175. ' ' O W. D. Crow et a/. Tetrahedron Letters 1972 223 1 2235 2239 3207; J. Amer. Chem. SOC.,94 4746; G. G. Vander Stouw A. R. Kraska and H. Schechter ibid. p. 1655; K. E. Krajca T. Mitsuhashi and W. M. Jones ibid. p. 3661 ;T. Mitsuhashi and W. M. Jones ibid. p. 677. R. T. Conley and S.Ghosh in ref. 1. p. 197. 'I2 D. V. Banthorpe in 'The Chemistry of The Azido Group'. ed. S. Patai Interscience New York 1971 pp. 397-440. 'I3 S. Ozaki Chem. Rev. 1972 72 457. 'I4 B. V. Ioffe and M. A. Kuznetsov Rum. Chem. Rev. 1972 41 131. 'I5 E. J. Moriconi and M. A. Stemniski J. Org. Chem. 1972 37 2035. l6 G. Just and M. Cunningham Tetrahedron Letters 1972 1151. 'I7 L. A. Paquette L. B. Anderson J. F. Hansen S.A. Lang and H. Berk J. Amer. Chem. SOC.,1972 94 4907. B. Sklarz and M. K. Sultan Tetrahedron Letters 1972 1319. Molecular Rearrangements 25 1 processes when compared with typical migratory aptitudes in thermal nitrene rearrangements clearly demonstrates the concerted nature of the deoxygenative processes.' l9 Sequential C-+N methyl migration occurs in the photolysis of 2,3-diazido-2,3-dimethylbutane. 2o Rearrangement processes observed in the photolysis of alkylideneisocyanates l2 ' and in the cycloaddition reactions of azirines with benzonitrile oxide122 are formulated as C+N alkyl [1,2]-shifts in alkylidene- and imino-nitrenes respectively.The formation of a-nitrosotoluene dimer by lead tetra-acetate oxidation of 0-benzylhydroxylamine is rationalized by an O+N benzyl shift in an 0-nitrene ir~terrnediate.'~~ An oxaziran interme- diate is excluded by the absence of benzaldehyde or benzamide among the rearrangement products. '23 Thermal Photochemical and Metal-catalySed Rearrangements.-Hydrocarbon thermal and photochemical rearrangements are the subject of two excellent Two illuminating article^'^^"^^ set forth the case for the incidence of sym- metry-forbidden but concerted pathways in sigmatropic rearrangements.The major conclusions reached in both papers are the same namely that whenever steric factors are unfavourable to the operation of orbital symmetry control (in terms of the Woodward-Hoffmann rules) intervention by other factors (e.g. subjacent orbital control involvement of low-lying excited states) can provide a concerted stereospecific but otherwise forbidden pathway for rearrangement. Orbital-symmetry-controlled sigmatropic rearrangements in which two a-bonds migrate intramolecularly are classified as 'dyotropic processes'. '27 The rarity of such rearrangements is attributed to the prohibitive activation energies needed for the simultaneous breakage of two o-bonds.lZ7 Rearrangements involving the trimethylenemethane biradical have been reviewed.12' Kinetic and deuterium-labelling studies demonstrate predominant retention of configuration in the degenerate thermal rearrangement of 2-methyl- 1-propylidenecyclobutane.This result implies that a symmetry-allowed but sterically unfavourable anturu-[1,2]-shift occurs to a significant extent in this rearrangement.'29 On the other hand the activation parameters (AH* = 44.8 k0.3 kcal mol- and AS* = +2.0 & 0.6 cal mol- ') for the new degenerate thermal methylenecycloalkane rearrangement [(67) G(68)] indicate a biradical process.'30 R. A. Abramovitch J. Court and E. P. Kyba Tetrahedron Letters 1972 4059.P. Margaretha and S. Solar Angew. Chem. Internat. Edn. 1972 11 1024. lZ1 C. J. Mikol and J. H. Boyer J.C.S. Chem. Comm. 1972 439. I ' ' V. Nair Tetrahedron Letters 1971 483 1. R. Partch B. Stokes D. Bergman and M. Budnik Chem. Comm. 1971 1504. "'J. J. Gajewski in ref. 1 p. I. J. E. Baldwin A. H. Andrist and R. K. Pinschmidt Accuunts Chem. Res. 1972,5,402. IZh J. A. Berson Accounts Chern. Res. 1972 5 406 (cf. also J. A. Berson and L. Salem J. Amer. Chem. Soc. 1972 94 8917). "' M. T. Reetz Angew. Chem. internat. Edn. 1972 11 129 130. P. Dowd Accounts Chem. Res. 1972 5 242. J. E. Baldwin and R. H. Fleming J. Amer. Chem. Soc. 1972 94 2140. I30 D. Hasselmann Tetrahedron Letters 1972 3465. 252 G.Tennant The controversy surrounding the mechanism of the thermal rearrangements of bicyclo[2,1,0]pent-2-enesto cyclopentadienes continues.The stereoselectivity reported' 31 for the thermal rearrangements of 1-methyl- and 2-methyl-bicyclo- [2,1,0]pentenes is consistent with the proposed'31 symmetry-allowed [,2 + ,2,] pathway. Contrary to these results however is the finding'32 that 2-methyl- and not 1-methyl-cyclopentadieneis the first trappable intermediate in the thermal rearrangement of 2-methylbicyclo[2,1,O]pentene.This observation coupled with a re-interpretation 133 of the original 13' kinetic data indicates non-stereoselectivity in these rearrangements in discord with the [,2 + ,2,] proposal. Compelling evidence also indicates a biradical mechanism as opposed to a [,2 + ,2,] path- way for the ready thermal rearrangement of a tricyclo[3,3,0,02~6]octa-3,7-diene to semibullvalene.34 Photochemical antara-[1,3]-carbon shift with inversion at the migrating carbon centre is exemplified by the novel bicyclo[3,1,0]hexene photoisomerization [(69)+(70)].135 The reversible unimolecular rearrangement of or-methylallyltrimethylsilane to trans-crotyltrimethylsilane at 500 "C in the gas phase or at 275 "C in solution represents the first example of an uncatalysed [1,3]-silicon shift in an allyl system.136 Acyclic concerted mechanism is implicated by the negative entropy of activation (AS* = -6.2 e.u. at 500 "C),the absence of substituent effects on rate and the lack of crossover products. Evidence for the anticipated suprafacial [1,3]-pathway with inversion at silicon is being sought.'36 Thermal sigmatropic [1,3]-nitrogen shift in an allyl structure is illustrated by the interesting hydroxyl- amine rearrangement [(71) -+ (72)].137 Several novel rearrangements involving sigmatropic [ 1,5]-carbon shifts have been described.The base-catalysed conversion of 4-bromobicyclo[3,2,O]hept- 2-ene into spiro[2,4]hepta-2,6-diene is rationalized by the intermediate formation ''I K. E. Baldwin and G. D. Andrews J. Amer. Chem. Sac. 1972,94 1775. 13* S. McLean D. M. Findlay and G. I. Dmitrienko J. Arrler. Chem. SOC.,1972,94 1380. M. C. Flowers and H. M. Frey J. Amer. Chem. SOC.,1972,94 8636. 134 A. Gold and W. T. Borden J. Amer. Chem. SOC.,1972 94 7179. 135 1972,94 3647. H. E. Zimmerman and G. A.Epling J.Amer. Chem. SOC. 136 H. Kwart and J. Slutsky J. Amer. Chem. SOC.,1972,94 2515. 13' R. L. Craigand J. S. Roberts J.C.S. Chem. Comm. 1972 1142. Molecular Rearrangements I + I a J"""l and thermal rearrangement of the elusive hydrocarbon bicyclo[3,2,0]hepta- 1,3-diene.l3 The latter process is formally a reverse vinylcyclopropane-methyl-enecyclobutane rearrangement and represents a rare thermal [1,5]-carbon shift in a ~yclopentadiene.'~~ A [1,5]-ester shift which occurs at temperatures as low as 100 "C has been reported. 39 The triplet-sensitized photochemical rearrange- ment [(73a) -+(74)] occurs by a formally forbidden [1,5]-carbon shift. 14* 0 0 1 K Ph Ph X (73) a X = CH (74) b;X=CO Acid-catalysed ovtho-Claisen rearrangements of allylphenyl ethers show a ca.105-fold rate enhancement compared with the purely thermal processes. 14' Enolate anions derived from ally1 esters rearrange under very mild conditions (-78 "C) to provide a valuable synthetic route to $-unsaturated acids.142 In related processes allenyl ethoxyvinyl ethers undergo smooth stereospecific [3,3]-sigmatropic rearrangement at 140 "C to afford diene esters in which the 13M N. K. Hamer and M. E. Stubbs Tetrahedron Letters 1972 3531. P. Schmidt R. W. Hoffmann and J. Backes Angew. Chem. Internaf. Edn. 1972 11 513. K. N. Houk and D. J. Northington J. Amer. Chern. SOC.,1971 93 6693. 14' U. Svanholm and V. D. Parker J.C.S. Chem. Comm. 1972 645. IJ2 R. E. Ireland and R. H. Mueller J. Amer. Chem. SOC.,1972 94 5897.254 G. Tennant new double bond is exclusively trans.'43 A highly stereospecific synthesis of unsaturated aldehydes involving sequential Claisen and Cope rearrangements has been described. 144 The reversal of the deep-seated Claisen rearrangement [(75)-+ (73b)] which occurs on irradiation involves a formally forbidden photo- chemical [3,3]-shift. 140 Thio-Claisen rearrangements in a simple ally1 vinyl sulphide16 and in allenyl ethynyl ~ulphides'~' have been reported. Novel examples of amino-Claisen rearrangements in quaternary nitrogen frameworks have also been described. '46*14' Elegant kinetic studies in conjunction with a new analytical approach to deuterium-labelling experiments have demonstrated a second degenerate Cope rearrangement of hexa-1,Sdiene.The transition state for the new process lies ca. 5.8 kcal mol- ' above that for the 'chair-like' Cope rearrangement and is tentatively assigned the 'boat-like' configuration. 148 A number of new re-arrangements attest to the ability of linearly constrained hydrocarbon structures to undergo concerted thermal reorganization. The degenerate thermal rearrange- ments of allenyl-propargyl derivatives exhibit kinetic parameters (E,= 30.8 * 0.3 kcal mol-' AS* = -11.7 cal mol-') consistent with their formulation as [3,3]-sigmatropic Cope-type proce~ses.'~~ Also amino-' and oxy-Cope ''O rearrangements have been demonstrated in allyl-propargyl structures. Pyrolysis of 1,6-dideuterio-cis-hexa-1,5-diyn-3-ene (76) in the gas phase at 300 "C results in complete label-scrambling between the acetylenic and vinylic positions [cf.(76)e(78)]. The free-radical character of this interesting rearrangement coupled H H . (77) with the isolation (in hydrogen-donor solvents) of benzene as by-product lends support to its formulation as a stepwise process in which para-benzyne (77) is a possible intermediate.I5' An ingenious attempt to demonstrate the 'ionic' Cope rearrangement [(79) (80) *(Sl)] has been reported. Clean thermal rearrangement of the dinitrile (79) occurred at 80 "C but resulted in the inversion 143 P. Cresson and M. Huche Comp. rend. 1972 274 C 2108. 144 R. C. Cookson and N. R. Rogers J.C.S. Chem. Comm. 1972 248. 145 J. Meijer and L. Brandsma Rec. Trav. chim. 1972,91 578.146 T. Laird and W. D. Ollis J.C.S. Chem. Comm. 1972 557. 141 J. B. Bapat D. St. C. Black R. F. C. Brown and C. Ichlov Austral. J. Chem. 1972,25 2445. 148 M. J. Goldstein and M. S. Benzon. J. Amer. Chem. Soc. 1972,94 7147 7149. 149 H. Hopf Tetrahedron Letters 1972 3571. 150 N. Manisse J. C. Pommelet and J. Chuche Bull. SOC.chim. France 1972 2423. 151 R. R. Jones and R. G. Bergmann J. Amer. Chem. SOC.,1972,94 660. 255 Molecular Rearrangements f NC NC NcP (79) (80) f NC+ NC NC of only one of the two ally1 units to give (82).‘52 The absence of a CIDNP effect excludes a radical pathway for this transformation. Conversely an ionic frag- mentation-recombination process [(79) (80)$(82)]is indicated by the isola- tion of anethole when the rearrangement is carried out in the presence of hydride i0n.153 A number of Cope rearrangements have been assigned ‘boat-like’ (as opposed to ‘chair-like’) transition states.The stereoselectivity observed lS4 in the re- arrangement of trans-1,2-di(prop-l ’-eny1)cyclopropane to 6,7-dimethylcyclo-hepta-1,4-diene is consistent with a concerted pathway involving a ‘boat-like’ transition state. A study of the influence of steric effects on rate provides general support for ‘boat-like’ transition states in the Cope rearrangements of ~is-1~2- dialkenylcyclobutanes. However in cis-propenyi derivatives steric inhibition of the boat-like’ transition state results in the operation of a new and possibly concerted symmetry-forbidden pathway155 (cf.refs. 125 and 126). The thermal rearrangement of truns-3,4-dimethyl-cis,truns-cyclo-octa-l,5-dieneto cis-3’4-dimet h yl-cis,cis-cyclo-octa- 1,5-diene involves cis- 1,2-trans,trans-dipropenylcyclo-butane as intermediate and is the result of two sequential highly stereospecific Cope rearrangements the first ‘chair-like’ and the second ‘boat-like’. 56 The first kinetic data for the degenerate homotropilidene rearrangement have been obtained from ‘H n.m.r. studies of an octadeuteriobicyclo[5,1,0]octa-2,5-diene. D. C. Wigfieid S. Feiner and K Taymaz Tetrahedron Letters 1972 891. 153 D. C. Wigfield S. Feiner and K. Taymaz Tetrahedron Letters 1972 895. C. Ultenius P. W. Ford and J. E. Baldwin J. Amer. Chem. Soc. 1972 94 5910. J. A. Berson and P. B. Dervan J.Amer. Chem. Soc. 1972,94 7597. J. A. Berson P. B. Dervan and J. A. Jenkins J. Amer. Chem. Sac. 1972 94 7598. 256 G. Tennant The observed activation parameters (AH* = 11.8 & 0.2 kcal mol-I and AS* = -8.0 f0.3 ex.) in conjunction with the results of n.m.r. studies again demonstrate a concerted 'boat-like' process.'57 Two new degenerate thermal rearrangements in C, hydrocarbons have been reported. Label scrambling observed at 185 "C in a hexadeuteriated cis-bicyclo- [6,2,0]deca-2,6-diene is attributed to a stereospecific Cope rearrangement [(83) (84)] originating in the less-stable folded conformation of the molecule H H (83) (841 and occurring in such a way that the methylene hydrogens maintain their con- figuration in relation to the attached ring.lS8 On the other hand the degenerate thermal rearrangement (revealed by label-scrambling at 510 "C) of a di-deuteriated cis-9,lO-dihydronaphthalene could occur either by a disrotatory ring- opening-ring-closure process or by sequential [3,3]-sigmatropic shifts of the rare antara-antara type.The saga of the thermal reorganizations of cis-bicyclo[6,1,0]nona-2,4,6-triene and its derivatives continues. The concerted symmetry-forbidden rearrangement (see previously) of bicyclo[5,2,0]nona-2,5,8-triene to all-cis-cyclononatetraene is proposed'60 as the key step in the problematical thermal rearrangement of cis-bicyclo [6,1 ,O]nona-2,4,6-triene to cis-3a,7a-di hydroindene. The elusive thermally degenerate [1,7]-sigmatropic carbon shift in the bicyclo-[6,1,0] nonatriene ring system has now been demonstrated.Thermal rearrange-ment of the epimeric cis-9-cyano-9-methylbicyclo[6,f,O]nona-2,4,6-trienes (85) at ca. 100 "Cresults in complete equilibration (Scheme 3) to the isomers [(86j- (88)l. The activation parameters and stereospecificity of this process are in accord with orbital-symmetry-controlled migration of C-9 (with inversion) round the periphery of the eight-membered ring. l6 ' The demonstration l6 ' that the stereochemical integrity of C-9 in (85) is maintained up to 180 "C is intriguing (to say the least !) in view of the reported'62 epimerization of the esters (85a and b ; C0,Et for CN H for Me in the cyclo-octane ring) at 160°C. The challenge posed by last year's theoretical predictions 163 regarding the Cope rearrange- ment which cannot be 'frozen out' may have been met by Paquette and his 15' H.Gunther J. B. Pawliczek J. Ulmen and W. Grimme Angew. Chem. Internat. Edn. 1972 11 517. 15' W. Grimme J. Amer. Chem. SOC.,1972 94 2525. 159 L. A. Paquette J. Amer. Chem. Sue. 1971 93 7110. 16' J. E. Baldwin A. H. Andrist and R. K. Pinschmidt J. Amer. Chem. SOC.,1972 94 5845. F. G. Klarner Angew. Chem. Internat. Edn. 1972 11 832. 16' M. B. Sohn M. Jones and B. Fairless J. Amer. Chem. SOC.,1972 94 4774. 163 R. Hoffmann and W.-D. Stohrer J. Amer. Chem. Suc. 1971,93,6941;M. J. S. Dewar Z. Nahlovska and B. D. Nahlovsky J.C.S. Chem. Comm. 1971 1377. Molecular Rearrangements (88) a; X = CN Y = Me b;X = Me.Y = CN Scheme 3 co-workers who have described an elegant synthesis of the potentially homo- aromatic Cope structure (89).164 U.V.and n.m.r. studies provide tentative support for the anticipated diamagnetic ring current in (89).164 Fluxional character in a bullvalene homologue has been demonstrated for the first time. Deuterium-labelling studies reveal the operation of the novel homobullvalenone equilibrium [(90) (91) S(92)]. 65 Interestingly none of the homotropilidene isomer (93) could be detected demonstrating a clear preference in the homo- bullvalene ring system for electron-withdrawing groups (i.e.C=O) to reside at L. A. Paquette R. E. Wingard and R. K. Russell J. Amer. Chem. SOC.,1972,94,4739. 16' M. J. Goldstein R. C. Krauss and S. H..Dai J. Amer. Chem.SOC.,1972 94 680. 258 G. Tennant C-516' [in contrast to bullvalenes where a C-1 preference is shown; cf Ann. Reports (B) 1971,68 2601. Ever increasing effort is being devoted to the elucidation of the detailed mechan- isms of metal-catalysed strained o-bond rearrangements. Two of the leading protagonists in the field have published full details of the work'66 reported in preliminary form last year [cf Ann. Reports (B) 1971 68 2621. Studies during the past year have centred mainly on the nature of the intermediates involved and the rnodus operandi of the metal catalyst. Thus n.m.r. st~dies'~~~'~* of the palla- dium(1r)-catalysed rearrangement of tricycl0[4,1,O,O~*~]heptanehave clearly demonstrated the initial formation of a carbenoid complex having at least some of the features of the metal-complexed-carbene-metal-bonded-carbonium-ion hybrid intermediate postulated for metal-catalysed strained a-bond rearrange- ments.The formation 169of dihydroazulene products in the rhodium(1)-catalysed rearrangements of phenyl-substituted bicyclo[ l,l,O]butanes is likewise consistent with insertion in an intermediate metal carbenoid. However products attri- buted ''O to the capture of a metal cyclopropylmethyl cation precursor by solvent have in fact been shown'67 to arise from uncatalysed carbonium ion processes promoted by acidic impurities in the metal catalyst. Kinetic studies also demonstrate the reversible formation of a metal complex in the silver(])-catalysed rearrangements of homocubanes.' ' The role played by the catalyst has been considerably clarified.A comprehen-sive study17' of the transition-metal-catalysed skeletal reorganizations of homo-cubanes shows that the course of rearrangement is dependent not only on the nature of the metal but also on the o-donor-n-acceptor capacity of the attached ligands. Product dependency on the nature of the catalyst is likewise apparent in the rhodium- and copper-catalysed rearrangements of bicyclo[ l,l,O]butanes.' 73 Solvent effects also appear to be important. Thus Ag'- and Rh'-promoted re- arrangements of bicyclobutanes follow similar courses in methanol in contrast to the divergent behaviour observed in aprotic media. 174 Highly stereospecific methyl shifts occur in the Ag'-catalysed rearrangements of tricyc10[3,2,0,0~~~]-heptane derivatives.'7s Rh'- and Ag'-catalysed rearrangements of tricyclo- [2,2,0,02*6]hexanehave been reported. '76 ' h6 P. G. Gassman and F. J. Williams J. Amer. Chem. SOC.,1972,94,7733; P. G. Gassman G. R. Meyer and F. J. Williams ibid. p. 7741 ;P. G. Gassman and T. J. Atkins ibid. p. 7749; P. G. Gassman T. J. Atkins and J. T. Lumb ibid. p. 7757; L. A. Paquette S. E. Wilson R. P. Henzel and G. R. Allen ibid. p. 7761 ;L. A. Paquette S. E. Wilson and R. P. Henzel ibid. pp. 7771 7780. 167 W. G. Dauben and A. J. Kielbania J. Amer. Chem. SOC.,1972 94 3669. IhS S. Masamune M. Sakai and N. Darby J.C.S. Chem. Comm. 1972,471. 169 P. G. Gassman and T. Nakai J. Amer. Chem. SOC.,1971 93 5897; P. G. Gassman Angew. Chem.Internat. Edn. 1972 11 323. P. G. Gassman and F. J. Williams J.C.S. Chem. Comm. 1972 80. 17' L. A. Paquette and J. S. Ward Tetrahedron Letters 1972 4909. 172 W. G. Dauben and A. J. Kielbania J. Amer. Chem. SOC.,1971 93 7345. 17' P. G. Gassman and T. Nakai J. Amer. Chem. SOC.,1972,94 2877. P. G. Gassman and T. Nakai J. Amer. Chem. SOC.,1972,94 5497. ''' L. A. Paquette and L. M. Leichter J. Amer. Chem. SOC., 1972 94 3653. R. J. Roth and T. J. Katz J. Amer. Chem. SOC.,1972 94 4770. Molecular Rearrangements 259 3 Aromatic Rearrangements Dienone-phenol and dienol-benzene transformations and rearrangements involving cyclohexadienyl anions cations and radicals have been reviewed.2 Topics covered in other review articles include spirocyclic rearrangements 77 aromatic valence-bond isomerizations 78 and the 'base-catalysed halogen dance'.179 The first unequivocal examples of anionic [1,2]-aryl shifts have been de- scribed. The operation of a [1,2]-biphenylyl shift in the alkali-metal-catalysed conversion of 2-p-biphenylyl-l-chloro-l,l-dideuterioethane into l-p-biphenylyl- ethane is demonstrated by label equilibration between C-1and C-2. A carbanionic (as opposed to radical) pathway is inferred from the inhibition of rearrangement by added t-butyl alcohol.'80 Alkali metals also promote anionic [1,4]-biphenylyl shift in l-p-biphenylyl-4-chloro-1,l-diphenylethane. The involvement of a spiro- [4,5]decadienyl anion intermediate in this rearrangement is indicated by the formation of 8-phenylspiro[4,5]-6,9-decadiene-8-carboxylicacid when l-p-biphenylyl-4-chlorobutane is treated with caesium-potassium-sodium alloy and subsequently carbonated.l8 In contrast to the apparent ease of biphenylyl migration [1,4]- and [1,5]-anionic phenyl shifts are only minor processes in the alkali-metal-catalysed transformations of 4,4,4-triphenylbutyl chloride and 5,5,5-trjphenylpentyl chloride. lg2 Hitherto unknown [1,4]- and [1,5]-phenyl shifts in 4,4-diphenylcyclohexyl and 4,4-diphenylcyclohexylmethylradicals have been reported. 83 The absence of similar phenyl migrations in the correspond- ing cyclopropyl cyclobutyl and cyclopentyl radicals is consistent with ring- strain effects and implies an intramolecular (bridged cyclohexadienyl radical) pathway for the [1,4]-phenyl shift.183 A novel [1,4]-tolyl shift from sulphur to carbon in a polycyclic structure is also explained in terms of a bridged cyclo- hexadienyl radical intermediate.lg4 The faster rate of ortho to para compared with ortho to meta t-butyl migration in the 1,2-di-t-butylbenzenium ion cannot be explained by consecutive shifts involving benzenium and benzonium ions. These and related alkyl shifts and isomerizations are ~onsidered"~ to be best rationalized by 'bond to bond' (polytopal) rearrangements [cf ref. 461 in benzonium ions. Hitherto unknown trimethylsilyl migrations are implicated in the formation of 1,3-and 1,4-di-(trimethylsily1)benzenes by acid-catalysed isomerization of 1,2-di(trimethylsilyl)- benzene. The absence of crossover in these rearrangements is consistent with the operation of intramolecular (and presumably polytopal) [1,2]-trimethylsilyl shifts 177 M.S. Newman Accounts Chem. Res. 1972,5 354. 178 E. E. van Tamelen Accounts Chem. Res. 1972,5 186. J. F. Bunnett Accounts Chem. Res. 1972 5 139. I8O E. Grovenstein and Y. M. Cheng J. Amer. Chem. SOC. 1972,94,4971. Is' E. Grovenstein S. Akabori and J. U. Rhee J. Amer. Chem. SOC.,1972,94,4734. 18* E. Grovenstein J. A. Beres Y. M. Cheng and J. A. Pegolotti J. Org. Chem. 1972,37 1281. 183 J. W. Wilt R. A. Dabek and K. C. Welzel J. Org. Chem. 1972 37 425. 184 R. Loven and W. N. Speckamp Tetrahedron Letters 1972 1567. 18' G. A. Olah R. H. Schlosberg R. D. Porter Y. K. Mo D. P. Kelly and G. D. Mateescu J. Amer. Chem. Soc.1972 94 2034. 260 G. Tennant in the 1,2-di(trimethylsilyl)benzeniumion. 86 The temperature dependence of the n.m.r. spectrum of the nitrohexamethylbenzenium ion is in accord with de- generate migration of the nitro-group round the periphery of the ring. The activation energy (E = 16.8 k 1.5 kcalmol-') for this novel process and the lack of nitration in added electrophiles are consistent with a benzenium-ion- benzonium-ion pathway [(94) (95) (96)] involving consecutive intra- NO* (94) (951 (96) molecular [1,2]-nitro-shifts. 18' The temperature independence of the n.m.r. spectrum of the chlorohexamethylbenzenium ion demonstrates the higher energy barrier to degenerate chlorine migrati~n.'~' Isomer ratios in the 'mixed acid' nitration of o-xylene vary with the concentration of the sulphuric acid used.This anomalous effect is due to inhibition of a competing pathway for ortho-nitration involving 'ipso' attack at C-1and subsequent [1,2]-nitro-shiftin the 1,2-dimethyl-l-nitrobenzenium ion produced.' 88 The kinetics of the acid-catalysed rearrangement of 4,4-dimethylcyclohexa- 2,4-dien- 1-one to 3,4-dimethylphenol are in accord with reversible protonation to a hydroxycyclohexadienyl cation followed by rate-determining [1,2]-methyl shift.lB9 The [1,2]-methyl shift invol~ed'~~~'~~ in the preferential rearrangement of 1'4-dimethylbenzene oxide to 2,4-dimethylphenol occurs under both neutral and acidic conditions and serves as a model for in viuo carbon migrations in arene oxides (NIH shifts).Zwitterionic intermediates are proposed to account for the uncatalysed process whereas acid-catalysed rearrangement is rationalized in terms of hydroxydimethylbenzenium ion intermediates produced directly from the protonated epoxide or after hydration to 1,4-dimethylcyclohexa-2,5-diene- 1,4-diol. ' Cyclopropylmethylcyclohexadienones rearrange thermally to phenols by formal [1,3]-cyclopropylmethyl shifts which occur even when the migration terminus is blocked and show a lo3 rate enhancement compared with the corre- spondingly methyl shift. '92 Since bimolecular and fragmentation-recombination pathways can be excluded it is suggested that these novel rearrangements should be classified as rare thermal [1,5]-carbon shifts which are not also formally [1,2]- processes.92 In contrast to the ease of thermal rearrangement acid-catalysed lB6D. Seyferth and D. L. White J. Amer. Chem. SOC.,1972,94 3132. 18' G. A. Olah H. C. Lin and Y. K. Mo J. Amer. Chem. SOC.,1972,94 3667. Ia8 P. C. Myhre J. Amer. Chem. SOC.,1972 94 7921. Ia9 V. P. Vitullo and N. Grossman J. Amer. Chem. SUC.,1972 94 3844. 190 E. A. Fehnel J. Amer. Chem. SOC.,1972,94 3961. 191 G. J. Kasperek T. C. Bruice H. Yagi N. Kaubisch and D. M. Jerina J. Amer. Chem. SOC.,1972 94 7876. 192 B. Miller and K. H. Lai Tetrahedron Letters 1972 517. Molecu Iar Rearrangements 261 cyclopropylmethyl migration in cyclohexadienones occurs only to a minor extent (ca. 10%) and is formulated on the basis of labelling experiments as a cyclopropylmethyl cation-phenol fragmentation-recombination process.How- ever the lack of skeletal reorganization in the cyclopropylmethyl moiety is surprising.'92 Normal [2,3]- and [1,5]-sigmatropic allyl shifts in P-naphtha- lenones would disrupt the conjugation in the benzene ring and so should be inhibited. In accord with this proposal '93 allyl-a-naphthalenones (97) are found to undergo novel acid-catalysed [3,4]-sigmatropic allyl shifts [cf (97)-+(98)] (97) a;R = H (98) a; R = H Me b;R=Me b:R=Me (99) unprecedented in dienone-phenol rearrangements of this type. Rearrangement of the P-naphthalenone (97b) also gave the [1,5]-shift product (99) despite the anticipated inhibition of this process.'93 Competing [1,2]- and [3,3]-sigmatropic allyl shifts are involved in the acid-catalysed dienol-benzene rearrangements of the highly crowded cyclohexadienols (100).194 Propargylcyclohexadienols (101) and ( 102)undergo novel acid-catalysed dienol-benzene rearrangements to afford (100) a; R' = RZ= Me (101) a; R = H (102) a; R = H b R' = Ph RZ = H b:R=Me b:R=Me c R' = Ph R2 = Me allenyl- and propargyl-benzene derivatives by a combination of [1,2]- [3,3]- and [3,4]-sigmatropic propargyl shifts in a common benzonium ion intermediate.'95 Spirocyclohexadienol-benzene rearrangements involving preferential migra- tion of nitrogen in competition with migration of carbon have been described.'96 The predominant inversion observed in the ortho-and para-s-butylphenols formed when optically active s-butyl phenyl ether is treated with aluminium bromide (Fries rearrangement) is inconsistent with a n-complex mechanism and Iy3 B.Miller and M. R. Saidi Tetrahedron Letters 1972 4391. 194 K. H. Lai and B. Miller J. Org. Chem. 1972 37,2505. 195 H. Heimgartner J. Szindely H. J. Hansen and H. Schmid Helv. Chim. Acta 1972 55 1113. Iy6 D. H. Hey G. H. Jones and M. J. Perkins J.C.S. Perkin I 1972 1162. 262 G. Tennant is rationalized by a bimolecular alkylation proces~.'~' On the other hand the increased ortho :para ratio and predominant retention in the ortho-product but racemization in the para-product observed when Fries rearrangement is carried out by adding the substrate to the catalyst (inverse addition) is inconsistent with both the x-complex and bimolecular pathways and is reconciled in terms of an ion-pair mechanism.197 Two investigations provide support for the caged- radical-pair mechanism of the photo-Fries rearrangement. A meticulous study 19' has shown that the composition of the hydrocarbon gas produced in the photo- Fries rearrangements of phenyl esters is consistent only with decarbonylation of intermediate acyl radicals. Flash photolysis experiments designed to detect these intermediates were inconclusive. '98 However similar studies' 99 have successfully demonstrated the intermediacy of the phenoxyl radical and the subsequently produced 6-acetylcyclohexa-2,4-dien-l-oneintermediate in the photochemical rearrangement of phenyl acetate. Radical intermediates were also detected in the photo-Fries rearrangements of anilides demonstrating a common caged-radical pathway.Contrary to the excited-state mechanism are the lack of rearrangementI9' in gas-phase photolysis and the in~ariance'~~ of the phenol phenolic ketone ratio with variation in the wavelength of the light used for irra- diation. The cis +trans isomerization observed in the photo-rearrangement of trans-but-2-enyl phenyl ether is also in accord with a radical pathway for the photo-Fries rearrangements of phenyl ethers.' 99 Abnormal photo-Fries re-arrangements of alkyl aryl phthalates200 and photo-Fries rearrangements of aryl methyl oxalates201 have been reported. The intramolecular character of the acid-catalysed rearrangement202 of N-meth y l-N-n itro-9-anthran ylamine to 10-ni tro-9-an throne is demonstrated by the lack of inhibition by added scavengers for nitrous acid and nitronium ion and by the lack of label incorporation in the presence of H15N0,.Transfer of the nitro-group in a concerted or caged-radical- or ion-pair process is sug-gested.202 However the abnormally low solvent isotope effect found (k,20/kH,o = 0.84) indicates a mechanistic dissimilarity with the intramolecular nitramine rearrangements of N-nitroaniline and N-nitro-1-naphthylamine for which kD20/kH20 = 2.5-3.3.202 Amines which react as the free base undergo nitra- tion by initial attack at the amino-group and subsequent intramolecular re-arrangement of the nitramine produced.203 The imidoyl chlorides (103) react with silver nitrate to give nitryloxyformamidines (104)which are spontaneously unstable and rearrange to mixtures of 0-and p-nitrophenylureas (105) and (106).204The low ortho :para ratio and nitration of added veratrole observed in these unusual rearrangements exclude 'cartwheel' and x-complex pathways but 19' P.A.Spanninger and J. L. Von Rosenberg J. Amer. Chem. SOC.,1972,94 1970 1973; M. J. S. Dewar and P. A. Spanninger J.C.S.Perkin II 1972 1204. 198 J. W. Meyer and G. S. Hammond J. Amer. Chem. Soc. 1972,94,2219. 19' C. E. Kalmus and D. M. Hercules Tetrahedron Lerters 1972 1575. *O0 A. S. Kende and J. L. Belletire Tetrahedron Letters 1972. 2145. 20' T. Inoue Y. Shigematsu and Y. Odaira J.C.S. Chem. Comm. 1972. 668. 202 D. V. Banthorpeand J. G. Winter J.C.S. Perkin 11 1972 1259. *03 J. H.Ridd and E. F. V. Scriven J.C.S. Chem. Comm. 1972 641. 204 W. Reid and W. Merkel Chem. Ber. 1972 105 1532. Molecular Rearrangements R I (103) R / R provide tentative support for a radical-pair mechanism.204 H N.m.r. studies in super-acid media indicate that the diprotonated dication (107)and the derived dehydrated azobenzene dication (108)are the key intermediates in the Wallach rearrangement of azo~ybenzene.~~~ Neither the oft-proposed oxadiaziridine intermediate nor its conjugate acid could be detected.205 On the other hand an oxadiaziridine process is considered to accord best with the results of detailed kinetic and labelling studies of the arenesulphonyl anhydride-catalysed re-arrangement of azoxybenzenes to p-arenesulphon yloxyazo benzenes.206 Despite the results of n.m.r. studiesZo5 in super-acid media provide strong support for the key CN-diprotonated intermediate (109) in the mechanistic rationale for the two-proton benzidine rearrangement proposed by Allantog last year. Reasoned arguments have been presented for205 and against209 the capacity of the intermediate (109)to account for the kinetics and NH,-NH NHz-NH + (109) *05 G. A. Olah K. Dunne D. P. Kelly and Y.K. Mo J. Amer. Chem. SOC.,1972,94,7438. 'Ob S. Oae and T. Maeda Tetrahedron 1972 28 2127. '07 D. V. Banthorpe Tetrahedron Letters 1972 2707. 'OB Z. J. Allan Tetrahedron Letters 1971 4225. '09 D. V. Banthorpe J.C.S. Perkin 11 1972 874. 264 G. Tennant product distributions observed in the two-proton rearrangement.In contrast aromatic proton exchange is not observed in the CF,CO,D-catalysed benzidine rearrangement of tetraphenylhydrazine thus implying the initial formation of an NN-diprotonated species.210 However compelling evidence supports the subsequent formation and rearrangemer. t of te trapheny lhydrazine radical-ca tion in this process and in the related photo-benzidine rearrangement of tetraphenyl- hydrazine.’ 4 Heterocyclic Rearrangements Nitrene-induced rearrangements involving spiro-heterocyclic structures have been reviewed.’ Nitrogen Heterocycles.-Kinetic and stereochemical data for the gas-phase thermal rearrangements of N-allylpyrroles to 2- and 3-allylpyrroles is consistent with the operation of [1,5]- and [3,3]-sigmatropic ally1 shifts and supports ‘chair-like’ transition states for the latter processes.2l3 The temperature dependence of the n.m.r.spectra of 1 -carboxamido-3,5-dimethylpyrazolesis interpreted in terms of a degenerate N-1 to N-2 acyl shift. An intermolecular pathway for transfer of the acyl group is indicated by the formation of crossover products.’ l4 The thermal rearrangement of 3-benzyl-3-cyanoindazole to 3-benzyl-1-cyanoindazolerepresents the first direct demonstration of a formal thermal [1,3]-shift in an indazole derivative. Migration of the cyano-group by sequential [1,2]-shifts rather than by a direct [1,3]-shift is suggested.21s 3-Ethoxy- carbonyl- and 3-cyano-pyrazolenines rearrange to the corresponding N-substi- tuted pyrazoles at temperatures as low at 80°C.The ease of these uncatalysed [1,5]-sigmatropic C4N shifts contrasts with the difficulty of effecting similar [1,5]-shifts in carbocyclic structures and is attributed to the driving force for attainment of aromatic stability.’ l6 Photo-hydration of N-methylpyridinium chlorides originates in the n-n* excited state and results in ring-contraction to 6-methylazabicyclo[3,l,O]hex-3-en-2-exo-01s (110). Deuterium-labelling studies suggest the intermediacy of azoniabenzvalenes (1 1 1) in these novel rearrangements.* Irradiation of 1H-2,3- benzodiazepines at 0 “C promotes electrocyclic ring-contraction to the unique fused diazacyclobutene derivatives (1 12).’ Heterocyclic valence isomeriza- tions’ l9 and skeletal rearrangements220 leading to azocines have been reported.2Lo U. Svanholm and V. D. Parker J.C.S. Chem. Comm. 1972,440. 211 U. Svanholm K. Bechgaard 0. Hammerich and V. D. Parker Tetrahedron Letters 1972 3675; U. Svanholm and V. D. Parker J. Amer. Chem. Soc. 1972,94 5507. 212 J. I. G. Cadogan Accounts Chem. Res. 1972 5 303. 213 J. M. Patterson J. W. de Haan M. R. Boyd and J. D. Ferry J. Amer. Chem. Soc. 1972,94,2487. 214 J. Castells M. A. Merino and M. Moreno-Manas J.C.S. Chem. Comm. 1972 709. 215 R. E. Bernard and H. Schechter Tetrahedron Letters 1972 4529. I M. Franck-Neumann and C. Buchecker Telrahedron Letters 1972 937. L. Kaplan J. W. Pavlick and K. E. Wilzbach J. .4mer. Chem. Sac. 1972 94 3283. 218 A. A. Reid J. T. Sharp and S. J. Murray J.C.S. Chem. Comm. 1972 827.219 D. Stusche M. Breuninger and H. Prinzbach Heh. Chim. Acta 1972 55 2359; P. G. Lehman Tetrahedron Letters 1972 4863. 220 A.Padwa P. Sackman E. Shefter and E. Vega J.C.S. Chem. Comm. 1972 680. Molecular Rearrangements A' Me R2 (1 13) (1 14) The highly strained triazoline derivative (113) undergoes thermal rearrangement at 195"C by unprecedented scission of the central cyclobutane bond to afford the remarkably stable triazonin (114).221 Oxygen and Sulphur Heterocycles.-The activation parameters for the highly stereospecific thermal valence isomerizations of cis-2,3-dialkenyl epoxides to oxepins are consistent with concerted [3,3]-sigmatropic processes involving 'boat-like' transition states.222 The stereospecificity observed in the corre-sponding thermal rearrangements of trans-2,3-dialkenyl epoxides to dihydro- furans cannot be reconciled with a biradical mechanism and is rationalized in terms of disrotatory ring-closure in a carbonyl ylide intermediate produced by conrotatory epoxide ring-opening.222 Remarkable valence isomerizations of the hitherto unknown benzene dioxide and trioxide have been described.Thus syn-benzene dioxide on heating above 50 "Cundergoes223 reversible symmetry- allowed [,2 + ,2 + ,2,] cycloreversion to afford 1,4-dioxocin which predominates to the extent of 9504 in the equilibrium. In contrast the symmetry-allowed [,2 + ,2 + ,2,] cyclore~ersion~~~.~~ in the syn-benzene trioxide (1 15) occurs only at 200 "C,and irreversibly to afford the novel hetero- cycle cis,cis,cis-1,4,7-trioxacyclononatriene (116).The operation of orbital symmetry control in these epoxide-oxonin valence isomerizations is demon- strated by the lack of rearrangement in the anti-benzene trioxide (1 17) even at 400-500°C in the gas phase.224 A radical fragmentation-recombination pathway is proposed to account for the interesting thermal isomerization of a rneta-dithiin to an ortho-dithiin.226 22' L. A. Paquette and R. J. Haluska J. Amer. Chem. SOC.,1972 94 534. 222 J. C. Pommelet N. Manisse and J. Chuche Tetrahedron 1972 28 3929. 233 E. Vogel H. J. Altenbach and D. Cremer Angew. Chem. Internal. Edn. 1972,11,935; H. J. Altenbach and E. Vogel ibid. p. 937. L24 E. Vogel H. J. Altenbach and C. D. Sommerfeld Angew. Citem. Internal.Edn. 1972 11 939. 225 R.Schwesinger and H. Prinzbach Angew. Chem. Internat. Edn. 1972 11 942. "' U. Eisner and T. Krishnamurthy Tefrahedron 1971 27 5753. 266 G. Tennant Cyclobutene adducts derived from thiophens and benzo[b]thiophens readily undergo thermal valence isomerization providing viable synthetic routes to the elusive thiepin and benzo[b]thiepin ring systems.227 In contrast irradiation is reported” to induce disrotatory electrocyclic ring-closure in benzo[b]thiepins affording benzo[b] thiophen-cyclobutene adducts. Pride of place for the year’s most unique molecular rearrangement must surely go to the remarkable homocubane transformation (118)-(119). A twelve-step mechanism involving no fewer than twenty bond-making and -breaking processes has been proposed ! O.8N-KOH Bu‘OH 10 min ; R.T.227 D. N. Reinhoudt and C. G. Kouwenhoven J.C.S. Chem. Comm. 1972 1232 1233. 228 H. Hofrnann and B. Meyer Tetrahedron Letters 1972,4597. 229 K. V. Scherer Tetrahedron Letters 1972 2077; R. Bau ibid. p. 2081.

ISSN:0069-3030    

DOI:10.1039/OC9726900235

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

7 Organometallic Compounds of the Transition Elements By J. P. CANDLIN and G. L. P. RANDALL Imperial Chemical Industries Limited Corporate Laboratory P.O. Box 71. The Heath Runcorn Cheshire and A. W. PARKINS Department of Chemistry. Queen Elizabeth College London W.8 I Introduction At least two techniques using organometallic compounds have been developed in the past few years which can now be used with confidence for synthetic ap- plications in organic chemistry catalytic hydrogenation and carbonylation despite the fact that both these topics were almost chemical curiosities ten years ago. New areas which are within this category for exploitation include dimeriza- tion (and oligomerization) and disproportionation of olefins. The use of organo- metallic compounds in organic asymmetric synthesis is a special case since it leans heavily on prior work.Use of these systems in natural product chemistry will undoubtedly follow. A new section has been introduced heterogenized homogeneous catalysis (Section 11). Although this topic considers heterogeneous examples the chemistry involved can be directly translated from analogous homogeneous systems. 2 Reviews Any assistance in encompassing the ever increasing and diversifying field of organometallic chemistry is very welcome and the ‘The Guide to the Literature of Organo-Transition Metal Chemistry 1950-1970’ gives great help in this area of chemistry.’ Gmelin’s Handbuch on organic compounds of chromium and vanadium2 and a book with 1879 references to mono-olefin complexes3 have been published.Two new review series4s5 provide more or less complete coverage of the organometallic literature. A new volume of ‘Organometallic Reactions’ has appeared.‘ ’ M. I. Bruce Adu. Organornetallic Chem. 1972 10 273. Gmelins Handbuch der Anorganischen Chemie (suppl. to 8th ed.) Verlag Chemie Weinheim 1971 vols. 2-3. M. Herberhold ‘Metal x-Complexes,’ Elsevier Amsterdam 1972 vol. 11 part I . ‘Organometallic Chemistry,’ ed. E. W. Abel and F. G. A. Stone (Specialist Periodical Reports) The Chemical Society London 1972 vol. 1 . ‘ ‘Organometallic Reactions,’ Wiley New York 1972 vol. 4. M.T.P. International Review of Science Butterworths London 1972. 267 268 J . P . Candlin G. L. P . Randall and A . W. Parkins Specific reviews on carbene carbon-carbon bond formation with metal intermediates," olefin oxidation with Group VIII complexes,' ' n-com- the aid of nickel catalyst^,^ synthesis of heterocyclic compounds zlia transition- plexes of transition metals,' catalysi~,'~ reactions of bis(cyclopentadieny1)transi- tion-metal compound^,'^ CT-n rearrangements,' olefin disproportionation,'6*1 bonding in olefin and acetylene complexes,' and organometallic compounds in the mass ~pectrometer'~ have appeared but this list is in no way exhaustive.3 Organometallic Compounds n-Complexes.-Although arenechromium compounds can form adducts with trinitrobenzene in which both faces of the arene ring are involved in the bond- ing," no such compounds are known in which cyclopentadienyl rings form n-bonds to two acceptor species.However the reaction of ( ~ c - C ~ H ~ ) ~ N ~ with Ph,C+BF,- or HBF gives a cation with the stoicheiometry (n-C,H,),Ni,+ which has been suggested as having a double sandwich structure.2'*22 A similar ion has been suggested as a species in the mass spectrum of (n-C,H5)2Ni.23 o-Complexes.-Alkyls continue to be studied very widely. The trend 24 towards using alkylating agents with no P-hydrogen atoms has continued. Particularly noteworthy are the preparation of W(CH3)6,25 (1-norbornyl),M where M = Ti V Cr Mn Zr or Hf,26 LiTi(CH,),(dioxan) ,,?and (n-C,H,)Ti(benzyl) .28 Nitrogen-containing Ligands.-(Ph,P),Pd(N,) undergoes a cycloaddition re- action with PhCN to give the tetrazole complex ( l).29 D. J. Cardin B.Cetinkaya and M. F. Lappert Chem. Rev. 1972,72 545. F. A. Cotton and C. M. Lukehardt Progr. inorg. Chem. 1972 16,487. H. Buchholz P. Heimbach H. J. Hey H. Selbeck and W. Weise Coordination Chem. Rev. 1972 8 129. l o C. W. Bird J . Organometallic Chem. 1973 47 281. ' ' R. Jira and W. Freiesleben Organometallic Reactions 1972 3 1. l 2 Papers in Fortschr. Chem. Forsch. 1972 28. l 3 Papers in Fortschr. Chem. Forsch. 1972 25. '' M. Hancock M. N. Levy and M. Tsutsui Organometallic Reactions 1972 4 1. E. G. Perevalova and T. V. Nikitina Organometallic Reactions 1972 4 163. '6 N. Calderon Accounts Chem. Res. 1972 5 127. " W. B. Hughes Organometallic Chem. Synth. 1972 1 341. l 6 F. R. Hartley Angew. Chem. internat. Edn. 1972 11 596. l 9 J. Muller Angew.Chem. Internat. Edn. 1972 11 653. 2 0 G. Huttner E. 0. Fischer R. D. Fischer 0. L. Carter A. T. McPhail and G. A. Sim Organometallic Chem. 1966 6 288. 2 1 H. Werner and A. Salzer Synth. Inorg. Metal-org. Chem. 1972 2 237. 2 2 A. Salzer and H. Werner Angew. Chem. internat. Edn. 1972 11 930 2 3 E. Schumacher and R. Taubenest Helv. Chim. Acta 1964,47 1525. 2 4 P. S. Braterman and R. J . Cross J.C.S. Dalton 1972 657. 2 5 A. Shortland and G. Wilkinson J.C.S. Chem. Comm. 1972 318. '' K.-H. Thiele K. Milowski P. Zdunneck J . Muller and H. Rau Z . Chem 1972 12 2 6 B. K. Bower and H. G. Tennet J. Amer. Chem. Soc. 1972,94,2512. 186. 2 8 G. A. Razuvaev U. N. Latyaeva G. A. Vasil'eva and L. I. Vyshinskaya Synth. inorg. Metal-org. Chem. 1972 2 33. 2 9 P.Kreutzer C. H. Weiss H. Boehme T. Kemmerich W. Beck C. Spencer and R. Mason Z . Naturforsch. 1972 27b 745. ' Ir ' Organometallic Compounds of the Transition Elements A series of compounds with ligands which are formally related to n-ally1 complexes but with the substitution of one3' or two3' of the C atoms by N have been prepared. The tungsten compound (2) can exist in two crystalline forms in which the ligand is either a 7c-aza-ally1 group (as shown) or a n-aza-allene group. N i i / \ p-Tol p-Tol -+ (n-C,H,)(CO),W- N C(p-Tol) (2) Carbene Complexes.-A new method for the preparation of the carbene com- plexes (3) involving a three-atom fragment oxidative addition has been reported.32 A route to the non-hetero-carbenes (4) using diphenylketen has appeared.II (~-CsH5)W(C0)3Cl + C N E N Ph,P PPh C1 Ph Ph /H / \ + Me,N=CHCI+ C1- -+ (Ph3P),Cl31r~C ->NMe (3) 0 I I CI An interesting contrast in reactivity is provided by the reaction between Fe(CO) and Al(NMe,), which yields34 a carbene complex (3 whereas the reaction between [(K-C H ,)Fe(CO),] with AlEt gives the adduct [(7c-C H ,)- Ph Ph 0 II Fe( CO) AIEt ,] ,. ' 3 0 H. R. Kealde and M. Kilner J.C.S. Dalton 1972 153. 3 1 T. Inglis M. Kilner and T. Reynoldson J.C.S. Chem. Comm. 1972 774. " B. Cetinkaya M. F. Lappert and K. Turner J.C.S. Chem. Comm. 1972 851. 3 3 P. Hong N . Nishi K. Sonogashira and N . Hagihara J.C.S. Chem. Comm. 1972,993. 3 4 W. Pete and G. Schmid Angew. Chem. Internat. Edn.1972 11 934. 3 5 A. Alich N. J. Nelson D. Strope and D. F. Shriver fnorg. Chem. 1972 11 2976. 269 C(p-To 1) 2 *> Y Rh / \ I / c \ C C / 0 II 270 ( 5 ) Olefin Complexes-An olefin complex (MeCN),Rh(C,H,) with three ethylene molecules co-ordinated to Rh' has been reported,36 and a detailed account of the chemistry of 1r(C2H4),C1 has appeared.37 4 Hydrogenation and Hydrogen-exchange Reactions MH where M = Cr,, Ti Zr or Hf,43 and Pd,(Ph,- Catalytic Hydrogenation.-The rate of discovery of new homogeneous hydrogen- ation catalysts appears to be diminishing more emphasis being put on the utilization of existing catalysts. The reduction of carbon-carbon unsaturation still dominates the field of hydrogenation. New catalysts introduced in this area include NiC1,-NaBH ,38 FeH ,N2( PPh,Et) ,39 [ Co(bipy) (PR 3)2H ,] + ,4° IrCl(CO)(PPh,),-H,O CoH(CO) { P(Bu") ) ,,' [Co(CN),I3 -?* (arene)Cr(CO) ,49 '(I~-C,H,),T~',~~ PCH2PPh,) ,44 together with the established catalysts IrCl(CO)(PR,) ,45,46 RhH(CO)(PPh,) ,5 (py),RhCI,-NaBH ,52 and NiC1,-NaBH,.5 2 The Ziegler- type systems e.g. Cot'- and Ni"-RLi have also been used to catalyse the hydrogenation of 1,4-polybutadiene and polystyrene5 (the latter compound is converted into the corresponding polycyclohexyl derivative). An interesting phenomenon occurs when the catalyst Pd"-NaBH was prepared in the presence of a reducible substrate. It was found that the resulting catalyst had increased activity and selectivity and it was suggested that the substrate participates in the formation of the active site leading to a particular hydrogenation path.54 36 F.Maspero E. Perrotti and F. Simonetti J. Organometallic Chem. 1972 38 C43. 3 7 A. L. Onderdelinden and A. Van der Ent Inorg. Chim. Acta 1972 6 420. 3 8 A. G. Hinze and D. J. Frost J. Catalysis 1972 24 541. 3 9 V. D. Biance S. Doronzo and M. Aresta J. Organometallic Chem. 1972 42 C63. 40 A. Camms C. Cocevar and G. Mestroni J. Organometallic Chem. 1972 39 355. 'I F. Van Rantwijk Th. G. Spek and H. Van Bekkum Rec. Trau. chim. 1972,91 1057. 4 2 V. V. Lunin G. V. Lisichkin A. E. Agronomov and A. A. Chertkov Doklady Akad. 4 3 G. V. Lisichkin V. V. Lunin A. E. Agronomov and A. A. Chertkov Russ. J . Phys. 1972 43 213. J . P. Candlin G. L. P.Randall and A . W. Parkins ,OAWMe2)2 Fe(CO) + Al(NMe,) -+ (CO),Fe-C \ NMe Nauk S.S.S.R. 1972 204 1356. Chem. 1972,46 620. 44 E. W. Stern and P. K. Maples J. Catalysis 1972 27 120 134. 4 5 W. Strohmeier and R. Fleischmann J. Organometallic Chem. 1972 42 163. 4 6 C. Y. Chan and B. R. James Proceedings of the 14th International Conference on '' G. Ferrari A. Andreetta G. F. Pregaglia and R. Ugo J. Organometallic Chem. Coordination Chemistry Toronto 1972 p. 70. 4 8 T. Funabiki M. Matsumoto and K. Tarama Bull. Chem. SOC. Japan 1972,45 2723. 5 3 J. C. Falk Makromol. Chem. 1972 160 291. 4 9 E. N. Frankel J. Org. Chem. 1972 37 1549; J. Catalysis 1972,24 358. E. E. van Tamelen W. Cretney N. Klaentschi and J. S. Miller J.C.S. Chem. Comm. 5 1 J.Hjortkjaer and Z. Kulicki J. Catalysis 1972 27 452. " P. Abley and F. J. McQuillin J. Catalysis 1972 24 536. 1972,481. 5 4 A. Canas-Rodriguez J.C.S. Perkin I 1972 554. Organometallic Compounds of the Transition Elements Ruthenium and rhodium complexes continue to dominate the field of hydrogen- geneous ~atalyst,~ ’ tends to be plagued by irreproducibility particularly when ation catalysts. RuHCl(PPh,) although a very active and selective homo- used in kinetic For this reason other ruthenium complexes have been used as catalysts e.g. RuC~,(DMSO)~’ and RuHJPR,) .58 The catalytic activity of the tetrahydride complex has been shown to be inhibited by N (also NH and CO) owing to the formation of the dinitrogen c o m p o ~ n d . ~ ~ . ~ ~ The nature of RhCl(PPh,) in solution has again been re-investigated and a further equilibria has been postulated K 2RhCIP (6) (ref.62) (8) Reaction with H yields the six-co-ordinate RhH,(Cl)(PPh,) with little or no dissociation of the phosphine ligand.6 Examples in which RhCl(PPh,) has been used as a catalyst for the selective hydrogenation of complex organic molecules [e.g. (6H9)] are given (arrows indicate position of hydrogenation). (ref. 65) 5 5 E. F. Litvin A. Kh. Freidlin and K. K. Karimov h e s t . Akad. S . S . S . R . Ser. khim. 1972 1853 (Nefiekhimiya 1972 12 318). 5(’ D. Evans J. A. Osborn and G. Wilkinson J . Chem. Soc. ( A ) 1968 3133. ” B. R. James R. S. McMillan and E. Ochiai Inorg. Nuclear Chem. Letters 1972,8,239. 5 8 S. Komiya A. Yamamoto and S.Ikeda J . Organometallic Chem. 1972 42 C65. 5 9 F. Pennella R. L. Banks and M. R. Rycheck ref. 46 p. 78. “ W. H. Knoth J . Amer. Chem. Soc. 1972 94 104. P. Meakin J. P. Jesson and C. A. Tolman J. Amer. Chem. Soc. 1972 94 3240. N. S. Crossley and R. Dowell J . Chem. Soc. (0 1971 2496. 6 3 M. Pardhasaradhi and G. S. Sidhu Tetrahedron Letrers 1972 4201. 64 H. C. Odom and A. R. Pinder J.C.S. Perkin I 1972 2193. A. Tanaka R. Tanaka H. Uda and A. Yoshikoshi J.C.S. Perkin I 1972 1721. [RhClP,] + 2P K - 4.10-4mo11-’ OH 0 OH 0 0 (ref. 63) (7) (ref. 64) (9) 27 1 2 7'2 J . P . Candlin G. L. P . Randall and A . W. Parkins All reductions appeared to give quantitative yields of the desired product. However the attempted reduction of the 9,lO-bond in ergocristine failed when RhCl(PPh,) was used as the catalyst.66 Tritiation ofergosta-1,4,22-trien-3-0ne~~ and stereoselective deuteriation of fumaric acid68 has been achieved.Selectivity hydrogenation studies on simple organic compounds using RhCl(PPh,) con- tinues to build up the necessary background information required for more com- plex systems ; in this category are papers on the reduction of substituted a l l e n e ~ ~ ~ cyclohexenol,70 and cy~lohexene.~' Preparative and mechanistic hydrogenation studies with various ligands e.g. PR'R2R3,72 (0-vinylpheny1)diphenylphos- ~ h i n e ~ SR ,74 DMS0,75 phenylanthranilic acid,76 N-f~rmylpiperidine,~~ and p h ~ s p h o l e s ~ ~ completes the rhodium catalysis area. Although there are many soluble catalysts for the hydrogenation of olefinic and acetylenic linkages unfortunately the number of catalysts which will reduce functional groups containing heteroatoms is very small particularly in view ofwhat can be achieved by using heterogeneous catalysts.This could be the result of deactivation of the catalyst by the substrate or merely that the conditions of the experiment have been incorrect. Only a few examples have appeared to remedy this situation e.g. cobalt(1r) tetra(p-sulphonatopheny1)porphine-NaBH, which will catalytically reduce oxide and cyanide (inorganic and organic) as well as acetylenic linkages.79 It would be of interest to know whether electro- chemically reduced cobalt(I1) tetraphenylporphine possesses similar activity since this system has shown to dehydrogenate cyclohexane into benzene ;" RhCl,(N- formylpiperidine) has also been shown to catalyse the hydrogenation of nitro- benzene to aniline.7 Hydrosilylation of 0lefins.-Both homogeneous and heterogeneous platinum catalysts have traditionally been used for this reactioq8' but recent work has been directed towards using rhodium(r) catalysts.As perhaps would be expected from hydrogenation studies the rate of hydrosilylation alters for different silanes and also for different catalysts e.g. RhCl(PPh,) and C O ~ ( C O ) . ~ ~ When 6 7 6 6 H. Cousse G. Mouzin and B. Bonnaud Bull. SOC. chim. France 1972 3131. B. Pelc and E. Kodicek J . Chem. Soc. (0 1371 3415. 6 8 R. H. Grubbs and T. K. Brunk J. Amer. Chem. SOC. 1972,94 2538.6 Q M. M. Bhagwat and D. Devaprabhakara Tetrahedron Letters 1972 1391. Y. Senda T. Iwasaki and S. Mitsui Tetrahedron 1972 28 4059. '' S. Siegel and D. Ohrt Inorg. Nuclear Chem. Letters 1972,8 15. " L. Horner and H. Siegel Annalen 1971 751 135. '' P. R. Brookes J. Organometallic Chem. 1972 43 415. 7 4 B. R. Jamesand F. T. T. Ng J.C.S. Dalton 1972 1321. 7 5 L. Kh Freidlin Yu. A. Kopyttsev N . M. Nazarova and T. I. Varava Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1972 1420. 7 6 L. Kh. Friedlin E. F. Litvin and L. F. Topuvidze BUN. Acad. Sci. U.S.S.R. 1971 20 336. 7 7 I. Jardine and F. J. McQuillin Tetrahedron Letters 1972 173. '' E. B. Fleischer and M. Krishnamurthy J. Amer. Chem. SOC. 1972 94 1382. 7 8 D. G. Holah A. N. Hughes and B. C. Hui Canad. J . Chem.1972 50 3714. * ' A. J. ChalK Trans. New York Acad. Sci. 1970 32 48 1 . H. Kageyama M. Hidai and Y. Uchida BUN. Chem. Sac. Japan 1972,45 2898. 8 2 P. Svoboda M. Capka J. Hetflejs and V. Chvalovsky Colf. Czech. Chem. Comm. 1972,37 1585. 273 Organometallic Compounds of the Transition Elements [RhCl(C,H,),] is used as the catalyst the variation in rate with different silanes is less marked.83 The high catalytic activity was noted for the hydrosilylation of carbonyls e.g. for cyclohexanone :84 \ CH-0-SiEt \ C=O + HSiEt -+ / / The hydrosilylation of various olefinic organic molecules containing func- tional groups using Pt catalysts has been ~ t u d i e d ~ e.g. allylamine [main product (R0),SiCH2CH2CH2NH2] ally1 esters of perfluoric acids and butadiene (main product CH,CH=CHCH,SiR,).The hydrogermylation of olefins and acetylenes has also been shown to be catalysed by standard hydrosilylation catalysts,86 e.g. PtCl,(PPh,) and RhCl(PPh,) . Catalytic hydrostannylation does not appear to have been attempted free-radical promoters are used with resultant mixtures of products. Hydrocyanation of 0lefins.-The industrially important reaction of HCN addition to olefins has been known for some time. Catalysts for this reaction include zerovalent palladium and nickel complexes (often containing phos- phite ligands) usually in the presence of a Lewis The mechanism proposed is NIL + HCN + MCI -+ [HNiL,]' [MCI,CN]- + L [HNiL,]' + RCH=CH -+ [RCH2CH2NiL3]' [RCH2CH2NiL,]+ + [MCl,CN]- -+ RCH,CH,CN + NIL + MC1 Although reports of the work are only disclosed in patents," the di-hydro- cyanation of butadiene using similar catalysts as above yields the commercially useful product NC(CH,),CN (adiponitrile) which is a key intermediate for the production of nylon 6,6.Metal-catalysed Intermolecular Hydrogen Exchange.-Transition-metal- catalysed exchange has probably reached the stage where it can be used as a recog- nized chemical technique for example in the isotopic hydrogen exchange with certain organic compounds. Thus with amino-acids and carboxylic acids H-D exchange occurs at the cc-hydrogen position,' and hydroxylic or imino-hydrogen 83 P. Svoboda M. Capka and J. Hetflejs CON. Czech. Chern. Comrn. 1972 37 3059. I. Ojima M. Nihonyahagi and Y .Nagai J.C.S. Chern. Cornrn. 1972 938. 8 5 Z. V. Belyakova V. N. Bochkarev S. A. Golubtsov Z . V. Belikova M. S. Yamova A. A. Ainshtein G. G. Baranova L. A. Efremova and K. K. Popkov J . Gen. Chem. (U.S.S.R.) 1972 42 848 852 879. 8 6 R. J. P. Corriu and J. J. E. Moreau J . Organornetallic Chem. 1972 40 5 5 73. E. S. Brown and E. A. Rick Chern. Comm. 1969 112. E. S. Brown E. A. Rick and F. D. Mendicino J . Organornetallic Chern. 1972 38 37. 89 B. W. Taylor and H. E. Swift J. Catalysis 1972 26 254. 9 0 E.g. Ger. Often. 2 055 747/1971 (Chem. A h . 1971,75,64 569); U.S.P. 3 631 191/1971 (Chem. Abs. 1972 76 87 990). 9 1 J. L. Garnett B. Halpern and R. S. Kenyon J.C.S. Chem. Cornm. 1972 135. R h J . P. Candlin G. L. P. Randall and A . W. Purkins pH) + 2 P h Scheme 1 Transition-metal hydride transfer continues to be in~estigated,~~ e.g.Scheme 2.98 Acidic organic alcohols e.g. catechol appear to be particularly effective in these reactions and unsubstituted olefins can be reduced to a saturated derivative R uCI *( PPh)f Scheme 2 G. G. Eberhardt M. E. Tadros and L. Vaska J.C.S. Chem. Comm. 1972 290. J. L. Garnett M. A. Long R. F. W. Vining and T. Mole J . Amer. Chem. SOC. 1972 274 can be transformed into the deuterium analogue by exchange with H in the presence of Group VIII complexes.92 An interesting and versatile preparation of deuteriated aromatic systems has been developed. Toluene (and alkyl-substituted aromatics) can be converted into [2H,]toluene by using homogeneous or heterogeneous platinum catalysts and into [2H,]toluene (aromatic-H exchange only) using C,D,-EtAlCl,.Reverse exchange of [’H ,]toluene (with C6H6-EtAlCl,) yields [,H ,]toluene (containing deuteriated alkyl g r ~ u p ) . ~ Instead of C6D6 as hydrogen-isotope source trace amounts of tritiated material can be used resulting in tritium-labelled organic corn pound^.^^ Deuterium exchange between C6D6 and substituted benzenes has also been found to be catalysed by anhydrous metal chlorides e.g. WCl or SbC1,.9s These compounds are Lewis acids and an obvious extension of this would be the use of ‘super-acids’ possibly in the presence of transition-metal complexes. A clue to the mechanism of hydrogen exchange may be obtained from the Rh’-catalysed disproportionation of cyclohexa-1,3-diene into benzene and cyclohexene.Deuterium substitution can arise from the equilibrium shown in Scheme l.96 92 9 3 9 4 94 8632. 94 5913. M. A. Long J. L. Garnett R. F. W. Vining and T. Mole J. Amer. Chem. SOC. 1972 9 5 J. L. Garnett M. A. Long R. F. W. Vining and T. Mole J.C.S. Chem. Comm. 1972 1172. 98 96 M. Green and T. A. Kuc J.C.S. Dalton 1972 832. 9 7 J. Blum Y. Sasson and S. Iflah Tetrahedron Letters 1972 1015. S. L. Regen and G . M. Whitesides J. Org. Chem. 1972 37 1832. 275 Organometallic Compounds of the Transition Elements e.g. cyclo-octa- 1 ,ti-diene into cy~lo-octene.~~ The general mechanism suggested is shown in Scheme 3. This could be the mechanism for the hydrogenation of oct-1-ene by HC02H catalysed by IrCl(CO)(PPh,) RuCl,(PPh,) etc."' hydride Hydrogen donor 4 - Hydrido-complex accepto; Product It H + other compounds + Catalyst Scheme 3 Examples exist where preformed hydride complexes have been used in a stoich- eiometric manner for the hydrogenation and deuteriation of ap-unsaturated can of carbonyl be produced compounds.in lo' - 100% Thus yield the saturated by using derivative Fe(CO),-OH- cholest-4-en-3-one (=hydrido-iron carbonyls). Inter- and Intra-molecular Oxidative Addition Reactions.-The mechanism of oxidative addition for alkyl halides to Ir' complexes has been suggested to be either an SN2 process (with Tr' acting as the nucleophile) or a one-step concerted three-centre reaction. New evidence using carefully prepared materials however suggests that these reactions occur via a free-radical process.l o 2 Thus small quantities of oxygen accelerates the reaction and radical scavengers (e.g. hydro- quinone) retards addition. These results could have important consequences for catalysed reactions in which oxidative addition reactions have been con- sidered as an intermediate step. More examples of intramolecular addition (internal metallation) have ap- peared. Factors which promote these reactions include bulky substituents on the ligand and the ease of formation of a five-membered ring. Both of these factors are present when ligands such as PBu',(o-tolyl) are attached to Pt'03 and Pd'04 centres. ortho-Metallation occurs with benzylideneaniline (Ph- CH=NPh) and various metal carbonyls e.g.Mn Ru or Rh ;Io5 azo-compounds e.g. PhN + X - with IrCl(CO)(PPh,) yield o-metallated di-imide products reduction of which gives hydrazine complexes ;Io6 and on heating Os,(CO),,- (PPh,) is converted into HOS,(CO)~(PP~~)(PP~,C,H,). lo7 9 9 T. Nishiguchi and K. Fukuzumi Bull. Chem. SOC. Japan 1972,45 1656. l o o I . S. Kolomnikov V. P. Kukolev V. 0. Chernyshev and M . E. Vol'pin Bull. Acad. Sci. U.S.S.R. 1972,21 661. l o ' R. Noyori I. Umeda and T. Ishigami J . Org. Chem. 1972 37 1542. I o 2 J. S. Bradley D. E. Connor D. Dolphin J. A. Labinger and J. A. Osborn J. Amer. lo' A. J. Cheney and B. L. Shaw J.C.S. Dalton 1972 860. ' 0 5 R. L. Bennett M. I. Bruce B. L. Goodall M. 2. Iqbal and F. G . A. Stone J.C.S.'06 D. Sutton A. B. Gilchrist and G . W. Rayner-Canham ref. 46 p. 88. l o ' C. W. Bradford R. S. Nyholm G. J. Gainsford J. M. Guss P. R. Ireland and Chem. SOC. 1972 94,4043. A. J. Cheney and B. L. Shaw J.C.S. Dalton 1972 754. Dalton 1972 1787. R. Mason J.C.S. Chem. Comm. 1972 87. 276 complexes. ' ' 5 Isomerization M M I J . P. Candlin G. L. P. Randall and A . W. Parkins The oxidative addition reaction in which oxidation of the metal by two units and addition of two ligands e.g. cannot occur when M" + is a six-co-ordinate complex e.g. IrX,L,. In these cases an anionic elimination mechanism has been suggested. lo* with Rh' and Ir' ' O9 and (x-C,H,),Ti(CO) ' ' and perfluoroacetone with Pd Intermolecular oxidative addition reactions include the addition of o-quinones Metal-catalysed C=C 1somerization.-Further evidence for the n-ally1 hydride mechanism for the isomerization of olefins catalysed by transition-metal com- plexes comes from spectroscopic identification of the intermediate n-ally1 hydride occurring in the reaction shown in Scheme 4 [M- = RhCI(PF)3),].''2 R1CH~CHCH2R2 R1CH2-CH=CHR2 5 [* RICH' 'i>HR:] CH Scheme 4 Other isomerization mechanisms however have been suggested e.g.a-olefin isomerization catalysed by [HNi{P(OEt)3)30r4]+ in which a metal-H addition and elimination l 4 By examining the effect of various substituents on the rate of equilibrium on the isomerization of 1,6diarylbutenes p-XC,H,CH,- CH,CH=CHPh it was concluded that the metal-H addition4imination mechanism was favoured.' However to explain the final distribution of deuterium-labelled pent-1-ene isomerized by RuHCl(PPh,) both the above mechanisms have been invoked.l 6 Nickel complexes are efficient olefin-dimerization catalysts but often the desired isomer is not obtained. Any information regarding the isomerization of olefin substances by nickel species is therefore valuable. The rate of isomeriza- tion of hex-1-ene by Ni(acac),-Al,Et,Cl,-PR catalysts can be altered by varying PR,. The isomerization of but-1-ene by NiCl,(PPh,),-SnC1 does not yield l o ' J. M. Duff and B. L. Shaw J.C.S. Dalton 1972 2219. I o 9 Y. S. Sohn and A. L. Batch J . Amer. Chem. Soc. 1972 94 1144. I " C. Floriani and G. F. Archinetti J.C.S. Chem. Comm. 1972 790.H. D. Empsall M. Green and F. G. A. Stone J.C.S. Dalton 1972 96. 'I2 J. F. Nixon and B. Wilkins J . Organometallic Chem. 1972 44 C25. ' * D. Bingham D. E. Webster and P. B. M'ells J.C.S. Dalton 1972 1928. 'I4 C. A. Tolman J. Amer. Chem. SOC. 1972 94 2994. ' * J. Blum and Y. Becker J.C.S. Perkin IZ 1972 982. D. F. Ewing B. Hudson D. E. Webster and P. B. Wells J.C.S. Dalton 1972 1287. ' I 7 Y. Sakakibara M. Mukai M. Sukai and N. Uchino Nippon Kagaku Kaishi 1972 1457. 277 Organometallic Compounds of the Transition Elements the equilibrium ratio of cis/trans-but-2-ene a high yield of the cis-isomer is produced. 118 The kinetics and mechanism to equilibrium of the cis-trans isomerization catalysed by CoHN,(PPh,), has been studied.' l 9 Metal-catalysed Carbon Skeletal 1somerization.-Linear OleBnic Substrates.An interesting although only stoicheiometric rearrangement has been shown to occur when secondary acyl compounds react with Ir' (Scheme 9.'" CH,CH,CH,Ir"' CHCOCI -t- Ir' -P H3C / H3C Scheme 5 The rearrangement of 2-me t h yl bu t-3-eneni trile into linear pen t-3-enenitrile is catalysed by cobalt and nickel phosphite complexes (Scheme 6).12' This is an important industrial reaction in the production of adiponitrile since 2-methylbut- 3-enenitrile arises as an undesired isomer in the hydrocyanation of butadiene. Scheme 6 Alicyclic Olejnic Substrates. Last year's rapid increase in activity in this area has subsided and more emphasis has been placed on the understanding of the mechanism of these skeletal rearrangements.Although a general scheme covering ail substrates and all catalysts is understandably impossible evidence is coming forward for a complexed carbene or carbonium ion intermediate for the metal-catalysed isomerization of substituted bicycle[ 1,l ,O]butanes (Scheme 7).'22-' 29 In the presence of protic solvents e.g. MeOH the reaction products are altered suggesting a change of mechanism.' 30 The [Rh(CO),Cl],-catalysed rearrangement of bicyclo[2,1,0]pentane is suggested as occurring through a rhodium hydride intermediate,' and the ' lR H. Kanai J.C.S. Chem. Comm. 1972,203. l 9 S. Tyrlik J. Organometallic Chem. 1972 39 371. l Z 0 M. A. Bennett and R. Charles J. Amer. Chem. Soc. 1972,94 666. E.g. Ger. Offen. 1 817 797/1971 (Chern.Abs. 1972,76,24 723); Ger. Offen. 2 221 113/ 1972 (Chem. Abs. 1973,78,42 868). ' 23 P. G. Gassman G. R. Meyer and F. J . Williams J . Amer. Chem. Soc. 1972,94 7741. l Z 2 P. G. Gassman and F. J . Williams J . Amer. Chem. SOC. 1972 94 7733. P. G. Gassman and F. J. Williams J.C.S. Chem. Comm. 1972 80. l Z 5 P. G. Gassman and T. J. Atkins J . Amer. Chem. SOC. 1972 94 7748. ' 2 7 W. G. Dauben and A. J. Kiebania jun. J. Amer. Chem. Soc. 1972,94 3669. S. Masamune M. Sakai and N. Darby J.C.S. Chem. Comm. 1972 471. l Z 8 L. A. Paquette R. P. Henzel and S. E. Wilson J . Amer. Chem. SOC. 1972,94 7780. l Z 9 P. G. Gassman and T. Nakai J. Amer. Chem. Soc. 1972,94 2877. 130 P. G . Gassman and T. Nakai J . Amer. Chem. SOC. 1972,94 5497. 1 3 ' P. G. Gassman T.J. Atkins and J. T. Lumb J . Amer. Chem. Soc. 1972 94 7757. J. P. Candlin G. L. P. Randall and A . W. Parkins L " J Scheme 7 Ag'-catalysed isomerization of tricyclo[4,1,0,0]heptanes' 3 2 1 3 3 and homocub- aneS 134913 5 is thought to proceed uia an argento-carbonium ion. Rearrangement of conjugated' 36 and unconjugated' have been studied e.g. Scheme 8. ' The ring-opening of alkylcyclopropenes cyclopropyl derivatives 278 14' 14' 14' 1972 17 E80. Scheme 8 to form substituted dienes is catalysed by Ag'; a mechanism involving an Ag' carbene has been suggested. 1 3 8 The rearrangement of organic molecules to form stable organometallic complexes initiated the above work catalytic usage of metal only being a recent development. The former stoicheiometric aspect is illustrated by the rearrange- ment of cyclododecatriene (by loss of a C fragment)' 39 and cyclo-~ctadiene'~~ into ruthenium and cobalt pentalenyl complexes.OleJin Disproportionation. This reaction has commercial applications and hence is a field which is being investigated by both industrial and academic workers. 14'- 143 Another area of overlap is that both homogeneous and hetero- geneous catalysts can be used. The principle catalytic metals for this reaction are 1 3 * L. A. Paquette S. E. Wilson R. P. Henzel and G. R. Allen jun. J. Amer. Chem. Soc. 1972,94,7761. 1 3 3 L. A. Paquette S. E. Wilson and R. P. Henzel J. Amer. Chem. SOC. 1972 94 7771. 1 3 4 L. A. Paquette and J. S. Ward Tetrahedron Letters 1972 4409. 1 3 5 L.A. Paquette R. S. Beckley D. Truesdell and J. Clardy Tetrahedron Letters 1972,4913. 136 R. Aumann Angew. Chem. Internat. Edn. 1972 11 522. 1 3 ' G. Albelo and M. F. Rettig J . Organometallic Chem. 1972 42 183. 1 3 ' J. H. Leftin and G. Gil-av Tetrahedron Letters 1972 3367. 1 3 9 S. A. R. Knox R. P. Phillips and F. G. A. Stone J.C.S. Chem. Comm. 1972 1227. S. Otsuka and T. Taketomi J.C.S. Dalton 1972 1879. 1 4 3 W. J. Kelly Preprints Amer. Chem. SOC. Diu. Petroleum Chem. 1972 17 B79; R. L. Banks Preprints Amer. Chem. SOC. Div. Petroleum Chem. 1972 17 A21. D. L. Crain and F. E. Reusser Preprints Amer. Chem. Soc. Div. Petroleum Chem. ibid. 1972 17 H32. 279 Organometallic Compounds of the Transition Elements and rhenium,' 46-' s6 together with either an acidic oxide e.g.A1,0, or an alkylating or hydride reagent. Com- binations of these metals have been used. ' 5 7 Efforts are now being madel'l to effect disproportionation of olefins carrying functional groups e.g. methyl oleate into octadec-9-ene and dimethyl octadec-9-enedioate. The fascinating use of these catalysts to prepare catenanes continues,' 58,1 5 9 as does the preparation of polymers by ring-opening polymerization. The mechanistic aspects of olefin disproportionation have again been illus- trated,'67 one of the theories being the formation of a tetracarbene intermediate. Indirect support for this comes from the heterogeneously catalysed dispropor- .tionation of CH,N, which yields CH,=CH (and N,),16* and also the for- mation of propene from ethylene.14' The novel use of disproportionation catalysts e.g. WCl,-EtAlCI, for aromatic transalkylation (polyalkylbenzene + benzene + monoalkylbenzene) could have synthetic utility. 169 1 4 4 E. S. Davie D. A. Whan and C. Kemball J . Catalysis 1972 24 272; R. F. Howe D. E. Davidson and D. A. Whan J.C.S. Faraday I 1972 2266; D. A. Whan M. 14' A. Uchida K. Kolsayashi and S . Matsuda Ind. and Eng. Chem. (Product Res. and Development) 1972 11 389. Barker and P. Swift J.C.S. Chem. Comm. 1972 198. 1 4 5 P. P. O'Neill and J. J. Rooney J . Amer. Chem. Soc. 1972 94 4383. 1 4 6 Y. Uchida M. Hidai and T. Tatsumi Bull. Chem. SOC. Japan 1972,45 1158. M. L. Khidekel V. I . Mar'in A. D. Shebaldova T. A. Bol'shinskova and I . V. Kalechits Bull. Acad.Sci. U.S.S.R. 1971 20 601. J.-L. Wang H. R. Menapace and M. Brown J. Catalysis 1972 26 455. 150 J . Chatt R. J. Haines and G . J. Leigh J.C.S. Chem. Comm. 1972 1202. 15' P. B. Van Dam M. C . Mittelmeijer and C. Boelhouwer J.C.S. Chem. Comm. 1972 1221. 1 5 2 J. A. Moulijn H. J. Reitsma and C. Boelhouwer J. Catalysis 1972 25 434. '53 P. A. Raven and E. J. Wharton Chem. and Ind. 1972 292. R. H. Grubbs and T. K. Brunck J . Amer. Chem. SOC. 1972,94 2538. T. Takagi T. Hamaguchi K. Fukuzumi and M. Aoyama J.C.S. Chem. Comm. 1972 838. 14' 1 5 4 ' 5 6 K. M. Minachev M. A. Ryashentseva G. V. Isagulyants and N. N. Rozhdestvenskaya Bull. Acad. Sci. U.S.S.R. 1972 21 675. W. R. Kroll and G . Doyle J. Catalysis 1972 24 396. 5 8 R. Wolovski and 2. Nir Synthesis 1972 134.E. Wasserman C. Batich S . Barer and D. A. Ben-Efraim Prrprints Amer. Chem. SOC. Div. Polymer Chem. 1972 12 920. H. Hocker and F. R. Jones Makromol. Chem. 1972 161 251. H. Hocker and R. Musch Makromol. Chem. 1972 157 201. 15' 160 I 6 I 1 6 ' G. Dall'asta Makromol. Chem. 1972 154 1 . G . Dall'asta G . Motroni and L. Motta J . Polymer Sci. Part A-1 Polymer Chem. 16' 1972 10 1601. 1 6 4 E. A. Ofstead and N. Calderon Makromol. Chem. 1972 154 21. V. A. Kormer I . A. Poletayeva and T. L. Yufa J. Polymer Sci. Part A-1 Polymer Chem. 1972 10 251. I b h Papers in Preprints Amer. Chem. SOC. Div. Polymer Chem. 1972 13 874 880 885 I h 7 897 910 914. F. D. Mango Preprints Amer. Chem. SOC. Dit Pol-vmer Chem. 1972 13 903. 1 6 ' P.P. O'Neill and J . J. Rooney J.C.S. Chem. Comm. 1972 104. 1 6 9 L. Hocks A. J. Herbert and P. Teyssie Tetrahedron Letters 1972 3687. R' R2 R' J . P. Cundlin G. L. P. Randall and A . W. Purkins Although a different mechanism may be operating. the disproportionation of electron-rich olehs by Rh' catalysts has been effected (Scheme 9). 1 7 * R' R 2 R 2 Scheme 9 6 Oligomerization of Unsaturated Hydrocarbons Diolefim-The oligomerization of butadiene by Nio catalysts has been summarized and more evidence has been put forward' 7 2 in favour of a multi- step mechanism for this type of reaction. The terpene dl-limonene has been prepared from isoprene using a Nio catalyst. Diallylplatinum halides which have been prepared recently produce dimers trimers tetramers and polymer from butadiene ; 7 3 however palladium acetate and Li,PtCI in the presence of formic acid and a base such as DMF dimerize butadiene to octa- 1,6-diene and octa- 1,7-diene respectively.1 7 4 A study of the effect of solvent and cobalt phosphine ratio upon the oligomeriz- ation of isoprene by the complex CoC1,-NaBH4-PPh catalyst indicate^'^ that in ethanol non-cyclic dimers are produced with 1,3-coupling when Co PPh is greater than 1 and 1.4- and 4,4-linked dimers for Co:PPh less than 1. Non- protic solvents gave cyclic dimers. Isoprene has also been trimerized and dimerized by Ni(acac) and Fe(acac) respectively in combination with triethylaluminium and a Schiff base.' 7 6 Another iron system which has been reported to oligomerize has been bis- (isoprene)iron carbonyl and a variety of diene- and tetraene-iron monocarbonyl complexes.The oligomerization of norbornadiene has been investigated photochemically in the presence of Ni(CO),,' '* where the dimer obtained is quite different to that obtained by thermal dimerization with the carbonyl. The reaction of norbor- nadiene with the (norbornadiene),Rh+ cation in the presence of hydrogen' 7 9 Amer. Chem. Soc. Diu. Petroleum Chem. 1972 17 B80. B. Barnett B. Blussemeier P. Heimbach P. W. Jolly C. Kruger I . Tkatchenko and G. Wilke Tetrahedron Letters 1972. 1457. D. J . Cardin M. J. Doyle and M. F. Lappert J.C.S. Chem. Comm. 1972 927. H. Buchholz P. Heimbach H.-J. Hey H. Selbek W. Wiese and G. Wilke Preprints and Catalysis ( U.S.S.R.) 1972 13 220.1 7 3 A. I . Lazutkima L. Ya. Gravrilina V. V. Malakhov and A. M. Luzutkin Kinetics 17' S. Gardner and D. Wright Tetrahedron Letters 1972 163. 1 7 5 K. Takabe K. Urata T. Katagiri and J. Tanaka Nippon Kagaku Kaishi 1972 1695. ' 7 6 C.-Y. Wu and H. E. Swift J . Catalysis 1972 24 510. ' 7 7 A. Carbonaro and F. Cambisi J . Organometallic Chem. 1972 44 171. G. E. Voecks P. W. Jennings G. D. Smith and C. N. Caughlan J . Org. Chem. 1972 37 1460. R . J. Roth and T. J. Katz Tetrahedron Letters 1972 2503. 280 "O 17' 7 2 28 1 Orgunornetullic Compounds of the Transition Elements and in its absence18' have been reported. Oligomerization by RhCI(PPh,) produces both dimers (64 %) and trimers (1 1 %).I8 I 8 0 ' Mono- and Poly-o1efins.-The effect of hydrogen on the induction period of involved in the catalysis.' 82 The rhodium-catalysed dimerization of ethylene ethylene dimerization with PdCl,(PhCN) indicates that a hydridic species is has also been investigated.'83 The effect of Lewis acids on the [(allyl)NiBr],- catalysed dimerization of ethylene indicates that of the series TiCI, AIBr and A1,Et,C13 the latter gives the highest activity ~ a t a 1 y s t . l ~ ~ Although bis-7r- allylpalladium dimerizes octa- 1,3,7-triene into normal hexadecapentaenes the addition of triphenylphosphine or a diphosphine gives 4-(but-3-enyl)dodeca- 1,6,8,1l-tetraene.l 85 Some very unusual spiro-compounds (10) and (1 I) have been synthesized by the Ni(cod),-catalysed dimerization of methylenecyclopropane.86 Propene has been oxidatively dimerized to hexa-2,4-diene and propane by the molybdenum allyl compound [(n-C,H,)(~n-C,H,)MoCl] and EtAlCl ; a com- plex containing a co-ordinated C linear hydrocarbon was isolated. ' 87 Although there has been little activity reported in the catalysed oligomerization of chloro-olefins two reports have been produced which shed more light on the subject of chloroprene oligomerization. PdCl,(PhCN) oligomerizes chloroprene to straight-chain products and a c8 palladium allyl complex has been isolated which is a catalyst for the above reaction. 188 Rh(C,H,),(acac) catalyses chloro- some 1,4-linked polymer is also formed. ' 89 prene dimerization to substituted vinylcyclohexenes and 1,2-divinylcyclobutanes ; Co-dimerization Reactions-The main work in this area has involved the co- dimerization of butadiene with ethylene and other olefins.The factors controlling the activity of nickel-aluminium-phosphine catalysts for the co-dimerization of M. Green and T. A. Kuc J.C.S. Dalton 1972 832. 1 8 1 N. Acton R. J. Roth T. J . Katz J. K. Frank C. A. Maier and I. C. Paul J. Amer. 182 Chem. SOC. 1972,94 5446. T. Kitamura K. Maruya Y . Moro-oka and A. Ozaki Bull. Chem. SOC. Japan 1972 45 1457. 183 I. Okura and T. Keii Nippon Kagaku Kaishi 1972 257. 184 S. G. Abasova A. I. Leshcheva E. A. Muchina V. Sh. Fel'dblyum and B. A. Krentsel Bull. Acad. Sci. U.S.S.R. 1972 21 608. 185 W. Keim and H. Chung J. Org. Chem. 1972,37,947. 186 P. Buiger Angew Chem. Internat. Edn.1972 11 309. 188 D. J. S. Guthrie and S. M. Nelson Coordination Chem. Rev. 1972 8 139. I89 K. Bouchal J. Skramovoka J. Coupek S. Pokorny and F. Hrabak Makromol. 187 M. L. H. Green G. G. Roberts J. Knight W. E. Silverthorn and L. C. Mitchard J.C.S. Chem. Comm. 1972,987. Chem. 1972 156. 225. 28 2 J. P. Candlin G. L. P. Randall and A . W. Parkins ethylene and butadiene have been reported.'" The effect of an alkylaluminium halide together with an alkylaluminium alkoxide or amide which itself is not a co-catalyst with nickel is to significantly increase selectivity towards formation of truns-hexa-l,4-diene. However NiC1,-(C,H,),Al catalysts can be modified with mono- or di-phosphines to give a great variety of co-oligomers of butadiene and ethylene depending on the phosphine nickel ratio.l 9 ' The same monomers when co-dimerized with a CoC1,-diphos-AlEt catalyst in 1 ,2-dichloroethane7 show a remarkable product dependence on the temperature of reaction. 19' From 80 to 100 "C cis-hexa-174-diene is formed in high yield; below 80 "C C materials form the main product; and above ll0"C both hexa-l,4-(and -2,4-)diene are formed. Titanium catalysts based on Ti(benzyl) or a Ti complex with AlR or LiR co-dimerize butadiene and ethylene to vinylcyclobutane. l9 Donor molecules e.g. bipyridyl phenanthroline or ethers improve yields of vinylcyclobutane. Butadiene and propene may be co-dimerized using the catalyst system [(n-C,H,)- PdCl],-AlCl,-PR in decalin containing nitrobenzene to give straight-chain C dienes.The selectivity to C products can be improved by using P(OR) in place of PR,.'94 High selectivity for co-dimerization of ethylene and propene to 3-methylpentene can be obtained using Ni-Al,R,Cl,. 195 The polyenes ( 1 2) (13) and (14) are produced by the co-dimerization of buta- diene with methyl methacrylate [for (12) and (13)]'96 or allene [for (14)].19' 190 A. C. L. Su and J. W. Collette J . Organometallic Chem. 1972 36 177. 1 9 1 Y. Inoue T. Kagawa Y. Uchida and H. Hashimoto Bull. Chem. Soc. Japan 1972 45 1996. 1 9 ' G. Henrici-Olive and S. Olive J . Organometallic Chem. 1972 35 381. 1 9 3 L. C. Cannell J. Amer. Chem. Soc. 1972,94,6867. ' 9 4 T. Ito T. Kawai and Y . Takami Tetrahedron Letters 1972 4775. 1 9 5 G. G. Eberhardt and H. K. Myers J .Catalysis 1972 26 459. 1 9 6 H. Singer W. Umbach and M. Dohr Synthesis 1972 42. 1 9 ' D. R . Coulson J . Org. Chem. 1972 37 1253. 283 Organometallic Compounds of the Transition Elements An unusual ring-opening co-dimerization of methylcyclopropane and methyl- acrylate to give 3-methylenecyclopropylcarboxylic acid methyl ester has been reported using Ni(CH,=CHCN) as catalyst.198 Allenes-The effect of phosphines and triaryl phosphites on the selectivity of the cyclo-oligomerization of allenes by NiO catalysts has been investigated. lg9 Excess phosphine or high catalyst concentrations increase selectivity of con- version to the tetramer whose structure has been reassigned as (15). Triaryl phosphites favour formation of the trimer 1,2,3-trimethylenecyclohexane.A mechanism has been proposed to describe these ligand effects.'yy~200 X-R aY structure investigations have confirmed the structure of an intermediate in the above Nio-catalysed oligomerization2" and also the structures of some oligoailene complexes of Fe,(CO) .202 Hexamerization of allene by [Rh(CO),Cl],-2PPh3 gave a product whose structure is proposed as ( 1 6).203 ( +)-Muscone has been synthesized by insertion of allene into dodecatrienyl- nickel followed by treatment with CO and hydrogenation of the unsaturated ketone produced.204 Acetylenes.-The catalytic trimerization of alkanediynes by Ni(CO),(PPh,) and (n-C,H,)Co(CO) to give the trisubstituted benzenes (17) in high yield has been rep~rted.~" Cyclo-octyne is also trimerized to the unusual benzene deriva- tive (18).206 The reaction of the diacetylene (19) with RC2R in the presence of RhCl(PPh3)3 gives the quinone (20).'07 However 0 S and Se may also be in- serted to give the heterocycle (21).Similar reactions have been reported with other diacetylenes and IrCl(PPh,) .,08 l o o 1 9 8 R. Noyori Y. Kumagai I. Umeda and H. Takaya J . Amer. Chem. SOC. 1972 94 4018. '99 S. Otsuka A. Nakamura T. Yamagata and K. Tani J . Amer. Chem. SOC. 1972 94 1037. M. Englert P. W. Jolly and G. Wilke Angew. Chem. Internat. Edn. 1972 11 136. ' " I B. L. Barnett C. Kruger and Yi-Hung Tsay Angew. Chem. Internat. Edn. 1972 11 137. 2 0 2 N. Yasuda N. Yasuoka Y. Kai N. Kasai and M. Kakudo J.C.S. Chem. Comm. 1972 157. ' 0 3 J . P. Shalten and H. J .van der Ploeg Tetrahedron Letters 1972 1685. 2 0 4 R. Baker B. N. Blackett and R. C. Cookson J.C.S. Chem. Comm. 1972 802. ' 0 5 A. J. Chalk and R. A. Jerussi Tetrahedron Letters 1972 61. *06 G. Wittig and S. Fischer Chem. Ber. 1972 105 3542. 20' E. Miiller and W. Winter Chem. Ber. 1972 105 2523. ' 0 8 E. Muller and C. Beissner Chem.-Zrg. 1972 96 170. 284 R n c*c*R R Ph- N J. P. Candlin G. L. P. Randall and A . W. Parkins O R P h - N a R I R V R 0 X = 0 Se or S C Ill c+c 'R C R I (17) (21) Oligomerizations of acetylenes which are stoicheiometric and in which the oligomer remains co-ordinated to the metal centre have been reported for a variety of cyclic d i a ~ e t y l e n e s ~ ~ ~ ~ ' ' flu oroalkyl-substituted acetylenes,2 ' ',' ' diaryl' and diakyl acetylenes,2 14*' ' and acetylenes with silyl substituents.' l 6 7 Insertion Reactions 'I7 The insertion of 1,3-dienes into 7c-allyl-palladium bonds has been investigated2 '' and it is concluded that carbon<arbon bond formation occurs outside of the '09 R.B. King and A. Efraty J . Amer. Chem. SOC. 1972 94 3021. "* R. B. King and I . Haiduc J . Amer. Chem. SOC. 1972,94 4044. I D. M. Barlex A C. Jarvis R. D. W. Kemmitt and B. Y . Kimura J.C.S. Dalton 1972 2549. 2 1 2 T. O'Conner A. J. Carty M. Mathew and G. J . Palenik J . Organometallic Chem. 1972 38 C15. 'I4 'I5 ' I 3 G. Ferraris and G. Grevasio J.C.S. Dalton 1972 1057. R. P. A. Sneeden and H. H. Zeiss J . Organometallic Chem. 1972,40 163. C.Calvo T. Hosokawa H. Reinheimer and P. M . Maithis J . Amer. Chem. SOC. 'I6 1972,94 3237. H. Sakurai and J. Hayashi J . Organornetallic Cheni. 1972 39 365. R. P. Hughes and J. Powell J. Amer. Chem. Soc. 1972 94 7723. NR 285 palladium co-ordination by an electrocyclic mechanism. The above mechanism is considered as being applicable to the polymerization of 1,3-dienes to give either 1,2- or l,4-polymers. The polymerization of norbornadiene by homo- allylic palladium compounds is not thought to have a similar It has been shown2I9 that diphenylacetylene may be inserted into one or two co-ordinated isocyanides either on Nio or Pd" catalytically to give the compounds (22) and (23). phnNR Stoicheiometric insertion of fluoro-olefins into a l k y l - g ~ l d ~ ~ ~ ' ~ ~ and silicon-mangane~e~~~ bonds have been reported and also the first insertion of an acetylene albeit hexafluorobut-2-ene into a n-allyl-palladium bond.z23 The photocheniically induced insertion of dioxygen and S (derived from Sa) into a variety of cobalt-carbon bonds in chelate complexes has CO has been inserted into the iron-hydrogen bonds of the complexes FeH,L and FeH2N,L (L = PEtPh,) to give formato-iron insertion into the Ptkarbon bond of aryl complexes is proposed'29 to involve prior attack of the sulphur dioxide on the metal.Rearrangements of the alkyl ligands occur when sulphur dioxide is inserted into cobalt-carbon bonds of cobaloxime complexes.230 The insertion of NO into tungsten-23' and zirc~nium-rnethyl~~~ bonds gives rise to a ligand containing two nitric oxide moeties (Scheme 10).SO2 R. P. Hughes and J. Powell J. Organometallic Chem. 1972 34 CSI. Y. Suzaki and T. Takizawa J.C.S. Chem. Comm. 1972 837. 2 2 0 A. Johnson R. J. Puddephatt and J. L. Quirk J.C.S. Chem. Comm. 1972. 938. C. M. Mitchell and F. G. A. Stone J.C.S. Dalton 1972 102. 2 2 2 H. C. Clark and T. L. Ham J. Organometallic Chem. 1972.42 429. 2 2 3 T. C. Appleton H. C. Clark R. C. Paller and R. J. Puddephatt J. Organometal~ic Chem. 1972,39 C13. C. Gianotti C. Fontaine and A . Gauderner J. Organometallic Chem. 1972 39 381. 2 3 2 Organometallic Compounds of the Transition Elements Ph Ph Ph N R (22) 2 2 ' Organometallic Chem. 1972 38 167. C . Gianotti C. Fontaine B. Septe and D.Dove J. Organometallic Chem. 1972 39. c74. "' F. Farone L. Silvestro S. Sergi and R. Pietropaulo J . Organometallic Chem. 1972 V. D. Bianco. S. Doronzo and M. Rossi J . Orgonometallic Chem. 1572 35 337. 42. 177. 130 C. J. Cooksey D. Dodd C. Gatford M. D. Johnson G. J. Lewis and D. M. Titch- 2 2 4 2 2 6 2 2 6 C. Gianotti B. Septe and D. Benlian 1. Organometallic Chem. 1972 39 C5. C. Fontake K. N. V. Duong Z Merienne A. Gauderner and C. Giannotti J . 2 2 1 2 3 1 marsh J.C.S. Perkin !I 1972 6 5 5 . S. R. Fletcher A. Shortland. A . K. Skapski and G. Wilkinson J.C.S. Chem. Comm. 1972 922. P. C. Wailes M. Weigold and A. P. Bell J Organometallic Chem. 1972 34 155. 286 NO Me I (n-C,H,),Zr -+ (C,H,),Zr-0-N-NO I Me Scheme 10 The insertion of dichlorocarbene into a tungsten-hydrogen bond,233 of an alkyl isocyanate into an iron-hydrogen bond,234 chlorosulphonyl isocyanate into an iron+-allyl) bond,235 and CS into a nickel-sulphur bond236 to give a tristhiocarbonate complex have been reported.8 Carbonylation Reactions Used in Organic Synthesis Many reports of carbonylation of organic species with transition-metal com- plexes have appeared this year. Cobalt and rhodium remain the two most com- monly studied elements but other metals are widely used in stoicheiometric reactions. Two reviews of the area have ap~eared.’~’.~~’ The most significant development is the use of asymmetric ligands in catalytic systems to give asymmetric products ;239,240 optically active phosphine ligands have also been used for this p ~ r p o s e .~ ~ ~ * ~ ~ ~ There have been many reports of detailed work directed towards influencing the normal is0 ratio of aldehyde product in the hydroformylation of olefins. Infrared measurements at high pressure have added to our understanding of this problem.243-245 A rhodium cluster compound ph,(CO) ,COEt]- has been isolated in the carbonylation of ethylene with Rh4(C0)12 as catalyst ;246 this is the first cluster acyl compound. Some recent work with nickel catalysts allows a coupling of two fragments to occur before carbonylation. In one case an allyl halide and an olefin couple prior 2 3 3 K. S. Chen J. Kleinberg and J. A. Landgreke J.C.S. Chem. Comm. 1972 295. 1 3 4 W. Jetz and R. J. Angelici J .Organometallic Chem. 1972 35 C37. 2 3 5 Y. Yamamoto and A. Wojcicki J . C . S . Chem. Comm. 1972 1088. 236 F. Sato K. lida and M. Sato. J . Organomerallic Chem. 1972 39 197. 13’ F. E. Paulik Catalysis Rev. 1972 6 49. 2 3 a M. Orchin and W. Rupilias. Caralysis Rec. 1972 6 85. * 3 9 P. Pino C. Botteghi G. Consiglio and S. Pucci ‘Proceedings of the Symposium on Hydroformylation,’ Veszpremi Vegyipari Egyetem Veszprtm Hungary 1972. p. I . *40 C. Botteghi G. Consiglio and P. Pino Chimia (Switz.) 1972. 26 141. 2 4 1 M. Tanaka Y. Watanabe T. Mitsudo K . Yamamoto and Y. Takegami Chem. Letters 1972 483. 2 4 2 I. Ogata Y . Ikeda Chem. Lerrers 1972. 488. 1 4 3 G. F. Pregaglia A. Andreetta. G. Gregorio G . Montrasi. and G. F. Ferrari ref. 239 p. 19; Chimica e Indusrria.1972 54 405. 24’ R. Whyman ref. 239 p. 24. ”’ D. E. Morris and H. B. Tinket Chem. Techno/ 1972. 2 554. 2 4 6 P. Chini S. Martinego and G. Garlaschelli J.C.S. Chem. Comm. 1972 709. 287 R ’ C 0 . R ’ Organometallic Compounds of the Transition Elements MeCH=CHCH,CI + CH,=CH CO-H,O MeCH=CH(CH,) ,CO,H Scheme 11 to carbonylation (Scheme 1 1),247 and in another example,248 acetylene is dimerized and then carbonylated (Scheme 12).248 The non-catalytic formation of muconyl chloride CICOCH=CHCH=CHCOCI from HCECH CO and PdCl, is already known.249 2HC=CH + CO + ROH [xN‘(c0)31; H,C=CHCH=CHCO,R Scheme 12 A synthesis of unsymmetric ketones using Na,Fe(CO) as a stoicheiometric reagent has been described (Scheme 13).250 This reaction is similar to the prepara- tion of aldehydes from alkyl halides251 or acyl halides,252 in which the inter- mediate is quenched with acetic acid.The above carbonyl synthesis has also been investigated using RLi-Fe(CO) combinations in the first stage.253 R L X + Na,Fe(CO) + [R’Fe(CO),]- Scheme 13 Further reports on the low-pressure carbonylation of methanol to acetic using rhodium catalysts (both homogeneous and heterogeneous) have appeared.’ 5-2 5 7 Decarbonylation of quite complicated organic molecules using RhCI(PPh,) has been reported e.g. steroidal aldehydes can be decarbonylated by refluxing with RhCl(PPh,) in boiling acetonitrile.62 2 4 7 2 4 8 9 Addition and Substitution Reactions of Unsaturated Hydrocarbons Co-ordinated to or Catalysed by Transition Metals This is an area where applications to synthetic organic chemistry may arise.Information concerning the steric control and reaction site specificity is forth- coming which may be of assistance in tailoring certain reactions. G. P. Chinsoli and G. Cometti J.C.S. C/rem. Comm. 1972. 1015. M. Foa and L. Cassar Garretta. 1972 102 85. 2 4 9 J. Tsuji M. Morikawa and N. Iwamoto J . Amer. Chem. Soc. 1964 86 2095. ”” J . P. Collman S. R. Winter and D. R. Clark J . Amer. Chem. Soc. 1972.94 1788. 2 5 1 M. P. Cooke J . Amer. Chem. Soc. 1970 92 6082. 15’ Y. Watanabe T. Mitsudo M. Tanaka. K. Yamamoto T. Okajima and Y. Takegami ”’ Y. Sawa M. Ryang and S. Tsutsumi J . Org. Chem. 1970,35 4183. 2 5 4 J. P. Candlin K. A. Taylor and A. W. Parkins Ann. Reports (B).1971 68 273. Bull. Chem. Soc. Japan 1971,44 2569. 2 5 5 A. Hershman K. K. Robinson J. H. Craddock and J. F. Roth Prepririrs. Amer. Chern. Soc. Dit,. Petroleum Chem. 1972. 17. E7. R. G. Schultz and P. D. Montaomery Preprinrs Amer. Chern. Soc. Div. Perroleurn Chem. 1972 17 B13. 2 5 ’ K. K. Robinson A. Hershman J. H. Craddock and J. F. Roth ref. 239 p. I I 1 J . P . Candlin G. L. P. Randall and A . W. Parkins 288 The main area of electrophilic addition reactions that have been reported are protonation reactions. Low-temperature protonation of cyclo-octatetraene- iron tricarbonyl and the methyl-substituted derivative has been to give a monocyclic dienyl complex which at - 60 “C isomerizes to the bicyclic dienyl derivative. Protonation of substituted butadieneiron tricarbonyls have been re-in~estigated,”~ together with hetero-dieneZ6’ and aldehyde-substituted butadiene26’ complexes.The latter two are protonated exclusively on the hetero- atom. Oxygen protonation also occurs in (cycloheptadienone)Fe(CO) but in (tropone)Fe(CO) the ring is protonated; exo protonation may be obtained using CF3C0,H.262 (x-C,H,)Fe(CO)2(u-cyclopropyl) complexes react with acids to open the cyclopropyl ring.263 Protonation of (n-C,H,)Rh(or Ir)(cyclohexadiene) occurs to give endo ad- dition to the C6 ring and this proceeds by metal p r o t o n a t i ~ n ~ ~ ~ . ~ ~ ~ whereas acetylation using AICI,-CH,COCI occurs with substitution on the C ring. The site of electrophilic substitution on heptafulvene may be directed to the C ring by co-ordination to Fe(CO) .266 The Cr(CO) grouping when co-ordinated to heptafulvenes promotes electrophilic attack at the exocyclic carbon atom as in the unco-ordinated heptaf~lvene.’~’ It has also been shown268 that products formed by addition of electrophilic olefins such as TCNE to co-ordinated polyenes followed by liberation of the ligand using Ce” are different to those formed by direct reaction.Electrophilic substitution of (cycloheptatriene)Fe(CO) using CH,COCI- AICI, gives a mixture of products derived from exo and endo addition; however CH3CO+ BF,- gives only exo attack.269 The structure of the intermediate obtained ’ by acetylation of (butadiene)Fe(CO) has been determined by X-ray methods and carbonyl-oxygen-iron interaction has been confirmed.270 The reactions of a series of (trimethylenemethane)Fe(CO) complexes with a variety of electrophiles have been rep~rted.~” In the area of nucleophilic reactions the addition of active-hydrogen-con- taining compounds to butadiene has been catalysed by PdCI,(PPh,),-NaOPh- ’” D.H. Gibson and R. L. Vonnahme J . Amer. Chem. Soc 1972.94 5090. M. Brookhart E. R. Davis and D. L. Harris J . Amer. Chem. Soc. 1972,94,7853. ’” ’” A. M . Brodie B. F. G. Johnson. P. L. Josty and J . Lewis J.C.S. Daltotr. 1972 2031. M. Brookhart and D. L. Harris J . Organometallic Chem. 1972.42 441. 2 6 2 D. F. Hunt G . C. Farrant and G . T. Rodeheaver. J . Orgonometallic Chem. 1972 38 349. 2 6 3 A. Cutler R. W. Fish W. P. Giering and M. Rosenblurn J . Amer. Chem.SOC. 1972 94 4354. 2 6 4 B. F. G. Johnson J . Lewis and D. Yarrow J.C.S. Chem. Comm. 1972 235. *6s B. F. G. Johnson J. Lewis and D. Yarrow J.C.S. Dalton 1972 2084. 2 6 6 B. F. G . Johnson J. Lewis. P. McArdle and G . L. P. Randall J.C.S. Dalton 1972 ’“ ’” J . A. S. Howell B. F. G. Johnson and J . Lewis J . Organometallic Chem. 1972 42 2076. c 5 4 . 2 6 8 D. J. Ehntholt and R. C. Kerber J . Orgatrometallir Chem. 1972 38 139. ’69 B. F. G. Johnson J. Lewis P. McArdle and G . L. P. Randall J.C.S. Daltot~ 1972,456. ’” A . D . V. Harvey and G. A. Sim J.C.S. Dalton 1972. 2305. ’” K. Ehrlich and G . F. Emerson J. Amer. Chem. Soc. 1972 94 2464. 289 Organometallic Compounds of the Transition Elements PPh 2 7 2 PdBr,(diph~s),-NaOPh,~~~ and n i ~ k e 1 ~ ~ ~ ~ ~ ~ and rhodium cata- 1 y ~ t ~ .~ ~ ~ ~ ~ ~ N one of these catalysed reactions give high specificity for terminal or internal substitution. High yields are obtained by the alkylation of olefins in the presence of alkylpalladium derivatives prepared in sit^.^^^^^^^ The addition of both MeOH280 and Et,NH281 to platinum-acetylene complexes to give substituted olefin complexes has been reported. n-Allylnickel halides have been used in the synthesis of coenzyme Q of plastoquinone by addition of an allyl residue to a quinone?82 and also in the synthesis of geranic Dodecat- rienylnickel reacts with aldehydes acid chlorides and allyl halides to give substituted t r i e n e ~ . ~ ~ A synthesis for substituted arenes has been reported ;285 this method involves CN- attack on (C,H,-,,Me,,)Mn(CO),+ to give a cyano- cyclohexadienyl derivative from which cyano-arenes are obtained by reaction with Ce'" which effects both hydride abstraction and metal removal.This latter step is an advance because normal hydride-abstracting agents remove the cyano-group in preference to the hydride. 10 Asymmetry and Asymmetric Induction Catalytic asymmetric hydrogenation has been reviewed.286 The catalyst for asymmetric hydrogenation may be either a modified Raney or a transition-metal complex with an optically active ligand. The ligands employed are often phosphines which contain either an asymmetric alkyl group e.g. the diphosphine (24),288 or have the phosphorus atom as the asymmetric centre e.g. R-benzylmethylphenylphosphine (P*).289 Complexes using the latter ligand Rh(norbornadiene) (P*) + have been used for the asymmetric reduction of ketones whilst the former class of ligands e.g. neomenthyldiphenylphosphine when complexed to Pd" are active asymmetric hydrosilylation catalysts.290 2 ' 2 K. Ohno T. Mitsuyasu and J. Tsiyi Tetrahedron 1972 28 3705. 2 7 3 K. Takahashi A. Miyake and G. Hata Bull. Chem. SOC. Japan 1972 45 1183. 2 7 4 D. Rose Tetrahedron Letters 1972 4197. 2 7 5 R. Baker D. E. Halliday and T. N. Smith J . Organometallic Chem. 1972 35 C61. 276 H. Kawazura and T. Ohmori Bull. Chem. SOC. Japan 1972 45 2213. 2 7 7 R. Baker and D. E. Halliday Tetrahedron Letters 1972 2773. 2 7 ) J R. F. Heck J. Organometallic Chem. 1972,37 389. 2 7 y R. F. Heck and J. P. Nolley J.Org. Chem. 1972 37 2320. "' D. M. Barlex R. D. W. Kemmitt and G . W. Littlecott J. Organometallic Chem. M. H. Chisholm and H. C. Clark J . Amer. Chem. SOC. 1972 94 1532. 1972 43 225. 2 8 2 L. S. Hegadus and E. L. Waterman J . Amer. Chem. SOC. 1972 94 7155. 2 R 3 K. Sato S. Inoue S . Ota and Y. Fujita J . Org. Chern. 1972,37,462. 2 8 4 R . Baker D. N. Backett R. C. Cookson R. C. Cross and D. P. Madden J.C.S. Chem. Comm. 1972 343. 2 8 5 P. J. C. Walker and R. J. Mawby J.C.S. Chem. Cornm. 1972 330. 2 8 6 W. S. Knowles M. J. Sabacky and B. D. Vineyard Chem. Technol. 1972 2 550. 2 8 7 T. Ninomya Bull. Chem. SOC. Japan 1972,45 2555. 2 8 8 H. B. Kagan and T . P. Dang J . Amer. Chem. SOC. 1972 94 6429. 2 8 9 P. Bonviani A. Levi G . Modena and G . Scorrano J.C.S.Chem. Comm. 1972 1188. 2 9 0 Y. Koso K. Yamamoto K. Tomao and M. Kumado J . Amer. Chem. SOC. 1972 94 4373. 290 J . P . Cundlin G. L. P . Randall and A . W. Parkins M exO TcH H z PPh CM,PPh Me 0 H (24) Asymmetric hydroformylation has been achieved using cobalt complexes containing optically active salen l i g a n d ~ . ~ ~ ~ A patent has now been granted for the oligomerization of ethylene to yield optically active hydrocarbon mixtures Which uses nickel catalysts containing chiral phosphine ligand~.'~' Further work has appeared on the asymmetric selection of DL-propene oxide using zinc catalysts. 292 Another example of asymmetric control by complexation is provided by the reactions of substituted arenechromium tricarbonyl derivative^.'^^.^^^ In these cases the optical activity arises when the Cr(CO) group (which can be removed by irradiation) is complexed to the arene ring.11 Heterogenized Homogeneous Catalysis This is a new section and concerns an aspect of work which has potential both for commercial exploitation and also for the possibility of novel catalytic effects. Essentially it is the transformation of homogeneous catalysts into heterogeneous analogues. There are two principle ways of achieving this (i) by addition of a metal-organic compound to a preformed organic or inorganic polymer either by condensation or ligand-exchange reactions and (ii) by preparation of a com- plex containing a ligand capable of being polymerized. Space does not permit a complete list of new reactions studied; however several recent reviews indicate the magnitude of the s ~ b j e c t .~ ~ ~ - ~ " 2 9 ' Ger. Ott'en. 2 039 12511972 (Chem. A h . 1972,76 126 478). 2 9 2 M. Nakaniva I. Kameoka R. Hirai and J. Furukawa Makromol. Chem. 1972 155 197. 2 9 s H. Heinemann Chem. Techno/. 1971 1 286. 2 9 3 G . Jaouen and R. Dabard Tetrahedron Letters 1971 1015. 294 A. Meyer and R. Dabard J . Organometallic Chem. 1972 36 C38. 296 N. Kohler and F. Dawans Reu. Inst. franc. Petrole 1972 27 105. 2 9 ' L. Sajus Mam. Sac. Roy. Sci. Liege 6' Serie 197 1,1,7 1 (Chem. A h . 1972,76 104 235). 2 9 8 J. Mannassen Fortschr. Chem. Forsch. 1972 25 1. 2 9 9 J. C . Bailar jun. Preprints Catalysis NATO Science Committee Conference Sar- dinia December 1972. 300 R. Ugo Englehard Tech. Bull. 1971 11 45.

ISSN:0069-3030    

DOI:10.1039/OC9726900267

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

8 Electro-organic Chemistry By K. KORINEK and T. F. W. McKlLLOP ICI Ltd. Corporate Laboratory P.O. Box 1 I The Heath Runcorn Cheshire 1 Introduction The advantages of electrochemical methods in synthesis and their applicability to a wide range of problems in organic chemistry are evident in numerous papers which have appeared during the year. Although progress is being maintained there is still considerable opportunity for improving the collaboration between organic chemists and electrochemists if the full potential of the techniques is to be realized. Regular exhaustive treatment of the subject is to be found in the Specialist Periodical Reports series on Electrochemistry. The aim of this review is to highlight those advances during the past year which to the authors seem most significant our principal concern being that the papers are of interest to both organic and electro-chemists.Consequently many papers of very high quality have been excluded since they deal only with mechanistic detail and conversely some papers of poorer quality are included on the strength of perhaps only one interesting observation. Continuing interest in the subject is shown by the number of genera12,3 and specialized reviews,4-6 and another volume of Fortschritte der chemischen Forschung has been devoted to two aspects of the subject uiz.the stereochemistry of electrochemical reductions and the oxidation of biologically important purines.’ In addition a new book by Fry has appeared dealing specifically with synthetic aspects of Organic Electrochemistry.* 2 Reductions Hydrocarbons.-Aromatic.Electrochemical reduction is generally recognized as a powerful technique in the preparation and study of non-benzenoid aromatics. The ton-electron system derived from cyclo-octatetraene and related compounds has been of particular interest recently [Ann. Reports (B) 1971,68,298]. It would ’ ‘Electrochemistry’ ed. G. J. Hills (Specialist Periodical Reports) The Chemical Society London vol. 1 1970 vol. 2 1972. J. H. P. Utley Chem. and Ind. 1972 230. ’ M. Janda and M. Nemec Chem. listy 1972,66,225. F. Beck Angew. Chem. Inrernat. Edn. 1972 11 760. ’ R. E. Dessy and L. A. Bares Accounts Chem. Res. 1972 5 415. ‘ L. I. Antropov Irogi Nauki Electrokhirn. 1971 6 5. Forschitte der chemischen Forchung 1972 vol.34. * A. J. Fry ‘Synthetic Organic Electrochemistry’ Harper and Row New York 1972. 29 I K. Korinek and T. I;. W.McKiilop now seem that much of the earlier work may be in need of serious revision. Anderson and Paquetteg have shown that benzo- and sym-dibenzo-cyclo- octatetraene in rigorously anhydrous aprotic solvents undergo a single reversible one-electron reduction to the radical anion which is not further reducible and does not undergo facile disproportionation. The presence of small numbers of protons leads to rapid protonation of the radical anion and further reduction waves are observed due to mono- or di-hydro-derivatives. In another paper Thielen and Anderson lo demonstrate similar behaviour for the unsubstituted cyclo-octatetraene.Previous studies have been interpreted as showing two one-electron-reduction waves leading to the formation of the aromatic 107~-electron dianion. The authors believe that the second wave represents reduc- tion of protonated radical anion and show that quaternary ammonium salts capable of undergoing Hofmann elimination can provide an adequate source of protons for such reactive radical anions. Thus it would seem that the potential gain in delocalization energy on going from radical anion to the dianion is insuffi- cient to compensate for the energy losses involved in forming the planar dianion. Of much more general significance however is the danger of mis-interpretation of polarographic data due to protonation. One system in which the dianion does seem to be obtained by electrochemical or chemical reduction is the [16]-annulene.In dimethylformamide with tetra-n-butylammonium perchlorate as supporting electrolyte two discrete one-electron waves are obtained which are both shown to be perfectly reversible by cyclic voltammetry. In addition excellent n.m.r. evidence is forwarded in support of the dianion structure. Olejins. Polarographic half-wave potentials have been obtained for a series of alkenes and alkynes bearing sulphonyl sulphinyl and sulphonium groups in methanolic solution with tetramethylammonium bromide as supporting electro- lyte.l2 Although of little fundamental significance to the electrochemist the authors point out that qualitative comparisons can be drawn with nucleophilic additions to these substrates and that the variation of half-wave potentials with pH reflects the ease of prototropy in the substrates.Perhaps such studies may provide the basis of more quantitative correlations for the physical-organic chemist interested in nucleophilic additions. Major interest still centres on the industrially important electrohydrodimeriza- tion of activated olefins. Beck has written a very detailed review4 and has pub- lished the results of some of his work on the hydrodimerization of acrylonitrile in undivided cells. l3 Decomposition of the supporting electrolyte is the major problem in using undivided cells. Beck simply lowers the concentration of electro-lyte and to overcome the problem of extremely high resistances has designed special ‘capillary gap cells’.High yields lower energy costs and facile work-up procedures are claimed as major benefits using this system. To a considerable L. B. Anderson and L. A. Paquette J. Amer. Chem. SOC.,1972,94 4915. lo D. R. Thielen and L. B. Anderson J. Amer. Chem. SOC.,1972 94 2521. J. F. M. Oth J. M. Gilles and G. Schroder J. Amer. Chem. Soc. 1972 94 3498. l2 R. W. Howsam and C. J. M. Stirling J.C.S. Perkin 11 1972 847. l3 F. Beck J. Appl. Electrochem. 1972 2 59. Electro-organic Chemistry extent the industrial future of organic electrochemistry will depend on con- tinuing improvements in such electrochemical engineering rather than on major advances in understanding. Asahi have also released some details of their industrial process for the production of adip~nitrile.'~ The catholyte is an emulsion of acrylonitrile in aqueous electrolyte with polyvinyl alcohol as emulsi- fier and by avoiding the use of expensive McKee salts they also claim gains in electrolyte costs lower power costs and easier product purification.Considerable effort is still being directed towards the mechanistic study of electro-hydrodimerization. Bard and Puglisi have extended the use of digital simulation techniques to rotating ring-disk electrodes and by this means have obtained further evidence to support their mechanism involving dimerization of the radical anion or its protonated counterpart." The compounds studied were dimethyl fumarate cinnamonitrile and fumaronitrile in tetra-n-butyl- ammonium iodide-dimethylformamide solution.Such studies however are carried out in very dilute solution and there is no guarantee that this is the dominant reaction pathway in the more concentrated solutions used for prepara- tive experiments. Radical dimerization is also advanced as the mechanism in the hydrodimerization of 4-styrylpyridine in aqueous solution. l6 Carbonyl Compounds.-Andrieux and Saveant have described the first successful pinacolization of x-substituted ketones. For ketones of the general structure I -CO-C-X- where X = N N +,P' 0,S or halide it was commonly believed I that reduction led to cleavage of the carbon-heteroatom bond. Previous studies have almost totally involved aqueous media maintained at acidic pH to prevent alkaline hydrolysis of the substituent.By carrying out controlled-potential reduction in solvents such as acetonitrile and keeping the medium neutral or slightly basic it has been shown that carbon-heteroatom cleavage is not univer- sal. Dialkylamino- or alkoxy-substituted acetophenones behave like the un- substituted ketone. As shown in Scheme 1 careful choice of conditions allows Ph-CO-CH3 + H-N / / \ &3*5 \/ OH Ph-CO-C H -N '~~t 3Pho-~A ~-Ph~ (64~%) v~ ~ I I/ OH N \ \ /\ +ot / Ph-CHOH-CH,-N (66%) \ Scheme 1 '' Chem. Eng. News,Oct. 16 1972 p. 11. V. J. Puglisi and A. J. Bard J. Electrochem. SOC. 1972 119 829 833. K. Alwair J. F. Archer and J. Grimshaw J.C.S. Perkin 11 1972 1663. C. P. Andrieux and J.M. Saveant Bull. SOC.chim. France 1972 3281. K. Korinek and T. F. W. McKiIlop the electroreduction to be directed towards bond cleavage yielding acetophenone hydrodimerization giving the first synthetic method for a,ct’-disubstituted pinacols or to substituted carbinols. The pinacolization does not give equal amounts of racemic and meso product and the authors explain this on the basis of intra-molecular hydrogen-bonding rather than any special electrode effect. Other substituents such as thiomethyl thiophenyl phenoxy and fluoro do undergo extensive bond cleavage however even under controlled conditions. An interesting stereochemical effect has been observed in the reduction of cyclohexane-1,4-dione. As part of their general investigation of diketones Kariv and Cohen have found that cyclohexane-1,4-dione (1) but not the acyclic hexane- 1,4-dione is reduced at a mercury cathode in water-propan-2-01 mixtures below 2.3 V (us.S.C.E.). In addition to the expected dimer (2) and the mono- reduced hydroxy-ketone (3) the authors find some ring-opened product (4). Of the diols (5) and (6) produced by reduction of both keto-groups the authors find 0 0 0 OH -OH OH OH X./y OH (5) E. Kariv and B. J. Cohen J.C.S. Perkin I/ 1972 509. Elect ro -organ ic Chemistry a preference for cis over trans which is potential-dependent but can be as high as 8 1. Since the separate reduction of hydroxy-ketone (3) gives a 1 1 mixture of diols the authors conclude that the diketone is probably adsorbed on the electrode in the boat conformation.Sadly the yields are too low to make this a useful synthetic procedure. Although one of the best known and well studied reductions in organic electro- chemistry is that of salicylic acid to the aldehyde very little is known about the mechanism. The reaction which is normally carried out in aqueous solution in the presence of borate ion has now been examined more thor~ughly.'~ The acid itself is not reduced the electroactive species being complexes of borate ion and salicylic acid. From polarography and cyclic voltammetry of the mono- and di-chelated complexes in dimethylformamide and aqueous systems the authors propose Scheme 2 for reduction of the complex. - etc. H,Ot + Ho\ -/B etc. (JoH MO/\ Scheme 2 Of more interest to the synthetic chemist faced with the notoriously difficult reduction of an acid to the aldehyde is the paper by Wagenknecht,20 who has studied a series of substituted aromatic acids.He describes a simple general method using aqueous solutions with either borate or phosphate added and involving the extraction of the aldehyde as it forms with benzene. Apart from salicylic acid the yields however are seldom better than 50% and acids with a strongly electron-donating para-substituent which is reflected in a pK of >4.40 or an E for the methyl ester more negative than -2.20 V (S.C.E.)in 50 % ethanol containing 0.1 M-tetraethylammonium perchlorate give very poor current efficiencies. Perhaps if some thought is given to the formation of more reducible complexes than the borate complex considerable improvements in this procedure might result.Another example of the use of the cheaper phosphate as a replacement for borate has been reported by Russian workers in the reduction of ~-ribono-y-lactone to D-ribose which is used as an intermediate in the produc- tion of riboflavin.2'*22 Reductive Cleavage of Halides.-An excellent review on the stereochemistry of electroreductions includes a useful section devoted to carbon-halogen bond cleav- age.7 The details of some interesting investigations referred to in Fry's review I9 J. Ch. Hofmann P. M. Robertson and N. Ibl Tetrahedron Letters. 1972 3433. 'O J. H. Wagenknecht J. Urg. Chem. 1972,37 1513. 2' I. A. Avrutskaya M.Y.Fioshin E. V. Gromova and V. T. Novikov Soviet Electro-chem. 1972 8 425. 22 M. Y. Fioshin I. A. Avrutskaya L. A. Muzychenko E. V. Gromova and V. T. Novikov Sovier Elecrrochem. 1972 8 593. K. Korinek and T. F. W. McKillop have been published.23 This involves the reduction of the halogenonorbornanes (7H10)in dimethylformamide at a mercury electrode with tetraethylammonium bromide as supporting electrolyte. All dihalides gave the same mixture of endo-norbornyl chloride (11) and nortricyclene (1 2) when reduced at a controlled potential under similar condi- tions -implying a common intermediate. By measuring the amount of labelled chlorine-36 remaining in the endo-chloronorbornane the rate of heterogeneous reduction of an exo-chloro-group was found to be thirteen times that for the endo-chloro.Thus as expected from chemical studies on the norbornane system the endo face would seem to be less sterically hindered and is preferentially oriented towards the electrode. Although under a given set of conditions all four compounds (7H10) give the same product mixture the ratio of nortricyclene to the endo-norbornyl chloride is very dependent on the presence of proton donors. Surprisingly however the authors find that water and methanol seem to be better donors in this system than acetic acid or phenol. The authors believe that the anions formed in this instance are highly reactive and consequently are protonated close to the electrode. The addition of sulphides usually regarded as perturbing the double layer together with a phenol can alter the balance of products but additionally leads to loss of both halogen atoms and the production of norbornane.The proposed mechanistic scheme to nortricyclene (12) and endo-chloronorbornane (11) is as shown and although the authors make some 23 A. J. Fry and R.G. Reed J. Arner. Chem. SOC.,1972 94 8475. Electro-organic Chew is try suggestions concerning possible intermediates for the formation of norbornane this clearly requires further investigation. Annino et have described their studies on the reduction of mono- and di-chloro-maleic and -fumaric acids in aqueous solution over a range of pH and interpret their results on the basis of reduction proceeding on the double bond followed by competition between the elimination of chloride ion and protona- tion.In the case of dichloro-derivatives chloride is eliminated to give chloro- maleic and chlorofumaric acids. As reduction continues at higher pH’s a second chloride elimination can occur to give maleic and fumaric acids which are finally reduced to succinic acid. The authors’ previously suggested ‘stereoselective protonation mechanism’ is much in evidence throughout the discussion. The two-electron reduction of dihalides leading to the loss of both halide groups has provided material for an interesting mechanistic debate as well as some useful electrosyntheses. Is the elimination concerted? What is the stereo- chemistry? Although these questions can now be answered with reasonable confidence several interesting papers still appear each year.The reduction of a vicinal dichloride has resulted in an alternative route (yie!d -50 %) to dimethyl- enecycl~butenes~~ as shown in Scheme 3. The reduction is carried out at a R R Scheme 3 mercury cathode at -1.4 V versus Ag(AgClICl-(O.l mol I-’) in a solution of 0.25 moll-tetraethylammonium perchlorate or lithium chlorate in dimethyl- formamide. It is well known that reduction of l73-dibrornoalkanes can give good yields of cyclopropanes [Ann. Reports (B) 1972 68 3041 and not surprisingly attempts have now been made to synthesize cyclopropanones by this route. In one paper Dirlam Eberson and Casanova26 have studied the reduction of 2,4-dibromo-2,4-dimethylpentan-3-one (1 3) at a stirred mercury cathode under a variety of conditions including the use of a several possible trapping agents for the cyclo- propanone which would be expected to be fairly unstable in th ionic media required for electrosynthesis.Fairly complex mixtures ot’ products resulted from the various attempts and these authors interpreted their results on the basis of the enol allylic bromide anion (14a) and the zwitterion (14b). Fry and 24 R. Annino R. J. Boczkowski D. J. Bolton W. E. Geiger jun. D. T. Jackson jun. and J. Mahler J. Electroana1q.t. C‘hem. lnterjacial Electrochem. 1972 38 403. 25 H. Doupeux and J. Simonet Tetrahedron Letters 1972 4899. 26 J. P. Dirlam L. Eberson and J. Casanova J. Amer. Chem. SOC. 1972,94 240. K. Korinek and T. F. W. McKillop 6- 0 (14a) (14b) 1 1 products products Scoggin~,’~ however have carried out very similar studies on the same dibromide and have been able to isolate the methyl hemiketal of the cyclopropanone in quantitative yield when electrolysis is carried out in methanol.They have also observed characteristic carbonyl stretching for a cyclopropanone in the i.r. during reduction of 2,4-dibromo-2-methyl-3-pentanone, so that it would seem certain that the cyclopropanone is indeed formed and undergoes further reaction. Sadly Dirlam et al. quote product ratios rather than yields making it difficult to assess whether other products may have been formed but not identified in their studies. Existing chemical methods of selectively introducing deuterium or tritium are often tedious expensive and can be very inefficient.The controlled manner in which specific carbon-halogen bonds can be electroreduced using very mild conditions offers considerable advantages. Cockrell and Murrayz8 have exa- mined the replacement of iodine by deuterium for 0-,m- and p-iodoanisole and o-iodotoluene. Yields of deuterium-labelled product were very high and could be as high as 99 % depending on the choice of solvent and supporting electrolyte. Using D,O as the deuterium donor they found dimethoxyethane to be prefer- able to acetonitrile as co-solvent and lithium perchlorate to tetraethylammonium perchlorate as supporting electrolyte. Protons derivable from the quaternary ammonium ion by Hofmann elimination would seem to compete even in the presence of considerable amounts of D,O indicating yet again the care which must be taken in using these salts in electroreductions.The advantages of introducing an isotopic label by electroreduction of a carbon-halogen bond are well exhibited in a recent comm~nication.~~ The authors required a series of deuteriated or tritiated spiro[2,3]-hexenes and -hexadienes such as (15) and (16). These could be obtained in a total yield of 88% with 98% isotopic purity by reduction at a stirred mercury cathode of the corresponding tetrachloro-compound (17) in dimethylformamide containing a 100% excess of DzO with lithium perchlorate as base electrolyte. Another topic of keen interest to synthetic chemists is the design of good protecting or activating groups and mild selective methods for their removal.Recently the halogenoethoxy-group has been used to modify carboxylic acids and halogenoethoxycarbonyls have been used to protect alcohols and amines. The controlled manner in which electroreduction cleaves these groups selectively 27 A. J. Fry and R. Scoggins Tetrahedron Letters 1972 4079. J. R. Cocknell and R. W. Murray J. Electrochem. Soc. 1972 119 849. 29 M. F. Semmelhack R. J. DeFranco and J. Stock Terrahedron Letters 1972 1371. Electro-organic Chemistry C1 CI CI 40 XI under mild conditions has been demonstrated in two communications.30~3 Undoubtedly more work is required to establish the full utility of the techniques and to popularize them especially among organic chemists. Apart from the intramolecular cyclization during the reduction of dibromides and the formation of carboxylic acids by the addition of carbon dioxide very few attempts have been made to utilize synthetically the carbanions resulting from reduction of a carbon-halogen bond.In a characteristically detailed paper Baizer and Chruma32 have examined the conditions necessary for nucleophilic addition of these anions to activated olefins. The products formed in these reactions were very dependent on the nature of the halide the olefin the working potential and the nature of the medium but in general were all derived from either (u)direct protonation (b)protonation after addition (c) cyclization with displacement of a halide or (d) by displacement of a halide from the starting polyhalogeno-compound.These are shown in an idealized form in Scheme 4. An electrocatalytic cyanoethylation of chloroform giving almost 350 % current yield was also observed during these investigations. :()60 C1 + CCY2 -C 0,R C1 C02R \/ +c1-C /\ CH2-CHX Scheme 4 Nitro- and Nitroso-compounds.-The controlled-potential reduction of a series of substituted nitro-naphthalenes has been described in a series of papers by Jubault and Peltie~-.~j The reductions were carried out at a mercury electrode 30 M. F. Semmelhack and G. E. Heinsohn J. Amer. Chem. SOC.,1972 94 5139. '' E. Kasafirek Tetrahedron Letters 1972 202 1. 32 M. M. Baizer and J. L. Chruma J. Org. Chem. 1972 37 1951. " M. Jubault and D. Peltier Bull.SOC.rhim. France 1972 1544 1551 1561. 300 K. Korinek and T. F. W. McKillop and employed either a 1 1mixture of 0.5 M-H,SO and ethanol or a 1 1 mixture of 1 M-ammonium acetate and ethanol with potassium nitrate as the supporting electrolyte. In general the nitronaphthalenes are reduced more easily than the corresponding nitrobenzenes and in acidic conditions they tend to undergo complete six-electron reduction to the amine suggesting that the naphthalenic ring is acting as a better electron-acceptor. By operating at neutral pH however it is usually possible to stop the reduction at the hydroxylamine stage. If reduction is controlled at the foot of the polarographic wave in acidic conditions the product derived from a Gatterman rearrangement is the hydroxy-amine or the ethyl ether when using ethanol.Scheme 5 summarizes the types of reaction observed for H,SO,:eihanol OH (or OEt) Scheme 5 a-nitronaphthalene. The effect of various substituents and their position with respect to the nitro-group is the subject of the second paper while the third paper deals in particular with a-nitronaphthalenes substituted at positions 2 or 8 or P-nitronaphthalenes substituted at positions 1 or 3 with groups such as -CONH, -CN or -NHCOMe. Both the hydroxylamine derived from partial reduction and the amine can undergo cyclizations with such substituents leading to various heterocycles. The yield of cyclized product depends on the nature and position of the substituent and on the pH of the medium. Further complications in such cyclizations have been observed by the same authors in a paper describing an electrosynthesis of 3-amino-2,1-benzisoxazoleby the reduc- tion of o-nitrobenz~nitrile.~~ At low temperatures better yields are obtainable than from normal chemical syntheses.The o-aminobenzamides and azoxy- derivatives normally encountered in such reactions are in the authors’ view the result of secondary redox reactions. An impressive example of the use of electroreduction as a synthetic procedure is found in the preparation of 4,5,9,10-tetra-azapyrene(18) and its N-oxide~.~~ Chemical methods used so far give at best only very poor yields of either the parent compound or the oxides. Electroreduction of 2,2’,6,6’-tetranitrobiphenyl on a mercury cathode [ -1.0V (S.C.E.); water :ethanol = 1 1 ;pH = 6.81 gave 34 M.Jubault and D. Peltier Bull. Soc. chim. France 1972 2365. 3s E. Laviron D. Bernard and G. Tainturier Tetrahedron Letters 1972 3643. Electro-organic Chemistry 30 1 the tetrahydroxylamine (19) which could be oxidized in situ either electro- chemically (+0.15 V) or using air to give excellent yields of oxides. The propor- tion of mono di- or tri-oxide obtained depended on the method of oxidation. Electroreduction of the crude mixture of oxides yielded either 65 % of the mono- oxide or 72 %of the tetra-azapyrene (yields based on tetranitrobiphenyl) depend- ing on the choice of the medium and the reduction potential. 0 QNHOH N F 7 N=N / \ / \ -I.OV / \ / \ +O.’5V> / \ / \ 0,N NO NHOH NHOH N’N + di- and tri-oxides.7 N =N N =N water acetone = 3 2 water DMF = 1 1 _______) ‘ pH 6.2; -0.45 V pH 6.8; -0.90 V N=N N=N Tautomerism between quinone mono-oximes and the p-nitrosophenol is well known and this has led to doubts concerning the nature of the electro-active species during reduction. By 0-methylation the tautomerism is blocked and using this approach it has been shown that the oxime is the form which undergoes red~ction.~~ The furoxans are an interesting class of compounds which have received very little attention from elecrrochemists. The results of constant-potential electrolysis on benzofuroxan (20) in aqueous solution over a stirred mercury cathode can be summarized as shown in Scheme 6.37 In addition to this relatively straightforward process considerable amounts of coupling to the 2,3-diaminophenazine (21) may occur but the details of this are as yet unclear.N”O zc-.,Oz,r QNOH 5fJNH + 2H,O NOH NH (20) (211 Scheme 6 ’’ J. Bonastre and A. Casteton Bull. Snc. chim. France 1972 362. 3i C. D. Thompson and R. T. Foley J. Elecrrochrm. Soc. 1972 119. 177. 302 K. Korinek and T. F. W. McKilIop Carbon-Nitrogen and Nitrogen-Nitrogen Double Bonds.-During the past fifteen years considerable effort has been expended on examining the mechanisms obtaining in the reduction of a wide variety of nitrogen-containing heterocycles. In the main this work has involved only polarographic studies in aqueous media and it is pleasant to report that the more recent investigations utilize a much wider range of techniques including controlled-potential electrolysis and product analysis.Much of this work is of very high quality and since nitrogen-containing heterocycles occupy such a key position in organic chemistry and biochemistry it could lead to better synthetic methods or improved understanding of electron- transfer reactions in biology. Sadly however the majority of these papers make little or no attempt to utilize the experimental data obtained and consequently seem only to confirm a mechanism extend its generality or introduce some minor modification. One communication which does indicate the usefulness of electroreduction to the organic chemist involves a study of furo[2,3-d]pyridazines (22; R’ R2,R3 = H Me or Et).38 Polarography in aqueous medium shows that the reduction waves are pH-dependent.Controlled-potential electrolysis of (22; R’ =R2= H R3 =Me) on mercury at pH 9 with E = -1.75 V (S.C.E.) gave 20% of the reduced form (23),and at pH 4.5 with the potential controlled at -1.35 V (S.C.E.) C0,Me 0 C0,Me 0 0-R2‘ ‘H R2 this was improved to 60%. The significance of this work however is that chemical reduction techniques appear to proceed exclusively on the furan ring rather than the pyridazine emphasizing that electrochemical methods can fre- quently provide another useful weapon in the synthetic chemist’s armoury. Reduction of Bonds involving Sulphur.-Reduction of aryl alkyl sulphones is known to lead to cleavage of the alkyl-sulphur bond in simple open-chain compounds and studies with a variety of P-disulphones confirm that this is the dominant form of cleavage.39 The same authors have also described experiments with some di- and tri-sulphones in basic media in which it is the carbanion formed on loss of a proton that is the electroreducible species.40 The addition of tetraethylammonium hydroxide to solutions of the trisulphone (24) in dimethyl- formamide containing tetraethylammonium bromide as supporting electrolyte gives the anion which undergoes two-electron reduction at high potentials to give the dissymmetric disulphone provided X itself is not an electro-active 38 A.Daver Compt. rend. 1972 274 C 244. 39 J. G. Gourcy G. Jeminet and J. Simonet Bull.SOC.chim. France 1972 2982. 40 G. Jeminet J. G. Gourcy and J. Simonet Tetrahedron Letters 1972 2975. Electro-organic Chemistry (X-Ar SO,),CH -SO ,CH 3(X-ArS02),C- S02CH (24) Ht2e-H+ X-ArS02-CH,S0,CH3 +-X-ArS02-CH-S02CH3 + X-ArS02 substituent. The authors claim that this could be a preferred method of synthesis. Contrary to expectation cleavage of the alkyl-sulphur bond is not the preferred mode however for cyclic sulphones such as (25). Reduction of these compounds (25;n = 1,2 or 3) in dimethylformamide at a mercury electrode leads predominantly by aryl- sulphur cleavage to the sulphinate ion C,H,CH,SO,-. Until recently organo- sulphur chemistry has been largely neglected by electrochemists but typical of a growing interest are several papers from Parker et al.41942 dealing with redox couples between sulphur heterocycles and dimeric products often of a dicationic structure.In some cases electrochemical methods are the only known means of preparing such compounds. Miscellaneous.-Although it is well known that solvated electrons can be generated electrochemically our knowledge of their formation properties and application is far from complete. Consequently it is encouraging that a number of papers have appeared recently dealing especially with the mode of formation and the stru~ture.~~~~~~ From the organic chemist’s viewpoint however there is a disappointing lack of activity especially since so much interesting chemistry was anticipated. Avaca and Bewick have published two papers in which they use the lithium chloride-hexamethylphosphoramide ~ystem.~’In one paper they have studied the reduction of anthracene and have found that reasonable selectivity can be obtained by choice of conditions.In the other paper they have examined the reduction of acetamide and observed that the ratio of the two main products ethanol and ethylamine depends on the acid-sensitive decomposition of the gem-hydroxy-amino intermediate. Using hydrochloric acid as proton donor they were able to direct the decomposition in favour of ethylamine. It 41 C. T. Pederson 0. Hammerich and V. D. Parker J. Electroanalyt. Chem. Interfacial Electrochem. 1972 38 1972. 42 C. T. Pederson and V. D. Parker Tetrahedron Letters 1972 767 771. 43 Y. Kanzani and S. Aoyagui J.Electroanalyt. Chem. Interfacial Electrochem. 1972 36 297. 44 L. I. Krishtalik N. M. Alpatova and M. G. Fomicheva Croat. Chem. Acra 1972,44 I. 45 N. M. Alpatova L. I. Krishtalik and M. G. Fomicheva Soviet Electrockem. 1972 8 516 46 N. M. Alpatova A. D. Grishina and M. G. Fomicheva Soviet Electrochem. 1972 8 248. 41 L. A. Avaca and A. Bewick J.C.S. Perkin II 1972 1709 1712. K. Korinek und T. F. W. McKillop is interesting that these investigators observed little hydrogen evolution under such conditions and this behaviour would seem to be unique to their system. Baizer et a/. have reported some of their results involving reductions in the presence of COz.48 In dipolar aprotic medium with a mercury cathode C02 is itself reduced at ca.-2.1 to -2.2 V (S.C.E.)via C021‘ to give oxalate as the major product. Reductions are carried out with the addition of various com- pounds such as activated olefins simple olefins and aralkyl and alkyl halides; numerous carboxylated products have been obtained. Clearly the mechanism obtaining in any given case depends on the ease of reducibility of the added substrate. As a final example of the electroreduction of interesting organic compounds it is worthwhile mentioning some Japanese work on cephalosporanic acids.49 A high-yield two-step conversion of various cephalosporins (26) into the corre- sponding desacetoxy-cephalosporins (27)is possible using electroreduction to the exomethylenes (28) followed by rearrangement. Although the electrochemistry has the distinct flavour of the 1920’s hopefully to be remedied by more detail in future publications this work does indicate the potential for electrochemistry in the fine chemicals area.3 Oxidations Hydrocarbons.-The mechanism of anodic substitution of aromatic hydro- carbons has been studied in great detail in recent years and it is now agreed that most reactions take place by a mechanism involving the oxidation of the organic compound followed by reaction between the oxidized species and nucleophiles present in the electrolysis solution. In certain cases selective substitution can be obtained. Some new anode reactions involving nitrate ions and a model com- pound mesitylene have been reported by Nyberg.” The electrolyses were carried out in nitromethane or acetonitrile containing tetrabutylammonium nitrate using carbon and platinum anodes.This study has clearly demonstrated the pronounced effect of the solvent and the anode material on the product distribution. Electrolysis in acetonitrile at a platinum anode produced mainly 3,Sdimethylbenzaldehyde and 3,Sdimethylbenzyl nitrate whereas at a carbon anode the major product was 2,4,6-trimethylphenol. In nitromethane at a carbon 48 D. A. Tyssee J. H. Wagenknecht M. M. Baizer and J. L. Chruma Tetrahedron Letters 1972,4809. 49 M. Ochiai 0.Aki A. Morimoto T. Okada K. Shinozaki and Y,Asahi Tetrahedron Letters 1972 2341. 50 K. Nyberg Actu Chem. Scund. 1971 25 3246. Electro-organic Chemistry 305 anode another major product was 2-nitromesitylene.A carbon anode leads to substitution of the aromatic ring whereas platinum gives products derived from side-chain oxidation. The products might arise through the formation of an aromatic carbonium ion or by the attack of products of nitrate oxidation. More detailed studies are necessary to clarify the reaction mechanism. Adsorption of a substrate on the electrode or shielding effects preventing further reaction of the intermediate or substrate are frequently encountered in electrochemical processes but their nature is still poorly understood. Eberson and co-~orkers~'-~~ have studied the possible effects of adsorption on the product distribution in the anodic oxidation of 2-t-butylindane and l-t-butyl- acenaphthene at platinum carbon and lead dioxide electrodes in acetic acid.According to present theories of the adsorption of aromatic hydrocarbons on anode surfaces the aromatic ring is orientated parallel to the electrode surface and therefore the cis-isomer should preferentially form during the sequence of events following the electron transfer. On the other hand a free cation should lead to the trans-isomer (Scheme 7). OAc / H Parallel alignment 1 Scheme 7 The predominant products in anodic acetoxylation of 2-t-butylindane at platinum and lead dioxide were side-chain acetates. The cis :trans ratio was significantly higher than in related homogeneous reactions indicating at least a partial control of the stereochemistry of the anodic acetoxylation. At a carbon anode 2-t-butylindene became the major product.Similar product composition was observed in the anodic acetoxylation of 1-t-b~tylacenaphthene.~~ The anodic addition of cyanide ion to aromatic compounds or olefins has not yet been successfully achieved. An interesting effect has been observed during the anodic addition of cyanide ion to 9,10-alkyI-substituted anthra~ene.'~ The structure of the product was consistent with (29) but the i.r. spectrum of the 51 L. Eberson and H. Sternerup Actu Chem. Scund. 1972 26 1431. 52 S. P. Dirlam L. Eberson and H. Sternerup Chem.-lng.-Tech. 1970,44 178. 53 J. P. Dirlam and L. Eberson Acta Chem. Scund. 1972 26 1454. 54 V. D. Parker and L. Eberson J.C.S. Chem. Comm. 1972 441. K. Korinek and T.F. W. McKillop crystalline product showed a nitrile band at 2200 cm-and an isonitrile band at 2125 cm- ’. A novel method of stabilizing aromatic cation radicals was reported by Parker et d55i56 Hexamethylbenzene is oxidized in trifluoroacetic acid (TFA) or methylene ~hloride-TFA~~ according to Scheme 8. ‘CH OCOCF OCOCF OCOCF, I I I TFA 3 -H’r 7 *\+; & 63$ \ \ \ \ CH,’ (30) Scheme 8 The radical cation (30)is stable as shown by cyclic voltammetry at slow sweep rates (150mV s-’). Similarly thianthrene yielded the extremely stable thian- threnium ion which can be isolated as a Thianthrenium ion reacts rapidly with water in aqueous acetonitrile but in TFA showed reversible redox behaviour the ratio of anodic to cathodic peak currents (at 150 mV s-I) being only slightly less than that existing when the solution contained 37 % of water.Stable radical cations were also observed during the oxidation of aromatic hydrocarbons in methylene chloride at -70 0C.57At this temperature radical cations of rubrene tetracene and thianthrene had half lifes of several minutes. The synthesis of biaryls by direct anodic oxidative coupling of aromatic com- pounds might be a useful alternative to purely chemical methods. N~berg~~ has shown that in the presence of mesitylene the anodic oxidation of naphthalene in acetonitrile-acetic acid mixtures produces 1,l’-binaphthyl and l-mesitylnaph- thalene. The product of mixed coupling which was produced in 20% current yield is believed to be formed by the attack of the naphthalene cation radical on mesi t y lene.Another interesting reaction which has been reported is the anodic trimeriza- tion of 1,2-dimethoxybenzene in TFA.59 The reaction appears to be quantitative ” U. Svanholm and V. D. Parker Tetrahedron Letters 1972 471. ’’ 0. Hammerich N. S. Moe and V. D. Parker J.C.S. Chem. Comm. 1972 156. 57 L. Byrd L. L. Miller and D. Pletcher. Tetrahedron Letters 1972 2419. 58 K. Nyberg Acta Chem. Scand. 1971 25 3770. s9 K. Bechgaard and V. D. Parker J. Amer. Chem. SOC.,1972 94 4749. Electro-organicChemistry 307 on the voltammetric concentration scale (1 mmol 1-') but on a larger scale gives only 50% current yield of hexamethoxytriphenylene (31). (31) can be easily oxidized in TFA-HS0,F (1 1)to a stable radical cation a dication with a limited stability which is a ground-state triplet and a trication only observable at temperatures below -50 "C.OMe During the past year several publications have appeared dealing with the anodic oxidation of olefins. Fleischmann and co-workers60,6 have continued their work on partial oxidation of hydrocarbons in aprotic solvents and have also reported a detailed kinetic study of the oxidation of propylene in acetonitrile. Provided that the platinum anode is maintained active by pulsing the reaction is governed by the rate of the irreversible charge-transfer process on the rising portion of the logi us. E curve and by the rate of diffusion of propylene in the plateau region of the wave. A careful product analysis has shown that alkylaceta- mide was the major product.Wendt et have studied olefin oxidation at platinum and carbon anodes and observed a shift of about -300mV in the half-wave potentials for irre- versibly oxidizable olefins when carbon was used instead of platinum. The authors have attributed this effect to the catalytic activation of the olefins due to specific adsorption on carbon anodes. The use of sodium toluene-p-sulphonate instead ofsodium perchlorate as supporting electrolyte in methanol also had an influence on the half-wave potential of the hydrocarbon oxidation and the slope of the i us. E curves. Toluene-p-sulphonate anions being of an aromatic character are thought to be adsorbed on carbon electrodes and to repress the adsorption of olefins.Other studies on functionalization anodic dimerization and allylic substitution of olefins have also been rep~rted.~~.~~ The electro-oxidation of hydrocarbons still promises to provide interesting synthetic applications. The investigations are however still at an early stage as many electrolysis parameters remain to be characterized. '' M. Fleischmann and D. Pletcher Chem.-1ng.-Tech. 1972 94 187. ' D. Clark M. Fleischmann and D. Pletcher J. Electroanalyt. Chem. Interfacial Electro- chem. 1972 36,137. '' M. Katz P. Riemenschneider and H. Wendt Electrochim. Acra 1972 17 1595. 63 H. Schafer and E. Steckhan Chem.-ing.-Tech. 1972 44 186. 64 T. Shono and A. Ikeda J. Amer. Chem. Soc. 1972 94 7892. K. Korinek and T.F.W.McKillop Alcohols Phenols and Carbonyl Compounds.-S~ndholm~~ has continued his studies on the oxidation of aliphatic alcohols and has reported half-peak poten- tials for a series of alcohols in acetonitrile propylene carbonate and methylene chloride using tetrabutylammonium tetrafluoroborate as supporting electrolyte. The oxidation potentials in different solvents were compared using ferrocene- ferrocenium couple as reference electrode. It was shown that the oxidation in methylene chloride takes place at a potential 100 mV more negative than in the other solvents. A shift of this magnitude is unlikely to be caused by uncertainty in the reference potential and implies a specific influence of the solvent on the reaction. The kinetics and mechanism of the oxidation of alcohols at oxide-covered nickel silver copper and cobalt electrodes were studied by Fleischmann et It was shown that these oxidations involve hydrogen abstraction from the substrate by an oxide species as the rate-determining step followed by a further oxidation of the radical intermediate.The oxidation products of primary and secondary alcohols were fatty acids and ketones respectively. There is a striking similarity between oxidations at metal oxide electrodes and the mode of opera- tion of transition-metal oxides as heterogeneous catalysts for the gas-phase or solution reactions between oxygen and organic compounds. The catalytic reactions are carried out at high temperatures and pressures causing at least the surface of the oxide to be converted into a higher oxide.In the electrochemical process electrical energy is used instead to convert the oxide into the active species under much milder conditions. On a larger scale a ‘nickel oxide’ electrode was successfully used in the oxidation of 2,3 :4,6-di-O-isopropylidene-~-sorbose to di-0-isopropylidene-L-xybhexulosonic acid.67 Both these compounds are intermediates in ascorbic acid synthesis. The oxidative coupling of phenols and aryl ethers is usually accompanied by extensive side-reactions and a further oxidation of products. The extraordinary stability of organic cations in TFA and TFA-HS0,F mixtures allows however the reaction to be stopped at the cation-radical stage of the coupred product (32) which can then be reduced back to (33).68 The synthetic application of this + OR (33) bS G.Sundholm Acra Chem. Scand. 1971 25 3188. 6b M. Fleischmann K. Korinek and D. Pletcher J.C.S. Perkin If,1972 1396. 67 G. Vertes G. Horanyi and F. Nagy Tetrahedron 1972 28 37 68 K. Bechgaard 0. Hammerich N. S. Moe A. Ronlan U. Svanholm and V. D. Parker Tetrahedron Letters 1972 227 1. Electro-organic Chemistry 309 method to anisole and other aryl ethers (other than para-substituted) led to a good yield of coupled products. Several examples of intramolecular couplings were reported last year. Another interesting communication on electrochemical oxidative cyclization has recently been pre~ented.~~ A controlled-potential oxidation of 4-(3,4-dimethoxybenzyl)-6,7-dimethoxyisochroman-3-one(34) in acetonitrile contain- ing sodium perchlorate as the supporting electrolyte produced spirocyclo- hexadienone (35) in 55% yield.MeO&o Meo&I Me0 / Meal I 0 Me0 Me0 \ 0 (34) (35) The anodic oxidation pathways of 2-substituted hydroquinones catechols and para-substituted phenols have been investigated by Adams et aL7' In general monosubstituted hydroquinones with an electron-withdrawing group undergo anodic hydroxylation at relatively low potentials in aqueous media (Scheme 9). OR bR OH OH \ .-2e- -2H'. I I H,-J QR *-2e-,-?HtL \ OH OH OH 0 OH 0 Scheme 9 The electron-withdrawing substituent generates a partial positive charge at the 3-position which facilitates nucleophilic attack by water.Of special interest is the hydroxylation of the para-substituted phenol tyrosine (36) to 3,4-dihydroxy- phenylalanine (DOPA) which is a pharmaceutically important compound. On a preparative scale the yield of DOPA was estimated at only lo% even though COzH COzH I I (36) DOPA '' M. Sainsbury and R. F. Schiruazi J.C.S. Chem. Comm. 1972 718. '' L. Paponchado G. Petrie and R. N. Adams J. Electroanalyt. Chem. Interfacial Electrochem. 1972 38 389. 310 K. Korinek and T. F. W.McKillop cyclic voltammetry has shown that the hydroxylating reaction is fast. The other products were not identified and are believed to be polymeric. Miller et a1." have reported an interesting study of the oxidation of benzylic alcohols ethers and esters at a platinum anode in acetonitrile.The phenyl- alkoxycarbonium ion (37) was the key intermediate which underwent further oxidative cleavage to carbonyl products and acids. In the presence of sodium tetrafluoroborate benzaldehyde and benzyl alcohol were formed during the anodic oxidative cleavage of dibenzyl ether in accordance with Scheme 10. The I R2 I R2 ?H (37) PhCOR'IR2 1R' = H PhC0,H + R'OH Scheme 10 use of LiClO as supporting electrolyte led to the formation of benzylacetamide as an additional product. Chemical studies showed that this is formed at least partially by acid-catalysed amidolysis of benzyl alcohol as shown in Scheme 11 PhCH,OH L:;o,) PhCH,' PhCH,NHCOMe Scheme 11 and does not occur to any significant extent in the presence of sodium tetrafluoro- borate.This is in contrast to previous investigation^'^ which suggested that tetrafluoroborate preferentially solvates water at or near the electrode surface and therefore enables the nucleophilic attack of water on the carbonium ion formed by the electrode reaction. Carboxylic Acids.-Synthesis of deuterio-hydrocarbons using conventional methods usually requires several steps under highly specific conditions and often low yields result. An anodic oxidation of trifluoroacetic acid in the presence of an excess of [2H3]acetate anions in CD3C0,H was rep~rted'~ to give 68 % yield of trifluoro[ 1,1 l-2H,]ethane. Pentaflu~ro[~H,]propane was prepared in yields of over 75 % under the same experimental conditions using pentafluoropropionic acid instead of trifluoroacetic.This is another example of how the controlled conditions of electrolysis can be manipulated in favour of the required products. E. A. Mayeda L. L. Miller and J. F. Wolf J. Amer. Chrm. SOC.,1972 94 6812. 72 K. Nyberg Chem. Comm. 1969 774. 73 R. N. Renaud and D. E. Sullivan Canad. J. Chrm. 1972 50 3084. Electro-organic Chemistry 31 1 Previously the Kolbe electrolysis of TFA in acetic acid led only to hexafluoro- ethane because under the conditions only TFA was oxidized. Other studies of Kolbe and related reactions have also been Nitrogen-containing Compounds.-The anodic oxidation of aliphatic and aromatic amines has been a subject of a number of papers but no fundamentally new principles have emerged.There has been a further move in applying electro- chemistry to the generation of intermediates which undergo interesting organic reactions. One example involves ring expansion during the anodic oxidation of 2,3,4,5- tetra-anisylpyrrole in nitr~methane.~~ The pyrrole ring was oxidized in two steps to the cation (38) which on heating produced ?,3,4,6-tetra-anisyl-pyridine. The anodic oxidation of a number of 1-arylmethylene semicarbazides (39) has been examined7* in acetonitrile-acetic acid containing sulphuric acid. This method provides a convenient preparative route to either oxadiazoles (40) or triazolines (41) depending on the conditions used for the reaction. In the presence N-NH XArCH=N-NHCONH /( ,k=o (39) XAr N H internal/ cyclization + (41)H +I XArCH-N=N-CONH XArC-N-N-CONH 0 OH II I XArCNH-NH-CONH $ XArC=N-NH-CONH kyclizat ion N -N (40) ''I.Minato T. Takasuka K. Kimura T. Sakakibara and Y. Odaira Bull. Chem. SOC. Japan 1972,45,965. l5 G. Maier and F. Boblet Tetrahedron Letters 1972 4483. 76 A. Laurent E. Laurent and M. Thomalla Compt. rend. 1972 274 C 1537. '' M. Libert C. Caullet and J. Huguet Bull. Soc. chim. France 1972 3639. '' 0. Hammerich and V. D. Parker J.C.S. Perkin I 1972. 1718. 312 K. Korinek and T. I;. W. McKillop of water a high yield of (40)was obtained while the presence of acetic anhydride in the solvent system resulted in the formation of (41). Cyclic voltammetry with high sweep rates showed that the cyclization was extremely rapid.The oxida- tion potentials correlate well with the value of up+for the para-substituent. The role of low concentrations of water in diverting the product from 100% (41) to 100% (40)could be due to the nucleophilic attack. A special case of an intramolecular nucleophilic substitution reaction on sterically hindered 8-t-butyl-l-(2-pyridyl)naphthalenes(42)was initiated via electrochemical ~xidation.'~ (42)can be anodically oxidized to an isolable perchlorate salt (43),which slowly eliminates the t-butyl group. Bu' &o. -e 0 Wo-OAc OAc dimer Q A detailed review of the mechanism of the electrochemical oxidation of a number of biologically important purines has appeared." The results were compared with the known biochemical data and it was shown that for some compounds the data are of value in the interpretation of biological observations.Miscellaneous.-Several authors have reported the oxidation of diary1 sul- phides.' l-' Electrochemical oxidation could be synthetically attractive if its extent could be controlled via the applied anode potential. Houghton and Humffray' have shown that controlled-potential oxidation of diphenyl sulphide in 80% acetic acid-water mixtures in the presence of perchlorate sulphate or chloride ions at a platinum anode produced diphenyl sulphoxide in nearly quanti- tative yield and coulombic efficiencies of nearly 100%. The mechanism is thought to involve adsorbed intermediates. In another study using acetonitrile Uneyama 79 G. Popp J.Org. Chem. 1972 37,3058. *' G.Dryhurst Furtschr. Chem. Fursch. 1972 34 49. A. A. Humffray and D. S. Houghton Electrochim. Acta 1972 17,1421 1435. 82 K. Uneyama and S. Torii J. Org. Chem. 1972 37,367. 83 S.Torii K. Uneyama K. Iida and K. Sasaki Tetrahedron Lerrers 1972 4513. Electro-organic Chemistry and Toriig3 report that the sulphonium perchlorate (44) was the main product of oxidation of diphenyl sulphide. In 10% aqueous acetonitrile however a considerable amount of diphenyl sulphoxide was also obtained. c10,-The widespread use of low-temperature environments for the observation of highly reactive intermediates has proved to be of considerable value in mechan- istic studies. Van Duyne and Reilleys4 have now published a series of papers on cyclic voltammetry in butyronitrile at low temperatures.Some of the generated radical cations had half-lives of at least two seconds. Lastly a useful but not new approach to predicting the products of organic electro-oxidation has been adopted by Miller et dg5The half-wave potentials were correlated with ionization potential data for 68 compounds. There have been many justified objections based on theoretical grounds against such a correla- tion but the practical results are surprisingly good indicating that it might be a useful pointer for the organic chemist. 84 R. P. Van Duyne and C. N. Reilley Analyt. Chem. 1972,44 142 153 158. 85 L. L. Miller G. D. Nordblau and E. A. Mayeda J. Org. Chem. 1972,36 916.

ISSN:0069-3030    

DOI:10.1039/OC9726900291

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

9 Photochemistry By A. GILBERT Chemistry Department The University Reading As with the previous two Reports in this series the present review of the literature is restricted to publications which in the author’s opinion either have general interest to organic photochemists or are of significance in a particular area of the subject. Even so lack of assigned space has meant that only about half of the originally selected reports from 1972 are mentioned here and the choice whether to include particular work or not was often arbitrary. The trend noted last year of general concern by organic photochemists with reaction mechanisms intermediates etc.,has again been evident in a large number of publications which appeared in 1972 and the detailed description of develop-ment in apparatus design and usage is to be welcomed.Recent publications in the latter area have spanned the range from the interesting account of preliminary experiments on frequency doubling of an output of 520-650 nm 1*2 to provide a CW laser light in the 260-325 nm region at an expected level of a few mW,3 to descriptions of simple apparati for quantitative photochemical studies4 and for the kinetic investigation of such reaction^.^ It is also encouraging that photo- chemical techniques and experiments are being described for teaching purposes.6 Stilbene continues to be the molecule most commonly used in cis Strans isomerization studies.’ One of the problems encountered in this area is the com- parative lack of efficiency of energy transfer to it by certain sensitizers.A re-examination of this aspect has revealed that in the cases of acetophenone benzophenone and fluorenone quenching of the triplet ketone by stilbene does not always occur with energy transfer to the olefin.8 Thus on collision with acetophenone triplets 5% of the time the stilbene does not become excited. Stilbene has also been used in a mechanistic study of the quenching of triplet states by di-t-b~tylnitroxide.~ The nitroxide quenches the stilbene in a way ’ S. A. Tuccio and F. C. Strome Appl. Optics 1972 11 64. A. Dienes E. P. Ippen and C. V. Shank IEEE J. Quantum Electron. 1972 QE-8 388. C. Gabel and M. Hercher iEEE J. Quantum Electron. 1972 QE-8 524. J. Rennert and P. H. Ginsburg Mol. Photochem. 1972,4 235. V. Rehak F.Novak and I. Cepciansky Chem. listy 1972 66 875. D. M. Goodall P. W. Harrison and J. H. M. Wedderburn J. Chem. Educ. 1972 49 669. J. Saltiel and E. D. Magarity J. Amer. Chem. SOC. 1972,94 2742. D. Valentine and G. S. Hammond J. Amer. Chem. Soc. 1972,94 3449. ’ R. A. Caldwell and R. E. Schwerzel J. Amer. Chem. Soc.. 1972,94 1035. 315 3 16 A. Gilbert which favours decay to the cis-isomer. Although the mode of reaction is still unknown an attractive theory is that the nitroxide reacts with the phantom twisted triplet of stilbene so that a different range of twist is adopted. The complex so formed may well prefer a cisoid conformation thus influencing the amount of cis-stilbene eventually formed. It has been suggested that a cyclic biradical intermediate could rationalize formation of some of the photoproducts of linear conjugated dienes.Boue and Srinivasan have now applied kinetic analysis to recent data of the mercury- sensitized isomerization of several alkyl-substituted buta-1'3-dienes and thus obtained evidence for the existence of such an intermediate." Further it is considered that the chemical properties of the species are best accommodated by a cyclopropyl biradical structure (1). The structure of the products as well as their subsequent reactions are greatly influenced by the vibrational energy which (1) possesses. Thus 'hot' (1) favours 1,4- and 1,2-shift of hydrogen over the normally preferred 1,3-shift and 'hot' alkyl cyclopropene reopens to a 1,2-diene a 1,3-diene or an acetylene.Since cyclopropenes are not detected as products from the sensitized reactions of dienes in solution it is suggested that in the triplet state vibrational energy is necessary for cyclization of the excited diene to (1). The light-induced ring-opening of substituted cyclohexa-1,3-dienes to geo- metric isomers of hexatrienes is still of interest and a study of the reaction with compounds of type (2) has been made with the view to confirming a postulate that the ground-state conformation of a molecule undergoing con- and dis-rota- tory ring-opening plays a major part in reaction mode. l1 The results in conjunc- tion with the observation that 5-alkyl substituents adopt pseudo-axial or pseudo-equatorial positions dependent upon their bulk appear to substantiate the original suggestion.Thus in derivatives with groups such as 5-t-butyl there is a distinct preference for pseudo-equatorial positioning and hence the cis,trans-hexatriene product is favoured whereas with the 5-methyl compound there is little to choose between the two conformers and more equal mixtures of the cis,cis-and cis,trans-hexatrienes are formed. H (2) The di-n-methane rearrangement is now a well-established reaction although further work concerning the fine details of the process remains to be done. Three centres of stereochemistry are involved in the conversion of the penta- 1,4-dienes lo S.Boue and R. Srinivasan Mol. Photochem. 1972,4 93. 'I C. W. Spangler and R. P. Hennis J.C.S. Chem. Comm. 1972,24. Photochemistry into the vinylcyclopropanes.It is known that the configuration at C-5in the cis-and trans-isomers of l,l-diphenyl-3,3-dimethylhexa-1,4-diene is retained and preliminary evidence has suggested that inversion of configuration is preferred at C-3 the stereochemistry at C-1 was however uncertain [see (3)]. The latter (3) aspect of the reaction has now been studied in detail by the direct photolysis of cis-and trans- 1-phenyl-3,3,5-trimethylhexa-1,4-dienes and most interestingly the di-n-methane rearrangement of the two dienes is stereospecific the situation at C-1paralleling that at C-5 in that cis-diene yields cis-product.12 Benzo- phenone-sensitized irradiation of the dienes only resulted in cis trans inter- conversion. The di-n-methane reaction appears to require central methyl groups and terminal phenyl substitution since derivatives without these features rearrange with reluctance.In order to determine the role of the methyl group the photochemistry of 1,1,5,5-tetraphenylpenta-1,4-diene(4)has been studied. l3 Again the desirability of methyl groups for facile reaction is noted and although the expected product (5)is observed its formation is via the alternative C-H CT + n mechanism. It is also demonstrated that the low reactivity of (4) is not merely due to rapid competitive decay but rather because of inherently low excited-state reactivity. Di-n-ethane and di-n-propane rearrangements are con- sidered to account for the products from the light-induced reactions of 1,1,2,2- tetra~henylethane'~ l5 and 1,1,3,3-tetraphenylpropanes.Kramer and Bartlett have studied in detail the p-acetonaphthone-sensitized cycloaddition of cis-and trans-but-2-enes to cyclopentadiene. l6 The product is composed of three stereoisomers of 5,6-dimethylnorbornene and four 6,7-dimethylbicyclo[3,2,O]heptenes. The olefins are reported to yield the same ratio of total cis-to total trans-isomers and of 1,2- to 1,4-cycloadducts but differences I* H. E. Zimmerman P. Baeckstrom T. Johnson and D. W. Kurtz J. Amer. Chem. SOC.,1972 94 5504. H. E. Zimmerman and J. A. Pincock J. Amer. Chem. SOC.,1972,94 6208. l4 W. C. Schumann D. B. Vashi J. A. Ross and R.W. Binkley J. Org. Chem. 1972,37 21. l5 R.W. Binkley and W. C. Schumann J. Amer. Chem. SOC.,1972,94 1769.B. D. Kramer and P. D. Bartlett J. Amer. Chem. SOC.,1972 94 3934. 318 A. Gilbert are observed in the products from the two butenes in the relative amounts of those having erythro and threo configurations with respect to C-5 and C-6 in the bicycloheptenes and C-1 and C-6 in the norbornenes. It is shown that the trans-1,4-adduct which could arise from either an erythro-or threo-biradical at 30°C is almost exclusively formed via the threo-intermediate and mainly so at -15 "C. Following the formation of the two diastereoisomeric triplet bi- radicals rotational equilibrium is established in such before spin inversion allows the final ring-closure. Similar types of product were reported from 5-bromo- acenaphthylene and cyclopentadiene but here the interest lay in the influence of heavy atoms on photochemical reactions.A kinetic analysis of the addition showed that surprisingly the reaction is insensitive to heavy-atom solvents the data are rationalized by proposing that the rate constant for triplet decay (kd) to ground state is important and it is concluded that the intramolecular heavy- atom effect increases kd. Such (2 + 4) addition to that noted above has also been observed in photo- dimerization in the solid state of compound (6).'* The authors point out that whereas (2 + 2) light-induced dimerizations are known in crystals theirs is the first example of a (2 + 4) process occurring in the solid state rather than in solution. 0,Me OMe MeOCH,OMOM' The stereochemistry of the 'Diels-Alder-like' cyclization of hexa-1,3,5-trienes to bicyclohexenes has been investigated with derivatives which have no strong steric constraints on cyclization are adequately labelled to indicate stereo- selectivity and which undergo cis trans conversions far slower than closure reactions." The process was found to be non-concerted although there appears '' B.F. Plummer and W. I. Ferree J.C.S. Chem. Comm. 1972 306. '' H. Achenback W. Karl and E. Schaller Angew. Chem. Internat. Edn. 1972 11 434. lYA. Seeley J. Amer. Chem. Soc. 1972 94 4378. Photochemistry to be no reason why this should be so. That in other instances at least partial electronic control has been demonstrated has led to the conclusion in the present work that 'one must not depend on orbital symmetry control of selectivity in exploiting this reaction'.Worthy of note in any consideration of Diels-Alder reactions are the reports which appeared this year describing both the thermal and photochemical reaction with thiophens.20 The benzenoid products reflect initial 1,6cycloaddi- tion followed by loss of sulphur. Again this year there have been numerous publications concerned with the many and varied aspects of carbonyl photochemistry ; in particular Norrish Type I1 and competing processes (Scheme 1) have received considerable atten- tion. It seems to be fairly well established that the Type I1 elimination and HO R' /A phG: PhU R ' \* OH R' \ Scheme 1 cyclization reactions of aryl alkyl ketones having y-hydrogens involve the inter- mediacy of a 1,4-biradical.Further studies however continue to provide interesting information concerning the nature of the 1'4-biradical and to this end the photochemistry of methyl-substituted butyrophenones has been investi- gated.21 The data show a pronounced substituent effect on biradical reaction and the results are explained in terms of the transition state for biradical cyclization and elimination. It appears that a-substituents increase the percentage of cyclization products whereas P-substituents have the opposite effect. Wagner and co-workers report on several aspects of the Type I1 process in a series of comprehensive papers the first of which offers further evidence for a biradical intermediate.22 Irradiation of P,y-diphenylbutyrophenoneled to no triplet stilbene which would have been formed in a concerted elimination ;from this it is concluded that y-hydrogen abstraction to give the 1,4-biradical is faster than the energetically allowed elimination.Further it is suggested that the 'triplet state leading to biradical and then on to product' sequence presents the facile path for triplet to singlet spin inversion which a concerted elimination cannot. An extensive study has been made on the effects of y-and 6-substituents on the rate of y-hydrogen abstraction by triplet states of such ketones.23 For example a y-methoxycarbony1 substituent gives little enhancement of the rate whereas 2o H. J. Kuhn and K. Gollnick Tetrahedron Letters 1972 1909; R. Helder and H. Wynberg ibid.p. 605. " F. D. Lewis and T. A. Hillard J. Amer. Chem. SOC.,1972,94,3852. '' P. J. Wagner P. A. Kelso and R. G. Zepp J. Amer. Chem. SOC.,1972,94,7480. 23 P. J. Wagner and A. E. Kemppainen J. Amer. Chem. SOC.,1972 94 7495. 320 A. Gilbert y-vinyl phenyl and cyano-groups increase the rate by a factor of ten and y-alkoxy-groups by approximately fifty-fold :this is presumed to reflect the charge- transfer stabilization of the transition state for hydrogen abstraction. That no correlation is obtained between quantum yields for Type I1 reactions and triplet- state reactivities is taken to provide kinetic evidence for the intermediacy of a 1,cbiradical. The same workers have also suggested that the biradical may be trappable by a thiol the evidence for this is provided from a kinetic study of the quenching of the Type I1 process of y-methoxybutyrophenone by dodecyl mercaptan and S-deuteriated n-butyl mer~aptan.~~ The observed effects on the photochemistry of phenyl alkyl ketones by electron-withdrawing and -donating groups in the phenyl ring and alkyl chain are suggested to result from charge transfer from the y-alkyl position to the oxygen in the transition state for reverse transfer of hydrogen back to the y-carbon which in fact is the major reaction of most l-hydroxy-1,4-biradicals.25 The steric effects which are observed are interpreted as reflecting the restricted rotation in the biradical elimination and cyclization processes occur before rotational equilibrium is established.Both competitive charge-transfer in a- y- and 6-dialkylamino-ketonesz6 and competi- tive &hydrogen abstraction” have received attention.The latter publication also describes work concerning the geometry of intramolecular hydrogen-atom transfers. For example geminal dimethyl substitution as in (7) significantly retards &hydrogen abstraction and this is considered to be due to the torsional strain in a seven-membered cyclic transition state. Such ring-strain considera- tions are suggested to account for the 1,5-rather than 1,6-hydrogen atom trans- fers. Other workers have commented upon the high dependence of excited-state reactivity towards y-hydrogen abstractions on conformational factors for both rigid (bicycloalkyl) and non-rigid (cyclopentyl and acyclic) phenyl ketones.’* These workers have also investigated the prediction that by careful choice of substituents a degree of control over product composition and stereochemistry could be obtained and have studied the photolysis of bicycloalkyl phenyl ketones.Support for the supposition was obtained and high-yield stereoselective syntheses of several novel bridged polycyclic alcohols [e.g.(8)] are reported. 24 R. G. Zepp and P. J. Wagner J.C.S.Chem. Comm. 1972 167; P. J. Wagner and R. G. Zepp J. Amer. Chem. SOC.,1972 94 287. 25 P. J. Wagner H. N. Schott and R. G.Zepp J. Amer. Chem. SOC.,1972,94,7506. 26 P. J. Wagner A. E. Kemppainen and T. Jelliner J. Amer. Chem. SOC.,1972 94 7512. 27 P. J. Wagner P. A. Kelso A. E. Kemppainen and R. G. Zepp J. Amer.Chem. SOC. 1972,94,7500. 28 F. D. Lewis R. W. Johnson and R. A. Ruden J. Amer. Chem. SOC.,1972,94,4292. Photochemistry 32 1 Reactions of the above type have been studied with aliphatic ketones in the presence of triplet q~enchers.~~ Photolysis of threo- and erythro-4-methyl- [5-’H]hexan-2-one leads to non-stereospecific formation of butenes from the singlet state. Olefin formation by hydrogen transfer is calculated to be 95 and 90 % from the erythro- and threo-ketones respectively. The results are interpreted in terms of a short-lived singlet biradical intermediate. Attempts have been made to determine the synthetic prospects of the Norrish Type I1 process with cyclopropane derivatives for example a-cyclopropoxy- acetophenone yields acetophenone and cyclopr~panone.~~ Introduction of two methyl groups into the cyclopropane residue however dramatically affects the reactivity of the system and (9) does not yield any dimethylcyclopropanone or significant amounts of acetophenone :the prin.cipa1 reactions are cyclization and isomerization of the cyclopropane ring to yield (10) and (1 1) respectively.The authors draw attention to the fact that such ‘remote’ methyl groups produce a surprising increase in the rate of radiationless decay to the ground-state ketone and suggest that this may result from electronic vibronic coupling in which the stretching of the C-2-C-3 bond induced by excited ketone ?-hydrogen inter- action acts as the energy sink. Pertinent here are studies on the selective photolysis of thiobenzoic acid 0-esters which leads to the formation of olefins and thioacids (Scheme 2).Attempts to Scheme 2 trap possible reaction intermediates failed but from nanosecond flash photolysis quenching and phosphorescence studies it has now been shown that the process proceeds mainly via the lowest nTt* triplet state :chemical evidence for the forma- tion of the 1,4-biradical intermediate is pre~ented.~ * Formation of oxetans continues to attract interest and two theoretical papers have dealt with donor-acceptor interactions in photocycloaddition reactions involving the carbonyl Irradiation of barbaralone in the presence of benzophenone affords an addition product but instead of the possibly expected 29 C. P. Casey and R.A. Boggs J. Amer. Chem. SOC., 1972,94 6457. 30 T. R. Darling and N. J. Turro J. Amer. Chem. SOC.,1972. 94 4366. 3’ D. H. R. Barton M. Bolton P. D. Magnus and P. J. West J.C.S. Chem. Comm. 1972 632. 32 N. D. Epiotis J. Amer. Chem. SOC.,1972 94 1941 1946. 322 A. Gilbert oxetan compound (12)is formed.33 Reports on the photolysis of py-unsaturated ketones usually describe 1,2-and 1,3-acyl shifts from the triplet and singlet states respectively and cyclobutanol formation if there is an abstractable y-hydrogen. Intramolecular oxetan formation has now been observed in such systems and 2-cyclo-oct-1-enylcyclo-octanone is reported to yield the novel compound (I 3).34 0 Ph ph@ In contrast the cyclobutanol (14) is formed from 2-cyclohex-1-enylcyclohexa-none.Further examples of the intramolecular process have been observed with the enone (15)35and bichromophoric molecules of type (16).36The former yields the oxetan and the py-unsaturated isomer of (1 5) whereas (16) forms the cis-isomer and the oxetan (17). In the latter case singlet excited states appear to be involved and the importance of intramolecular complexation is recognized. lVlC Me-Me Two groups of workers have been concerned with the photochemistry of cyclo- pentenone derivatives. Rearrangement of aryl-substituted compounds in a mode similar to the di-n-methane reactions of 4,4-diarylcyclohexenones has been de- ~ribed.~’ The process proceeds via the 1,3-biradical (18) and although experi- ments suggest triplet intermediates the singlet-triplet splitting of only 1-2 kcal mol-in cyclopentenes is noted.Zimmerman and Little have studied the reactions of (19) and in benzene solution have obtained evidence for formation of the keten (20).38 33 K. Kurabayashi and T. Mukai J.C.S. Chem. Comm. 1972 1016. 34 R. C. Cookson and N. R. Rogers J.C.S. Chem. Comm. 1972 809. ’’ L. E. Friedrich and G. B. Schuster J. Amer. Chem. SOC.,1972,94 1193. 36 S. R. Kurowsky and H. Morrison J. Amer. Chem. SOC.,1972,94 507. 3’ S. Wolff and W. C. Agosta J.C.S. Chem. Comm. 1972 226. 38 H. E. Zimmerman and R. D. Little J.C.S. Chem. Comm. 1972 698. Photochemistry The photolysis of 6,6-diphenylbicyclo[3,l,O]hex-3-en-2-oneis known to yiefd 2,3- and 3,4diphenylphenols the ratios of which are a function of soIvent polarity.6-Bromo-5,5-diphenylcyclohex-2-ene-1-one has been synthesized as a precursor for the postulated zwitterionic intermediate in this phot~reaction.~ Indeed reaction of the bromo-compound with potassium-t-butoxide is now reported to yield the phenols in ratios which have the same solvent dependence as in the light-induced reaction and thus convincing proof is provided for the intermediacy of the zwitterion in the rearrangement of bicyclo[3,l,0]hexenone derivatives. Steroidal and other naturally occurring enones are still extensively studied but the area is too large for comment here. The photochemistry of the former class has been re~iewed,~' and the usual incredible number of publications concerned with light-induced reactions of pyrimidine derivatives have again appeared.* Seemingly cyclohexa-2,5-dienone photochemistry has been far from exhaust- ively studied and one particularly interesting example within the year has con-cerned the light-induced processes of the dienone (21).41 Novel in this area is the formation of the cyclopentadienone (22) ;formation of this is accompanied BU'O 5 + Bu' BU' OMe Me0 OMe (22) Bu' OMe $.Dimer by the isomeric dienone (23). It is suggested that (22) arises by concerted cheleo- tropic elimination of dimethylcarbene from either (21) or the usual zwitterionic intermediate or the latter's triplet precursor. Surprisingly the reaction is not general and the isomer (24) gave onIy the ester (25) in methanol. * The reader is referred to Chapters 2 and 6 in Part 111 of Volume 4 of the Chemical Society's Specialist Periodical Report on Photochemistry (ed.D. Bryce-Smith) for reviews of the literature in this area. 39 H. E. Zimmerman and G. A. Epling J. Amer. Chem. SOC.,1972,94 7806. 'O J. A. Waters Y. Koudo and B. Witkop J. Pharm. Sci. 1972,61 321. " D. G. Hewitt and R. F. Taylor J.C.S.Chem. Comm. 1972,493. 324 A. Gilbert Once again the ubiquitous benzophenone and its derivatives have been the subject of a variety of reports. Its photoreactivity in water has been reinvesti- gated.42 Quantum yields are low and in oxygen-free solution benzpinacol is formed whereas in aerated solution 2- 3- and 4-hydroxybenzophenones are produced very inefficiently. The primary products are the ketyl radical and a radical adduct of benzophenone.Approximate thermochemical calculations indicate that simple hydrogen abstraction from water by triplet ketone may indeed be occurring. Continuing with his extensive studies on the photoreduc- tion of aryl ketones by amines Cohen has reported his findings on the process with 2-naphthaldehyde and 2-acetonaphthone which both have lowest m* triplet states.43 The aldehyde is far more efficiently reduced than the ketone and only a tertiary amine gives a rapid rate of reaction. Generally the results parallel those reported earlier for fiuorenone and p-aminobenzophenone and appear to be characteristic of carbonyl compounds with low-lying m* triplet states. Davidson and co-workers have also contributed many worthwhile studies to this area and have now reported details of their kinetic and flash photolysis evidence for the electron-transfer mechanism of interaction between tertiary aryl amines and ben~ophenone.~~ A new technique has been described for the determination of intersystem crossing yields.45 The method is really a combination of two previously reported procedures involving cis Strans isomerism and the flash spectroscopic tech- nique which uses heavy-atom enhancement of intersystem crossing.This combination is suggested to be most useful for benzenoid compounds since in these cases the earlier methods are only marginally applicable. Basically the technique involves measuring the relative fluorescence intensities of the benzene derivative along with the yields of isomerization of the ‘triplet counter’ in solu- tion.Concentrations of the counter and aromatic compound are held constant while that of the heavy-atom fluorescence quencher is varied. New examples of 1,2-? 1,3- and 1,4-light-induced cycloadditions of olefins and dienes to aromatic compounds continue to appear. The 1,3-process with norbor- nene and benzene has been described in and the reaction with 3,4- dimethylcyclobutene is reported to yield both 1,3- and 1,4-adducts in the ratio 42 M. B. Ledger and G. Porter J.C.S. Faraday I 1972,68 539. 43 S. G. Cohen G. A. Davis and W. D. K. Clark J. Amer. Chem. SOC.,1972,94 869. 44 R. F. Bartholomew R. S. Davidson P. F. Lambeth J. F. McKellar and P. H. Turner J.C.S.Perkin II 1972 577. 45 1972 94 6246. F. A. Carroll and F. H. Quina J. Amer. Chem. SOC. 46 R. Srinivasan J. Phys. Chem. 1972 76 15. Photochemistry of 4 l.47 On the other hand when allenes are used as the olefin the 1,4-cyclo- adduct (26) predominates over the 1,3-adduct (27):48 this is unusual in such pro- cesses. The photoreaction of benzene with 1,2-dichloroethylenes leads to cis-and trans-P-chlorostyrenes olefin dimers and the two 1 1 adducts (28) and (29):the latter are suggested to arise via acid-catalysed rearrangement of the primary products although these were not i~olated.~’ (28) (2% Two groups have reported on the photoaddition of conjugated dienes to anthracene and its derivative^.^^-^^ Both describe the (4+ 4) cycloaddition of cyclohexa-1,3-diene but Kaupp’s main concern was with the (2 + 4)and (4+ 4) cycloadditions of cyclopentadiene to anthracene.” The American workers also report on the reactions of dienes with anthracenes substituted with electron- withdrawing groups stereospecific 1,2-additions of the diene are observed and exciplex intermediates are in~oked.’~ The basic stilbene-phenanthrene conversion has been applied to numerous specialized examples.Such usage for the photochemical synthesis of isoquinoline alkaloids has been re~iewed.’~ Reports continue to appear concerned with preparation of helicenes by this method,54 and recent descriptions of the further success of formation of helicene optical isomers using circularly polarized light are of great intere~t.~’ The mechanism of the reaction has been studied and convincing evidence has been obtained for the ‘bond rotation’ hypothesis as against the earlier proposed pathway of ‘partial photoresolution’ of the dihydro- intermediates.” The cyclization process with aryl dienes trienes tetraenes and pentaenes has received detailed examination from Leznoff and Hayward.’ As 47 R.Srinivasan J. Amer. Chem. SOC.,1972 94 81 17. 48 D. Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 664. 49 D. Bryce-Smith B. E. Foulger and A. Gilbert J.C.S. Chem. Comm. 1972 769. 5o G. Kaupp Angew. Chem. Internat. Edn. 1972. 11. 718. ’’ N. C. Yang and J. Libman J. Amer. Chem. Sac. 1972,94 1405. 52 N. C. Yang J. Libman L. W. Barrett M. H. Hui and R. L. Loeschen J.Amer. Chem. Soc.,1972 94 1406. s3 T. Kametani and K. Fukumoto Accounrs Chem. Res. 1972 5 212. 54 W. H. Laarhoven and R. G. M. Veldhuis Tetrahedron 1972,28 181 I 1823. 55 W. J. Bernstein M. Calvin and 0.Buchardt J. Amer. Chem. SOC. 1972,94,494,2195. s6 R. J. Hayward A. C. Hopkinson and C. C. Leznoff Tetrahedron 1972 28,439; C. C. Leznoff and R. J. Hayward Canad. J. Chem. 1972,50 528. 326 A. Gilbert with the stilbenes the CF* values for the terminal atoms involved in the cycliza- tion are accurate guides in predicting modes of reaction and again for reaction to occur the value must be greater than unity. Recently there has been some discussion concerning the mechanism of the formation of carbazole from di- phenylamine. It now however seems clear that the product is derived from the earlier described ‘610 nm’ transient5’ and further the same workers have described a new transient from their flash studies of this system.58 This transient absorbs at 430 nm and is assigned to the excited triplet state of the intermediate photoproduct 11,12-dihydrocarbazole.Problems concerning the mechanism of the photo-Friedel-Crafts reaction have also apparently been resolved. In a most comprehensive paper Meyer and Hammond describe their efforts in this area and clearly demonstrate that the reaction pathway followed is one of cage combination of radicals to yield re- arrangement products and diffusion from the solvent cage by the radicals to form phenols.59 In these studies attempts to observe short-lived intermedidates by both e.s.r.and flash photolysis were unsuccessful but other workers using phenyl acetate 2,6-dimethylphenyl acetate and acetanilide were able to observe transient species.60 Since the earlier reports by Buchardt and co-workers on the photolability of heteroaromatic N-oxides each year more workers seem to become interested in some aspect of the process. Much effort continues to be directed towards identifi- cation of intermediates,6 and both conventional and laser photolytic studies have been applied to the reaction of isoquinoline N-oxides.62 It is difficult to choose highlights from the vast amount of literature concerned with photoelimination reactions but Chapman’s successful synthesis of the a-lactone (31) by loss of carbon dioxide from (30)is of outstanding interest.63 (30) The year has been most eventful in the area of photo-oxidation reactions particularly those involving simple olefins.Discussions of the mechanism(s) involved in the formation of allylic peroxides continue with the ‘ene’ mechanism seemingly coming back into favour possibly at the expense of the perepoxide ” E. W. Foerster and K. H. Grellmann J. Amer. Chem. SOC.,1972 94 634. 58 E. W. Foerster and K. H. Grellmann Chem. Phys. Letters 1972 14 536. 59 J. W. Meyer and G. S. Hammond J. Amer. Chem. Sue. 1972,94 2219. ‘O C. E. Kalmus and D. M. Hercules Tetrahedron Letters 1972 1575. 61 For recent work in this area see 0.Buchardt C. L. Pedersen and N. Harrit J. Org. Chem. 1972 37 3592; I.Ono and N. Hata Bull. Chem. SOC.Japan 1972 45 2951 ; S. Yamada M. Ishikawa and C. Kaneko Tetrahedron Letters 1972 971 977. 62 C. Lohse J.C.S. Perkin II 1972 229. 63 0. L. Chapman P. W. Wojkowski W. Adam 0. Rodriquez and R. Ruckstaschel J. Amer. Chem. SOC.,1972 94 1365. Pholochemistry pathway. The groups of both Gollnick and Foote have contributed to this topic but these reports are considered to be too important to summarize in a sentence or so and the reader is referred to the original articles.64 Other noteworthy accounts within the year describe the use of a pulsed ruby laser to investigate the effect of azide ion on singlet oxygen reactions6’ and further results on the stereo- specificity of addition of singlet oxygen to vinylidene diethers.66 Within the year there have been several accounts of the more applied and unusual aspects of photochemistry.These have included a description of the identification of organo-chlorine pesticide residues by U.V. solid-phase irradia- ti~n,~’ photochemical oxidation of organic pollutants in waste water,68 and photochemical smog and its related problems.69 A most readable and interest- ing account of the photochemistry of interstellar molecules has appeared and this includes a brief description of an interstellar photochemical labor at or^.^' The work involving the photochemistry in the atmospheres of comets has been described and further experiments are suggested for laboratory and astronomical observations in order to obtain a more detailed understanding of comets.’* Finally the importance of sunlight in bringing about not only photosynthesis but also other photo-effects has been re~iewed.’~ In this all too short account more effort is urged to be devoted to storage of the sun’s light energy.‘Ground rules’ for searching for suitable and convenient solar photoreactions are presented and future possibilities are suggested. 64 K. Gollnick D. Haisch and G. Schade J. Amer. Chem. Soc. 1972,94 1747; C. S. Foote T. T. Fujimoto and Y. C. Chang Tetrahedron Letters 1972 45. 65 N. Hasty P. B. Merkel P. Radlick and D. R. Kearns Tetrahedron Letters 1972 49. A. P. Schaap and N. Tontapanish J.C.S. Chem. Comm. 1972 490. b7 D. E. Glotfelty Ana/yt. Chem. 1972 44 1250. C. Y. Cha and J. M. Sjith Ind.and Eng. Chem. (Process Design),1972 11 451. 69 N. Yamaki J. Jap. Petrol. Inst. 1972 15 540. ’O L. J. Stief Mol. Photochem. 1972 4 153. ” W. M. Jackson Mol. Photochem. 1972,4 135. ’’ F. Daniels,Biophys. J. 1972 12 723.

ISSN:0069-3030    

DOI:10.1039/OC9726900315

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

10 General Methods By W. B. MOTHERWELL and J. S. ROBERTS Chemistry Department University of Stirling Stirling 1 Alkanes Isotopically labelled substrates play a vital role in delineating the course of many biosynthetic and mechanistic pathways. Organoaluminium dihalide catalysts are particularly useful for the specific labelling of a wide variety of aromatic compounds regardless of steric factors. Perdeuteriobenzene and tritiated water act as convenient isotope sources and equilibrium is generally attained within minutes at room temperature. Some polycyclic aromatic hydrocarbons do however require more forcing conditions and therefore constitute a theoretically interesting exception. The versatile chlorosulphoxide grouping can be reduced stepwise to a mono- di- or tri-deuteriomethyl group.* The halogen atom of an allylic halide may be replaced by using controlled-potential electrolytic reduc- tion and only a small excess of isotopically enriched water,3 while reductive elimination of the halogen atom of a-halogeno-esters can be smoothly accomp- lished with zinc and deuterium oxide in polar aprotic solvent^.^ The exchange reaction between isotopically substituted water and the a-hydrogen atoms of a carboxylic acid or amino-acid is efficiently catalysed by DCl or TCI.Boro-hydride reduction then gives the correspondingly labelled alcohol^.^ Two reports have appeared describing improved procedures for the preparation of deuterio-diazomet hane.6 2,4,6-Tri-isopropylbenzenesulphonylhydrazide is a readily prepared reagent which decomposes at a much lower temperature than toluene-p-sulphonyl hydrazide to provide an experimentally convenient source of the hydrogenating agent ‘di-imide’.’ In an effort to devise a suitable sequence for the introduction of the gem-dimethyl moiety Posner et a1.* have studied the reaction of benzal and other ’ J.L. Garnett M. A. Long R. F. W. Vining and T. Mole J. Amer. Chem. SOC.,1972 94 5913,8632. G. C. Joshi and E. W. Warnhoff J. Org. Chem. 1972,37,2383. M. F. Semmelhack R. J. DeFranco and J. Stock Tetrahedron Letters 1972 1371. L. S. Trzupek E. R. Stedronsky and G. M. Whitesides J. Org. Chem. 1972,37 3300. J. L. Garnett B. Halpern and R. S. Kenyon J.C.S. Chem. Comm. 1972 135. ’ S. M. Hecht and J. W. Kozarich Tetrahedron Letters 1972 1501; J.R. Campbell Chem. and Ind. 1972 540. ’ N. J. Cusack C. B. Reese and B. Roozpeikar J.C.S. Chem. Comm. 1972 1132. G. H. Posner and D. J. Brunelle Tetrahedron Letters 1972 293. 329 W.B. Motherwell and J. S. Roberts benzylic halides with lithium dimethylcuprate and found surprisingly that a stepwise mechanism is not operative in the case of the geminal dihalide. Full details of the modified Clemmensen reduction using activated zinc powder in common organic solvents saturated with hydrogen chloride have now been published. The value of cyclopropyl compounds as synthetic intermediates continues to stimulate endeavour in this area. Palladium-acetate-catalysed cyclopropana-tion with diazoalkyl compounds can be virtually quantitative under mild thermal conditions.lo Conia and co-workers" have reported that the replacement of copper by silver appears to curb the intractability of the Simmons-Smith reaction especially with functionally substituted olefins. A further modifica- tion suggested by these authors is the addition of pyridine to remove zinc salts thus allowing the preparation of hydrolytically labile compounds. An attractive alternative to this classical method is the use of a stable organomercury car- benoid benzylmercurio-iodomethane which reacts under mild conditions with olefins to give cyclopropanes in high yield.12 A new and efficient route to gem-dimethylcyclopropanes has been described which proceeds via the 1,4-addition of triphenylphosphonium isopropylide to up-unsaturated esters.The ability of the carbon-tin a-bond to stabilize a y-carbonium ion has been exploited in a unique approach to a variety of cyclopropylcarbinyl systems (2),involving addi- tion of electrophilic reagents to but-3-enyltributyltin ( l)I4 (Scheme 1). E = CI Br I RS,or HgCl Scheme 1 Two new routes to biaryl compounds have been reported which do not use the halogenoaromatic precursor necessary for the Ullmann reaction. Electron- rich aromatic compounds react with tellurium tetrachloride to give bis(ary1)- tellurium dichlorides (3) which on treatment with degassed Raney nickel afford symmetrical biaryls. Exchange reactions unfortunately prohibit the exclusive formation of unsymmetrical compounds. An unusual approach to the biaryl system has been discovered by Ollis et al.who have shown that amines of type (4)rearrange thermally to the biaryls (5) in good yield. M. Toda M. Hayashi Y. Hirata and S. Yamamura Bull. Chem. SOC.Japan 1972,45 264. lo R.Paulissen A. J. Hubert and P. Teyssie Terrahedron Letters 1972 1465. (a)J. M. Denis C. Girard and J. M. Conia Synthesis 1972 549; (6)J. M. Denis and J. M. Conia Tetrahedron Letters 1972 4593. R. Scheffold and U. Michel Angew. Chem. Internat. Edn. 1972 11 231. l3 P. A. Grieco and R. S. Finkelhor Tetrahedron Letters 1972 3781. l4 D. J. Peterson and M. D. Robbins Tetrahedron Letters 1972 2135. l5 J. Bergman Tetrahedron 1972 28 3323. R. W. Jemison T. Laird and W. D. Ollis J.C.S. Chem. Comm. 1972 556. General Methods 331 c1 Ph Me,N (3) (4) 2 Alkenes The use of polymeric reagents for the Wittig olefin synthesis has now emerged to challenge the established procedure by offering many practical advantages and comparable or improved yields from a wide variety of phosphoranes.” A convenient alternative to the Wittig methylenation reaction which may prove to be especially valuable in the case of hindered ketones involves the reductive elimination of readily prepared phenylthiomethylcarbinyl esters with lithium in liquid ammonia.* Among other impressive advantages the methylenation of cis-a-decalone without epimerization and the specific conversion of an ester into an isopropenyl group (Scheme 2) are particularly noteworthy. -% R-C RC0,Me 2R-C /OCOPh ”. HCH2 \ Reagents i PhCH ,SLi; ii benzoylation; iii Li-NH Scheme 2 The inaccessibility of p-lactones has in the past restricted their consideration as viable intermediates for olefin synthesis.However the recent availability of b-hydroxy-acids by condensation of a-metalated carboxylate salts with alde- hydes or ketones and the discovery that the reaction of these with benzene- sulphonyl chloride in pyridine affords p-lactones in high yield have now com- bined to make an extremely attractive procedure. Decarboxylation into olefins occurs at moderate temperatures with retention of the initial geometry and without isomerization of the double bond.” Alcohols may be simply converted into their thiocarbonate or N-methyl-4-alkoxypyridiniumiodide derivatives,21 both of which undergo smooth pyrolytic elimination to give alkenes in high yield.A novel 1,4 Hofmann elimination reation has been used to prepare s-trans dienes (6) from the readily prepared amines (7).22 The reaction of the prostanoid intermediate (8) with either (a) W. Heitz and R. Michels Angew. Chem. Internal. Edn. 1972 11 298; (6) S. V. McKinly and J. W. Rakshys jun. J.C.S. Chern. Comm. 1972 134. Is R. L. Sowerby and R. M. Coates J. Atner. Cliem. Sac. t 972 94 4758. W. Adam J. Baeza and J.-C. Liu J. Amer. Chem. Sac. 1972 94 2000. l9 2o H. Gerlach T. T. Huong and W. Muller J.C.S. Chem. Comm. 1972 1215. ’I G. H. Schmid and A. W. Wolkoff Canad. J. Chem. 1972,50 1181 1188. 22 M. R. S* ,rt J. Org. Chem. 1972 37 2201. W.B. Motherwell and J. S. Roberts (7) methanesulphonyl chloride2 or phosphorus o~ychloride~~ in pyridine gives the olefin from the iodohydrin in very high yield without recourse to any reducing agent. Full experimental details have now been published on the remarkable dialkoxydiphenylsulphurane (9) which acts as a mild dehydrating agent for secondary and tertiary alcohols at low temperature^.^^ This reagent may succeed where others have failed as in the case of the labile olefin (10). Treatment of the Ph OC(CF,),Ph \/ S Ph/\OC(CF,),Ph (9) dimesylates of vicinal diols with sodium anthracene or naphthalene in THF or DME affords a rapid and high-yield conversion into the alkene in a non- stereospecific reaction.26 In a similar vein vicinal exocyclic dimethylene hydro- carbons can be prepared by base-catalysed double elimination from trans-1,2-bis(hydroxymethy1)alkyltoluene-p-s~lphonates.~~ The reductive deoxygenation of organic compounds continues to grow in stature as a preparatively useful method with the discovery that lower-valent tungsten halides prepared by the reaction of tungsten hexachloride with n-butyl- lithium are exceptionally powerful deoxygenating agents.Olefins are formed in a highly stereoselective manner from both epoxides and vicinal dialkoxide anions while the intermolecular deoxygenation of aromatic aldehydes and ketones is virtually without precedent. Thus benzaldehyde may be converted into trans-stilbene in 76% yield.28 A milder organometallic method for the conversion of epoxides into olefins involves reaction with sodium (cyclopentadieny1)dicarbonyl-ferrate and subsequent reaction of the intermediate alkoxide anion with fluoro- boric acid to generate an olefin complex which can be easily decomposed with sodium iodide in acetone.29 Aldehyde and ester functions are unaffected and once again there is a high percentage of retention of configuration.23 E. J. Corey and P. A. Grieco Tetrahedron Letters 1972 107. 24 P. Crabbe and A. Guzman Tetrahedron Letters 1972 115. ’’ R. J. Arhart and J. C. Martin J. Amer. Chem. SOC.,1972,94,5003. 26 J. C. Carnahan jun. and W. D. Closson Tetrahedron Letters 1972 3447. 27 D. N. Butler and R. A. Snow Canad. J. Chem. 1972,50 795. 28 (a)K. B. Sharpless and T. C. Flood J.C.S. Chem.Comm. 1972,370;(b)K. B. Sharpless M. A. Umbreit M. T. Nieh and T. C. Flood J. Amer. Chem. SOC.,1972,94,6538. ’’ W. P. Giering. M. Rosenblum and J. Tancrede J. Amer. Chem. Sac. 1972 94 7170. General Methods 333 The well-known allylic sulphoxide +sulphenate ester interconversion has been used in an ingenious manner to vary the substituent pattern in a Diels- Alder reaction. Thus the reaction of the diene sulphoxide (1 1) with an electron- rich olefin gives an adduct which may thermally rearrange and subsequently be SOPh SOPh Scheme 3 cleaved to the allylic alcohol (12)30 (Scheme 3). This [2,3]sigmatropic rearrange- ment has also been extended to a stereospecific synthesis of trisubstituted ~lefins.~~ The addition of vinyl and ally1 organocopper reagents to acetylenic esters occurs in a stereospecifically cis manner providing a flexible approach to the synthesis of 1,3- and 1,4-diene~.~~ Lithium aluminium hydride reduction of acetylenic alcohols of type (13) proceeds via the tertiary allenic alcohol to give tetrasubstituted butadiene derivatives (14).33 A novel though limited tetrasubstituted olefin synthesis involves the reaction of a compound containing an active methylene group with thionyl chloride.34 Ethyl cyanoacetate for example gives the olefin (15) as a mixture of geometrical isomers probably through the intermediacy of an episulphide.A convenient synthesis of trans-olefins from alkynes via hydroboration-cyanohalogenation has been reported.35 Wilkinson’s catalyst is a highly stereoselective reagent for 30 D.A. Evans C. A. Bryan and C. L. Sims J. Amer. Chem. SOC.,1972,94,2891. P. A. Grieco J.C.S. Chem. Comm. 1972 702. 32 E. J. Corey C. U. Kim R. H. K. Chen and M. Takeda J. Amer. Chem. SOC.,1972,94 4395. 33 A. Claesson and C. Bogentoft Acta Chem. Scand. 1972 26,2540. 34 C. J. Ireland and J. S. Pizey J.C.S. Chem. Comm. 1972 4. 35 G. Zweifel R. P. Fisher J. T. Snow and C. C. Whitney J. Amer. Chem. SOC.,1972 94 6560. W.B. Motherwelland J. S. Roberts the reduction of allenes to 01efins.~~ Gram quantities of trans-cyclo-octene are now readily prepared by photolysis of the cis-isomer in the presence of cuprous chloride.37 A useful review has been published on the olefin metathesis reaction. * 3 Alkynes and Allenes Corey and Fuchs3’ have developed a simple and expeditious homologation sequence for effecting the formyl-ethynyl interconversion (Scheme 4).The RCHO RCH=CBr % R-C-C-Li Reagents i Ph,P CBr,; ii 2BuLi Scheme 4 base-catalysed decomposition of 5,5-disubstituted 3-nitroso-oxazolidone deriva- tives (16) can now be used to prepare cyclopropylacetylenes (17; R = Ph Me or cycl~propyl).~~ Eschenmoser and his colleagues4 have also widened the LN \ NO (16) scope of their powerful fragmentation reaction by the simple expedient of thermolysing the hydrazones derived from UP-epoxy-ketones and 2-phenyl- or trans-2,3-diphenyl-1-amino-aziridines. Many previously unsuccessful fragmenta- tions can now be realized ; as for example in the preparation of acetylenic alde- hydes (Scheme 5).A base-induced toluene-p-sulphonylhydrazohefragmentation VPh + N + PhCH=CH N Reagents i I ,ether 0 “C;ii A NH Scheme 5 36 M. M. Bhagwat and D. Devaprabhakara Tetrahedron Letters 1972 1391. ” J. A. Deyrup and M. Betkouski J. Org. Chem. 1972,37 3561. 38 N. Calderon Accounts Chem. Res. 1972 5 127. j9 E. J. Corey and P. L. Fuchs Tetrahedron Letters 1972 3769. 40 M. S. Newman and S. J. Gromelski J. Org. Chem. 1972 37 3220. 41 D. Felix R. K. Muller U. Horn R. Joos J. Schreiber and A. Eschenmoser Helv. Chim. Acta. 1972. 55. 1276. General Methods reaction has also been used to prepare allenic alcohols e.g. (18) from cyclic toluene-p-sulphonylhydrazoneethers e.g. (19).42 a-Diazo-P-hydroxy-esters and -ketones are useful intermediates which can now be simply prepared by hydroxide-catalysed condensations of ethyl diazo- acetate and diazo-ketones with aldehydes in mcthanolic solution.The reaction of these derivatives with boron trifluoride in ether-acetonitrile leads smoothly to the formation of the corresponding acetylenic ester or ketone.43 An equalIy effective way of preparing a/?-acetylenic esters involves formation of the 3- substituted-5-pyrazolone derivative (20; R3 = H) of a P-keto-ester and subse- quent oxidation with two equivalents of thallium nitrate in methanol.44 Under the same conditions or-alkyl-P-keto-esters give rise to allenic esters (21) via the pyrazolone (20; R3 = alk~l).~~ R' R2 C0,Me H (211 (20) 1-Halogeno-allenes and 3-chloro-alk- 1 -ynes react with dialkylcopperlithium reagents to give 1,3-di- and 1,3,3-tri-alkyl-substitutedallene~.~~ Vinylallenes can be prepared by the reaction of 5-chloropent-3-en-1-yneswith methyl- magnesium iodide.47 gem-Dibromocyclopropylcarbinols give a-allenic alcohols on treatment with an alkyl-lithi~rn.~~ Activated ketones of the type (22) react with dibromotriphenylphosphine in the presence of triethylamine to give allenic compounds (23 ;R2= COMe CO,Et or CN) in good yield.49 42 A.M. Foster and W. C. Agosta J. Org. Chem. 1972 37 61. 43 (a) E. Wenkert and C. A. McPherson Synthetic Comm. 1972 2 331 ; (b) J. Amer. Chem. SOC.,1972,94 8084. 44 E. C. Taylor R. L. Robey and A. McKillop Angew. Chem. Interoat. Edn. 1972,11,48.45 E. C. Taylor R. L. Robey and A. McKillop J. Org. Chem. 1972,37 2797. 46 M. Kalli P. D. Landor and S. R. Landor J.C.S. Chem. Comm. 1972 593. 47 J. Gore and J. P. Dulcere J.C.S. Chem. Comm. 1972 866. 48 R. Maurin and M. Bertrand Bull. SOC. chim. France 1972 2349. 49 G. Buono Tetrahedron Letters 1972 3257. 336 W.B. Motherwell and J. S. Roberts 4 Alkyl Halides Some helpful comments have been published on the Cristol-Firth modification of the Hunsdiecker reaction.” Potential users of this method should however take note that even alkanes react with bromine and mercuric oxide to give preparatively useful yields of alkyl halide^.^ A new general and exceptionally mild method for the preparation of chlorides from thiols involves successive reaction with chlorocarbonylsulphenyl chloride and triphenylph~sphine.’~ New reactions of the versatile triphenylphosphine-carbon tetrachloride combination continue to appear.Epoxides react readily to give the correspond- ing cis-1,2-dichloroalkane in good yield and in a highly stereospecific manner with inversion of configuration at both carbon atoms.53 Enolizable ketones have also succumbed ; with acyclic and six-membered-ring compounds the enyl chloride is formed while five- and four-membered rings give the corresponding exocyclic dichloromethylene compound. 54 In addition this potent combina- tion has now been successfully used for the conversion of secondary allylic alcohols into the chlorides without rearrangement.s5 The new reagent (24) formed by the reaction of dimethyl sulphide with N-chlorosuccinimide has been developed by Corey et al.lo8 as a mild system for the oxidation of alcohols.However in the presence of alcohols which can form stable carbocations the reaction takes a different course and offers an extremely regiospecific conversion into the chloride as evidenced by the high-yield trans- formation of (25; R = OH) into (25; R = Cl).56 0 / HO CH,R (251 The reaction of an iodo-alkane with the reagent (26) provides a useful homolog- ation sequence for iodomethylation (26; R2 = H) and iodopropenylation (26; R2 = (Scheme 6). Iodides can be prepared from primary and secondary alcohols in good yield using N-me thy1-NN’-dicyclohex ylcarbodi-imidium iodide. 5o J. Cason and D.M. Walba J. Org. Chem. 1972,37 669. N. J. Bunce Canad. J. Chem. 1972 50 3109. 52 D. L. J. Clive and C. V. Denyer J.C.S. Chem. Comm. 1972 773. ’’ N. S. Isaacs and D. Kirkpatrick Tetrahedron Letters 1972 3869. 54 N. S. Isaacs and D. Kirkpatrick J.C.S. Chem. Comm. 1972 443. 55 E. I. Snyder J. Org. Chem. 1972 37 1466. 5h E. J. Corty C. U. Kim and M. Takeda Tetrahedron Letters 1972 4339. 57 K. Hirai and Y. Kishida Tetrahedron Letters 1972 2743. ’’ R. Scheffold and E. Saladin Angew. Chem. Internat. Edn. 1972 11 229. General Methods ii,ii / Me Reagents i R'I; ii Me1 Scheme 6 Phenyltetrafluorophosphorane has been recommended as a selective reagent for the fluorination of alcohols as their TMS ethers.59 Bisfluoroxydifluoro- methane shows promise as an economic and efficient reagent for electrophilic fluorination.60 Interest in the generation of dihalogenocarbenes has been reawakened by the application of phase-transfer catalytic methods.Cetyltrimethylammonium chloride and the commercially available detergent Cetrimide are now recom- mended as the most efficient catalysts for use in this technique.61 The generation of dihalogenocarbenes from phenyl(trihalogenomethy1)mercury compounds has been reviewed6* and cyclohexyl(trihalogenomethy1)mercurials are now also claimed to be excellent sources.63 A variation on the more established methods for generating dihalogenocarbenes is the use of thallous ethoxide which does not require rigorously anhydrous condition^.^^ A1t houg h mono bromocyclopropanes are normally prepared by reduction of the dihalogeno-compound a direct method has now been devised (Scheme 7).65 + CH,Br + (Me,Si),NNa x AH 1 Br + (Me,Si),NH + NaBr Scheme 7 59 D.U. Robert and J. G. Riess Tetrahedron Letters 1972 847. 6o D. H. R. Barton R. H. Hesse M. M. Pechet G.Tarzia H. T. Toh and N. D. Westcott J.C.S. Chem. Comm. 1972 122. G. C. Joshi N. Singh and L. M. Pande Tetrahedron Letters 1972 1461. 62 D. Seyferth Accounts Chem. Res. 1972 5 65. 63 D. Seyferth and C. K. Hoas J. Organometallic Chem. 1972 46 C33. 64 C. M. Hall Synthetic Comm. 1972 2 121. '' B. Martel and J. M. Hiriart Angew. Chem. Internat. Edn. 1972 11 326. W. B. Motherwell and J. S. Roberts 5 Alcohols Thiourea dioxide in alkaline solution is an efficient and economical reagent for the reduction of ketones to alcohols.66 The reducing agent lithium tri-s-butyl- borohydride is easily prepared in situ and displays unusually high stereo- selectivity; e.g.96.5% (27; R = H) from 4-t-butyl~yclohexanone.~~ A dramatic reversal of stereochemistry has been reported in the reduction of 4-t-butylcyclo- hexanone with trimethylaluminium.68 When the trimethylaluminium/ketone ratio is 1 1 the axial alcohol (27; R = Me) is formed predominantly. However when the ratio is 2 1 or greater the equatorial alcohol is formed in 90% yield. Further advances in the preparation of optically active secondary alcohols by asymmetric reduction of an achiral ketone include the use of (+)-tris[(S)-2-methylbutyl]aluminium etherate,69 chiral magnesium alcoholates from N-meth~lephedrine,~~ and homogeneous hydrogenation using a rhodium complex with an optically active phosphine ligand.71 Several five-membered-ring cyclic ethers have been reductively opened in high yield to the corresponding alcohol by using lithium tri-t-butoxyalumino- hydride in the presence of triethylb~rane.~’ The ability of iron tricarbonyl compounds to act as diene traps has been used to achieve selective hydroboration as in the conversion of (28) into (29).73 woH Mercuration-reduction is an inefficient and non-stereoselective method for the formation of diols from allylic alcohols.The addition of chloral however directs the reaction through an intermediate hemiacetal and hence offers a highly regio- and stereo-specific conversion of allylic alcohols into cis-vicinal di01s.’~ Phase-transfer catalytic methods have also been employed for the 66 K.Nakagawa and K. Minami Tetrahedron Letters 1972 343. ” H. C. Brown and S. Krishnamurthy J. Amer. Chem. Soc. 1972,94 7159. 68 E. C. Ashby S. H. Yu and P. V. Roling J. Org. Chern. 1972,37 1918. 69 R. A. Kretchmer J. Org. Chem. 1972 37 801. 70 J.-P. Battioni and W. Chodkiewicz Bull. Soc. chim. France,1972 2068. P. Bonvicini A. Levi G. Modena and G. Scorrano J.C.S. Chem. Comm. 1972 1188. 72 H. C. Brown S. Krishnamurthy and R. A. Coleman J. Amer. Chem. Soc. 1972 94 1750. 73 C. H. Mauldin E. R. Biehl and P. C. Reeves Tetrahedron Letrers 1972 2955. l4 L. E. Overman J.C.S. Chem.Comm. 1972. 1196. General Methods preparation of cis-1,2-diols from endocyclic 01efins.~ The hydroboration of specifically generated enolates provides a simple route to tran~-1,2-diols.'~ The reaction of aromatic aldehydes with trimethylsilyl chloride and magnesium in HMPT can be controlled to give high yields of the pinacol type product in a reaction which does not involve the intermediacy of a ketyl radical.77 The problem of alcohol protection is an area of intense investigation. Sodium thioethoxide in DMF has been developed as a powerful new reagent for de- methylating aryl methyl ethers cleanly and in high yield.78 Naphtho- and benzo-hydroquinone dimethyl ethers may be oxidatively cleaved to paraquinones under exceptionally mild conditions in the presence of many functional groups by using silver oxide.79 The preferential cleavage of an aromatic methylene- dioxy-group in the presence of methoxy-groups has been achieved with boron trichloride." For non-aromatic alcohols two achiral sulphur-containing protect- ing groups have been reported.81 The thioether acetal(30) is slightly more labile Me0 OR 0 (30) than the methoxy-tetrahydropyranyl ether but if necessary may be oxidized to the corresponding sulphone which is much less labile.The NNN'N'-tetra- methylphosphorodiamidate group has been recommended as a useful function for the protection of alcohols. Reductive elimination with lithium and ethyl- amine however leads to the hydrocarbon thus offering a two-step sequence which is claimed to have certain advantages over the Wolff-Kishner reduction.82 The t-butyldimethylsilyl group is an extremely useful one being stable to aqueous or alcoholic base hydrogenolysis and mild chemical reduction but rapidly regenerated by brief treatment with tetra-n-butylammonium fluoride in THF at room temperat~re.~~ Controlled-potential electrolysis may be used for the selectively controlled stepwise removal of various groups of the 2-halogeno- ethoxy type in neutral media.84 75 W.P. Weber and J. P. Shepherd Tetrahedron Letters 1972 4907. 76 J. Klein R. Levene and E. Dunkelblum Tetrahedron Letters 1972 2845. 77 T. H. Chan and E. Vinokur Tetrahedron Letters 1972 75. 78 G. 1. Feutrill and R. N. Mirrington Austral. J. Chem. 1972 25 1719 1731.79 C. D. Snyder and H. Rapoport J. Amer. Chem. SOC.,1972,94,227. 80 S. Teitel J. O'Brien and A. Brossi J. Urg. Chem. 1972 37 3368. 81 J. H. van Boom P. van Deursen J. Meeuwse and C. B. Reese J.C.S. Chem. Comm. 1972,766. 82 R.E. Ireland D. C. Muchmore and U. Hengartner J. Amer. Chem. Soc. 1972 94 5098. 83 E. J. Corey and A. Venkateswarlu J. Amer. Chem. Soc. 1972 94 6190. 84 M. F. Semmelhack and G. E. Heinsohn J. Amer. Chem. Soc. 1972,94 5139. 340 W. B. Motherwell and J. S. Roberts 6 Ethers Elevated temperatures may be employed in epoxidation with rn-chloroperbenzoic acid if a small amount of a radical inhibitor is present.85 A general oxiran synthe- sis involves the reaction of an aldehyde or ketone with an a-bromo-lithium compound generated from the dihalide.86 Photolysis of 1,2-dioxolans also yields epoxides as major products.*’ An excellent method for the preparation of optically active epoxides has been published88 (Scheme 8).A rapid and stereo- specific conversion of oxirans into thiirans has been conveniently achieved with Me Me. H Me ,,CO*H i’Me40Ac 7HmMe Me Cl 0 Me H Reagents i PCl,; ii base Scheme 8 phosphine ~ulphides.~~ Alicyclic olefins can be simply transformed into the corresponding episulphide with iodine thiocyanate and subsequent base hydroly~is.’~ A general synthesis of ethers from aldehydes and ketones by reduction with trialkylsilanes in alcoholic acidic media has been described.” Cyclic ethers may be prepared by radical-induced reduction of lactones with trichloro~ilane.~~ In contrast to normal alkyl-lithium reagents which are unreactive towards isolated double bonds chloromethoxymethyl-lithium adds by cis-addition to give tetrahydrofurans.7 Amines Highly reactive dialkylchloroboranes are readily prepared via hydroboration of olefins with chloroborane diethyl ethe~-ate,~~ and unlike their trialkyl counter- parts rapidly form secondary amines on treatment with azide~.~~ The reaction of dialkyltrichloromethylamines with Grignard reagents provides a convenient route to tertiary amines possessing a tertiary alkyl Substantially improved yields from the Clarke-Eschweiler reductive methylation of primary and secondary amines are obtained by treatment with aqueous formaldehyde and sodium cyan~borohydride.~’ 85 Y.Kishi M. Aratani H. Tanino T. Fukuyama T. Goto S. Inque S. Sugiura and H. Kakoi J.C.S. Chem. Comm. 1972,64. 86 G. Cainelli N. Tangari and A. U. Ronchi Tetrahedron 1972 28 3009. 87 W. Adam and N. Duran Tetrahedron Letters 1972 1357. 88 M. S. Newman and C. H. Chen J. Amer. Chem. SOC.,1972,94,2149. 89 T. H. Chan and J. R. Finkenbine J. Amer. Chem. SOC.,1972,94,2880. 90 J. C. Hinshaw Tetrahedron Letters 1972 3567. 91 M. P. Doyle D. J. DeBruyn and D. A. Kooistra J. Amer. Chem. SOC.,1972,94 3659. 92 R. Nakao T. Fukumoto and J. Tsurugi J. Org. Chem. 1972,37 76. 93 M. B. Groen and E. H. Jacobs Tetrahedron Letters 1972 4029. 94 H. C. Brown and N. Ravindran J. Arner. Chem. SOC.,1972,94 21 12. 95 H. C. Brown M. M.Midland and A. B. Levy J. Amer. Chem. SOC.,1972,94,2114. 96 V. P. Kukhar and V. I. Pasternak Synthesis 1972 61 1. 97 R. F. Borch and A. I. Hassid J. Org. Chem. 1972,37 1673. General Methods 341 An efficient route for the conversion of optically active alcohols into amines has been developed (Scheme 9). The overall inversion of configuration observed implies that triphenylphosphine oxide is displaced from an intermediate phos- phonium salt.98 1 ROH + Ph3P + dNI-I dN-R 0 0 J;r RNH Reagents i Et0,C-N:N-C0,Et; ii NZH Scheme 9 Two excellent methods are now available for the preparation of anilines from phenols.99 Methanolic solutions of dodecacarbonyltri-iron specifically reduce nitro- groups to amines in high yield in the presence of a wide variety of functional groups.The effective reducing agent is purported to be the hydridoundeca- carbonyltriferrate anion. loo Chain extension at the a-carbon atom of secondary amines has now been achieved by quantitative lithiation of the corresponding nitrosamide with lithium di-isopropylamide reaction with an electrophile and final regeneration of the amine from the N-nitroso-derivative. lo' Amine hydrochlorides can be prepared directly by catalytic reduction of azides nitriles oximes and nitro-compounds in a solvent containing a small amount of chloroform.'02 This procedure also allows the preparation of amine hydrochlorides containing acid-labile functional groups. Optically active primary amines may be converted into their 4,5-diphenyl-3- oxazolin-2-one derivatives without racemization.These highly crystalline compounds can be efficiently regenerated by low-pressure hydrogenolysis or mild oxidative conditions but are inert to the majority of reactions used to remove protecting groups. lo3 Isopropenyl formate selectively formylates an amino-acid ester under neutral conditions in the presence of a hydroxy-group. Primary and secondary aliphatic and aromatic amines can be regenerated from their sulphonamides in acceptable yield using sodium bis-(2-methoxyethoxy)- aluminium hydride."' Other useful protecting groups to be devised include a 0.Mitsunobu M. Wada and T. Sano J. Amer. Chem. Soc. 1972,94,679. '' (a) R. A. Rossi and J. F. Bunnett J. Org. Chem. 1972 37,3570; (6) R.A. Scherrer and H.R. Beatty ibid. p. 1681. loo J. M. Landesburg L. Katz and C. Olsen J. Org. Chem. 1972 37,930. lo' D. Seebach and D. Enders Angew. Chem. Internat. Edn. 1972 11 301 1101. Io2 J. A. Secrist and M. W. Logue J. Org. Chem. 1972 37,335. lo3 J. C. Sheehan and F. S. Guziec jun. J. Amer. Chem. SOC.,1972,94,6561. lo' J. E. W. van Melick and E. T. M. Wolters Synthetic Comm. 1972 2 83. '05 E. H. Gold and E. Babad J. Org. Chem. 1972 37 2208. W.B. Motherwell and J. S. Roberts photosensitive group for aniline protection lo6 and the 1-oxy-2-picolyl group which may be advantageous when the benzyl group is particularly inefficient.'" 8 Aldehydes and Ketones Corey and Kim'08 have developed a new high-yield method of oxidizing primary and secondary alcohols to their carbonyl counterparts.This can be achieved in two ways using either the complex (31) derived from dimethyl sulphide and chlorine or more conveniently the complex (32) starting from N-chloro- succinimide and dimethyl sulphide. The aldehyde or ketone is obtained by treatment of the resultant sulphoxonium salt (33) with triethylamine over a short period (Scheme 10). A striking example of the recent emergence of poly- meric reagents in organic synthesis is the use of a carbodi-imide linked to a cross- linked polystyrene matrix (34)and functioning in the Pfitzner-Moffatt oxidation R1R2CHOH 7R'R2CHOiMe2 C1- 7R'R2C0 (33) (R2 = H or alkyl) 0 0 Scheme 10 CH2N=C=NCHMe2 (34) of primary and secondary alcohols.109 Excellent yields have been obtained by this method with the added bonus that the polymeric reagent can be regenerated (with some loss of activity).The use of 1-methyl-4-piperidone as the hydride acceptor in the Oppenauer oxidation of alcohols is recommended.'" The more readily prepared and stable oxidant sodium ruthenate has some advantages over potassium ferrate although it is more vigorous.' Carbon monoxide reacts lo6 B. K. Barnett and T. D. Roberts J.C.S. Chem. Comm. 1972 758. 107 y . M.izuno W. Limn K. Tsuchida and K. Ikeda 3. Org. Chem. 1972,37 39. E. J. Corey and C. U. Kim J. Amer. Chem. Soc. 1972,94 7586. lo9 N. M. Weinshenker and C.-M. Shen Tetrahedron Letters 1972 3281 3285. R. Reich and J. F. W. Keana Synthetic Comm. 1972 2 323. D. G. Lee D. T.Hall and J. H. Cleland Canad. J. Chem. 1972,50 3741. General Methods with lithium di-n-butylcuprate to form 5-nonanone in 77% yield thus paving the way for a new synthesis of symmetrical ketones. 'l2 Methods of converting amines into ketones and aldehydes are steadily on the increase as witnessed by three recent reports. Ca16 et al.' l3 have described a method which hinges upon the same principle as used by Corey and Achiwa,'14 uiz. the base-catalysed prototropic rearrangement of the Schiff bases (35) ob-tained from amines and benzothiazole-2-carbaldehyde.Hydrolysis of the isomeric Schiff bases can be effected with oxalic acid to give the carbonyl com- pound in high yield. Another method involves the photolysis of N-phenacyl-amines (36) (as salts in the case of primary amines) which by a Norrish Type I1 process generate acetophenone and the imine which is further hydrolysed by acid.' In a different approach Andersen and Uhl l6 have solvolysed ditosyl- amines (generated from primary amines in two high-yield steps) with potassium iodide and acetate in HMPT to give the corresponding acetate which in turn can be hydrolysed and oxidized to the aldehyde.This route works best for unhindered amines but future interest lies in the more direct conversion of suphonimides into aldehydes. Good yields of aldehydes and ketones have been obtained from the Grignard reaction on S-methylthioamidium iodides (37).''' In the case of NNSS'-tetra- methyldithiocarbamidium iodide (37; R' = Me R2 = SMe) two consecutive reactions with different Grignard reagents give the unsymmetrical ketone in varying yields (20-40%) (Scheme 1I).(37) R3R4C0 Reagents i R3MgX; ii H,O'; iii R4MgX Scheme 11 J. Schwartz Tetrahedron Letters 1972 2803. V. Calo L. Lopez and P. E. Todesco J.C.S. Perkin I 1972 1652. 'I4 E. J. Corey and K. Achiwa J. Amer. Chem. SOC.,1969,91 1429. 'I5 J. A. Hyatt J. Org. Chem. 1972 37 1254. 'I6 N. H. Andersen and H.4. Uh Synthetic Comm. 1972 2 297. T. Yamaguchi Y. Shimizu and T. Suzuki Chem. and ind. 1972 380. 344 W. B. Motherwell and J. S. Roberts Two interesting papers have highlighted the use of disodium tetracarbonyl- ferrate(-11) (prepared from iron pentacarbonyl) as an inexpensive and efficient reagent for the high-yield conversion of aliphatic halides and toluene-p-sul- phonates into aldehydes and ketones (Scheme 12).1'89' l9 An additional advantage of this method is that the R'X substrates can also have ester and cyano functional groups.Na,Fe(CO) Fe(CO) [R'Fe(CO),]-[R'CFe(CO),L] -[R 'CFe( CO),] - II 0 O II 1.i R'R~CO R'Ch'O Reagents i R'X; ii L = CO or PPh,; iii R'COCl; iv R'Li; v R2X; vi H+ Scheme 12 Functionalized Aldehydes and Ketones.-Methyl methylthiomethyl sulphoxide (38)l2' is proving to be a versatile reagent by its further use in the synthesis of a-hydroxy-aldehydes which in the past have been difficult compounds to prepare. The reaction of the anion derived from (38) with both ketones and aldehydes gives the derivatives (39) which yield the a-hydroxy-aldehydes on acid hydrolysis.With benzaldehyde and Triton B as base the intermediate product (40)is converted into ethyl phenylacetate by reaction with ethanolic hydrochloric / MeSCH,S(O)Me R R2 C(OH)CH(SMe)S(O)Me PhCH =C (38) (39) 'SMe (40) acid. '21 As an alternative route to a-hydroxy-aldehydes the reaction of ketones with dichloromethyl-lithium and mild basic hydrolysis of the resultant dichloro derivative (41) gives reasonable yields.122 Further reaction of the lithium alkoxide with excess n-butyl-lithium generates the chloro-enolate (43) via intra-molecular carbene insertion of the P-oxido-carbenoid (42).'23 Thus starting with benzaldehyde phenacyl chloride is obtained in 72 % yield and cyclo- pentanone can be converted into 2-chlorocyclohexanone in 64 % yield.*I8 J. P. Collman S. R. Winter and D. R. Clark J. Amer. Chem. SOC. 1972 94 1788. W. 0. Siegl and J. P. Collman J. Amer. Chem. SOC. 1972 94 2516. *20 K. Ogura and G. Tsuchihashi Bull. Chem. SOC. Japan 1972,45 2203. 2L K. Ogura and G. Tsuchihashi Tetrahedron Letters 1972 268 1 1383. 122 P. Blumbergs M. P. LaMontagne and J. I. Stevens J. Org. Chem. 1972 37 1248. lZ3 H. Taguchi H. Yamamoto and H. Nozaki Tetrahedron Letters 1972 4661; J. Villieras C. Bacquet and J. F. Normant J. Organometallic Chem. 1972,40 CI. General Methods 345 R1R2C(O-jCHCI R1R2C(O-)C(Li)Cl2 R1C=CR2CI I (41) (42) -0 Li' (43) A regiospecific method of converting olefins into a-halogeno-ketones involves treatment of enol borinates (derivable from a trialkylborane and an a-diazo- ketone) with N-bromo- and N-chloro-succinimide.'24 Further ramifications of this method include the formation of a-halogens-esters from the corresponding a-diazo-ester and halogenation of the enol borinate obtained from trialkyl- borane addition to @-unsaturated ketones (Scheme 13). \/ c=c + R:B R:BOC=CHR' R2CCH(XjR1 /\ I II R2 0 I 0 iil II R'CH,CH=C-OBR; 3R'CH2-CH(BrjCMe I Me Reagents i R'COCHN,; ii NXS; iii CH,=CHC(O)Me iv NBS Scheme 13 The formation of certain a-ketols can be achieved by addition of the acetyl synthon 1-ethoxyvinyl-lithium CH2=C(Li)OEt to aldehydes followed by hydroly~is.'~~ 2-Methoxy-l,3-dithiolan has been used as a masked formyl group in the TiC1,-catalysed Friedel-Crafts reaction with certain indole derivatives.l2 a-Chloro-enamines e.g. (44) are also reactive acyl synthons as demonstrated by their electrophilic addition to electron-rich aromatic compounds. '27 Addi-tional synthetic interest in these compounds lies in the silver-ion-promoted elimination of C1- to generate ketenimmonium salts which undergo a facile (2 + 2) cycloaddition to olefins thus providing an entry into substituted cyclo- butanones (Scheme 14). \/ Reagents i Et,N furan; ii H,O+; iii AgBF,; iv C=C ;v OH-/\ Scheme 14 J. Hooz and J. N. Bridson Canad. J. Chem. 1972,50 2387.' U. Schollkopf and P. Hanssle Annalen 1972 763 208. lZ6 P. Stutz and P. A. Stadler Heiv. Chim. Acra 1972 55 75. J. Marchand-Brynaert and L. Ghosez J.Amer. Chem. Soc. 1972,94 2869 2870. W. B. Motherwell and J. S. Roberts The [2,3]sigmatropic rearrangement of a number of ylides (45)has once again been in evidence as an effective method of preparing unsaturated aldehydes (and acids). These are summarized in Scheme 15.'28-' 30 A related process involves the rearrangement of the carbene (46) to the dithio-ester (47).I3l R' R2 (45) X = NMe,,Y = CHSPh12* X = SPh Y = CHSPh'28 X = NMe,,Y = CHOPh129 x Y = "3 TS130 Scheme 15 )p*-Jp+-TCGS c SMe c; SMe \ (44) SMe (47) Keten thioacetals (50; R' = alkyl R2 = H) have already been shown to be extremely useful synthetic reagents but their scope has been limited by the fact that until now they have only been available from the Wittig reaction of alde-hydes with the ylide (48).Three groups working independently have made the important discovery that the lithio-dithian (49) reacts with a wide variety of n n sYs Li SiMe (49) ketones to give the keten thioacetals (50; R' = R2 = aryl or alkyl) in high yield.'32 Scheme 16 illustrates some of the synthetic applications of these deriva- tives ;the applicability can be extended since routes A and B both proceed via lz8 S.Julia C. Huynh and D. Michelot Tetrahedron Letters 1972 3587. lZ9 C. Huynh S. Julia R. Lorne and D. Michelot Bull. SOC.chim. France 1972 4057. 130 E. Hunt and B. Lythgoe J.C.S. Chem. Comm. 1972 757; see also G. Andrews and D. A. Evans Tetrahedron Letters 1972 5121. 131 J. E. Baldwin and J. A. Walker J.C.S.Chem. Comm. 1972 354. 132 F. A. Carey and A. S. Court J. Org. Chem. 1972 37 1926; P. F. Jones and M. F. Lappert J.C.S. Chem. Comm. 1972 526; D. Seebach B.-Th. Grobel A. K. Beck M. Braun and K.-H. Geiss Angew. Chem. Internat. Edn. 1972 11 443. General Methods (50) Bliv.iii R'R'R'CCHO Reagents i H +;ii R,SiH; iii hydrolysis; iv R3Li Scheme 16 the corresponding dithian which can be further metalated. Meyers and Strickland'33 have also observed the formation of the keten thioacetal (51; R = H) from the reaction of 2-formyl-l,3-dithian with excess cyanomethylene- phosphorane (the conjugated nitrile could only be obtained by using excess dithian). Alkylation of the anion derived from either isomer with ethyl iodide gave predominantly the keten thioacetal(51; R = Et) as the thermodynamically more stable isomer.R Carey and Court's interest in keten thioacetals is closely associated with their research on vinyl thioethers and phosphonates which relates to the earlier work of Peter~0n.l~~ They have now shown that both (52) and (53) react with alde- hydes and ketones to generate the vinyl derivatives [54; X = P(O)(OEt) or SPh] R' Me SiCH( Li)P(O)( OEt) Me,SiCH(Li)SPh \ /H c=c (53) /-\x R2 (54) which in turn can be hydrolysed to the corresponding a1deh~des.l~' Another interesting vinyl ether synthesis is the reaction of the carbene complex phenyl- methoxycarbenepentacarbonyltungsten(0) (55 ; R3= Ph) with Wittig reagents (Scheme 17).136 More recently it has been shown that anions generated a to the A.I. Meyers and R. C. Strickland J. Org. Chem. 1972 37 2579. 134 D. J. Peterson J. Org. Chem. 1968 33 780. 135 F. A. Carey and A. S. Court J. Org. Chem. 1972 37 939. C. P. Casey and T. J. Burkhardt J. Amer. Chem. Soc. 1972 94 6543. W. B. Motherwell and J. S. Roberts R3 R' \ / + C=PPh,/ \(OC),W=C OMe R2 -+ C-OMe \ R '-C-PPh,/ +R2 (55) (R'or R2 = H) I4 Me0 R' \/ /C=C\ + PPh,W(CO) R3 R2 Scheme 17 carbene centre e.g. (55;R3 = CH;) can be alkylated or acylated and also react with aldehydes the latter reaction ultimately leading to @-unsaturated esters.' 37 Silyl enol ethers (56)can be used as precursors of substituted @unsaturated ketones and 1,3-diketones by reaction with polyhalogeno-compounds and acid chlorides respectively (Scheme 18).13* R2 = CCl or CHCI (56) X = Y = CI or Br X = C1 Y = CN or C0,Et Reagents i R'COCl; ii CX3Y Scheme 18 Last year it was reported that 2,4-dimethylthiazole can be used in aldehyde syntheses.Meyers et now report that 2-methyl-Zthiazoline (57)can also be employed in a similar manner according to Scheme 19. In a related area a conve- nient 'one-pot' procedure for the two-carbon homologation of alkyl iodides and bromides to aldehydes has been described (Scheme 20).140 An excellent review of the chemistry of 5,6-dihydro-4H-l,3-oxazines has appeared. 14' Further work on the addition of olefins to ketones in the presence of manganic and ceric acetates demonstrates that when acetophenone is used as the ketonic substrate useful yields of a-tetralones can be obtained (Scheme 21).'42 In a related the catalytic effect of copper(r1) acetate has been noted in the 13' C.P. Casey R. A. Boggs and R. L. Anderson J. Amer. Chem. SOC. 1972,94 8947. 138 S. Murai Y. Kuroki T. Aya N. Sonoda and S. Tsutsumi J.C.S. Chem. Comm. 1972 741; S. Murai Y. Kuroki K. Hasegawa and S. Tsutsumi ibid.,1972 946. 139 A. I. Meyers R. Munavu and J. Durandetta Tetrahedron Letters 1972 3929. A. I. Meyers and N. Nazarenko J. Amer. Chem. Soc. 1972,94 3243. 141 R. R. Schmidt Synthesis 1972 333. 14' E. I. Heiba and R. M. Dessau J. Amer. Chem. SOC., 1972,94 2888. 143 G. I. Nikishin M. G. Vinogradov and G. P. Il'ina Synthesis 1972 376. General Methods (57) ii 1 iiil OHCCH ,C(OH)R3 R4 OHCCH,R' OHCCHRIRZ Reagents i.BuLi; ii R3R4CO; iii AI-Hg; iv HgCl,; v R'X; vi R2X Scheme 19 1. RCH,CHO Reagents i MeI; ii NaH; iii RX; iv BH,-; v H,O+ Scheme 20 PhCOMe -b PhCOCH,CH,kHR -+ R R Scheme 21 W. B. Motherwelland J. S. Roberts R'CH2CO(CH2)3R2+ R2(CH2),CH-CHO I /r R' R TH,CHO 1;1 R~CH=CHCH,CHCHO I R' Reagents i Mn3+ R2CH2CH=CH2; ii Mn3+-Cu2 +,RZCH,CH=CH Scheme 22 homologation of aldehydes with olefins (Scheme 22). Multi-carbon homologa- tion of olefins to ketones can be achieved in good yields starting from thexyl- borane (Scheme 23).'44 Reagents i olefin; ii CH,=CH(CH,),OAc; iii CO H,O; iv H,O, NaOAc Scheme 23 Propargylic alcohols can be converted into c$-unsaturated aldehydes by treat- ment of the corresponding tetrahydropyranyl derivative with n-butyl-lithium and subsequent quenching with aqueous methanol containing potassium carbonate.14' This procedure regenerates about 30 % of the starting tetrahydro- pyranyl ester but the desired allene derivative RCH=C=CHOTHP can be selectively hydrolysed to a mixture of cis-and trans-@unsaturated aldehydes. Pyrolysis of propargyl esters gives rise to ene-diones which undergo further rearrangement if R' or R2 = H to yield @unsaturated ketones (Scheme 24).'46 The ylide enolate (58) reacts with ketones to produce py-unsaturated ketones (59).'47 In contrast the ylide anion (60) is alkylated (with R'X) at the terminal position and the resultant alkylated ylide can then undergo a normal Wittig reaction with aldehydes (R2CHO) to give @-unsaturated ketones (61).14* An interesting example of the use of Claisen and sequential Cope rearrange-ments in the field of aldehyde synthesis has been reported by Cookson and 44 E.Negishi and H. C. Brown Synthesis 1972 196. 145 E. J. Corey and S. Terashima. Tetrahedron Letters 1972 18 15. W. S. Trahanovsky and P. W. Mullen J. Amer. Chem. SOC.,1972 94 5086. 14' C. Broquet and M. Simalty Tetrahedron Letters 1972 933. 14* J. D. Taylor and J. F. Wolf J.C.S. Chem. Comm. 1972 876. General Methods 351 R' R' 0 R2 Scheme 24 +-Ph,P-C=C-Ph R1CH=CCH,COPh I I 0-Li+ R2 (58) (59) +-Ph,P-CH-C-CH Lif R~CH=CH-CCH~R' II II 0 0 (60) (61) Rogers'49 (Scheme 25). All three aldehydes are formed with a high degree of stereospecificity and the isomer ratio can be controlled to a large extent by the operating temperature.+ OEt R3 UCHO Reagents i (EtO),CH CHMeCHMe(0Et) Scheme 25 149 R. C. Cookson and N. R. Rogers J.C.S. Chem. Comm. 1972 248. 352 W. B. Motherwell and J. S. Roberts New methods of effecting selective reduction of afi-unsaturated ketones and aldehydes include the use of iron pentacarbonyl in aqueous methanolic sodium hydroxide solution (this method can be adapted for P-incorporation of deute-rium)' and hydrosilane-rhodium(1) complexes (followed by hydrolysis of the resultant silyl enol ether). l5 Once again attention has been focussed on the synthesis of the valuable perfumery compound cis-jasmone (62).' 52,15 Some recent successes in this area have in turn provided new methods of obtaining substituted 1,4-diketones (Scheme 26).'54-156 In this connection it has been noted that y-nitro-ketones Me0 U n-C6H 13 I [(PhS),C],CuLi -n-C6H ,C(SPh),CH,CH,COMe dihydro-(62) + ;ii > Reagents i EtOCH :CH,,H MgBr; iii HIO,; iv OH -;v HS(CH,),- SH,H+; vi BuLi,MeI; vii BuLi -Br; viii HgCl,,CdCO,; ix KI; x CH,:CHC(O)Me; xi CuCl,,CuO,H,O Scheme 26 do undergo the Nef reaction if it is carried out in ethanolic sodium hydroxide.I5' In the presence of cuprous chloride the addition of excess vinylmagnesium chloride to carboxylic acids and esters gives 1,6-diketones in varying yields of 150 R.Noyori I. Umeda and T. Ishigami J. Org. Chem.1972 37 1542. 15' I. Ojima T. Kogure and Y. Nagai Tetrahedron Letters 1972 5085. W. F. Berkowitz J. Org. Chem. 1972 37 341. 153 H. C. Ho T.-L. Ho and C. M. Wong Cunad. J. Chem. 1972,50 2718. 154 S. M. Weinreb and R. J. Cvetovich Tetrahedron Letters 1972 1233. R. A. Ellison and W. D. Woessner J.C.S. Chem. Comm. 1972 529 cf. W. D. Woessner and R. A. Ellison Tetrahedron Letters 1972 3735. 156 T. Mukaiyama K. Narasaka and M. Furusato J. Amer. Chem. SOC.,1972,94 8641. 15' D. St.C. Black Tetrahedron Letters 1972 1331. General Methods which the best is obtained with n-valeric acid giving rise to $10-tetradeca- dione. a The carbonyl function can be regenerated from the protecting thioacetal group (in the form of 1,3-dithiolans and 1,3-dithians) under an ever-increasing number of conditions of which methyl iodide,' 59*16* methyl fluorosulphonate 160 l6 ' thallium(rI1) trifluoroacetate,' 62 ceric ammonium nitrate,' 63 silver oxide '64 and chloramine T' 65 are recent additions.Bis(alky1thio)cyclopropanes (63) which can be obtained either from an epoxide or an ap-unsaturated aldehyde undergo different ring-opening reactions according to the conditions (Scheme 27).166 Reagents i HgCl, HgO aq. MeOH; ii aq. HC02H Scheme 27 Barton et ~1.'~~ have published full details of the use of trityl tetrafluoroborate for the removal of ethylene acetals conversion of acetonides into a-ketols and the deprotection of benzyl and benzyloxycarbonyl ethers. The prior protection of a keto-group as a metal enolate before lithium aluminium hydride reduction has been demonstrated in the case of a number of 1l-keto-steroids.l6* 9 Acids Hindered methyl esters can be converted into the corresponding acids by treat- ment with DBN at 165 "C.169 The reaction of 2-substituted 2-lithio-1,3-dithians with methyl disulphide gives orthothioformates which can then be hydrolysed to acids (and esters).' 70 An interesting method of homologating aldehydes and ketones to acids has been described by Schollkopf and Schroder (Scheme 28).17 ' 15' S.Watanabe K. Suga T. Fujita and Y. Takahashi Canad. J. Chem. 1972 50 2786. H.-L. W. Chang Tetrahedron Letters 1972 1989. 16' M. Fetizon and M. Jurion J.C.S. Chem. Comm. 1972 382. 16' T.-L. Ho and C. M. Wong Synthesis 1972 561. 16' T.-L.Ho and C. M. Wong Canad. J. Chem. 1972,50 3740. 163 T.-L. Ho H. C. Ho and C. M. Wong J.C.S. Chem. Comm. 1972 791. 164 D. Gravel C. Vaziri and S. Rahal J.C.S. Chem. Comm. 1972 1323. 165 W. Huurdeman H. Wynberg and D. W. Emerson Synthetic Comm. 1972 2 7. D. Seebach and M. Braun Angew. Chem. Internat. Edn. 1972 11 49. 16' D. H. R. Barton P. D. Magnus G. Smith G. Streckert and D. Zurr J.C.S. Perkin I 1972 542. Ih8 D. H. R. Barton R. H. Hesse M. M. Pechet and C. Wiltshire J.C.S. Chem. Comm. 1972 1017. D. H. Miles and E. J. Parish Tetrahedron Letters 1972,3987; For a review of DBN and DBU see H. Oediger F. Moller and K. Eiter Synthesis 1972 591. R. A. Ellison W. D. Woessner and C. C. Williams J. Org. Chem. 1972 37 2757. I 'I U. Schollkopf and R.Schroder Angew. Chem. Internat. Edn. 1972 11 3 1 1. 354 .. W.B. Motherwell and 1.S. Roberts C \N Li' ICH-S0,Ar 7 RZ/\S0,Ar 1 R'R~CHCO~H Reagents i R R 'CO Scheme 28 The major product from the attempted acyloin condensation of (64)with sodium in liquid ammonia is the ring-opened diester (65).'72 These conditions appear to be general for cleavage of both cis-and trans-l,Zdiesters in contrast to the other classical acyloin methods. CO,Me cC0,Me i C0,Me Me Me Functionalized Acids.-A one-step reductive hydrolysis of azlactones gives the corresponding N-acyl amides which can then be further hydrolysed to the corresponding a-amino-acids. An alternative method of a-amino-acid synthesis involves amination of a-lithiated acid salts with O-methylhydroxyl- amine.74 The asymmetric formation of a-amino-acids continues to be a field of active research. Rhodium(I)-diene complexes mixed with two chiral phosphine ligands where the chirality is on the phosphorus e.g.optically active o-anisylmethylcyclo- hexylphosphine are very effective hydrogenation catalysts for a-acylamino- cinnamic acids,17' e.g. optical purities of >80 % for the derived cr-acylamino-3- arylpropionic acids have been achieved. Another approach is the reaction of di-isopinocamphenylborane with a nitrile to form a ketiminoborane which reacts further with HCN to form the aminoborane (66). Methanolysis of (66) followed by acid-catalysed hydrolysis of the intermediate a-amino-nitrile yielded for instance (R)-(-)-valine in 45% overall yield with an optical purity of 12.4%176 Of even greater potential is the reaction of the optically active iron carbonyl-imine complex (67)with alkyl bromides.Hydrogenation of the product L72 P. G. Gassman and X. Creary J.C.S. Chem. Comm. 1972 1214 cf. J. J. Bloomfield, R.A. Martin and J. M. Nelke ibid. p. 96. 173 A. Badshah N. H. Khan and A. R. Kidwai J. Org. Chem. 1972,37 2916. S. Yamada T. Oguri and T. Shioiri J.C.S. Chem. Comm. 1972 623. W. S. Knowles M. J. Sabacky and B. D. Vineyard J.C.S. Chem. Comm. 1972 10. U. E. Diner M. Worsley J. W. Lown and J.-A. Forsythe Tetrahedron Letters 1972 3 145. General Merhods Me Me R I* * I* I* H-C-N CH-CO,Et H-C-N-CH-C0,Et Lh '%< 111 II Fe Ph Fe(CO),Br (CO) (68) (67) (68) followed by alkaline hydrolysis gives the a-amino-acid in reasonable yield with up to 95 % optical purity.' 77 a-Keto-acids can be obtained in good yield by a Ritter reaction on an alde- hyde cyanohydrin followed by oxidation and hydrolysis (Scheme 29).17' RCH(0H)CN -bRCH(OH)C(O)NHCMe '2RC(0)C02H Reagents i Me,COH H,SO,; ii CrO, H+; iii H,O+ Scheme 29 Two have reported that anions of allylic esters (69) undergo a [3,3]-sigmatropic rearrangement to give $-unsaturated acids (70).This process is remarkably facile although in certain cases it is advantageous to trimethyl- silylate the lithio-enolate before warming. A synthetically useful modification 0-0 0 of this process is the conversion df the enol ether analogue (71) into dihydro- jasmone (Scheme 30).@-Anions of a-substituted carboxylic acids react with formaldehyde to form hydracrylic acids which can be dehydrated to form a variety of a-substituted acrylic acids. As structural equivalents of metalated carboxylic acids silylated keten acetals (72)react thermally with aromatic aldehydes to give after hydrolysis J. Y. Chenard D. Commereuc and Y. Chauvin J.C.S. Chem. Comm. 1972 750. '" J. Anatol and A. Medete Bull. SOC.chim. France 1972 189. 179 R. T. Arnold and C. Hoffman Synthetic Comm. 1972 2 27; R. E. Ireland and R. H. Mueller J. Amer. Chem. SOC.,1972 94 5897. lE0 P. E. Pfeffer L. S. Silbert and J. M. Chirinko jun. J. Org. Chem. 1972 37 451; P. E. Pfeffer E. Kinsel and L. S. Silbert J. OrK. Chem. 1972. 37 1256.W. B. Motherwell and J. S. Roberts o5 &c5H1 1 . .. + C5H 1 1. I1 O 0 F 0 iii. ivi Reagents i LiICA Me,SiCI; ii H+;iii (Bu'),AIH; iv OH-Scheme 30 p-hydroxy-acids.''' Crotonic acid reacts with ketones in the presence ofdiethyl-amine and lithium naphthalene to yield 5,5-dialkyl-5-hydroxy-2-pentenoic acids.''* Making use of the well-known acylaziridine -+ 2-oxazoline rearrangement Meyers et ~1.''~have converted a number of acids into the acylaziridines which rearrange to 5,5-dimethyl-2-oxazolineson acid catalysis. These derivatives are inert to LiAlH and Grignard reagents and can be reconverted into the starting acid by dilute sulphuric acid (Scheme 31). Carboxylic acids can be protected RC0,H RC0,H 4 Scheme 31 against acidic and basic conditions by conversion into the corresponding acyl- hydrazides with NN'-di-isopropylhydrazine.184 The acids can be regenerated by a variety of oxidants of which Pb(OAc) is the preferred one. 181 P. L. Creger Tetrahedron Letters 1972 79. 18 K. Suga S. Watanabe and T. Fujita Austral. J. Chem. 1972 25 2393. D. Haidukewych and A. I. Meyers Tetrahedron Letters 1972 303 1. lS4 D. H. R. Barton M. Girijavallabhan and P. G. Sammes J.C.S.Perkin I 1972 929 cf. T.-L. Ho H. C. Ho and C. M. Wong Synthesis 1972 562. General Methods 10 Esters Useful esterification techniques and modifications (Scheme 32)'85-'90 have been described with particular emphasis on hindered acids and alcohols. (R ' CO),O R'C0,Me A R'C0,H R'CO,RZ Reagents i RZX KOH EtOH HMPA,IE5 or Me,N OH- R'X DMF,IE6 or R'X Cu,O C,H,,NC:'*' ii Mel DMSO CaO 188or 25% aq.NaOH Me,S0,;'89 iii R 'OH 4-dimethylaminopyridine.90 Scheme 32 Functionalized Esters.-A convenient high-yield process for converting /I-keto- esters into a-bromo-esters (and acids) has been described. 19' The use of lithium bis(trimethylsily1)amide as the base in the Darzens glycidic ester synthesis is recommended. 192 As an alternative to the Reformatsky reaction P-hydroxy- esters are obtained in good yield according to Scheme 33.'93 A convenient synthesis of ethyl 1,3-dithian-2-carboxylate,a precursor for a-keto-esters has been reported. 94 Ti(OR*),+ R2R3C0 + CH,=C=O -+ R2R3C(OH)CH2C0,R' Scheme 33 Enolate anions of ap-unsaturated esters are readily obtained using lithium N-isopropylcyclohexylamide (LiICA) as base.These can be quenched to give predominantly the unconjugated isomer alkylated at the a-position and react with ketones to give /I-hydroxy-a-vinyl ester^."^ Propargylic esters (RC-CCH,CO,Et) (and hence y-keto-esters) are easily synthesized by the reaction of a trialkynylborane with ethyl diazoacetate. 196 The replacement of dialkylchloroboranes for trialkylboranes in the reaction with ethyl diazoacetate (two-carbon homologation of an olefin) results in increased yields especially for P. E. Pfeffer T. A. Foglia P. A. Barr I. Schmeltz and L. S. Silbert Tetrahedron Letters 1972,4063. Is6 J. H. Wagenknecht M. M. Baizer and J. L. Chruma Synthetic Comm. 1972,2,215. I*' T.Saegusa and I. Murase Synthetic Comm. 1972 2 1 ; CJ A. H. Lewin and N. L. Goldberg Tetrahedron Letters 1972 491. G. Mehta Synthesis 1972 262. J. Grundy B.G. James and G. Pattenden Tetrahedron Letters 1972,757. I9O G. Hofle and W. Steglich Synlhesis 1972 619. I9l P. L. Stotter and K. A. Hill Tetrahedron Letters 1972 4067. 192 R. F. Borch Tetrahedron Letters 1972 3761. 193 L. Vuitel and A. Jacot-Guillarmod Synthesis 1972 608. 194 E. L. Eliel and A. A. Hartmann J. Org. Chem. 1972,37 505. Ig5 M. W. Rathke and D. Sullivan Tetrahedron Lerters 1972 4249. 196 J. Hooz and R. B. Layton Canad. J. Chem. 1972,50 1105. W.B. Motherwell andJ. S. Roberts bulky alkyl groups. 197 Low-temperature reaction of ethyl acetate anion with cuprous iodide gives ethoxycarbonylmethyl copper which reacts with allylic bromides to generate yd-unsaturated esters.'98 The same technique can be used to generate cyanomethylcopper leading to yh-unsaturated nitriles.199 Ladones.-The 3c-methylene-y-butyrolactone moiety is a common feature of many naturally occurring cytotoxic sesquiterpenoids. The synthesis of this unit has been the subject of a number of papers of which those by Ourisson et ~1.~'' are noteworthy (Scheme 34). Alternatively the anion from the nor-methyl trans-lactone can be trapped with formaldehyde to generate the cr-hydroxy- methyl derivative which on mesylation and elimination yields (73).20* Another method of forming y-lactones involves the addition of a metalated carboxylic acid to an epoxide.202 Reagents i Ph,CLi; ii Br(CH,),Br; iii DBN; iv (PhCO,) Scheme 34 11 Amides Nitriles Carbamates Isocyanates and Isonitriles The new and readily prepared reagent diphenylphosphorylazide is very effective for the conversion of acids into carbamates and amides (Scheme 35).'03 The use of this reagent provides an excellent alternative to the classical methods of converting acids into mines in view of the mild conditions and it can also be utilized for peptide synthesis without racemization.Another useful modifica- tion of the Curtius reaction involves the reaction of an acid chloride or anhydride with trimethylsilylazide to give the isocyanate dire~tly.''~ R'CONHR3 &-R'C0,H + N,PO(OPh) * R'NHC02R2 Reagents i R3NH, Et,N; ii RZOH,Et,N A Scheme 35 19' H.C. Brown M. M. Midland and A. B. Levy J. Amer. Chem. SOC. 1972,94 3662. 19* I. Kuwajima and Y. Doi Tetrahedron Letters 1972 1163. 199 E. J. Corey and I. Kuwajima Tetrahedron Letters 1972 487. A. E. Greene J.-C. Muller and G. Ourisson Tetrahedron Letters 1972 2489 3375. 'O1 P. A. Grieco and K. Hiroi J.C.S. Chem. Comm. 1972 1317. *02 P. L. Creger J. Org. Chem. 1972,37 1907. ,03 T. Shioiri K. Ninomiya and S. Yamada J. Amer. Chern. Soc. 1972 94 6203. ,04 S. S. Washburne and W. R. Peterson jun. Synthetic Comm. 1972 2 227; H. R. Kricheldorf Synthesis 1972 55 1. General Methods New or improved routes to nitriles include dehydration of aldoximes with cyanuric chloride,205 treatment of toluene-p-sulphonylhydrazones with CN-,206 reaction of N-trimethylsilylamides with an acid chloride,207 and dehydration of amides with phosphonitrilic chloride.208 a-Keto-nitriles can be obtained from the corresponding acid chloride with cuprous cyanide.20g Phase-transfer catalysis is proving to be an effective synthetic procedure from several standpoints.Thus glycidic nitriles can be readily prepared from ketones and chloroacetonitrile in aqueous sodium hydroxide solution using benzyl- trimethylammonium chloride as the catalyst.2 lo This technique has also been successfully applied to the Hofmann carbylamine synthesis of isonitriles from primary amines.* Isonitriles can also be prepared by low-temperature dehydration of formamides with chlorodimethylformiminiumchloride (Vilsmeier reagent)2 and in variable yields by the reaction of a formamide with triphenyl- phosphine and diethyl azodi~arboxylate.~' A new simple method of convert- ing isonitriles into carbamates involves thallium(I1r) nitrate trihydrate as the oxidant .2 l4 12 Alkylation and Coupling Reactions Two highly ingenious and powerful synthetic weapons have been developed by Trost and his co-workers.The 'transfer alkylation' sequence achieves selective alkylation at the y-or &-position of a polyenolate system by a process in which the normal roles of nucleophile and electrophile are reversed (Scheme 36). The Me0,C' C0,Me Me0,C' C0,Me RDI = Br/h-o Me Me0,C C0,Me 0 (74) Scheme 36 20s J. K. Chakrabarti and T. M. Hotten J.C.S. Chem. Comm. 1972 1226. 206 S.Cacchi L. Caglioti and G.Paolucci Chem. undInd. 1972,213. *'' M. L. Hallensleben Tetrahedron Letters 1972 2057. 208 J. C. Graham and D. H. Marr Cunad. J. Chem. 1972,50 3857. '09 J. F. Normant and C. Piechucki Bull. Soc. chim. France 1972,2402. 210 A. Jonczyk M. Fedorynski and M. Makosza Tetrahedron Letters 1972 2395. 'I1 W. P. Weber and G. W. Gokel Tetrahedron Letters 1972 1637; W. P. Weber G. W. Gokel and I. K. Ugi Angew. Chem. Internat. Edn. 1972 11 530. 212 H. M. Walborsky and G. E. Niznik J. Org. Chem. 1972 37 187. 'I3 B. Beijer E. Von Hinrichs and I. Ugi Angew. Chem. Internat. Edn. 1972 11 929. F. Kienzle Tetrahedron Letters 1972 1771. W. B. Motherwell and J. S. Roberts bromomalonate ester (74) functions in the first instance as a brominating agent to give an intermediate species (75) which is subsequently attacked in SN2’ fashion by the counter-ion released in the bromination sequence.2 l5 ‘Secoalkylation’ in which the formal electronic sense of the Michael acceptor is reversed offers a procedure complementary to the famous Robinson annela- tion.216 The key steps illustrated by a typical annelation sequence (Scheme 37) are the rearrangement of the spiro-epoxide (76)and the unmasking of a disguised diketone system (77).Me&o Me -!+ MeA0-Me&* Me Me Me Me (76) Me /111 IV Me Me Me Me Me (77) Reagents i Ph,; base; ii H+; iii MeLi; iv AcCl; v LiCIO,; vi base 4 Scheme 37 In the realm of more-established annelation sequences an excellent review has been published by Lansbury217 on the chloro-olefin annelation reaction and a method has been outlined for obtaining significantly higher yields in the Stork isoxazole annelation.* L. S. Melvin jun. and B. M. Trost J. Amer. Chem. Soc. 1972,94 1790. B. M. Trost and M. J. Bogdanowicz J. Amer. Chem. Soc. 1972,94,4777. 217 P. T. Lansbury Accounts Chem. Res. 1972 5 31 1. 218 J. W. Scott B. L. Banner and G. Saucy J. Org. Chem. 1972 37 1664. General Methods 36 1 The metalation of limonene with the complex of n-butyl-lithium and tetra- methylethylenediamine leads uniquely to the organometallic species (78) which can react with a wide variety of electrophiles to give products in which optical activity is retained.219 The full synthetic potential of this remarkable reagent remains to be assessed.Li + (78) The widespread use of dithian derivatives has been paralleled by a general resurgence of interest in the power of sulphur building-blocks for organic synthesis. An excellent three-step synthesis of p-santalene features the novel use of a sultone as a carbon-activating group and a novel desulphurization proce- dure220 (Scheme 38). Two conceptually identical approaches to the synthesis of Cecropiajuvenile hormone have also favoured this type of strategy.22 Reagents i BuLi; ii Br; iii AIH,; iv POCI Scheme 38 The steadily increasing importance of organocuprates is reflected in two useful review^,'^^,^^^ and by a growing tendency not only to perform reactions on 'Iy R. J. Crawford W. F. Erman and C. D. Broaddus J.Amer. Chem. SOC.,1972 94 4298. 220 J. Wolinsky R. L. Marhenke and R. Lau Synthetic Comm. 1972 2 165. (a) K. Kondo A. Negishi K. Matsui D. Tunemoto and S. Masamune J.C.S. Chem. Comm. 1972 131 1 ;(b)P. L. Stotter and R. Hornish 164th A.C.S. National Meeting; New York Aug. 27-Sept. I 1972 Organic Section 75. 222 G. H. Posner Organic Reactions 1972 19 1. 223 J. F. Normant Synthesis 1972 63. W.B. Motherwell and J. S. Roberts polyfunctional molecules224 but also to prepare organocopper reagents which are themselves highly fun~tionalized.~~’ In order to combat the wastage problem which arises when a valuable synthetic intermediate (R‘) is employed in organo- cuprate form Corey and Beames226 have devised a mixed acetylenic cuprate (79) which operates with selective transfer of R‘ (Scheme 39).Among the newer iii. iv (79) 0 Scheme 39 reactions of organocopper compounds are the homoconjugate addition to cyclo- propanes possessing electron-withdrawing groups2’ and the occurrence of a novel cyclization mediated by organocuprates.22 In addition stereospecifically- generated vinyl-copper reagents have been alkylated in the presence of triethyl phosphite and HMPT with retention of configurati~n.~~~ Cuprous t-butoxide has been recommended as a metalation agent.230 The organometallic reagents (80)and (81)are useful in Grignard-type reactions and permit the introduction of a hydroxypropyl group without cornplicati~n.~~ The new Meerwein reagent (82) is reported to be of comparable reactivity to methyl fluorosulphonate.’ ’ 224 G.H. Posner C. E. Whitten and P. E. McFarland J. Amer. Chem. SOC.,1472 94 5106. 225 (a) A. F. Kluge K. G. Untch and J. H. Fried J. Amer. Chem. SOC.,1972 94 7827; (6)P. E. Eaton and R. H. Mueller ibid. p. 1014; (c) C. J. Sih R. G. Salomon P. Price G. Peruzzoti and R. Sood J.C.S. Chem. Comm. 1972,240. 226 E. J. Corey and D. J. Beames J. Amer. Chem. SOC.,1972,94 7210. 227 (a)E. J. Corey and P. L. Fuchs J. Amer. Chem. Suc. 1972,94,4014;(b)G. Davidaud and P. Miginiac Tetrahedron Letters 1972 997. ’** J. A. Katzenellenbogen and E. J. Corey J. Org. Chem. 1972,37 1441. 229 J. F. Normant G.Cahiez C. Chuit and A. Alexakis J. Organometallic Chem. 1972 40 c49. 230 T. Tsuda T. Hashimoto and T. Saegusa J. Amer.Chem. SOC.,1972,94 658. 231 P. E. Eaton G. F. Cooper R. C. Johnson and R. H. Mueller J. Org. Chem. 1972,37 1947. 232 A. J. Copson H. Heaney A. A. Logun and R. P. Sharma J.C.S. Chem. Comm. 1972 315. General Methods Me BF,-(82) A facile new method for the alkylation and alkenylation of heterocyclic com- pounds involves the reaction of a chloroheterocycle with two equivalents of a Wittig reagent to form a new ylide which is either hydrolysed to give an alkyl- substituted heterocycle or subjected to the normal Wittig reaction with a carbonyl compound to give an olefinic ~ide-chain.~~~ Aromatic compounds carrying a nucleofugic substituent react with potas- sium acetonate in liquid ammonia to give a mixture of the 1-aryl-2-propanols and the arylacetone~.~~~ The alkylation of easily enolizable compounds can be accomplished in accept- able yield by intermolecular dehydration with an alcohol using triphenyl- phosphine and diethyl azodi~arboxylate.~~~ A selective cross-coupling reaction for the formation of 1,5-dienes utilizes the novel generation of an allyl-lithium by reduction of an allylic mesitoate ester.236 A new nickel-phosphine complex has been found to catalyse the selective cross- coupling of a Grignard reagent with a vinyl or aryl halide.237 Yields in phenolic oxidative coupling are markedly improved by the use of a new iron complex [Fe(DMF),CIJ [FeC1,].2 38 13 Miscellaneous Through the years the Diels-Alder reaction has played an important role in many aspects of organic synthesis.This role continues unabashed. A beautiful illustra- tion of its utility is seen in the first of three papers by Eschenmoser et who have generated the ‘enophilic’ diene (83) a conjugated nitrosonium ion by de- chlorination of the P-chloro-nitrone (84). In the presence of an olefin a facile (4+ 2) cycloaddition takes place and treatment of the resultant iminium salt 233 E. C. Taylor and S. F. Martin J. Amer. Chem. SOC. 1972 94 2874. 234 R.A. Rossi and J. F. Bunnett J. Amer. Chem. Soc. 1972,94 683. 235 M. Wada and 0. Mitsunobu Tetrahedron Letters 1972 1279. 236 J. A. Katzenellenbogen and R. S. Lenox Tetrahedron Letters 1972 1471. 237 K. Tamao K. Sumitani and M. Kumada J. Amer. Chem. SOC.,1972 94 4374. 238 S. Tobinaga and E. Kotani J.Amer. Chem. Soc. 1972,94 309. 239 U. M. Kempe T. K. Das Gupta K. Blatt P. Gygax D. Felix and A. Eschenmoser Helu. Chim. Acta 1972 55 2187; T. K. Das Gupta D. Felix U. M. Kempe and A. Eschenmoser ibid. p. 2198; P. Gygax T. K. Das Gupta and A. Eschenmoser ibid. p. 2205. W. B. Motherwell and J. S. Roberts (85) with CN-followed by base-catalysed elimination of HCN and hydrolysis of the imino-lactone gives the lactone (86) in high yield. Additional synthetic interest in the adduct (85) (as the Ph,B- salt) is manifest by its base-induced deprotonation to produce the enamine (87) which undergoes a retro-Diels-Alder reaction to generate after hydrolysis the @unsaturated aldehyde (88)(Scheme 4). (83) R3 (85) iiil .1 R2 R3 1 R3 A R3 (88) Reagents i AgBF,; ii CN-; iii Bu'O-; iv H,O'; v K,CO,; vi SiO Scheme 40 General Methods Another elegant example of the Diels-Alder reaction is the use of 1,3-dithi- enium fluoroborate (89) as a dienophile to produce the salt (90).Rearrangement of the ylide derived from (90)yields the cyclopropane derivative (91),which in turn thermally rearranges to the cyclopentenone thioacetal (92) and by subsequent hydrolysis may be transformed into the cyclopentenone (Scheme 41).240 In a nutshell this amounts to the addition of carbon monoxide to a conjugated diene. (92) (91) Reagents i BuLi; ii heat Scheme 41 Vinylketen thioacetals are reactive dienes as illustrated by the conversion in Scheme 42.241 Methyl cc-bromovinyl sulphone (93) functions as an effective synthon for the formal addition of allene to a diene -the final step being the well-known Ramberg-Backlund reaction (Scheme 43).242 Further examples of dichlorovinylene carbonate acting as a dienophile have been Scheme 42 (93) Scheme 43 240 E.J. Corey and S. W. Walinsky J. Amer. Chem. SOC.,1972 94 8932. 241 F. A. Carey and A. S. Court 164th ACS Meeting 1972 Abstracts ORGN 139. 242 J. C. Philips and M. Oku J. Amer. Chem. SOC.,1972,94 1012. 243 H.-D. Scharf and W. Kiisters Chem. Ber. 1972,105 564. W. B. Motherwell and J. S. Roberts Dichloromaleic anhydride isomerizes 1-methoxycyclohexa- 1,4-dienes without participation in an ensuing Diels-Alder reaction and thus acts as a convenient catalyst for the addition of dienophiles to the non-conjugated cyclic diene~.~~~ The useful annelating agents (94; R' = R2 = H; R' = H R2 = Me; R' = Me R2 = H) are most readily obtained by a retro-Diels-Alder reaction of the bicyclic keto-esters (95).245 A rigorous study of the effect of various pelleted and powdered molecular sieves on enamine and ketimine formation has shown that the best conversions are achieved with powdered molecular sieve together with 25%(w/w) of com- mercially available silica-alumina catalyst.246 Type 5A molecular sieve is also useful for acetal formation with secondary alcohols.247 A procedure for preparing highly reactive magnesium metal has been described.248 The black magnesium powder thus obtained forms Grignard reagents under very mild conditions.A convenient chemical source of singlet oxygen is potassium perchromate which liberates the gas by decomposition in aqueous medium.249 Crown polyethers two of which are now commercially available continue to show interesting properties from the point of view of organic synthesis.250 A new and very effective method of preparing solutions of diborane has been described.25 ' 244 A. J. Birch and K. P. Dastur Tetrahedron Letters 1972 4195. 245 G. Stork and R. N. Guthikonda Tetrahedron Letters 1972 2755. 246 D. P. Roelofsen and H. van Bekkum Rec. Trav. chim. 1972 91 605. 247 D. P. Roelofsen and H. van Bekkum Synthesis 1972,419. 248 R. D. Rieke and P. M. Hudnall J. Amer. Chem. SOC.,1972 94 7178. 249 1972,94, J. W. Peters J.N. Pitts jun. I. Rosenthal and H. Fuhr J. Amer. Chem. SOC. 4348. C. J. Pedersen and H. K. Frensdorff Angew. Chem. Znternat. Edn. 1972,11 16; D. J. Sam and H. E. Simmons J. Amer. Chem. SOC.,1972 94 4024; R. N. Greene Tetra-hedron Letters 1972 1793; M. J. Maskornick ibid. p. 1797. 251 A. Brandstrom U. Junggren and B. Lamm Tetrahedron Letters 1972 3173.

ISSN:0069-3030    

DOI:10.1039/OC9726900329

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

11 Aliphatic Compounds By R. S. ATKINSON Department of Chemistry The University Leicester LEl 7RH and E. W. COLVIN Department of Chemistry University of Glasgow Glasgow GI 2 8QQ 1 Acetylenes Recent reviews on areas of acetylenic chemistry include synthetic routes to average ring size cycloalkynes,' a theoretical investigation of bonding in metal- acetylene complexes,2 transition-metal complexes of a~etylene,~ intramolecular cyclization reactions with acetylenic bond parti~ipation,~ oligomerization of acetylenes induced by metals of the nickel triad,5 and a book on preparative acetylenic chemistry.6 Several recent studies have been concerned with thermal rearrangements of acetylenes. cis-Hexa-1,5-diyn-3-ene (l) on gas-phase pyrolysis in a flow system at 300 "C,undergoes a degenerate thermal rearrangement.Thus pyrolysis of the dideuteriated analogue (2) causes scrambling of the deuterium between acetylenic and vinyl positions but with either both vinyl positions or both acetylenic posi- tions labelled only [(2) and (3)]. The results demand that the intermediate or R H / \ \ R H / (1) R = H (3) R = D (2) R = D ' H. Meier Synthesis 1972 235. F. R. Hartley Angew. Chem. Internat. Edn. 1972 11 596. L. D. Pettit and D. S. Barnes Fortschr. Chem. Forsch. 1972 28 85. E. E. Voyakovskaya L. N. Shil'nikova N. V. Koshmina and F. Ya. Perveev Reakts. spos. org. Soedinenii 197 1 97. P. M. Maitlis Pure Appl. Chem. 1972 30 427. L. Brandsma 'Preparative Acetylenic Chemistry' Elsevier New York 1971.367 R. S. Atkinson and E. W. Colvin transition state in this scrambling has C-I C-3 C-4 and C-6 equivalent. This intermediate shows radical-type behaviour hence the 1,4-benzenediyl diradical (4)is assumed for its structure rather than a p-benzyne structure (S).7 Phenyl propargyl ether (6) gives indan-2-one and benzocyclobutene on low- pressure gas-phase pyrolysis (-450 "C). The mechanism of formation of indan-2-one (Scheme 1) is based on the work of Schmidt' and supported by employing -0-m J CH=C=O Scheme I the deuteriated propargyl ether (7) which yielded the appropriately labelled indanone (8).9 Pyrolysis studies on (9)" (lo),and (11)" have also been made. Me + D@O D* = 19 "/ deuterium = lOO%deuterium H D C D I (8) H (7) A & fiC HC HC (9) (10) -R.R. Jones and R. G. Bergman J. Amer. Chem. SOC.,1972,94,660. H. J. Hansen and H. Schrnid Chimia (Switz.) 1970 24 89. W. S. Trahanovsky and P. W. Mullen J. Amer. Chem. Soc. 1972 94 591 1. lo M. B. D'Arnore R. G. Bergman M. Kent and E. Hedaya J.C.S. Chem. Comm. 1972 49. 'I T. J. Henry and R. G. Bergman J. Amer. Chem. SOC.,1972,94 5103. Aliphatic Compounds An unusual o-Claisen rearrangement is observed with the sulphoxide (12)which is converted into the thioalcohol (13) by a 2,3- followed by a 3,3-sigmatropic rearrangement without isolation of the allenyl sulphenate (14).12 CH,R Ill js'" -Q R = p-C1-phenoxy Metal- Acetylene Reactions.-Treatment of thiacycloheptyne (15)' with (PhCN),PdCl in tetrahydrofuran affords the yellow cyclobutadiene complex (16)(80%).A suspension of (16)in chloroform reacts with ethylene-bis(dipheny1- phosphine) under nitrogen to give the stabilized cyclobutadiene (17) as a yellow crystalline oxygen-sensitive solid. A distinction between (17a) and (17b) has yet to be made.14 + (PhCN),PdCI -+ S sx Cyclotrimerization of acetylenes with olefins has been accomplished ; N-methylmaleimide and phenylacetylene gave the 2 1 adduct (18) and also the 2 :2 adduct (19).15 l2 K. C. Majumdar and B. S. Thyagarajan J.C.S. Chem. Comm. 1972 83. l3 J. Haase and A. Krebs Z. Naturforsch. 1972 27 624; A. Krebs and H. Kimling Tetrahedron Letters 1970 76 1. l4 H. Kimling and A.Krebs Angew. Chem. Internat. Edn. 1972,11 932. Is A. J. Chalk J. Amer. Chem. SOC.,1972 94 5928. 370 R.S. Atkinson and E. W. Colvin Ph-C-CH ‘ + Ni(COl,(PPh,) ’ Ph &NMe 4-MezaNMe 0 000 0 0 (18) (19) I Me Diethylalkyriylalanes react with a range of simple conjugated enones to give fair to good yields of y6-acetylenic ketones e.g. (20) not accessible by Michael addition of acetylenic anions. The organoalane (21) is easily obtained by converting the terminal acetylene into its lithium salt and then treating with diethylaluminium chloride. A plausible pathway for the reaction involves the intramolecular delivery of the alkynyl group through prior complexation with the carbonyl oxygen as in (22).16 PhC-CLi + Et,AICI --* Et,AIC=CPh + LiCl (21) I R (22) Cycloaddition Reactions.-The 1,3-dipolar addition of mesoionic systems to dimethyl acetylenedicarboxylate (DMAD) is typified by synthesis of pyrroles from A2-oxazolium-5-olates (munchnones) (23) -P (24).’ This reaction has been Me Me I Me0,C phQph C0,Me (24) ’’ J.Hooz and R. B. Layton J. Amer. Chem. SOC.,1971,93 7320. ” R. Huisgen H. Gotthardt H. 0. Bayer and F. C. Shaefer Chem. Ber. 1970,103,2611. Aliphatic Compounds 37 1 \o7-;OzH ii,I (McCOI,ODMAD ’ C0,MeQTlC0,Me H H Me (25) ti,DMAD M I. (MeCOI,O ’ OT H G \I e 0-q~~ N CO,H (26) Me02C C0,Me extended to elaborate fused and functionalized pyrrole rings on to the piperidine rings of (25)and (26) in moderate yields.’ 2-N-Methylaminopyridine adds to carbon suboxide giving (27) which under- goes a 1,4-dipolar addition with DMAD yielding the quinolizone (28) after extrusion of methyl isocyanate.’ Similarly pyridones (29) are obtained from NN’-disubstituted amidines (30).20 Me Me Me I 0 +I I /C/ /C \N + p30-p7 \N 0 0 (27) @COzMe C0,Me @COzMe C0,Me \N \ No 0 0 (28) R2 C0,Me RZ \ DMAD C=NR~ + c,02-+ I -MeNCO -Meo2cOo NHPh 0 R2 I (30) Rl (29) F.M. Hershenson J. Org. Chem. 1972 37 3 1 1 1. l9 K. T. Potts and M. Sorm J. Org. Chem. 1971,36 8. 2o K. T. Pottsand M. Sorm J. Org. Chern. 1972,37 1422. 372 R. S. Atkinson and E. W. Colvin Reaction of the naphthotriazine (31) with DMAD may be construed as a 1,11-dipolar cycloaddition giving the acenaphthotriazine (32) after spontaneous dehydrogenation.2 Me Me (31) Me0,C CO,Me (32) Among additions of DMAD to dienes is its reaction with the arsabenzene (33) to yield arsabarralenes (34).It appears that their reactivity as dienes increases in the order N < P < As in these Group V heterobenzenes.22 C0,Me (33) Ph (34) The first non-condensed thiepin (35) has been obtained by reaction of the pyrrolidinylthiophen (36) with DMAD. An intermediate (36) is detected by n.m.r. monitoring of the reaction mixture where (35) also decays by sulphur ___) MbNA nMAD (37) \ N Me&CO,Me '' C. W. Rees R. W. Stephenson and R. C. Storr J.C.S. Chem. Comm. 1972 1281. 22 G. Markl J. Advena and H.Hauptmann Terrahedron Letters 1972 3961. Aliphatic Compounds extrusion to the benzene derivative (38). The potentially antiaromatic thiepin is stabilized by electron-withdrawing groups.23 Miscellaneous Reactions.-Several papers show that silylation is a useful protective method in synthesis of poly-ynes. Thus the triethylsilyl-protected bromoalkyne (39) can be used successfully in Cadiot-Chodkiewicz coupling of acetylenes (40)+(41); addition of alkali liberates the diyne quantitatively. CU’ NaOH ArCrCH + BrC-CSiEt EtNH,I Ar(CrC),SiEt +Ar(C-C),H DMF-(40) (39) (41) The triethylgermanium compounds behave similarly with the added bonus that the germanium-acetylene bond may also be cleaved by acids without con- comitant hydration of the triple bond (unlike the silicon analogue) a factor which could be useful in synthesis of alkali-sensitive polyene~.~~ Triethylsilyl-protected terminal alkynes can be coupled oxidatively the coupled products partially desilylated (monitoring by u.v.) and then recoupled and completely desilylated (42)-(43).25 The reagents Et,Si(CcC),H (m= 1 2 or 4) may be used in H(C EC)8H +-Et ,Si(C =C),Si Et (43) excess as one component in mixed oxidative couplings allowing extension of terminal poly-yne chains by up to four yne units in a single step.Again the tri- ethylsilyl group is easily removable using base.26 Mechanisms of nucleophilic displacement at acetylenic carbon RCECX + NUC-+ RC-C-NUC + X have been discussed with particular reference to the reactions of phosphines and amines with halogenoacetylenes.Nucleophilic attack using amines occurs on the acetylenic carbon but with phosphines attack on halogen also compete^.^' 2 Alkanes Electrophilic substitutions at alkanes and in alkyl-carbonium ions have been reviewed.28 The barriers to rotation and rotamer preferences in various acyclic alkanes have been determined.29 Effects of halogenoalkanes as solvents on the 3Cn.m.r. spectra of n-alkanes have been studied.30 ” D. N. Reinhoudt and C. G. Kouwenhoven J.C.S. Chem. Comm. 1972 1232 1233. 24 R. Eastmond and D. R. M. Walton Tetrahedron 1972 28,4591. 25 R. Eastmond T. R. Johnson and D. R. M. Walton Tetrahedron 1972 28 4601. 26 T. R. Johnson and D. R. M. Walton Tetrahedron 1972 28 5221. 27 J.I. Dickstein and S. I. Miller J. Org. Chem. 1972 37 2168 2175. ** D. M. Brouwer and H. Hogeveen Progr. Phys. Org. Chem. 1972,9 179. 29 C. H. Bushweller and W. G. Anderson Tetrahedron Letters 1972 181 1. 30 G. Bergmann and J. Dahm Angew. Chem. Internat. Edn. 1972 11 1032. 374 R.S. Atkinson and E. W. Colvin 3 Allenes Syntheses of the allene functional group have been included in a recent v~lume.~ Acetylene- and allene-containing animal alkaloids have been reviewed.32 Cycloaddition of partially resolved cyclonona-1,2-diene and dimethylketen gives an optically active product (44) which is converted into (45) on hydrogena- tion and treatment with base conditions not affecting the chirality of C*. From the absolute configuration of (45) (by 0.r.d.) and knowing the absolute configura- tion of starting cyclononadiene it can be deduced that addition takes place at least preferentially with the keten acting antarafacially and the allene supra- facially (Woodward-Hoffman-allowed .2 + .2J assuming that the two reaction components approach in the least sterically demanding way.33 The case of dimethylketen addition to optically active 1,3-dimethylallene has also been examined and the possibility of suprafacial addition of the keten and antara- facial addition of allene is suggested as a competing (minor) reaction.34 Cyclo- nona-l,2-diene (partially resolved) also reacts with t-butylcyanoketen giving optically active products.The major product (46) has a cis relationship between the t-butyl group and the adjacent CH, which again suggests a synchronous reaction via a .2 (allene) and .2 (keten) addition as shown in (47).35 Y C t C I1 u 0 NC-Secondary deuterium isotope effects support a multistep pathway for all allene [2 + 21 cycloadditions (including dimerizations) but concerted mechanisms for [2 + 31 and [2 + 41 cycl~additions.~~ Support for Dolbier and Dai’s interpreta- tion of secondary deuterium isotope effects has come from study of a reaction known to be a [2 + 21 cycloaddition proceeding via a biradical.Thus when 31 ‘Organic Functional Group Preparations’ ed. S. R. Sandler and W. Karo Academic Press New York 1971 Vol. 2. 32 B. Witkop Experientia 1971 27 1121. 33 M. Bertrand J.-L. Gras and J. Gore Terrahedron Letters 1972 1189.34 M. Bertrand J.-L. Gras and J. Gore Tetrahedron Letters 1972 2499. 35 W. Weyler L. R. Byrd M. C. Caserio and H. W. Moore J. Amer. Chem. SOC.,1972 94 1027. 36 S.-H. Dai and W. R. Dolbier J. Amer. Chem. SOC., 1972 94 3946. Aliphatic Compounds 375 cyclohexa- 1,2-diene is generated from dibromonorcarane (48)it rapidly dimerizes to an intermediate diallylene (49)which can either cyclize to diene (50)or dimerize to two C24H32 stereoisomers. (48) R = H (51) R = D (49) Labelled dibromide (51) was converted into diene [2H2]-(50)whose deuterium distribution was assayed using 2H n.m.r. The spectra obtained correspond to deuterium on vinyl or tertiary positions with a small excess of deuterium on the vinyl position.The magnitude and direction of the isotope effect (kH:k = 1.04) is comparable with that observed by Dolbier and Dai (k,:k = 1.06).37 Thermal cycloaddition of many substituted allenes yielding 1,2-dimethylene- cyclobutanes is believed in many cases to proceed via the intermediate bisallyl biradical(52). The preference for formation of those stereoisomers having larger groups (R) on the ‘inside’ positions of the double bonds of the product (53) can be explained on this model by assuming that they occupy the least-hindered inward positions in (52). Dimerization of diadamantylallene was studied since models suggested a product such as (53 ;R = adamantyl) would be impossible to R H I 2RHC=C=CHR RH C/R RHaC‘R I RH H (53) form. Dimerization was unsuccessful but heating a 3 ”/ solution in di-isopropyl- benzene gave (54) in 60 ”/ yield.This constitutes added evidence for the two-step biradical mechanism for allene dimerization -the biradical in this case being too sterically hindered to undergo ring-closure but instead abstracting hydrogen atoms from di-is~propylbenzene.~~ RCH=C-C=CHR 11 RH,C CH,R (54) R = adamantyl ” W. R. Moore P D. Mogolesko and D. D. Traficante J. Amer. Chem. SOC.,1972,94 4753. 38 T. L. Jacobs and R. C. Kamrnerer J. Amer. Chem. SOC.,1972,94 7190. 376 R. S. Atkinson and E. W.Colvin flCH2 flcCHI CH\\ C\ + ,RlR Rg!R R CH2 (57) R = C0,Me Bisallenyl(55) and DMAD (56)react together to give [2,2]paracyclophane (57) in 32 % yield. Thus [2 + 41 addition to the diene system is followed by dimeriza- tion of the p-quinodimethane (58).Pure biallenyl is unnecessary for the reaction and the crude C,H hydrocarbon mixture from dimerization of propargyl bromide containing -40 % of (55)can be used directly.39 A sector rule for chiral allenes has been derived relating the position of the substituent to the sign of the lowest energy Cotton effect and a physical basis proposed for the Lowe-Brewster rule which relates the configuration of a chiral allene to the sign of its D-line r~tation.~’ 4 Olefins Recent reviews involving olefin chemistry include olefin reactions catalysed by transition-metal compounds ;41 transition-metal complexes of olefins and acety- lenes ;42 transition-metal-catalysed homogeneous olefin disproportisnation ;43 rhodium(1)-catalysed isomerization of linear butenes ;44 catalytic olefin dispro- portionation;45 oxidation of olefins with mercuric salts;46 the syn and anti steric course in bimolecular olefin-forming eliminations ;47 isotope-effect studies of elimination reactions ;48 stereochemistry of Hofmann eliminations ;49 diene synthesis by boronate fragmentation,” Friedel-Crafts acylation of alkenes ;’ olefin oxidation and related reactions with Group VIII noble metal com-pounds ;52 lead tetra-acetate oxidation of ole fin^,^^ epoxidation of olefins by hydroperoxides ;54 mechanism of addition of singlet oxygen to olefins :55 theory 39 H.Hopf Angew. Chem. Internar. Edn. 1972 11 419. 40 P. Crabbe E. Velarde H. W. Anderson S.D. Clark W. R. Moore A. F. Drake and S. F. Mason Chem. Comm. 1971 1261. 41 C. W. Bird Topics Lipid Chem. 1971 2 247. 42 L. D. Pettit and D. S. Barnes Forrschr. Chem. Forsch. 1972 28 85. 43 W. B. Hughes Organometallic Chem. Syn. 1972 1 341. 44 R. Cramer Ann. New York Acad. Sci. 1971 172 507. 45 R. L. Banks Fortschr. Chem. Forsch. 1972 28 39. 46 H. Arzoumanian and J. Metzger Synthesis 1971 527. 47 J. Sicher Angew. Chem. Internat. Edn. 1972 11 200. 48 A. Fry Chem. Soc. Rer. 1972 1. 163. 49 J. L. Coke Selectice Org. Transform. 1972,2 269. 50 J. A. Marshall Synthesis 1971 229. ’’ J. K. Groves Chem. Soc. Reti. 1972 I 73. ” R. Jira and W. Freiesleben Organometallic Reactions 1972 3 1. 53 R. M. Moriarty Selective Org. Transform. 1972,2 183.54 R. Hiatt in ‘Oxidation’ ed. R. L. Augustine Dekker New York 1971 Vol. 2. 55 C. S. Foote Pure Appl. Chem. 1971 27 635. Aliphatic Compounds of cycloaddition reactions ;56 reactions of electron-rich olefins with proton active compounds ;' and the stereochemical features of vinylic radicals.'* Metal-Olefin Reactions.-Few asymmetric syntheses proceeding under catalytic conditions have been reported and high optical purity has been observed only in hydrogenation using optically active catalysts in which the chiral centres are formed as a result of C-H bond formation. A catalyst and con- ditions have been developed which allow the conversion of cyclo-octa-1,3-diene and ethylene into 3-vinylcyclo-octene (59) having an optical purity of 70 %.The chiral catalyst was prepared using an optically active phosphine having chiral R groups.59 Although the transfer of oxygen from a transition metal to an organic sub- strate is well known the reverse is less common. Lower-valent tungsten halides prepared by addition of lithium alkyls to tungsten hexachloride can (a)transform dialkoxides into olefins,6' (b)bring about reductive coupling of aldehydes and ketones to olefins and (c) cause stereoselective reduction of epoxides to olefins in high yield.61 An alternative reagent for stereospecific conversion of epoxides into olefins with retention of configuration is sodium (cyclopentadieny1)dicarbonylferrate(see p. 332).62 A possible pathway in selenium dioxide oxidation of olefins e.g. (60),involves an allylseleninic acid (61).63 Ally1 selenoxides have now been shown64 to undergo ready 2,3-sigmatropic rearrangement to give the selenium(I1) ester ; the isolated R R R H,O ,5 -0 HOSeO CH,OH (60) R = n-butyl (61) 56 W.C. Hernden Chem. Rev. 1972 72 157. 57 J. Hocker and R. Merten Angew. Chem. Internat. Edn. 1972 11 964. L. A. Singer Selective Org. Transform. 1972,2 239. 59 B. Bogdanovic B. Henc B. Meister H. Pauling and G. Wilke Angew. Chem. Internat. Edn. 1972 11 1023. 6o K. B. Sharpless and T. C. Flood J.C.S. Chem. Comm. 1972 370. 61 K. B. Sharpless M. A. Umbreit M. T. Nieh and T. C. Flood J. Amer. Chem. SOC. 1972,94,6538. " W. P. Giering M. Rosenblum and J. Tancrede J. Amer. Chem. SOC.,1972 94 7170. 63 K. B. Wiberg and S.D. Nielsen J. Org. Chem. 1964 29 3353. 64 K. B. Sharpless and R. F. Lauer J. Amer. Chem. SOC.,1972,94 7154. R. S.Atkinson and E. W. Colvin ph\gek I PhSe -v 0-(62) products are derived by hydrolysis of the latter. Thus oxidation of (62) with hydrogen perioxide gives the E-alcohol(63) as the major product. Significantly it is this E-alcohol which is the major product in selenium dioxide oxidation of (60),65 suggesting that the corresponding seleninic acid (61) may undergo a similar rearrangement. A possible mechanism for oIefin disproportionation is the simultaneous rup- ture of both Q-and n-bonds of the chemisorbed olefins by the metal followed by recombination.66 This avoids the postulate of a cyclobutane intermediate,67 for which there is little evidence but suggests that the reaction at some stage has the character of a tetracarbene-metal complex.It has been found6* that a com- mercial cobalt molybdate catalyst shows a similar activity in propene dispropor- tionation as in ethylene formation from diazomethane implying that the sites responsible for both reactions are the same. Oxymercuration-demercuration of olefins is a useful route to alcohols and ethers with Markovnikoff orientation in the addition.69 The relative reactivities of a number of olefins have been determined in order to assess the possibility of selectiveoxymercuration-demercuration of one olefin in the presence of another. The reactivities can be rationalized in terms of carbonium ion stability strain in the double bond and steric interaction^.^' Intramolecular aminomercuration [(64)-+ (65)]has been studied as a function ofvarious parameters including length of chain and effects of substituents on the nitrogen in the chain and on the double bond.” In the case of olefin (66) the n.m.r.spectrum of the intermediate mer- curial shows it to be exclusively one of the two possible diastereoisomers (67). h5 G. Biichi and H. Wiiest Helv. Chim. Acta 1967 50 2440. “ G. S. Levandos and R. Petit Tetrahedron Letters 1971 789. h7 C. P. Bradshaw E. J. Howman and L. Turner J. Curulysis 1967,7,269;F. D. Mango Ah. Catalysis 1969 19 291. 68 P. P. O’Neill and J. J. Rooney J.C.S. Chem. Comm. 1972 104. ‘9 H. C. Brown and P. J. Geoghegan J. Org. Chem. 1970 35 1844; H.C. Brown and M.-H. Rei J. Amer. Chem. Sac. 1969 91 5646. ’O H. C. Brown and P. J. Geoghegan J. Org. Chem. 1972 37 1937 1941. ” J. J. Perie J. P. Laval J. Roussel and A. Lattes Tetrahedron 1972 28 675. Aliphatic Compounds 379 Although a complexing of mercury by nitrogen could be envisaged leading to cis-addition in fact the reaction proceeds by a formal tuans-addition of mercury and nitrogen across the double bond as is shown by detailed examination of the n.m.r. spectra of products from cis-and trans-substituted amino-olefins. Reduc- of (67) with sodium borohydride yields a mixture of (68) and (69) for which an intermediate aziridinium ion is held to be responsible. (67) NaRH4+ The ally1 Grignard reagent (70) cyclizes on heating via a Cope-like transition state (71).The product after hydrolysis was predominantly the cis-cyclopentyl- olefin (72) also obtained in the Cope rearrangement of the corresponding diene.73 A non-cyclic mechanism (73) is known to operate in intermolecular addition where intramolecular electrophilic assistance is available in the ~lefin.~~ CYH2 CC"; ., 'MgBr .-'MgBr I' 70) (71) Cycloaddition Reactions.-Although a common photochemical reaction the thermal cycloaddition of olefins to cyclobutanes is rare in hydrocarbons. Pyrolysis of 1,8-divinylnaphthalene (74) at 425 "C or in solution above 180 "C 72 J. Roussel J. J. Perie J. P. Laval and A. Lattes Tetrahedron 1972 28 701. 73 H. Felkin J. D. Umpleby E. Hagaman and E. Wenkert Tetrahedron Letters 1972 2285.74 H. Felkin and C. Kaeseberg Tetrahedron Letters 1970 4587. R.S. Atkinson and E. W.Colvin gave (75) and (76) in quantitative yield. The driving force for this conversion is the strain caused by enforced close proximity of the n-clouds of the peri-vinyl substituents. A study of the reactions of (77) and (78) suggests that the reaction is a radical one.75 (74) R = H (75) (76) (77) R = D (78) R = Ph A combination of [4+ 21 cycloaddition and 2,3-sigmatropic rearrangement has been used with effect in the synthesis of hasubanan derivatives. Thus the 1-butadienyl phenyl sulphoxide (79) reacts with enamine (80) to yield (81) as a mixture of diastereoisomeric sulphoxides. Heating (81) in the presence of a thiophilic species drives the equilibrium over to the sulphenate (2,3-sigmatropic shift) as the latter is converted into amino-alcohol (82).This annelation sequence may also be used with electron-deficient dienophiles (maleic anhydride methyl vinyl ketone) by employing 1-butadienyl phenyl sulphide (83).76 The only [2 + 2,] concerted cycloadditions of alkenes observed are those involving antarafacial components such as ketens or allenes in which one of the carbon atoms involved has no protruding substituents to hinder close approach of the alkene. To assess whether such a [2 + 2,] reaction is feasible using two 75 S. F. Nelson and J. P. Gillespie J. Amer. Chem. Sac. 1972 94 6237; J. Meinwald and J. A. Kapecki ibid. p. 6235. '6 D. A. Evans C. A. Bryan and C. L. Sims J. Amer.Chem. Sac. 1972,94 2891. Aliphatic Compounds 381 alkenes the separate reactions of cis-and trans-dideuterioethylene with tri- fluoroethylene were studied. The i.r. spectra of the reaction products from both isomers were identical and significantly different from that of authentic (84). This result is that expected from a stepwise cyclsaddition involving biradical intermediates with high loss of c~nfiguration.~~ Oxyallyl cations can be generated from e.g. 2-dimethylamino-4-methylene-1,3-dioxolans (85)7* or from ad-dibromoketones on debromination in situ with zinc in glyme. In the presence of traces of acids (85) liberates dimethylamine to generate the dioxolenium ion (86) which opens to the W-shaped allylic cation (87) stereospecifically.Addition of the latter to cyclopentadiene or furan takes place preferentially by a boat transition state (88) to give bicyclic ketones (89).79 Z = 0 or CH, R = CHO This is in contrast to the simple 2-methylallyl cation which adds preferentially via a chair transition state.*' The synthesis of 2-methoxyallyl halides (90) has been reported by two independent routes the reaction of 2-methoxypropene P. D. Bartlett G. M. Cohen S. P. Elliot K. Hummel R. A. Minns C. M. Sharts and J. Y. Fukunaga J. Amer. Chem. Sac. 1972,94,2899. H. M. R. Hoffmann K. E. Clemens E. A. Schmidt and R. H. Smithers J. Amer. Chem. Sac. 1972,94 3201. H. M. R. Hoffmann K. E. Clemens and R. H. Smithers J. Amer. Chem. SOC.,1972 94 3940. H. M. R. Hoffmann and D. R. Joy J. Chem.SOC. (B) 1968 1182. 382 R.S. Atkinson and E. W. Colvin with N-halogeno-succinimides and the pyrolysis of l-halogeno-2,2-dimethoxy-propane.8 These halogenomethoxypropenes also provide precursors for 2-methoxyallyl cations on treatment with silver trifluoroacetate.82 CH,X (90) X =C1 or Br Addition of a mixture of isomeric nitronic esters (91) and (92) to cis-olefins yields endo-adducts. With dimethyl fumarate isoxazolidines (93) and (94) are obtained.83 These isoxazolidines both consist of single invertomers at nitrogen,84 and the cycloaddition apparently proceeds under kinetic control since boiling in toluene yields the thermodynamically more stable isomers in excess by inversion at nitrogen. Hence in the cycloaddition rehybridization of the nitrogen lone pair takes place specifically on to one side of the plane of the original dipole.85 -0 0-‘\ /H N=C N=C /+ \H /+ \ Me0 Me0 C0,Me Me0,C H H-tf -CO Me N C0,Me MeO-N /o .‘C-C0,Me MeO-N /o OMe H’ HxC02Me . (931 Me0,C H r(/ Me02C H-$02Me C0,Me O\N ‘tH OMe (94) G. Greenwood and H. M. R. Hoffmann J. Org. Chem. 1972,37 61 1. 82 H. M. R. Hoffmann Angew. Chem. Internat. Edn. 1972 11 324. ” R. Gree and R. Carrie Tetrahedron Letters 1971 41 17. 84 K. Muller and A. Eschenmoser Helv. Chim. Acta 1969,52 1823;G. V. Lagodzinskaya Zhur. strukt. Khim. 1970 11 1. 85 R. Gree and R. Carrie Tetrahedron Letters 1972 2987. Aliphatic Compounds 1,3-Dipolar reagents and the classical Diels-Alder addition of dienes both usually require activated olefins and few enophiles are known which cycloadd readily to isolated double bonds.Such an enophile is the N-alkyl-N-vinyl- nitrosium ion (95) generated in situ by silver(1)-induced ionization of a-chloro-nitrones (96). Cycloaddition takes place in the presence of olefins to give (97).8b-88 Further ramifications of this method are discussed on p. 363. (96) (97) Other Reactions of 0lefins.-In spite of its popularity in undergraduate textbooks the oxidation of olefins to cis-l,2-glycols by alkaline potassium permanganate is a poor reaction giving with a few exceptions low yields. A modification uses phase-transfer catalysts (benzyltriethylammonium chloride- methylene dichloride-aqueous base) to yield 1,2-glycols in 50 % yield.' Potassium permanganate is solubilized in benzene by adding crown polyethers to form reagents which are also useful oxidants for olefins.Internal olefins are converted into carboxylic acids (if disubstituted) or ketone-acids (if trisubsti-tuted." The stereochemistry of solvolytic displacement at unsaturated (vinyl sp2) carbon has been studied using trifluoromethanesulphonates (triflates) as leaving groups. Solvolysis of Z-and E-triflates (98) and (99) in trifluoroethanol gave rise to different ratios of products (100) and (101); a greater proportion of (101) is formed from (99) than from (98). The results are best accommodated by assuming the intervention of ion pairs in which the departing triflate group shields the molecule from attack by solvent." uic-Dialkylidenecycloalkanes (102) are chiral although attempts at resolu- tion have been unsuccessful.92 The rate of racemization of (103) prepared by isomerization of (104) has been measured using n.m.r.and the coalescence of the diastereotopic methylene protons. Close approach of the inside methyl groups is entirely responsible for the activation energy of the process (88 kJ mol-') 86 U. M. Kempe T. K. Das Gupta K. Blatt P. Gygax and A. Eschenmoser Helu. Chim. Acra 1972 55 2 187. '' T. K. Das Gupta D. Felix U. M. Kempe and A. Eschenmoser Helu. Chim. Acfa 1972,55 2198. P. Gygax T. K. Das Gupta and A. Eschenmoser Helv. Chim. Acta 1972 55 2205. 89 W. P. Weber and J. P. Shepherd Tetrahedron Letters 1972 4907. 90 D. J. Sam and H.E. Simmons J. Amer. Chem. SOC.,1972,94 4024. 91 T. C. Clarke D. R. Kelsey and R. G. Bergman J. Amer. Chern. Soc. 1972 94 3626; T. C. Clarke and R. G. Bergman ibid. p. 3627; R. H. Summerville and P. von R. Schleyer ibid. p. 3629. 92 K. B. Alberman R. N. Haszeldine and F. B. Kipping J. Chern. SOC.,1952 3287; F. 9.Kipping and J. J. Wren ibid. 1957 3246. R.S. Atkinson and E. W. Colvin -0Tf Bu Bu Bu \ /OTf \ + \ /Me c=c --+ C=C-Me + c=c / /\ Me./\Me Me Me OCH,CF 1 Bu / \+ (loo) Me/C=C-Me t \ Bu \ /Me Bu \ Bu ,OCH,CF + Me./c=c\ OTf + Me / C=C-Me + Me/c=c\ Me -OTf (99) Me (103) and as expected the barrier is significantly higher than that (<50 kJ mol- ’) of (105).93 5 Carboxylic Acids Data presented94 for the product composition in the pyrolysis of calcium decanoate allow the prediction of the complete set of pyrolytic products for a given saturated monoacid salt ;unsaturated acid and diacid salts give much more diverse and numerous products.The mechanism of dehalogenative decarboxyla- tion of P-bromoacids has been subjected to kinetic ~crutiny,~’ which confirmed its assignment to the general class of heterolytic fragmentations. 93 G. F. Kiefer J. J. Levek and T. T. Bopp J. Amer. Chem. SOC.,1972 94,4751. 94 R. A. Hites and K. Biemann J. Amer. Chem. SOC.,1972,94 5772. 95 W. R.Vaughan W. F. Cartwright and B. Henzi J. Amer. Chem. SOC.,1972,94,4978. Aliphatic Compounds A polystyrene resin containing the chiral complex of copper(i1) with N-methoxycarbonyl-L-valine effects the partial resolution96 of a-aminoacids ; the L-acid co-ordinates preferentially.Equilibrium constants have been deter- mined in a number of solvents for the ring-chain tautomerism of substituted cis-3-benzoyL9 and cis-3-acyl-acrylic acids.98 The anion of methyl hydrogen di-isopropylmaleate is rapidly hydrolysed by an unassisted nucleophilic dis- placement of methoxide ion by the carboxyl ion ;this phenomenon is ascribed99 to the effect of strain in the system increasing the concentration of the tetrahedral intermediate (106) sufficiently to make the relatively unfavourable loss of methoxide significant. Pr' (106) The formation of fl-lactones as kinetic products in the halogeno-lactonization of py-unsaturated carboxylic acids is now considered'00 to be quite general.The absolute configurations of (-)-carlosic acid"' and (+)-tropic acidlo2 have been shown to be (S) and (R),respectively. 6 Carboxylic Acid Esters The hitherto elusive a-lactones have been synthesized photochemically by irra- diation of substituted malonyl peroxides at 77 K in an i.r. ce11;'03 the a-lactone produced shows carbonyl absorption at 1895 cm-and C-0 absorption at 1163 cm-;secondary photoproducts are as shown in Scheme 2. Treatment of malonyl peroxide with triphenylphosphine lo4 generates malonyl anhydrides and the corresponding ketens by two different decomposition modes of the intermediate (107) (Scheme 2). In a definitive series of papers,'" the marked acceleration by certain additives such as dimethylformamide or 1,2-dimethoxyethane on the rate of alkylation of the sodium enolate of diethyl n-alkylmalonates in benzene solution has been studied with a view to identifying the reactive species; the evidence obtained 96 R.V. Snyder R. J. Angelici and R. B. Meck J. Amer. Chem. SOC.,1972,94 2660. '' K. Bowden and M. P. Henry J.C.S. Perkin II 1972 201. 98 K. Bowden and M. P. Henry J.C.S. Perkin II 1972 206. 99 M. F. Aldersley A. J. Kirby and P. W. Lancaster J.C.S. Chem. Comm. 1972 834. loo W. E. Barnett and W. H. Sohn J.C.S. Chem. Comm. 1972,472; Tetrahedron Letters 1972 1777. J. L. Bloomer and F. E. Kappler J.C.S. Chem. Comm. 1972 1047. M. B. Watson and G. W. Youngson J.C.S. Perkin I 1972 1597. lo' 0. L. Chapman P.W. Wojtkowski W. Adam 0.Rodriquez and R. Rucktaschel J. Amer. Chem. SOC.,1972 94 1365. W. Adam and J. W. Diehl J.C.S. Chem. Comm. 1972 797. G. H. Barlow and H. E. Zaugg J. Org. Chem. 1972 37 2246; H. E. Zaugg J. F. Ratajczyk J. E. Leonard and A. D. Schaefer ibid. 2249; H. E. Zaugg and J. E. Leonard ibid. 2253. R. S. Atkinson and E. W. Colvin 0 hv 77"KI CO + -P CO + R,CO R 0 1 Ph,P v (1 07) Scheme 2 suggests either a solvent-separated ion pair or a contact ion pair with a larger interionic distance than that in the solvated ion pair and which is in equilibrium with the micellar system In a kinetic and mechanistic investigationlo6 of the isomerization of allylic esters in acetic acid catalysed by palladium(1r) chloride it has been found that two pathways are operative one involving simultaneous exchange and isomeriza- tion the other isomerization without exchange (Scheme 3).In the latter pathway labelling shows the involvement of a 1,3-acyloxonium ion. MeCH=CHCH,OCOEt S MeCH=CHCH,OAc OAc OCOEt I I CH,=CHCHMe -CH,=CHCHMe Scheme 3 The products observed in the pyrolysis of lactone toluene-p-sulphonyl- hydrazone sodium salts can be accounted for by formation of an intermediate oxycarbene.lo' Controversy'08 continues over the precise nature of the transition state in the p-cis thermolytic ester elimination reaction. Whereas one group"' interpret lob P. M. Henry J. Amer. Chem. Sac. 1972 94 1527 5200. lo' A. M. Foster and W. C. Agosta J. Amer. Chem.SOC.,1972 94 5777. log A. Tinkelenberg E. C. Kooyman and R. C. Louw Rec. Trav. chim. 1972,91 3. lo9 H. Kwart and J. Slutsky J.C.S. Chem. Comm. 1972 1182. A liphatic Compounds 387 their data as indicative of a symmetrical non-heterolytic non-planar transition state (108),analogous to other pericyclic fragmentations another author '* proposes charge development (log) defining the order of extent of movement of electrons as 1 > 2 > 3. I C HH Ar" The effect of structure on the rate of gas-phase pyrolysis of acetates and car- bonates*''has been found to be similar the faster rate of the latter being derived equally from lower activation energies and more positive entropies of activation. The ct-elimination of acetic acid from methyl acetate derivatives has been observed' '*in gas-phase pyrolysis ; a five-membered cyclic transition state is proposed the other products being interpretable on the basis of unirnolecular fragmentation and/or isomerization of the co-produced carbene (Scheme 4).lX ox H.. . . . . C X II I i'Y MeC-0-CH 0. 0 + MeC0,H + :C-Y 1 Y -C+ I Me X = H Y = OMe OPh SMe X = OMe OEt Y = OMe OEt Scheme 4 With the analogous S-methoxymethyl thioacetates a novel p-elimination reaction occurs' (Scheme 5). When 2-chloro-1-methoxyethyl acetate or thioacetate is 0 II MeC-SCH,OMe --+ MeC0,Me + CH,=S + MeCOSMe Scheme 5 pyrolysed in the gas phase hydrogen chloride elimination with rearrangement is observed114 (Scheme 6). ' R. Taylor J.C.S.Perkin 11 1972 165. D. B. Bigley and C. M.Wren J.C.S. Perkin ZI 1972 926 1744. P. C. Oele and R. Louw Tetrahedron Letters 1972,3623 (printed correctly on p. 4941). P. C. Oele and R. Louw J.C.S. Chem. Comm. 1972 848. P. C. Oele R. Louw and A. Tinkelenberg Tetrahedron Letters 1972 5159. R. S. Atkinson and E. W. Colvin ClCH=CHOMe + MeCOSH CICH,CH \ SCOMe MeCOSCH=CHOMe + HCI Scheme 6 The hindered amino-diester (1 10) has been synthesized (Scheme 7) and shows' significant activity in oivo on lympbocytic leukemia; the preferred conformation (110a) is such that its mode of action bridging RNA or DNA strands is probably similar to that of the well-known nitrogen mustard dialkylat- ing agents. 0 (1 IOa) Scheme 7 The hydrolysis of isopropenyl acetate is catalysed by organomercury(1r) and bis(organo)thallium(w) ions ; an oxymetalation-demetalation sequence is pro-posed.'' Whereas the reduction of dimethyl bismethylmalonate with sodium in xylene is reported"' to yield the keten acetal (111) in the presence of trimethylchloro- silane when the solvent is ammonia a mixture of products is formed,' l8 includ-ing the cyclopropanediol di(trimethylsily1)ether (1 12).This diol on treatment with sodium methoxide gives the products shown (Scheme 8) possibly by Cannizzaro disproportionation of the intermediate aldehyde (113). ' P. Y. Johnson and I. Jacobs J.C.S. Chem. Comm. 1972,925. P. Abley J. E. Byrd and J. Halpern J. Amer. Chem. SOC.,1972 94 1985. 'I' Y. N. Kuo F. Chen C.Ainsworth and J. J. Bloomfield Chem. Comm. !971 136. 'I8 F. Chen and C. Ainsworth J. Amer. Chem. SOC. 1972 94 4037. Aliphatic Compounds OMe Me,C(CO,Me) 4Me,C=C' \ OSiMe, (111) \ OSiMe '0-H CH,OH + XCH2OH XCH20H C0,H t-XCH0 CH20H The first case of a C-0 migration of an ethoxycarbonyl group has been observed' l9 (Scheme 9). -+ CHO OH -0 J Scheme 9 According to MO-LCAO-SCF calculations,' 2o the magnitude of the shift of the carbonyl stretching frequency in the i.r. of a-lithio-esters as compared with the non-metalated analogues indicates the degree of ionic character of the C-Li 119 F. W. Lichtenthaler and G. Bambach J. Org. Chem. 1972 37 1621. 12' J. KfiZ and P. Schmidt Tetrahedron 1972 1033. R.S. Atkinson and E. W.Coluin bond; the results obtained support a model of a strongly polarized covalent C-Li bond as opposed to the simple ionic picture. A novel synthesis12' of 0-methyl thioformate has been described. The direct introduction of the acyloxy- group has been reviewed. ' Rubrenolide (1 14) and rubrynolide (I 13 two structurally novel lactones have been isolated' 23 from the Amazonian tree Neactandra rubra. cis-Dec-5-en-l-yl-3-methyl butanoate (116) has been identified as the sex pheromone'24 of the Pine Emperor Moth (Nudaurelia cytherea cytherea Fabr.) a pest indigenous in South African pine plantations. (114) R = CH=CH2 (115) R = C-CH 7 Carboxylic Acid Amides The question of 0-us. N-protonation of amides has recently been raised anew.' Liler,'26 using U.V.spectroscopy has suggested that benzamide is 50 :50 0 :N protonated in 80% sulphuric acid. However a considerable body of evidence has been produced to show that protonation or Lewis acid complexation of amides consistently occurs by co-ordination with oxygen. Studies have included the acidity dependent changesI2' in the n-n* and n-n* bands of aliphatic amides; kinetic evidenceI2' based on rate data for acid-catalysed amide hydrolysis in both dilute and concentrated acid ; an n.m.r. study' 29 of proton-exchange rates where for N-methylacetamide the molar ratio of 0 :N protonated species is greater than lo6; a "N n.m.r. of adducts of boron trifluoride and antimony pentachloride with '5N-labelled ureas where the hybridization- dependent "N-H coupling constants were inconsistent with N-co-ordination ; Iz1 D.H. Holsboer and H. Kloosterziel Rec. Truu. chim. 1972 91 1371. 22 D. J. Rawlinson and G. Sosnovsky Synthesis 1972 1. N. C. Franca 0. R. Gottleib D. T. Coxon and W. D. Ollis J.C.S. Chem. Comm. 1972 514. 124 H. E. Henderson F. L. Warren 0. P. H. Augustyn B. V. Burger 0. F. Schneider P. R. Boshoff H. S. C. Spies and H. Geertsema J.C.S. Chem. Comm. 1972 686. 125 M. Liler J.C.S. Perkin IZ 1972 816. 126 M. Liler J.C.S. Chem. Comm. 1972 527. '' H. Benderly and K. Rosenheck J.C.S. Chem. Comm. 1972 179. C. R. Smith and K. Yates Canud. J. Chem. 1972,50 771. 129 R. B. Martin J.C.S. Chem. Comm. 1972 793. 130 P. Stilbs Tetrahedron Letters 1972 227. Aliphatic Compounds 39 1 i.r.and n.m.r. evidence ;13' catalysis of the imidate-imide rearrangement'32 by Lewis acids and alkyl halides ;and finally the effect of alkali-metal cations on the torsional barrier about the N-C(0) bond in amides,'33 when it was found that lithium ions served to increase this barrier an effect ascribed to complexa- tion with oxygen increasing the importance of (1 17) as a contributing structure complexation with nitrogen would decrease the barrier by preventing delocaliza- tion of the N lone pair of electrons into the N-C(0) bond. R 0-\t / R /N=C\ R Several N-t-butoxyamido radicals' 34 and amido free radicals' 35 themselves have been generated and observed by e.s.r. spectroscopy; a n ground state is unanimously assigned.The paramagnetic species generated and studied by a French group,136 who interpreted them as amido radicals have now been shown to be the corresponding acyl nitroxides.135" A combination of 3C Fourier transform and partially relaxed Fourier trans- form spectroscopy has been utilized' 37 to determine steric compression shifts in aliphatic amides and oximes allowing a sensitive study of internal rotation etc. The synthesis of ON-bis(trimethylsily1)formamidehas been reported,' 38 and some of its reactions have been determined (Scheme 10). OSiMe, /OSiMe3 RCOCH,COR 1 HCONH 1,H-C R-C=CHCOR \\ \-NSiMe + HCONHSiMe, RCH,CN R \ C=CHNH + Me,SiOSiMe, / NC' Reagent i Me,SiCi-Et,N Scheme 10 L. Hendriksen and B. Baltzer Tetrahedron Letters 1972 2485.32 B. C. Challis and A. D. Frenkel J.C.S. Chem. Comm. 1972 303. W. Egan T. E. Bull and S. Forsen J.C.S. Chem. Comm. 1972 1099. 134 T. Koenig J. A. Hoobler and W. R. Mabey J. Amer. Chem. Soc. 1972,94 2514. 135 (a) W. C. Danen and R. W. Gellert J. Amer. Chem. Soc. 1972 94 6853; (6) Y. L. Chow and R. A. Perry Tetrahedron Letters 1972 53 1. 136 P. Tordo E. Flesia and J. M. Surzur Tetrahedron Letters 1972 183; P. Tordo E. Flesia J. M. Surzur and G. Labrot Tetrahedron Letters 1972 1413. '" G. C. Levy and G. L. Nelson J. Amer. Chem. SOC.,1972 94,4897. 13* W. Kantlehner W. Kugel and H. Bredereck Chem. Ber. 1972 105 2264. 392 R.S. Atkinson and E. W. Colvin An unsymmetrical concerted transition state has been assigned to the thermal p-elimination reaction of alkyl NN-dimethylcarbamates ; both and systems lead to the conclusion of a unimolecular reaction involving heterolytic rather than homolytic fission.A new type of basic amide hydrolysis characterized by N-alkyl fission has been rep~rted,'~' and is exemplified in Scheme 11. N=NPh -+ RCON=C(Me) Scheme I1 8 Nitriles A proposed step in the prebiotic polymerization of HCW the dimerization of iminoacetonitrile (118) to the HCN tetramer diaminomaleonitrile (119),has been achieved142 with the N-t-butyl derivative of (118). Di-iminosuccinonitrile (120) an oxidized form of the tetramer has been prepared'43 as shown (Scheme 12); catalytic reduction afforded diaminomaleonitrile. Ti i NC-CN + 2HCN -!+ NcHNH HN CN Reagents i Et,N; ii H2-Pd/C ( 120) Scheme 12 9 Ketones The search for the hypothetical oxyallyl zwitterion (121) and the postulations of its intermediacy continue unabated ;reductive transformations of aa'-dibromo-ketones plausible precursors of the zwitterion have been extensively studied.IJ9 H. Kwart and J. Slutsky J.C.S. Chem. Comm. 1972 552. 14' N. J. Daly and F. Ziolkowski J.C.S. Chem. Comm. 1972 911; CJ refs. 15 16 17 ''I F. H. Stodola J. Org. Chem. 1972 37 178. H. Dabek R. Selvarajan and J. H. Boyer J.C.S. Chem. Comm. 1972 244. 143 0. W. Webster Angew. Chem. Internat. Edn. 1972 11 153. Aliphatic Compounds 393 Reduction of such ketones with a Zn-Cu couple in DMF yields 2-dimethyl- amino-4-methylene-l,3-dioxolans(122) by formal 1,3-dipolar addition of solvent to the hypothetical zwitterion ;144 such species are reported'45 to decompose spontaneously in the presence of suitable dienes to give adducts whose structures suggest the intermediacy of the parent zwitterion.Interest-ingly mild acid-catalysed hydrolysis of the dioxolan (122) affords 2,4-dimethyl- penta-l,3-dien-3-01(123) as a relatively stable aliphatic enol ;146such stability has been rationalized in an ab initio MO of the effects of a-substitution on keto-enol tautomerism when it was found that for all the substituents studied the enol structure was stabilized preferentially to the keto form owing to greater interaction with the double bond. When methanol is used as solvent the reduc- tion of ad-dibromoketanes with a Zn-Cu couple leadsI4* to the parent ketone and the a-methoxyketone :once again the oxyallyl zwitterion is implicated.OH H Me+H Me Me Electrochemical reduction of m'-dibromoketones has been studied' 49 as a potential route to cyclopropanones; no such species was detected and the products obtained were rationalized in terms of an oxyallyl zwitterion or an enol allylic bromide. Later however cyclopropanone hemiacetals were isolated '50 from such reductions in methanol. The oxyallyl zwitterion is involved in a mechanistic s~herne'~' to explain the production of cycloheptenones cyclopentanones and cyclopentenones' 52 by the reaction of ad-dibromoketones with iron carbonyl (Scheme 13). The intermediacy of organoiron halide complexes is proposed 153 to account for the products observed in the reaction of a-halogenoketones with iron pentacarbonyl.has presented a correlation between the conjugate addition of lithium dimethylcopper to ap-unsaturated carbonyl compounds and their L44 H. M. R. Hoffmann K. E. Clemens E. A. Schmidt and R. H. Smithers,J. Amer. Chem. SOC.,1972 94 3201. 145 H. M. R. Hofmann K. E. Clemens and R. H. Smithers J. Amer. Chem. SOC.,1972 94 3940. 146 H. M. R. Hofmann and E. A. Schmidt J. Amer. Chem. Soc. 1972,94 1373. 14' W. J. Hehreand W. A. Lathan J.C.S. Chem. Comm. 1972 771. 14' H. M. R. Hofmann T. A. Nour and R. H. Smithers J.C.S. Chem. Comm. 1972,963. 149 J. P. Dirlam L. Eberson and J. Casanova J. Amer. Chem. Soc. 1972,94,240. I5O A. J. Fry and R. Scoggins Tetrahedron Letters 1972 4079.151 R. Noyori Y. Hayakawa M. Funakura M. Takaya S. Murai R. Kobayashi and S. Tsutsumi J. Amer. Chem. SOC.,1972 94 7202. 152 R. Noyori K. Yokoyama S. Makino and Y. Hayakawa J. Amer. Chem. SOC.,1972 94 1772. H. Alper and E. C. H. Keung J. Amer. Chem. SOC.,1972,94 2566. 54 H. 0.House and M. J. Umen J. Amer. Chem. SOC.,1972,94 5495. 1540H.0.House L. E. Huber and M. J. Umen J. Amer. Chem. SOC.,1972 94 8471. R. S. Atkinson and E. W. Coluin A Br Br r6 R 0 Scheme 13 polarographic reduction potentials. Such a correlation is compatible with the first step of the mechanism (Scheme 14)proposed for this reaction and in conjunc-tion with empirical rules for the estimation (with an accuracy of k0.l V) of the reduction potentials of such unsaturated compounds 54a offers the ability to predict both when conjugate addition will be successful and when reduction of the olefinic linkage will be a serious competitor.A plausible explanation of the preference for 1,2-or 1,4-addition1 55 or reduction' s6 processes of o$-unsaturated carbonyl compounds has been advanced based on Pearson's theory of hard and soft acids and bases the 4-carbon being considered softer than the 2-carbon; rational variation of reagent then permits alteration of this preference. -+ [A]'R-C=C-CHR I 0-Me R-CH-CH=C-R + (MeCu) + 0.5 (LiCuMe,) + 1 I Me 0Li R 0-Scheme 14 The reduction of P-chlorovinyl aldehydes by zinc powder leads to selective removal of the @-chlorine substituent but if the reduction is performed in the 15s 0.Eisenstein J.-M. Lefour C. Minot N. T. Anh and G. Soussan Compt. rend. 1972 274 C 1310. 156 J. Bottin 0.Eisenstein C. Minot and N. T. Anh Tetrahedron Letters 1972 3015. Alip hatic Compounds presence of air reductive coupling is observed'57 (Scheme 15). The mechanism of such one-electron reductive coupling reactions has been subjected to further ~crutiny.'~' R' R' \/ H/C=C\CHO R' --< \/ /c=c\R2 C1 CHO CH =C(R2K;f2 [K Scheme 15 The first example has been reported'" of a normal base-catalysed intermolecu- lar condensation of a diazomethyl ketone with two potential reaction sites (Scheme 16). PhCH2COCHN2 + PhCHO -+ PhCH,COCCH(OH)Ph It N2 Scheme 16 The influence of the structure of the aldehyde on the regioselectivity of base- catalysed mixed ketolization with methyl methylene ketones has been studied.In the base-induced bromination of butan-2-one in aqueous solution each hydro- gen on the 1-carbon and the 3-carbon is attacked equally fast to form an enolate,16' the resulting enolates rapidly giving bromoform and propionate and lactate salts respectively (Scheme 17). Contrary to currently held views the variation in enol content as a function of structure in fl-diketones is due to varia- tion in energy of the ketones and not of the enols; the situation is reversed however in Ij-ketoesters.'h2 The heats of formation of aliphatic ketones have been further studied,'63 and an improved force-field method for the calculation of the structures and energies of carbonyl compounds has been described.Ab initio calculation^'^^ on acetone show that the methyl groups have hydrogen eclipsing oxygen in the ground state and that the rotational barrier is 0.99 kcal mol- '. '" A. Hara and M. Sekiya Chem. and Pharm. Bull. (Japan),1972 20 309. '" S. L. Thuan and J. Wiemann Bull. SOC.chim. France 1972 1861. 159 N. F. Woolsey and M. H. Khalil J. Org. Chem. 1972 37 2405. 16' J.-E. Dubois and P. Fellmann Tetrahedron Letters 1972 5085. 16' C. G. Swain and R. P. Dunlap J. Amer. Chem. SOC.,1972,94 7204 Ih2 P. Alcais and R. Brouillard J.C.S. Perkin 11 1972 1214. 163 J.-E. Dubois and H. Herzog J.C.S. Chem. Comm. 1972 932. 164 N. L. Allinger M. T. Tribble and M. A. Miller Tetrahedron 1972 1173.165 N. L. Allinger and M. J. Hickey Tetrahedron 1972 2157. R.S. Atkinson and E. W. Colvin 0 0 i-CHBr 1 + Scheme 17 The utility of bifunctional reagents such as oxalic acid monohydrazide for the resolution of racemic carbonyl compounds has been reported.'66 A four-centred transition state (124) is proposed'67 for the bimolecular substitution of P-halogenoketones and related compounds (Scheme 18). 0 0 ] II RCCH,CH,X + Y-+ [R-cIyH2 -+ RCCH,CH,Y II YfiXCH --Xb-+ X-Scheme 18 Glyoxal reacts with ethyl carbamate under acid catalysis to give 1,1,2,2-tetra(ethoxycarbony1amino)ethanel6*(125) and not as previously reported glyoxal bis(ethoxycarbony1imide) (126). Gyrinal (127) has been identified'69 as an arthropod defensive substance.0 CH(NHCO,Et) CH=NCO,Et I I CH(NHCO,Et) CH=NCO,Et (125) (126) (127) 10 Amines Even a complete thermodynamic analysis'" of the anomalous order of amine basicities in solution utilizing gas-phase basicities' 71-1 72 and thermodynamic 166 H. Kaehler F. Nerdel G. Engemann and K. Schwerin Annalen 1972 757 15. *" E. N. Trachtenberg and T. J. Whall J. Org. Chem. 1972 37 1494. 16' G. F. Whitfield R. Johnson and D. Swern J. Org. Chem. 1972. 37.95. L69 H. Schildknecht H. Neumaier and B. Tauscher Annalen 1972 756 155. E. M. Arnett F. M. Jones M. Taagepera W. G. Henderson J. L. Beauchamp D. Holtz and R. W. Taft J. Amer. Chem. SOC.,1972 94 4724. D. H. Aue H. M. Webb and M. T. Bowers J. Amer. Chem. SOC.,1972,94,4726.172 J. P. Briggs R. Yamdagni and P. Kebarle J. Amer. Chem. SOC.,1972 94 5128. Aliphatic Compounds dissolution properties has failed to provide an easy interpretation of this phenomenon. Much attention has been paid to aliphatic azoxy-compounds partly due to their potent physiological properties which range from therapeutic antibiotic behaviour to high carcinogenicity. Following a model of isomeric azoxy-compounds and a revision' 74 of absolute stereochemistry the antibiotics elaiomycin (128) and LL-BH 872a (129) have now been both assigned the Z stereochemistry for the azoxy-group. The methyl diazonium cation (130) has been identified'25 as the reactive methylating species of the toxin aglycone Z-methyl-ONN-azoxymethanol (13 1). A previously reported process' 76 has H MH Me(CH,),CH N=N CH,OMe J 'c( 0 I CHOH HH Me(CH,),CH >=(N=N.,COCH,OH Me + \ (131) been generalized' 77 to provide a flexible and directed synthesis of azoxyalkanes (Scheme 19). The Z-E thermal isomerization of aliphatic azodioxy-compounds Et \R2 Reagents i Et,OBF,-CH,CI,; ii R21-HMPA Scheme 19 K. G. Taylor and T. Riehl J. Amer. Chem. SOC.,1972,94,250. W. J. McGahren and M. P. Kunstmann J. Org. Chem. 1972,37 902. M. H. Benn and P. Kazmaier J.C.S. Chem. Comm. 1972 887. "' R. A. Moss and M. J. Langdon Tetrahedron Letters 1969 3897. R. A. Moss M. J. Langdon K. M. Luchter and A. Mamantov J. Amer. Chem. SOC. 1972,94,4392. R. S. Atkinson and E. W. Colvin (dimeric nitrosoalkanes) has been shown to occur exclu~ively'~~ by a dissocia- tion-recombination mechanism.In the reductive rearrangement of t-nitrosoalkanes with trialkyl phosphites the nitrene (132) was invoked' 79as a possible intermediate. Later results indicatedI8' that the phosphate ester (133) decomposed directly in a concerted process to the observed products. R3C-N=0 + (EtO),P --+ P-+R,C-N-bO-P(OEt)3I S R + R,C-N (132) + (EtO),PO (133) (EtO),PO + R,C=NR + R,CO + RNH Scheme 20 It has been shown18' by isotopic labelling that the exchange reaction of nitroso- alkanes with nitric oxide occurs via oxygen-atom transfer and not by a displace-ment mechanism (Scheme 21). Scheme 21 Microwave spectroscopic studies on aminoacetonitrile and some deuteriated analogues indicate'82 that the amino and methylene groups are trans to each other (134).H H H35H The preparation of pure di-imine by high-vacuum pyrolysis of the sulphon- amide salt (135) has been reportedIg3 (Scheme 22); it is a yellow liquid unstable above -150 "C. Th. A. J. W. Wajer and Th. J. De Boer Rec. Trav. chtm. 1972,91 565. B. Sklarz and M. K. Sultan Tetrahedron Letters 1972 1319. R. Abramovitch J. Court and E. P. Kyba Tetrahedron Letters 1972 4059. 181 P. J. Carmichael B. G. Gowenlock and C. A. F. Johnson J.C.S. Perkin 11 1972 1379. I a2 J. N. Macdonald and J. K. Tyler J.C.S. Chem. Comm. 1972 995. lS3 N. Wiberg H. Bachhuber and G. Fischer Angew. Chem. Internat. Edn. 1972,11,829. Aliphatic Compounds 399 \ Tos ,N-N H Tos /' \ -L N-N /H-J!+ HN=NH H \H Na / \H (135) Reagents i NaN(SiMe,),; ii heat Scheme 22 11 Alcohols A general method lS4 for distinguishing threo from erythro diastereoisomers of certain a-glycols and related compounds has been described based on the detection of W long-range coupling or an intramolecular Overhauser effect respectively in the n.m.r.spectra of the corresponding acetonides (Scheme 23). R' RZ 'W' XH -Me YH RZ 'NOE' Scheme 23 Chiral alcohols have significant Cotton effects between 185 and 198 nm. Where the preferred conformation of the hydroxy-group cannot be defined unambiguously Cotton effects are small but for the majority of compounds with clearly preferred conformations the observed sign'85 agrees with the prediction.A new procedure'86 for the identification and analysis of A3- A4- or A5-unsaturated long-chain alcohols and acids has been reported ; intramolecular participation of the alcohol function (after reduction of the acid) in an oxy-mercuration-demercuration sequence affords 2-alkyltetrahydropyranyl and/or 2-alkyltetrahydrofuranyl ethers. K. Nakanishi D. A. Schooley M. Koreeda and I. Miura J. Amer. Chern. SOC.,1972 94 2865. D. N. Kirk W. P. Mose and P. M. Scopes J.C.S. Chem. Comm. 1972 81. F. D. Gunstone and R. P. Inglis J.C.S. Chem. Comm. 1972 12. R. S. Atkinson and E. W.Colvin 12 Ethers The catalysed formation of dimethyl ether from hydrogen and carbon dioxide has been rep~rted.'~' Non-bonded 0-0 interactions in straight-chain diethers have been investigated.lg8 13 Sulphur Compounds Low-valency sulphur acids continue to arouse interest.The fundamental yet elusive sulphenic acids (136) have now been generated and trapped (Scheme 24).189 H C0,Me \/ /c=c\ RS(0) H Scheme 24 An analogous route has been de~eloped''~ for the synthesis of the hitherto unknown sulphoxylic acids (1 37) (Scheme 25). /L\ H C0,Me \/ RSS(0) Scheme 25 S. Naito 0. Ogawa M. Ichikawa and K. Tamaru J.C.S. Chem. Comm. 1972 1266. '" M. Mansson Acta Chem. Scand. 1972 26 1707. E. Block J. Amer. Chem. SOC.,1972 94 642. l9' E. Block J. Amer. Chem. SOC.,1972 94 644. Aliphatic Compounds The facile cleavage of alkyl disulphides by silver nitrate and sodium methane- sulphinate provides a new route'" to thiolsulphonate esters (Scheme 26).The reduction of sulphoxides to sulphides with sodium hydrogen sulphite has been R-S-S-R + Ag' 0 1 RSAg + MeS0,SR Scheme 26 subjected1g2 to mechanistic investigation. Convenient methods for the prepara- tion of sulphilimines' 939194 and sulphoximine~~ 94 have been reported. The undeca-1,3,5-trienes present in the essential oil of the seaweed Dicty-opteris could conceivably arise from two new comp~nents'~~ of this oil the disulphide (138) and the thioacetate (139). \ 12 (138) (139) 14 Miscellaneous A MIND0/2 calculation of the reaction path for the methyl isocyanide to acetonitrile rearrangement predictsIg6 a stable triangular intermediate with the properties of a n-complex rather than an ion pair.Ab initio SCF calculations '97 substantiate this proposal. Further MIND0/2 calculation^'^^ have been carried out on the rotational barrier about C-C single bonds; in an ab initio study,lg9 the calculated energies of rotation were rationalized in terms of contributions from three principal effects staggered arrangements of bonds are preferred ;the axis of a lone-pair orbital prefers to be coplanar with an adjacent electron-withdrawing polar bond or orthogonal to an adjacent lone pair ;dipole moment components perpendicular to the internal rotation axis tend to be opposed. '')I M. D. Bentley I. B. Douglass and J. A. Lacadie J. Org. Chem. 1972 37 333. 19' C. R. Johnson C. C. Bacon and J. J. Rigau J. Org.Chern. 1972 37 919. 193 N. Furukawa T. Omata Y. Yoshimura T. Aida and S. Oae Tetrahedron Letters 1972 1619. '94 Y.Tamura K. Sumoto J. Minamikawa and M. 1. Keda Tetrahedron Letters 1972 41 37. '95 R. E. Moore J. Mistysyn and J. A. Pettus J.C.S. Chem. Comm. 1972 326. 196 M. J. S. Dewar and M. C. Kohn J. Arner. Chem. SOC.,1972 94 2704. 19' D. H. Liskow C. F. Bender and H. F. Schaefer J. Amer. Chem. SOC.,1972,94 5178. 19' M. J. S. Dewar and M. C. Kohn J. Amer. Chem. SOC.,1972,94 2699. '99 L. Radom W. J. Hehre and J. A. Pople J. Amer. Chem. SOC.,1972,94 2371. R. S. Atkinson and E. W. Colvin A 'general' route reported"' for the preparation of a-diazoesters (Scheme 27) while successful for ethyl diazoacetate has been found to fail2'' with ethyl a-diazopropionate.R' R' \ \ C=PPh + TosN -+ C=N, / / R202C R202C Scheme 27 G. R. Harvey J. Org. Chem. 1966 31 1587. M. B. Sohn M. Jones jun. M. E. Hendrick R. R. Rando and W. von E. Doering, Tetrahedron Letters 1972 53.

ISSN:0069-3030    

DOI:10.1039/OC9726900367

出版商:RSC    

年代:1972

数据来源: RSC

摘要:

12 Alicyclic Compounds By J. M. MELLOR Department of Chemistry University of Southampton Southampton SO9 5NH The Chemical Society Specialist Periodical Report gives a comprehensive survey of literature published in 1970 and 1971 and a further volume will cover 1972. The selectivity of this present review has precluded coverage of annulenes,2 the synthesis of prostaglandin^,^ cy~loalkynes,~ cyclob~tenediones,~ macro-cyclic compounds,6 and the extensive studies of the sigmatropic rearrangements of polycyclic hydrocarbons of which a part the rearrangement of (CH) com-pounds' and of (CH), hydrocarbons,8 has been excellently reviewed. The chemistry of carbocyclic four-membered-ring compounds has been extensively reported.' 1 Structural Aspects The development of a new spectroscopic technique that permits an unambiguous structural assignment where all other techniques have failed is noteworthy.The structure of the triene (l),obtained by alkylation of the pentachlorocyclopenta- dienyl anion could only be determined" by ESCA. Although differences in (1) carbon 1s electron binding energies in hydrocarbons are small (ethane 290.6 ethylene 290.7 and acetylene 291.2 eV) in carbonium ions the positive charge leads to increased 1s binding energies and the differences can be resolved ' 'Aliphatic Alicyclic and Saturated Heterocyclic Chemistry' ed. W. Parker (Specialist Periodical Reports) The Chemical Society London vol. 1 1973. F. Sondheimer Accounts Chem. Res. 1972 5 81. W. Ried and A. H. Schmidt Angew.Chem. Internat. Edn. 1972 11 997. W. P. Schneider Prostaglandins Prog. Res. 1972 293. H. Meier Synthesis 1972 235. P. R. Story and P. Busch Adv. Org. Chem. 1972 8 67. ' L. T. Scott and M. Jones Chem. Rev. 1972,72 181. S. Masamune and N. Darby Accounts Chem. Res. 1972,5,272. 'Methoden der Organischen Chemie' (Houben-Weyl) ed. E. Muller Georg Thieme Verlag Stuttgart 1971 vol. IV/4. '" D. T. Clark W. J. Feast M. Foster and D. Kilcast Nature 1972 236 107. 403 404 J. M. Mellor experimentally. Spectra have been recorded ’’of the cyclopentyl l-methylcyclo- pentyl norbornyl and 2-methylnorbornyl cations as salts in frozen solutions of AsF,. In contrast to the other cations which have considerable charge localiza- tion the norbornyl cation has an ESCA spectrum indicative of extensive charge delocalization.The ‘non-classical’ nature of the ion is thus confirmed; if there were equilibration between classical structures the relatively fast time-scale of the photoionization process would lead to a spectrum characteristic of the classical structure with little charge delocalization. The use of photoelectron spectroscopy using a U.V. source is now well estab- lished as a technique which can show intramolecular interactions and help to distinguish between ‘through-bond’ and ‘through-space’ effects. Heilbronner’ has elegantly analysed these effects in (2). There is greater conjugation between the exocyclic double bond and the cyclopropane ring than between the two double bonds. The development of an understanding of how ‘through-bond’ and ‘thrcrugh-space’ effects control chemical reactivity may be expected.The ready photochemical ring-closure of norbornadiene to give quadricyclane (3) may be explained by the promotion of an electron from an orbital which is anti- bonding with respect to the isolated double bonds to an orbital which is bonding. Hypostrophene (4)fails to undergo photocycloaddition. The view is advanced that through-bond coupling between high-lying CT levels and the .n level leads to a reversal in order of the two 7r levels and hence the photocycloaddition is no longer symmetry-allowed. The spectrum of hypostrophene and MIND0/2 calculations give some support to this view. Recent contact-shift studies [Annual Reports (B) 1971 68 5671 and now the e.s.r.spectra of the 1-adarnantyl (5)and l-bicyclo[2,2,2]octy1 (6) radicalsI4 show the important geometrical requirements for through-bond interactions. The (2.69) & (3.08) (0.89) (5) G. A. Olah G. D. Mateescu and J. L. Riemenschneider J. Amer. Chem. Soc. 1972 94 2529. E. Heilbronner and H.-D. Martin Hefv. Chim. Acta 1972,55 1490. l3 W. Schmidt and B. T. Wilkins Tetrahedron 1972 28 5649. l4 P. J. Krusic T. A. Rettig and P. von R. Schleyer J. Amer. Chem. Soc. 1972 94 995. AIicyclic Compounds large coupling to the axial hydrogens in (5) by a favoured pathway contrasts with that to the equatorial hydrogens (see observed G values). Applications of n.m.r. spectroscopy are described elsewhere [Annual Reports (B),1972,69 191 but the use of shift reagents now re~iewed,'~ is well established and the possibility of obtaining first-order 'H spectra from deuteriated com- pounds' b containing a small amount of randomly distributed 'H is promising.The importance of through-bond effects in determining the chiroptical proper- ties of substituted ketones is now recognized [Annual Reports (B) 1971,68 1031. Such effects make hazardous the assignments of configuration to strained or caged ketones based upon modified octant rules. This is illustrated by the case of twistane (7)and the related ketones (8)and (9). Absolute configurations have been reassigned' on the basis of chemical correlations. Application of Brewster's conformational dissymmetry model or of the simple octant rule to (8)or (9) leads to incorrect assignments.It is noted that the helical conductor model of optical activity" gives the absolute configuration correctly. Snatzke19 analyses the failure of the simple octant rule as applied to (8),(9),and (10)and concludes that a correct application of the rule using the method of spheres with recognition that in (10) a situation similar to that of a p-axially substituted cyclohexanone applies also gives the configurations correctly. In such a cyclohexanone 'the sign of the contribution to the Cotton effect is usually opposite to that predicted from the original octant rule'.' As through-bond substituent effects will have strong conformational dependence the analogy of (10)to a simplecyclohexanone is doubtful.The difficulty ofpredicting whether a substituent will have an octant or anti-octant effect is highlighted. The synthesis of compounds in which chirality derives from deuterium substitu- tion is well established. Now the first alicyclic compound in which chirality derives from "0 substitution is reported.20 The c.d. spectrum of the quinone (11) has structured bands at 280 and 480 nm of opposite sign. The capacity to measure such weak spectra is noteworthy. Theoretical analyses have been made of systems in which 'bond stretch isomerism' might be possible. EH calculations suggest that tricyclo[2,2,2,0]- '' R. von Ammon and R. D. Fischer Angew. Chem. Internat. Edn. 1972 11 675. Ib J. J. Katz G. N. McDonald and A. L. Harkness J.C.S. Chem. Comm. 1972 542. '' M.Tichy Tetrahedron Letters 1972 200 1. J. H. Brewster Tetrahedron Letters 1972 4355. G. Snatzke and F. Werner-Zamojska Tetrahedron Letters 1972 4275. W. C. M. C. Kokke and L. J. Oosterhoff J. Amer. Chem. Soc. 1972,94 7583. 406 J. M. Mellor (11) octane has two energy minima,2 representing the strained propellane (1 2) and the stabilized biradical (13). Similarly bond-stretch isomerism may relate (14)22 and (15). The energy barrier to interconversion of (12) and (13) a simple bond- stretchingprocess is expected to be significant because of the constraints of orbital symmetry. Of the symmetric and antisymmetric combinations of the orbitals at C-1 and C-4 the former is stabilized by through-space interaction and the latter by through-bond interactions.Subsequent ab initio calculation^^^ suggest minima at -1.52 and 2.60 A separated by an energy barrier of -29 kcal mol-’. Significantly although (16) shows high reactivity with many reagents24 it is thermally remarkably stable (t+ at 195°C -20h). However all attempts to prepare (12) failed :only (17) was obtained suggesting a failure of a 1,4-biradical to close. Aspects of propellane chemistry have been reviewed :25 syntheses of higher propellanes are reported later. (13) (15) (17) Ab initio calculations give accurate data for low-molecular-weight hydro- carbons” and details are now given of their use to chart the nature of an organic transition state the species involved in the ‘geometrical isomerization’ of cyclo- propane27[cfi Annual Reports (B) 1971 68,821.Does this herald future redun- dancies for experimental chemists? Following improvements to MIND0/2 application2* to classical carbonium ions leads to results in good agreement with experiment but analysis of the C3H7 species the edge-protonated cyclo- + propane (18) the corner-protonated cyclopropane or n-complex (19) and the 21 W. D. Stohrer and R. Hoffmann J. Amer. Chem. SOC.,1972,94 779. 22 W. D. Stohrer and R. Hoffmann J. Amer. Chem. SOC. 1972,94 1661. 23 M. D. Newton and J. M. Schulman J. Amer. Chem. SOC. 1972,94,4391. 24 K. B. Wiberg and G. J. Burgmaier J. Amer. Chem. SOC.,1972,94 7396. l5 D. Ginsburg Accounts Chem. Res. 1972 5 249. 26 D. R. Whitman and J. F. Chiang J. Amer. Chem. SOC.,1972,94 1126; M. D. Newton and J.M. Schulman ibid. p. 767 773; L. Radom J. A. Pople V. Buss and P. von R. Schleyer ibid. p. 3 11 ;J. S. Wright and L. Salem ibid. p. 322. 27 J. A. Horsley Y. Jean C. Moser L. Salem R. M. Stevens and J. S. Wright J. Amer. Chem. SOC. 1972,94,279. 28 N. Bodor M. J. S. Dewar and D. H. Lo J. Amer. Chem. SOC.,1972,94 5303. Alicyclic Compounds 407 centre-protonated cyclopropane (20) leads to different conclusions from those obtained by ab initio methods. MIND0/2 methods show (18) to be more stable than the n-propyl cation and to be the only stable species :ab initio method^^^.^^ show two stable species the n-propyl cation and a corner-protonated species. Force field calculations have now been applied to olefins3’ and to ketones.31 With minor improvements of parametrization heats of formation and strain energies are now available for ketones containing up to 60 atoms.CH t3 H ( 19.) (18) The influence of heteroatom substituents upon conformational equilibria requires further investigation. The preference of the diaxial conformer of trans- 1,4-dichlorocyclohexane over the diequatorial conformer has been explained32 by attractive chlorine-hydrogen interactions but in 3,3-dimethylhalogenocyclo-hexanes it is suggested3 that the repulsive syn-axial methyl-halogen interaction considerably destabilizes that conformer with axial halogen. In the extensive studies of conformational equilibria of cr-halogenocyclohexanones,34results are interpreted on the basis of the relative polarizabilities of the halogens.The geometrical requirements of orbital interactions may determine preferred conformations. The importance of possible minor changes in the geometry of a cyclohexyl ring by introduction of a t-butyl group has been much discussed. Crystallo- graphic analyses of (21)35and (22)36show in contrast to earlier predictions that there is little twisting of the t-butyl group. Minor differences in the two analyses are to be noted but distortion is slight and only a small flattening of the cyclohexane ring occurs. Greater flattening occurs in (23)37and (24).38 Analysis of (25) indicates3’ equilibration between chair and twist-boat conformers and shows the hazard of assuming that a t-butyl group freezes a cyclohexane ring in a chair conformation.29 L. Radom J. A. Pople V. Buss and P. von R. Schleyer J. Amer. Chem. SOC.,1971 93 1813. 30 N. L. Allinger and J. T. Sprague J. Amer. Chern. SOC.,1972,94 5734. 31 N. L. Allinger M. T. Tribble and M. A. Miller Tetrahedron 1972 28 1173. 32 R. J. Abraham and Z. L. Rossetti Tetrahedron Letters 1972 4965. 33 D. S. Bailey J. A. Walder and J. B. Lambert J. Amer. Chem. SOC., 1972 94 177. 34 J. Cantacuzene R. Jantzen and D. Ricard Tetrahedron 1972 28 717. 35 P. L. Johnson C. J. Cheer J. P. Schaefer V. J. James and F. H. Moore Tetrahedron 1972,28 2893. 36 R. Parthasarathy J. Ohrt H. B. Kagan and J. C. Fiaud Tetrahedron 1972 28 1529. ” P. L. Johnson J. P. Schaefer V. J. James and J. F. McConnell Tetrahedron 1972,28 2901. 38 H. J. Geise F. C.Mijlhoff and C. Altona J. Mol. Structure 1972 13 21 1. 39 P. L. Barili G. Bellucci G. Ingrosso F. Marioni and I. Morelli Tetrahedron 1972 28 4583. 408 J. M. Mellor (21) X = OTS (22) X = OBB Bicyclo[2,2,2]octane (26) is known to have a broad energy minimum for twist- ing about the C-1-C-4 axis. Now force field calculations show4’ a similar flexibility in bicyclo[3,2,2]nonane (27) bicyclo[3,3,2]decane (28) and homo- adamantane (29). Preferred conformations ofsubstituted compounds will depend upon the nature of the substituent. A detailed analysis of factors controlling rates of formation of lactones from bicyclic hydroxy-acids is used4’ to support Koshland’s controversial concept of orbital steering. Although the importance of orientation in determining reaction rate is of great importance Koshland’s work still seems to underestimate the possible consequences of twisting in both bicyclo[2,2,2]octanes and bicyclo[2,2,l]heptanes.Bredt’s Rule has continued to provoke doubting synthetic chemists. Although decarb~xylation~’ of (30) does not proceed uia the strained enol(31) bisdehalo- genati~n~~ of (32) or (33) gives a similar ratio of the adducts (34) and (35). Reasonably the intermediacy of (36) is concluded although the nature of the bonding in this species is not clear. Pyrolysis44 of the thionocarbonate (37) in the presence of 1,3-diphenylisobenzofuran gave adducts of bicyclo[3,2 lloct- 1-ene (38). Similarly good evidence for the intermediacy of adamantene is 40 E.M. Engler L. Chang and P. von R. Schleyer Tetrahedron Letters 1972 2525. 41 D. R. Storm and D. E. Koshland J. Amer. Chem. SOC.,1972 94 5805 5815. ‘* G. L. Buchanan N. B. Kean and R. Taylor J.C.S. Chem. Comm. 1972 201. 43 R. Keese and E. P. Krebs Angew. Chem. Internat. Edn. 1972 11 518. 44 J. A. Chong and J. R. Wiseman J. Amer. Chem. Soc. 1972 94 8627. Alicyclic Compounds 409 S \I &&f&&&?$ (34) (35) (36) (38) (37) provided by isolation of dimeric hydrocarbons from treatment of 1,2-di-iodo- adamantane45 or l-brom0-2-iodoadamantane~~with n-butyl-lithium. The strained olefin (39)is surprisingly stabie4’ (ft at -20 “C = 70 h). Convincing evidence is presented for the intermediacy of (40)in the silver-ion-promoted ring- opening48 of the dibromide (41),which gives the ketone (42) and of (43)in the formation of the dimer (44)from thermal decomp~sition~~ of (45).Wiseman’s Cl Cl helpful modification of Bredt’s Rule now appears to overestimate the strain in the more strained olefins.Twisting about a double bond may occur in order to avoid steric compression as in cis-di-t-butylethylene or to relieve strain as in trans-cyclo-olefins. Analysis’ of the consequences of partial rehybridization which would lead to better rc overlap for the double bond suggests that trans-cyclo-olefins should more readily undergo cis-addition but that olefins such as cis-di-t-butylethylene should be more susceptible to trans addition. Little experimental evidence is available to support these interesting suggestions and it is possible that the effects of rehybridization upon the stereochemistry of attack are only minor.45 D. Grant M. A. McKervey J. J. Rooney N. G. Samman and G. Step J.C.S. Chem. Comm. 1972 1186. 46 D. Lenoir Tetrahedron Letters 1972 4049. 47 G. Kobrich and M. Baumann Angew. Chem. Internat. Edn. 1972,ll. 52. 48 C. B. Reese and M. R. D. Stebles J.C.S. Chem. Comm. 1972 1231. 49 P. Warner R. LaRose C. Lee and J. C. Clardy J. Amer. Chem. SOC.,1972,94,7607. 50 W. L. Mock Tetrahedron Letters 1972 475. 410 J. M. Mellor The exceptional lack of reactivity of 5-iodocyclopentadiene to silver-ion-promoted solvolysis is attributed5 to the destabilization associated with the antiaromatic character of a cyclopentadienyl cation.Spiroaromaticity fails52 to stabilize the ion (46) and e.s.r. spectra of the radical anion of (47) that electron transfer from one dienone to the other is slow. CI CI (47) (46) 2 Synthesis Three-and Four-membered Rings.-A 1though so1u t io n s of c yc lopro pen one have been previously obtained the first isolation of pure crystalline material is now reported.54 Many interesting additions are discussed,55 including that of methylmagnesium bromide to give 2-methylresorcinol (Scheme 1). Many methods of reduction of 2,3-diphenylcyclopropenone fail to give 1,2-?MgBr I Scheme 1 diphenylcyclopropene but reduction with trimethylamine-borane is most effective56(90% yield). A potentially most useful conversion of 3-butenyl halides into cyclopropylcarbinyl derivatives is described5’ (Scheme 2).XMg.CH,CH,CH=CH Bu3SnX* Bu,SnCH,CH,CH=CH % ACH,Br Scheme 2 A synthesiss8 of cyclobutanones by [,2 + .2,] cycloaddition uses keten- immonium cations as a keten equivalent (Scheme 3). Pyrolysis of l-vinylcyclo- propanols gives cyclobutanones in good yield by an intramolecular stereospecific 51 R. Breslow and J. M. Hoffman J. Amer. Chem. SOC.,1972,94,2110. 52 M. F. Semmelhack R. J. DeFranco Z. Margolin and J. Stock J. Amer. Chem. SOC. 1972 94 21 15. 53 F. Gerson R. Gleiter G. Moshuk and A. S. Dreiding J. Amer. Chem. SOC.,1972 94 2919. 54 R. Breslow and M. Oda J. Amer. Chem. SOC.,1972,94,4787. 55 R. Breslow M. Oda and J. Pecoraro Tetrahedron Letters 1972,4415 4419. 5b W. C.Perkins and D. H. Wadsworth J. Org. Chem. 1972,37,800; Synthesis 1972,205. ” D. J. Peterson and M. D. Robbins Tetrahedron Letters 1972. 2135. 58 J. Marchand-Brynaert and L. Ghosez J. Amer. Chem. SOC.,1972 94 2870. Alicyclic Compounds 41 1 c1 / Me,C=C 4Me,C=C=&Me BF,-\ NMe i ii Reagents i AgBF4-CH2Cl, -60 "C;ii CH,=CH-CH=CH,; iii OH Scheme 3 cis additi~n,'~ and Conia and Salaun" have reviewed the reverse process viz. the ring contraction of cyclobutane derivatives. At last the spectral characterization of cyclobutadiene has been achieved. Photolysis of (48) at -175"Cgave61 a transient [A,, -300 nm (E -loo)] and photolysis of (49) gave62 a transient the i.r. spectrum of which was recorded. Both transients gave the dimer (50) by thermal dimerization and it is concluded that cyclobutadiene is the transient in each case.Advances have also been made in the synthesis of simple alkyl-substituted cyclobutadienes. Decomp~sition~~ of the dimer (51)afforded (52) the first crystalline alkyl-substituted cyclobutadiene (Scheme 4). Although substituted cyclobutadienes are not produced64 by photo- lysis of anhydrides of cyclobutenedicarboxylic acids (Scheme 5) evidence is given6' for their formation by anodic oxidation of the corresponding acids. Attempts to prepare dianions of cyclobutadienes have been unsuccessful but now reaction of cis-dichlorocyclobutene with sodium naphthalide in tetrahydro- furan followed by quenching with MeOD has been shown to give 3,4-dideuterioI cyclobutene.66 This is interpreted as evidence for the intermediacy of the cyclobutadiene dianion.Other chemical or spectroscopic information to confirm this view would be welcome. 59 J. R. Salaun and J. M. Conia Tetrahedron Letters 1972 2849. 60 J. M. Conia and J. R. Salaun Accounts Chem. Res. 1972,5 33. 61 S. Masamune M. Suda H. Ona and L. M. Leichter J.C.S.Chem. Comm. 1972 1268. h2 C. Y. Lin and A. Krantz J.C.S.Chem. Comm. 1972 I1 11. 63 H. Kimling and A. Krebs Angew. Chem. Internat. Edn. 1972 11 932. 64 G. Maier and F. Bosslet Tetrahedron Letters 1972 1025. 65 G. Maier and F. Bosslet Tetrahedron Letters 1972 4483. 66 J. S. McKennis L. Brener J. R. Schweiger and R. Pettit J.C.S. Chem. Comm. 1972 365. 412 J. M. Mellor Sxs (PhCN) PdCI ...s (52) Scheme 4 h 11+ 0 Scheme 5 Five-and Six-membered Rings.-The synthesis of cyclopentanes by a [,2 + *4J cycloaddition of an allyl anion to an olefin has been achieved by the ring-opening of a cyclopropyl anion to give an intermediate allyl anion.67 Isolation" of 1,2,4-triphenylcyclopentane from reaction of a-methylstyrene with lithium di-isopro- pylamide in the presence of stilbene has also been interpreted as a [,2 + .4,] '' G. Boche and D. Martens Angew. Chem. Internat. Edn. 1972 11 724. 68 R. Eidenschink and T. Kauffmann Angew. Chem. Internal. Edn. 1972 11 292 Alicyclic Compounds addition. Full experimental details are now given69 for the analogous addition of 2-oxyallyl cations to dienes (Scheme 6). Following the synthesis of cyclo-heptenones [Annual Reports (B),1971,68,355] the reaction of aa’-dibromoketones Scheme 6 with iron carbonyls in the presence of enamines has been shown7’ to give ad-dialkylcyclopentenones in good yield.Reaction of $-unsaturated acyclic aldehydes with tris-( tripheny1phosphine)- chlororhodium gives both cyclopentanones and cyclopropane derivatives,’ and similarly citronella1 undergoes cyclization but gives72 (53) and (54)(Scheme7). A A. OH (53) (54) Scheme 7 A further pyrolytic preparation of cyclohexanones has been de~eloped’~ (Scheme 8). R = H Yield = 35% R = Me Yield = 60 % R = Ph Yield = 90% Scheme 8 69 H. M. R. Hoffmann K. E. Clemens and R. H. Smithers J. Amer. Chem. Soc. 1972 94 3940. 70 R.Noyori K. Yokoyama S. Makino and Y. Hayakawa J. Amer. Chem. Soc. 1972 94 1772. ” K. Sakai J. Ide 0.Oda and N. Nakamura Tetrahedron Letters 1972 1287. 72 K. Sakai and 0.Oda Tetrahedron Letters 1972 4375. ” G. Moinet J. Brocard and J. M. Conia Tetrahedron Letters 1972,4461. 414 J. M. Mellor Q0 Ref. 74b Ref. 74e no p Qo + '50"cb 0 C0,Me Me0,C 0 COlH 0 S Ph2 Reagents i HC0,H; ii K2C03-MeOH-H,0; iii H'; ; v MeLi; vi AcC1; iv A vii LiClO,; viii NaOEt-EtOH-C,H,; ix CH,=CHX; x A,250 "C; xi LiNH,-NH,-Bu'O,C(CH,),CO,Bu'; xii Ppa 100 "C Scheme 9 Alicyclic Compounds 415 Several important annelation procedures are rep~rted'~ (Scheme 9) and the use of chloro-olefins has been reviewed.74b Full experimental details of hydro- azulene syntheses from cyclode~adienols~ and from 9-methyl- l-decalyl to~ylates~~ have appeared.Large Rings.-trans-Cyclo-octene (19 % yield 99 % purity) is ~btained'~ by photoisomerization of the cis-cyclo-octene-Cu2C1 complex. The possible existence of trans,trans-1,5-cyclo-octadiene in conformations with double bonds perpendicular or parallel has been noted. Now the cyclopropyl analogues (55) and (56)have been ~ynthesized~~ and their structures proved by X-ray analysis. Under certain conditions cyclic olefins are known to be formed by auto-oxidation of bis-ylides. Now Be~tmann~~ gives details whereby low yields of C,, C,, C,, and C,o cyclic hydrocarbons are obtained from (57). The exploitation of nickel complexes is exemplified by the low-yield synthesis" of muscone (58) (Scheme 10).Allene-E 1,O CO Et,O -zF? 0 "C 0 0 Scheme 10 Polycyclic Systems.-Elegant studies are reported of the synthesis and interest- ing chemistry of homobenzenes and benzene epoxides. The reaction of the known 74 (a) K. E. Hardingand W. D. Nash Tetrahedron Letters 1972,4973; (b)P. T. Lansbury Accounts Chem. Res. 1972 5 31 1 ; (c)B. M. Trost and M. J. Bogdanowicz J. Amer. Chem. Soc. 1972,94,4777; (d)D. A. Evans W. L. Scott and L. K. Truesdale Tetra-hedron Letters 1972 137; (e) D. S. Watt and E. J. Corey ibid. p. 4651 ; (f)R. G. Carlson and E. G. Zey J. Org. Chem. 1972,37,2468. l5 J. A. Marshall W. F. Huffman and J. A. Ruth J. Amer. Chem. SOC.,1972,94,4691. '' C. H. Heathcock R. Ratcliffe and J.Van J. Org. Chem. 1972 37 1796. 77 J. A. Deyrup and M. Betkouski J. Org. Chem. 1972,37 3561. '* J. A. Deyrup M. Betkouski W. Szabo M. Mathew and G. J. Palenik J. Amer. Chem. Soc. 1972 94 2147. 79 H. J. Bestmann and H. Pfuller Angew. Chem. Infernal. Edn. 1972 11 508. 416 J.M. Mellor (59) (59)with diazomethane" gave (60) and (61) with diazomethane82 gave both the bishomobenzene derivatives (62) and (63) and also the trishomo-derivative (64). Quantitative thermal rearrangemer~t~~,~~ of (65) at 45 "C gave the anti-trioxide Br I $' Br Br 0 Br Br 167) Reagents i CCI,-NBS-azodi-isobutyronitrile; ii KMnO,; iii sodium glycolate Scheme 11 (66) whereas the syn-trioxide (67) was ~repared~~,~~ by Scheme 11. The syn- trioxide (67) by a [,2 + ,2 + ,2,] cycloreversion gives trioxacyclononatriene (68)at 200 "C,but in contrast (66) is stable at 200 "C,and (69)and (70)*6 are in thermal equilibrium at 60 "C (ratio 5 :95).R. Baker B. N. Blackett and R. C. Cookson J.C.S.Chem. Comm. 1972 802. M. Engelhard and W. Luttke Angew. Chem. Internat. Edn. 1972 11 310. 82 H. Prinzbach and R. Schwesinger Angew. Chem. Internat. Edn. 1972 11 940. C. H. Foster and G. A. Berchtold J. Amer. Chem. Soc. 1972 94 7939. 84 E. Vogel H. J. Altenbach and C. D. Sommerfeld Angew. Chem. Internat. Edn. 1972 11 939. '' R. Schwesinger and H. Prinzbach Angew. Chem. Internat. Edn. 1972 It 942. a6 H. J. Altenbach and E. Vogel Angew. Chem. Internot. Edn. 1972 11 937. Alicyclic Compounds 417 Certain macrobicyclic systems are capable of isomerism involving atropo- isomers [e.g.(71H73)]. Although heterocyclic analogues are well known rational syntheses of the carbocyclic examples (72) and (73)8' and (74)88 are now described. Further syntheses of outside-inside and inside-inside bicyclics can be expected. The synthetic utility of diazocyclopentadienes continues to be (Scheme 12). Norcaradiene-cycloheptatriene rearrangements have been exa- mined9' for the systems (75)-(77). There is sufficient strain in (75) for (75a) to be the preferred isomer but for (76) and (77) the cycloheptatrienes are the more stable. Full details are published of the synthesis of numerous substituted diaman- tane~~~ and of extensions of the elegant n-route to substituted adamantane~.~~ 87 C.H. Park and H. E. Simmons J. Amer. Chem. SOC.,1972,94 7184. 88 P. G. Gassmann and R. P. Thummel J. Amer. Chem. SOC.,1972 94 7183. 219 H. Durr and B. Ruge Angew. Chem. Inrernar. Edn. 1972 11 225. 90 H. Durr and H. Kober Tetrahedron Letters 1972 1259. 91 H. Durr B. Heu B. Ruge and G. Scheppers J.C.S. Chem. Comm. 1972 1257. 92 E. Vogel W. Wiedemann H. D. Roth J. Eimer and H. Gunther Annalen 1972 759. 1. 93 T. Courtney D. E. Johnston M. A. McKervey and J. J. Rooney J.C.S. Perkin I 1972 2691. 94 F. Blaney D. Faulkner M. A. McKervey and G. Step J.C.S. Perkin I 1972 2697. 418 J. M. Mellor Ph Ph Me0,C.C-C CO,Me 40x C0,Me Ref. 89 Ph Ph C02Me Ph Ph Ph Ph Ph Ph Ph Ph Ref. 91 Ph Ref.91 Ph I Ph I Scheme 12 b (75) n = 3 (76) n = 4 (77) n = 5 3 Reactions Metal-promoted Reactions.-Comprehensive reviews of transition-metal carbene complexes,95 a most useful review96 of carbon-carbon bond forming processes via n-ally1 nickel complexes and a somewhat dated review of o~ymetalation~~ have appeared. Mechanistic aspects of transition-metal-promoted isomerizations of hydro-carbons have been extensively investigated. Now that it is clear that catalysis 95 D. J. Cardin B. Cetinkaya and 'M. F. Lappert Chem. Rev. 1972 72 545. 96 M. F. Semmelhack Org. Reactions 1972 19 115. 97 W. Kitching Organometallic Reactions 1972 3 3 19. Alicyclic Compounds by relief of orbital symmetry constraints permitting concerted processes is not important the emphasis has been on establishing the nature of organometallic intermediates in a multistep process.Spectroscopic evidence for the interme- diacy of a carbene-metal complex e.g. (78) or possibly (79),has been given.98i99 (78) (79) Further kinetic evidence establishes that silver(1)-catalysed rearrangement of homocubanes is a multistep process and a pre-equilibrium involving a hydro- carbon-metal ion complex is suggested."' Full experimental details of the extensive product and kinetic studies by Gassman"' and Paquette"' show that products are critically dependent upon the nature of the metal on the oxidation state and on the ligands attached to the metal. A further complication is the solvent dependence of theprod~cts''~ as shown in Scheme 13.In methanol. both [Rh(CO),CI],-MeOH 30 "C + or AgBF,-MeOH 30°C * 95 x [Rh(CO),CI],-CHCI I Ph Ph [Rh(CO),CI],- MeOH Ph or AgBF MeOH 40% 52 % Scheme 13 9n S. Masarnune M. Sakai and N. Darby J.C.S. Chem. Comm. 1972 471. 9y W. G. Dauben and A. J. Kielbania J. Amer. Chem. SOC., 1972 94 3669. loo L. A. Paquette and J. S. Ward Tetrahedron Letters 1972. 4909. lo' P. G. Gassman and F. J. Williams J. Amer. Chem. SOC.,1972,94,7733;P. G. Gassman and T. J. Atkins ibid. p. 7748; P. G. Gassman G. R. Meyer and F. J. Williams ibid. p. 7741 ;P. G. Gassman T. J. Atkins and J. T. Lumb ibid. p. 7757. lo' L. A. Paquette S. E. Wilson R. P. Henzel and G. R. Allen J. Amer. Chem. Soc. 1972 94 7761 ;L.A. Paquette S. E. Wilson and R. P. Henzel ibid. pp. 7771 7780. Io3 P. G. Gassrnan and T. Nakai J. Amer. Chem. SOC.,1972,94 5497. 420 J. M. Mellor silver fluoroborate and the rhodium complex give products by cleavage of the same carbon<arbon bonds. In contrast in chloroform silver ion promotes a cleavage of different bonds from those cleaved by the rhodium complex. Deuteri- ation experiments establish that in methanol no metal-complexed carbene-type intermediate is involved whereas their intermediacy in chloroform is estab- lished."' The products in methanol (Scheme 13) suggest the intermediacy of carbonium ions which may be produced by an acid-promoted cleavage of the bicyclobutane. The use of n-ally1 derivatives to effect asymmetric synthesis is beautifully illu~trated''~by a preparation of 3-vinylcyclo-octene having up to 70 % optical c" Scheme 14 purity (Scheme 14).Bulky chiral phosphanes give the optimum chiral induction. The use of nickel(0) complexes is further exemplified'05 by the oligomerization of allene to give inter alia (80). Cycloadditions.'06-Emphasis has been on understanding the mechanism of cycloadditions involving ketens and allenes and on the development of the synthetic utility of some less-used modes of cycloaddition. The stereochemistry of addition of allenes to ketens has been nicely exposed (Scheme 15) the observed optical activity of products and their stereochemical integrity suggest that cycloaddition of ketens to allenes is a one-step process. However full details are now given of the study of secondary deuterium isotope effects which have been interpreted as evidence of multistep pathways for the dimerization of allenes and of their addition to olefins."' A small secondary deuterium isotope effect is observed" in dimerization of (81).The existence of small secondary isotope effects in [,2 + ,2,] cycloadditions of allenes is clearly demonstrated but the Io4 B. Bogdanovic B. Henc B. Meister H. Pauling and G. Wilke Angew. Chem. Internat. Edn. 1972 11 1023. lo5 M. Englert P. W. Jolly and G. Wilke Angew. Chem. Internat. Edn. 1972 11 136. '06 W. C. Herndon Chem. Rev. 1972,72 157. lo' M. Bertrand J. L. Gras and J. Gore Tetrahedron Letters 1972 2499. M. Bertrand J. L. Gras and J. Gore Tetrahedron Letters 1972 1189.Io9 W. Weyler L. R. Byrd M. C. Caserio and H. W. Moore J. Amer. Chem. SOC.,1972 94 1027. 'lo S. H. Dai and W. R. Dolbier J. Amer. Chem. SOC. 1972 94 3946. Alicyclic Compounds 421 C + i-iii o& Ref. 108 ca II 0 H -0 Ref. 109 Reagents i A;ii H2-Pt0,; iii OH-Scheme 15 necessary interpretation of multistep processes is still questionable. In contrast the retro-Diels-Alder reaction of some bridged anthracenes is deduced from a study’ l2 of secondary isotope effects to be concerted. The paucity of non-solvolytic kinetic data establishing the importance of through-bond interactions makes the report of such effects in a [,4 + ,2,] cycloaddition welcome. Addition’ l3 of (82) to (83)--(85) (endo addition remote from any 9-substituent in each case) is influenced by long-range electronic effects (relative reactivity of 84 85 is 1 :600).The analysis is supported by calculations using perturbation theory and might be additionally clarified by the photo- electron spectra of (83H85). Miscellaneous cycloadditions are shown in Scheme III W. R. Moore P. D. Mogolesko and D. D. Traficante J. Amer. Chem. Soc. 1972,94 4753. 112 M. Taagepera and E. R. Thornton J. Amer. Chem. SOC.,1972,94 1168. I13 M. N. Paddon-Row and R. N. Warrener Tetrahedron Letters 1972 1405. I I4 S. Ito H. Ohtani S. Narita and H. Honma Tetrahedron Letters 1972 2223. 11s H. Tanida and T. Tsushima Tetrahedron Letters 1972 395. 1 I6 I. Tabushi K. Yamamura and Z. Yoshida J. Amer. Chem. SOC.,1972,94,787.422 J. M. Meiior 0 ,OMe W 16. The ratios obtained1I7 in [2 + 2 + 21 additions with norbornadiene are taken as further evidence for the importance of non-bonding attractive forces but alternative explanations based on electronic interactions cannot be dis- counted. 0 0 c+(J++& -Ref. 114 0 0 Ref. 115 Ref. 116 CN Me Scheme 16 l7 Y. Kobuke T. Sugimoto J. Furukawa and T. Fueno J. Amer. Chem. SOC.,1972,94 3633. Alicyclic Compounds Miscellaneous Reactions.-The view that 'calculations are so quick and reliable that they obviate synthetic exploration' [Annual Reports (B),1971,68,335 ref. 141 has not daunted experimental chemists.- Successful isolation and characterization of all 12 possible products of cyclopropanation of biallenyl (86) with diazo- methane' 's emphasizes the importance of g.c.techniques. In synthesis of many alicyclic systems particularly with prostaglandins liquid chromatography is an invaluable method of analysis and of separation. A further development of stereospecific reducing agents is the use of lithium tri-s-butylborohydride,' l9 which with 4-t-butylcyclohexanone gives 96.5 % cis-alcohol at -78 "C(compar-able with the stereospecificity in enzymatic reductions with NADH). Oxidation of cyclohexanone at room temperature with thallium(m) nitrate gives 2-hydroxy- cyclohexanone (84 % yield) yet at 40 "Ccyclopentanecarboxylic acid is isolated' 2o (84%). The intermediacy in both cases of (87) is suggested although the reaction is performed in acetic acid.Further evidence for the intermediacy of (87) would be welcome. The chemistry of valence isomers of aromatic systems has been reviewed,I2' numerous additions to Dewar benzenes have been reported,'22 and the influence of substituents upon the rate of rearrangement of Dewar benzenes to aromatic compounds has been studied.'23 The rates for (88H91) (0 :5 :464 1860) are X I (88) X = Y = C1 (89) X = Y = H (90) X = H Y = C1 (91) X = H Y = F Y 'I8 F. Heinrich and W. Luttke Angew. Chem. Internat. Edn. 1972 11 234. ' l9 H. C. Brown and S. Krishnamurthy J. Amer. Chem. SOC. 1972,94,7159. I2O A. McKillop J. D. Hunt and E. C. Taylor J. Org. Chem. 1972 37 3381. 12' E. E. Van Tamelen Accounts Chem. Res. 1972,5 186. H. Hogeveen and P.W. Kwant Tetrahedron Letters 1972 2197; G. R. Krow and J. Reilly ibid. pp. 3129 3133; L. A. Paquette S. A. Long S. K. Porter and J. Clardy ibid. p. 3137; L. A. Paquette R. J. Haluska M. R. Short L. K. Read and J. Clardy J. Amer. Chem. SOC.,1972 94 529; L. A. Paquette S. A. Long M. R. Short B. Parkinson and J. Clardy Tetrahedron Letters 1972 3 141. '23 R. Breslow J. Napierski and A. H. Schmidt J. Amer. Chem. SOC. 1972 94 5906. 424 J. M. Mellor interpreted as evidence for stabilization of the antiaromatic transition state by a push-pull effect. Other biradicals of unknown multiplicity include (92) and (93). Decompo~ition'~~ of (94) in the presence of olefins gives adducts (95) and (96) via the intermediacy of (92) and decomp~sition'~~ of (97) gives (93) which readily dimerizes or cycloadds (Scheme 17).The chemistry of trimethylene-methane has been reviewed. 26 Re-examination' 27 of the thermodynamic parameters relating trans-and cis-2-decalone establishes the error in earlier work. trans-2-Decalone is the more stable (AG4g8 =2.2 kcal mol-AH488= 2.5 kcal mol-',and As488 = 0.6 cal K-' mol-') and no anomalous entropy effects are observed. K \/ hy 4/17. N a 0 + 8 .. .. (97) (93) Scheme 17 J. A. Berson D. M. McDaniel L. R. Corwin and J. H. Davies J. Amer. Chem. SOC. 1972,94 5508. 125 C. S. Chang and N. C. Bauld J. Amer. Chem. SOC.,1972,94 7593 7594. Iz6 P. Dowd Accounts Chem. Res. 1972,5 242. N. L. Allinger and J. H. Siefert J. Amer. Chem. SOC.,1972,94 8082.

ISSN:0069-3030    

DOI:10.1039/OC9726900403

出版商:RSC    

年代:1972

数据来源: RSC