摘要:

5 Arynes Carbenes Nitrenes and Related Species By J. T. SHARP Chemistry Department University of Edinburgh West Mains Road Edinburgh EH9 3JJ 1 Arynes Generation.-The generation of reactive intermediates by the vapour-phase pyrolysis of heterocyclic systems continues to produce interesting results. The benzo-1,2,3-triazine (1) undergoes complete fragmentation at >500 "C to give benzyne but at lower temperatures eliminates only nitrogen to produce the highly reactive red benzazete (2).' The 2,3-benzoxazin-l-one (3) however was stable at 560 "C but underwent complete fragmentation at 850 "C losing benzonitrile and carbon dioxide to give benzyne. Benzoxazin-4-ones were even more stable e.g. (4) was little decomposed at 800"C.z It is suggested that the first bond to be broken in these molecules is between atoms 2 and 3 so that the ob- served order of stability parallels the order of bond energies N-N < N-0 < C-0.A 4,5-dehydropyridazine (5) has now been generated for the first time by the pyrolysis of a triazine precursor at 420°C and predictably like 3,4-pyridyne it fragments rather than dimerizes losing nitrogen to give diphenyl- butadiyne in 54% yield.3 Dinitrobenzenes when pyrolysed at 550°C first lose NO to give nitrophenyl radicals which to some extent lose the remaining NO to give products formally derived from phenylene biradicals. There is evidence for some concerted loss of both nitro-groups from o-dinitrobenzene to give ben~yne.~ The photolysis of phthaloyl peroxide provides a convenient route to benzyne in solution and benzpropiolactone (6) is of interest as a possible intermediate ' B.M. Adger M. Keating C. W. Rees and R. C. Storr J.C.S. Chem. Comm. 1973 19. M. P. David and J. F. W. McOmie Tetrahedron Letters 1973 1361. T. L. Gilchrist G. E. Gymer and C. W. Rees J.C.S. Chem. Comm. 1973 819. E. K. Fields and S. Meyerson J. Org. Chem. 1972 37 3861. 177 178 J. T. Sharp in this reaction and in the decomposition of benzenediazonium-2-carboxylate. It has now been shown that (6) is produced together with (7) in the irradiation of a phthaloyl peroxide matrix at 8 K.’ Attempts to define the thermal chemistry of (6) were not successful. Prolonged irradiation of the phthaloyl peroxide matrix however produced benzyne which was identified by infrared comparison with benzyne produced by the irradiation of benzocyclobutenedione (8) under similar conditiom6 This is the first report of the i.r.spectrum of benzyne. The formation of benzyne from (8) involves reversible conversion to the bisketen (9) and possibly the intermediacy of benzocyclopropenone (10). Kolc’ has also shown that (8) can be decarbonylated to give benzyne in an EPA matrix at 77 K. a O --+ D Benzyne \ \ “0 I t The highly reactive 8nelectron molecules 3,6-dehydro-oxepin (11; X = 0)8 and its sulphur analogueg have been prepared by thermal rearrangements of (12; X = 0 or S). In the latter case rearrangement competes with sulphur extrusion. These molecules exist in the closed form in contrast to lP-dehydro- benzene in which the ‘dehydro’ bond is open.The chemistry of these and related 1,4dehydroaromatic intermediates has been reviewed. 2,3-Dehydrobiphenylene has been generated from 2-biphenylenediazonium 3-carboxylate and trapped with tetracyclone ; this intermediate however failed to dimerize or couple with benzyne.’ ’ The cycloalkyne (13) was too unstable to isolate and was trapped with phenyl azide.” Its instability compared with the 0.L. Chapman C. L. McIntosh J. Pacansky G. V. Calder and G. Orr J. Amer. Chem. SOC.,1973.95 4061. 0. L. Chapman K. Mattes C. L. McIntosh J. Pacansky G. V. Calder and G. Orr J. Amer. Chem. SOC.,1973,95 6134. ’ J. Kolc Tetrahedron Letters 1972 5321. * K. P. C. Vollhardt and R. G. Bergman J.Arner. Chem. SOC. 1972,94 8950. K. P. C. Vollhardt and R. G. Bergman J. Amer. Chem. SOC. 1973 95 7538. lo R. G. Bergman Accounts Chem. Res. 1973 6 25. ” E. N. Losey and E. LeGoff J. Org. Chem. 1973,38 3812. It A. Krebs and G. Burgdorfer Tetrahedron Letters 1973 2063. Arynes Carbenes Nitrenes and Related Species isolable carbocyclic and S analogues was attributed to the short C-0 bond length. 1-Halogenocycloheptenes react with potassium t-butoxide in DMSO and THF to give cycloheptyne and cyclohepta-1,2-diene which dimerize or react together to give the major products rather than add base to give l-t-butoxy- cycloheptene.’ Similar reactions with sodium pyrrolidide however gave 1-(1-pyrro1idino)cycloheptenein fair yield. The formation of similar mixtures of the 2- and 3-t-butyl ethers in reactions of 2- and 3-halogenobicyclo[3,2,l]oct-2-enes with potassium t-butoxide in THF points to (14) as the intermediate but reactions in DMSO appear to involve the cyclic allene also.l4 Structure and Reactiom-Ab initio calculations of the equilibrium geometry electronic structure and heat of formation have been carried out for the singlet ground-state of o-benzyne.’’ The molecule is predicted to have an ‘acetylenic’ C(l)-C(2) bond (122pm) and a C(4)-C(5) bond which is slightly elongated compared with benzene. The heat of formation which is important in assessing the hydrogen-abstracting capability of benzyne was estimated to be ca. 500 kJ mol- ’ implying that abstraction of hydrogen from an alkane methylene group by ground-state o-benzyne would be nearly thermoneutral but abstraction from benzene would be endothermic by ca.80 kJ mol- ’.” It has been suggested that benzyne resembles singlet oxygen in its reactions with alkenes leading to an approach like (15) with interaction between the HOMO of the alkene and the LUMO of benzyne.I6 This is a similar approach to that which emerged recently from some EH calculations (see last year’s report p. 216). Some INDO calcula-tions on benzyne and its 3-and 4-lithio-derivatives and anions have rationalized the differences in reactivity between benzyne itself and 3-lithio-1,Zdehydro- benzyne (16).” The presence of a lithium atom in the 3-position has a more marked effect on the charge distribution in the ‘dehydro’ bond than one in the 4-position and it is predicted that in polar addition reactions (16) will have a (15) (16) l3 A.T. Bottini K. A. Frost B. R. Anderson and V. Dev Tetrahedron 1973 29 1975. l4 A. T. Bottini and B. Anderson Tetrahedron Letters 1973 3321. M. D. Newton and H. A. Fraenkel Chem. Phys. Letters 1973 18 244. l6 S. Inagaki and K. Fukui Bull. Chem. SOC.Japan 1973,46 2240. ” F. M. Stoyanovich Ya L. Gol’dfarb I. A. Abronin and G. M. Zhidomirov Tetrahed-ron Letters 1973 1761. 180 J. T. Sharp higher reactivity than the Clithio-derivative and benzyne itself while the reverse will be true for Diels-Alder reactions. Benzyne with its symmetrical singlet ground-state undergoes stepwise [2 + 23 cycloaddition with alkenes concerted [4 + 21 cycloaddition with 1,3-dienes and also ene cycloadditions which may or may not be concerted.Some of the factors influencing these reactions have been examined by using substrates with which all three reactions are possible. For a series of six- to eight-membered ring polyenes e.g. (17) the partitioning of reaction between the three modes has been correlated with the ring conformations." It was concluded that the concerted [4 + 21 cycloaddition is very sensitive to the skew angle of the diene unit e.g. (19) in which the diene is planar reacts exclusively by this mode whereas (17) which has a skew angle of 40° gives no [4 + 21 adduct but similar yields of the [2 + 21 and ene products. Evidence is also presented for a concerted mechanism for the ene reaction.The addition of very low concentrations of Ag' has a profound effect on the product ratios in these reactions e.g. the relative amount of (18) is increased from 0% to 100% by the ene [2 + 21 addition of silver fluoroborate. This effect has previously been ascribed to the capture of Ag+ by benzyne to give a more electrophilic complex but it is now suggestedIg that a Ag+-olefin complex reacts with the benzyne precursor to give (20) which decomposes to the benzyne-olefin complex (21). Finally a stepwise electrophilic attack leads to (22) which collapses to products via partial silver (20) (21) (22) bridging to the polyenyl cation. The nature of the final products from cyclic 1,Zdienes seems to be controlled by the distances between the benzene carbon bound to silver and the polyene ring carbons.Benzyne showed no tendency to add across several vinylcyclopropane systems but rather reacted by ene and P. Crews and J. Beard J. Org. Chem. 1973 38 522. l9 P. Crews and J. Beard J. Org. Chem. 1973 38 529. Arynes Carbenes Nitrenes and Related Species [2 + 21 cycloadditions,20 whereas enthynylcyclopropane gave (24) probably via (23).21 Tetrachlorobenzyne adds to 1,2-dimethoxynaphthaleneto give the normal 1,Cadduct but unexpectedly the major product in reaction with 1,2,3,4-tetramethoxynaphthalene is the semibullvalene (25).22 The dienolate anions e.g. (26) derived from ajl-unsaturated carbonyl compounds bearing y-hydrogen atoms behave as diene components and react with benzyne to give naphthols e.g.(27) and naphthalene^.^^ These reactions probably involve a non-concerted cycloaddition followed by loss of methanol or water during work-up (see also ref.24). Me0 CI Ph H / H Me0 Me0 (23) (24) (25) 0-M+ MeOk Benzyne ~ Me0 OHa--+ The relative reactivities of ammonia and amide ion in addition to Cchloro- benzyne have been determined:25 amide ion is 2.4 x lo2 times as reactive as ammonia at the 2-position and 49 times as reactive at the 1-position. The amide ion is equally reactive to the two positions but ammonia is 4.92 times as reactive toward the 1-position as to the 2-position. This data supports a stepwise mechan- ism for the addition reactions and leads to the conclusion that there is very little C-N bond formation in the transition states for amide ion addition but con- siderable bond formation in the transition states for the addition of the weaker nucleophile ammonia.In the latter case there will therefore be more develop ment of negative charge on carbon and a substantial orientation effect. Caubere's group continue their st~dies~~,~~-~~ of the reactions of benzyne with a range of ketone enolates to give a variety of products e.g. anthranols (28) lo V. Usieli and S. Sarel J. Org. Chem. 1973 38 1703. ' V. Usieli and S. Sarel Tetrahedron Letters 1973 1349. 22 F. Serratosa and P. Sola Tetrahedron Letters 1973 821. 23 P. G. Sammes and T. W. Wallace J.C.S. Chem. Comm. 1973 524. L4 P. Caubere and G. Guillaumet Bull. SOC. chim. France 1972 4643. l5 J.F. Bunnett and Jhong Kook Kim J. Amer. Chem. SOC. 1973,95 2254. 2b P. Caubere and G. Guillaumet Bull. SOC. chim. France 1972 4649. 27 P. Caubere and G. Guillaumet Compt. rend. 1972 275 C 463. l8 P. Caubere M. S. Mourad and G. Guillaumet Terrahedron 1973 29 1843. l9 P. Caubere M. S. Mourad and G. Guillaumet Tetrahedron 1973 29 1857. 30 P. Caubere M. S. Mourad and D. Canet Tetrahedron Letters 1973 2221. 182 J. T. Sharp can be formed in aprotic media. Similarly the reaction of malonic ester with arynes under basic conditions in HMPT provides a route to homophthalic acids and homophthalimides in addition to phenylmalonic ester.3 Tetrafluoro-benzyne reacts with its precursor tetrafluoroanthranilic acid to give (29),which can be ring-closed to octafluoroacridone (65 %) ; tetrachloroanthranilic acid gave the analogous acridone directly and no intermediate like (29)was isolated.32 Halogenobenzenes react with cyclopropyl-lithium to give cyclopropylbenzenes and biphenyls and also undergo considerable dehal~genation.~~ Introduction of the cyclopropyl groups may proceed via direct nucleophilic substitution as well as viaaryne intermediates and the dehalogenation of the chloro- and fluoro- benzenes is thought to take place uia hydride transfer to benzyne.The reported reaction of (30) with benzyne to give (32)34has now been shown to be a reaction of (31) not (30).35 Intramolecular nucleophilic addition to the 'dehydro' bond in benzyne has proved a fruitful source of heterocyclic compounds and now similar reactions have been demonstrated for hetarynes e.g.(34) from (33) in the presence of lithium amide~.~~ The high-yielding synthesis of phenanthridenes e.g. (36) from o-halogenoanils involves a fast addition of amide ion to the and to give the anion (35) which is now sterically capable ofcyclization on to the subsequently formed aryne to give (36).37 Some 2,lO- 3,lO- and 4,lO-diazaphenanthrene~~~ 3' M. Guyot and D. Molho Tetrahedron Letters 1973 3433. 32 S. Hayashi and N. Ishikawa Nippon Kagaku Kaishi 1973 1319 (Chem.Abs. 1973,79 78 576). 33 W. Kurtz and F. Effenberger Chem. Ber. 1973 106 560. 34 N. Dennis A. R. Katritzky S. K. Parton and Y. Takeuchi J.C.S. Chem. Comm. 1972 707. 35 N. Dennis B. Ibrahim A. R. Katritzky and Y. Takeuchi J.C.S.Chem. Comm. 1973 292. 36 T. Kauffmann and H. Fischer Chem. Ber. 1973 106 220. ''I S. V. Kessar R. Gopal and M. Singh Tetrahedron 1973 29 167 177. 3a S. V. Kessar and G. S. Joshi Tetrahedron 1973 29 419. Arynes Carbenes Nitrenes and Related Species H and benzo[i]phenanthridenes3 have been similarly prepared. There are reports of the syntheses of the following compounds by benzyne reactions apomor- ~hine,~'.~~ tetrahydrodibenzopyrrocoline alkaloid^,^' rnorphinandien~ne,~'.~~ 6a,7-didehydroap0morphine:~ indolo[2,1 -a]isoq~inolines,4~ ( f)-cryptamto-line,43 ( )-thali~orphine,~~ dibenzindolizinium alkaloid^,^ and mefenamic acid and related Ethers are known to be cleaved by the highly electrophilic tetrahalogeno- benzynes and this reaction e.g.(37) has now been observed for benzyne itself,45 giving phenetole in up to 40% yield but only when the reaction is carried out in a solvent other than the ether itself. Et +PhOEt + CH,=CHZ (37) 39 S. V. Kessar B. S. Dhillon G. S. Joshi Indian J. Chem. 1973 11 624. O0 T. Kametani A. Ujiie K. Takahashi T. Nakoro S. Susuki and K. Fukumoto Chem. and Pharm. Bull. (Japan) 1973 21 766. " S. V. Kessar R. Randhawa and S. S. Gandhi Tetrahedron Letters 1973 2923. 42 T. Kametani S. Shibuya and S. Kano J.C.S. Perkin I 1973 1212. O3 T. Kametani K. Fukumoto and T. Nakano J. Heterocyclic Chem. 1972 9 1363. *' T. Kametani K. Kigasawa M. Hiiragi K. Wakisaka and S. Saito Yakugaku Zasshi 1972 92 1547 (Chem. Abs. 1973 78 57 972). O5 G.D. Richmond and W. Spendel Tetrahedron Letters 1973 4557. 184 J. T. Sharp 2 Nitrenes Structure and Reactivity.-Nitrenes (R-N ) and nitrenium ions (R-fi-R) like carbenes can exist in either singlet or triplet electronic states with the triplet usually lower in energy but interactions with certain substituents may perturb the energy levels so that the singlet becomes the ground state. A recent ++ + cal~ulation~~ compares the electronic states of NH, NF, and NHF with the + + isoelectronic carbenes :CH, :CHF and :CF,. The results for NH and NF agree with earlier ~ork~~*~~ in predicting a triplet ground state for &H (150")188 + kJmo1-' below the singlet (120")and a singlet ground state for NF (105") 138 kJ mol- 'below the triplet (122").As in :CHF the singlet and triplet states + of NHF are almost degenerate. The substitution of fluorine has a marked effect on the charge distribution e.g. the charge on the N atom increases from +0.10 + + for NH to +0.96 for NF,. INDO Calculations on unconstrained methyl- + + and dimethyl-nitrenium ions MeNH and Me,N show that the triplet states (-linear) are more stable than the singlets (122" and 118" respectively) by CQ. 109 kJ mol- ' at minimum energy configuration^.^^ The chemical implications of these results are discussed e.g. if $ for (38)were constrained to 109"the singlet and triplet energies would be very close and singlet to triplet interconversion could take place rapidly thus explaining the unexpected exclusively triplet reactivity of (39).'O The addition reactions of aminonitrenes e.g.(40) to olefins to give aziridines has been much studied in recent years and it has been shown that olefin reactivity + +. H,CTN ,N. (38) (39) is increased by electron-withdrawing substituents and nitrene reactivity re- duced by such groups. These effects have been correlated with frontier orbital energies." The stereospecificity of these addition reactions has led to the suggestion that the aminonitrenes react in a singlet state and that this is also the ground state. However some recent ab initiu calculation^^^ have shown that at their optimum geometries the triplet aminonitrene (H,N-N) is more stable than the singlet species by ca. 105 kJ mol-' but when the triplet is con- strained to singlet geometry the separation is negligible.It is predicted that more refined calculations would show a ca. 42 kJ mol-' separation between the triplet " J. F. Harrison and C. W. Eakers J. Amer. Chem. SOC.,1973 95 3467. 47 S. T. Lee and K. Morokuma J. Amer. Chem. SOC.,1971 93 6863. 48 A. B. Cornford D. C. Frost F. G. Herring and C. A. McDowell J. Chem. Phys. 1971 54 1872. 49 G. F. Koser J.C.S. Chem. Comm. 1973,461. P. G. Gassman and G. D. Hartman J. Amer. Chem. SOC.,1973,95 449. '' H. Person F. Tonnard A. Foucaud and C. Fayat Tetrahedron Letters 1973 2495. N. C. Baird and R. F. Barr Cunud. J. Chem. 1973 51 3303. A rynes Carbenes Nitrenes and Related Species 185 ground state and the singlet state at optimum geometries and that in most cases the singlet state would lie below the triplet at optimum singlet geometry.Hence the observed ‘singlet’ reactivity of these species could due to the initially formed aminonitrene being ‘trapped’ in a singlet well and only undergoing very slow crossing to the triplet ground state.52 Some INDO calculation^'^ for a number of heteroatom (0,N or S) substituted nitrenes have also predicted that the singlet species of HON CH,ON and NH2N will be more stable than the triplets at optimum singlet geometry but that the separation will be much greater than suggested above.52 The electronic spectra of phenylnitrene and 1-nitrenopyrene and their azide precursors have been measured and correlated with calculated transition energies and oscillator strengths.54 The n-electron structure of phenylnitrene was shown to be similar to those of benzyl or anilino radicals.Calculations on carbonylnitrenes suggest that alkanoylnitrenes have triplet ground states and bigger singlet-triplet separation than alkoxycarbonylnitrenes which may be ground-state singlets. It is suggested that this could result in the latter showing a reduced tendency to undergo intersystem crossing and could have some bearing on the very different effects which methylene chloride has on the reactivity of alkanoyl- and ethoxycarbonyl-nitrene~.’~ The mechanisms of abstraction and insertion reactions are discussed and it was concluded that although direct insertion of the lowest singlet nitrene into o-bonds is forbidden in the Woodward-Hoffmann sense the reaction does proceed in a concerted manner so explaining the high degree of selectivity and stereospecificity ob- served.’’ Photochemical generation of nitrenes can result in the direct formation of triplet as well as singlet species but in the case of benzoylnitrene it is the singlet which is the primary product of the photolysis of benzoyl aide and of (41).56 (Me),i-NCOPh (41) Stereoselective addition (ca.98 %) to olefins gave N-benzoylaziridines in 25-65 % yield with ca.30% of phenyl isocyanate -the Curtius rearrangement product. On photolysis of benzoyl azide in a glass at 77 K no e.s.r. signal due to triplet benzoylnitrene was observed but phenyl isocyanate was formed which de- composed to phenylnitrene on further irradiation.57 Singlet ethoxalylnitrene is also predominant in the photolysis of its azide precursor; it inserts into the 0-H but not C-H bonds of alcohols and is reported to be more selective in its insertion into hydrocarbon C-H bonds than etho~ycarbonylnitrene.’~ 53 L. J. Hayes F. P. Billingsley and C. Trindle J. Org. Chem. 1972 37 3924. 54 M. Kashiwagi S. Iwata T. Yamaoka and S. Nagakura Bull. Chem. SOC.Japan 1973 46 417. 55 P. F. Alewood P. M. Kazmaier and A. Rauk J. Amer. Chem. SOC.,1973 95 5466. s6 Y. Hayashi and D. Swern J. Amer. Chem. SOC.,1973 95 5205. ” V. J. Kuck E. Wasserman and W. A. Yager J. Phys. Chem. 1972,76 3570. 58 T. Shingaki M. Inagaki M. Takebayashi and W. Lwowski Bull. Chem. SOC.Japan 1972,45 3567. 186 J. T. Sharp Methanesulphonylnitrene is less selective than either in its insertion reactions into C-H bonds perhaps because it is less resonance stabilized.It is however more reactive than. ethoxycarbonylnitrene in insertion into 0-H bonds owing to its higher electr~philicity.~~ CIDNP results6' are consistent with an earlier proposal that triplet ethoxycarbonylnitrene inserts into the tertiary C-H bond in trans-decalin via the triplet radical pair (42). Tetrafluoropyridylnitreneunder-goes non-stereospecific insertion into C-H bonds.61 The photolysis of p-cyanophenyl aide in dimethylamine gives (44) and (45) both uia the singlet nitrene (43). It has now been shown that the product ratio is EtOCONH N NHNMe2 NMe CN CN (43) (44) (45) wavelength-dependent and this effect is rationalized in terms of excess of excita-tion energy producing a hot nitrene.62 It is suggested that wavelength may be an important variable in other photoreactions of aryl azides.The presence of carboxylic acids is well known to affect the course of the reac- tions of aromatic nitro- and nitroso-compounds with trialkyl phosphites and it has now been shown that trifluoroacetic acid profoundly affects the photo- chemical and thermal decomposition of aryl a~ides.~~ A number of substituted azides were decomposed in mesitylene containing ca. 8 % of trifluoroacetic acid to give substantial yields of diphenylamines e.g.(46). The thermal reaction is thought not to involve a nitrene but to go uia electrophilic attack on the sub-strate by a protonated or hydrogen-bonded aryl azide molecule (47);a nitrenium ion pathway may be a competitive process for phenyl azide but not for other azides studied.63 The presence of acid also affects the reactions of phthalimido-nitrene (40; R = phthalimido-); when generated from (48) it reacted with 1,3-dimethoxybenzene to give mostly (49),but when produced by the Pb(OAc) oxidation of N-aminophthalimide it gave mostly the insertion product (50).It is proposed that the acetic acid present in the latter case diverts (51) to (50).64 59 T. Shingaki M. Inagaki N. Torimoto and M. Takebayashi Chem. Letters 1972 1181. 6o M. R. Brinkman D. Bethell and J. Hayes Tetrahedron Letters 1973 989. 61 R. E. Banks and G. R. Sparkes J.C.S. Perkin I 1972 2964. 62 R. A. Odum and G.Wolf J.C.S. Chem. Comm. 1973 360. 63 R. J. Sundberg and K. B. Sloan J. Org. Chem. 1973 38 2052. 64 D. W. Jones J.C.S. Chem. Comm. 1973,67. Arynes Carbenes Nitrenes and Related Species Me Me \N (46) (47) OMe (49) OMe (50) Generationand Reactions-The phosphinoylidene(52)and its thio-analogue have been generated by dechlorination reactions and reacted with disulphides to give (53) and with benzil to give (54) reactions which are similar to those earlier reported for phosphinidene (R-P:).65 Attempts to adapt the methods used ‘i-c”O,p,O + R-P=O -+ 0 II RPSEt R’ *O (52) (53) (54) for generating N-nitrenes for the production of 0-nitrenes e.g. the Pb(OAc) oxidation of 0-substituted hydroxylamines produced some interesting chemistry but no conclusive evidence for the intermediacy of free RON or RO&H.66 It is suggested that the primary reaction is the formation of (55) and that the observed 0-N migrations and addition to alkenes could be concerted with the cleavage of the N-Pb bond in (55).Other workers however have ass& the intermediacy RONH + Pb(OAc) -D RONHPb(OAc) + AcOH (55) 65 M. Yoshifuji S. Nakayama R. Okazaki and N. Inamoto J.C.S. Perkin I 1973 2065. 66 F. A. Carey and L. J. Hayes J. Org. Chern. 1973 38 3107. 188 J. T. Sharp of alkoxynitrenes in the formation of a range of alkoxyaziridines in similar reaction^.^' It is clear that the results of Pb(OAc) reactions with amino-com- pounds in general must be interpreted with caution e.g.the formation of the little known triphenylarsinimines (57) by the reaction of Pb(OAc) and tri- phenylarsine with methane- or toluene-p-sulphonamides might appear to go via a nitrene but in fact does not and involves the primary formation of (56).68 However the formation of (57) by the decomposition of azides and other nitrene RNH, Ph,As + Pb(OAc) -Ph,As(OAc) +RN=AsPh (56) (57) f AsPh RN RN precursors in the presence of triphenylarsine does appear to involve nitrenes. 68 Cyclization of the nitrenium ion (59) formed under acidic conditions from o-biphenylhydroxylamine generated in situ was last year reported to give carba- zole but surprisingly (58) itself had been reported not to undergo a similar ‘NH H (58) (59) reaction. This conflict has now been resolved by a reinvestigation of the latter reaction which shows that (58) is converted into carbazole in 10% yield by 20% sulphuric acid.69 In another cyclization involving a nitrenium ion (60) was converted to ( f)-glaziovine (61).” NN-Dialkylhydroxylamines (62) have been found to be useful precursors for nitrenium ions via conversion of the hydroxy-group to a leaving group such as a benzoate or sulphonate ester.50 0‘ RZN-OH 67 B.V. Ioffe and E. V. Koroleva Tetrahedron Letters 1973 619. 68 J. I. G. Cadogan and I. Gosney J.C.S. Chem. Comm. 1973 586. 69 T. B. Patrick and J. A. Schield Tetrahedron Letters 1973 445. 70 T. Kametani K. Takahashi K. Ogasawara and K. Fukumoto Tetrahedron Letters 1973,4219. Arynes Carbenes Nitrenes and Related Species 189 O-Sulphonate esters were generally found to be too unstable and 3,Sdinitro- benzoates were found to be the most useful derivatives for subsequent heterolytic cleavage of the N-0 to produce nitrenium ions.Similarly the rearrangement of O-(arenesulphony1)phenylhydroxylamines to O-arenesulphonyl-o-bnzamido-phenols takes place uia an intimate nitrenium-tosylate ion pair. 71 There is continued synthetic and mechanistic interest in the production of heterocyclic compounds by the reductive cyclization of aromatic nitro- and nitroso-compounds and the thermal decomposition of the analogous azides. Holliman has adapted a useful deuterium-labelling technique to investigate the nature of the intermediates in such reactions e.g.the cyclization of (63; X = N3) to (64) and (65) or the phosphite-induced cyclization of (63;X = NO) to similar prod~cts.’~ These reactions go by different mechanisms in decalin and chloro- benzene but both proceed oia a common intermediate most likely (66),in triethyl phosphate as solvent. A similar intermediate was postulated for the reaction of (67; X = N3) with triethyl phosphate. It was also shown that reactions of 2- nitrobiphenyls with triethyl phosphite proceed by way of the nitroso-com- pounds.’* The reductive cyclization of (68) and (69) and related compounds (67) (68) (69) provides routes to 2-aryl-1N-[1]benzothieno[2,3-b]pyrroles 2-aryl[ llbenzo- thieno[3,2-~]pyrazoles and similar systems.73 2-Azido-3-vinyl-l,4-quinones likewise cyclize to indole-4,7-dione~.~~ Jones has extended his work on the 71 D.Gutschke and A. Heesing Chem. Ber. 1973 106 2379. 72 P. K. Brooke R. B. Herbert and F. G. Holliman Tetrahedron Lerters 1973 761. ” K. E. Chippendale B. Iddon and H. Suschitzky J.C.S. Perkin I 1973 125 129. 74 P. Germeraad and H. W. Moore J.C.S. Chem. Comm. 1973 358. 190 J. T. Sharp cyclization of 2-azidodiphenylmethanes to analogous heterocyclic systems e.g. (70)-(71) and (72) the latter being formed by an unusual ring-opening of the thiophen ring followed by recyclization. 75 The nitrene (73) ring-closes on to both C and N in the pyridine ring unlike 2-(2-nitrenophenyl)pyridinewhich gives exclusively N-N bond formation.76 Phthalirnid~-~~ have been added to cyclo- and methoxycarbonyl-nitrene~’~ butenes to give 5-azabicyclo[2,1,O]pentanes.The full report on the addition of phthalimidonitrene to alkynes has now appeared ;79 1H-azirines are the primary products which rearrange to 2H-azirines most likely via (74). Methoxycarbonyl-nitrene generated by the photolysis of methyl azidoformate in isocyanates added to the C=N bond of the isocyanate to give (75) while the azide reacted with two molecules of the isocyanate to give (76).80 A range of substituted 3H-azepines has been prepared by the reaction of substituted nitrobenzenes with tervalent phosphorus reagents in the presence 75 G. R. Cliff G. Jones and J. McK. Woollard Tetrahedron Letters 1973 2401. 76 R. Y. Ning P. B. Madan and L. H. Sternbach J. Org. Chem. 1973 38 3995.l7 A. G. Anderson and D. R. Fagerburg Tetrahedron 1973 29 2973. 78 D. H. Aue H. Iwahashi and D. F. Shellhamer Tetrahedron Letters 1973 3719. 79 D. J. Anderson T. L. Gilchrist G. E. Gymer and C. W. Rees J.C.S. Perkin I 1973 5 50. S. M. A. Hai and W. Lwowski J. Org. Chem. 1973,38 2442. Arynes Carbenes Nitrenes and Related Species 191 RY-fo RN-NC0,Me RNCO 4-MeOCON -+ Nyo+ oANAo R OMe (76) (75) of various amines." These reactions involve intramolecular ring closure of the 'nitrene' to give a strained azirine followed by attack of the amine and ring expansion. The transitory presence of l-azirines in the thermal decomposition of vinyl azides has been demonstrated by using a cyclopentadienone trap which diverted the reactions from their usual courses to give 3H-a~epines.~~ In (77) when R = H the molecule decomposes solely by Curtius ring expansion but when R = alkyl the intermediate azirine is sufficiently stabilized to allow the formation of (78) as the exclusive product.83 &13 -5: MeOH &..2 OMe OMe (77) (78) Thermolysis of cyclopropyl azides e.g.(79) takes place readily at much lower temperatures than alkyl azides. The elimination of nitrogen is facilitated by the electron-donor effect of the cyclopropane system to the empty p-orbital on the nitrene nitr0ge1-1.~~ This charge transfer weakens the bonds to the carbon bearing the nitrene and facilitates the rearrangement to the azetidine (80; 76 %) [XPh qph 'loocb + PhCN + CH,=CH, N-N3 or the chelotropic elimination of benzonitrite (21 %) to give ethylene.Using deuterium-labelled cyclopropyl azides it was shown that the alkene formation follows a stereospecific cis course (> 97 %) confirming the concerted cleavage of the cyclopropyl bonds.85 The nitrenium ion (81;R = H),formed by Pb(OAc) oxidation of the cyclopropylamine ring opens to give (83),86 but in the formation8' '' F. R. Atherton and R. W. Lambert J.C.S. Perkin I 1973 1079. 82 D. J. Anderson and A. Hassner J. Org. Chem. 1973 38 2565. 83 Y. Tamura Y. Yoshimura T. Nishimura S. Kato and Y. Kita Tetrahedron Letters 1973 351. 84 G. Szeimies U. Siefken and R. Rinck Angew. Chem. Internat. Edn. 1973 12 161. 85 G. Szeimies and J. Harnisch J.C.S. Chem. Comm. 1973 739. T.Hiyama H. Koide and H. Nozaki Tetrahedron Letters 1973 2143. '' J. A. Deyrup and R. B. Greenwald Tetrahedron Letters 1973 4771. 192 J. T. Sharp of the same product from (84) the analogous nitrenium ion (81 ; R = tosyl) is not thought to be involved but to be 'concertedly by-passed' in favour of (82; R = tosyl) owing to the effect of the tosyl group in opposing the development of PhCHPh-v-NR- CH=NR -+ PhCH=CHCHO H H (81) (82) (83) positive charge on nitrogen. The formation86 of benzonitrile and ethylene from (85) represents a new type of fragmentation of nitrenium ions similar to the nitrene reaction discussed above.84 Further study of the diazene-hydrazone Ph rearrangement has shown that azo-compounds (86) are primary products ; however it is possible that (87) may also be formed in another way.88 R'CH \ N-N -+ R2N=NCH2R' * R2NH-N=CHR' / R2 (86) (87) There are three reports of the unusual insertion of arylnitrenes into amine N-H bonds to give azo-compounds.89-9' The photolysisg2 of t-butyl azide gives(88)rather than 2,2-dimethylaziridine as reported earlier.An intermolecular reaction between a nitrene and an azide is reported in which ethoxycarbonyl- nitrene reacts with n-hexyl azide to give (89) which rearranges to the hydra~one.'~ B. V. Ioffe and L. A. Kartsova Tetrahedron Letters 1973 623. E. F. V. Scriven and H. Suschitzky Tetrahedron Letters 1973 103. 90 R. E. Banks and A. Prakash Tetrahedron Letters 1973 99. 91 P. A. S. Smith and H. Dounchis J. Org.Chem. 1973 38 2958. 92 S. Solar E. Koch J. Leitich P. Margaretha and 0.E. Polansky Monatsh. Chem. 1973 104,220. 93 H. H. Gibson C. H. Bundy and H. R. Gaddy Tetrahedron Letters 1973 3801. Arynes Carbenes Nitrenes and Related Species 193 C,H ,,N=NCO,Et Alkoxy~arbonylnitrenes~~ and aromatic substitution by nitrenes carbenes and free radicals9' have been reviewed. 3 Carbenes Some interesting and extensive theoretical studies on carbenes and their reactions have been reported this year. The energies of the So,S ,and 7'' states of :CH have been calculated as a function of bond angle using the MIND0/2 method and it is predicted that the separation between the So state and the 7'' ground state should decrease with decreasing bond angle to a crossing at a bond angle of 80".96 Cyclopropylidene where the bond angle is constrained to 60" has a much lower singlet-triplet separation than methylene but the ground state is still 7'' ;however vinylidene where the bond angle is formally zero is calculated to have a singlet ground state.The preferred reaction paths for the reactions of singlet and triplet methylene with ethylene and methane have also been calculated ; the approaches of singlet methylene are generally similar to earlier predictions from EH calculations. It is interesting to note that the 'forbidden' approach of singlet methylene to ethylene [shown in (go)] is predicted to have quite a low activation energy (ca. 40kJ mol-') and so may be feasible for 'hot' carbene~.~~ The results of CNDO INDO and AVECIS CF calculations on methylene and its fluoro-derivatives have been ~ompared.~' The presence of an a-mercuric group in carbenes greatly accelerates intersystem crossing to the ground state by an internal heavy-atom effect.98 In view of the easy synthesis99 of a-mercuridiazo-compounds as carbene precursors this effect provides a useful kinetic probe for determining the ground-state multi- plicity of ~arbenes.~~ A heavy-atom effect is also observed in the reactivity of halogenoethoxycarbonylcarbenes generated by the photolysis of ethyl bromo- iodo- or chloro-diazoacetates.loo Even when generated in the presence of 94 H. S. M. Abdul Pakistan J. Sci. Ind. Res. 1972 15 258. 95 S. R. Challand in 'Aromatic and Heteroaromatic Chemistry' ed.C. W. Bird and G. W. H. Cheeseman (Specialist Periodical Reports) The Chemical Society London 1973 Vol. 1 p. 260. " N. Bodor M. J. S. Dewar and J. S. Wasson J. Amer. Chem. SOC.,1972 94 9095. 97 V. Menendez and J. M. Figuera Anales de Quim. 1973,69 1. 98 P. S. Skell S. J. Valenty and P. W. Humer J. Amer. Chem. Soc. 1973 95 5041. 99 P. S. Skell and S. J. Valenty J. Org. Chem. 1973 38 3937. loo M. Reetz U. Schollkopf and B. Banhidai Annafen 1973 599. 194 1.T. Sharp triplet sensitizers these carbenes add stereospecifically to olefins owing to the accelerated intersystem crossing to the singlet ground state before addition occurs (see also ref. 168). Generation.+-Elimination provides a new route to keto-carbenoids e.g. the reaction of (91) with diethylzinc in benzene gave (92) by intramolecular inser- tion :lo' the analogous diazo-ketone gave the same product.The keto-carbenoid from (93) has been trapped by cycloaddition to alkenes.'02 Reactions of benzyl V v PhCOCPhBr (93) halides have in the past failed to provide a good route to phenylcarbene; however the use of lithium 2,2,6,6-tetramethylpiperidide as a strong proton-specific base enabled arylcyclopropanes and 3-arylcyclopropenes to be synthesized in high yield.Io3 An unusual photochemical cleavage of benzoylmethylenetriphenylphos-phorane provides a useful route to benz~ylcarbene.'~~ It has previously been suggested that the photochemical extrusion of carbenes from cyclopropanes occurred via trimethylene biradicals or oia excited states having much biradical character but some elegant experimental work has now shown that the elimina- tion of diphenylcarbene from (94) is stereospecific and proceeds via the singlet state by a concerted pro~ess."~ The cyclopropane (95) is formed by the gas- phase addition of dichlorocarbene to tetrafluoroethylene at 140 "C in a 'chemi- cally activated' state with an excess of vibrational energy which leads to its Ph Ph /q [F*&J OAc OAc (95) decomposition to difluorocarbene unless rapidly deactivated by collision.' O6 The competition between difluorocarbene extrusion and rearrangement has been studied for a range of halogenopolyfluorocyclopropanes.'O7 Both benzo- lo' L.T. Scott and W. D. Cotton J. Amer. Chem.SOC.,1973 95 2708. Io2 L. T. Scott and W. D. Cotton J. Amer. Chem. SOC.,1973,95 5416. '03 R. A. Olofson and C. M. Dougherty J. Amer. Chem. SOC.,1973 95 581. Io4 R. R. da Silva V. G. Toscano and R. G. Weiss J.C.S. Chem. Comm. 1973 567. '05 S. S. Hixon J. Amer. Chem. SOC.,1973 95 6144. lo6 J. M. Birchall R. N. Haszeldine and D. W. Roberts J.C.S. Perkin I 1973 1071. lo' J. M. Birchall R. Fields R. N. Haszeldine and N. T. Kendall J.C.S. Perkin I 1973 1773. Arynes Carbenes Nitrenes and Related Species cyclopropene"* and phthalide'08~'09cleave like indazole on pyrolysis to give (96),which rearranges to fulvenalleneand ethynylcyclopentadiene. The phthalide isomer ar-coumaranone decomposes by a different path losing carbon monoxide rather than carbon dioxide;'08 this difference in behaviour is explained by thermochemical estimates.' 0°C"' Full details are now available of Newman's improved method for generating unsaturated carbenes e.g.the reaction of (97) with sodium hydroxide in the presence of alkenes gives alkylidenecyclopropanesin high yield.' ' The highly unsaturated carbene (99) was produced by the reaction of (98) with potassium t-butoxide and added to alkenes to give rather fragile cyclopropanes.' '' CH,N(NO)COMe -* A kinetic investigation into the gas-phase decomposition of (100) led to the conclusion that the rate-determining step is the unimolecular transfer of a-fluorine to silicon via the three-centre transition state (101) to give difluoro-methylfluorocarbene' ' (see also ref.114). Kinetic evidence also indicates the .:F .. CHFzCFzSiF3-+ CHF,CF.; -+ CHF,CF + SiF .. (100) (101) 'SiF3 presence of free dichlorocarbene in the thermolysis CCl,SiCl and related compounds.' '' Dichlorocarbene is similarly formed from CC13GeC13 and diphenylcarbene from Ph,CClGeCl .'' Organomercury compounds continue lo' C. Wentrup and P. Muller Tetrahedron Letters 1973 2915. Io9 U. E. Wiersum and T. Niewenhuis Tetrahedron Letters 1973 2581. lo C. Wentrup Tetrahedron Letters 1973 2919. I I' M. S. Newman and Z. ud Din J. Org. Chem. 1973 38 547. J. Gore and A. Doutheau Tetrahedron Letters 1973 253. 'I3 R. N. Haszeldine P. J. Robinson and W. J. Williams J.C.S. Perkin 11 1973 1013. ' l4 R. N. Haszeldine C. Parkinson and P.J. Robinson J.C.S. Perkin II 1973 1018. 'Is E. Lee and D. W. Roberts J.C.S. Perkin 11 1973 437. l6 S. P. Kolesnikov B. L. Perl'mutter and 0. M. Nefedov Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1973 1418 (Chem. Abs. 1973,79 92 339). 196 J. T. Sharp to provide new sources of halogenocarbenes e.g.CF,CF,' '' :CFCl,' l8 :CFBr,' ' :CF2,'20*'2' :CC12,'22 :CBr2,'22 and :CClBr.'22 Makosza's method has been used for the preparation of the versatile gem-dibromocyclopropanes'23*'24 and for 1-fluoro- 1-iodocyclopropanes.'25 Further mechanistic details are now available on the acceleration of the diethylzinc-methylene halide cyclopropanation of olefins by oxygen light and radical initiators ;a free-radical chain mechanism is proposed for the generation of the methylene-transfer reagent followed by a conventional transfer step.' 26 The zinc-methylene iodide reagent which converts benzaldehyde into styrene is thought to be CH,(ZnI) and there may be a reversible reaction between this and the Simmons-Smith reagent (ICH2ZnI).127 The first example has been reported of a benzonitrile oxide adding to an olefin via the carbene structure Ph-C-N=O to give an unstable cyclo- propane.'28 Further evidence has been presented for the formation of the interesting aminocyanocarbenes in the basic decomposition of t-octylamino-malononitrile.' 29 Dinitrocarbene is postulated as an intermediate in the forma- tion of (102) in a silylation reaction in the presence of cycl~hexene.'~~ The full report in the formation of 1,3-dithiolium carbenes from carbon disulphide and electron-deficient acetylenes has now appeared.13' There are recent reviews on the preparation and reactions of diazomalonic esters' 32 and the generation I I7 D. Seyferth and G. J. Murphy J. Organometallic Chem. 1973 52 C1. 1 I8 D. Seyferth and G. J. Murphy J. Organometallic Chem. 1973 49 117. 1 I9 D. Seyferth and S. P. Hopper J. Organometallic Chem. 1973 51 77. 120 L. T. Knunyants F. Ya. Komissarov B. L. Dyatkin and L. T. Lantseva Izuest. Akad. Nauk S.S.S.R. Ser. khim. 1973 943 (Chem. Abs. 1973,79 42 635). 121 D. Seyferth and S. P. Hopper J. Org. Chem. 1973,37,4070. 122 D. Seyferth and C. K. Haas J. Organometallic Chem. 1972 46 C33. I23 L. Skattebd G. A. Abskharoun and T. Greibrokk Tetrahedron Letters 1973 1367.I24 M. Makosza and M. Fedorynski Synthetic Comm. 1973 3 305. I25 P. Weyerstahl R. Mathias and G. Blume Tetrahedron Letters 1973 61 1. 126 S. Miyano and H. Hashimoto Bull. Chem. SOC. Japan 1973 46 892. 127 S. Miyano T. Ohtake H. Tokumasu and H. Hashimoto Nippon Kagaku Kaishi 1973 381 (Chem.Ah. 1973,78 159 784). 128 G. Lo. Vecchio G. Grassi F. Risitano and F. Foti Tetrahedron Letters 1973 3777. I29 L. de Vries J. Org. Chem. 1973 38 2604. I30 S. L. Ioffe L. M. Makarenkova M. V. Kashutina V. A. Tartakovskii N. N. Rozhdes-tvenskaya L. I. Kovalenko and V. G. Isagulyants Zhur. org. Khim. 1973 9 905 (Chem. Abs. 1973,79 53 436). 131 H. D. Hartzler J. Amer. Chem. SOC.,1973 95 4379. I32 B. W. Pearce and D. S. Wulfman Synthesis 1973 137.Arynes Carbenes Nitrenes and Related Species of carbenes from diazo-compounds' 33 and by photo-cycloelimination' 34 reactions. Rearrangements.-Methylcarbene ( 103)rearranges to ethylene so fast that it never undergoes intermolecular addition and insertion reactions. It has now been calculated that cis migration (of H-5or H-6) would require ca. zero activation energy and should occur in the time of a molecular vibration whereas trans migration (of H-4) would be highly unfa~ourable.'~~ It is also predicted that the axial hydrogen in cyclohexylidene should migrate more readily than the equat0ria1.I~~Other calculations on the rearrangement of (103) also predict low activation energy. 36 The facility of this hydrogen-transfer reaction allows methyl groups to be used for the detection of carbene intermediates e.g.in the related deuterium migration converting (104)-P (105) at 700 "C the methyl group functions as an effective (94%) trap for the carbene and there is little leakage to the biradical (106). Similarly biradicals like (106) are effectively trapped by the methyl group and do not significantly convert to (104).137Other workers however have proposed that olefins e.g. (log) are not formed directly from carbene precursors e.g. (107) in a concerted process but by migration of the hydrogen atom to give a discrete intermediate thought to be the phantom 133 W. J. Baron M. R. De Camp M. E. Hendrick M. Jones R. H. Levin and M. B. Sohn in 'Carbenes' ed. M. Jones and R. A. Moss Wiley 1973 Vol.I p. 1. ' 34 G. W. Griffin and N. R. Bertoniere in 'Carbenes' ed. M. Jones and R. A. Moss Wiley 1973 Vol. 1 p. 305. IJs N. Bodor and M. J. S. Dewar J. Amer. Chem. SOC.,1972,94 9103. V. Menendez and J. M. Figuera Chem. Phys. Letters 1973 18 426. 13' W. D. Crow and M. N. Paddon-Row Tetrahedron Letters 1973 2217. 198 J. T. Sharp singlet state (108) in which the groups about the 'double' bond are perpen- di~u1ar.l~~ This intermediate would then decay to give the E and 2 olefins in a ratio independent of the nature of their carbene precursor. The carbene (1 lo) generated by photolysis of trimethylsilyldiazoacetate in part rearranges by methyl migration to give the unusual intermediate (1 11) with a silicon-carbon double bond which adds alcohol to give (1l2).l3' The Me,SiCCO,Et + Me,Si=CMeCO,Et -* Me,Si(OR)CHMeCO,Et (1 10) (1 11) (1 12) ring expansion of molecules containing a carbene at a bridgehead position provided a means of generating some anti-Bredt olefins which either rearrange or dimerize.' 40 There has been much interest recently in carbenes with a-unsaturated groups e.g.(113; X = 0,NR or CR,) and particularly in the participation of the iso- meric cyclopropene and its hetero-analogues (1 14) in their reactions. Strausz X X X II R~CCR~ s R~CCRZ II e AR2 R' has now published full details of his work on the search for transient oxirenes (1 16) in the Wolff rearrangement of dia~o-ketones.'~' The labelled keto- carbenes (1 15) showed a surprising difference in reactivity depending on the mode of generation when formed by photolysis the carbene (115) equilibrated with (117) via the oxiren (116) thus leading to both ketens (118) and (119) but no 0 0 0 0 II N2 /\ .. I1 RC-CR -+ R#CR R$=CR R$CR label scrambling and no oxiren participation were detected in the thermally induced reaction. Extended Huckel calculation^'^^ on the system showed that 13' M. Pomerantz and T. H. Witherup J. Amer. Chem. SOC.,1973,95 5977. 139 W. Ando T. Hagiwara and T. Migita J. Amer. Chem. SOC.,1973,95 7518. I4O M. Farcasiu D. Farcasiu R. T. Conlin M. Jones and P. v. R. Schleyer J. Amer. Chem. SOC.,1973 95 8207; A. D. Wolf and M. Jones ibid. p. 8209. 14' J. Fenwick G. Frater K. Ogi and 0.P. Strausz J. Amer. Chem. SOC.,1973 95 124.14' I. G. Csizmadia H. E. Gunning R. K. Gosavi and0. P. Strausz J. Amer. Chem. SOC. 1973 95 133. Arynes Carbenes Nitrenes and Related Species the oxiren state lay at higher energy than the transition state for keten formation and the experimental results were rationalized by the suggestion that only carbenes in the vibrationally excited state resulting from the photochemical decomposition of the diazo-ketone would have sufficient energy for oxiren formation to complete effectively with rearrangement to the keten (118). Con- certed formation of ketenes in a single step from diazo-ketones now seems un- likely in view of the high calculated activation energy (477 kJ mol-').'42 Other recent results conflict with some aspects of this work for example another group has demonstrated oxiren participation in both photolytic and thermal reactions of diazo-ketones.' 43 Theoretically too there is a difference of opinion regarding the relative stability of the oxiren (116) and the isomeric carbene (1 15); whereas EH ~alculations'~~ predict that the oxiren is inherently unstable with respect to C-0 bond cleavage MIND0/3 results'44 show that (116) is ca.80 kJmol-' more stable than (115). In general the latter results show that oxirens and similar anti-aromatic compounds (114; X = NH or S) should occur as stable reaction intermediates rather than short-lived transients. It is also predicted that keto-carbenes should convert without activation to ketene~.'~~.'In other ~alculations,'~~ 44 oxiren and formylcarbene are found to have almost identical energies both being less stable than keten by ca.290 kJ mol-'. Comparison of the products from the peroxy-acid oxidation of cyclo- alkynes and the decomposition of related diazo-ketones provides further evi- dence for the intermediacy of keto-carbenes or keto-carbenoid transition states in the oxidation rea~ti0ns.I~~ An elegant new synthesis of /I-lactams with obvious applications in the preparation of antibiotic analogues has been achieved by Wolff ring-contraction of 3diazopyrrolidine-2,4-diones.147 a-Iminocarbenes (113; X = NR) have been generated by the pyrolysis of 1,2,3-tria~oles'~~* 149 and product studies show that these species can also equili- brate via (114; X = NR). N-Alkyliminocarbenes undergo the Wolff rearrange- ment but also react by a new 1,Qhydrogen transfer'49 to give isoquinolines e.g.the formation of 3-methylisoquinoline from both 1,4-dimethyl-5-phenyl- triazole and 1,5-dimethyl-4-phenyltriazole via (120) and (121). 14' '43 S. A. Matlin and P. G. Sammes J.C.S. Perkin I 1972 2623. 144 M. J. S. Dewar and C. A. Ramsden J.C.S. Chern. Comm. 1973,688. 14' A. C. Hopkinson J.C.S. Perkin If 1973 794. P. W. Concannon and J. Ciabattoni J. Amer. Chern. SOC.,1973,95 3284. 14' G. Lowe and D. D. Ridley J.C.S. Chem. Comm. 1973 328. 14* T. L. Gilchrist G. E. Gymer and C. W. Rees J.C.S. Perkin I 1973 555. 149 T. L. Gilchrist G. E. Gymer and C. W. Rees,J.C.S. Chem. Comrn. 1973 835. 200 J. T. Sharp Several papers on the thermal and photolytic ring-opening of cyclopropenes (114; X = CH,) have demonstrated ring cleavage to vinylcarbenes (113; X = CH,).On pyrolysis the optically active cyclopropene (123) racemizes faster than it converts to products and thermochemical analysis suggests that ring cleavage and bond' rotation occur simultaneously to give (122) or (124) which either ring close to give (123) or its enantiomer or undergo hydrogen shifts to give products. lS0 Product studies also show that (125) is thermally cleaved to HC-Etc-) A,.H wEt,x r\ Et E Et give the two possible vinylcarbene~'~' but a large negative AS* is not consistent with simple ring opening and may be due to a bond cleavage concerted with migration or the formation of a more highly structured precursor of the two carbenes.Other cyclopropenes' ",'53 have been cleaved photochemically to give vinylcarbenes. Full details have now appeared of Crow's work on the high-temperature rearrangement reactions of arylcarbenes and on the mechanism of indazole pyrolysis.'54 A full report has also appeared from W. M. Jones' group on their extensive work on the rearrangements of arylcarbenes ;'55 it is concluded that all evidence points to the cyclopropene mechanism for carbene interconversions rather than a one-step Wolff-type mechanism.' 55 The 10 n-electron aromatic carbene (127) is produced by the rearrangement of (126) in solution at 135 0C.156 The rearrangement of acenaphthylcarbene to phenalenylidene (at 410 "C) is the first carbene-car bene rearrangement featuring expansion of a five-membered ring.' " 150 E.J. York W. Dittmar J. R. Stevenson and R. G. Bergman J. Amer. Chem. SOC. 1973,95 5680. 15 I R. D. Streeper and P. D. Gardner Tetrahedron Letters 1973 767. 152 J. A. Pincock R. Morchat and D. R. Arnold. J. Amer. Chem. SOC.,1973 95 7536. 153 L. Schrader and W. Hartmann Tetrahedron Lctrers 1973 3995. 154 W. D. Crow and M. N. Paddon-Row Ausrrrrl. J. Chem. 1973 26 1705. 155 W. M. Jones R. C. Jokes J. A. Myers T. Mitsuhashi K. E. Krojka E. E. Waali T. L. Davis and A. B. Turner J. Amer. Chem. SOC.,1973 95 826. I56 P. H. Gebert R. W. King R. A. LaBar and W. M. Jones J. Amer. Chem. SOC.,1973 95 2357. 157 T. T. Coburn and W. M. Jones Tetrahedron Letters 1973 3903.Arynes Carbenes Nitrenes and Related Species 201 The cyclohexadienylidene (128) undergoes a remarkable fragmentation in the gas phase at 380 "C to give p-xylene and toluene. Crossover experiments with the diethyl analogue showed that the carbenes fragment to give radicals which abstract and recombine to give the products.15* This reaction has been adapted to the preparation of [6]1 and [7]-paracyclophane~.~~~ The ketones (130)formed in the thermal rearrangement of the oxycarbene (129)are formed via a biradical intermediate in w.hich the stereochemical integrity of (129) is lost; however the concurrent fragmentation to an olefin and ketene proceeds with retention of configuration probably via a [4+ 21 cyc1oreversion.l6' Me Me Q -(9-..-0+6 / \ \ Me Me Me Me Addition.-The first intermolecular addition of a 'foiled' methylene to an olefin has been carried out ;carbene (1 31) was found to be very unreactive towards cis-4-methylpent-2-ene compared with its saturated analogue owing to the stabilizing interaction in (131) between the carbene centre and the double bond which facilitates intramolecular reaction.' 61 The application of relative reactivity studies to carbene-lefin additiqp reactions has been reviewed.62 The presence of a mercury atom in MeCOCHgMe changes the chemistry compared with that of MeCOCH so that the former does not undergo Wolff rearrangement but adds stereospecifically to alkenes in high yield.'63 This is a potentially 15* T. E. Berdick R. H. Levin A. D. Wolf and M.Jones J.Amer. Chem. SOC., 1973,95 5087. I A. D. Wolf V. V. Kane R. H. Levin and M. Jones J. Amer. Chem. SOC.,1973,95 1680. 160 A. M. Foster and W. C. Agosta J. Amer. Chem. SOC.,1973 95 608. 16' R. A. Moss and U.-H. Dolling Tetrahedron Lerrers 1972 51 17. 162 R. A. Moss in 'Carbenes' ed. M. Jones and R. A. Moss Wiley 1973 Vol 1 p. 153. P. S. Skell and S. J. Valenty J. Amer. Chem. SOC.,1973 95 5042. 202 J. T. Sharp useful synthetic reaction since the carbon-mercury bond in the products can be cleaved by electrophilic reagents. Steric factors are particularly important in determining the course of the reaction of diphenylcarbene with alkenes e.g. propene and isobutene give only cyclopropanes whereas more hindered olefins such as 2-methylbut-2-ene react largely by abstraction-recombination.In addition to enynes the double bond is favoured.’64 It has been suggested that ‘steric attraction’ due to secondary orbital interactions is at least partly res- ponsible for the contra-thermodynamic stereoselectivity observed in the addition of unsymmetrical carbenes to cis-olefins.’ 65 It was recently suggested that the lack of synergism in the addition of di- chlorcarbene to compounds such as (132) might be due to the forbidden character of the intramolecular front-side displacement of the CCl fragment from the oxygen carrier. This idea has now been tested with compounds like (133) in which back-side displacement could occur (134) but these compounds too Q Q 0U 0U showed no evidence for prior co-ordination of :CCI to the oxygen atom.166 Either oxygen atoms are not nucleophilic enough to compete with the double bond for the highly selective carbene or any 0-ylide formed takes an alternative reaction path to (134).The sulphur atom in vinyl sulphides however competes very effectively with the double bond for methoxycarbonylcarbene e.g. (135) gives only 5 % of the cyclopropane and 39 % of (1 36) formed by elimination from the sulphur ylide. 16’ /SEtMe,C=C ‘Me -+ SCH,CO,Me/ \Me,C=C Me + CH,=CH (135) (136) Cyclopentadienylidene has a triplet ground state but is so reactive that it adds to alkenes in its singlet state faster than its intersystem crosses to the ground state ; the tetrabromo-analogue however crosses more rapidly owing to the 164 W.J. Baron M. E. Hendrick and M. Jones J. Amer. Chem. SOC.,1973,95 6286. 165 R. Hoffmann C. C. Levin and R. A. Moss,J. Amer. Chem. SOC.,1973,9S 629; see also Theor. Chim. Acta 1972 26 43. 166 R. A. Moss and C. B. Mallon Tetrahedron Letters 1973 4481. 16’ W. Ando H. Fujii T. Takeuchi H. Higuchi Y. Saiki and T. Migita Tetrahedron Letters 1973 21 17. Arynes Carbenes Nitrenes and Related Species heavy-atom effect (see also refs. 98 and 100) and adds non-stereospecifically to olefins.' 68 Similarly non-stereospecific additions of triplet di- and tetra-phenylcyclopentadienylidenes can be observed if the diazo-precursor is photo- lysed in the presence of a triplet sensitizer ;direct photolysis gives stereospecific additi011s.I~~The addition of cyclopentadienylidenes to benzene has already been much studied but further investigation has provided strong evidence for the intermediacy of the novel bisnorcaradiene (138) in the equilibration of (137) and (139) which convert to products by hydrogen shifts.'70 The irradiation of substituted diazocyclopentadienes in pyridines gave stable ylides as major products and also for example (140) from 2,6-dimethylpyridine.A similar reaction in dimethylthiophen gave (141).' R4 ~3 R4 R3 R4 Me The use of copper catalysts in cyclopropanation reactions using diazo- compounds leads to rather complex chemistry. However it is clear that olefin- metal co-ordination is important in copper triflate catalysed reactions in which the cyclopropanation of the least alkylated olefin is favoured whereas most copper catalysts favour the most nucleophilic olefin.172 The differing reactivity patterns observed in the copper-catalysed addition of ethyl diazoacetate to the two double bonds of dicyclopentadiene have also been related to differing abilities of the catalysts to co-ordinate to double bonds." Copper-isocyanide complexes provide useful cyclopropanation reagents for electron-deficient olefins. 74* '" E. T. McBee and K. J. Sienkowski J. Org. Chem. 1973 38 1340. H. Diirr and W. Bujnoch Tetrahedron Letters 1973 1433. I7O H. Diirr H. Kober I. Halberstadt U. Neu T. T. Coburn T. Mitsuhashi and W. M. Jones J. Amer. Chem. SOC.,1973 95 3818. I ' H. Diirr B. Heu B. Ruge and G. Scheppers J.C.S. Chem. Comm. 1972 1257.17' R. G. Salomon and J. K. Kochi J. Amer. Chem. SOC.,1973,95 3300. 173 T. Sato T. Mori and J. Shinoda Bull. Chem. SOC.Japan 1973,46 1833. T. Saegusa K. Yonezawa and Y. Ito Synthetic Comm. 1972 2 431. 17' T. Saegusa K. Yonezawa I. Murase T. Konoike S. Tomita and Y.Ito J. Org. Chem. 1973,38 2319. 204 J. T. Sharp The cyclopropanation of trimethylsilyl enol ethers with the Simmons-Smith reagent provides a convenient route from aldehydes and ketones to cyclo- propanols and also to the @-methyl carbonyl compounds.' 76*177 Dimethoxy-carbene adds two molecules of aryl isocyanates to give 5,5-dimethoxyhydan- toins.'78 Phenalenylidene is reported to add to cycloheptatriene by a unique 1,6-addition. 79 Further evidence for the importance of charge-transfer complexes in the stereospecific addition of the triplet carbene (142) to olefins has been provided 0 1 (CN),C= C(CN) by the e.s.r.detection of the TCNE radical anion."' 4,9-Methano[l llannuleny- lidene shows the typical reactivity of a singlet aromatic carbene; like cyclo- heptatrienylidene it dimerizes and adds readily to electron-deficient olefins. Cycloheptatrienylidene has also been added to dienes styrene and phenyl- acetylene' 82 to give adducts which rearrange readily. Insertion.-The full report on the insertion of phenyl( bromodichloromethy1)- mercury-derived dichlorocarbene into the benzylic C-H bond of ( + )-2-phenylbutane confirms that the reaction occurs with predominant retention of c~nfiguration."~ This and a Hammett study using substituted cumenes show that the insertion is concerted and involves a transition state like (143).Di- chlorocarbene derived from the same source also inserts into the Si-C bond of l-butyl-1,3-dimethyl- 1-silacyclobutanes largely with retention of configuration most likely uia a transition state like (lM).'84 The insertion of diazomethane- ( I 43) ( 144) J. M. Conia and C. Girard Tetrahedron Letters 1973 2767. G. M. Rubottom and M. I. Lopez J. Org. Chem. 1973,38 2097. R. W. Hoffmann K. Steinbach and B. Dittrich Chem. Ber. 1973 106 2174. I. Murata T. Nakazawa and T. Imanishi Tetrahedron Letters 1972 5089. I8O Y. Yamamoto S-I. Murahashi and I. Moritani Tetrahedron Letters 1973 589. 181 R. A. LaBar and W. M. Jones J. Amer. Chem. SOC.1973,95,2359. E. E. Waali and W. M. Jones J. Amer. Chem. SOC.,1973,95,8114; J. Org. Chem. 1973 38 2573. D. Seyferth and Ying Ming Cheng J. Amer. Chem. SOC. 1973,95 6763. D. Seyferth Houng-Min Shih J. Dubac P. Mazerolles and B. Serres J. Organo-metallic Chem. 1973 50 39. Arynes Carbenes Nitrenes and Related Species 205 derived methylene into the Si-C bond of (145) however is thought to take place via (146),which undergoes migration of the trimethylsilyl group with concerted loss of nitrogen.' Me,SiCH=CHCO,Me + Me,Si + Me,SiCH,CH=CHC02Me (145) (146) The irradiation of dimethyl diazomalonate in benzene is reported to give a high yield (65 %) of dimethyl phenylmalonate;'86 this however conflicts with a recent report that the same reaction gave 7,7-dimethoxycarbonylcyclohepta-triene and dimethyl phenylmalonate in a 2.7 :1 ratio.18' Bismethoxycarbonyl-carbene is much more selective in its reactions than ethoxycarbonylcarbene e.g.the former reacts selectively with cyclohexane in mixtures containing benzene whereas the latter reacts equally with both solvents. Decomposition of the diazomalonic ester in toluene gave 33 %of side-chain insertion and 67 %insertion into the aromatic C-H bonds with attack at the 0-and p-positions predominat- ing.'86 Intramolecular insertion reactions of the carbenoids derived from mixed alkyl methyl diazomalonates e.g. (147),provide a route to butyrolactones'88 which proceeds with retention of configuration.' 89 Methyl benzyl diazo- malonates give lactones of 7-carboxy-l-hydroxymethylcycloheptatriene~.~~~ Ethoxycarbonylcarbene inserts into the 0-H bonds of alcohols water and weak acids in high yield in the homogeneous rhodium-catalysed decomposition of ethyl diazoacetate.' 91 Alkylation with carbenes has been reviewed.' 92 '*' R.F. Cunico Hong Mee Lee and J. Herbach J. Organometallic Chem. 1973,52 C7. H. Ledon G. Linstrumelle and S. Julia Bull. SOC. chim. France 1973 2065. M. Jones W. Ando M. E. Hendrick A. Kulczycki P. M. Howley K. F. Hummel and D. S. Malament J. Amer. Chem. SOC. 1972 94 7469. H. Ledon G. Linstrumelle and S. Julia Bull. SOC. chim. France 1973 2071. H. Ledon G. Linstrumelle and S. Julia Tetrahedron Letters 1973 25. Igo H. Ledon G. Linstrumelle and S. Julia Tetrahedron 1973 29 3609. I9l R.Paulissen H. Reimlinger E. Hayez A. J. Hubert and Ph. Teyssie Tetrahedron Letters 1973 2233. 192 T. L. Gilchrist Chem. and Ind. 1973 881.

ISSN:0069-3030    

DOI:10.1039/OC9737000177

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

6 Molecular Rearrangements By G. TENNANT Department of Chemistry University of Edinburgh West Mains Road. Edinburgh EH9 aJ 1 Introduction The continuing importance of molecular rearrangements in mechanistic and synthetic organic chemistry is demonstrated by the multitude of publications which appeared in the literature during 1973. Unhappily lack of space permits the discussion of only some 250 of the more than 1500 relevant papers and review articles noted. Heterocyclic rearrarlgements have not been allocated a separate section this year. The originator of a now well-known class of ylide rearrangements is co-author of an excellent new monograph on molecular rearrangements.' Two authoritative articles concerned with the application of CIDNP as a mechanistic probe for rearrangement processes have been published.2 Aliphatic Rearrangements Anidc Rearrangements.-A theoretical treatment of sigmatropic rearrange- ments involving polar transition states indicates that configuration interaction can reverse the stereoselectivity of [1,2] but not of [1,4] anionic shift^.^ Ultimate formation of the stable aromatic dianion (3) provides the driving force for the hitherto unknown [1,2] anionic shift of carbon involved in the transformation of the spiro[2,7]decatrienyl anion (1) into the bicyclo[6,2,0]decatrienyl anion (2).4 ' T. S. Stevens and W. E. Watts 'Selected Molecular Rearrangements' Van Nostrand Reinhold London 1973. * A. R. Lepley in 'Chemically Induced Magnetic Polarization' ed. A. R. Lepley and G.L. Closs Wiley-Interscience New York 1973 pp. 324-375; D. Bethel1 and M.R. Brinkman Adv. Phys. Org. Chem. 1973,10 115-120. N. D. Epiotis J. Amer. Chem. SOC.,1973 95 1206. S. W. Staley G. M. Cramer and W. G. Kingsley J. Amer. Chem. SOC.,1973,95 5052. 206 Molecular Rearrangements The facility of this otherwise forbidden carbon migration is explicable in terms of a concerted pathway involving a [1,8] carbon shift in an aromatic (ten electron) transition state. The rearrangements (4) +(5) and (6)-B (7) exemplify novel [1,2] and [1,4] C +C trimethylsilyl shifts to carbanion sites.’ Formation of (7) by a direct [1,3] anionic trimethylsilyl shift in (4) was excluded and competing phenyl shifts were not observed.’ Similar [1,2] N +C trimethylsilyl shifts to \ SiMe ‘Ph H Ph (4) BuLi 1 Li + CH,SiMe, I Ph Li+ Li’ SiMez \-c-c -/ / I -\ H ‘Ph H Li’ Ph (7) benzyl carbanion centres in acetamidine derivatives have also been reported.6 The identical product mixtures obtained from the reaction of lithium benzo- phenone ketyl with hex-5-enyl iodide and the Wittig rearrangement of benzhydryl hex-5-enyl ether show similar ratios for products derived by cyclization of hex-5-enyl radicals and subsequent coupling with ketyl but divergent ratios for products of direct hex-5-enyl-ketyl coupling.These results are interpreted in terms of competing intermolecular and intramolecular radical pathways for the Wittig process.’ Further evidence has accumulated in support of non-concerted pathways for Meisenheimer rearrangements.The lack of stereoselectivity observed in the [1,2] Meisenheimer rearrangements of (2)-and (E)-N-benzhydryl nitrones is consistent with the operation of a radical-pair process.8 Likewise the low cage effect (33%) observed in the Meisenheimer rearrangement of N-benzyl-N-methyl- aniline N-oxide is in accord with a predominant non-geminate radical-pair pathway to product.’ On the other hand the highly stereospecific character of the thermal reorganizations of N-ally1 amine N-oxides demands their formula- tion as concerted [2,3] sigmatropic processes.’o9 ’’ Thus the transformation of J. J. Eisch and M.R. Tsai J. Amer. Chem. SOC.,1973,95,4065. 0. J. Scherer and G. Schnabl J. Organometallic Chem.1973 52 C18. J. F. Garst and C. D. Smith J. Amer. Chem. Soc. 1973,95,6870. * T. S. Dobashi and E. J. Grubbs J. Amer. Chem. SOC. 1973,95 5070. J. P. Lorand R. W. Grant P. A. Samuel E. M. O’Connell J. Zaro J. Pilotte and R. W. Wallace J. Org. Chem. 1973 38 1813. Y. Yamamoto J. Oda and Y. Inouye J.C.S. Chem. Comm. 1973 848. lo V. Rautenstrauch Helv. Chim. Acta 1973 56 2492. 208 G. Tennant the chiral amine N-oxide (8) into the hydroxylamine (9) proceeds with essentially complete conservation of optical activity in keeping with rearrangement via a cyclic five-membered half-chair transition state. ' Aldoxime 0-ally1 ethers H I WIY1v ' 0N-NMe undergo the reverse thermal [2,3] shift to nitrones to the complete exclusion of alternative [3,3] sigmatropic pathways.l2 In contrast hydroxylamine 0-ally1 ethers rearrange by [1,3] shifts in presumed radical-pair processes." Meisen-heimer and Stevens rearrangements at the bridgehead in homoadamantane derivatives have been reported.' The competing [1,2] (Stevens) and [2,3] (Sommelet-Hauser) rearrangements of dibenzyl sulphide carbanion have been shown to be markedly solvent- and temperature-de~endent.'~ The concomitant formation of products derived by coupling (e.g. bibenzyl) and the predominance of Stevens rearrangement at high temperatures and in non-polar media support a radical-pair mechanism for the [1,2] shift. Conversely a concerted pathway for the [2,3] shift is indicated by its prevalence at low temperatures in polar media.14 Competition between Stevens and Sommelet-Hauser rearrangements is also observed in sulphur ylides generated in situ by the reaction of aryl carbenes with dialkyl sulphides.15 The formal nitrogen counterpart of this type of process is represented by the 'one pot' reaction of an aniline with t-butyl hypochlorite in the presence of a dialkyl sulphide.This elegant new synthetic method leads in high yield by exclusive Sommelet-Hauser rearrangement in an intermediate sulphilimine ylide to the corresponding specifically ortho-alkylated aniline.'6*' ' The considerable syn- thetic potential of this novel process is demonstrated by its application to the alkylation of aminopyridines'* and in a general synthesis of indole derivatives.16 Considerable experimental effort continues to be devoted to the study of rearrangements undergone by ammonium and sulphonium ylides.Particularly noteworthy are the high-order rearrangements of a variety of ammonium ylides reported by Ollis and his co-workers. Of these pride of place goes to the remark- able high-yield (ca.60%) rearrangements of ally1 (pentadienyl) ammonium ylides l2 S. Ranganathan D. Ranganathan R. S. Sidhu and A. K. Mehrotra Tetrahedron Letters 1973. 3577. " B. L. Adarns and P. Kovacic J. Amer. Chem. SOC.,1973 95 8206. l4 J. F. Bielmann and J. L. Schmitt Tetrahedron Letters 1973 4615. l5 W. Ando M. Yamada E. Matsuzaki andT. Migita J. Org. Chem. 1972 37 3791. l6 P. G. Gassman T. J. Van Bergen and G. Gruetzmacher J. Amer.Chem. SOC.,1973 95 6508 and other recent papers cited therein. P. G. Gassman and H. R. Drewes J.C.S. Chem. Comm. 1973,488. P. G. Gassman and C. T. Huang J. Amer. Chem. SOC.,1973,95,4453. Molecular Rearrangements (10) to octatrieneamines (l2).I9 Except in two instances [i.e. (10d) and (loe)] by-products derived by [2,3] sigmatropic and radical-pair pathways did not exceed 5%. The lack of crossover observed in these rearrangements excludes R' R2 Y R3 (10) R' R2 R3 a; H b; Me Me H Ph Ph c; Me d; H Me H Ph Ph e H Me H R3 NMe mechanisms based on sequential [2,3] and [3,3] migrations and supports their formulation as concerted [4,5] sigmatropic shifts involving aromatic (ten electron) bishomoazonine transition states.l9 Interestingly ethynyl (pentadienyl) ammo- nium ylides rearrange by competing [1,2] (Stevens) and [2,3] proces~es.'~ The [2,3] shift typical of carbonyl-stabilized N-allyiammonium ylides is largely suppressed in favour of other rearrangement modes when the ylide system is part of an aromatic framework. Rearrangements of this are exemplified by the thermal transformations of the 2-oxyanilinium ylides (14) into a mixture of the ethers (15) (79%) and the phenols (16) (11 %).20 Deuterium-labelling studies2' demonstrate that the ethers (15) are the result of a single concerted [1,4] sigmatropic shift whereas the phenolic products (16) stem from consecutive b; R=Ph (15) I9 T Laird and W. D. Ollis J.C.S. Chem. Comm. 1973 658. 2o S. Mageswaran W. D.Ollis I. 0. Sutherland and Y. Thebtaranonth J.C.S. Chem. Comm. 1973,651. " W. D. Ollis I. 0.Sutherland and Y. Thebtaranonth J.C.S. Chem. Comm. 1973 653. 22 W. D. Ollis 1. 0.Sutherland and Y. Thebtaranonth J.C.S. Chem. Comm. 1973 654. 210 G. Tennant [2,3] and [3,3] ally1 migrations. However the detection of CIDNP emission and deuterium scrambling between C-1 and C-3 of the ally1 sidechain in the rearrange- ment (14a) +(15a) + (16a) reveals a concomitant though minor radical-pair pathway to products.22 These observations emphasize the need for caution when using CIDNP as a mechanism probe. In contrast to their ortho counter-parts para-oxyanilinium ylides rearrange to products derived either by consecu- tive concerted [2,3] and [3,3] shifts or by radical fragmentation-recombination pathways the outcome depending on the allylic substitution pattern.20 ortho-and para-N-Pentadienylox yanilinium ylides also contrast in their rearrangement behaviour.Whereas the paru-compounds undergo orthodox stepwise [2,3] and [3,3] sigmatropic shifts ortho-N-pentadienyloxyanilinium ylides (17) afford mixtures of ethers (18) and phenols (19) derived respectively by novel concerted [1,4] and [4,5] shifts.2J The transition state for the latter process is constrained to a geometry (20) which differs from that propo~ed'~ for the acyclic [4,5] shift R' R2 R3 (17) a; H H Me b; Me H H (19) c; H Me H [cf. (1311. The lower rates of rearrangement of six-membered cyclic allylic ammonium and sulphonium ylides are most readily interpreted in terms of concerted processes involving strained 'bicyclic' transition The apparent [2,3J sigmatropic rearrangement of N-ethynylammonium ylides is not inhibited when the ylide system is part of a bicyclic structure.This result contrasts with the inhibition of rearrangement already noted for similar bicyclic N-allyl- ammonium ylides [cf Ann. Reports (B) 1971 68 2431 and suggests a non-23 W. D. Ollis R. Somanathan and I. 0.Sutherland J.C.S. Chem. Comm. 1973 661. '' S. Mageswaran W. D. Ollis and I. 0.Sutherland J.C.S. Chem. Comm. 1973 656. Molecular Rearrangements 21 1 concerted zwitterionic pathway for rearrangement of the acetylene derivative^.^' The rearrangement of cyclic allylammonium ylides derived from pyrrolidines provides the key step in a new general route to By-unsaturated aldehydes which is claimed to be superior to that based on the corresponding rearrangements of sulphonium ylides.26 Lithioderivatives [e.g.(2111 of 1-benzyl-4vinyl-azetidin-2-ones rearrange at -78 "C by allylic participation and concomitant ring expansion yielding azepinones [e.g. (22)]. The failure of N-allyl-N-benzyl-benzamide to rearrange similarly lends weight to the suggestion that this interest- ing /3-lactam ring expansion derives its driving force from relief of ring l train.^' The creation of chirality in an allylic sulphonium ylide and its transfer to quaternary carbon by sigmatropic rearrangement has been demonstrated.28 Thus treatment of S-methyl-S,S-bis-(y,ydimethylallyl)sulphoniumfluoroborate with (R)(-)-2,2,2-trifluorophenylethoxidein (R)(-)-2,2,2-trifluorophenylethanol at -10"Cgenerates the chiral ylide (23) which spontaneously rearranges to the thioether (24) with [cx]~'= -1.45 +_ 0.12'.Despite the low optical yield these results demonstrate the suitability of allylic sulphonium ylide rearrangement as a model for the process involved in the in vivo construction of head -P head-linked polyisoprenoids.28 The high optical induction (>94 %) observed in the [2,3] sigmatropic rearrangement of adamant-1-ylallylethylsulphonium ylide is attributed to concerted rearrangement via the most conformationally stable half- chair transition state.2g Similar stereoselectivity is exhibited by the [2,3] shifts undergone by allylic sulphonium ylides derived from conformationally fixed cyclohexylidene derivative^.^' Allylic sulphonium ylide rearrangement also " W.D. Ollis 1. 0.Sutherland and Y. Thebtaranonth J.C.S. Chem. Comm. 1973,657. 26 L. N. Mander and J. V. Turner J. Org. Chem. 1973 38 2915. '' T. Durst R. Van Den Elzen and M. J. Le Belle J. Amer. Chem. SOC.,1972 94 9261. 28 B. M. Trost and W. G. Biddlecom J. Org. Chem. 1973 38 3438. 29 B. M. Trost and R. F. Hammen J. Amer. Chem. SOC.,1973 95,962. 30 G. Andrews and D. A. Evans Tetrahedron Letters 1972 5 12 1. 212 G. Tennant provides one of the key steps in an elegant general synthesis of A3-cyclopent- en one^.^' The reaction of benzyne with 1-alkynyl or allenyl sulphide anions to give terminal acetylenes is rationalized by [2,3] sigmatropic rearrangement of intermediate allenyl sulphur ylide~.~ The negative entropies of activation observed for the allyl sulphoxide-ally1 sulphenate rearrangements of enantiomeric steroidal 6fl-sulphoxides demonstrate the concerted nature of such processes.The substantially lower rate of rearrangement observed for the (R) 6fl-sulphoxide is attributed to steric crowding in the transition state and illustrates the strictly suprafacial character of the allyl sulphoxide-ally1 sulphenate transf~nnation.~~ The selenium analogue [(25) +(2611 of the thiosulphinate-thiosulphoxylate equilibrium [cf:Ann. Reports (B) 1971 68 2441 has been demonstrated and as anticipated occurs at a faster rate than the sulphur system.34 The novel [2,3] sigmatropic rearrangement of allylseleninic acids to allyl selenium esters is implicated in a mechanistic rationale for the selenium dioxide oxidation of 01efins.~’ Novel Pummerer rearrangements of intermediate sulphinylsulphonium cations are invoked to account for the products of the thermal disproportionation reactions of alkyl thiol~ulphinates.~~ Attempts to trap the oxosulphonium cation intermediates proposed for Pummerer rearrangements have been un- successf~l.~ ’ Several new versions of the Favorskii rearrangement have been reported.The ring contraction of 2-acetylcyclopentanones to 2-(2’-chlorovinyl)cyclo-butene carboxylic acids which occurs on treatment with chloral in the presence of potassium carbonate is rationalized by a mechanism involving the enolate- induced rearrangement of trichloroethylidenecyclopentanone intermediate^.^^ The precise mechanism of these interesting (though low-yield) reactions which also occur in acyclic s~bstrates,~’ awaits the outcome of further experimentation.Deuterium-labelling studies support the involvement of cyclopropanone inter- mediates in the sodium methoxide-catalysed transformation of terminal alkyne 3’ E. J. Corey and S. W. Walinsky J. Amer. Chem. SOC.,1972 94 8932. 32 L. Brandsma S. Hoff and H.D. Verkruijsse Rec. Trav. chim. 1973 92 272. 33 D. N. Jones J. Blenkinsopp A. C. F. Edmonds E. Helmy and R. J. K. Taylor J.C.S. Perkin I 1973 2602. 34 K. B. Sharpless and R. F. Lauer J. Org. Chem. 1972 37 3973. 35 K. B. Sharpless H.P. Jensen D. Arigoni and A.Vasella J. Amer. Chem. SOC.,1973 95 791 7; K. B. Sharpless and R. F. Lauer ibid. 1972 94 7154. ’‘ E. Block and J. O’Connor J. Amer. Chem. Soc. 1973,95 5048. 37 T. Durst K. C. Tin and M.J. V. Marcil Cunad. J. Chem. 1973 51 1704. 38 A. Takeda S. Tsuboi F. Sakai and M. Tanabe Tetrahedron Letters 1973 4961. 39 A. Takeda and S. Tsuboi J. Org. Chem. 1973,38 1709. Molecular Rearrangements 213 diazotates to rearranged esters.40 Major pathways for these rearrangements involving planar oxyallyl cation intermediates are excluded by the demonstration that rearrangement of a chiral diazotate occurs with predominant (88%) inver-~ion.~~ The failure of sodium methoxide to catalyse the Favorskii rearrangements of a'-chloro-a-phenylketones which occur readily in the presence of secondary amines is adduced as evidence for the intermediacy of cyclopropanimonium cations (as opposed to cyclopropanones) in the amine-catalysed transforma- tion~.~' Ketens have been eliminated as viable intermediates in certain homo- Favorskii rearrangement^.^^ Relief of ring strain accounts for the base-catalysed conversion of cyclo- propylmethyl tosylate or bromide in moderate yield into cyclob~tene.~~ The lack of rearrangement in the absence of base and the exclusion (by deuterium- labelling studies) of rearrangement (cyclopropylcarbene ring expansion) induced by initial a-elimination is adduced as evidence for the bimolecular character of these new cyclopropylmethyl ring expansion^.^^ Ring expansion of lithio halohydrin intermediates rationalize^^^ the lithium halide-catalysed rearrange- ments of oxaspiropentanes to cyclob~tanones.~~~~~ Kinetic studies suggest that the rearrangements of 2-bromo- and 2-tosyloxycyclobutanone to cyclopropane- carboxylic acid in neutral media are best explained in terms of the formation and ring contraction of the corresponding cyclobutanone hydrates.46 The reaction of hydroxycyclobutenones with amines results in ring contraction to hydroxycyclopropane carbo~amides.~~ The pattern of deuterium uptake observed in the sodium methoxide-catalysed conversion of the diol (27) in [O-2H]methanol into the rearranged diketone (30) is rationalized by a novel 'double-barrelled' cyclobutane ring contraction-ring expansion (28) -+ (29) preceded and followed by hom~ketonization.~~ The intermediacy of geminal (rather than vicinal) dichlorosulphones and the derived thiiren S-dioxides in the reactions of bis(primary alkyl) sulphones with potassium hydroxide-carbon tetrachloride to give alkenesulphonic acid salts has been established conclu- sive]~.~~ Reactions of a-chloro- and a,a-dichlorodibenzyl sulphides with tri- phenylphosphine followed by potassium t-butoxide provide useful routes to stilbene and tolane derivatives respectively.These transformations exhibit features at varislnce with their formulation as Ramberg-Backlund-type rearrange- men ts. s' 40 W. Kirmse A. Engelmann and J. Hesse Chem. Ber. 1973,106 3073; W. Kirmse and A. Engelmann ibid. p. 3086. 4' F. G. Bordwell and J.Almy J. Org. Chem. 1973 38 571. 42 R. H. Bisceglia and C. J. Cheer J.C.S. Chem. Comm. 1973 165; S. Wolff and W. C. Agosta ibid. p. 771. " W. R. Dolbier and J. H. Alonso J.C.S. Chem. Comm. 1973 394. 44 D. H. Aue M. J. Meshishnek and D. F. Shellhamer Tetrahedron Letters 1973 4799. 45 B. M. Trost and M. J. Bogdanowicz J. Amer. Chem. Soc. 1973 95 5321. 4b J. Salaun B. Garnier and J. M. Conia Tetrahedron 1973 29 2895. 47 W. Reid A. H. Schmidt and H. Medem Annalen 1973 1530. 48 R. D. Miller and D. Dolce Tetrahedron Letrers 1973 1151. 49 C. Y. Meyers L. L. Ho G. J. McCollum and J. Branca Tetrahedron Letters 1973 1843. 50 R. H. Mitchell J.C.S. Chem. Comm. 1973 955; Tetrahedron Letters 1973 4395. 214 G. Tennant H / (29) (30) Cationic Rearrangements.-Photochemical valence isomerizations in carbo- cation^,^ ’ rearrangements accompanying Koch-Haaf carboxylati~n,~~ and the application of physical methods (i.r.Raman n.m.r. and ESCA spectroscopy) to the study of carbocation rearrangements5’ have been reviewed. The current position regarding the respective roles of edge- and corner- protonated cyclopropane intermediates in secondary and tertiary alkyl cation rearrangements is discussed in an authoritative article by the Yale group.54 It has been suggested on theoretical grounds that configuration interaction in the polar transition states for cationic rearrangements can reverse the stereo- selectivity of [1,4] but not of [1,2] cationic shifts3 Retention of configuration at the migrating centre for [1,2] cationic shifts in non-sterically constrained frame- works has been demonstrated for the first time.55*56 This stereoselectivity is in accord with the orbital-symmetry prediction that a thermal cationic [1,2] shift will be allowed in the [,O + a2J mode corresponding to a ‘slither’ type transition state (31) but disallowed in the LO + ,2,] mode corresponding to the ‘pivotal’ type transition state (32).55 The flaws inherent in the traditional methods for establishing migratory preferences in [1,2] cationic shifts particularly in relation to quantitative aspects have been pointed out.” Despite considerable ingenuity 51 P.W. Cabell-Whiting and H. Hogeveen Ado. Phys. Org. Chem. 1973 10 130-144. 52 H. Hogeveen Adu. Phys. Org.Chem. 1973 10.29-52. 53 G. A. Olah Angew. Chem. Internat. Edn. 1973 12 173. 34 M. Saunders P. Vogel E. L. Hagen and J. Rosenfeld Accounts Chem. Res. 1973,6,53. 53 T. Shono K. Fujita and S. Kumai Tetrahedron Letters 1973 3123. st W. Kirmse and W. Gruber Chem. Ber. 1973 106 1365; W. Kirmse W. Gruber and J. Knist ibid. p. 1376. 37 D. Howells and S. Warren J.C.S. Perkzn ]I 1973 1645. Molecular Rearrangements the construction of a suitable reference framework for the study of competing methyl and diphenylphosphinyl shifts was not entirely successful. However the demonstrably negligible difference in migratory aptitude for methyl uersus diphenylphosphinyl tends to support the contention that it is the ability of the non-migrating group not the migrating group to stabilize a positive charge which determines migratory preferen~e.~' A minimum free energy difference of 6.9 kcal mol-' has been estimated for competing em and endo [3,2] hydride shifts in methylated norbornyl cations.'* This result demonstrates the highly stereospecific nature of such shifts and implies an activation energy for [3,2] endo hydrogen and alkyl shifts ca.3 kcal mo1-l in excess of that required for alterna- tive sequential Wagner-Meerwein and [6,2] hydride shifts. However the question of [3,2] endo-methyl shifts in norbornyl cations is still to some extent contentious. Recent evidence59 purporting to support the operation of such a shift has been rejected.60 The possible involvement of a [3,2] endo-methyl shift in the cationic sultone rearrangement (33) +(34) has been shown by deuterium-labelling studies to be unlikely.61 The circumvention of the normal [3,2] exo-methyl shift in bicyclo[2,2,l]heptyl systems forms the basis of a new synthetic route to 8-substituted camphor derivatives.62 Rearrangements associated with vinyl cations have been re~iewed.~' Solvolysis of the trans-triflates (35) in 60% aqueous ethanol results in ca.50% phenyl shift [to rearranged ketone (40)] and occurs cu. 20-40 times faster than similar solvolysis of the cis-isomers (38) in which allene rather than ketone formation 58 L. Huang K. Ranganayakulu and T. S. Sorensen J. Amer. Chem. SOC.,1973,95 1936. 59 C. W. David B. W. Everling R. J. Kilian J. B. Stothers and W. R. Vaughan J. Amer. Chem.SOC.,1973 95 1265. 6o C. J. Collins and M. H. Lietzke J. Amer. Chem. SOC.,1973 95 6842. 6' D. R. Dimmel and W. Y. Fu J. Org. Chem. 1973,38 3778 3783. 62 C. R. Eck R. W. Mills and T. Money J.C.S. Chem. Comm. 1973,911. " P. J. Stang in 'Progress in Physical Organic Chemistry' ed. A. S. Streitwieser and R. W. Taft Wiley-Interscience New York 1973 Vol. 10 p. 205. 216 G.Tennani b; R' = CD, R2 = Me \ C=C=CH, c; R' = Me R2 = CD / Me (37) I Ph R2 \/ c=c R' /\ OTf (38) (39) + R2 \ CHCOR' / Ph (41) Scheme 1 predominate^.^^ These results are interpreted in terms of divergent solvolytic pathways for the trans-and cis-triflates (Scheme 1) involving respectively the bridged vinylidene phenonium ion (39) (the unsaturated analogue of the now well documented ethylene phenonium ion) and the open cation (36).The former cation leads directly to ketone whereas the latter mainly undergoes deprotonation to allene and to a minor extent sdvolysis either directly or via (39),to ketone.64 In contrast the kinetic nature and essentially non-stereospecific character of the anisyl shift which occurs in the course of the silver-promoted acetolysis of 2,2- dianisyl- 1-phenylvinyl bromide militate against the intermediacy of a bridged cation.65 The ratio of ring expansion (to cyclobutenes and alkylidenecyclo- butanes) to substitution in 1-cyclopropylvinyl cations increases with increasing electron donation at C-2. This effect is attributed to preferential charge stabiliza- tion in the transition state leading to rearrangement.66 The direct ring contraction of a cyclobutenyl cation (43) to a cyclopropylidene cation (44)is implicated in the solvolytic rearrangement of 1-bromo-2-phenylcyclobutene(42) to cyclopropyl phenyl ketone (45).An alternative mechanism involving protonation of (42) and subsequent cat ionic cyclobutykyclopropylmethyl ring contraction is excluded by the lack of deuterium uptake in deuteriated solvents.67 64 P. J. Stang and T. E. Dueber J. Amer. Chem. SOC.,1973,953 2683 2686. 6s Z. Rappoport A. Gal and Y. Houminer Tetrahedron Letters 1973 641. 66 D. R. Kelsey and R. G. Bergman J.C.S. Chem. Comm. 1973 589. 67 J. L. Derocque F. B. Sundermann N. Youssif and M. Hanack Annufen 1973. 419 Molecular Rearrangements 217 N.m.r.studies in super-acid media continue to reveal deep-seated yet otherwise undetected carbocation reorganizations. Cyclopropylallyl cations rearrange at -95 to -25 "C in FS0,H by smooth first-order processes to give hexadienyl and cyclohexenyl cations.68 Formation of the latter is envisaged as occurring via bicyclo[3,l,0]hex-2-enylcations (49) produced by an allowed but hitherto unprecedented [,2 + .2,] or [,2 + .2,] cycloaddition of a polarized a-bond to an allyl cation [(46) -* (47)-+ (4911 (Scheme 2). The corresponding stepwise 0 + ..... . .... . .... Scheme 2 process [(46) -+(48)-+ (4911 (Scheme 2) would require the energetically unfavour- able conversion of an allyl cation (46) into a primary cyclopropylmethyl cation (48).68 Low-temperature n.m.r.studies in FS0,H have also revealed a remark- able series of rearrangements interrelating methylated cyclohexenyl and norbornyl cation^.'^ A thorough kinetic study has completely unravelled the complex series of [1,2] hydrogen and methyl shifts and skeletal bond reorganiza- tions involved.58 Acid-catalysed rearrangements of readily accessible oxaspiro- pentanes provide general high-yield routes to cyclobutanone deriva-tives.44,45.69-7 1 These formal cyclopropylmethyl-cyclobutylcation ring expan- sions occur with preferential migration of the most electron-rich cyclopropyl K. Rajeswari and T. S. Sorensen J. Amer. Chem. SOC.,1973 95 1239. 69 B. M. Trost and M. J. Bogdanowicz J. Amer. Chem. SOC.,1973,95 531 1. 70 B.M. Trost and M. J. Bogdanowicz J. Amer. Chem. Soc. 1973,95,2038. " C. R. Johnson and E. R. Janiga J. Amer. Chem. SOC.,1973,95 7692. G.Tennant 218 bond and are highly stereospecific involving inversion at the migration termin~s.~'*~~* 70 Despite the apparent analogy between the purely thermal rearrangements of oxaspiropentanes to cyclobutanone~~~ and of a 2-azaspiro- [2,2]pentane to a cycl~butanimine,~' and the spiropentane methylenecyclobutane rearrangement the heterocyclic processes occur at much lower temperatures and consequently are suggested7' to be cationic in nature. Interestingly ring-unsub- stituted 2-azaspiro[2,2]pentanes are stable to heat and under a variety of acidic conditions give ring-opened rather than ring-expanded products.73 Cationic mechanisms are also proposed for the chlorinative ring expansions of l-vinyl- cycloalkanols to cycloalkanones.74 The closely related acid-catal ysed rearrange- ments of 1-vinylcyclopropanols and pinacolic rearrangements of a variety of 1 -substituted cyclopropanols occur readily and afford cyclobutanones in high In contrast more stringent conditions (5 % H2S04 100"C 30 min) are required for the corresponding acid-catalysed ring contractions of 2-alkyl-idenecyclobutanols to 1-alkylcyclopropyl carbonyl derivative^.^^ Skeletal reorganizations associated with adamantane and its derivatives continue to attract attention. Particular effort has been devoted to the elucida- tion of the complex structural changes involved in the Lewis-acid-catalysed hydrocarbon rearrangements leading to adamantane structures.The magnitude of the problem is indicated by the ob~ervation~~ that even if only [1,2] alkyl shifts are involved there are still no fewer than 2897 pathways available for the Lewis-acid-catalysed rearrangement of a tricyclodecane to adamantane ! The report77 that a new theoretical approach (based on molecular mechanics calcula- tions) correctly predicts the low-energy pathway for the aluminium bromide- catalysed rearrangement of endu-tetrahydrocyclopentadiene to adamantane represents an important break through in this area. Protodiamantane has been shown experimentally to be the most likely penultimate intermediate in the acid-catalysed rearrangement sequence leading from tetrahydro Binor-S to diamantane.78 Treatment of 1,l'- or 2,2'-biadamantane with aluminium bromide in cyclohexane at 60"C leads to an equilibrium mixture in which the 2,2'-isomer largely predominate^.^^ The implied greater stability of a 2-substituted adaman- tane in comparison with a- 1-substituted adamantane is unprecedented but is in accord with predictions based on molecular mechanics calculations. '' The aluminium bromide-catalysed rearrangement of 2,2'-binoradamantane affords [2]diadamantane the parent of a new class of adamantane derivatives." The 72 J. K. Crandall and W. W. Conover J.C.S. Chem. Comm. 1973 33. 7J D. H. Aue R. B. Lorens and G. S. Helwig Tetrahedron Letters 1973 4795. 74 C. R. Johnson and R. W. Herr J. Org. Chem. 1973,38 3153.75 J. P. Barnier B. Garner C. Girard J. M. Denis J. Salaun and J. M. Conia Tetra-hedron Letters 1973 1747; B. M. Trost D. Keeley and M. J. Bogdanowicz J. Amer. Chem. Soc. 1973,95 3068. 76 J. P. Barnier J. M. Denis J. R. Salaun and J. M. Conia J.C.S. Chem. Comm. 1973 103. 77 E. M. Engler M. Farcasiu A. Sevin J. M. Cense and P. von R. Schleyer J. Amer. Chem. Soc. 1973,95 5769. 78 T. M. Gund and P. von R. Schleyer Tetrahedron Letters 1973 1959. 79 J. Slutsky E. M. Engler and P. von R. Schleyer J.C.S. Chem. Comm. 1973 685. W. D. Graham P. von R. Schleyer E. W. Hagaman and E. Wenkert J. Amer. Chem. SOC.,1973 95 5785. Molecular Rearrangements protoadamantane-adtane rearrangement is exemplified by the reaction of 4methyleneprotoadamantane with formic acid to give 1-methyl-2-adamantyl formate.'' The acid-catalysed conversion of the diol(50) into the ketone (51) is interpreted in terms of consecutive [1,3] and [1,2] hydride shifts.The persistence of these novel rearrangements under conditions of high dilution demonstrates their intramolecular character.82 The preference for migration of axial hydrogen and the results of deuterium-labelling studies likewise conclusively demonstrate the intramolecularity of the novel [1,3] hydride shift implicated in the rearrange- ment (52) +(S1).82 The absence of label scrambling in the [42H]homotropilium ion at elevated temperatures (65-80 "C) in acidic media (FS03Hor H2S04)83places a minimum value on the energy barrier to circwnambulatory rearrangement in the cyclo- heptatrienylmethyl cation of 26-27 kcal mol-' which agrees well with the value (37 kcal mol- ') predicted on theoretical ground^.'^ This prohibitively high energy barrier to orbital symmetry-allowed rearrangement can be attributed to the loss of aromatic stabilization which would accrue in the course of the degenerate shift.The demonstration of two new degenerate rearrangements of bicyclo[3,2,1 Ioctadienyl cations is of considerable interest in this context." Changes in the 'H n.m.r. spectrum of the nonamethylbicyclo[3,2,1 Joctadien-Zyl cation (53) in FSOJHSOzCIF at -100 to -50 "C in the course of which only the basal methyl groups at C-1 and C-5 retain their integrity are ascribed to the novel circumambulation of the one carbon bridge and its attendant bridgehead carbons round the residual five carbon framework [cf.Scheme 3; (53) (54) (55)]." On the other hand the complete scrambling of the basal methyl groups observed in specifically labelled (53) at -78 "C demonstrates the concomitant 81 B. L. Adams and P. Kovacic J.C.S. Chem. Comm. 1972 1310. 82 E. Boelema J. H. Wieringa H. Wynberg and J. Strating Tetrahedron Letters 1973 2377. 83 J. A. Berson and J. A. Jenkins J. Amer. Chem. SOC.,1972,94. 8907. 84 W. J. Hehre J. Amer. Chem. SOC.,1972 94 8908. 85 M. Kuzuya and H. Hart J. Amer. Chem. SOC.,1973 95 4096; Tetrahedron Letters 1973 3887. 220 G. Tennant operation ofa slower degenerate process involving a [1,2] methano bridge shift [cf:Scheme 3; (53)$(56) *(57)].85 8v9 8v9 4J& 3 + 4 6&' 3 2 (54) (55) "v9 8 9 (53) = 6&2 5 5 Scheme 3 As anticipated a number of studies relating to polytopal cation rearrangements have followed hot on the heels of the theoretical predictions of Stohrer and Hoffmann regarding the (CH),' energy surface [cj Annual Reports (B) 1972 69 2451.The Stohrer-Hoffmann prediction that a cation of square-pyramidal C, geometry [cf Ann. Reports (B) 1972 69 2451 is the sole stable species on the (CH),' potential energy surface is only partly substantiated by recent theoretical studies.86-88 Although supporting the stability of the C4"configura-tion CND0,86 MIND0/3,87 and ab initioB8methods are in agreement that a non-planar version of the cyclopentadienyl cation also corresponds to an energy minimum.However the three methods are at variance in their respective estimates of the relative stabilities of the C,, and cyclopentadienyl species.86-88 Experimental support for the Stohrer-Hoffmann predictions is provided by the demonstration that esters of homotetrahedran-3-01s undergo smooth solvolytic rearrangement to cyclopentene derivative^.^^ On the other hand evidence for the intermediacy of the C4, cation in these rearrangements was inconclusive in so far as deuterium-labelling studies failed to demonstrate the associated polytopal rearrangement process [(58) (59) (60)JB9 More encouraging evidence for the incidence of polytopal cations of the type (58)comes from studies of the solvolytic rearrangements of labelled substrates at low temperatures in super-acid media using 13C n.m.r.as a structure probe. Thus the pattern of 86 H. Kollmar H. 0.Smith and P. von R. Schleyer J. Amer. Chem. SOC.,1973,95 5834. M. J. S. Dewar and R. C. Haddon J. Amer. Chem. SOC.,1973,95 5836. W. J. Hehre and P. von R. Schleyer J. Amer. Chem. SOC.,1973 95 5837. 89 S. Masamune M. Sakai and H. Ona J. Amer. Chem. SOC.,1972,94. 8955. Molecular Rearrangements 22I D p. p:::.:. 7 .. ..... D-C.’.’.............. .......!:.C-H ..... .... H (59) D (60) label scrambling and 13C n.m.r. absorption observed for solutions of homo-tetrahedran-3-01,’’ endo-tricyclo[3,2,0,02~7]heptan-4-0191 and related bridged and their derivatives in S0,ClF-FS0,H or S0,ClF-SbF at -1 15 to -50 “Careplausibly interpreted in terms ofcollapse to square-pyramidal cations [c$ (SS)] or their bishomo-anal~gues.’@-~~ Similar studies of the solvolytic rearrangements of bicyclo[2,l,l]hexane and tricyclo[3,1,0,02~6]hexane derivatives provide compelling e~idence’~ for the existence of the remarkable polytopal dication of C5”symmetry (61).‘H and 13C n.m.r. studies have also c. 2+ .’+. ........ . .of ’.., c, ........... .’ ... ... . : .’..... *. ......... demonstrated the operation in FS0,H at -150 to -20“C of the remarkably facile five-fold degenerate double circumambulatory rearrangement [(62) (63) etc.] which occurs by the formal corner alkylation of a cyclopropane by an ally1 ~ation.’~ The suggestiong4that the novel polytopal cation (64)(termed the [3,5,3]armilenium cation by the authors and corresponding to the co-ordina-tion of two mutually perpendicular allylic cation units above and below the plane of a cyclopentadienyl anion) is an intermediate in the process [(62)* (63) etc.] awaits further substantiation.Exclusive ortho-migration of the arene sulphonyloxy-group is observed in the spontaneous rearrangements of arenesulphonyloxy-N-phenylbenzohydrox-amic acids in aprotic media whereas in methanol the para-rearrangement 90 S. Masamune M. Sakai H. Ona and A. J. Jones J. Amer. Chem. SOC.,1972,94 8956. 91 S. Masamune M. Sakai A. V. Kemp-Jones H. Ona A. Vernot andT. Nakashima Angew. Chem. Internat. Edn. 1973 12 769. 92 H. Hart and M. Kuzuya J.Amer. Chem. SOC.,1972 94 8958; Tetrahedron Letters 1973,4123. 93 H. Hogeveen and P. W. Kwant Tetrahedron Letters 1973 1665; H. T. Jonkman and W. C. Nieuwpoort ibid. p. 1671; H. Hogeveen and P. W. Kwant ibid. p. 3747. 94 M. J. Goldstein and S. A. Kline J. Amer. Chem. Sor. 1973 95 935. 222 G.Tennant H H product predominates.” The lack of crossover and the results of ‘80-labelling experiments are interpreted in terms of a caged phenylnitrenium-sulphonate ion pair mechanism for the ortho-rearrangement.” Nitrenium cation inter- mediates are also postulated to account for the skeletal reorganizations of N-aroyloxy- and N-arenesulphonyloxy-azabornanederivatives?6 and in the ring expansion of a hydroxycyclopropylhydroxylaminetosylate to a #?-lactam.” The formally analogous oxidative ring expansions of hydroxycyclopropyl toluenesulphonylhydrazidesto #?-lactams have also been rep~rted.’~ a-Peroxyamine-based rearrangements have been re~iewed.~’ The extent of alkyl migration in the Baeyer-Villiger oxidations (by peroxytrifluoroacetic and peroxymaleic acids) of simple ketones increases with increasing bulk of the alkyl group to a value of 13for neopentyl versus methyl.’ O0 Baeyer-Villiger rearrange- ments of cyclobutanones to y-butyrolactones occur with oxidizing agents (basic hydrogen peroxide sodium hypobromite) which are normally incapable of effecting such transformations in larger cyclic ketone^.^^'^' Ring expansions of this type show the same stereospecificity (retention of configuration at the migrating centre) and migratory preferences (tertiary > secondary > primary alkyl) as acid-catalysed Baeyer-Villiger processes and owe their facility to relief of strain in the four-membered ring.44*45 The controversial Story mechanism 9s D.Gutschke and A. Heesing Chem. Ber. 1973 106 2379. q6 P. G. Gassman and G. D. Hartman J. Amer. Chem. SOC.,1973,95,449. ’’ H. H. Wasserman. E. A. Glazer and M. J. Hearn Tetrahedron Letters 1973,4855. 98 F. D. Greene R. L. Camp V. P. Abegg and G. 0.Pierson Tetrahedron Letters 1973 409 1. q9 E. G. E. Hawkins Angew. Chem. Internat. Edn. 1973 12 783. loo M. A. Winnick and V. Stoute Canad. J. Chem. 1973,51 2788. Molecular Rearrangements for ozonolysis [cf:Ann. Reports (B) 1971 68 257; 1972 69 2481 now appears to be untenable in the light of recent critical studies.The key role attributed to the Staudinger molozonide by Story is refuted by the demonstration"' that a primary (trioxolan) ozonide can readily effect the Baeyer-Villiger oxidation of propionaldehyde to propionic acid and by the lack of firm evidence for the formation of dioxetan by-products.' O'J O2 In addition products (e.g.6-hexano-lide) supposedly produced by Baeyer-Villiger oxidation of added carbonyl compounds by the molozonide are shown not to be primary but to arise by subsequent peroxide decomposition.' 02*'O3 Carbene Nitrene and Related Rearrangements.-Rearrangement processes associated with carbenes are included in a new textbook'04 and in a survey"' of the reactions of atomic carbon.The products of the thermal and photochemical decomposition of 1,2-diphenyl- 1-diazopropane stem from competing hydrogen and phenyl shifts originating in the singlet and triplet states respectively of a common carbene intermediate. '06 The formation of the same product mixture in the photolysis or thermolysis of 2,2-diphenyl-l-diazopropaneis explained by a [1,2] phenyl shift in (1-phenethy1)- phenylcarbene to a transient intermediate which is tentatively formulated as the 'phantom' singlet state of cr-methyl~tilbene.'~~ The demonstration'06 that c1 % methyl migration occurs in the thermal and direct photochemical decomposition of 1-([4'-2H]phenyl)-2-phenyldiazopropane is in accord with the aptitude H >Ph >Me for migration to a carbene centre.A carbene pathway for the [1,2] methyl shift observed in the photolysis of ethyl trimethylsilyldiazoacetate is supported by the demonstration that carbonium ion and silacyclopropane intermediates are not inv~lved.'~' Ring expansion of bridgehead carbenes generated in the gas phase provides a potentially valuable method for the synthesis of bridgehead olefins (notably Bredt violators). Thus the thermolysis of norbornane- and adamantane- 1-carboxaldehyde tosylhydrazone salts affords products which require the intermediacy of the Bredt violators bicyclo[2,2,2]- oct-1-ene and homoadamant-3-ene respecti~ely.'~~~'~~ Thermolysis of 4,4-dimethyl- and 4,4-diethyl-cyclohexadienylideneat 380 "Cin the gas phase gives moderate yields (35-40 %)of the alkyl-shift products p-xylene and 1,4-diethyl- benzene respectively.' The co-formation of monoalkylbenzenes and the P.S. Bailey T. P. Carter C. M. Fisher and J. A. Thompson Canad. J. Chem. 1973 51 1278. lo' K. R. Kopecky P. A. Lockwood J. E. Eilby and R. W. Reid Cunad. J. Chem. 1973 51 468. '03 D. R. Kerur and D. G. M. Diaper Canad. J. Chem. 1973,51 31 10. Io4 'Carbenes' Vol. 1 ed. M. Jones jun. and R. A. Moss Wiley-Interscience New York 1973. Io5 P. S. Skell J. J. Havel and M. J. McGlinchey Accounts Chem. Res. 1973 6 97. Io6 M. Pomerantz and T. W. Witherup J. Amer. Chem. SOC. 1973,95 5977. lo' W. Ando T. Hagiwara and T. Migita J. Amer. Chem. SOC. 1973 95 7518. lo' A. D. Wolf and M. Jones jun. J. Amer. Chem. SOC. 1973,95 8209. M. Farcasiu D.Farcasiu R. T. Conlin M. Jones jun. and P. von R. Schleyer J. Amer. Chem. SOC. 1973,95 8207. 'IoT. E. Berdick R. H. Levin A. D. Wolf and M. Jones jun. J. Amer. Chem. SOC. 1973 95. 5087. 224 G. Tennant demonstration of crossover in these rearrangements support their formulation as carbene-radical fragmentation-recombination processes. The extension of such rearrangements to spirocyclic cyclohexadienylidenes provides an elegant route to small-ring paracyclophanes (e.g. [7]paracyclophane).' ' New examples of carbene<arbene rearrangements have been reported. The formation of ring- expanded dimeric products in the solution-phase (135 "C,diglyme) thermolysis of a methanoannulene carboxaldehyde tosylhydrazone salt is rationalized by the novel arylcarbene-arylcarbene transformation [(65)-+ (66)] cited as the first example of such a process under non gas-phase The rearrange- ment of acenaphthylcarbene to phenalenylidene (the first example of a five- to a six-membered arylcarbene ring expansion) is implicated in the formation of peropyrene and phenalene by gas-phase thermolysis of the sodium salt of acenaphthylene-1-carboxaldehyde tosylhydrazone.Several studies provide unequivocal support for vinylcarbene intermediates in thermolytic and photolytic cyclopropene transformations. The activation parameters and product distribution noted for the thermolysis of 1-t-butyl-3,3- dimethylcyclopropene are rationalized in terms of ring-opening in both possible directions to vinylcarbenes which subsequently undergo competing insertion and hydrogen and methyl migration.'14 The faster rate of racemization compared with product formation in the thermolysis of 1,3-diethylcyclopropene is a measure of the lower rates of [1,2] and [1,4] hydrogen shifts compared with ring closure in vinylcarbenes.' l5 [1,4] Hydrogen shift in a vinylcarbene intermediate also accounts for the photolytic conversion of 1,2-diphenyl-3,3-dimethylcyclopropene into a mixture of cis-and trans-1,2-diphenyl-3-methylbuta-1,3-diene.Con-versely the products of the photolysis of 2,3,3-triphenylcyclopropene-l-carbox-aldehyde are most readily explained by Wolff rearrangement in an intermediate vinylketocarbene. l7 Rearrangement in vinylketocarbenes produced by nitrogen loss from intermediate vinyldiazoketones also accounts for the photodecomposi- A.D. Wolf V. V. Kane R. H. Levin and M. Jones jun. J. Amer. Chem. Soc. 1973,95 1680. I12 P. H. Gebert R. W. King R. A. LaBar and W. M. Jones J. Amer. Chem. Soc. 1973 95 2357. I13 T. T. Coburn and W. M. Jones Tetrahedron Letters 1973 3903. I14 R. D. Streeper and P. D. Gardner Tetrahedron Letters 1973 767. I I5 E. J. York W. Dittmar J. R. Stevenson and R. G. Bergman J. Amer. Chem. SOC. 1973,95 5680. I16 J. A. Pincock R. Morchat and D. R. Arnold J. Amer. Chem. SOC.,1973 95 7536. Ill L. Schrader and W. Hartmann Tetrahedron Lerrers 1973 3995. Molecular Rearrangements 225 tion of 5-acyl-3,3-dimethyl-3H-pyrazolesto vinyl-ketens. '**' However in the light of the work discussed before,' '1-acylcyclopropenes (significantly the major products of 4-acyl-3,3-dimethyl-3H-pyrazole photolysis' '8 should also be considered as vinylketocarbene precursors in pyrazole photolyses.Interestingly products derived by competing [1,4] hydrogen shift in the alkylated vinylketo- carbene intermediates were not isolated in these reactions though they may lurk in the polymeric' l9 by-products. Bicyclopropenyl to vinylcyclopropenyl- carbene ring-opening and subsequent ring expansion to a Dewar benzene provides the basis for a new mechanistic rationale for the bicyclopropenyl to benzene rearrangement. 2o The intermediacy of oxiren intermediates in photo-Wolff rearrangements and their absence (except possibly at elevated temperatures) in the corresponding thermal processes previously' established experimentally [cf Ann.Reports (B) 1972 69 248-2491 has now received theoretical support.'21 It is suggested that the energetically favourable but otherwise symmetry-forbidden singlet excited ketocarbene to oxiren transition is overcome by internal conversion to a ground-state singlet which still has sufficient vibrational energy to permit access to oxiren.' 2' However this interpretation is based on the calculated' 2' greater stability of the ketocarbene relative to the oxiren. This theoretical prediction is at variance with a MIND0/3 study'22 which suggests that keto- carbenes are substantially less stable than their oxiren counterparts and should rearrange to the latter without activation. On the other hand ab initio calculations indicate that the difference in stability between oxiren and formylcarbene is only CQ.0.49 kcal mol-' in favour of the former species.'23 Attempts to matrix- trap the oxirens produced by photolysis of diazoacetaldehyde and ethyl diazo- acetate have been unsuccessful. '24 In contrast ethoxycarbonylcarbene is trapped (as ethyl propionate) by hydrogen transfer in hydrocarbon matrices.' 24 These experimental findings appear to conflict with the predictions of the MIND0/3 study.' 22 * 3C-Labelling studies,' 25 which demonstrate label scrambling and its absence in ketens derived respectively by photo- and thermal Wolff rdrrange- ments supplement studies reported last year [cf Ann. Reports (B) 1972 69 2491. The differing product ratios observed for the identical reaction mixtures resulting from the peracid oxidative rearrangement of cycloalkynes and the thermal rearrangement of the corresponding diazocycloalkanones may be attributed to the involvement of oxiren intermediates in the former reactions and their absence in the latter.126 Migratory aptitudes have been determined A.C. Day A. N. McDonald B. F. Anderson T. J. Bartczak and 0.J. R. Hodder J.C.S. Chem. Comm. 1973,247. I I' M. Franck-Neumann and C. Buchecker Tetrahedron Letters 1973 2875. I2O R. Weiss and S. Andrae Angew. Chem. Internat. Edn. 1973,12 150 152. I I. G. Csizmadia H. E. Gunning R. K. Gosavi and 0.P. Strausz J. Amer. Chem. Soc. 1973 95 133. 122 M. J. S. Dewar and C. A. Ramsden J.C.S. Chem. Comm.1973 688. I 23 A. C. Hopkinson J.C.S. Perkin II 1973 794. 12d A. Krantz J.C.S. Chem. Comm. 1973 670. J. Fenwick G. Frater K. Ogi and 0.P. Strausz J. Amer. Chem. SOC.,1973 95 124. P. W. Concannon and J. Ciabattoni J. Amer. Chem. SOC.,1973 95 3284. 226 G. Tennant for therma1'2'~'28 and photochemical'28 [1,2] shifts in diketocarbenes. In accord with rearrangements occurring by anionotropic shift to electron-deficient termini the migratory aptitude in both processes increases with increasing nucleophilicityin the migrating group." However for the thermal rearrange- ment steric as well as electronic effects appear to be important and tend to dominate when the two effects are opposed.'28 The failure of the ketocarbene (67) to rearrange to the keten (68) is ~uggested'~' to reflect a ring strain barrier to successful photo-Wolff ring contraction of ca.50 kcal mol- '. The failure of methylmercuricdiazoacetoneto undergo photo-Wolff rearrangement is attributed to excited state to ground state relaxation induced by the proximity of the heavy metal atom to the carbene centre in methylmercuriketocarbene.'30 The products of the gas-phase (600"C)pyrolysis of 1-alkyl and l-aralkyl-l,2,3-triazolesare rationalized by competing [1,2] and [1,4] shifts in iminocarbene intermediates.13' A new version of the Schmidt rearrangement accounts for the reactions of malonic acid half esters with diphenylphosphoryl azide to give high yields of a-amino-acid derivatives.' 32 The failure of added alkenes to intercept nitrene intermediates is cited' 33 as evidence for the concerted nature of photochemically induced C-N methyl shifts in t-alkyl azides.The exclusive methyl shift (in competition with acetonyl shift) observed in the photolysis of 4-azido-4-methyl- pentan-2-one implies that electronic as well as steric factors are important in determining migratory preference in such rearrangements.' 33 New examples of ring expansion and ring contraction originating in thermal and photochemical azide decompositions have been reported. The activation parameters for the high-yield thermolytic rearrangements of cyclopropyl azides to azetines are inconsistent with ring bond shift being concerted with nitrogen loss. The facility of these novel ring expansions is explained in terms of stepwise mechanisms involving cyclopropyl-stabilized nitrene intermediates.'34 The interesting 12' K.P. Zeller H. Meier and E. Muller Tetrahedron 1972 28 5831. G. Heyes and G. Holt J.C.S. Perkin I 1973 1206. 12' B. M. Trost and P. L. Kinson Tetrahedron Letters 1973 2675. P. S. Skell and S. J. Valenty J. Amer. Chem. SOC.,1973,95 5042. 13' T. L. Gilchrist G. E. Gymer and C. W. Rees J.C.S. Chem. Comm. 1973 835. 13' S. I. Yamada K. Ninomiya and T. Shioiri Tetrahedron Letters 1973 2343. '33 S. Solar E. Koch J. Leitch P. Margaretha and 0.E. Polansky Monarsh. 1973 104 220. t34 G. Szeimies U. Siefken and R. Rinck Angew. Chem. Internut. Edn. 1973 12 161. Molecular Rearrangements 227 photochemical and thermal ring contractions of 1-azidocyclobutenes to cyano- cyclopropanes are similarly considered to involve ene-nitrenes which can be trapped by added dipolarophile~.'~~ However the low temperatures which suffice to promote the ring contractions of 2-azidopyridine N-oxides to 2-cyano N-hydroxypyrroles militate against the involvement of nitrene intermediates.' 36 Ring-opening in concert with nitrogen loss and electrocyclization of the nitroso- nitriles produced accounts satisfactorily for these synthetically valuable trans- formations.' 36 The nitrene-nitrene rearrangement of 2-quinolylnitrene to 1-isoquinolylnitrene is to be involved in oxadiazolo- and tetrazolo- quinoline thermolyses.Evidence has been obtained for the intermediacy of N-nitrenes in diazene-hydrazone rearrangements.* Thermal Photochemical and Metalcatalysed Rearrangements.-Valence iso-merizations of fluxional molecules' and photochemical rearrangements of cyclohe~adienones'~~ and cy~loheptadienones'~' have been reviewed. A new theoretical treatment of concerted sigmatropic rearrangements has been published'42 and the steric course of dyotropic rearrangements [cf Ann. Reports (B) 197269,2511 has been di~cussed.'~~ Di-n-methane and oxa-di-n-methane rearrangements have been reviewed.'44 Biradical intermediates are proposed to account for the non-stereospecificity of singlet-state photo-[ 1,2] methyl shifts in /3-t-butyl styrenes.'4s The inhibition of the [1,2] methyl shift by electron-donating groups in the aryl nucleus reveals the operation of polar effects in the n-n* excited states for such rearrangements akin to those governing [1,2] shifts in carbocation~.'~~ The successful photo- rearrangement of a rigid 1,Cdiene system constrained to react by the syn-disrotatory mode demonstrates that access to the normally preferred anti-disrotatory pathway is not a prerequisite of successful di-n-methane rearrange- ment.'46 Preferential ethynyl (sp)shift in competition with vinyl (sp2)migration is observed in the singlet-state photo-rearrangements of cis-and trans-l,5-diphenyl- 3-methyl-3-methoxypent-l-en-4-yne. The facility of these novel stereospecific [l,2] ethynyl shifts contrasts with the difficulty of the corresponding alkyl and hydrogen migrations and supports their formulation as di-n-methane-like [,2 + ,2 + ,2 J cycloadditions.'47 The success of these transformations 13' G.Buhr Chem. Ber. 1973 106 3544. 136 R. A. Abramovitch and B. W. Cue J. Org. Chem. 1973 38 173. 13' R. F. C. Brown F. Irvine and R. J. Smith Austral. J. Chem. 1973 26 2213. 13' B. V. Ioffe and L. A. Kartsova Tetrahedron Letters 1973 623. 139 B. Decock-Le-Reverend and P. Goudmand Bull. SOC. chim. France II 1973. 389. I4O G. Quinkert Angew. Chem. Internat. Edn. 1972 11 1072. 14' H. Hart Pure Appl. Chem. 1973,33 347. 142 .I.Mathieu Bull. Soc. chim. France 11 1973 807. 143 M. T. Reetz Tetrahedron,1973 29 2189. 144 S. S. Hixson P. S. Mariano and H. E. Zimmerman Chem. Rev. 1973,73 531. 14' S. S. Hixson and T.P. Cutler J. Amer. Chem. SOC. 1973 95 3031 3032. P. S. Mariano and R.B. Steitle J. Amer. Chem. SOC. 1973.95 61 15. 14' J. Perreten D. M. Chihal G. W. Griffin andN. S. Bhacca J. Amer. Chem. SOC.,1973 95 3427. 14' 228 G. Tennant contrasts with the lack of di-lr-methane rearrangement reported'48 for hexa- 1,2,5-trienes. The demonstration that the photoisomerization of a sterically unconstrained By-unsaturated ketone occurs with retention at C-2 requires that if concerted the oxa-di-7c-methane rearrangement must be formulated as a [,2 + .2J rather than a [,2 + .2,] cycloaddition.'49 The question of electron-repulsive destabilization of pericyclic transition states for sigmatropic rearrangements has been Considered.' Theoretical calculations of electron repulsion energies indicate that although electron-repulsive destabiliza- tion cannot reverse the order of transition state energies for the concerted- forbidden and non-concerted pathways open to [1,3] carbon shift this need not invariably be so.A case in point is the concerted-forbidden [,2 + .2,] thermal methylenecyclopropane rearrangement repulsive interaction in the transition state for which is predicted to be destabilizing compared with the biradical pathway.' 'O A detailed kinetic and stereochemical analysis of thermal [1,3] carbon shifts in em-and endo-bicyclo[4,3,0]oct-2-enesdemonstrates that steric blockade of the allowed suprafacial-inversion (si) mode permits the operation of the forbidden concerted suprafacial-retention (sr) pathway. ''' The gradual increase in the sr :si ratio for [1,3] carbon shift along the series bicyclo[2,1,1]- hexenyl < bicyclo[3,2,0]heptenyl < bicyclo[4,2,0]octenyl < vinylcyclobutyl is indicative of subjacent orbital control commensurate with a gradual increase in the efficiency of overlap between the front lobe on the migrating carbon atom and the C-2 suprafacial lobe of the allylic framework."' The racemization and lack of antarafacial participation associated with thermal vinylcyclobutane to cyclohexene rearrangements are best explained in terms of competing allowed (si) and forbidden (sr) pathways for these thermal [1,3] carbon shift~.''~ Kinetic parameters for the thermal equilibrations and degenerate automerizations of ethylidenecyclobutanes are also considered to be inconsistent with fully stepwise processes involving biradical intermediates.'' Conversely the demonstrably greater rate of racemization (k = 4.93 k1.13 x lO-'s-l) compared with deuterium scrambling (degenerate automerization) (kds= 8.15 & 0.6 x lo-' s-') in (2)-1-ethylidene-2-methylcyclobutane is construed as evidence for ca. 40% antarafacial allylic participation in the [1,3] carbon shift.lS3 A detailed kinetic and stereochemical analysis of the thermal rearrangement of l-(Z)-( 1-deuterio- ethylidene)-2-methyl-trans-3,4,4-trideuteriocyclobutane indicates a ratio of 77 :23 for the allowed and disallowed rearrangement pathways.' 53 In contrast a kinetic and stereochemical analysis of thermal methylenecyclopropane re- arrangements clearly excludes allowed and forbidden concerted paths for such [1,3] carbon migrations.' 54 Other studies provide compelling evidence for 14* D.C. Lankin D. M. Chihal G. W. Griffin and N. S. Bhacca Tetrahedron Letters 1973,4009. 149 J. I. Seeman and H. Ziffer Tetrahedron Letters 1973,4413. W. T. Borden and L. Salem J. Amer. Chem. SOC.,1973,95,932. Is' J. A. Berson and R. W. Holder J. Amer. Chem. Soc. 1973 95 2037. '52 J. A. Berson and P. B. Dervan J. Amer. Chem. SOC.,1973 95 267 269. IS3 J. E. Baldwin and R. H. Fleming J. Amer. Chem. Soc. 1973,95 5249 5256 5261. lS4 W. von E. Doering and L. Birladeanu Tetrahedron 1973 29 499. Molecular Rearrangements 229 tetramethylene methane biradical intermediates in dimethylenecyclobutane and methylenespiropentane rearrangements.' 55 Activation parameters for the thermal rearrangements of 6-methylenebicyclo[3,2,0]pent-2-ene'56 and aryldi- methylvinylidenecyclopropanes'5 'accord best with stepwise processes involving orthogonal biradical intermediates.Interestingly 1-dimethylvinylidene-2-iso-propylidene-3,3-dimethylcyclopropane which embodies structural features requisite both for vinylidene- and methylene-cyclopropane rearrangement under- goes biradical mediated shift not to the anticipated radialen but to the corres- ponding cross-conjugated dienyne.' 58 Although activation parameters fail to differentiate concerted ([ 1,3] or [3,3]) and biradical pathways for the vinyl- methylenecyclopropane to 3-methylenecyclopentene isomerization the pattern of label scrambling observed in this rearrangement is consistent with a non- concerted process.' 59 Thermal methyleneaziridine automerizations and methyl- eneaziridine-cyclopropanimine interconversions have been conclusively demon- strated for the first time.'60 It now seems probable that the predominant pathway for the thermal rearrangement of 2-methylbicyclo[2,1,0]pent-2-eneto 1-and 2-methylcyclopentadienesinvolves stepwise rupture of the C-1 -C-4 cross-link and subsequent [1,5] hydrogen shifts in the vibrationally activated 2-methylcyclo- pentadiene produced.' 61 However reports'61,'62 that rearrangement to crossed product still occurs to a significant extent in solution requires the vibrationally activated intermediate to exhibit longevity unprecedented for solution-phase isomerizations.Examples of new light-induced [1,3] shifts have been reported.The triplet- state rearrangement of 1,1,4-triphenyl-3,3-dimethylpenta-1 ,Cdiene occurs by [1,3] vinyl shift to the exclusion of the alternative di-n-methane rearrangement.' 63 Photolysis of 3,4benzotropilidene and its 7,7-dimethyl derivatives affords products derived by rare [1,3] hydrogen and methyl migration which thus compete to a significant extent (ca. 10%) with the more commonplace [1,7] shifts.'64 The first-order [1,3] benzyl shift involved in the thermal rearrangement of 1,4dibenzyl-1,4-dihydro-2,6-diphenylpyrazineto the 1,2-dibenzyl-1,2-di-hydro-isomer occurs intramolecularly and with 295 % inversion at the migrating centre. This interesting transformation is claimed as the first unequivocal example of suprafacial [1,3] migration in a nitrogen-containing ally1 frame- Concerted allylic [1,3] shifts of selenium34 and boron ligands'66 have W.R. Roth and G. Erker Angew. Chem. Internat. Edn. 1973,12,503,505; W. Grimme and H. J. Rother ibid. p. 505. 156 D. Hasselmann Tetrahedron Letters 1973 3739. Is' ' ' I. H. Sadler and J. A. G. Stewart J.C.S. Perkin II 1973 278. G. Kobrich and B. Posner Tetrahedron Letters 1973 203 1. J. C. Gilbert and D. P. Higley Tetrahedron Letters 1973 2075; W. E. Billups K. H. Leavell E. S. Lewis and S. Vanderpool J. Amer. Chem. SOC., 1973 95 8096. 160 H. Quast and W. Risler Angew. Chem. Internat. Edn. 1973 12 414. 16' J. I. Brauman W. E. Farneth and M. B. D'Amore J. Amer. Chem. SOC.,1973,95,5043. 16' G. D.Andrews M. Davalt and J. E. Baldwin J. Amer. Chem. SOC.,1973,95 5044. 163 H. E. Zimmerman D. W. Kurtz and L. M. Tolbert J. Amer. Chem. SOC.,1973 95 8210. L64 K. A. Burdett D. H. Yates and J. S. Swenton Tetrahedron Letters 1973 783. 165 J. W. Lownand M. H. Akhtar J.C.S. Chem. Comm. 1973 511. L66 K. G. Hancock and J. D. Kramer J. Amer. Chem. SOC.,1973,95 3425 6463. 230 G. Tennant also been reported. Conversely the demonstration' 67 of concomitant I3C CIDNP emission demonstrates a radical fragmentation-recombination pathway for the [1,3] O-+S shift involved in the oxime thionocarbonate rearrangement.I6' Activation parameters for thermal acyl shifts in 1-acyl-1-methylcyclohexa-2,4-dienes are consistent with their formulation as concerted [1,5] sigmatropic processes.Trapping experiments excluded alternative sequential [1,3] acyl shifts for these novel rearrangements which are shown to exhibit the migratory preference CHO > H x COMe > C02Me.16* Analogous [1,5] shifts likewise rationalize acetyl migrations accompanying the Claisen rearrangements of ortho-allyloxyacetophenones.'69 The interesting thermal rearrangement of bicyclo[3,2,0]hepta-1,3-diene (69; n = 2) to spiro[2,4]hepta-4,6-diene(70; n = 2) illustrates the greater rate of [1,5] carbon shift compared with the corresponding hydrogen shift in this cyclopentadiene system. The facility of the process (69) n = 2 or 3 (70) (69;n = 2)-+ (70; n = 2) may be a consequence of relief of ring strain and the development of a degree of aromatic character in (70; n = 2).I7O The formal reversal of this [l,5] carbon shift (70; n = 3)-(69; n = 3) occurs in the gas phase at 400 "C.'" The label equilibration undergone by octadeuteriobicyclo- [5,l,O]octa-2,4-diene (71) at 110 "C is explained in terms of novel butadienylcyclo- propane equilibria (71) $(72) and (73) (74) interconnected by a reversible [1,5] hydrogen shift (72) $(73) [cf:Scheme 41.' 72 The stereospecificity observed for the degenerate butadienylcyclopropane rearrangement in (7 1) is consistent with its formulation as an antarafacial [1,5] carbon shift involving inversion at the migrating centre.' 72 Interestingly the degenerate isomerization exhibited by the iron tricarbonyl derivative of (71) appears to involve a different transition- state stereochemistry.' 73 Several new examples of thermal and photochemical [1,7] carbon and hydrogen shifts have been reported.'74s'75 Enynols react at -10 "C with diethyl chlorophosphite to give intermediate enynol phosphites which rearrange spontaneously by [2,3] sigmatropic shift to afford ene-allenyl- phosphonates.'76 In the case of 3-methylhex-2-en-Qynol the intermediate phosphite (75) can be isolated but rearranges at 130 "C by a rare and presumably concerted [2,5] shift to give the phosphonate (76).' " Allylsulphones undergo 167 C.Brown R. F. Hudson and A. J. Lawson J. Amer. Chem. SOC.,1973,95 6500. '68 P. Schiess and P. Funfschilling Tetrahedron Letters 1972 5191 5195. C. P. Falshaw S. A. Lane and W. D. Ollis J.C.S. Chem. Comm. 1973,491.M. Oda and R. Breslow Tetrahedron Letters 1973 2537. 171 A. de Meijere and L. U. Meyer Angew. Chem. Internat. Edn. 1973 12 858. W. Grimme and W. von E. Doering Chem. Ber. 1973 106 1765. 173 R. Aumann Angew. Chem. Internat. Edn. 1973 12 574. 174 K. A. Burdett T. J. Ikeler and J. S. Swenton J. Amer. Chem. SOC.,1973,9!5,2702. 17) E. E. Waali and W. M. Jones J. Amer. Chem. SOC.,1973,95,8114; J. Org. Chem. 1973 38,2573. * 76 M. Huche and P. Cresson Tetrahedron Letters 1973 4291. Molecular Rearrangements 231 D D H D D (71) D D D D D .D (74) (73) Scheme 4 clean high-yield (80%) fragmentation-rearrangement at 300 "Cin the gas phase providing a new and probably stereospecific method for constructing carbon- carbon bonds.' Evidence has been obtained for biradical-mediated pathways in the Cope rearrangements of hexa-1,5-dienes.78 2,5-Diphenylhexa-l,5dene undergoes Cope rearrangement two thousand times faster than hexa-1,5-diene. This rate enhancement is inexplicable in terms of a [3,3] sigmatropic shift and is interpreted as evidence for a stepwise mechanism involving a biradical intermediate (1,4-diphenyl-1,kyclohexylene). '* The Cope rearrangement of a structure involving rare doubly bound silicon [(77)$(7831 has been ingeniously demonstrated by the ready stereomutation (E = 39.2 Ifr 0.4 kcal mol-') of cis-and trans-pro- penylallyldimethylsilane [(77) (78) (7911at 500 "C in the gas phase. The facility of this novel transformation (which requires only CQ. 3 kcal mol-I more activation than the allcarbon analogue) is attributed to the ability of silicon to stabilize the pericyclic transition state.' 79 The apparently low energy barrier '" J.B. Hendrickson and R. Bergeron. Tetrahedron Letters 1973 3609. l" M. J. S. Dewar and L. E. Wade. J. Amer. Chem. SOC.,1973 95 290. 179 J. Slutsky and H. Kwart J. Org. Chem. 1973 38,3658. 232 G. Tennant to diaza-Cope rearrangement in 1,2-diarylideneamino- 1,2-diarylethanes lends weight to theoretical predictions [cf Ann. Reports (B) 1972 69 2561relating to transition-state energies for Cope rearrangements.' Deuterium-labelling studies suggest that the formal diaza-Cope rearrangements of N-allylhydrazones are not entirely concerted but follow to the extent of ca. 18% a radical-pair pathway.181 Remarkably (and contrary to previous belief) rearrangement of cis-1,2-divinylcyclopropane to cycloheptadiene is slow enough between 5 and 20 "C to allow kinetic measurements which reveal an activation free energy of ca.20 kcal mol- Unprecedented Cope rearrangement involving a benzene nucleus is proposed to account for the thermal base-catalysed conversion of cis-Zphenylvinylcyclopropaneinto l-phe11ylpenta-l,3-diene.l~~ The sterically difficult thermal transformations of trans- 1,2-divinylcyclobutanes into cyclo-octa- 1,5-dienes have been shown to occur (at least in part) by prior stereomutation to the cis-isomers and subsequent Cope rearrangement. The reluctance of the trans-isomers to rearrange directly to product is a measure of the unfavourable strain barrier to cyclization in the presumed biradical interrnediate.la4 Divinyl- cyclopropane rearrangement provides a mechanistic rationale for the biogenesis of the naturally occurring cyclohepta-1,5-dienes dictyopterins C and D.185 Thermal rearrangements of sterically constrained diacetylene frameworks' 86 are exemplified by the interesting valence isomerizations (80) -+ (8l).' ' (80)X = 0 or S The thermal rearrangements of fused cis-divinylcyclopropanescontinue to stimulate much experimental effort.Preferential rearrangement from folded conformations leading to boat-like transition states accounts for the kinetic 180 F. Vogtle and E. Goldschmit Angew. Chem. Internat. Edn. 1973 12 767. I8 1 R. V. Stevens E. E. McEntire W.E. Barnett and E. Wenkert J.C.S. Chem. Comm. 1973,662. 182 J. M. Brown B. T. Golding and J. J. Stofko J.C.S. Chem. Comm. 1973 319. 183 E. N. Marvel1 and C. Lin Tetrahedron Letters 1973 2679. 184 J. A. Berson and P. B. Dervan J. Amer. Chem. SOC.,1972 94 8949. 185 W. Pickenhagen F. Naf G. Ohloff P. Muller and J. C. Perlberger Hefu. Chim. Acta 1973,56 1868. I86 R. G. Bergman Accounts Chem. Res. 1973 6 25. 187 K. P. C. Vollhardt and R. G. Bergman J. Amer. Chem. Soc. 1972,94,8950; ibid. 1973 95. 7538. Molecular Rearrangements 233 parameters and stereospecificity exhibited by the new heteroanalogous homo- tropilidine valence isomerization (82) S(83)’88 and by new bicyclo[6,1 ,O]nona- 2,6-diene to bicyclo[5,2,0]octa-2,5-diene inter conversion^'^^ and their hetero- analogues.’90 Stereochemical control is also manifest in the thermal rearrange- ments of bicyclo[6,1,O]nona-2,4,6-trienescontaining a bulky C-9 substituent.(82) (83) Thus the stereospecific high yield (9&94 %) thermal rearrangements of syn-and unti-9-t-butylbicyclo[6,1 ,O]nonatrienes to truns-and cis-8,9-dihydroindenes respectively are consistent with previous mechanistic proposals [cf Ann. Reports (B) 1971 68 2611 for these controversial rearrangements.”’ The reluctance of syn-9-cyanobicyclo[6,1,0]nona-2,4,6-trieneto undergo thermal rearrangement can be attributed to stabilization of the C-1-C-8 cross-link lending weight to the contention [cf:Ann. Reports (B),1971,68,261] that syn-9-substituted bicyclo- r6,l ,Olnonatrienes rearrange by direct symmetry-allowed C- 1-C-8 rupture in extended conformations.lg2 The stabilizing effect of a syn-9-cyano-group may also account for the apparent dichotomy in bicyclo[6,1,0]nonatriene isomeriza-tions noted last year [cf Ann. Reports (B),1972,69 2561. Theoretical studies [cf:Ann. Reports (B) 1972 69 2561 predict that electron- releasing substituents will destabilize the transition state for the Cope rearrange- ments of biallyl systems. In accord with this prediction is the demon~tration’~~ that complexation with pentacarbonyltungsten (equivalent to net electron release) raises the activation energy for the degenerate semibullvalene rearrange- ment. Despite other evidencelg4 to the contrary a reinterpretation’” of 13C n.m.r.data’ 96 indicates the absence of homoaromatic character in the hydro- carbon (84). It follows that the ‘Cope rearrangement which cannot be frozen out’ has yet to be demonstrated. (84) I88 H. Klein W. Kursawa and W. Grimme Angew. Chem. Internat. Edn. 1973 12 580. 189 W. Grimme J. Amer. Chem. SOC.,1973,952381. I90 W. Grimme and K. Seel Angew. Chem. Internat. Edn. 1973 12 507. I91 A. G. Anastassiou and R. C. Griffith J. Amer. Chem. SOC.,1973,95 2379. 192 A. G. Anastassiou and R. C. Griffith Tetrahedron Letters 1973 3067. I93 R. M. Moriarty C. L. Yeh E. L. Yeh and K. C. Ramey J. Amer. Chem. SOC.,1972 94 9229. 194 G. P. Ceasar J. Green L. A. Paquette and R. E. Wingard Tetrahedron Letters 1973 1721. 195 E. Vogel U. H. Brinker K. Nachtkamp J.Wassen and K. Mullen Angew. Chem. Internat. Edn. 1973 12 758. I96 E. Wenkert E. W. Hagaman L. A. Paquette R. E. Wingard and R. K. Russell J.C.S. Chem. Comm. 1973 135. 234 G. Tennant The use of Claisen-type rearrangements for the synthesis of yd-unsaturated carbonyl compounds and their thioanalogues continues to attract attention. An elegant variation on this general theme (the so-called Reformatskyxlaisen reaction) involves the in situ [3,3] sigmatropic rearrangement of zinc enolates derived from a-bromo-ally1 or -propargyl esters and provides a new route to yd-unsaturated acids avoiding both strongly acid and strongly basic conditions. 97 Similar rearrangements of enol phosphates derived from allyl trichloroacetates have also been described.lg7 In accord with fully concerted processes involving chair-like transition states the [3,3] sigmatropic rearrangements of allyl vinyl ethers to y6-unsaturated carboxylic acid derivatives proceed with >90% optical induction.The enhanced optical stereospecificity of these rearrangements recommends them as methods for carbon-carbon bond formation at asymmetric centres. 98 Thio-Claisen rearrangements of allyl vinyl sulphides and related substrates likewise provide methods for the generation of double bonds with a high degree of stereochemical The thermal rearrangement of allyl 1,1-dimethylbut-3-enyl ethers (85) to 7-methyloct-6-enol (86) competes to a 35% significant extent with alternative retro-ene cleavage or cyclization. The timing of the bond-making and -breaking processes in this novel formal [4,4] sigma- tropic shift has yet to be established."' Rearrangements involving metal-carbene complexes,2o' and those induced by thallium salts,202 have been reviewed.The known facilitation of the thermally difficult [3,3] sigmatropic propargyl ester-allenyl ester shift [(87) S (91)] by Ag' ions is of interest in relation to orbital symmetry considerations. This Ag'-catalysed process has now been subjected to detailed mechanistic scrutiny by Schmid and his co-workers with notable results.203 Rearrangement is shown to involve the prior formation of an Ag' ion n-complex (88 W),and occurs intra-molecularly and with high stereospecificity. These features prompt its formulation as a fully concerted process (cf Scheme 5) involving a cyclic metal-bonded transition state (89) unperturbed in an orbital symmetry sense by the presence of the metal cation which merely serves in a polar capacity to accelerate the 19' J.E. Baldwin and J. A. Walker J.C.S. Chem. Comm. 1973 117. R. K. Hill R. Soman and S. Sawada J. Org. Chem. 1972 37 3737. 199 H. Takahashi K. Oshima H. Yamamoto and H. Nozaki J. Amer. Chem. Soc. 1973 95 2693 5803; K. Oshima. H. Yamamoto and H. Nozaki ibid. p. 4446. E. N. Marvel1 and M. Fleming Tetrahedron Letters 1973 3789. zoi D. J. Cardin B. Cetinkaya M. J. Doyle and M. F. Lappert Chem. SOC. Rev. 1973,2 99. 202 A. McKillop and E. C. Taylor Chem. Brit. 1973 9 4. 203 H. Schlossarczyk W. Sieber M. Hesse H.-J. Hansen and H. Schmid Helv.Chim. Acta 1973 56 875. 19' Molecular Rearrangements Ar Ar I I / C\o -I / C\o 0 Ag+ . 0 I R'-C fast R' -C R2/\C R2/\c=c-...... 2; \R3 (87) jl slow Ar I C ON \o I slow C -R c(';...*g ......... LCN I I R3 Ar I L ON '0 I I R2 Scheme 5 otherwise sluggish thermal [3,3] shift (i.e.in Schmid's n~menclature~'~ a 'charge induced' [3,3] sigmatropic shift). The probability that such metal-catalysed transformations will not be restricted to the propargyl ester-allenyl ester '04 U. Widmer J. Zsindely H.-J. Hansen and H. Schmid Helu. Chim. Acta 1973,s. 75. 236 G. Tennant equilibrium is substantiated by the dernon~tration~~' of similar Ag'-ion catalysed [3,3] shifts in propargyl phenyl ethers.The nature of the intermediates and the precise mechanisms involved in transition-metal-catalysedstrained a-bond rearrangements are still controversial. It has been suggested206 that the hitherto unconsidered ylide canonical form (93) makes a significant contribution to the hybrid structures of the metal-complexed- carbene intermediates [cf.Ann. Reports (B),1971,68,262; 1972,69,258] proposed for such isomerizations. The course of the observed skeletal reorganizations can then be explained in terms of the relative contributions from (92t(94) which in turn depend specifically on the interplay between the n-donor and a-acceptor capacities of the particular metal. Consequently overriding n-donor capacity should favour the ylide form (93) and overriding a-acceptor capacity the carbonium ion form (94).It follows that metal-carbene intermediates in Rh'- (high n-donor capacity) catalysed rearrangements ought to exhibit ylide character (93). In support of these suggestions is the observation that metal- carbene intermediates in Rh'-catalysed bicycle[l,l,O]butane isomerizations can be trapped (as cyclopropane derivatives) by electron-poor but not by electron- rich 01efins.~" On the other hand there is now little doubt that products of Rh'- catalysed isomerizations in methanol ostensibly derived from metal-carbonium ion precursors [cf. Ann. Reports (B) 1971 68 262; 1972,69 2581 are in fact the result of pure acid-catalysed carbonium ion processes.208~209 Evidence for the nature of the acid-producing species in bicycle[l,l,O]butane-Rh'-methanol systems has been presented.208 Products of the Rh'-catalysed isomerization of naphtho[1,8]tricyclo{4,1,0,02~7]heptenein methanol attributed to the inter- mediacy of metal-carbonium ion species presumably have a similar origin.210 However the intramolecular nature of the Ag'- and Rh'-catalysed rearrangements of exo-l,2,3,4,5,6-hexamethyltricyclo[2,2,O,O2~6]heptane to cis-hexamethylcyclo- hexa- 1,3-diene is most readily explained by [1,2] hydride shifts in intermediate metal-complexed carbonium ions2' In contrast the 2-em hydrogen dependence 2os U.Koch-Pomeranz H.-J. Hansen and H. Schmid Helv. Chim. Acta 1973 56 2981. ''' R. Noyori Tetrahedron Letters 1973 1691. 207 P.G. Gassman and R. R. Reitz J. Organometallic Chem. 1973 52 C51. '08 W. G. Dauben A. J. Kielbania and K. N. Raymond J. Amer. Chem. SOC.,1973 95 7 166. 209 P. G. Gassman and R. R. Reitz J. Amer. Chem. Soc. 1973,95 3057; L. A. Paquette S. E. Wilson G. Zon and J. A. Schwartz ibid. 1972,94,9222; E. Muller Tetrahedron Letters 1973 1201 1203; G. F. Koser P. R. Pappas and S.-M. Yu ibid. p. 4943. 'lo '" I. Murata K. Nakasuji and H. Kume Tetrahedron Letters 1973 3401 3405. H. Hogeveen and J. Thio Tetrahedron Letters 1973 3463. Molecular Rearrangements and deuterium exchange found for Rh'-catalysed isomerizations of bicyclo[2,1,0]- pentanes to cyclopentenes accord best with hydrogen transfer in an allylrhodium hydride intermediate.' '' 3 Aromatic Rearrangements Dienone-phenol2 and arene oxide' rearrangements have been reviewed.The use of e.s.r. spectroscopy has permitted the first direct observation of the 2-methyl-2-phenylpropyl to 1-methyl-2-phenylethyl (neophyl) radical rearrange- ment. The activation energy (E = 43 & 9 kJ mol-') for this [1,2] radical shift is inconsistent with its formulation as a simple dissociation-recombination process.215 New examples of rare thermal [1,4] aryl shifts have been described. Diarylmethane formation in the thermal decomposition of benzyl triarylacetates is rationalized in terms of [ 1,4] aryl shifts which exhibit a ca. twofold preference for p-tolyl as opposed to phenyl migration.'16 Formation of a resonance-stabilized carbanion provides the driving force for the [1,2] phenyl shift which occurs in the base-catalysed rearrangement of 1,2,3,4,5-pentaphenylcyclopenta-2,4-dien- 1-01 to 2,3,4,5,5-pentaphenylcyclopent-2-en-1-one.' ' Novel ortho semibenzene-benzene rearrangements of 2-allenyl- and 2-pro- pargyl-1-methylenecyclohexa-3,5-dienesare implicated in the thermal rearrange- ments of dimethylenetricyclo[3,2,1,02~7]oct-3-enes to butynylbenzenes and in the Wit tig reactions of 2-propargy lcyclohexa-3,5-dienes with tripheny lp hos- phonium methylide to give 4-arylbuta-l,2-dienes.' ' The formulation of these rearrangements as concerted [3,3] sigmatropic processes is supported by the lack of by-products due to radical fragmentation-recombination and the side- chain inversion demonstrated by deuterium labelling.' ' In contrast radical- pair pathways are proposed for the [3,3] shifts undergone by 4-propargyl-l- methylenecyclohexa-2,5-dienes.2' The dimerization of a-fluorobenzyl radicals to give 1,2-difluorobibenzyl has been shown to occur by the intermediate forma- tion and subsequent semibenzene rearrangement of a fluoromethylenecyclohexa-2,4-diene.' ' Semibenzene rearrangements in intermediate cyclohexadienyl zwitterions account for the concomitant group shifts observed in the cyclo- addition reactions of l-methylenecyclohexa-2,5-dienes with tetracyanoethylene.The preference for [ 1,2] phenyl as opposed to methyl migration in these rearrange- ments is a measure of the greater migratory aptitude of phenyl compared with methyl in carbonium ion-induced shifts.220 Crossover experiments support an intermolecular radical fragmentation-recombination mechanism for the semi- phosphabenzene-phosphabenzene isomerization (95)-+ (96).2211 -Nitro-4-acet- 212 K.B. Wiberg and K. C. Bishop Tetrahedron Letters 1973 2727. 'I3 R. S. Ward Chem. Brit. 1973 9 447. 'I4 J. W. Daly D. M. Jerina and B. Witkop Experientia 1972 28,1129. 'I5 E.J. Hamilton and H. Fischer Helu. Chim. Acta 1973 56 795. 'I6 W. S. Trahanovsky D. E. Zabel and M. L. S. Louie J. Org. Chem. 1973 38,757. A. K.Youssef and M. A. Ogliaruso J. 3rg. Chem. 1973 38,2023. 2'8 P.Gilgen J. Zsindely and H. Schmid Helu. Chim. Acta 1973 56 681. 'I9 D. Bethell M. R. Brinkman and J. Hayes J.C.S. Chem. Comm. 1972 1324. ''O N. K. Hamer and M. E. Stubbs J.C.S.Perkin I 1972 2971. 22' G. Mark1 and D. E. Fischer Tetrahedron Letters 1973 223. 238 G. Tennunt PhvPh oxycyclohexa-2,5-dienes undergo novel thermal [1,3] nitro shifts the intra- molecular nature of which is demonstrated by the lack of nitration in added mesitylene.222 Formal [1,2] and [1,3] nitro shifts likewise account for the acid- catalysed conversion of the ‘ipso’ adduct (97) into the isomeric nitro-compounds (98) and (99). The concurrence of [1,2] carbon shifts in these rearrangements was excluded by deuterium labelling.223 A kinetic study indicates that the [1,2] shift rather than the protonation step is rate determining. in simple dienol- benzene rearrangement^.^^^ The acid-catalysed rearrangements of hydroxy-pentadienyl-1,2-dihydronaphthalenes occur almost entirely by [1,2] pentadienyl shift.The hoped for high-order [5,5] and [5,6] pentadienyl migrations were not observed.22 Trifluoroacetic acid-catalysed rearrangement of allylcyclohexadienones affords allylphenols derived exclusively by concerted [3,3] sigmatropic allyl shifts. Conversely the rearrangement of allylcyclohexadienones in the presence of trifluoroacetic anhydride or acetic anhydride-sulphuric acid yields allylphenols derived by competing [1,2] and [3,4] as well as [3,3] sigmatropic allyl shifts.204 The discovery of this dichotomy in the allylcyclohexadienone-allylphenol rearrangement has led to a new classification for charge-mediated sigmatropic processes.204 It is proposed that the [3,3] shifts (designated ‘charge induced processes’) originate in oxygen-protonated allylcyclohexadienone intermediates and involve charge-stabilized but otherwise thermally equivalent cyclic transition states.Conversely the [1,2] and [3,4] shifts (designated as ‘charge controlled processes’) are suggested to stem from acyloxybenzenium ion intermediates and consequently to involve highly polarized transition states having quite different orbital symmetry requirements compared with the purely thermal processes.204 222 A. Fischer and C. C. Greig J.C.S. Chem. Comm. 1973 396. 223 R. C. Hahn and M. 9. Groen J. Amer. Chem. SOC. 1973 95 6128. 224 V. P. Vitullo and M. J. Cashen Tetrahedron Letters 1973 4823. 22s H. Greuter H.-J. Hansen and H. Schmid Helv. Chim. Acta 1973 56 2479.Molecular Rearrangements 239 The dienone-phenol rearrangements of 2-propargylcyclohexa-3,5-dienones catalysed by trifluoroacetic anhydride-boron trifluoride or acetic anhydride- sulphuric acid occur by exclusive ‘charge-controlled’ [1.21 and [3,4] pathways affording 3-propargyl- and 3-allenyl-phenols respectively. 226 The novel Ag’- ion-catalysed dienone-phenol rearrangement of a 2-allenylcyclohexa-3,5-dienone to a 3-allenylphenol may likewise be classified as a ‘charge controlled’ [1,2] shift involving a silver benzonium ion complex.205 A kinetic study of the acid- catalysed rearrangement of 4-ethyl-4-methylcyclohexa-2,5-dienone to 3-ethyl-4- methylphenol reveals a twenty-five-fold greater preference for ethyl shift compared with methyl shift.22 Deuterium-labelling studies support a purely intramolecular (as opposed to addition-elimination) pathway for the dienone-phenol rearrange- ments of ortho-quinol acetates.228 A detailed kinetic study has revealed that the rearrangements involved in the conversion of indane 8,9-oxide into indan-4-01 and indan-5-01 occur mainly by an arene oxide-arene oxide (‘oxygen-walk’) pathway and only to a minor extent by alternative spirodienone-phenol rearrange- ment.229 The novel thermal arene oxide-arene oxide rearrangement (100) -+ -(101) occurs readily at 80-100 “C (E = 28.2 f0.8 kcal mol l) and shows a lack of rate dependence on solvent polarity which prompts its formulation as an allowed suprafacial [1,5] shift akin to the cycloheptatriene-norcaradiene valence i~omerization.~~~ The interesting photochemical rearrangement of 9,10-dihydro-9,10-epoxyphenanthrene(102) to 1,2 :3,4-benzoxepin (104) may also be explained by an ‘oxygen-walk’ mechanism involving a concerted suprafacial [1,5] shift [(102) +(10311 (allowed in the first excited state with inversion at oxygen) and subsequent thermal valence isomerization [( 103)* (104)].23’ [1,2] Shifts in iminonium cation intermediates are proposed to account for alkyl migrations which accompany the potassium ferricyanide oxidation of aniline derivatives to q~inone-anils.~~~ that peroxidase The dem~nstration’~~ induces similar shifts in a variety of alkyl groups (e.g.methyl t-butyl cyclohexyl) in uitro implies a possible relationship with similar enzymically promoted rearrangements.226 U. Widmer H.-J. Hansen and H. Schmid Helu. Chim. Acra 1973 56 1895. 227 J. W. Pilkington and A. J. Waring Tetrahedron Letters 1973 4345. 228 H. Budzikiewicz and J. Gunavan Monatsh. 1973 104 876. 229 G. J. Kasperek P. Y. Bruice T. C. Bruice H. Yagi and D. M. Jerina J. Amer. Chem. SOC.,1973 95 6041 1673. 230 F. G. Klarner and E. Vogel Angew. Chem. Internat. Edn. 1973 12 840. ”* N. E. Brightwell and G. W. Griffin J.C.S. Chem. Comm. 1973 37. 232 S. L. Goldstein and E. McNelis J. Org. Chem. 1973 38 183. 233 P. B. Baker V. R. Holland and B. C. Saunders Tetrahedron 1973 29 85. 240 G. Tennant &-[;:I,& --*& -( 102) ( 103) (104) The trifluoromethane-sulphonic acid-catalysed transformations of ortho-hydroxybenzophenones to phenyl benzoates exemplify ret ro-Fries rearrangements which are not therefore so rare as previously thought.The implication that acid- catalysed Fries rearrangements are subject to thermodynamic control requires the reappraisal of mechanistic proposals for such transformations the reversibility of which has not been considered hitherto.234 The observation of CIDNP effects attributable to the intermediacy of aryloxy and aroyl radicals in the photochemical rearrangement of para-cresyl 2~chlorobenzoate to 2-(2’-chloro- benzoy1)-p-cresol provides convincing evidence for the radical-pair mechanism of the photo-Fries rearrangement.235 The acylamino fragmentation which ac- companies photo-acyl shift in NN-diacylanilines and NN-diacylnaphthylamines likewise supports radical-pair pathways for these ph~to-rearrangements.~~~ On the other hand the absence of phenolic by-products and products derived by para-substitution in the photo-rearrangement of 2-phenoxybenzimidazole to 2-(2’-hydroxypheny1)-benzimidazoleis tentatively suggested to imply a con-certed mechanism.237 The latter reaction is claimed237 as the first example of a photo-Fries rearrangement to involve the preferential migration of a heterocyclic nucleus.Photo-Fries rearrangement of N-acyl- and N-alkoxy-indoles provides routes to relatively inaccessible 3,4-and 6-substituted indole derivatives.238 The site specificity of the high-yield triplet-state photo-rearrangement of ortho- phenoxybenzoic acid to phenyl salicylate excludes a radical fragmentation- recombination pathway.A mechanism involving intramolecular nucleophilic aromatic substitution in a polar excited-state complex is suggested to account for this novel photoisomerization.239 It is of interest that the reverse of this process appears to be involved in the photochemical transformation of benzyl salicylate into ortho-benzyloxybenzoic The inhibition of .rearrangement in the presence of a radical scavenger (nitric oxide)241 and the observation242 of CIDNP emission associated with phenoxyl and ally1 radicals clearly support a radical fragmentation-recombination mechanism for the photo-Claisen re-arrangement. ortho- And para-products of such rearrangements originate in 234 F. Effenberger H. Klenk and P. L. Reiter Angew.Chem. Internat. Edn. 1973 12 775. 235 W. Adam J. Arce de Sanabia and H. Fischer J. Org. Chem. 1973 38 2571. 236 Y. Katsuhara H. Maruyama Y. Shigemitsu and Y. Odaira Tetrahedron Letters 1973 1323. 237 P. D. Hobbs and P. D. Magnus J.C.S. Perkin I 1973,469. 238 M. Somei and M. Natsume Tetrahedron Letters 1973 2451. 239 N. C. Yang P. Kumler and S. S. Yang J. Org. Chem. 1972 37 4022. 240 M. Afzal Chem. and Ind. 1973 37. 241 F. A. Carroll and G. S. Hammond J. Amer. Chem. SOC.,1972 94 7151. 242 W. Adam H. Fischer H.-J. Hansen H. Heimgartner H. Schmid and H. R. Waespe Angew. Chem. Internat. Edn. 1973 12 662. Molecular Rearrangements 241 singlet radical-pair encounters whereas rneta-products are the result of encounters in triplet radical pairs.242 An elegant stereochemical study has shown that thermal and boron trifluoride-catalysed para- ortho Claisen rearrangements of steroidal allylcyclohexadienones exhibit identical optical stereospecificity consistent with their formulation as [3,3] concerted sigmatropic processes involving essentially identical and largely ( >95%) chair-like transition states.243 This significant study supports the ont tent ion^^^,^^^ that ‘charge-induced’ sigmatropic shifts in oxygen- co-ordinated allylcyclohexadienones involve essentially unperturbed ‘thermal’ transition states the presence of the charge merely acting as a stabilizing and therefore rate-enhancing influence.Boron trichloride-catalysed ortho-Claisen rearrangements of allyl aryl ethers show all of the mechanistic features associated with their purely thermal counterparts thereby permitting their formulation as true concerted [3,3] sigmatropic shifts.244 Consequently the 10’o-fold Jate en- hancement exhibited by such processes compared with their thermal analogues is a measure of the charge induction resulting from allyl shift in a boron trichloride- co-ordinated allylcyclohexadienone as opposed to an unco-ordinated species.244 Charge induction resulting from co-ordination at nitrogen likewise accounts for the facility of the zinc chloride-catalysed amino-Claisen rearrangements of N-allylanilines compared with the difficulty of the corresponding thermal processes.245 A kinetic has shown that the rates of ortho-Claisen rearrangements of allyloxynaphthoquinones are accelerated in protic media.These results which amplify observations reported last year [cf:Ann. Reports (B) 1972 69 2131 are attributed to the stabilization of the polar transition state for rearrangement by hydr~gen-bonding.~~~ Kinetic evidence indicates that the products of the thermal rearrangements of 3-substituted prop-Zynyl tropolone ethers result from novel [3,7] sigmatropic shifts.247 An example of a thermal [5,5] pentadienyl shift in an aryl pentadienyl ether has been reported.23 Lack of crossover demonstrates the intramolecular character of the thermal 0-arylhydrazonate-hydrazide rearrangement [cf Ann. Reports (B) 1971 68 267].248*249 The polar effects of substituents on the rate of these rearrangements differ from those of the formally analogous Chapman rearrangement of aryl imidates and prompt a radical-pair mechanism for the hydrazonate processes.248 Benzidine rearrangements of N-acetyl- and NN-dimethyl-hydrazobenzene have been rep~rted.~~~,~~’ Deuterium-labelling studies demonstrate the intra-molecularity of the rearrangement of N-acetylhydrazobenzene in strong acid to give by a ‘one proton’ mechanism N-acetylbenzidine.It is suggested that 243 A. Wunderli J. Zsindely H.-J. Hansen and H. Schmid Helv. Chim. Acta 1973 56 989. 244 J. Borgulya R. Madeja P. Fahrni H.-J. Hansen and H. Schmid Helv. Chim. Acta 1973 56 14. 245 M. Schmid H.-J. Hansen. and H. Schmid Helv. Chim. Acta 1973,56 105. 246 J. A. Miller and C. M. Scrimgeour J.C.S. Perkin II 1973 1137.247 R. M. Harrington and J. D. Hobson J.C.S. Perkin I 1973 1960. 248 A. F. Hegarty J. A. Kearney and F. L. Scott J.C.S. Perkin II 1973 1422. 249 A. S. Shawali and H. M. Hassaneen Tetrahedron 1972 28 5903. 250 J. R. Cox and M. F. Dunn J. Org. Chem. 1972 37,4415. 251 D. V. Banthorpe and M. O’Sullivan J.C.S. Perkin II 1973 551. 242 G.Tennant the N-acetyl group aids N-N bond heterolysis in the conjugate acid and consequently that rearrangement follows a n-complex path~ay.~” The lack of ring deuteriation observed in the course of these studies is cited2s0 as evidence against the CN-diprotonation hypothesis proposed [cJ:Ann. Reports (B) 1972 69,2631 to account for the courses of ‘two proton’ benzidine rearrangements. Despite the formation of products (semidines fission amine) attributable to radical fragmentation-recombination the absence of crossover and the ineffec- tiveness of radical scavengers support an intramolecular polar transition state mechanism for the acid-catalysed benzidine rearrangement of NN-dimethyl- hydra~obenzene.~” Deuterium- and ’N-labelling studies demonstrate that ortho-and para-semidine formation in ‘one proton’ benzidine rearrangements occurs intramolecularly.2s2 The acid-catalysed rearrangement of the cyclic hydrazobenzene (105) affords (106) the first isolable ortho-semidine inter- (105) ( 1 06) mediate.253 The preferential formation of this product is attributed to steric restraints in (105) which inhibit alternative benzidine andpara-semidine rearrange- ment~.~ The acid-catalysed arenesulphenanilide-aminodiarylsulphide re-s arrangement has been shown to be quite general and to exhibit features in common with the benzidine and nitramine rearrangements.The available evidence supports an intramolecular radical-pair mechanism for these rearrange- ment~.~~~ It has been shown that the photo-Wallach rearrangement is preceded by protonation in the first excited state.2ss The acid-catalysed rearrangement of 2,2’,4,4’,6,6’-hexamethylazoxybenzene to 4-hydroxymethyl-2,2‘,4,6,6’-penta-methylazobenzene illustrates a new variant of the Wallach rearrangement.2s6 The ring contraction (107) +( 108)76 is proffered as the year’s most unusual molecular rearrangement albeit in a kinetic rather than a structural sense ! 2s2 A.Heesing and U. Schinke Chem. Ber. 1972 105 3838. lS3 W. W. Paudler A. G. Zeiler and M. M. Goodman J. Heterocyclic Chem. 1973 10 423. 254 F. A. Davis E. R. Fretz and C. J. Horner J. Org. Chem. 1973 38 690; F. A. Davis C. J. Horner E. R. Fretz and J. F. Stackhouse ibid. p. 695. *” R. H. Squire and H. H. Jaffe J. Amer. Chem. Sac. 1973,95 8188. 256 R. A. Cox and E. Buncel Canad. J. Chem. 1973 51 3143.

ISSN:0069-3030    

DOI:10.1039/OC9737000206

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

7 Organometallic Compounds of the Transition Elements By J. D. JONES R. PEARCE and G. L. P. RANDALL lCl Ltd. Corporate Laboratory P.O. Box 1 I The Heath Runcorn WA7 4QE 1 Introduction We have adopted approximately the same format as in previous years with emphasis being given to modification and transformation of organic compounds by organometallic complexes of the transition metals and to the mechanisms of the more important catalytic reactions. The reader’s attention is drawn to the fact that those aspects of catalytic processes that involve either stereoselective synthesis or the transformation of homogeneous catalysts into a form that is readily separable from the product mixture (heterogenized) are dealt with in separate sections. 2 Reviews Areas of chemistry involving organometallic transition-metal complexes that have been reviewed include :homogeneous hydrogenation ‘y2 hydrosilyla-asymmetric ~ynthesis,~ ti~n,~ cyclo-oligomerization,5 atmospheric pressure carboxylation,6 skeletal isomerization,7 heterocyclic synthesis,* the role of carbene complexes as reaction intermediate^,^ synthetic applications of metal 7r-allyl~,’~ ‘the nickel effect’,’ CO insertion,’ organocobalt compounds,’ ’ B.R. James ‘Homogeneous Hydrogenation’ Wiley New York 1973. ’R. E. Harmon S. K. Gupta and D. J. Brown Chem. Rev. 1973,73 21. C. S. Cundy B. M. Kingston and M. F. Lappert .4dv. Organometallic Chem. 1973 11 253. B. Bogdanovic Angew. Chem. Internat. Edn. 1973 12 954. P. Heimbach Angew. Chem. Internat.Edn. 1973 12 975. L. Cassar G. P. Chiusoli and F. Geurrieri Synthesis 1973 509. ’J. R. Anderson Adv. in Catalysis 1973 23 I. C. W. Bird J. Organometallic Chem. 1973 47 281. B. Cetinkaya D. J. Cardin M. J. Doyle and M. F. Lappert Chem. Soc. Rev. 1973 2,99. lo R. Baker Chem. Rev. 1973 73 487. ‘I K. Fischer K. Jonas P. Misbach R. Stabba and G. Wilke Angew. Chem. Internat. Edn. 1973 12 943. I’ A. Wojcicki Adv. Organometallic Chem. 1973 11 88. l3 D. Dodd and M. D. Johnson J. Organometallic Chem. 1973 52 I ;H. Bonnemann Angew. Chem. Internat. Edn. 1973 12 964; Gmelins Handbuch der Anorganischen Chemie Verlag Chemie 1973. 243 244 J. D. Jones R. Pearce and G. L. P. Randall palladium and platinum ~hemistry,'~ the role of palladium in addition reactions of butadiene,' exchange and isomerization'6 and aromatic substitution of olefins," metal-induced carbonium ions in platinum complexes,18 r-ligand tran~fer,'~ mass spectrometry,20 organometallic electrochemistry,2' and stability and reactivity of metal alkyls.22 3 Organometallic Complexes a-Complexes.-The chemistry of o-hydrocarbyls especially of the binary com- plexes is an expanding area.Contributions this year may be placed in three categories (i) new complexes including R3M,2THF (R = Me3CCH or Me,SiCH,; M = Scor Y),23(Me3SiCH2),TaCl -,,(n = 2or 3),24(Me,SiCH2C~)t5 with a novel square-planar arrangement of copper atoms with singly bridging CH2-/ alkyls,26 and phosphorous ylide complexes based on the bidentate Me2P \ CH2-(R) R2M2 (M = Cu Ag or Au),~' R,Ni2,28 and R,Cr;29 (ii) full details of previously reported complexes including silylmethyls and neopentyls of Ti Zr and Hf,,' Nb Ta Cr and Mo,,l Me6W,32 ArAg and Ar2AgLi (Ar = e.g.o-M~,NCH,C,H,-),~~ and R Ri-,TiLi ;34 (iii) Studies of decomposition l4 F. R. Hartley 'The Chemistry of Platinum and Palladium' Applied Science London 1973. J. Tsuji Accounts Chem. Res. 1973 6 8. I6 P. M. Henry Accounts Chem. Res. 1973 6 16. " I. Moritani and Y. Fujiwara Synthesis 1973 524. M. H. Chisholm and H. C. Clark Accounts Chem. Res. 1973 6 202. l9 A. Efraty J. Organometallic Chem. 1973 57 I. 2o M. R. Litzow and T. R. Spalding 'Mass Spectrometry of Inorganic and Organometallic Compounds' Elsevier Amsterdam 1973.2' H. Lehmkuhl Synthesis 1973 377. 22 P. S. Braterman and R. J. Cross Chem. SOC.Rev. 1973 2 271 ;F. Calderazzo Pure Appl. Chem. 1973,33,453. 23 M. F. Lappert and R. Pearce J.C.S. Chem. Comm. 1973 126. 24 S. Moorhouse and G. Wilkinson J. Organometallic Chem. 1973 52 C5. 25 M. F. Lappert and R. Pearce J.C.S. Chem. Comm. 1973 24. 26 J. A. J. Jarvis B. T. Kilbourn R. Pearce and M. F. Lappert J.C.S. Chem. Comm. 1973,475. 27 H. Schmidbaur J. Adlkofer and W. Buchner Angew. Chem. Internat. Edn. 1973 12 415; H. Schmidbaur and R. Franke ibid. p. 416. 2R H. H. Karsch and H. Schmidbaur Angew. Chem. Internat. Edn. 1973 12 853. 29 E. Kurras U. Rosenthal H. Mennenga and G. Oehme Angew. Chem. Internat. Edn. 1973 12 854. 30 M. R. Collier M. F. Lappert and R.Pearce J.C.S. Dalton 1973 445; P. J. Davidson M. F. Lappert and R.Pearce J. Organomerallic Chem. 1973 57 269. W. Mowat and G. Wilkinson J.C.S. Dalton 1973 1120; W. Mowat A. J. Shortland N. J. Hill and G. Wilkinson ihid. p. 770. 32 A. J. Shortland and G. Wilkinson J.C.S. Dalton 1973 872. " A. J. Leusink G. van Koten J. W. Marsman and J. G. Noltes J. Organometallic Chem. 1973 55 419; A. J. Leusink G. van Koten and J. G. Noltes ibid. 1973 56 379. " J. Muller H. Rau P. Zdunneck and K.-H. Thiele Z. anorg. Chem. 1973,401 113. Organometallic Compounds of the Transition Elements 245 pathways particularly P-and reductive elimination in complexes e.g. (PhCH,),-Zr,,' (Ph,P),RhR,36 R'R2R3AuPR3 ,37 (Ph3P)2Pt(CH2),,38 and (Ph,P),Pt(I)- Me,.39 The observation that addition of aluminium alkyls accelerates the decomposi- tion of transition-metal hydrocarbyls points to the need for extreme care in the preparation of these complexes if meaningful data are to be obtained and also to the possible function of aluminium compounds in Ziegler catalysis in labilizing a metal-carbon bond.40 New methods of M-C synthesis include (i) interaction (with subsequent metallation) of phosphorous ylides with metal halides ;27728 (ii) halogen-alkyl exchange involving alkyls of Sn,,l Zn,24 Hg,4' and Ag42 where competing side reactions with RLi are avoided and partial alkylation may be achieved; (iii) direct synthesis from metal vapour and aryl halides ;43 and from carbocation sources e.g.MeS0,F (giving novel cationic metal alkyl~).~~ A recently discovered synthetic method that is now becoming accepted by organic chemists and has been successfully applied to the synthesis of e.g.prostaglandins juvenile hormones and pheromones is the formation of carbon- carbon bonds by organocopper reagents usually of the type R,CuLi. The anion in these reagents has considerably reduced nucleophilic character compared with that in lithium or magnesium reagents permitting their use in reactions with compounds containing carbonyl groups where attack at this function is much suppressed and avoiding some other side reactions associated with organo- lithiums. Their use in synthesis has been re~iewed.~' The two main areas of application are in (i) coupling e.g. with alkyl/aryl halides acid halides a-halo- genoketones and tosylates ;46 (ii) addition to unsaturated systems e.g.cis-addition to acetylenic compounds to give specifically substituted ole fin^,,^ and conjugate addition to aP-unsaturated carbonyl compounds.48 35 K.-H. Thiele E. Kohler and B. Adler J. Organometallic Chem. 1973 50 153. 36 C. S. Cundy M. F. Lappert and R. Pearce J. Organometallic Chem. 1973 59 161. 37 A. Tamaki S. A. Magennis and J. K. Kochi J. Amer. Chem. SOC. 1973 95 6487; A. Tamaki and J. K. Kochi J. Organometallic Chem. 1973 61 441; J.C.S. Chem. Comm. 1973,423. 38 J. X. McDermott J. F. White and G. M. Whitesides J. Amer. Chem. SOC. 1973 95 445 1. " M. P. Brown R. J. Puddephatt and C. E. E. Upton J. Organometallic Chem. 1973 49 C61. 40 T. Yamamoto and A.Yamamoto J. Organometallic Chem. 1973 57 127. 41 C. Santini-Scampucci and J. G. Riess J.C.S. Dalton 1973 2436; C. J. Cardin D. J. Cardin M. F. Lappert and K. W. Muir J. Organometa/lic Chem. 1973 60 C70. 42 R. L. Bennett M. I. Bruce and R. C. F. Gardner J.C.S. Dalton 1973 2653. 43 K. J. Klabunde and J. Y. F. Low J. Organometallic Chem. 1973 51 C33. 44 J. L. Peterson T. E. Nappier jun. and D. W. Meek J. Amer. Chem. SOC. 1973 95 8195; D. Strope and D. F. Shriver ibid. p. 8197. 45 J. F. Normant Synthesis 1972 63; G. H. Posner Org. Reactions 1972 19 1. C. R. Johnson and G. A. Dutra J. Amer. Chem. Soc. 1973 95 7777 7783; G. H. Posner and J. J. Sterling ibid. p. 3076. S. B. Bowlas and J. A. Katzenellenbogen Tetrahedron Letters 1973 1277; J. F. Normant G.Cahiez C. Chuit and J. Villieras ibid. p. 2407; J. Organometallic Chem. 1973 54 c53. 48 E. J. Corey and R. H. K. Chen Tetrahedron Letters 1973 1611; P. A. Grieco and R. Finkelhar J. Org. Chem. 1973 38 2100. 246 J. D. Jones R. Pearce and G. L. P. Randall Two important extensions of this work are the increase in the range of R groups that may be used to include secondary and tertiary alkyl groups and the increase in yield with respect to the amount of R converted into product this latter aspect being important in synthesis of complex molecules. These objectives have been achieved with reagents of the type RCUXL~,~~ readily prepared from RLi and CuX where X is eg. Bu‘C=C CN Bu‘O PhS. Varying X affects the thermal stability of the reagent its solubility in the reaction medium and its selectivity in reaction PhS appearing to be the most versatile.While organocopper species do not react readily with carbonyl group^,^^'^^ the use of these reagents in the derivatization of aldehydes and ketones to tertiary and quaternary carbon centres has been investigated. Reactions studied include those with p-alkoxy and thioalkoxy ab-unsaturated carbonyl corn pound^,^ readily prepared from 1,3-diketones and with ab-ethylenic sulphur corn pound^.^^ n-Complexes.-Simple olefin complexes have now been prepared. Both tris- (ethylene)nickel obtained from C2H and all-trans-1,5,9-~yclododecatriene-nickel(^),^^ and tris(norb0rnene)nickel are reasonably stable the latter having an approximately trigonal planar arrangement of the olefinic units around nickel.54 (Z-c&I&Ti has been prepared by condensation of benzene and titanium vapour at 77 K ; a symmetrical sandwich structure was propo~ed.’~ Variable-temperature ‘H and ’3C n.m.r.measurements on Os(CO)NO(C,H,)- (PPh,),PF have allowed an assignment of the mode of rotation of the olefin within the ~omplex.’~ The rotation axis is that of the metalhlefin bond and not the carbon-carbon double bond. Fluxional behaviour was also found in the analogous acetylene complex but the data did not allow an assignment in this case.57 Further insight has been gained into the structure of the elusive titanocene in some structural studies on cycldpentadienyl complexes of Ti Nb and Mo.~~ These show that the unit C5H4 is a general feature being found as a bridging ligand and as the fulvalene ligand (see also ref.59). 49 G. H. Posner C. E. Whitten and J. J. Sterling J. Amer. Chem. Soc. 1973 95 7788; H. 0.House and M. J. Umen J. Org. Chem. 1973 38 3893; J.-P. Gorlier L. Hamon J. Levisalles and J. Wagnon J.C.S. Chem. Comm. 1973 88; G. H. Posner and C. E. Whitten Tetrahedron Letters 1973 1815. 50 CJ L. T. Scott and W. P. Cotton J.C.S. Chem. Comm. 1973 320. ” G. H. Posner and D. J. Brunelle J.C.S. Chem. Comm. 1973 907; C. P. Casey D. F. Marten and R. A. Boggs Tetrahedron Letters 1973 2071. ’’ G. H. Posner and D. J. Brunelle Tetrahedron Letters 1973 935; J. Org. Chem. 1973 38 2747. ’3 K. Fischer K. Jonas and G. Wilke Angew. Chem. Internat. Edn. 1973 12 565. 54 C.Kruger B. L. Barnett D. J. Brauer and Y.-H. Tsay Ah. Sixth Internat. Con$ Organometallic Chem. 1973 80. 55 F. W. S. Benfield M. L. H. Green J. S. Ogden and D. Young J.C.S. Chem. Comm. 1973 866. 56 B. F. G. Johnson and J. A. Segal J.C.S. Chem. Comm. 1973 1312. ” J. Ashley-Smith B. F. G. Johnson and J. A. Segal J. Organometallic Chem. 1973 49 C38. 58 L. J. Guggenberger and F. N. Tebbe J. Amer. Chem. Soc. 1973,95,7870; J.C.S. Chem. Comm. 1973 227; L. J. Guggenberger Inorg. Chem. 1973 12 294; R. A. Forder M. L. H. Green R. E. Mackenzie J. S. Poland and K. Prout J.C.S. Chem. Comm. 1973 426. An improved synthesis of fulvalene complexes of the late transition metals is described in A. Davison and J. C. Smart J. Organometallic Chem. 1973 49 C43. Organometallic Compounds of the Transition Elements 247 Carbene Complexes.-The carbene group in metal complexes is generally un- reactive but a number of papers have reported possible uses in organic synthesis.These involve carbene transfer to silanes a method complementing the halo- alkylmercury route ;60 reactions of anions generated CI to the carbene carbon ;61 and coupling with the carbenoid fragment in diazo alkanes and phosphorous ylides.62 Evidence has been presented for the intermediacy of carbene complexes in metal-catalysed addition of amines to isocyanides to give for ma mi dine^.^^ A new departure is in the preparation of complexes of the other Group IVB element^,^^,^' e.g. stannylenes as in [(Me,Si),CH]2SnCr(CO),.64 Reactions of Group I11 metal-carbenes have given two novel complexes reaction of (OC),-WC(0Me)R with BX gives the carbyne complexes RC=W(CO)4X66 and reaction of (OC),CrC(OMe)Ph with diazabicyclo[2,2,2]octane gives the N-ylide (q.67 /F77 (CO)5CrC-,OMe N \ WN Ph 4 Hydrogenation The subject has now reached a state of maturity with papers addressing them- selves more to an understanding of the workings of existing systems than to the reporting of new catalysts.A problem in catalysis not restricted to hydrogena- tion is that the concentration of the reactive species (usually produced by neutral ligand dissociation) that participate in the catalytic cycle is usually small e.g. the equilibrium RuH,(PPh,) @ RuH,(PPh,) + PPh lies well to the left. The new apparently simple technique of reverse osmosis offers a solution.By a suitable choice of membrane free phosphine may be removed to give the reactive RuH,(PPh,), and to illustrate the utility of the technique the authors also report the isolation of new complexes by this method.68 6o J. A. Connor P. D. Rose and R. M. Turner J. Organomerallic Chem. 1973 55 11 1. 6' C. P. Casey R. A. Boggs and R. L. Anderson J. Amer. Chem. SOC., 1972,94 8947. 62 C. P. Casey S. H. Bertz and T. J. Burkhardt Tetrahedron Letters 1973 1421. 63 J. A. McCleverty and M. M. M. da Mota J.C.S. Dalton 1973 2571; J. E. Parks and A. L. Balch J. Organometaffic Chem. 1973 57 C103. 64 P. J. Davidson and M. F. Lappert J.C.S. Chem. Comm. 1973 317. 65 T. J. Marks and A. R. Newman J. Amer.Chem. SOC. 1973,95 769; M. D. Brice and F. A. Cotton ibid. p. 4529. 66 E. 0. Fischer G. Kreis C. G. Kreiter J. Muller G. Huttner and H. Lorenz Angew. Chem. Internat. Edn. 1973 12 564. '' F. R. Kreissl E. 0.Fischer C. G. Kreiter and K. Weiss Angew. Chem. Internat. Edn. 1973 12 563. 68 L. W. Gosser W. H. Knoth and G. W. Parshall J. Amer. Chem. SOC.,1973.95 3436. 248 J. D.Jones R. Pearce and G. L. P. Randall Both (Ph,P),RhCl and (Ph,P),Rh,CI are lo4 times less reactive towards H2 than (Ph,P),RhCl. (Ph,P),Rh,CI now appears to be of no significance in the mechanistic scheme for hydr~genation.~~ Earlier assumptions on the reversibility of H uptake by the (Ph,P),RhCl system are now shown to be invalid. In both C6H6 and CHC1,-EtOH solvents H uptake was irreversible the kinetically non-labile (Ph,P),RhHCl being identified in the latter system.70 Stereochemical criteria for mechanistic studies have been proposed wherein the variation of the isomeric ratio of the hydrogenation products of 4-t-butyl- methylenecyclohexane with H pressure may allow an assignment of the rate- determining step in the catalytic cycle.For (Ph,P),RhCI it is the formation of the hydridoalkyl from the olefin complex whereas at high H pressure for HRh(CO)(PPh,) it is the formation of the olefin complex.71 The importance of the solvent in determining reaction mechanism is brought out in a study of Co(CN) - where in glycerol-methanol a radical mechanism prevails whereas in water organocobalt species are involved.72 Other complexes studied include (7c-C5H,),MoH ,73 Mn2(C0),o,74 (Ph,P),Fe(N,)H ,75 (n-areneRuCl,) ,76 (Ph,P),Ru(CO)CI ,77 (P~,P),Ru,+,~~ (Ph,P),OsH and (Ph,P),OsH ,79 CO(CN),~-," (Ph,P),CoH(N) with NaCloH,,81 (Ph,P),M(NO) (M = Co Rh or Ir),* Rh,(OAc) ,83 (Ph,P),RhC1,84 (py),RhCI,-NaBH ,85 H,Ir(CO)-(PPh,) ,86 (Ph,P),Ir(CO)C1,87 and (cyclo-octene),Ir2C1 .88 The high-boiling solvent technique using the system PtC1,-Et,NMCl (M = Ge or Sn) offers a useful alternative to techniques employing either polymer- supported catalysts or product separation using selective membranes.1,5,9-Cyclododecatriene was selectively hydrogenated to cycl~dodecene~~ (see also ref. 90). Selective hydrogenation of both orb-unsaturated carbonyl and nitrile compounds has been achieved using Rh6(CO) 6-CO-H,0,91 while addition 69 J.Halpern and C. S. Wong J.C.S. Chem. Comm. 1973 629. 70 G. G. Strathdee and R. M. Given J. Catalysis 1973 30 30. S. Siege1 and D. W. Ohrt Tetrahedron Letters 1972 5 155. 72 T. Funabiki M. Mohri and K. Tarama J.C.S. Dalton 1973 1813. 73 A. Nakamara and S. Otsuka J. Amer. Chem. SOC. 1973,957262; Tetrahedron Letters 1973 4529. 74 T. A. Weil S. Meltin and I. Wender J. Organometallic Chem. 1973 49 227. 75 E. Koerner von Gustorf I. Fischler J. Leitich and H. Dreeskamp Angew. Chem. Internat. Edn. 1972 11 1088. l6 A. G. Hinze Rec. Trav. chim. 1973,92 542; R. Iwata and I. Ogata Tetrahedron 1973 29 2753. 77 D. R. Fahey J. Org. Chem. 1973 38 3343. 7H R. W. Mitchell A. Spencer and G. Wilkinson J.C.S. Dalton 1973 846.79 B. Bell J. Chatt and G. J. Leigh J.C.S. Dalton 1973 997. 8o J. Basters C. J. Groenenboom H. van Bekkum and L. L. van Reijen Rec. Trav. chim. 1973 92 219. S. Tyrlik J. Organometallic Chem. 1973 50 C46. 82 G. Dolcetti Inorg. Nuclear Chem. Letters 1973 9 705. 83 B. C. Y. Hui W. K. Teo and G. L. Rempel Inorg. Chem. 1973 12 757. " B.Bayeri M. Wahren and J. Graefe Tetrahedron 1973 29 1837. 85 C. J. Love and F. J. McQuillin J.C.S. Perkin I 1973 2509. 86 M. G. Burnett and R. J. Morrison J.C.S. Dalton 1973 632. M. G. Burnett R. J. Morrison and C. J. Strugnell J.C.S. Dalton 1973 701. 8H C. Y. Chan and B. R. James Inorg. Nuclear Chem. Letters 1973,9 135. 89 G. W. Parshall J. Amer. Chem. SOC. 1972 94 8716. 90 D. H. Fahey J. Org. Chem. 1973 38 80.91 T. Kitamura N. Sakamoto and T. Joh Chem. Letters 1973 379. Organometallic Compounds of the Transition Elements 249 of base to (Ph,P),RuCl allows greater selectivity in the hydrogenation of a number of steroids such that the method now appears synthetically useful.92 Interestingly (Ph,P),Ir(CO)Cl selectively hydrogenates 1,3- and 1,4-cyclohex- adiene to cyclohexene without competing isomerization or disprop~rtionation.'~ Not strictly within the scope of hydrogenation are a number of potentially useful reductions involving transition-metal complexes e.g. reduction of enones by (X-C,H,),F~-HC~,~~ of cyclic acid anhydrides to aldehyde acids by Na2- Fe(C0)4,95 and selective reduction of the functions -C=CX (X = OAc C1 or CHO) to -C=CH and AcO-C-C=O to H-C-C=O by Fe(CO) .96 5 Hydrosilylation Hydrosilylation of unsaturated functions containing heteroatoms (e.g.C=O C=N) is a method that complements hydrogenation in organic synthesis e.g. R2 R2 I 0 R' / \c/ OSiX II \ /c //o XW* C/ -R'CH2CHR2CR3 c c RICH, I I 1 H R3 R3 This year significant contributions have been made in this area using Rh and Pd complexes particularly (Ph,P),R hC1 as homogeneous catalysts. Hydrosilyla- tion of ap-unsaturated terpene carbonyl compounds is said to be the most selective method for reduction in these systems with predominantly 1,4-addition and without isomerization or hydrogenation of other areas of unsaturation within the rn01ecule.~' Stereoselective reduction of terpene ketones was also in~estigated.~~ Enamines and isocyanates are readily hydrosilylated to give ~ilylamines~~ and N-silylformamidines loo respectively which may be further transformed by cleavage (e.g.with MeCOCl) of the Si-N bond.Contrary to earlier findings nickel compounds [e.g.(Ph,P),NiCl,] are now found to be effective in the hydrosilylation of olefins. They are operative above ca. 90 "C and show similar reactivity to the Pt complexes."' With the silanes R,SiH ,some unusual products arising via SiH-SIC1 exchange were obtained.lo2 92 S. Nishimura T. Ichino A. Akimoto and K. Tsuneda Bull. Chern. SOC.Japan 1973 46 279. 93 J. E. Lyons J. Catalysis 1973 30 490. 94 K. Yamakawa and M. Moroe J. Organometallic Chern. 1973 50 C43. 95 Y. Watanabe M. Yamashita T.Mitsudo M. Tanaka and Y. Takegami Tetrahedron Letters 1973 3535. 96 S. J. Nelson G. Detre and M. Tanabe Tetrahedron Letters 1973 447. 97 I. Ojima and T. Kogure Tetrahedron Letters 1972 5035. 98 I. Ojima M. Nihonyanagi and Y. Nagai Bull Chern. Sor. Japan 1972.45 3722. 99 I. Ojima T. Kogure and Y. Nagai Tetrahedron Letters 1973 2475. I00 I. Ojima and S. Inaba Tetrahedron Letters 1973 4363. I01 Y. Kiso M. Kumada K. Tamao and M. Umeno J. Organornetallic Chern. 1973 50 297. I02 Y. Kiso M. Kumada. K. Maeda K. Sumitani and K. Tamao J. Organornetallic Chem. 1973 50 3 1 1. J. D. Jones R. Pearce and G. L. P. Randall (Ph,P),NiCl also catalyses the addition of R,SiH2 to dienes to give 1,4- products by both Si-H and surprisingly Si-Si addition and to acetylenes to give silacyclopentadienes.'O3 Silenoid intermediates are proposed (cf. ref. 104). Selective hydrosilylation of styrene by (Cp)Ni(CO) to give l-silylphenyl- ethanes has been rep~rted,"~ and a chelating carbaboranyldiphosphine nickel complex gives an unusually high proportion of the internal product in hydrosilyla- tion of cr-oIefins.lo6 A new departure in this field is in the use of Ziegler-Natta catalysts. Differ- ences from the conventional Group VIII catalysts were found the most notable being the production of the 2:l adduct H,C=CRC(R)=CHSiX with acety- lenes. O7 (Ph,P),RhCl has been used as a catalyst for the related reaction of dehydro- genative condensation of silanes with alcohols. The reaction is surprisingly not complicated either by disproportionation of silane or by isomerization and/or hydrogenation (by the produced H2) of other functions in the molecule.lo8 6 Metal-catalysed Hydrogen Exchange Progress this year was essentially confined to mechanistic aspects.Platinum catalysts continue to be studied.'09-' l3 Attempts have been made without success to exploit the activation of saturated hydrocarbons by Pt" by diverting the possible hydridoalkyl or platinum-alkene intermediates into pathways other than that leading to simple H/D exchange. The authors concluded that the correlation of reactivity in the H/D exchange reaction already noted is an index of the polarizability of the C-H bonqs within the molecule and that H/D exchange is probably a synchronous process the reactive Pt species being of the type DPtC12+.lo9 On the other hand proof of the formation of Pt-alkyl intermediates in this reaction has been claimed from a study of the reaction of Pt" complexes with alkylmercury compounds under the H/D exchange con- ditions.' '' Application of a microwave spectroscopic technique to H/D ex- change in propene points to a mechanism involving metal hydride addition- elimination rather than the formation of n-ally1 species by allylic C-H oxidative addition.' l4 Io3 H.Okinoshima K. Yamamoto and M. Kumada J. Amer. Chem. SOC. 1972 94 9263. I. Ojima S. Inaba T. Kogure and Y. Nagai J. Organometallic Chem. 1973 55 C7. lo' P. Svoboda P. Sedlmayer and J. Hetflejs Coll. Czech. Chem. Comm. 1973 38 1783.'06 M. Kumada K. Sumitani Y. Kiso and K. Tamao J. Organometallic Chem. 1973 50 3 19. lo' M. F. Lappert and S. Takahashi J.C.S. Chem. Comm. 1972 1272. Io8 I. Ojima T. Kogure M. Nihonyanagi H. Kono and S. Inaba Chem. Letters 1973 501 ; R. J. P. Corriu and J. J. E. Moreau J.C.S. Chem. Comm. 1973 38. Io9 G. W. Littlecott and F. J. McQuillin Tetrahedron Letters 1973 5013. 'lo N. F. Gol'dshleger I. I. Moiseev M. L. Khidekel and A. A. Shteinman Proc. Acad. Sci. U.S.S.R. 1972 206 694. R. J. Hodges D. E. Webster and P. B. Wells J.C.S. Dalton 1972 2571 2577. G. E. Calf and J. L. Garnett Tetrahedron Letters 1973 51 1. C. Masters J.C.S. Chem. Comm. 1972 1258; 1973 191. Il4 C. A. Tolman and L. H. Scharpen J.C.S. Dalton 1973 584. Organometallic Compoundsof the Transition Elements 25 1 (Ph,P),R hC1 catalyses the hydrogen transfer from dioxan to cyclopentene with the formation of cyclopentane and dioxene.The rate-determining step was reasonably assigned to the activation of the C-H bond of dioxan by oxida- tive addition to Rh.' l5 Full details of hydrocarbon activation by (Cp),MH (M = Nb or Ta) have now appeared.' ' 7 Oxidative Addition and Reductive Elimination Further evidence has appeared regarding the free-radical nature of oxidative addition reactions e.s.r. spectroscopic evidence is offered to show that the formally two-electron oxidation of Pto to Pt" by addition of alkyl halide proceeds in discrete one-electron steps involving free-radical intermediates. ''' A number of publications deal with the process of reductive elimination free-radicals are generated under mild conditions by the reaction of diethyl fumarate with dialkyl Pt" complexes,' and deuteriation studies indicate that the reductive elimination of ethane from (Me,PPh),PtMe,I proceeds via a first-order intra- molecular reaction' ' while the cis-elimination of ethane from EtMe,Au"'(PPh,) proceeds via initial phosphine dissociation.' 2o However it is also reported that the elimination of ethane from cationic dimethyl Au" complexes is favoured by co-ordination of bulky phosphine ligands,' and elimination from trimethyl Pt" cations is favoured if two of the three accompanying ligands are of high trans-influence.'22 A number of organic syntheses involving oxidative addition to and reductive elimination from transition metals have been reported.These are :the conversion of aryl halides into nitriles by NaCN'23 and the formation of alkylene carbonates from epoxides and CO '24 catalysed by zerovalent nickel complexes ; acrylic esters and propionaldehyde acetals from acrolein and alcohols in the presence of low-valent Rh Ru and Ir complexes ;125 carbon-carbon bond formation via Rh' complexes;'26 the condensation of aryl halides with olefins catalysed by Pdo;'27*'28 a range of unsymmetrical ketones prepared by the treatment of acid halides with alkyl Rh' c~mplexes;'~~ and the coupling of alkenyl halides in the presence of a Nio complex.'3o T. Nishiguchi K. Tachi and K. Fukuzumi J. Amer. Chem. SOC. 1972,94 8916. l6 U.Klabunde and G. W. Parshall J. Amer. Chem. SOC. 1972,94 9081. I I' M. F. Lappert and P. W. Lednor J.C.S. Chem. Comm. 1973 948. I18 N. G. Hargreaves R. J. Puddephatt L. H. Sutcliffe and P. J. Thompson J.C.S. Chem. Comm. 1973 861. M. P. Brown R. J. Puddephatt and C. E. E. Upton J. Organometallic Chem. 1973 49 C61. IZo A. Tamaki S. A. Magennis and J. K. Kochi J. Amer. Chem. SOC. 1973 95 6487. I2I C. F. Shaw J. W. Lundeen and R. S. Tobias J. Organometallic Chem. 1973 51 365. 122 H. C. Clark and L. E. Manzer Inorg. Chem. 1973 12 362. 123 L. Cassar J. Organometallic Chem. 1973 54 C57. I 24 R. J. De Pasquale J.C.S. Chem. Comm. 1973 157. 12' M. Hidai K. Ishimi J. Iwase E. Tanaka and Y. Uchida Tetrahedron Letters 1973 1189. M. F. Semmelhack and L. Ryono Tetrahedron Letters 1973.2967. ''' M. Julia and M. Duteil Bull. SOC. chim. France 1973 2790. 12' M. Julia M. Duteil C. Grard and E. Kuntz Bull. SOC. chim. France 1973 2791. L. S. Hegedus S. M. Lo and D. E. Bloss J. Amer. Chem. SOC. 1973 95 3040. I3O M. F. Semmelhack P. J. Helquist and J. D. Gorzynski J. Amer. Chem. SOC. 1972 94 9234. 252 J. D. Jones R. Pearce and G. L. P. Randall More examples of internal metallation have appeared. The first examples of internally metallated S-donor ligand complexes have been reported for Fe' and Ru'~~ and in addition the first example involving ortho-B-H bond cleavage.' 33 Crystal structures of internally metallated Ir' complexes have been determined' 34 and whereas arsines will form internally metallated complexes with Pt none was detected with Pd.'35 Previously prepared Pt cluster compounds derived from Pt(PPh,) have been recharacterized as associated Pt" complexes [Pt(PPh2)(C,H4PPh2)I2* 3,0r4 formed through internal meta1lati0n.l~~A new type of internally metallated Mn derivative has been reported.' A new percyanovinyl complex of Pt" is formed by irradiation of the corres- ponding tetracyanoethylene complex.' 38 Other alkenyl transition-metal corn- plexes are produced by electrophilic additions to hexafluorobut-2-yne complexes of Pt Ir and Rh.13' The migration of an alkyl group from a nitrogen atom to a metal ion in a new Rh'-porphyrin complex proceeds concertedly with oxidation to Rh111 140 The cleavage of carbon-carbon bonds with concomitant increase in metal oxidation state by low-valent metal species has been reported.Thus zerovalent Pt and Pd cleave carbonxarbon bonds of ring system^,'^^,'^^ Fe behaves ~irnilarly,'~~ and Pto Ir' and Ru" cleave carbon-carbon bonds in compounds with electron-accepting substituents. 144 Oxidative additions involving the cleavage of other bonds include the nitrogen- chlorine bond of nitrosyl chloride by Mo Wj145or Ni,146 and sulphur-sulphur bond in disulphides by Pt or Pd'47 and the sulphur-hydrogen bond in benzene- thiols by Ir.148 I3l H. Alper and A. S. K. Chan J. Amer. Chem. Soc. 1973,954905. 132 H. Alper and A. S. K. Chan J. Organometallic Chem. 1973 61 C59. 133 E. L. Hoe1 and M. F. Hawthorne J. Amer. Chem. SOC.,1973 95 2712. 13' G. Perego G.Delpiero M. Cesari M. G. Clerici and E. Perrotti J. Organometallic Chem. 1973 54 C5 1. 13' B. L. Shaw and R. E. Stainbank J.C.S. Dalton 1973 2394. IJ6F. Glockling T. McBride and R.J. I. Pollock J.C.S. Chem. Comm. 1973 650. R. J. McKinney B. T. Huie C. B. Knobler and H. D. Kaesz J. Amer. Chem. SOC. 1973 95 633. L38 0.Traverso V. Carassiti M. Graziani and U. Belluco J. Organometallic Chem. 1973 57 c22. 139 R. D. W. Kemmitt B. Y. Kimura and G. W. Littlecott J.C.S. Dalton 1973 636. loo H. Ogoshi T. Omura and 2.Yoshida J. Amer. Chem. Soc. 1973,95 1666. I4l J. A. Evans G. F. Everitt R.D. W. Kemmitt and D. R. Russell J.C.S. Chem. Comm. 1973 158. M. Lenarda R. Ros J. Graziani and U. Belluco J. Organometallic Chem. 1972 46 C29. L43 R. M. Moriarty K.-N.Chen C.-L. Yeh J. L. Flippen and J. Karle J. Amer. Chem. SOC.,1972 94 8944. I. S. Kolomnikov P. Svoboda and M. E. Vol'pin Bull. Acad. Sci. U.S.S.R. 1972 21 2752. M. Deane and F. J. Lalor J. Organometallic Chem. 1973 57 C61. 146 M. Hidai M. Kokura and Y. Uchida Bull. Chem. SOC. Japan 1973 46 686. 147 R. Zanella R. Ros,and M. Graziani Inorg. Chem. 1973 12 2736. 148 J. L. Herde and C. V. Senoff Cunad. J. Chem. 1973,51 1016. Organometallic Compoundsof the Transition Elements 8 Oligomerization and Polymerization Homo-oligomerization.-Further studies of the catalytic cycloaddition of allene with Nio phosphine systems have shown that the selective catalytic reaction path leading to the cyclic trimer (2) tetramer (3) or pentamer (4),is primarily (2) (3) (4) dependent on the nature of the phosphine ligand.'49 The reduction of Ni" phosphine complexes with borohydride or alkoxide produces catalysts for the cyclodimerization of butadiene to either 2-methyl-vinylcyclopentane or n-octatrienes depending upon reduction conditions.' The products from the Rh-catalysed cyclodimerization of butadiene depend on the solvent employed' ' and temperature is important with reduced nickel catalysts.' s2 Modification of a Co"' acetylacetonate-AlEt catalyst with dianil increases the linear dimerization of butadiene from 60 % without modification to >80 %.' s3 Butadiene is oligomerized by (1,5-cod),Ni1s4 and isoprene is dimerized using either Co(acac),-AlEt 'ss or a PdBr,(Ph,PCH,CH,PPh,~sodium phenoxide-phenol system.' s6 A review on the catalysis and mechanism of olefin dimerization by transition- metal complexes has appeared.' s' Both the basicity and bulkiness of the phos- phine favour the formation of 2,3-dimethyl butene rather than hexanes or pentenes from the dimerization of propene by Ni(acac),-Et3A1,C1,-tertiary phosphine catalysts.' '* [(Pr',P),Ni(C,H,),] was isolated from the reaction of ethylene with [(Pr',P),NiHCl] ;further treatment with Lewis acids oligomerizes the co- ordinated ethylene to dimers and trirner~."~ a-Olefins are produced by the oligomerization of ethylene in the presence of Zr complexes' 60,1 and increasingly the acidity of the Phillips catalyst (SO,-149 S.Otsuka K. Tani and T. Yamagata J.C.S. Dalton 1973 2491.150 J. Kiji K. Yamamoto S. Mitani S. Yoshikawa and J. Furukawa Bull. Chem. SOC. Japan 1973,46 1791. 15' P.S. Chekrii M. L. Khidekel I. V. Kalechits 0. N. Eremenko G. I. Karyakina and A. S. Todozhokova Bull. Acad. Sci. U.S.S.R. 1972 21 1521. * F. Furukawa K. Yamamoto S. Mitani and S. Yoshikawa Chem. Letters 1972 121 1. 15' R. Giezynski and S. Pasynkiewicz Przemysl Chem. 1972 51 804. N. Yamazaki and T. Ohta Polymer J. 1973 4 616. I" J. Beger C. Duschek and D. Paul Z. Chem. 1973 13 133. K. Takahashi G. Hata and A. Miyake Bull. Chem. SOC. Japan 1973,46 600. " J. Hetflejs and J. Langova Chem. listy 1973 67 590. 15* Y. Sakakibara T. Tagano M. Sakai and N. Uchino Bull. Inst. Chem. Res. Kyoto Univ. 1972 50 375. N. V. Petrushanskaya A. E. Kurapova and V.Sh. Feldblium Proceedings of the 15th International Conference on Coordination Chemistry Moscow 1973. I6O C. J. Attridge R. Jackson S. J. Maddock and D. T. Thompson J.C.S. Chem. Comm. 1973 132. P. Longi F. Greco and U. Rossi Chimica e Industria 1973 55 252. J. D. Jones R. Pearce and G. L. P. Randall supported chromium oxide) by addition of oxides such as ZnO WO, or MOO produces effective oligomerization catalysts for ethylene.' 62 Diphenylacetylene is catalytically cyclotrimerized to hexaphenylbenzene by (n-C,H,)Rh(CO),.' 63 Evidence for a non-concerted mechanism in cycloadditions involving the stepwise formation of metal-carbon o-bonded intermediates followed by reductive elimination of the hydrocarbon is provided by the isolation of nor- bornadiene-iridium complexes in which two norbornadiene rings form a metallo- cycle by Ir insertion and from which a norbornadiene dimer can be di~p1aced.l~~ Co-o1igornerization.-The substituted olefin 3-methyleneheptane-2,6dionewas obtained by the dimerization of methyl vinyl ketone using various transition- metal triphenylphosphine complexes,' 65 1,4-benzoquinones can be made by the Ni(1,S-cod),-catalysed dimerization of 2,3-disubstituted cyclopropenones,'66 and P-propiolactone is oligomerized by a Cu,O-isocyanide complex.' 67 The pyrrolidone enamine of cyclohexanone reacts with butadiene in the presence of Pd complexes to produce after hydrolysis 2-(2,7-octadienyl)cyclo-hexanone.' 68 The addition of Ph,P to Ni( 1,5-cod) produces a highly selective catalyst for the [2 + 21 cross-addition of norbornadiene and methylenecyclo- propane to (5).'69 The formation of pent-2-ene and 2-methylbut-1-ene from the codimerization of ethylene and propene in the presence of a Ni catalyst is con- sistent with the process of insertion of propene into an ethyl-Ni bond followed by /?-hydrogen elimination.' 70 Certain Rh compounds are effective catalysts for the codimerization of styrene with ethylene propene or butenel'l and the codimerization of a-olefins with 1,3-dienes.l7' The codimerization of butadiene with dicyclopentadiene to form (6) is catakysed by a borohydride-reduced Nil' 16' G. Henrici-Olive and S. Olive Angew. Chem. Internar. Edn. 1973 12 754. 163 S. A. Gardner P. S. Andrews and M. D. Rausch Inorg. Chem.1973 12,2396. 164 A. R. Fraser P. H. Bird S. A. Bezman J. R. Shapley R. White and J. A. Osborn J. Amer. Chem. SOC.,1973 95 597. T. Miyakoshi H. Omiti and S. Saito Nippon Kagaku Kaishi 1973 123. 166 R. Noyori I. Umeda and H. Takaya Chem. Letters 1972 1189. 167 T. Saegusa I. Murase M. Nakai and Y. Ito Bull. Chem. SOC.Japan 1972,45 3604. 16' J. Tsuji Bull. Chem. SOC.Japan 1973 46 1896. 16' R. Noyori T. Ishigami N. Hayashi and H. Takaya J. Amer. Chem. SOC.,1973 95 1674. I'O K. Maruya T. Mizoroki and A. Ozaki Bull. Chem. SOC.Japan 1973,46 993. 17' A. Umezaki Y. Fujiwara K. Sawara and S. Teranishi Bull. Chem. SOC.Japan 1973 46 2230. A. C. L. Su and J. W. Collette J. Organometallic Chem. 1972 46 369. 255 Organometallic Compounds of the Transition Elements phosphine complex,' and alternating co-oligomers are produced by the action of VO(acac),-Et,Al-Et,AlCl on an isoprene-propene mixture.'74 Oligomerization with Addition.-Nickel catalyses the oligomerization of buta-diene to alkoxy-octadienes in the presence of alcohols'75 and to phenoxy- octadienes in the presence of and the reaction of allene with amines or active methylene compounds to give compounds of the type (7).177 Pd compounds catalyse the selective reaction of acetic acid with butadiene to give acetoxy~ctadienes'~ * and the hydrodimerization of butadiene to octa-1,7- diene.I7 Polymerization Reactions Catalysed by Non-Ziegler Systems.-Tris-n-allyl-chromium supported on silica-alumina will copolymerize isoprene-butadiene mixtures,"' Ni Zr Cr Mo and Co allyls polymerize 2-alkylb~ta-l,3-dienes,'~~ n-allylic Ni complexes stereospecifically polymerize 2,3-dimethylbutadiene and cyclohexa- 1,3-diene and copolymerize butadiene with other dienes.' 82 The relative reactivities of a series of dienes and butadiene-styrene copolymerizations using various catalysts based on transition-metal allylic compounds have been 173 S.Yoshikawa S.Nishimura J. Kiji and J. Furukawa Tetrahedron Letters 1973,3071. 174 J. Furukawa S. Tsuruki and J. Kiji J. Polymer Sci. Part A-1 Polymer Chem. 1973 11 1819. 175 J. Beger C. Duschek and H. Fullbier 2. Chem. 1973 13 59. 176 F. J. Weigert and W. C. Drinkard J. Org. Chem. 1973 38 335. 177 R. Baker and A. H. Cook J.C.S. Chem. Comm. 1973,472. D. Rose and H.Lepper J. Organometallic Chem. 1973 49 473. P. Roffia G. Gregorio F. Conti G. F. Pregaglia and R. Ugo J. Organometallic Chem. 1973 55 405. Ieo V. L. Shmonina N. N. Stefanovskaya E. I. Tinyakova and B. A. Dolgoplosk Proc. Acad. Sci. (U.S.S.R.),1973 209 227. Is' V. A. Vasiliev N. A. Kalinicheva V. A. Kormer M. I. Lobach and V. I. Klepikova J. Polymer Sci. Part A-I Polymer Chem. 1973 11 2489. "* B. A. Dolgoplosk S.I. Beilin Yu. V. Korshak; G. M. Chernenko L. M. Vardanyan and M. P. Teterina European Polymer J. 1973 9 895. 256 J. D. Jones R. Pearce and G. L. P.Randall studied.lS3 The reaction products from Zr tetra-ally1 and TiC14 will initiate the cationic polymerization of isoprene. Transition-metal complexes are proving particularly useful for the polymeriza- tion and copolymerization of polar monomers; thus Rh' complexes in the presence of organic halides effect the free-radical polymerization of methyl methacrylate (MMA),'85 iron complexes will initiate the polymerization of styrene (ST) acrylonitrile (AN) methacrylonitrile (MAN) and MMA,ls6 and it has been demonstrated using deuteriated MMA that opening of the double bond occurs in the polymerization catalysed by diethylbis(bipyridy1)- iron.' 87 Cobalt hydrido and methyl complexes will initiate the polymerization of AN MAN and MMA'88 and the reactivity of polar monomers to polymeriza- tion effected by Co carbonyl complexes has been re~0rted.l~~ Tetrabenzyl Zr polymerizes ST by a co-ordinated anionic mechanism.' 90 Carbonyl complexes of Mn and Re are active photoinitiators of the free-radical polymerization of tetrafluoroethylene ;in addition these carbonyls in the presence of low concen-trations of tetrafluoroethylene will photoinitiate polymerization of ST.MMA and AN.'91 The free-radical polymerization of AN and MMA is also initiated by a-amino-acid ester Cu" systems.' 92 Poly(Schiff bases) are produced by the nickel-catalysed polymerization of isocyanides.' 93 9 Insertion Reactions The complex [PtH(C2H4)(PEt,),]BPh4 an intermediate in the insertion of ethylene into Pt-hydride bonds has been isolated'94 and other insertions into metal-hydride bonds have been reported. The insertion of functionally substi- tuted allylic compounds into cationic Pt" hydrides involves an initial migration of the double bond,lg5 the ortho-olefinic moiety of a chelating phosphine ligand will insert into Pt-H bonds,'96 and olefins and acetylenes insert into Ir"' hydride~.'~' The insertion of substituted acetylenes into R-methyl bonds B.A. Dolgoplosk S. I. Beilin Yu. V. Korshak K. L. Makovetsky and E. I. Tinyakova J. Polymer Sci. Part A-1 Polymer Chem. 1973 11 2569 V. A. Kormer V. A. Vasiliev N. A. Kalinicheva and 0.I. Belgordskaya J. Polymer Sci. Part A-1 Polymer Chem. 1973 11 2557. N. Kameda and N. Itagaki Bull. Chem. SOC. Japan 1973,46 2597. Y. Kubo A. Yamamoto and S. Ikeda J. Organometallic Chem. 1972 46 C50. T. Yamamoto A. Yamamoto and S. Ikeda J. Polymer Sci. Part B Polymer Letters 1972 10 835. "' Y. Kubo A. Yamamoto and S. Ikeda J.Organometallic Chem. 1973 59 353 lS9 G. Palyi F. Baumgartner and I. Czajlik J. Organometallic Chem. 1973 49 C85. I9O D. G. H. Ballard J. V. Dawkins J. M. Key and P. W. van Lienden Makromol. Chem. 1973 165 173. l9I C. H. Bamford and S. U. Mullik Polymer 1973 14 38. 192 K. Azuma Y. Inaki and K. Takemoto Makromol. Chem. 1973 166 189. '93 R.J. M. Nolte R. W. Stephany and W. Drenth Rec. Trav. chim. 1973 92 83. '94 A. J. Deeming B. F. G. Johnson and J. Lewis J.C.S. Dalton 1973 1848. '95 H. C. Clark and H. Kurosawa Inorg. Chem. 1973 12 357. '96 P. R. Brookes J. Organometallic Chem. 1973 47 179. 197 H. C. Clark and R. K. Mittal Canad. J. Chem. 1973 51 151 1. Organometallic Compounds of the Transition Elements 257 proceeds by a free-radical mechanism,' 98 insertion of 1,2-dienes1 99 and strained olefins2" into Pd-ally1 bonds proceeds viao-ally1 intermediates and interestingly the insertion of isoprene into n-crotyl-Ni bonds differs from that of n-allylic palladium in that the n-crotyl group bonds to the methylene carbon of the diolefin.20' The insertion of isocyanide into the Pt-alkyl or -aryl bonds of dialkyl or diary1 Pt phosphine complexes202 is dependent upon the nature of the phosphine and the isocyanide and is not so general as those into monoalkyl complexes.203 The crystal structure of a typical isocyanide insertion product has been deter- mined.204 Insertion of SO into Fe- Mo- Mn- and Re-alkyl bonds to form sulphur- bonded sulphinates involves the initial formation of oxygen-bonded inter- mediate~,~~~,~~~ and studies indicate that SO insertion into an Fexarbon bond occurs with retention of configuration at Fe.,07 Insertion of SO into Ptxarbon bonds is faster than that of carbon monoxide.208 New metal-acyl complexes of Ni209 by CO insertion into a Ni-methyl bond and of Cr,," by treatment of phosphines with h5-C5H5Cr(CO),Me have been prepared.The decarbonylation of (h5-C H 3-1-Me-3-Ph)Fe(CO) (PPh,) (COMe) is highly stereospecific.2 '' Other insertions of small molecules into metalkdement bonds include chloro- sulphonylisocyanate into Fexarbon, ' CO into Cu-methyl,2' aldehydes into Co-and Ni-ally1,2'4 0 into Coxarbon of alkyl cobaloxime~,~'~,~~~ carbon dioxide2l7 and carbon disulphide218 into Ru-hydride the =C(CF,) group 19' T.G. Appleton M. H. Chisholm and H. C. Clark J. Amer. Chem. Soc. 1973 94 8912. IQ9 R. P. Hughes and J. Powell J. Organometallic Chem. 1973 60 409. 2oo R. P. Hughes and J. Powell J. Organometallic Chem. 1973 60 387. 201 V. A. Vasl'ev V. I. Klepikova G. P. Kondratenkov V. A. Kormer and M. I. Lobach Proc. Acad. Sci. U.S.S.R. 1973 206 719. 202 P. M. Treichel and K. P. Wagner J. Organometallic Chem. 1973 61 415. '03 P. M. Treichel K. P.Wagner and R. W. Hess Inorg. Chem. 1973 12 1471. 204 K. P. Wagner P. M. Treichel and J. C. Calabresse J. Organometallic Chem. 1973 56 C33. 'OS S. E. Jacobson P. Reich-Rohrwig and A. Wojcicki Inorg. Chem. 1973 12 717. 206 S. E. Jacobson and A. Wojcicki J. Amer. Chem. Soc. 1973 95 6962. 207 T. C. Flood and D.L. Miles J. Amer. Chem. Soc. 1973 95 6460. *08 F. Faraone L. Silvestro S. Sergi and R. Pietropaolo J. Organometallic Chem. 1972 46 379. 209 H.-F. Klein Angew. Chem. Internat. Edn. 1973 12 402. 210 K. W. Barnett D. L. Beach and T. G. Pollman Inorg. Nuclear Chem. Letters 1973 9 131. 211 T. G. Attig P. Reich-Rohrwig and A. Wojcicki J. Organometallic Chem. 1973 51 c21. 'I2 Y. Yamamoto and A. Wojcicki Inorg. Chem. 1973 12 1779. 213 A. Miyashita and A. Yamamoto J. Organometallic Chem. 1973 49 C57. 214 G. Agnes G. P. Chiusoli and A. Marraccini J. Organometallic Chem. 1973 49 239. 21s F. R. Jensen and R. C. Kiskis J. Organometallic Chem. 1973 49 C46. C. Giannotti and B. Septe J. Organometallic Chem. 1973 52 C36. 217 S. Komiya and A. Yamamoto J.Organometallic Chem. 1972 46,C58. 2'a R. 0. Harris N. K. Hota L. Sadavoy and J. M. C. Yuen J. Organometallic Chem. 1973 54 259. 258 J. D. Jones R. Pearce and G. L. P. Randall into Pt- and Pd*hl~rine,~ l9 keto- and vinyl-carbenes into Pd-chlorine.220 and nitric oxide into C-lefin bonds.221 The expansion of a four-membered ring bound to Fe by insertion of a -CF group into a carbon4arbon bond has been reported222 and a series of five- membered-ring-metal complexes have been prepared by the insertion of various organofluoro-compounds into three-membered-ring Pt and Ni c~mplexes.~~~,~~~ One of these organofluoro-compounds (CF,),CO also inserts into carbon- hydrogen bonds of organic moieties bonded to Fe Rh,225 and Ru.226 The insertion of an ethylacetatocarbene species into the 0-H bond of alcohols is catalysed by homogeneous Rh 10 Asymmetric Induction and Stereospecific Syntheses Transition-metal catalysts are increasingly being employed for the preparation of specific stereoisomeric compounds.Further work has appeared on the asym- metric reduction of ketones to alcohols catalysed by a chiral phosphine-rhodium complex,228 and the crystal structure of a Co complex used in the asymmetric hydrogenation of various unsaturated has been determined.230 The modification of catalyst surface enhances the activity for asymmetric hydro- genation reactions. Treatment with either L-glutamic or D-tartaric acid protects the surface of Raney Ni from corrosion. Whereas the protective effects are greater with the amino-acid the activity for the hydrogenation of acetylacetone is higher for the hydroxy-acid ;231 in addition the activity for a hydroxydicarbo- xylic acid modified catalyst in the hydrogenation of methylacetoacetonate is maximized at pH 5-8.232 D-tartaric acid m0dified.R~ catalysts have been used for the hydrogenation of the carbonyl group in acetoacetic ester,z33 and boro- hydride-reduced Ni modified with ethylenediamine greatly increases the cis-trans ratio in the stereospecific hydrogenation of alkynes to alkene~.~~~ The first 219 J.Clemens M. Green and F. G. A. Stone J.C.S. Dalton 1973 1620. 220 N. Yoshimura S.-I. Murahashi and J. Moritani J. Organometaffic Chem. 1973 52 C58. 221 H. Brunner and S. Loskot J. Organometaffic Chem.1973 61 401. 222 A. Bond M. Green and S. H. Taylor J.C.S. Chem. Comm. 1973 112. 223 J. Browning H. D. Empsall M. Green and F. G. A. Stone J.C.S. Dalton 1973 381. 224 P. K. Maples M. Green and F. G. A. Stone J.C.S. Dalton 1973 388. zzs M. Green and B. Lewis J.C.S. Chem. Comm. 1973 114. 226 T. Blackmore M. I. Bruce F. G. A. Stone R. E. Davis and N. V. Raghavan J. Organo- metallic Chem. 1973 49 C35. 227 R. Paulissen H. Reimlinger E. Hayez A. J. Hubert and Ph. Teyssie Tetrahedron Letters 1973 2233. 22a M. Tanaka Y. Watanabe T. Mitsudo H. Iwane and Y. Takegami Chem. Letters 1973 239. 229 S. Takeuchi Y. Ohgo and J. Yoshimura Chem. Letters 1973 265. 230 Y. Ohashi Y. Sasada Y. Tashiro Y. Ohgo S. Takeuchi and J. Yoshimura Buff. Chem. SOC. Japan 1973,46 2589.23' T. Tanabe Bull. Chem. SOC.Japan 1973 46 1482. 232 T. Tanabe K. Okuda and Y. Izumi Bull. Chem. SOC. Japan 1973,46 514. 233 E. I. Klabunovskii N. P. Sokolova A. A. Vedenyapin Yu. M. Talanov N. D. Zubareva V. P. Polyakova and N. V. Gorina Bull. Acad. Sci. U.S.S.R.,1972 21 2306. 234 C. A. Brown and V. K. Ahuja J.C.S. Chem. Comm. 1973 553. Organometallic Compounds of the Transition Elements 259 report of a Ni-catalysed selective reduction of exocyclic methylene groups important synthetically and industrially has a~peared.~ Asymmetric hydrosilylation reactions are proving useful for the preparation of specific organic products. Chiral Rh complexes catalyse the asymmetric hydrosilylation of ketones ;236.23 optically active carbinols are produced by the Pt"-catalysed addition of a silane to ketones :238 whereas an insoluble chiral polymer-supported Rh complex related to soluble Rh'diop has a low efficiency for asymmetric hydrogenation both the insoluble and soluble catalysts are very efficient for the asymmetric silylation of ketones.239 Rhl-diop is also used in the preparation of asymmetric amine~,~~' and the asymmetric hydroformylation of aliphatic olefinsZ4l and styrene.242 The asymmetric hydrocarboxylation of olefins can be achieved using chiral Pd complexes.243 The first catalytic asymmetric synthesis in which a chiral centre results from carbon-carbon bond formation has been reported for the Ni-catalysed co- dimerization of cyclo-octa- 1,3-diene and ethylene ;244 butadiene is dimerized stereospecifically with a Zr complex245 and whereas the polymerization of butadiene with a Ni complex gives mainly the cis-1,4 an ally1 Cr catalyst gives predominantly trans-l,4-~tructures.~~~ Certain cross-coupling reactions proceed stereospecifically ;Grignard reagents react with monohalogeno-olefins in the presence of Ni248 and the addition of alkyl halides to vinyl-Rh' complexes is followed by a reductive elimination yielding trisubstituted 01efins.~~' The first report of a high stereo- and regio-selective Vor Mo catalysed epoxida- tion in a complex molecule synthesis provides a remarkable demonstration of the use of a transition-metal catalyst to give products not obtainable with any other reagent .250 235 J.H. P. Tyman and S. W.Wilkins Tetrahedron Letters 1973 1773. 236 I. Ojima T. Kogure and Y. Nagai Chem. Letters 1973 541. 237 K. Yamamoto T. Hayashi and M. Kumada J. Organometallic Chem. 1973 54 c45. 238 K. Yamamoto T. Hayashi and M. Kumada J. Organometallic Chem. 1972 46 C65. 239 W. Dumont J.-C. Poulin T.-P. Dang and H. B. Kagan J. Amer. Chem. Soc. 1973 95 8295. 240 N. Langlois T.-P. Dang and H. B. Kagan Tetrahedron Letters 1973 4865. 24' G. Consiglio C. Betteghi C. Salomon and P. Pino Angew. Chem. Internat. Edn. 1973 12 669. 242 C. Salomon G. Consiglio C. Botteghi and P. Pino Chimia (Switz.) 1973 27 215. 243 C. Botteghi G. Consiglio and P. Pino Chimia (Switz.) 1973 27 477. B. Bogdanovic B. Henc B. Meister H. Pauling and G. Wilke Angew. Chem. Internat. Edn. 1972 11 1023.245 H.-J. Kablitz and G. Wilke J. Organometallic Ckm. 1973 51 241. 246 I. Ya. Ostrovskaya and K. L. Makovetskii Proc. Acad. Sci. (U.S.S.R.),1973,208 110. 247 V. L. Shmonina. N. N. Stefanovskaya E. I. Tinyakova. and B. A. Dolgoplosk, Vysokomol. Soedineniya (A) 1973 15 647 (Chem. Abs. 1973 79 53 851r). 24a K. Tomao M. Zembayashi Y. Kiso and M. Kumada J. Organometallic Chem. 1973 55 C9 1. 249 J. Schwartz D. W. Hart and J. L. Holden J. Amer. Chem. SOC. 1972 94 9269. 250 K. B. Sharpless and R. C. Michaelson J. Amer. Chem. SOC. 1973 95 6136. 260 J. D. Jones R. Pearce and G. L. P.Randall 11 Isomerization Carbon-Carbon Double-bond 1somerizations.-The majority of papers in this area have no immediate application to synthetic organic chemistry ; however they do extend the scope of these isomerization~.~~'-~~* Mechanistic information on these reactions has been extended by isotope studies of homogeneous iso- meri~ations~~~ and also by the determination of the relative stabilities ofpalladium pentene complexes.260 Of particular interest is the isomerization of cis,rrans-cyclodeca- 1,6-diene to cis,tmns-and cis,cis-cyclodeca- 1,5-diene which is cata- lysed by RuCl,,xH,O in this conversion cannot be carried out thermally.The catalytic isomerization of allyl to propenyl ethers has been used as a step in a new method of protecting a hydroxy-group.262 The hydroxy-function to be protected is converted into an allyl ether which can only be cleaved by very strong acid or base or by SeO oxidation.When the hydroxy-group is to be regenerated the allyl ether is catalytically isomerized to a propenyl ether which can be cleaved under dilute acid conditions (pH2). In the procedure described above R hCl(PPh,) was used as the isomerization catalyst ; however Pd(PhCN),Cl also catalyses the reaction.263 The isomerizations of ligands which remain co-ordinated to a metal are of interest in so far as they can indicate how related catalytic transformations occur. Amongst these are the conversion of the n-ally1 in x-allylirondicarbonyl iodide into a a-propenyl ligand under the influence of the co-ordination of a trispyrazolyl- borate ligand,264 the rearrangement of a a-but-3-enyl group co-ordinated to nickel to a C4 allyl by heat or irradiation,265 and the conversion ofa 1,l-dimethyl- n-ally1 ligand on Co or Rh into a 1,2-dimethyl-n-ally1.266 Isomerization of Carbon Skeletons.-There has been a large amount of informa-tion concerning the extension of these reactions and the mechanistic route by which they proceed.The reactions are divided into three types. '" B. Corain and G. Puosi J. Catalysis 1973 30 403. ' J. E. Lyons J. Catalysis 1973 28 500. 2s3 B. R. James L. D. Markham B. C. Hui and G. L. Rempel J.C.S. Dalton 1973 2247. 254 C. Eaborn N. Farrell and A. Pidcock J.C.S. Chem. Comm. 1973 766. 255 W. Strohmeier J. Organometallic Chem. 1973 60,C60. W. Strohmeier R. Fleischmann and W. Rehder-Stirnweiss J. Organometallic Chem. 1973 47 C37. 257 Y. M. Zhorov G. V. Demidovich and G. M. Panchenkov Kinetika i Kataliz 1973 14 809.258 B. Corain and G. Puosi J. Catalysis 1973 30 403. '" B. I. Cruikshank and N. R. Davies Austral. J. Chem. 1973 26 1935. 260 G. F. Pregaglia F. Conti B. Minasso and R. Ugo J. Organometallic Chem. 1973,47 165. 26' D. L. Schmitt and H. B. Jonassen J. Organometallic Chem. 1973,49,469. 262 E. J. Corey and J. W. Suggs J. Org. Chem. 1973,38 3224. 263 P. Goldborn and F. Scheinmann J.C.S. Perkin I 1973 2870. 264 R. B. King and A. Bond J. Organometallic Chem. 1973 46 C53. 265 J. M. Brown and K. Mertis J.C.S. Perkin II 1973 1993. 266 M. A. Cairns J. F. Nixon and B. Wilkins J.C.S. Chem. Comm. 1973 86. Organometallic Compoundsof the Transition Elements 26 1 lsomerizations which Generate Additional Carbon Rings.The influence of transition-metal co-ordination on the equilibrium between cyclo-octa- 1,3,5- triene and bicyclo[4,2,0]octa-2,4-diene has been demonstrated by co-ordination to Fe(CO) .267 The co-ordinated polyene favours the monocyclic isomer as compared with the unco-ordinated which prefers the bicyclic structure. In the field of catalytic synthesis bicyclo[3,3,0]oct-2-ene has been made from 1,5-cod using Ni 2-ethylhexanoate and C2H,AIC12 as catalyst.268 Isomerizations which Reduce the Number of Carbon Rings. The ring-opening reactions of bicyclobutanes have received a great deal of attention. N~yori~~' has rationalized the catalytic transformations which occur with a variety of metals on the basis of the relative electrophilic or nucleophilic nature of the carbene intermediates.A labile carbene intermediate generated in the Rh- catalysed isomerization of 1-methylbicyclo[ l,l,O]butane has been trapped by reaction with methyl acrylate :270 Me3 + CH,=L-CH,CH-Rh + CH,-4C0,Me A marked substituent effect has been noted in the product distribution obtained in the silver-ion-catalysed isomerizations of tricycloheptanes which may be related to carbene intermediate^.^" The understanding of the mechanism of iron-carbonyl-catalysed isomerizations of cyclopropanes has been improved by the isolation of some trimethylenemethane-Fe(CO) complexes from one of these reaction^.^^^.^^^ Among extensions of these isomerizations are the conversion of quadri-cyclene derivatives into norbornadienes by univalent Ag274 and Rh275 and the unusual cleavage of cyclohexadiene by Ru,(CO), to give a linear p-ally1 co- ordinated to a Ru carbonyl Isomerizations Maintaining the Same Number of Carbon Rings.Mechanistic information on the bond reorganization of homocubanes by homogeneous 267 M. Brookhart N. M. Lippman and E. J. Reardon J. Organometallic Chem. 1973 54 247. 26a N. A. Maly H. Menapace and M. F. Farona J. Catalysis 1973,29 182. 269 R. Noyori Tetrahedron Letters 1973 1691. 'lo P. G. Gassman and R. R. Reitz J. Organometallic Chem. 1973 52 C51. 27' G. Zon and L. A. Paquette J. Amer. Chem. SOC. 1973,95 4456. 272 W. E. Billups L. P. Lin and B. A. Baker J. Organometallic Chem. 1973 61 C55. 273 I. S. Koull J. Organometallic Chem. 1973 57 363. 274 G. F. Koser P.R. Pappas and S.-M. Yu Tetrahedron Letters 1973,4943. 275 M. Hogeveen and B. J. Nusse Tetrahedron Letters 1973 3667. 276 T. H. Whitesides and R. A. Budnik J.C.S. Chem. Comm. 1973 87. J. D. Jones R. Pearce and G. L.P. Randall silver salts suggests that reversible Ag-C bond formation occurs and that there is little evidence for carbonium ion intermediate^.^^' A deuteriation study of the stereospecific addition of acetic acid to norborna- diene catalysed by Pt complexes indicates that a bond reorganization has oc- c~rred.~~~ A ring-expansion reaction has been described when 1.2-divinyl-cyclobutane co-ordinates to Pd to give a 1,5-cod complex.279 12 Carbonylation Reactions Used in Organic Synthesis There have been a large number of reports of transition-metal-catalysed carbonyl-ations this year.The use of complexes of metals other than Co and Rh is in- creasing although no significant improvements have been noted. Reviews on olefin hydroformylation280 and CO insertion into transition-metalkarbon bonds28 have been published. High-pressure i.r. spectroscopy has been a useful technique for determining the species present under hydroformylation conditions. Iridium carbonyl phosphine catalysts have been shown to break down to monomeric carbonyl hydrides from Ir clusters under reaction conditions.282 It has also been demon- strated that cyclohexenyl hydroperoxide activates the (PPh,),Rh(CO)Cl hydro- formylation catalyst by promoting the formation of cis-(PPh,)Rh(CO),Cl which is a very active catalytic species.283 The use of the tetracarbonylferrate anion in organic syntheses involving carbonylation has been extended and closely related anions have also been used for similar reactions.The general intermediate (CO),FeCOR made by the reaction of an alkyl bromide or tosylate followed by treatment with CO may be converted into a variety of carboxylic acid derivatives (Scheme 1). These (CO),FeCOR' R'C02H 1 R'CO~R~ R 'C02NRZR Scheme 1 reactions give isolated yields of 80%.284The reaction conditions do not affect either ester or ketone functions as demonstrated by the transformation shown in 2" L. A. Paquette and J. S. Ward Tetrahedron Letters 1973 4909. 278 E. F. Magoon and L. H. Slaugh J. Organometallir Chem. 1973,55,409. 279 P.Heimback and M. Molin J. Organometallic Chem. 1973 49 483. J. Falbe Propylene and its industrial Derivatives 1973 333. 281 A. Wojcicki Adv. Organometallic Chem. 1973 11 88. 282 A. J. Drakesmith and R. Whyman J.C.S. Dalton 1973 362. 283 H. B. Tinker and D. E. Morris J. Organometallic Chem. 1973 52 C55. 284 J. P. Collman. S. R. Winter and R. G. Komoto J. Amer. Chem. SOC. 1973 95 249. 263 Organometallic Compounds of the Transition Elements Scheme 2. The tetracarbonylferrate reagent may be used to make hemifluorinated ketones containing nitrile or chloro functions285 and also in the preparation of cyclic ketones from ethylenic bromides and allenic bromides.286 The related 0 0 II 0-c/\1 Scheme 2 anion CpFe(CO),-may be used in a similar manner but a cupric salt is required as an oxidizing agent in the carbonylation and metal removal step.287 Nickel tetracarbonyl has been used as a catalyst in the synthesis of l-alkyl-2-phenylindolin-3-ones from phenyl iodide and N-benzylidene-alkylamines :288 0 PhI + PhCH=NMe -+ (PhCONMeCHPh) The same carbonyl is used to carbonylate aryl halides in aprotic solvents with base and one atmosphere of C0.289 Olefins and saturated hydrocarbons may be converted into tertiary carboxylic acids under one atmosphere of CO at ambient temperature by a cuprous salt in concentrated sulphuric acid.290*291 The cation Cu(CO) + is suggested as the catalyst in these reactions.The rhodium catalyst (PPh,),RuCl has been used to decarbonylate an aldehyde selectively in the presence of an ester292 and silver perchlorate has also been used as a decarbonylation catalyst for strained-ring ketones.293 13 Addition and Substitution Reactions at Co-ordinated Ligands This subject has been the topic of a large number of publications recently and has been divided into two sections.lH5 J. P. Collman and N. W. Hoffman J. Amer. Chem. SOC.,1973,95 2689. J. Y. Merour J. L. Roustan C. Charrier J. Collin and J. Benaim J. Organomerallic Chem. 1973,51 C24. K. M. Nicholas and M. Rosenblum J. Amer. Chem. Soc. 1973,95 4449. 2H8 M. Ryang Y. Toyoda S. Murai N. Sonoda and S. Tsutsumi J. Org. Chem. 1973 38 62. 289 L. Cassar and M. Foa J. Org. Chem. 1973 51 381. ‘90 Y. Souma H. Sano and J. Iyoda J. Org. Chem. 1973 2016. 291 Y.Souma and H. Sano J. Org. Chem. 1973 3633. 292 B. M. Trost and M. Preckel J. Amer. Chem. SOC..1973 95 7862. 293 H. Ona M. Sakai M. Suda and S. Masamune J.C.S. Chem. Comm. 1973,45. J. D. Jones R. Pearce and G. L. P. Randall The Understanding aod Discovery of New Reactions of Functional Groups Pro-moted by Co-ordination to a Transition Metal.-The modification of the reactivity of co-ordinated nitriles and isocyanides has received much attention. An in- vestigation of the addition of hydrazine to the ligands in (RNC),PtZ+ has shown that the products (8) consist of two isocyanide residues linked by an N-N Nucleophiles such as thiols alcohols and amines produce co-ordinated carbenes by attack at isocyanide arbo on.^^^-^^' It has been proposed that the catalytic homogeneous hydration of nitriles proceeds by a related attack of hydroxide at the carbon of a co-ordinated nitrile followed by proton transfer and hydrolysis.298 The activation of olefins and dienes towards nucleophilic attack is an interesting result of co-ordination to transition metals.The Pt-olefin system has been investigated by n.m.r.,299 and it has been found that olefins co-ordinated to Fe" react with enolate anions to give the corresponding iron alk~1.~'' Reversible nucleophilic addition to butadiene which is co-ordinated to Mo has been demon- strated in the complex (9).301 Q Nucleophilic addition of PPh to cyclohexadienyl- and cycloheptadienyl- Fe(CO) complexes gives phosphonium derivatives and pyridine adds in a similar way.3o2 In the related open-chain pentadienyl complexes the product of reaction with nucleophiles either addition or H+ removal is dictated by the substituents on the pentadienyl chain.303 Electrophilic addition of C2(CN) 294 W.M. Butler J. H. Enemark J. Parks and A. L. Balch Inorg. Chem. 1973 12 451. 295 W. M. Butler and J. H. Enemark Inorg. Chem. 1973 12,540. 296 B. Crociani T. Boschi G. G. Troilo and U. Croatto Inorg. Chim. Acta 1972 6 655. 297 J. Chatt R. L. Richards and G. M. D. Royston Inorg. Chim. Acta 1972 6 669. 298 M. A. Bennett and T. Yoshida J. Amer. Chem. Soc. 1973,95 3030. 299 D. Hollings M. Green and D. V.Claridge J. Organometallic Chem. 1973 54 399. 300 A. Rosan M. Rosenblum and J. Tancrede J. Amer. Chem. SOC. 1973 95 3062. 301 M.L. H. Green L. C. Mitchard and W. E. Silverthorn J.C.S. Dalton 1973 1952. 302 J. Evans D. V. Howe B. F. G. Johnson and J. Lewis J. Organometallic Chem. 1973 61 C48. '03 P. McArdle and M. Sherlock J. Organometallic Chem. 1973 52 C29. Organometallic Compounds of the Transition Elements to (cycloheptatriene)Fe(CO) gives an exo-l,3-addition and it has been shown that in (ditropyl jFe(CO) the unco-ordinated cycloheptatriene ring is the first to react with C,(CN),.305 The Synthetic Use of Transition-metal-promoted Additions and Substitutions.-There have been several very interesting reports covered by this heading. The synthesis of Vitamin K has been carried out by the use of a n-allyl-Ni derivative.,06 In this reaction the bromine of an aryl halide is replaced by a substituted ally1 group in high yield.The addition of an amine to a n-allyl-Ni complex produced by a Ni-H addition to butadiene is suggested as the important step in the synthesis of butenylamine from butadiene and amine in 60% yield.," Iron tricarbonyl complexes have found use in the preparation of cyclopentanones from 1,l'-dibromoketones and 01efins.~'~ The oxyallyl ligand is suggested as the intermediate to which olefin adds. Terpene complexes of Fe(CO) may be isomerized and have addition reactions carried out on them which do not occur in the unco-ordinated materials. These properties have been used in some terpene transformation^.^^^ Birch and his co-workers have studied the removal of Fe(CO), which is an essential step in any synthesis involving an Fe(CO) Besides the efficient removal of the Fe(CO) group under mild conditions a chemical transformation of the ligand can be achieved by careful choice of reagents (Scheme 3).CH,COMe A FeCI inw/ethanolic lpb(o\ d3 c=o I c=o Scheme 3 304 M. Green S. Heathcock and D. C. Wood J.C.S. Dalton 1973 1564. 305 P. McArdle J.C.S. Chem. Comm. 1973 482. 306 K. Sato S. Inoue and K. Saito J.C.S. Perkin I 1973 2289. 307 J. Kiji K. Yamamoto E. Sasakowa and J. Furukawa J.C.S. Chem. Comm. 1973 770. 308 R. Noyori. K. Yokoyama and Y. Hayakawa. J. Amer. Chem. SOC.,1973,95 2722. 309 D. V. Banthorpe H. Fitton and J. Lewis J.C.S. Perkin I 1973 2051. 3 10 A. J. Birch K. B. Chamberlain M. A. Haas and D. J. Thompson J.C.S.Perkin I 1973. 1882. 266 J. D. Jones R. Pearce andG. L. P.Randaii The Pd-catalysed addition of secondary amines to both carbon-carbon double bonds and carbon-nitrogen double bonds has been used to prepare pyrimidenes in 75 % yield from secondary amines and a,~-diamines.~ 14 Heterogeneous Catalysts Derived from Known Homogeneous Systems The modification of homogeneous catalysts to operate in a heterogeneous mode has the attraction of possibly being able to combine the most attractive features of each system. The adaptation of homogeneous hydroformylation catalysts to heterogeneous systems by supporting on a polymer backbone has been discussed in a review of hydrof~rmylation.~' The preparation of polymer-attached hydrogenation catalysts as a means of making them heterogeneous has received some attention.The hydrogenation catalyst (PPh,),RhCl has been made heterogeneous by replacing a PPh by a cross-linked phosphinated poly~tyrene.~ The catalytic activity is reduced by a factor of 16 but the catalyst is readily removed from the reaction medium by simple filtration. It shows selectivity for small substrates and may be used as a suspension in a polar solvent to hydrogenate selectively non-polar olefins in the presence of polar ones. Platinum and palladium hydrogenation catalysts have been made heterogeneous in a similar way. ' Polymer-supported palladium catalysts have also been used in the following reaction :315 / Me,SiOH + -+ Me,SiO The polymerization catalyst PhNi(bipy)Cl has been supported by replacing the monomeric phenyl group by that of polystyrene derived from 4-chloro- p~lystyrene.~'~ The polymerization activity is very poor but with added EtAlCl as an activator ethylene and propylene may be dimerized.15 Metathesis of Olefins and Acetylenes The majority of publications on this subject involve the use of WCl ,EtAlCl as an olefin metathesis catalyst ;however there have been some useful developments with applications. The alternative co-catalyst LiAlH has been used with WC1 to convert hept-3-ene into oct-4-ene and hex-3-ene. l7 The attraction of this co-catalyst is its stability towards oxygen. It has been found that the extent to which alkylation of aromatic solvent occurs using WCl ,EtAICI catalysts is very dependent on the ratio of olefin to Polybutadiene has been ' N.Yoshimura I. Moritani T. Shimamura and S.-I. Murahashi J. Amer. Chem. SOC. 1973 95 3038. 'I2 P. W. H. L. Tjan Chem. Weekblad 1973 69 K11. 313 R. H. Grubbs L. C. Kroll and E. M. Sweet J. Macromof. Sci. Chem. 1973 7 1047. 3'4 H. Bruner and J. C. Bailar Inorg. Chem. 1973 12 1465. 'I5 M. Capka P. Svoboda and J. Hetflejs Cofl.Czech. Chem. Comm. 1973 38 1242. '' S. Ikeda and T. Harimoto J. Organometaffic Chem. 1973 60 C67. 3'7 S. A. Math and P. G. Sammes J.C.S. Chem. Comm. 1973 174. 318 L. Hocks. A. J. Hubert and P. Teyssie Tetrahedron Letters 1973 2719. OrganometallicCompoundsof the TransitionElements 267 used as a substrate for metathesis with oct-4-ene319 and d0dec-6-ene.~~' The same polymer has been disproportionated using both WCl ,EtAIC12 and (C4H,),Mo,EtA1C12 catalysts to give lower molecular weight polymer together with cyclo-octadiene and cycl~dodecatriene.~~' The structure determination of a complex obtained from the reaction of a cyclic diyne with Fe,(CO), indicates that the ligand is a product of the meta- thesis of the di~ne.~~~ 319 K.Hummel and W. Ast Die Makro. Chem. 1973 166 39. 320 K. Hummel D. Wewerka F. Lorber and G. Zeplichal Die Makro. Chem. 1973 166 45. 321 E. N. Kropacheva D. E. Sterenzat Y. A. Patrushin and B. A. Dolgoplosk Proc. Acad. Sci. (U.S.S.R.),1973 206 776. 322 H. B. Chin and R. Bau J. Amer. Chem. SOC. 1973 95 5068.

ISSN:0069-3030    

DOI:10.1039/OC9737000243

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

8 Organophosphorus Chemistry By S. TRIPPETT Department of Chemistry The University Leicester LEI 7RH The significant advances in organophosphorus chemistry are reviewed annually in the Specialist Periodical Reports series.’ This article seeks to answer the question ‘What has been happening in the past three years?’. The answer must be subjective and is doubtless biased by the interests of the Reporter. However most organophosphorus chemists would agree that the major publishing event has been the start to publication of the new ‘Kosolapoff ’2 which promises to be as indispensable as its predecessor. Chemically the most significant advances have been in the general area of quinquecovalent phosphoranes. The importance of these as reactive intermediates3 has made an understanding of their properties both chemical and fluxional essential to further developments in mechanism and stereochemistry.1 Structural Aspects Numerous ab initio and semi-empirical MO calculations have been rep~rted.~ In general they agree that the inclusion of d-orbitals in the basis set gives a better bonding picture but the improvement in some cases is small. The calculations on simple trigonal-bipyramidal phosphoranes confirm that the more electro- negative substituents will prefer to occupy apical positions and that atoms having a lone-pair of electrons will prefer to be bonded at the equatorial position. They further predict4b that groups having a low-lying vacant orbital will be more stable in the apical position and that an equatorial substituent with a single donor orbital will prefer to have that orbital in the equatorial plane.This preferred orientation can give rise to a considerable barrier to rotation round equatorial bonds which is well established experimentally in the case of P-N,5 ‘Organophosphorus Chemistry’ ed. S. Trippett (Specialist Periodical Reports) The Chemical Society London 197&-1974 Vols. 1-5. ‘Organic Phosphorus Compounds’ ed. G. M. Kosolapoff and L. Maier Wiley- Interscience New York 1973 Vols. 1-4. ’ P. Gillespie F. Ramirez I. Ugi and D. Marquarding Angew. Chem. Internat. Edn. 1973 12 91. E.g. (a) A. Rauk L. C. Allen and K. Mislow J. Amer. Chem. Soc. 1972 94 3035; J. B. Florey and L. C. Cusachs ibid. 3040; A. Strich and A. Veillard ibid.1973 95 5574; (6) R. Hoffman J. M. Howells and E. L. Muetterties ibid. 1972 94 3047; (c) P. Gillespie P. Hoffman H. Klusacek D. Marquarding S. Pfohl F. Ramirez E. A. Tsolis and I. Ugi Angew. Chem. Innternat. Edn. 1971 10 687. E. L. Muetterties P. Meakin and R. Hoffman J. Amer. Chem. Soc. 1973 95 974 and earlier papers cited therein. 268 Organophosphorus Chemistry and P-S,6 but not P-0,' bonds. Ab initio calculations' on methylenephos- phorane H,P=CH, show no barrier to rotation round the C-P bond and this distinction between four- and five-co-ordinate phosphorus could be of considerable significance. The generally assumed trigonal-bipyramidal geometry has been confirmed in X-ray analysis of numerous phosphoranes among them the 1,2-oxaphosphetan (1),9 the dioxyphosphorane (2)," and the spirophosphorane (3).' However the tetrathiospirophosphorane (4)12has a geometry intermediate between trigonal bipyramidal and square pyramidal while the spirophosphorane (5)13 is essentially square pyramidal with CPO bond angles of 154 and 148".Ph Ph' 1 Ph Ph. ..;) ;Yo C,H,Br-p (3) 2 Five- and Six-Co-ordinate Species Pseudorotation Processes-The exact route by which one trigonal-bipyramidal phosphorane (7) is transformed into its isomers is still contr~versial.~,~~ The two favoured mechanisms both of which satisfy the requirements of the famous Whitesides and Mitchell experiment,14 are Berry pseudorotation (BPR) which involves a square-pyramidal intermediate or transition state (8) and turnstile rotation (TR) which proceeds uia a 30" (2 + 3) species (6) shown in Newman S.C. Peake and R. Schmutzler J. Chem. SOC. (A) 1970 1049. ' S. C. Peake M. Fild M. J. C. Hewson and R. Schmutzler Znorg. Chem. 1971,10,2723; D. U. Robert D. J. Costa and J. G. Riess J.C.S. Chem. Comm. 1973 745. I. Absar and J. R. Van Wazer J. Amer. Chem. SOC.,1972 94 2382. Mazhar-ul-Haque. C. N. Caughlan F. Ramirez J. F. Pilot and C. P. Smith J. Amer. Chem. SOC.,1971,93 5229. lo D. D. Swank C. N. Caughlan F. Ramirez and J. F. Pilot J. Amer. Chem. SOC.,1971 93. 5236. I' M. Sanchez J. Ferekh J. F. Brazier A. Munoz and R. Wolf Roczniki Chem. 1971 131. '' M. Eisenhut R. Schmutzler and W. S. Sheldrick J.C.S. Chem. Comm. 1973 144. l3 J.A. Howard D. R. Russell and S. Trippett J.C.S. Chem. Comm. 1973 856. l4 G. M. Whitesides and H. L. Mitchell J. Amer. Chem. SOC.,1969 91 5384. 270 S. Trippett projection. Numerous calculations (e.g.refs. 4a b) suggest that in simple sym- metrical phosphoranes BPR is the easier route but of course this may not be so in less symmetrical and particularly in highly strained systems. The geometries and energies of the two species (6) and (8) are so similar that for the experimental chemist distinction between BPR and TR processes is probably academic. 3 (7) The pseudorotations of many stable phosphoranes have been studied using dynamic n.m.r. techniques and some have been interpreted in terms of the varying apicophilicities of the groups moving between apical and equatorial positions and the variation in strain as small-membered rings move from apical- equatorial to diequatorial positions.The dangers inherent in such experiments have been emphasized by the ob~ervation'~ that the fluorine exchange in Ph,PF is intramolecular when monitored by n.m.r. in Teflon tubes but intermolecular and of lower energy in Pyrex. The intermolecular processes observed here and in other cases16 could be due to impurities caused by chemical reaction with the glass and it may be that all previous data on fluorophosphoranes are suspect for the same reason. Only a few of the d.n.m.r. studies on phosphoranes can be mentioned. The 31Pand 'H spectra of the difluorophosphorane (9) at -100 "C show clearly17 that the phosphorane is a 2.3 :1 mixture of (9a) and (Sc) equilibration via the high-energy (9b) being slow on the n.m.r.time-scale at this temperature. The apicophilicity of fluorine is balancing the increased strain involved in placing the four-membered ring diequatorial. The 19F d.n.m.r. spectra of the spiro- phosphoranes (10) give data on the relative apicophilicities of the groups R.18 The pseudorotation that can be followed is (10)S(11) and AG*varies as the apicophilicity of R. The results give an order Ph < CH=CMe < Pr' < Me < Me2N < PhO < H. A large difference in apicophilicity between R2N and PhO is supported" by studies on the adducts (12) and data on the hexafluorobiacetyl adducts (13) give information on the apicophilicities of R relative to phenyl.,' Is C.G. Moreland G. 0.Doak and L. B. Littlefield J. Amer. Chem. SOC.,1973 95 255. l6 T. A. Furtsch D. S. Dierdorf and A. H. Cowley J. Amer. Chem. Soc. 1970,92 5759. " N. J. De'ath D. Z. Denney and D. B. Denney J.C.S. Chem. Comm. 1972 272. '' R. K. Oram and S. Trippett J.C.S. Perkin I 1973 1300. l9 S. Trippett and P. J. Whittle J.C.S. Perkin I 1973 2302. J. I. Dickstein and S. Trippett Tetrahedron Letters 1973 2203. Organophosphorus Chemistry 27 1 F (94 R OPh PhO.. I The spirophosphoranes derived from the various ephedrines provide particu- larly intriguing exercises in following pseudorotation processes.* Of those formed from (-)-ephedrine one (14) is obtained pure by crystallization and equilibration with its isomer (15) can be followed polarimetrically in solution.The resulting activation parameters agree with those previously obtained from n.m.r. studies. Isomers (14) and (15) can equilibrate either by routes involving five successive pseudorotations on which the highest energy tbps are of type (16) or by routes involving seven pseudorotations on which the highest energy tbps are of type (17). One would expect the latter to be the preferred pathways. With the accumulation of data on the energetics of pseudorotation processes it should soon be possible to predict the relative stabilities of isomeric quinque- covalent phosphoranes and to apply this knowledge to prediction of the pathways of reations at phosphorus involving such intermediates. The first kinetic evidence for the involvement of a quinquecovalent intermediate in the alkaline hydrolysis of a phosphate ester has been reported.22 Hydrolysis of methyl di-isopropyl- phosphinate shows an induction period and the overall kinetics are consistent with Scheme 1 where k N k N O.lk-A similar long-lived intermediate has been invoked23 to account for the racemization observed in the alkaline hydrolysis *' A.Klaebe J. F. Brazier F. Mathis and R. Wolf Tetrahedron Letters 1972 4367. '' R. D. Cook,P. C. Turley C. E. Diebert A. H. Fierman and P. Haake J. Amer. Chem. SOC.,1972 94 9260. '' L. P. Rieff L. J. Szafraniec and H. S. Aaron Chem. Comm. 1971 366. 272 S. Trippett Ph MGo 0 \ Ph Ph of the ester (18) and the loss of stereospecificity observed during demethylation of the methoxyphosphonium salt (20) has been ascribed24 to racemization (by repeated pseudorotation) of the phosphoranes (19) formed in parasitic equilibria.The Arbuzov reaction” is among others in which the reversible formation of intermediate quinquecovalent phosphoranes has been postulated. Chemistry of Phosphoranes.-Fragmentation of the cis-3-phospholen (21) is 99 % stereospecific26 and probably a concerted disrotatory process. Hoffman showed4* that the concerted reactions (22) e(23) are symmetry allowed for Scheme 1 l4 K. E. DeBruin and S. Chandrasekaran J. Amer. Chem. SOC..1973,95 974. 25 C. L. Bodkin and P. Simpson J.C.S. Perkin II 1972 2049. *‘ C. D. Hail J. D. Brarnblett and F. F. S. Lin J. Amer.Chem. SOC.,1972 94 9264. 273 0rganophosphorus Chemistry Ph OMenthyl Ph OMenthyl Ph OMenthyl \I \+ / \/ P-OMee P +N-+ +MeN /\ Me/I N Me OMe Me (19) (20) apical-apical or equatorial-equatorial loss or addition. Similar reasoning pre- dicts that the allowed process for loss of diene from (21) is from an apical- equatorial position. The 1,3-dipoles (25) formed on thermolysis or photolysis of the 1,3,5-oxaza- phospholens (24) have been trapped27 with a wide variety of dipolarophiles including olefins acetylenes nitriles isocyanides and carbonyl compounds. qR1 (CF3),C=NCOR’ +(R20),P -(CF& (,O *(CF3),C-N=6R‘ +(R20),po P (OR2)3 (25) (24) The possibility that six-co-ordinate species might be intermediates in reactions at phosphorus has long been recognized.Firm kinetic evidence has now been obtained for such an intermediate or transition state in the hydrolysis of penta- phenoxyphosphorane,28 and the base-catalysed exchange of alkoxy-groups in the oxaphosphetan (26) probably proceeds via attack of nucleophile in the ’’K. Burger and K. Einhellig Chem. Ber. 1973 106 3421 and earlier papers. 28 W. C. Archie jun. and F. Westheimer J. Amer. Chem. SOC.,1973 95 5955. 274 S. Trippett equatorial plane.29 A growing number of stable six-co-ordinate phosphorus- containing anions e.g.(27) have been pre~ared.~'.~ ' Although these are doubt- less stabilized by the presence of small rings their very existence suggests that similar anions may be more important as intermediates than has hitherto been appreciated.The triethylammonium salt of (27) on heating' gave the phos- phorane (28) and hydrogen! 1 Among other substitution reactions of five-co-ordinate phosphorus reported are those shown in Scheme 2.32 Nothing is known so far about the stereo- chemistry of such reactions. The equilibria between the tetrasubstituted phosphoranes (29) and the cor- responding P'll species (30) have been studied for X = Y = 0,33for X = 0 Y = N,34for X = Y = N,34 and for X = 0,Y = S.35 In general substitution of the rings favours the Pv form so that for example there is no evidence for the Pi''form in solutions of (31). However when Y = S the compounds exist entirely as (30; Y = S). 29 F. Ramirez G. V. Loewengart E. A. Tsolis and K.Tasaka J. Arner. Chern. SOC. 1972 94 353 1. 30 E.g. B. C. Chang D. B. Denney R. L. Powell and D. W. White Chern. Cornrn. 1971 1070; L. Lopez M. T. Boisdon and J. Barrans Cornpt. rend. 1972 275 C 295; R. Burgada D. Bernard and C. Laurenco ibid. 1973 276 C 297. 31 M. Wieber and K. Foroughi Angew. Chern. Internat. Edn. 1973 12 419. 32 D. Bernard and R. Burgada Tetrahedron Letters 1973 3455. 33 D. Bernard C. Laurenco and R. Burgada J. Organornetallic Chern. 1973,47 113. 34 C. Laurenco and R. Burgada Cornpt. rend. 1972 275 C 237. '' D. Bernard P. Savignac and R. Burgada Bull. SOC.chirn. France 1972 1657. Organophosphorus Chemistry NMe2 OCOPh 00 OMe Scheme 2 H Me Phosphoranes containing at least two alkoxy-groups undergo exchange re- actions with 1,2- and 1,3-glycols e.g.(32)-+ (33).36 The synthesis of phosphoranes using dialkyl peroxides has been extended to include the use of the dioxetan (34)37 and the dithiet (35).38 cy/OEt P(OEt) + HOCH2CH20H -+ 0-P (32) 01 + Ph3P benzene Ph3P 0 (34) '' B.C. Chang W. E. Conrad D. B. Denney D. Z. Denney R. Edelmann R. L. Powell and D. W. White J. Amer. Chem. SOC.,1971 93 4004. 37 P. D. Bartlett A. L. Baumstark and M. E. Landis J. Amer. Chem. Soc. 1973,95,6486. 38 N. J. De'ath and D. B. Denney J.C.S. Chem. Comm. 1972 395. 276 S. Trippett + -@h (35) 3 Stereochemistryand Mechanism The barrier to inversion in phosphines normally increases with the electro-negativity of the ligand~.~'The lower barrier in PhPr'PSi(OMe) than in PhPr'PSiMe is ascribed to negative hyperconjugation in the former.40 Data on the alkaline hydrolysis of phosphonium salts which it is not at present possible to rationalize continue to accumulate.Hydrolysis of the salts (36) with loss of R proceeds with partial inversion or retention at phosphorus de-pending on the nature of R and on whether the reaction is carried out under homogeneous or heterogeneous condition^.^^ Hydrolysis of the alkoxy(alky1thio) salts (37) with formation of thiol involves retention at phosph~rus.~~ Among hydrolyses involving interesting rearrangements are those of (38)43 and (39).44 The stereochemistry of the products from the hydrolysis of the phosphetanium salts (40; X =RO Me,N MeS or C1) has been accounted for in terms of the varying apicophilicity of X.45 Me .+ Ph -,P -R Bu' (36) R = PhCH ,p-F,CC,H,CH Ph,CH or CH,=CHCH Ph \+/ OMenthyloH-Ph\p/OMenthyl OMenthyl OH \A/P + Ph\D/oMenthyl /\\ Me SMe Me / \\o PhaPh 5 Phmph P P /\ Ph CH,I P Ph u A (38) 39 R.D. Baechler and K. Mislow J. Amer. Chem. Soc. 1971 93 773. 40 R. D. Baechler and K. Mislow J.C.S. Chem. Comm. 1972 185. 41 R. Luckenbach Phosphorus 1972 1 223 229,293. 42 N. J. De'ath K. Ellis D. J. H. Smith and S. Trippett Chem. Comm. 1971 714. 43 A. N. Hughes and C. Srivanavit Canad. J. Chem. 1971,49 879. 44 F. Mathey Tetrahedron 1972 28 4171; 1973 29 707. 45 K. E. DeBruin A. G. Padilla and M.-T. Campbell J. Amer. Chem. Soc.1973,95,4681. Organophosphorus Chemistry 277 Whereas methanolysis of (41) with displacement of thiolate ion involves inversion at the reaction of (42) with phenylmagnesium bromide also with loss of thiolate ion gives retenti~n.~’ The latter observation which is not at present understood required the revision of several previously accepted configurational assignments. MexMe ,SMe / SMe O=P. O=P-Me i-* OM en thy1 \ OPr’ Me P Me Ph RO# kx SbCI (40) (41) (42) The reactivities of cyclic tervalent phosphorus compounds relative to their acyclic analogues have been discussed48 in terms of the changes in ring strain between ground state and transition state. An S,1 (P)mechanism has been sug- gested for the aqueous solvolysis of di-t-butylphosphinic ~hloride.~’ The stereochemistry of nucleophilic displacements on phosphorus is being intensively studied using cyclic compounds such as (43)” and (44)” in which the configuration at phosphorus can be deduced from n.m.r.measurements. The outcome depends critically upon the nucleophile and is affected markedly by the presence of added salts. In an area where strain and apicophilicity factors are finely balanced it is difficult to rationalize the results satisfactorily. Studies continue on intramolecular catalysis in the solvolysis of phosphate esters. General acid catalysis has been identified in the hydrolysis of the dianions (45)52 and (46),53 and among examples of intramolecular nucleophilic catalysis are the solvolyses of the phosphonylated hydroxamic acids (47)where the rates CI CH,CI I 04pyz-?A C1 Me0 OMeOMe (43) “ W.B. Farnham K. Mislow N. Mandell and J. Donahue J.C.S. Chem. Comm. 1972 ’’ J. 120. Donahue N. Mandell W. B. Farnham R. K. Murray K. Mislow and H. P. Benschop J. Arner. Chem. Soc. 1971 93 3792; G. R. Van den Berg D. J. H. M.Platenburg and H. P. Benschop Rec. Trao. chim. 1972 91 929. R. Greenhalgh and R. F. Hudson Phosphorus 1972 2 1. 49 P. Haake and P. S. Ossip J. Amer. Chem. SOC.,1971 93 6919. 50 W. S. Wadsworth jun. J. Org. Chem. 1973,38 2921. 51 T. D. Inch and G. J. Lewis Tetrahedron Letters 1973 2187. ’’ Y. Murakami J. Sunamoto and H. Ishizo Bull. Chem. SOC.Japan 1972 45 590. 53 R. H. Bromilow and A. J. Kirby J.C.S. Perkin If 1972 123. 278 S.Trippett Pop EtO I R’ \o I HON=CAr (45) 0 (47) (46) are relatively insensitive to steric hindran~e.’~ Metaphosphates’ and meta- pho~phorimidates’~ continue to be postulated as reactive intermediates in order to account for kinetic and product data. Micelle formation can hinder or assist the hydrolysis of phosphate esters. The elimination of p-nitrophenate anion from (48) in base is strongly catalysed by micelles of long-chain quaternized ethanolamines ;57 in contrast the alkaline hydrolysis of bis-(2,4-dinitrophenyl) phosphate is inhibited by micelles of an uncharged detergent possibly by adsorption of the ~ubstrate.’~ Gels of yttrium hydroxide catalyse hydrolysis of (49) with the hydroxide perhaps acting as both general acid and nu~leophile.’~ 0 No II MeP-0 -C H,NO 2-p (Ph0)2P\ I OC6H4NOZ-p 0-(48) (49) MO calculations suggest that the lowest energy pathway for the Wittig olefin synthesis is via a 1,2-oxaphosphetan which undergoes P-C bond cleavage considerably in advance of P-0 cleavage.60 Kinetic data have been variously interpreted in terms of betaine formation6’ and of direct 1,2-oxaphosphetan formation via a four-centred transition state of low polarity.62 The intermediates in Wittig reactions have been detected at low temperature^^^ by Fourier transform ‘P n.m.r.;from their chemical shifts they are undoubtedly 1,2-oxaphosphetans and it is suggested that they are formed directly in (z2a + n2s) reactions involving orthogonal approach of ylide and carbonyl compound.The isolation of the aminotetraoxyphosphorane (52) from the reaction of the nitro-compound (50) with trimethyl phosphite provides evidence for the 54 J. I. G. Cadogan and D. T. Eastlick J.C.S. Chem. Comm. 1973 238. 55 E.g. D. G. Gorenstein J. Amer. Chem. SOC.,1972 94 2523. 56 E.g. M. A. Fahmy A. Khasaninah and T. R. Fukuto J. Org. Chem. 1972 37 617. 57 C. A. Bunton and L. G. Ionescu J. Amer. Chem. SOC.,1973,95 2912. C. A. Bunton A. Kamego and L. Sepulveda J. Org. Chem. 1971,36 2566. 59 F. McBlewett and P. Watts J. Chem. SOC.(B) 1971 881. 6o C. Trindle J.-T. Hwang and F. A. Carey J. Org. Chem. 1973 38 2664. 61 I. F. Wilson and J. C. Tebby J.C.S. Perkin I 1972 271 3. 62 G. Aksnes and F. Y. Khali Phosphorus 1972 2 105; P. Frcayen Acta Chem.Scand. 1972 26 2163. 63 E. Vedejs and K. A. J. Snoble J. Amer. Chem. SOC.,1973 95 5778. Organophosphorus Chemistry formation of spirodienyl intermediates eg. (51) in deoxygenations of nitro-compounds involving rearrangement^.^^ Phosphinidenesand Related Species.-Phosphinidenes :,have been postulated as intermediates in the thermal decomposition of cyclopolyphosphines65 and of the anhydride (53).66 They have been trapped with dienes diphenylacetylene biphenylene ally1 ethyl sulphide and benzil. Phenylphosphinidene sulphide PhPS formed from phenylphosphonothioic dichloride and magne~ium,~~ and the disulphides RPS, formed from the anhydrides (54),68 have been trapped with similar reagents. The oxide PhPO has been generated by pyrolysis of the phosphine oxide (55).69 Ph (Phi),O 5 PhP(OH) + PhP:b%' II 0 0 64 J.I. G. Cadogan D. S. B. Grace P. K. K. Lim and B. S. Tait J.C.S. Chem. Comm. 1972 520. 65 A. Ecker and U. Schmidt Chem. Ber. 1973 106 1453. 66 M. J. Gallagher and I. D. Jenkins J. Chem. SOC.(0,1971 593. 67 S. Nakayama M. Yoshifuji R. Okazaki and N. Inarnoto Chem. Comm. 1971 1186. 68 H. Ecker I. Boie and U. Schmidt Angew. Chem. Infernat. Edn. 1970 10 191. 69 J. K. Stille J. L. Eichelberger J. Higgins and M. E. Freeburger J. Amer. Chem. SOC. 1972 94 476 1. 280 S. Trippett Reactive Centres a and to Phosphorus.-Carbene or carbenoid centres adjacent to phosphoryl groups have been generated from the corresponding diazo- compounds by photolysis7' or by thermolysis in the presence of ~opper.~ They add to olefins to give cyclopropanes and undergo the expected insertions and rearrangements as in Scheme 3.Irradiation of the aide (56) in methanol gave the esters (57) and (58) consistent with the formation of a nitrene inter- mediate.72 I OMe Scheme 3 Me Me Me Me2 P n r '\ Me (56) (57) (59) Carefully designed experiments on the generation of carbonium centres p to a diphenylphosphinyl group show that this group and methyl migrate com- petitively with the Ph,PO migration slightly pred~minating.~~ 'O H. Scherer A. Hartmanu M. Regitz B. D. Tunggal and H. Gunther Chern. Ber. 1972 105 3357. D. Seyferth and R. S. Marmer J. Org. Chern. 1971 36 128. 72 M. J. P. Harger Chern. Comm. 1971 442." D. Howells and S. Warren J.C.S. Perkin II 1973 1472. Organophosphorus Chemistry 28 1 ?-Irradiation of phosphonium salts gives radicals by loss of a hydrogen atom from an a-~arbon.~~ These from their e.s.r. spectra show little interaction be- tween the radical centre and phosphorus. In contrast when the radical centre is fl to phosphorus there is strong interaction which is believed to be hypercon- jugative and at a maximum for the conformation shown in (59)." Phosphorus Radicals.-Phosphinyl radicals e.g. (60) have been shown to be configurationally stable.76 A large number of phosphoranyl radicals R,P* have been generated by addition of radicals to phosphines or by hydrogen ab- straction from tetraoxyphosphoranes. The pseudorotation processes of some have been followed by e.s.r.spectroscopy; the barrier between (61) and (62) is 16-21 kJ mol-' with an energy difference between the two of 2.9 kJ mol-'. It is not possible to harmonize the results on phosphoranyl radicals with those on phosphoranes and clearly there is a fundamental difference between the two. 0 0. 0 II 11 H,C=CH 11 hv P-Ph *ip\H Ph-1 0 ph-f\ Et EtO EtO EtO The a-and /3-scission reactions (see Scheme 4)of phosphoranyl radicals have been extensively studied.78 a-Scission is thought to occur preferentially from an apical position and p-scission preferentially from an equatorial. These preferences lead to an intimate interplay between the rates of pseudorotation processes and scissions. Thus the radicals (63) are comparatively stable since both a-and /3-fission require a highly unfavourable pseudorotation to (64).78 /3-Scission leading to the formation of a P=O bond would normally be expected to be thermodynamically more favourable than a-scission but kinetically this l4 A.R. Lyons G. W. Neilson and M. C. R. Symons J.C.S. Faruday II 1972 68 807. l5 A. R. Lyons and M. C. R. Symons J.C.S. Faraday II 1972 68 622; A. G. Davies D. Griller and B. P. Roberts J. Arner. Chern. Soc. 1972 94 1782; A. L. J. Beckwith Aitsrral. J. Chern. 1972 25 1887; B. C. Gilbert J. P. Larkin R. 0. C. Norman and P. M. Storey J.C.S. Perkin II 1972 1508. l6 G. R. Van den Berg D. H. J. M. Platenburg and H. D. Benschop Chem. Cornrn. 1971 606. l7 R. W. Dennis and B. P. Roberts J.Organornetallic Chern. 1973 47 C8. E.g. A. G. Davies D. Griller and B. P. Roberts J.C.S. Perkin II 1972 2224. 282 S. Trippett is not always so. It may be that the transition state for /3-scission has little P=O character." /p\ OR2 Scheme 4 4 OrganophosphorusCompounds in Synthesis The halogenophosphonium ions (65) formed from phosphines and carbon tetrahalides have found extensive application as sophisticated and more con- venient forms of phosphorus pentahalides e.g. in the conversion of alcohols into halides" and in a wide variety of dehydration reactions.8' The ylides formed from subsequent reaction of the anions (66) with more phosphine can be used in olefin synthesis82 and the anions can also be trapped with carbonyl compounds.Thus the anions from trichloroacetic esters lead to the glycidic esters (67).83 - R3P + CX R36X + CX,- R,P=CX (65) (66) - 0- R3P + Cl,CCO,Et CI,CCO,Et R1R2d-CC1.C02Et I 1 Cl0 /\R 1R2C-CCl. C0,Et (67) '9 W. G. Bentrude E. R. Hansen W. A. Khan T. B. Min and P. E. Rogers J. Amer. Chem. SOC.,1973,95 2286. 'O E.g. E. I. Snyder J. Org. Chem. 1972 37 1466. " R. Appel K. Warning and K.-D. Ziehn Chem. Ber. 1973 106 3450. a2 E.g. E. J. Corey and P. L. Fuchs Tetrahedron Letrers 1972 3769. 83 J. Villieras G. Lavielle and J.-C. Combret Bull. Soc. chim. France 1971 898. Organophosphorus Chemistry Condensations between alcohols and active-hydrogen compounds have been achievedg4 using the complex between triphenylphosphine -and diethyl azo- dicarboxylate (Scheme 5) when HX = phthalimide MeCOCH,CO,Et CH,(CN), (RO),PO,H and benzoic acid.85 The last is useful for the epi- merization of alcohols.86 Carboxylic acids have also been activated particularly in relation to peptide synthesis with trisdimethylarninoph~sphine-CCl~ ,8 triphenylphosphine-bis-(2-pyridyl) disulphide (Scheme 6),88 phosphites and pyridine in the presence of mercuric chloride (Scheme 7),8 diphenyl phosphoryl a~ide,~' and diethyl phosphoryl ~yanide.~ Ph,P + (Et0,CN l2 + Ph3hN-NC02Et Ph3 ;N-NHCO,Et I 1 C0,Et C0,Et X-J.0.Ph,PO + RX Ph,GOR + (EtOJNH) Scheme 5 Ph,P + RSSR -+ Ph,;SR SR lXOH Ph,PO + XY 3 Ph,;OX + RSH X = RCO or ROP0,H; Y = RO ROPO,H or RNH Scheme 6 Scheme 7 Thiirans (68 ;X = S) are obtained from oxirans and phosphine sulphides in the presence of trifhoroacetic acid.92 The same reaction using triphenylphosphine selenide gives the corresponding olefins stereo~pecfically.~~ A synthesis of allylic alcohols involves cleavage of the sulphenate esters (70) formed by a [2,3]-sigmatropic shift from the sulphoxides (69).'" 84 M.Wada and 0. Mitsunobu Tetrahedron Letters 1972 1279. 85 G. Alfredsson and P. J. Garegg Arta Chem. Scand. 1973 27 724. 8h A. K. Bose B. Lal W. A. Hoffman and M. S. Manhas Tetrahedron Letters 1973 1619. '' S. Yamada and Y. Takeuchi Tetrahedron Letters 1971 3595; T. Wreiand and A. Seeliger Chem. Ber. 1971 104 3992. 88 T. Mukaiyarna and M. Hashimoto J. Amer. Chem. Soc. 1972 94 8528. *' N. Yamazaki and F.Higashi Bull. Chem. Sou. Japan 1973.46 1235 1239. 90 T. Shioiri K. Ninomiya and S. Yamada J. Amer. Chem. SOC.,1972 94 6203. 91 S. Yamada Y. Kasai and T. Shioiri Tetrahedron Letters 1973 1595. 92 T. H. Chan and J. R. Finkenbine J. Amer. Chem. Sor. 1972 94 2880. 93 D. L. J. Clive and C. V. Denyer J.C.S. Chem. Comm. 1973 253. 94 D. A. Evans G. C. Andrew T. T. Fujimoto and D. Wells Tetrahedron Letters 1973 1385 1389. 284 S. Trippett (70) The Wittig olefin synthesis has been extensively developed but mention can be made only of 'one-pot' syntheses in which alkyl halide phosphine and car- bony1 compound are allowed to react together in the presence of an epoxide as the source of base.95 The cyclic acylphosphate (71)is both highly selective and very reactive towards hydroxylic nu~leophiles.~~ The resulting acetoin esters are readily hydrolysed to give phosphate diesters.RO 0 ROH I MeCOpf -MeCOCHMe,OR % \p/ 0 P/o -co 0-P \ MeO' \OH OH 'OMe OMe (71) Finally reference must be made to Khorana's heroic synthesis of the structural gene of tRNAA'" from yeast a double-stranded DNA by a combination of chemical and enzymic methods." '' E.g. G. P. 2 132 032 (Chem.Abs. 1972 76 99 874). '' F. Ramirez S. Glaser P. Stern P. Gillespie and 1. Ugi Angew. Chem. Internat. Edn. 1973 12 66. '' H. G. Khorana K. L. Agarwal H. Biichi M. H. Caruthers N. K. Gupta K. Kleppe A. Kumar E. Ohtsuka U. L. RajBhandary J. H. van de Sande V. Sgaramella T. Terao H. Weber and T. Yamada J.Mol. Biol. 1972 72 209.

ISSN:0069-3030    

DOI:10.1039/OC9737000268

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

9 Electro-organic Chemistry By K. KORINEK and T. F. W. McKlLLOP lCl Ltd. Corporate Laboratory P.O. Box 11 The Heath Runcorn Cheshire 1 Introduction Although the literature in organic electrochemistry continues to expand at an ever-increasing rate much of the published work constitutes rather trivial ex- emplification. The emphasis throughout this review has been placed on those papers which indicate the utility of electrochemical methods in organic synthesis or in the study of organic systems. As in previous years a more comprehensive treatment of the subject is to be found in the Specialist Report on Electro- chemistry.’ The application of electrochemical methods to organometallic chemistry is a growing subject which has considerable promise as indicated in a brief review.2 The substantial number of papers involving organometallic electrochemistry have not been included in this or in previous reviews.Progress continues to be made in developing techniques for the characteriza- tion of intermediates and this is leading to a better understanding of their nature and role in electrode reactions. Advances in this subject formed the basis of a Faraday Discu~sion.~ Several booksL8 and a number of reviewsg-’ have also appeared dealing with a wide range of topics in organic electrochemistry. In addition several established review series have devoted chapters to organic electrode reaction^'^.'^ and to the electrochemistry of biologically interesting compounds.’ 1 ‘Electrochemistry’ ed. G. J. Hills (Specialist Periodical Reports) The Chemical Society London Vol.1 1970 Vol. 2 1972 Vol. 3 1973; ed. H. R. Thirsk Vol. 4 1974. 2 H. Lehmkuhl Synrhesis 1973 377. 3 Discussions of The Faraday Society Vol. 56. 4 ‘Techniques in Electrochemistry’ ed. E. Yeager and A. J. Salkind Wiley New York 1972. 5 ‘Organic Electrochemistry’ ed. M. M. Baizer Dekker New York 1973. 6 ‘Techniques of Electro-Organic Synthesis’ ed. A. Weissberger and N. L. Weinberg Wiley New York 1973. 7 A. P. Tomilov S. G. Mairanovskii M. Y. Fioshin and V. A. Sirnov ‘The Electro- chemistry of Organic Compounds’ Halsted Press New York 1972. 8 ”on-aqueous Electrolytes Handbook’ ed. G. J. Janz and R. P. T. Tomkins Academic Press New York Vol. 1 1972. 9 M. S. Bourbien and J.-J. Rameau Bull.SOC. chim. France 1973 1268. 10 G. Popp Eastman Organic Chemical Bulletin 1973 45 I. 11 L. Eberson and K. Nyberg Accounts Chem. Res. 1973 6 106. 12 L. A. Mirkind and Y. M. Tyurin Elektrosintez. Mekh. Org. Reakts. 1973 181. 13 A. A. Humffray in ‘Modern Aspects of Electrochemistry’ ed. J. O’M. Bockris and B. E. Conway Butterworths London 1972 Vol. 8. 14 M. Fleischmann and D. Pletcher Ado. Phys. Org. Chem. 1973 10 55. 15 ‘Electroanalytical Chemistry’ ed. A. J. Bard Dekker New York Vol. 6 1973. 285 K. Korinek and T. F. W. McKillop 2 Reduction Hydrocarbons.-Aromatic. In recent years the study of aromaticity and related phenomena has provided some intriguing problems for the physical-organic chemist. It is encouraging to see electrochemistry playing a useful part in such studies.For instance Breslow and Chu16 have dzscribed a novel thermo- dynamic method of determining the pK,'s of weak acids such as substituted cyclopropenes. This requires a measurement of the pK + for the corresponding cations and the reduction potentials for the conversion of cation through radical into anion. Although the method can only give approximate values the results for cyclopropenes lend support to the concept of the anions being destabilized or antiaromatic. Using the same approach but reversing the thermodynamic cycle the pK,+ for antiaromatic cations such as cyclopentadienyl can also be obtained.' ' The general utility of the method is shown by the use of a similar series of measurements to determine the acidity of a bicyclic trialkylhydrazine." Electron delocalization can be observed in some unsaturated systems in which one or more methylene groups interrupts conjugation the phenomenon being described as homoaromaticity.Anderson Broadhurst and Paquette' have studied the use of cyclic voltammetry and polarography in non-aqueous solvents in an attempt to quantify such-effects. Although this approach did not provide a polarographic criterion for homoaromaticity the authors indicate that the techniques may be useful in studying the ground-state conformations of poly- olefins. Activated Olefins. In recent years considerable effort has been expended in studying the mechanism of the industrially important reductive coupling of activated olefins [Ann.Reports (B) 1971 68 2991. Saveant and co-workers2' have now reported in full their studies on 13 compounds in solvents of low acidity (acetonitrile dimethylformamide and alkaline ethanol). The kinetics are derived from the variation in peak potential in linear-sweep voltammetry with sweep rate initial concentrations and the amount of proton donor in solution. Where protonation is unimportant the dimerization occurs purely by radical coupling of two anion-radicals and although some ambiguity still exists in the interpretation of the kinetics it would seem that radical coupling still predominates in media of higher acidity. These results are in complete agree- ment with those reported previously by Bard and co-workers. The French group have also published two valuable theoretical papers.The first paper2' treats the interference of solution electron transfer with electrode electron l6 R. Breslow and W. Chu J. Amer. Chem. SOC. 1973 95 41 1. R. Breslow and S. Mazur J. Amer. Chem. SOC. 1973 95 584. S. F. Nelson and R. T. Landis jun. J. Amer. Chem. SOC. 1973 95 5422. I' L. B. Anderson M. J. Broadhurst and L. A. Paquette J. Amer. Chem. SOC. 1973,95 2198. 2o E. Lamy L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 189. 21 C. P. Andrieux L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 223. Elect ro-organic Chemistry 287 transfer in the particular case of electrohydrodimerization and derives diagnostic criteria of mcchanism and procedures for determining rate constants from linear- sweep voltammetry rotating-disc voltammetry and classical polarography.In the second paper22 the authors consider the effect of proton-transfer reactions on the same systems. Although these papers are concerned specifically with electrohydrodimerization the approach employed is of general interest. If electrochemical reduction is effected on a mixture of two activated olefins cross-coupling reactions may occur and it is studies of these reactions which have provided some of the evidence against the radical-radical coupling mechan- ism. Puglisi and Bard23 have reported their investigations on the reduction of dimethyl fumarate in the presence of cinnamonitrile and acrylonitrile.Various voltammetric and coulometric techniques using rotating ring disc electrodes were used to study solutions in NN-dimethylformamide with tetrabutylammon- ium iodide as supporting electrolyte. The authors conclude that contrary to previous reports crossed coupling competes with direct dimerization only when the ratio of non-reduced olefin to reduced olefin is very large and that crossed coupling is more efficient when both olefins are reduced. Although evidence was found for oxidation-reduction reactions in solution under certain conditions it seems that radical-radical coupling is still the major mechanistic pathway. Carbony1s.-The presence of the electrode surface allows the possibility of introducing stereochemical control during electrochemical reactions.Although well recognised few attempts have been made to utilize this possibility. Stemming largely from an interest in the detailed mechanism involved in the reduction of carbonyls several papers have appeared substantiating and extending Horner’s previous studies in which asymmetric synthesis of the carbinol is obtained by the addition of an optically active compound to the reduction medium. Scheme 1 shows a simplified picture of the possible steps involved in the reduction of a carbonyl-containing compound to the carbinol(1) and pinacol(2). Horner and S~hneider*~ have reported their studies on the reduction of acetophenone in which they examined the ratio of carbinol to pinacol and the optical yield of carbinol as a function of temperature electrode material solvent pH potential and the structure and configuration of the optically active base electrolyte.Mercury and cadmium seem to be the preferred electrodes and as expected lower temperatures tend to give higher optical yields. The effects obtained with different solvent systems and the increase in optical yield on addition of HCl up to 0.2mol1-’ are less easy to explain. Kariv Terni and Gileadi2’ have also studied the reduction of acetophenone but in this case they employed pre-reduced quinidine as the optically active agent. Considerably enhanced asymmetric induction occurs using the more strongly adsorbed alkaloids 22 L. Nadjo and J. M. Saveant J. Electroanalyt. Chem. Interfacial Electrochem. 1973,44 327. ’’ V. J. Puglisi and A.J. Bard J. Electrochem. SOC. 1973 120 748. 24 L. Horner and R. Schneider Tetrahedron Letters 1973 3133. 25 E. Kariv H. A. Terni and E. Gileadi Electrochim. Acta 1973 18 433. K. Korinek and T F. W. McKillop OH I R' -C-R2 I H OH (1) I R' -G-R2 (4) Scheme 1 but essentially the same trends are observed. The authors interpret their results as evidence for an adsorbed complex between the alkaloid and acetophenone or one of its reactive intermediates (3) or (4).One can probably take the conclusions a step further however and this has been done recently by a French group who studied the reduction of phenylglyoxalic acid to mandelic acid in aqueous con- ditions on a mercury cathode.26 As these authors point out the final stereo- chemistry of the alcohol must depend essentially on the environment of the carbanion (4)during its life-time and therefore it is the influence of the alkaloid on the carbanion which is of critical importance.Such a mechanism would also lend itself better to an understanding of the effect of pH and addition of other solvents. Not surprisingly if adsorption is important this same group reports that the efficiency of the stirring has a profound effect on the optical yield higher optical yields being obtained as the stirring is improved. Andrieux and Saveant2' have continued their study of the reduction of M-substituted ketones and have reported the successful pinacolization of tri- fluoroacetophenone using acetonitrile as solvent and a mixture of lithium and tetraethylammonium perchlorates as supporting electrolytes.The yield obtained (35%) is double that achieved by photochemical methods. Traditionally the electrochemist has endeavoured to keep the system as simple as possible when studying an organic compound and this has undoubtedly restricted his view of the potential use to which electrochemical methods may be put. Perhaps indicative of a change in attitude is a paper by Lund and Simonet28 in which they study the reduction of some ketones and imines in the presence of a less easily reduced alkyl halide. Since reduction leads to radical anions and 26 M. Jubault E. Raoult and D. Peltier Compt. rend. 1973 277 C 583. 21 C. P. Andrieux and J. M. Saveant Bull. SOC. chim. France 1973 2090. 28 H.Lund and J. Simonet Bull. SOC.chim. France 1973 1843. Electro-organic Chemistry 289 possibly dianions it is hardly surprising that in the presence of good electro- philes substitution reactions ensue. In such a complex system there are clearly a number of competing reaction pathways and as the authors indicate their studies are at a preliminary stage yet it is quite conceivable that substitution reactions of synthetic use could be derived from such an approach. After ail the organic chemist usually has to contend with complex systems and competing react ions. Reductive Cleavage of Halides.-The first step in the reduction of haIogeno-aromatic compounds is the formation of the corresponding radical anion. The stability of this radical anion and the chemistry which results from its formation depend on a variety of factors such as the medium employed the electrochemical condition of reactions and the nature and position of the halide and any other substituent present.Indeed some of the papers dealing with this subject provide elegant examples of the sophistication and experimental subtlety needed to cope with such molecular perversity. For example Hawley and co- worker~~~ have examined the decomposition of the radical anions obtained from 2- 3- and 4-fluorobenzonitrile in NN-dimethylformamide. As shown in Scheme 2 each compound behaves differently 2-fluorobenzonitrile (5) giving rise to benzonitrile (6) via a dimerization and disproportionation sequence whereas 3-fluorobenzonitrile (7) undergoes elimination of the cyano-group followed by hydrogen abstraction to give fluorobenzene (8).Benzonitrile and 4,4'-dicyanobiphenyl(I 0) are the products of reduction of 4-fluorobenzonitrile (9). The biphenyl is believed to result from dimerization of the fluorobenzonitrile radical anion followed by rapid loss of the two fluoride ions while benzonitrile is formed by loss of fluoride ion and hydrogen abstraction. Alwair and Grimshaw have also been interested in the factors which control the carbon-halogen fragmentation of halogenoaromatics and have reported their findings in two papers dealing with 4-(chlorostyryl)pyridines30and deriva- tives of quinoline quinoxaline and phena~ine.~ The authors conclude that the stability of the carbon-halogen bond in a halogenated radical anion depends upon the strength of this bond and the redox potential of the substrate-radical anion couple.Further they have shown that within a given series the rate of cleavage can be correlated with the free electron density in the radical anion at the carbon terminus as calculated by Hiickel MO methods. Of more interest to the organic chemist is a paper by Nelson Carpenter and Leo32 dealing with the reduction of monohalogenated nitrobenzenes in non- aqueous media. As well as studying the mechanistic features these authors have attempted to examine the synthetic possibilities already demonstrated or potentially obtainable from the reduction of halogenonitrobenzenes (Scheme 3). 29 K. H. Houser D. E. Bartak and M. D. Hawley J.Amer. Chem. Soc. 1973,95 6033. 30 K. Alwair and J. Grimshaw J.C.S. Perkin It 1973 1150. I K. Alwair and J. Grimshaw J.C.S. Perkin It 1973 181 I. " R. F. Nelson A. K. Carpenter and E. T. Seo J. Electrochem. Soc. 1973 120 206. K. Korinek and T. F. W.McKillop .bcN][N=cwc=N] dimerize, e (5) disproportion I FacN -* (7) [.=CmC=N] Scheme 2 Electro-organic Chemistry 29 1 X = p-Br;o-Br AN-TEAP b -2.0 v CD3CN-TEAP b AN-TEAP I CN NO aprotic media I b AN = MeCN; TEAP = Et4N+C104-Scheme 3 The electroreduction of simple alkyl halides is of considerable importance in the production of lead alkyls and this has given rise to detailed mechanistic studies which are now forming the basis of synthetic methods for other organo-metallic compounds.In the context of organic chemistry however it is the reduction of dihalogenated compounds leading to unsaturated or cyclized products which holds most interest. The concertedness or otherwise of 1,3-elimination reactions has been one of the most controversial subjects in organic chemistry in recent years and this has been equally true of electrochemical 1,3-eliminations [Ann.Reports (B) 1971 68,2041. K Korinek and T. F. W.McKillop Fry and Britt~n~~ have now reported in full their results obtained from the reduction of meso-and dl-2,4-dibromopentane (1 1) in DMSO using a mercury cathode. That both isomers yield essentially the same amounts of the cis-and trans-dimethylcyclopropanes (1 2) and (13) argues strongly against a concerted mechanism.The authors have extended their investigation by preparing the (+)-(2S,4S) stereoisomer of (1 1). On reduction this yielded the (-)-(lR,2R)-form of (13) of high optical purity which the authors interpret as cyclization occurring from the carbanion via a semi-VV transition state with inversion. Azizullah and Grim~haw~~ have studied the reduction on mercury of endo-2 endo-6-dibromobornane (14) in which it is impossible to achieve the semi-W transition state. Aqueous ethanol and NN-dimethylformamide were the solvents used with either tetrapropylammonium perchlorate or tetraethylammonium bromide as supporting electrolytes and under these conditions the cyclized compound tricyclene (15) was usually the major product with bornane (16) as the only other hydrocarbon produced in significant quantity.The authors showed that bornane arises by stepwise reduction of each bromo- group to the carbanion which is then protonated and that tricyclene does not arise by any solvolytic process. Clearly tricyclene must arise either by front-face displacement of a bromine ion by the carbanion or via a more concerted process possibly involving a biradical ; the authors prefer the latter explanation. Until recently the reduction of am-dibromides has always been discussed in terms of non-adsorbed intermediates yet it is well known that chemisorption of radical intermediates occurs to a significant extent on mercury cathodes. Brown and Gonzalez3’ have re-examined the reduction in non-aqueous media of.1,3-dibromopropane and 174-dibromobutane on mercury and aluminium cathodes. Product analysis and kinetic measurements have shown that reduction on mercury proceeds through adsorbed radicals whereas on aluminium products are best explained in terms of free-radical reactions initiated by solvated electrons. 33 A. J. Fry and W. E. Britton J. Org. Chem. 1973 38 4016. 34 Azizullah and J. Grimshaw J.C.S. Perkin I 1973 425. 35 0. R. Brown and E. R. Gonzalez J. Electroanalyt. Chem. Interfacial Electrochem. 1973 43 215. Electro-organic Chemistry 293 Nitro- and Nitrosocompounds.-Although the electroreduction of nitro-substituents to amines is an extremely convenient and well-studied procedure organic chemists continue to avoid its use often preferring more complicated and less specific chemical methods.Perhaps by examining one or two cases from among the numerous recent examples organic chemists will be encouraged to try electrochemical methods. A very straightforward preparation of p-aminobenzyl cyanide from the corresponding nitro-compound has been des- ~ribed.~~ Using a mercury cathode and a solvent system of 1 1 4N-HCl in EtOH 95 % chemical yield can be obtained with almost 100 % current efficiency. Tryptophan can be readily produced by reduction of indolyl nitroacrylate in 89 % EtOH the best yields being achieved at pH 1-3 with a lead ~athode.~' Provided that a suitable ortho-substituent is present the reduction of aromatic nitro-groups can often be directed towards cyclized products as is the case with some a-(o-nitropheny1)ketones (17).38 These compounds usually show two discrete waves by polarography.Acidic or slightly basic solutions of these compounds on electrolysis at the first reduction potential using a mercury cathode give quantitative yields of the hydroxyindole (18) by a four-electron reduction as shown in Scheme 4. At higher potentials however a six-electron R' R' R' 1-H20 I H reduction occurs leading directly to the indoles (19). Although chemical reduc- tion methods have been employed it can be extremely difficult to stop reduction at the hydroxylamine stage if hydroxyindole is required. Carbon-Nitrogen and Nitrogen-Nitrogen Double Bonds-As in previous years an enormous amount of work has been carried out on the reduction of various heterocyclic systems especially those which are of biological interest.Unfor-tunately they deal solely with detailed examinations of the electrochemical 36 P. E. Iversen J. H. P. Utley and S. 0. Yeboah Org. Prep. Proced. Internat. 1973 5 129. '' 1. A. Avrutskaya K. K. Babievskii V. M. Belikov E. V. Zaporozhets and M. Ya. Fioshchin Elektrokhimiya 1973 9 1363. 38 R. Hazard and A. Tallec Bull. SOC.chim. France 1973 3040. 294 K. Korinek and T. F. W.McKillop mechanisms which are of little general interest to the organic chemist. One of the few exceptions is a short paper by Grimshaw and Tro~ha-Grimshaw~~ on the reduction of some styrylpyrazole derivatives. The polarogram of 5-phenyl-3-styrylpyrazole (20) in anhydrous NN-dimethylformamide shows two discrete waves and by controlled-potential reduction at the first wave in moist NN-dimethylformamide reasonable yields of 5-phenyl-3-( 2-phenylethy1)pyrazole (21) were obtained.The diphenyl-substituted pyrazole (22) however showed three waves in the polarogram and on reduction at a potential near the diffusion plateau of the first two waves afforded the tetrahydro-product (23). The authors point out that these electrochemical reductions are an alternative to catalytic hydrogenation and may be especially valuable in compounds containing some other functionality prone to catalytic reduction. CH,CH,Ph 3Ph J--i I I H +4e mCHzCH2Ph I I Ph Ph (23) In recent years one or two examples have been reported which indicated that electrochemical reduction of oximes yielded opposite stereochemistry to that obtained by dissolving metal reductions.A cautionary note is sounded by Allen- mark and Helgee4' who have studied the reduction of 1,2-(~-ketotetramethylene)-ferrocene oxime. They found that electrolysis gives better yields of amine but at the expense of selectivity. With both electrochemical and sodium in ethanol reductions however the endo-isomer predominates. Since the earlier reports offered considerable promise to the organic chemist it would seem well worth- while trying to determine the various factors involved in such reductions. Reduction of Bondsinvolving Sulphur.- The reductive cleavage of carbon-sulphur bonds continues to receive attention.For instance Japanese workers have reported the conversion of derivatives of S-methylmethionines into a-amino- butyric acid with reasonable yields.41 Thioethers and tertiary sulphonium 39 J. Grimshaw and J. Trocha-Grimshaw J.C.S. Perkin I 1973 1275. 40 S. Allenmark and B. Helgee Acta Chem. Scand. 1973 27 1816. 41 T. Iwasaki M. Miyoshi M. Matsuoka and K. Matsumoto Chem. and Ind. 1973 1163. Electro -organ ic Chemistry ion derivatives of /3-mercaptopyruvic acid provide an interesting series of com- pounds in which reduction can occur either at the carbonyl group or with cleavage of the a-s~bstituent.~~ In aqueous or aqueous-alcoholic media at a mercury electrode the sulphonium salts undergo carbon-sulphur cleavage whereas the direction of reduction with thioethers R’SCH2-CO-C0,R2 depends on the nature of the R’ group.When R’is aryl cleavage of the S-CH bond still occurs but when R’ is alkyl reduction of the carbonyl group takes place. A similar dependence on the nature of R has been found in the reduction of a-arylamino-esters which can proceed with high yields.43 With tosylates cleavage can be observed at the sulphur-oxygen bond rather than the carbon-sulphur bond. Gerdi144 has examined the reduction of various ditosylates in aprotic media such as NN-dimethylformamide with a particular interest in the intramolecular cyclization shown in Scheme 5. Not surprisingly Scheme 5 the reaction occurs most efficiently for n = 2 to give high yields of ethylene oxide.From all the data obtained the author concludes that it is the orientation of the molecule at the electrode which determines the efficiency of ring closure. Perhaps the most interesting paper to appear recently involving electro- chemistry and organic sulphur compounds is that involving a new synthesis of s~lphones.~’ Sulphur dioxide is reduced in aprotic media at a platinum electrode to give the blue radical anion which reacts with alkyl halides to give the sulphone as shown in Scheme 6. The yields can be as high as SO-SO% and provided that SO2 + e-+ S02-Sol-+ RX -+ RSOl + X-RSOl + S03-+ RS0,-+ SO2 RSO2-+ RX -+ R-SO,-R + X-Scheme 6 the electrochemical conditions can be kept simple this could be the preferred route to various sulphones.With certain dihalides e.g. (24) the sultine (25) is the product of electrosynthesis but this can easily be converted into sulphone (26) by treatment with silica gel in ethanol. 42 J. Moiroux and M. B. Fleury Electrochim. Acta 1973 18 691. 43 K. Matsumoto M. Suzuki T. Iwasaki and M. Miyoshi J. Org. Chem. 1973,37 2731. 44 R. Gerdil Helv. Chim. Acta 1973 56 1859. 45 D. Knittel and B. Kastening J. Appl. Electrochem. 1973 3 291. K. Korinek and T. F. W.McKillop o",'40 \ 0 Miscellaneous.-The possible involvement of hydrated electrons in many cathodic reduction processes has been the subject of considerable debate in recent years. An earlier claim to have measured the concentration of hydrated electrons near an electrode using an optical method has now been challenged and has resulted in a dial~gue.~~,~' Bewick and Avaca have studied the physical and mechanistic aspects of formation of electrons generated in HMPA.48 Another intermediate of possible synthetic interest which is electrochemically generable is the superoxide ion.Mayeda and Bard4' have used this intermediate in a novel way to produce singlet oxygen. Superoxide ion from oxygen and ferricenium ion from ferrocene are produced alternately in the same acetonitrile solution by pulsing between the reduction potential of oxygen and the oxidation potential of ferrocene. The reaction sequence shown in Scheme 7 then occurs in solution leading to singlet oxidation which was identified by chemical quench- ing. Fer % [Fer].+ [Fer].' + 01-* Fer + '02 Scheme 7 The separate electroreductions of ethylene and carbon dioxide have been well studied. Gambino and Silvestri'' have recently described the production of succinic acid and oxalic acid when reduction is carried out in the presence of both gases in aprotic media. By controlling the partial pressures they showed that electrosynthesis can be directed towards either of the two acids. This is an intriguing system both from the mechanistic point of view and from its implica- tions for commercial processes and it clearly merits further study. 3 Oxidatious Hydrocarbons.-A great deal of interest has recently been shown in the anodic oxidation of aromatic compounds in non-nucleophilic media. Many different solvents and a variety of experimental conditions were used to obtain information 4h A.Bewick B. E. Conway and A. M. Tuxford J. Electroanalyt. Chem. Interfacial Electrochem. 1973 42 App. 11-15. 47 D. C. Walker J. Electroanalyt. Chem. Interfacial Electrochem. 1973,42 App. 17-18. L. A. Avaca and A. Bewick J. Electroanalyt. Chem. Interfacial Electrochem. 1973,41 395. 49 E. A. Mayeda and A. J. Bard J. Amer. Chem. SOC. 1973,95,6223. 50 S. Gambino and G. Silvestri Tetrahedron Letters 1973 3025. Electro-organic Chemistry 297 on the inherent stability of cation radicals generated in these reactions. Greatly enhanced stability of radical cations has been observed at low temperatures in trifluoroacetic acid and in AlCl melts. Discussion about the mechanism of anodic oxidation of aromatic hydrocarbons has centred on two possible reaction mechanisms the eecc and the ecec mechanisms.It has also been shown that reactions of certain radical cations proceed to a dication (27) through a fast disproportionation equilibrium ArH-' ArH2+ + ArH (27) but the extent to which this reaction contributes to the overall process has been questioned. Parker and Hammerich have now shown5' that it is possible to observe irreversible behaviour for both the oxidation and reduction step of 4,4'-dimethoxybiphenyl thianthrene and 9,lO-di-p-anisylanthracenein aceto-nitrile nitromethane dichloroethane and other solvents containing a suspension of neutral alumina which is believed to remove the last traces of water. In solvents containing trifluoroacetic acid or the corresponding acid anhydride stable solutions of radical cations and dications were prepared.The dispropor- tionation constants calculated from the reversible electrode potentials are very dependent upon the solvent used and the cationic system in question. The mechanistic aspects of anodic substitution reactions in the presence of the nucleophile have been the subject of another paper by Parker and Jen~en.~~ Cyclic voltammograms of 9-phenylanthracene in the presence of pyridine have shown a pre-peak the height of which was in direct relation to the stoicheio- metric ratio of 9-phenylanthracene to pyridine. Parker had previously suggested that this observation could be consistent with a nucleophile-assisted electron transfer.Using a digital simulation technique it was found that an ecec mechan-ism gave substantial cathodic shifts of the peak potential for the oxidation of a substrate in the presence of a reactant and if the rate constants were sufficiently high not only a cathode shift but also a pre-peak was observed on simulated voltammograms. The experimental observations cannot therefore be used as evidence for an assisted mechanism. The details of the reaction mechanism are however only of limited importance in the practical applications of anodic aromatic substitution which has been reviewed by Eberson and Nyberg." The method offers a unique way of intro- ducing functional groups into cheap starting materials such as hydrocarbons. Another type of synthetically useful reaction the mixed oxidative coupling of aromatic compounds has been investigated by N~berg.~~ Oxidation of napththalene at a platinum anode in the presence of alkylbenzene in MeCN- MeC0,H (volume ratio 9 1)containing 0.1 M tetrabutylammonium tetrafluoro- borate formed two main products 1-arylnaphthalene and 1,l'-binaphthyl.The relative yield of arylnaphthalene follows the relative nucleophilicity of the 5' 0.Hammerich and V. D. Parker Electrochim. Acta 1973 18 537. 52 B. S. Jensen and V. D. Parker Electrochim. Acta 1973 18 665. 53 K. Nyberg Acta Chem. Scand. 1973 27 503. K. Korinek and T. F. W.McKillop alkylbenzene which suggests an electrophilic reaction between naphthalene cation radicals and alkylbenzenes.The preparative electrolysis of naphthalene in the presence of the more nucleophilic hydrocarbons isodurene and penta- methylbenzene produced the mixed coupled products 1-(2,3,4,6-tetramethyl- pheny1)naphthalene and l-(pentamethylpheny1)naphthalene in 42 and 56 % isolated yields respectively. The fact that the products of oxidative coupling are often more easily oxidized than the original substrates has been limiting the usefulness of oxidative cycliza- tions. Ronlan Hammerich and Parker have now reporteds4 a study of oxidative intramolecular cyclization of methoxybibenzyls and bis-(3-methoxyphenyl)- methane in acetonitrile and in acetonitrile containing trifluoroacetic acid. In acetonitrile the yields of dimerized products were low but in the presence of TFA the cation radicals of the coupled products were stable and often on subse- quent reduction produced substituted dihydrophenanthrene (28) or fluorene (29) in high yields.Me -OmO-M e MeOmOMe An interesting influence of halide ions (Cl- Br- I -) on the electroluminescence (ecl) of 9,lO-diphenylanthracene (DPA) has been reported by Kihata Sukihara and Honda.” They have used a controlled-potential double-step method in which the potential of the electrode is changed in a stepwise manner and have shown that in a solution containing DPA. -and no halide ions the ecl emission appears at a potential corresponding to the first anodic wave of DPA as shown in Scheme 8. In the presence of halide ions the ecl emission in acetonitrile DPAs-+ DPA.’ + 2DPA + hv Scheme 8 containing DPA.- appears at the potentials of the first anodic wave of the halide ion.It is suggested that (DPA-X-)* is an important intermediate in the ecl emission the mechanism being shown in Scheme 9. This mechanism is also DPAs-+ X,-+ DPA -X. + 2X-DPA -X. + DPAs--+ DPA -X-* + DPA DPA -X-* + DPA* + X-DPA* -+ DPA + hv Scheme 9 54 A. Ronlan 0. Hammerich and V. D. Parker J. Amer. Chem. Soc. 1973 95 7132. 55 T. Kihata M. Sukihara and K. Honda Electrochim. Acra 1973 18 639. Electro-organic Chemistry 299 supported by the low intensity of the ecl of the DPA-iodide system since the heavy atom will play a role in the energy-dissipation process of the excited species. The Southampton groups6 has reported studies on oxidation of fluoro-aromatic hydrocarbons in fluorosulphonic acid containing acetic acid (0.1 moll-I) which in this medium functions as a base.Most of the compounds studied have shown an initial one-electron oxidation to form a cation radical which is stable on the time-scale of cyclic voltammetry. Results of some prepara- tive studies have shown that fluorosulphonate esters may be prepared using this technique. One report on the oxidation of alkanes in similar media has already appeared showing that ap-unsaturated ketones are the major products. This work has now been extended” to show that in fluorosulphonic acid solutions which contain less of the added base the anodic oxidation of butane propane and ethane becomes possible. These experiments have clearly demonstrated that the proton-donating ability of the medium is determining the electrode and chemical behaviour of alkanes.A 50-60% yield of ctp-unsaturated ketone (30) was achieved in the anodic oxidation of cyclohexane in the presence of acetic propionic or hexanoic acid. 01.15 M RCOZH?YcoR High yields of acetamidated adamantyl derivatives were also achieved by the anodic oxidation of adamantane and 1-halogenoadamantanes in acetonitrile containing lithium perchlorate as supporting electrolyte.’’ Clark Fleischmann and Pletcher” have also reported a comparative study of the anodic oxidation of a series of aliphatic hydrocarbons in six solvents containing tetrafluoroborate as supporting electrolyte. Scheme 10 shows the steps involved in product forma- tion from propylene.It was found that the basicity of the solvent determines the stability of the cation radical to loss of proton and the degree to which the production of the starting material is important. The results have also suggested that in sulpholan and propylene carbonate protonation was much more important than in aceto- nitrile and nitroethane. Furthermore the nucleophilicity also determines the relative importance of the reactions of intermediates with the solvent. The strong apparent nucleophilicity of tetrafluoroborate is interesting. Reasonable yields (45 % on current) of a fluorinated product l-chloro-2-fluorocyclohexane 56 J. P. Coleman M. Fleischmann and D. Pletcher Electrochim. Acta 1973 18 331. ’’ J. Bertram J. P. Coleman M.Fleischmann and D. Pletcher J.C.S. Perkin ZI 1973 374. 58 V. R. Koch and L. L. Miller Tetrahedron Letters 1973 9 693. ’’ D. B. Clark M. Fleischmann and D. Pletcher J. Electroanalyr. Chem. Interfacial Electrochem. 1973 42 133. 300 K. Korinek and T. F. W.McKillop F F I I Me-CH-CHi -e-Hf~ Me-CH -CH,OAc BF4-T CH,=CH=CH,OAc Me-CH=CH 3 MekH-eH 3 H+ + CH CH2=CH-CH,F Me-CH=CH, li CH2 Me ’+ ‘MeH% Me2CHOAc Scheme 10 were also observed6’ in the oxidation of chloride ion in the presence of cyclo-hexene in methylene chloride-tetrabutylammonium tetrafluoroborate. Schafer and Koch6’ have reported on the anodic dimerization of substituted olefins containing both the electron-withdrawing group -CO-R and the electron-donating group -NH-R in methanolic sodium perchlorate solutions.The products of dimerization of enamino-ketones or enamino-esters were substituted pyrroles formed by radical dimerization of the primary oxidation product followed by the intracyclization and elimination of the dication (Scheme 11). X x* x-/ It H-C H-C-C-H II -2e C MeOH NaCIO,’ Me-c I &-Me I Me’ \NH NH NH I I I R R R X C-CHX II ‘C-Me -RNH \ bx C I Me N Me Me’ \NH NH I I I RR R Scheme 11 6o V. R. Koch L. L. Miller D. B. Clark M. Fleischmann T. Joslin and D. Pletcher J. Electroanalyt. Chem. Interfacial Electrochem. 1973 43 3 18. 61 D. Koch and H. Schafer Angew. Chem. 1973,85,264. Electro-organic Chemistry 30 1 Alcohols Phenols and Carbonyl Compounds-The electrochemical oxidation of saturated aliphatic alcohols is known to occur at highly positive potentials.During the oxidation of 2-methoxyethanol which contains both alcohol and ether functions in the presence of tetraethylammonium tetrafluoroborate as supporting electrolyte attack predominantly occurs on the ether rather than on the alcohol function.62 Formaldehyde bis-(2-methoxyethyl)formal,and 2-hydroxyethyl-2’-methoxyethylformal were the products. Recently there has been an increased interest in electrosynthetic reactions involving phenols. Nilsson Ronlan and Parker63 have studied the anodic hydroxylation of phenols at a lead dioxide electrode in aqueous sulphuric acid. In all cases studied the substitution occurred at the 4-position.The 4-substituted phenols gave 4-substituted 4-hydroxycylohexadienoneswhereas phenol itself was oxidized to p-benzoquinone. Lead dioxide anodes were found to be superior to carbon nickel and platinum anodes in hydroxylation reactions. The sug- gested mechanism proceeds via an electrolytic step generating surface species which chemically oxidize the phenol and are regenerated electrochemically. Phenoxonium ion is the probable intermediate. The method offers a convenient high-yield synthesis of 4-allyl-4-hydroxycyclohexa-2,5-dienones from 4-allyl- phenols. Data correlating ionization potentials electron densities and oxidation potentials for 20 phenol derivatives were also reported.64 During the investigation of anodic cleavage of benzyl ketones it was observed that tetrafluoroborate ion is not inert6’ and that high yields of fluorinated products result from its participation with reactive intermediates.The elec- trolysis of benzhydryl p-toluyl ketone in dry acetonitrile containing 0.1 M tetramethylammonium tetrafluoroborate gave 65 % yield of p-toluyl fluoride. Carboxylic Acids.-The electrochemical oxidative decarboxylation of aliphatic carboxylate anions can lead either to coupled products derived from radical reactions as in Kolbe electrosynthesis or to the so-called Hoffer-Moest products derived from the reactions of carbonium ions. It is known but not generally recognized among electro-organic chemists that the electrode material greatIy influences the nature of the products.Brennan and Brettle65 have now reported results from preparative electrolysis of triethylammonium heptanoate using different types of carbon as anodes in protic solvents. The Kolbe product dodecane was the major product at a vitreous or baked carbon or platinum anode but the major products at a graphite anode were three heptanoates(1- methylpentyl- 1-ethylbutyl- and 2-ethylbutyl-heptanoate) which arose from the rearrangements of the hexyl cation. Similarly when a platinum electrode was used the Kolbe dimer was the major product of anodic oxidation of phenyl-acetic acid phenoxyacetic acid and ethyl and methyl hydrogen succinates. At a graphite anode only a very small amount of the Kolbe product was formed. The 62 S. D. Ross J. E. Barry M. Filkenstein and E.J. Rudd J. Amer. Chem. Soc. 1973,95 2 193. ‘’ A. Nilsson A. Ronlan and V. D. Parker J.C.S. Perkin I 1973 2337. 64 A. E. Lutskii Y. 1. Beilis and V. I. Fedorchenko Zhur. ohshchei Khirn. 1973,43 101. 65 M. P. J. Brennan and R.Brettle J.C.S. Perkin I 1973 257. 302 K. Korinek and T. F. W.McKillop divergence in behaviour between a platinum and a carbon anode in the electrolysis of phenylacetic acid had been previously attributed to the presence of paramag- netic centres which bind the initially formed radical and promote the second electron transfer. This theory may now be rejected since the baked carbon electrode which is also likely to contain paramagnetic impurities gives rise to the Kolbe dimer. Marked differences have however been observed for pyrolytic graphite electrodes depending on whether the electrode surface is parallel (pyrolite face) or perpendicular (pyrolite edge) to the cleavage plane.The number of adsorption sites on the pyrolytic edge electrode is much greater and therefore the greater concentration of adsorbed radical intermediate favours dimerization. Thus some forms of carbon but not soft graphite can be used successfully in the Kolbe electrosynthesis. Whether the Kolbe reaction proceeds via free or adsorbed radical intermediates . has been the subject of much controversy in recent years. Utley et ~1 have ~~ now reported a study of anodic oxidation of 4substituted cyclohexane- and cyclohexene-carboxylates at a platinum anode in methanol. The coupling between conformationally biased cyclohexyl radicals should allow a distinction between the free and adsorbed radical intermediates.The ratio of the three coupled stereoisomers of 4,4-di-t-butylcyclohexyl (a :a; a :e; e :e) was 1 :2:1 which indicates that the recombination is in a random fashion and strongly suggests that the role of adsorption is negligible. The distribution of electrolysis products of cis-4-phenylcyclohex-2-enecarboxylatessuggested that for un-saturated carboxylates the intermediates may be adsorbed at the anode. Similar conclusions have been reached by Eberson and Ryde-Petter~on,~’ who have studied the mixed coupling of monoethyl (+)-ethylmethylmalonate and isovaleric acid in methanol. The mixed coupling product ethyl ethylisobutylmethylacetate (31)was 99.98% racemic.Since adsorbed radicals might be expected to give Me COZEt Me C0,Et \/ \/ +Me,CHCH,CO,-2i%,C C* b /\ /\ Et co,-Et CH,-CHMe (31) coupling products with at least partial retention of configuration the formation of racemic product supports the theory of free radicals. Another example of a successful use of mixed Kolbe electrolysis is the one-step synthesis of the housefly sex attractant (Z)-tricos-Pene (32).68 Anodic oxidation of the mixture of (Z)-octadec-9-enoic acid and n-heptanoic acid in methanolic sodium methoxide afforded the expected dimers. Isolated yields for a typical uncontrolled and not optimized synthesis are shown. Renaud and Sullivan69 have discussed the probable mechanism of the mixed Kolbe reaction of difluoroacetic acid and propionic acid.The mixed dimer 66 G. E. Hawkes J. H P. Utley and G. B. Yates J.C.S. Chem. Comm. 1973 305. 67 L. Eberson and G. Ryde-Petterson Acta Chem. Scand. 1973 27 1159. 68 G. W. Gribble J. K. Sanstead and J. W. Sullivan J.C.S. Chem. Comm. 1973 735. 69 R. N. Renaud and D. E. Sullivan Canad. J. Chem. 1973 51 772. 303 Electro-organic Chemistry Me(CH2),CH=CH(CH2),C02H + Me(CH2),C02H Me(CH,),,Me 20% \ + Me(CH,),CH=CH(CH,) ,Me 14% (32) + Me(CH,),CH=CH(CH,),,CH=CH(CH,),Me 7 trifluoropropane was not formed but instead 3,3,3-trifluoropropene 1,Zbis- (trifluoromethyl)ethane and 1,2,4-tris( trifluoromethy1)butane were obtained The probable mechanism could involve attack of the CF radical on ethylene formed by oxidation of propionic acid followed by further reaction with CF radical or ethylene.If electrochemical methods are to make a significant contribution to syn- thetic chemistry it is clear that a more systematic approach is needed to the study of electro-organic reactions. It is essential that electrolysis should first be investigated using controlled-potential methods before attempting to scale up or use constant-current conditions. It is therefore disappointing that most of the studies reported sn Kolbe reactions have not adopted this systematic approach especially since the necessary equipment is now readily available and reasonably cheap. In the case of mixed coupling reactions controlled- potential studies are absolutely essential. Anodic oxidation of aromatic carboxylic acids has not been investigated in great detail.Recently however the controlled-potential and controlled-current oxidation of benzoic acid in acetonitrile and propionitrile was reported?' The main product in acetonitrile was anthranilic acid and in propionitrile it was N-propionylanthranilic acid with N-propionylbenzamide also being formed (Scheme 12). Electrochemical methods can be very useful in extending the range of syn- thetic approaches available to the organic chemist. Since one usually employs mild conditions reactive and strained compounds can be prepared. The electro- synthesis of one such unstable intermediate 1,4bis(methoxycarbonyl)bicyclo-[2,2,2]octa-2,5-diene (33) was effe~ted,~' by electrolytic decarboxylation of the adduct of maleic anhydride and dimethyl cyclohexa- 1,3-diene- 1,4dicarboxylate (34).In the presence of a radical inhibitor 4-t-butylcatechol the yield of (33) was increased to 65%. Decarboxylations are only one type of important 'de- gradative reaction which may be amenable to electrochemical oxidation. Nitrogen-containing Compounds.-The oxidative cycliza tion of substituted diphenylamines has received further attention. Nelson and Berkenk~tter~~ 70 Y. Matsuda. K. Kimura C. Iwakura and H. Tamura Bull. Chem. SOC.Japan 1973 46 430. 7' C. R. Warren J. J. Bloomfield S. S. Chickos and R. A. Rouse J. Org. Chem. 1973 38 401 1. 72 P. Berkenkotter and R. F. Nelson J. Electrochem. Soc. 1973 120 346. K. Korinek and T. F. W.McKiIlop R = Me or Et IRCN I I-e j+H. 0 0f-NH-C-EtII 0 FHCOR Scheme 12 partial electrolytic ' decarboxylation C0,Me OC'O have shown that the anodic oxidation of NNN'-triphenyl-o-phenylenediamine in acetonitrile leads to 5,10-dihydro-5,10-diphenylphenazinein nearly quantita- tive yields (Scheme 13). This intramolecular cyclization is the key intermediate step in the formation of dihydrophenazines from substituted diphenylamines. An extensive study of substituted diphenylamines has shown that for efficient cyclization the para ring positions must be substituted with electron-donating neutral or weakly electron-withdrawing groups which do not undergo elimination during the Electro-organic Chemistry R 9 / R \ R R Scheme 13 reaction.Cauquis and Serve73 have measured electrochemical and spectro- scopic properties of o-phenylenediamines and studied their oxidative cyclization to dihydrophenazines in acetonitrile containing 0.1 M-Et,NClO,. They con- cluded that the basicity of the medium influenced the anodic cyclization and suggested in agreement with Nelson and Berkenk~tter,~~ that o-phenylene- diamines are possible intermediates in the oxidation of diphenylamines. Other studies on the mechanism of the oxidation of diphenylamine derivatives in aprotic solvents,74 and in various aqueous media75 have been reported. Behret76 has patented a new continuous electrochemical route to aniline from benzene and ammonia using iodobenzene as catalyst.A diaryliodonium ion is found as the product of the anodic oxidation of aromatics in acetonitrile in the presence of iodobenzene and reacts with NH to form the aromatic amine. Several authors have reported anodic oxidations of amine~~’.~~ and pyridination of Schiff bases79 in acetonitrile. The recent trend of applying electrochemistry to more complex organic systems has continued. Further progress has been made in the electrosynthesis of alkaloids by intramolecular coupling of non-phenolic tetrahydrobenzyliso- quinoline precursors. Miller Stermitz and Falck” have published a full paper ” G. Cauquis and D. Serve Tetrahedron Letters 1973 2695. 14 T. M. H. Saber G. Farsang and L. Ladanyi Microchem. J. 1973 18 66. 75 G. Farsang V. Vass L. Ladanyi and T.M. H. Saber J. Electroanalyt. Chem. Inter- facial Electrochem. 1973 43 391. 76 H. Behret G.P. 2 154 348 1973. 7’ M. P. J. Brennan and 0. R. Brown J. Appl. Electrochem. 1973 3 231. 78 H. Say0 and M. Masui J.C.S. Perkin II 1973 1640. 7q M. Masui and H. Ohmori J.C.S. Perkin II 1973 11 12. ‘O L. L. Miller F. R. Stermitz and J. R. Falck J. Arner. Chem. Soc. 1973,95 2651. K. Korinek and T. F. W. McKillop on the synthesis of morphadienones (35).The electrolysis was carried out at a platinum electrode in acetonitrile containing lithium perchlorate or tetra-methylammonium tetrafluoroborate at 0 "C and yields for a typical substituted 0R' o-benzylflavinantine (R' = R3 = Me R2 = CH,Ph) were 53% of isolated product. This compares extremely well with the available chemical routes whose yields seldom exceed 10%.A common side-reaction in biogenetic-type syntheses is the formation of C-0-C coupled products. This is completely avoided in electro-oxidative cyclization. The Pschorr synthesis which is based on the decomposition of diazonium salts can be adapted for both phenolic and non- phenolic substrates. Several steps are involved however and overall yields are usually very low. Electrochemical coupling has a very definite advantage in such situations and by modifying the conditions excellent yields have been obtained. Kotani and Tobinagas' have also reported the synthesis of morpha- dienone alkaloids. By modifying the conditions and using fluoroboric acid as supporting electrolyte an even higher yield (86%) was obtained for o-benzyl- flavinantine.The same Japanese group has describeda2 oxidative ring closure in the electrosynthesis of 5,lO-ethanophenanthridinering system. The anodic oxidation of trifluoroacetic derivatives of N-(4-methoxyphenethy1)-3,4-methyl-enedioxybenzylamine (36)in acetonitrile containing fluoroboric acid yielded (37) in 62% yield. Alkaline hydrolysis afforded (i-)-oxocrinine (38) which can be readily transformed into the Amaryllidaceae alkaloid ( -t)-crinine (39). This electrochemical approach offers a novel and attractive route to the synthesis of certain alkalbids giving selectivity high yields and adaptability to both phenolic and non-phenolic substrates. Although the synthetic and stereochemical implications of organic electrode reactions occurring at heterogeneous surfaces are widely recognized they are still only poorly understood.Pinesa3 has investigated the controlled-potential oxidation of (S)-2-acetamido-2-(3,4-dimethoxybenzyl)propionitrile at a platinum electrode in sodium acetate-acetic acid containing acetic anhydride. It was hoped to illustrate some differences between homogeneous and electrode E. Kotani and S. Tobinaga Tetrahedron Letters 1973 4759. 82 E. Kotani N. Takeuchi and S. Tobinaga J.C.S. Chem. Comm. 1973 550. 83 S. H. Pines J. Org. Chem. 1973 38 3854. Electro-organic Chemistry OMe 0' COCF / processes which coufd be attributed to the heterogeneous nature of the electrode surface. The major products were (4R,5S)-and (4R,5R)-4-cyano-5-(3,4-di-methoxyphenyl)-2,4-dimethyl-2-oxazolines(40) and (41) respectively in a 3.5 1 ratio.Similar ratios but lower yields can be achieved in the homogeneous reaction of (42) with Mn(OAc) . Me Me CN CH NHCOMe * [ArMq + I-ly-Ar + / NHCOMe NHCOMe Ar (42) - OAfiZ,,, -k Ar H Me Me (40) (41) It seems therefore that in this case the intermediates are not strongly adsorbed and the cyclization step occurs after their desorption from the electrode. Miscellaneous.-An increased use of sulphonium salts and sulphonium ylides in organic synthesis has led to studies on their electrochemical preparation. Some Japanese workers84 have now reported the synthesis of sulphonium salts 84 S. Torii Y. Matsuyama K. Kawasaki and K.Uneyano Bull. Chem. SOC.Japan 1973 46,291 I. K. Korinek and T. F. W.McKillop of the type (43) by anodic oxidation of alkyl phenyl sulphides at a platinum electrode in anhydrous acetonitrile containing lithium perchlorate as supporting electrolyte. The sulphonium perchlorates were formed in 60-70 % yield the precise yield depending on the nature of the R group. As part of their continuing investigation of the anodic oxidation of aryl sulphides Humffray and Imberger” have reported the oxidation of thianthrene in acetic acid-water mixtures (80:20) containing perchloric acid. The controlled- potential electrolysis at 1.15 V us. Ag-AgCl promises to be a useful synthetic reaction yielding 98% of the thianthrene monoxide. At higher potentials a mixture of products was formed with cis-and trans-dioxides predominating.The conventional thiocyanation of double bonds is carried out using thio- cyanogen generated in situ. Direct electrochemical generation of thiocyanogen offers the advantages of minimizing side-product formation. The utility of this method has been demonstratedB6 in the synthesis of vicinal dithiocyanates from olefins and thiocyanate salts in acidic media high yields of dithionate addition products being achieved. The detailed observation led the authors to propose a new radical mechanism for the thiocyanation as shown in Scheme 14. \/ \/ c-Y :> x+y-o;:;g* I c-x /\ /\ X = -SCN Y = -SCN or other nucleophile Scheme 14 A new procedure for alkyl coupling which might accommodate a wide variety of functional groups has been achieved by anodic oxidation of triallylboranes.The reaction was carried out in methanolic sodium hydroxide using platinum electrodes and good yields of coupled products were reported.87 85 H. E. Imberger and A. A. Humffray Electrochim. Acta 1973 18 373. a6 W. J. De Klein Electrochim. Acta 1973 18 413. T. Taguchi M. Itoh and A. Suzuki Chem. Letters 1973 7 719. Electro-organic Chemistry An electrochemical method has been developed for bonding electrically conductive adherents.88 The procedure is based on the electrochemical genera- tion of a curing agent from otherwise chemically unreactive precursor mixed with an epoxy resin and usually a supporting electrolyte sandwiched between the bonding members.Some of the advantages of electrochemical curing are obvious-no need to mix components long-term storage stability relatively rapid curing at room temperatures and ease of manipulation during fabrication. '' N. L. Weinberg G. M.Blank H. A. Aulich A. K. Hoffman andT. B. Reddy J. Appl. Electrochem. 1973 3 227.

ISSN:0069-3030    

DOI:10.1039/OC9737000285

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

10 Photochemistry By W. M. HORSPOOL Department of Chemistry The University Dundee This chapter covers a few photochemical topics selected from the many thousands reported which in the author's opinion are either of general interest to photo- chemists or else are of special interest in a specific area of the subject. Inevitably the report is subjective and tends to be biased towards the author's own interests. Interest in benzene photochemistry continues unabated. Several reports during the past year are worthy of selection. West et a[.' have reported the photoisomerization of 1,2,4,5-tetrakis(trimethylsilyl)benzene by irradiation in ether. The product mixture which differed somewhat in the distribution of pro-ducts from that obtained from alkyl benzene rearrangement did not contain 'Dewar' benzenes but gave 1,2,3,5-tetrakis(trimethylsilyl)benzene (12 %) the benzvalenes (la) (19 %) and (lb)(47%) the fulvene (2) (5 %) and another fulvene of uncertain structure.The first report of photoisomerization in the naphthalene series has been made' whereby 1,3,6,8-tetra-t-butyl naphthalene is converted (94%) into the 'Dewar' naphthalene (3). a; R' = SiMe, R2 = H (3) b; R' = H R2= SiMe (1) Ohashi3 has reported the enhanced ( x 10) formation of the 1,2-adduct between acrylonitrile and benzene when the irradiation is carried out in the presence of ' R. West M. Furue and V. N. Mallikarjuna Rao Tetrahedron Letters 1973 91 I. ' ' W. L. Mandella and R. W. Franck J. Amer. Chem. SOC.,1973,95971. M. Ohashi Tetrahedron Letters 1973 3395.310 Photochemistry 31 1 zinc chloride. Cornelisse and Srinivasan4" have studied the addition of cyclo- butene and cyclopentene to benzene and from quantitative measurements have come to the conclusion that the first excited singlet (IB2J of benzene is involved in the addition rather than a vibrationally excited ground state or excited states of valence isomers. This mode of addition has provided a route to an asterane (4) by the addition of cyclopentene to benzene yielding the adduct (5)(25% 0 = 0.011). This is transformed thermally or by action of acid into the asterane (4).4b A triplet biradical mechanism is involved in the addition of phenanthrene to dimethyl fumarate dimethyl maleate and maleic anh~dride.~ However the mechanism is complicated by the formation of only one product (with maleic anhydride) and the suggestion is that a triplet exciplex is formed which leads to the formation of only one of the two possible biradical inter- mediates.Complex formation (charge-transfer) is also important in the photo- chemical addition of tetracyanoquinodimethane to toluene.6 Irradiation into this band gives a single product (6) in high yield. This reaction is greatly ( x 9) enhanced in the presence of trifluoroacetic acid. Acid catalysis such as this has previously been noted in other benzene systems7 Bryce-Smith et aL8 have reported the formation of adducts (7) by meta-cyclization following the irradiation of phenylpropyl (or butyl) dimethylamine. -NMe H (7) n = 3 or 4 Interest in bichromophoric systems has continued during the past year.De Schryver and his co-workers' have examined the reactions of 1,l'-and 2,2'-linked dianthracenes. With the 1,l'-dianthracenes (8a and b) only one photo- product was obtained in each case but although head-to-head linking resulted no conclusion could be reached as to whether the products were the result of syn-or anti-addition. The 2,2'-linked dianthracenes (9a and b) give both head- to-tail and head-to-head dimers whereas the shorter chain-length linkage of (9c) gave exclusively the head-to-head dimer. The X-ray structure of the bis- thymine (10) has shown that the two thymine units lie over each other in the (a) J. Cornelisse and R. Srinivasan Chem. Phys. Letters 1973 20 278; (b) C.S. Angadiyvar J. Cornelisse V. Y. Merritt and R. Srinivasan Tetrahedron Letters 1973 4407. R. A. Caldwell J. Amer. Chem. SOC.,1973 95 1690. K. Yamasaki A. Yoshino T. Yonezawa and M. Ohashi J.C.S. Chem. Comm. 1973,9. D. Bryce-Smith M. T. Clarke A. Gilbert G. Klunklin and C. Manning Chem. Comm. 1971 916. D. Bryce-Smith A. Gilbert and G. Klunklin J.C.S. Chem. Comm. 1973 330. F. C. De Schryver M. De Brackeleire S. Toppet and M. Van Schoor Tetrahedron Letters 1973 1253. W.M. Horspool X X X= CO(CH,),OCIt0It 0 (8) a; n = 2 b;n=3 (9) a; n = 7 b;n= 9 c; n=5 0 correct environment to produce a trans-syn-dimer.' Solid-state irradiation of the bisthymine (10) does indeed lead to a material with trans-syn cyclobutane linkages but as the product was polymeric little could be done to fully character- ize the material.' ' Nevertheless the results from the photolysis and the structure determination clearly show that the thymine fragments are aligned in a stack within the crystal which permits interaction in a specific fashion.Irradiation of the same bisthymine (10) in acetone-water (1 :9) gave the cis-syn-dimer (1l).' Intramolecular addition has also been examined in the bismaleimides (12).12 The reaction arises from the triplet state and the intersystem crossing efficiency increased (to 0.15 from 0.025) with increasing chain length. The intramolecular cycloaddition of biscoumarins (1 3)' has also been studied. lo J. K. Frank and 1. C. Paul J. Amer. Chem. Soc. 1973,95,2324.N. J. Leonard R. S. McCreadie M. W. Logue and R. L. Cundall J. Amer. Chem. SOC. 1973 95 2320. l2 J. Pat and F. C. De Schryver J. Amer. Chem. Soc. 1973 95 137. l3 L. H. Leenders E. Schouteden and F. C. De Schryver J. Org. Chem. 1973 38 957. Photochemistry I/ (CH,) (12) n = 3,4 5,6 or 7 (11) Barltrop and his co-w~rkers'~.' have studied the photoreactions of pyrilium salts (14a and b) in aqueous solution. Both compounds afford ring-opened keto-aldehydes (15) as products. Pyrilium salt (14b) also gives the syn-and anti-isomers of the cyclopentenone (16).15The results are rationalized in terms of an oxoniabenzvalene intermediate (17). Other worked6 have examined the photochemistry of the pyrilium salts (14c and d) in sulphuric acid.Benzvalene R3 R' R2 R3 R4 R5 (15) (16) (14) a; Me b; Me c; Me d; H H H H Me Me Et HO Me H H H H Me Me Me HO fi e; Me g; Mef; Me Ph Me Ph HO HO HO Ph Me H Me Me Me 9 :fiR3+/OH R' R2 0 R3 (I7) (18) a; Me Ph Ph b; Me Ph H c; Me Me Me intermediates do not seem to fit for these reactions. However in a later report Pavlik and KwongI7 propose the rearrangement of the salts (14e f and g) into the isomeric species (18a-c) via an oxoniabenzvalene intermediate. l4 J. A. Barltrop K. Dawes A. C. Day and A. J. H. Summers J.C.S. Chem. Comm. 1972 1240. l5 J. A. Barltrop K. Dawes A. C. Day S. J. Nuttall and A. J. H. Summers J.C.S. Chem. Comm. 1973 410. l6 J. W. Pavlik and E. L. Clennan J. Amer. Chem. Soc. 1973,95 1697. " J. W. Pavlik and J. Kwong J.Amer. Chem. Soc. 1973 95 7914. W.M. Horspool Over recent years interest has been shown in the nature of the excited state involved in nucleophilic aromatic substitutions. Present work has established that the photosubstitution of m-nitroanisole by hydroxide ion can be sensitized by benzophenone or quenched by oxygen. These results suggest the inter- mediacy of a n+ n* triplet state." On the other hand photosubstitution in anthraquinones (19) arises from the singlet manif01d.'~ Wubbels et ~1.~'have examined the photoreduction of nitrobenzene in HC1-water-propan-24. Contrary to an earlier report there is no evidence for protonation of the excited state and the reduction is effected by electron transfer from the chloride ion. 0 a; R' = MeO R2 = H b; R' = H R2 = Me0 The question of non-vertical energy transfer has been re-examined.Originally the concept arose from the examination of the kinetics for the cis-trans-photo- sensitized isomerization of stilbene.2 Yamauchi and Azumi22 have questioned the original interpretation and suggested that although the non-vertical concept is valid the process can be accommodated within the normal spectroscopic framework. The authors22 deduce that the spectrum of cis-stilbene tails below 48.6 kcal mol-' (204 kJ mol-') whereas the spectrum of the trans-isomer cuts off sharply at this value. In view of this fact and also that the rate of energy transfer from sensitizer to stilbene is dependent on the overlap between the spectra of the stilbene and that of the sensitizer then the transfer rate to the trans-isomer will fall off more rapidly than that to the cis.A comparison of the irradiation of cycloalkenes in the presence of hydroxylic solvents by both direct and sensitized photolysis has shown that there are similarities of beha~iour.~~ Thus 1-methylcyclo-hex-,-hept-,and -oct-enes give a similar range of products by either mode of irradiation. However 1-methylcyclopentene forms ethereal products upon direct irradiation but not under sensitized irradiation. The dimerization of norbornene yielding (20) has been carried out in high yield (88 %) by irradiation in the presence of copper(1) trifluoromethanesulphonate as catalyst.24 This catalyst is superior to other Cu' halides normally used for this purpose.J. den Heijer T. Spee G. P. de Gunst and J. Cornelisse Tetrahedron Letters 1973 1261. l9 J. Griffiths and C. Hawkins J.C.S. Chem. Comm. 1973 11 1. 2o G. G. Wubbels J. W. Jordan and N. S. Mills J. Amer. Chem. SOC.,1973 95 1281. 21 G. S. Hammond and J. Saltiel J. Amer. Chem. SOC.,1963 85 2516. 22 S. Yamauchi and T. Azuni J. Amer. Chem. SOC.,1973,95 2709. " P. J. Kropp E. J. Reardon,jun. Z. L. F. Gaibel K. F. Williard and J. H. Hattaway J. Amer. Chem. Sac. 1973 95 7058. 24 R. G. Salomon and J. K. Kochi Tetrahedron Letters 1973 2529. 315 Photochemistry A re-in~estigation~~ of the photochemical reaction of /I-t-butylstyrenes has been made.26 Using the p-cyano-derivative for ease of analysis the photo- chemical conversion of either the cis- or the trans-olefin gives a trans-cyclo- propane (21) uia a singlet-state reaction.This clearly shows the reaction to be non-concerted. Further study showed that the substituent on the aryl group of the styrene can influence the ease with which the methyl group migrate^.^' Considerable effort has been expended upon the solving of the mechanistic and stereochemical problems posed by the remarkable di-n-methane rearrange- ment. The studies have revealed that in the conversion of (22) into (23),there is a preferential formation of the three-membered ring by disrotatory motion of the orbitals at C-1and C-3anti to the migrating olefinic group. This conversion will occur only if the stereochemistry of the molecule will permit. The recognition of a syn-disrotatory conversion has been achieved in the direct irradiation (the sensitized reaction afforded no products) of the diene (24) which yielded the cyclopropyl olefin (25).28 It is reasoned that in the absence of steric restraints the preference for a given path lies in the better orbital overlap from the back of C-3with C-1.This situation does not obtain in the present example and an alternative reaction mode is followed.Zimmerman and Pincock2’ have examined the photochemistry of 3,3-dimethyl-l,5-diphenylpenta-1,4-diyne, which does not follow a di-n-methane route to the products 1,l -dimethyl-3,4-diphenylcyclopenta-2,4-diene and the ene-yne (26). The products arise uia a biradical intermediate and hydrogen abstraction from the solvent. Another potential di-n-methane reaction was sought for in the irradiation of the allene (27).In this example [2 + 21 cycloaddition predominates giving (28) and the di-n-methane path is of 25 S. S. Hixson and T. P. Cutler J. Amer. Chem. SOC., 1973 95 3031. 26 H. Kristinsson and G. W. Griffin J. Amer. Chem. SOC.,1966 88 378. 27 S. S. Hixson and T. P. Cutler J. Amer. Chem. SOC., 1973,95 3032. 28 P. S. Mariano and R. B. Steitle J. Amer. Chem. SOC.,1973 95 61 14. 29 H. E. Zimmerman and J. A. Pincock J. Amer. Chem. SOC.,1973,95 3246. W.M. Horspool minor imp~rtance.~’ The discovery of a ‘walk rearrangement’ of a cyclopropane ring in a heterocyclic system (29) has been reported?1 This rearrangement affords (30) (which is also photoreactive) as the primary photochemical product from the singlet excited state of (29).Mazzocchi and Lust$’ have observed the photochemical conversion of optically active l-deuterio-2,2-dimethyl-l-phenylcyclopropane into 4-deuterio-2-methyl-4-phenylbut-l-ene with complete retention of optical activity. This is interpreted as arising by fission of the C-l- C-2 bond concomitant with rotation about the C-2-C-3 bond which brings the methyl groups on C-2 into close proximity with the developing free radical at C-1. The Norrish Type I1 reaction of ketones in their nn* excited states is probably the most studied photochemical reaction. Related to this reactivity is the report of the irradiation (253.7nm) of a methanol solution of l,l-dideuterio-2-phenyl-2-o-tolylethylene which gave product (31) where an exchange of a deuterium atom had taken place.33 The reaction is thus similar to a Norrish Type I1 hydrogen abstraction in carbonyl compounds and the authors33 suggest that there is a similarity between the reactivities of the nn* state of a carbonyl group and of the nn* state of the ethylene.CHD CHzD Another problem associated with the interpretation of results from Norrish Type I1 reactions of ketones has been uncovered following a re-in~estigation~~ of the photochemistry of 4-methyl- 1-phenylpentan-1 -one. Usually care is taken in such experiments to exclude oxygen to prevent quenching of the ketonic triplet ~tate.~’,~~ However Grotewold et have observed that oxygen does not 30 D. C. Lankin D. M. Chihal G. W.Griffin and N. S. Bhacca Tetrahedron Letters 1973 4009. 3’ H. E. Zimmerman and W. Eberbach J. Amer. Chem. Soc. 1973,95 3970. 32 P. H. Mazzocchi and R. S. Lustig J. Amer. Chem. Soc. 1973 95 7178. 33 F. Scully and H. Morrison J.C.S. Chem. Comm. 1973 529. 34 J. Grotewold C. M. Previtali D. Soria and J. C. Scaiano J.C.S. Chem. Comm. 1973 207. 35 J. A. Barltrop and J. D. Coyle J. Amer. Chem. Soc. 1968 90 6584. 36 F. D. Lewis and T. A. Hilliard J. Amer. Chem. Soc. 1972,94 3852. Photochemistry 317 quench the reaction of the ketone and in fact the yields of acetophenone cyclo- butanol (32) and 2-methylpropene are increased. The authors34 suggest that PhHb (32) (34) (33) a; R' = OCOCH,Ph R2 = H b; R' = H RZ = OCOCH,Ph c; R' = CH,COMe R2 = H the triplet ketone reacts with oxygen producing a biradical species which sub- sequently breaks down to products.A peroxide is formed in the reaction and this appears to support the mechanism. Lewis and his co-~orkers~~ have examined the differences in the photochemical reactivity of some arylalkyl ketones. Their results indicate that radiationless decay of the triplet states of the ketones will not compete with y-hydrogen abstraction in the Norrish Type I1 process since the triplet state is too short-lived. They reason37 that the differences in reactivity result from differences in entropy. The singlet-state-induced Norrish Type I1 elimination reactions of the esters (33a and b) give a low yield of the highly reactive adamantene (34).38The success of this route is to be contrasted with the failure of the triplet-state reaction of (33c) which yielded cycl~butanols.~~ Kanaoka et uI.~'.~' have reported the photochemical reactions of N-substituted phthalimides.Norrish Type I reactions of the cyclic ketones (35) have been used in the synthesis of some rna~rolides.~~ 0 0 ,Li3 4 (35) n = I. 2 3 or 4 MO treatment of the photochemistry of fl,y-unsaturated ketones has been reported.43 This demonstrates that in the singlet state (m*) of the model com- pound (36) there is weak bonding between C-2and C-5 but antibonding between C-2 and C-4 with considerable weakening of the C-2-C-3 bond. Thus in this 37 F. D. Lewis R. W. Johnson and D. R. Kory J. Amer. Chem. Soc. 1973 95 6470. " J.E. Gano and L. Eizenberg J. Amer. Chem. Soc. 1973 95 972. 39 R. B. Gagosian J. C. Dalton and N. J. Turro J. Amer. Chem. Soc. 1970 92 4752. 40 Y.Kanaoka Y.Migita K. Koyama Y.Sato H. Nakai and T. Mizoguchi Tetrahedron Letters 1973 1193. 41 Y. Kanaoka and Y. Migita Tetrahedron Letters 1973 51. 42 R. G. Carlson J. H.-A. Haber and D. E. Henton J.C.S. Chem. Comm. 1973 223. 4' K. N. Houk D. J. Worthington and R. E. Duke jun. J. Amer. Chem. Soc. 1972 94 6233. W. M. Horspool state a-fission or 1,3-aryl shifts are predicted. The triplet nn* state is similar. However the triplet nn* state has little weakening of the C-2-C-3 bond but the interaction between C-2 and C-4 is bonding. Thus from this state 1,2-aryl shifts are predicted. The explanation given here is different from that proposed by Schuster et Interest in the difference between singlet and triplet reactivity of enones has continued.Direct irradiation of 2-methyl-2-(cyclopent- 1-enyl) cyclopentanone has been shown to give the hydrazulene (37) by a 1,3-migration whereas acetone sensitization gives the tricyclic ketones (38) by a 1,2-migration (an oxa-di-n-methane rea~tion).~' The structural effects in the triplet state rearrangement of some /?,penones have been Wolff and Agosta4' have continued their examination of the intramolecular hydrogen abstracting capabilities of the triplet state ofa,/?-unsaturated ketones. The current report deals with the formation of the bicyclic ketone (39) from the irradiation of the enone (40). This ketone (39) arises by loss of methylene from the biradical intermediate (41) followed by cyclization.The effect of the electron- withdrawing groups on the photochemistry of cross-conjugated cyclohexa- dienones has been reported.48 0 I1 n 0 I (43) A 1,4-addition product (42) has been isolated from the argon-laser-induced photoaddition of p-benzoquinone (n +n* excitation) to cy~lo-octatetraene.~~ This structure (42) is a reassignment of an earlier report where 1,2-addition was proposed.'' An enedione (43) has been isolated from the mixture of products formed from the irradiation of 2,6-di-t-butyl benzoquinone.' ' The continuation of the elegant studies of Chapman and his co-workers on matrix isolation photochemistry of organic molecules merits special mention.44 D. I. Schuster G. R. Underwood and T. P. Knudsen J. Amer. Chem. SOC.,1971 93 4304. 45 R. G. Carlson R. L. Coffin W. W. Cox and R. S. Givens J.C.S. Chem. Comm. 1973 501. 46 H. Sato K. Nakanishi J. Hoyashi and Y. Nakadaira Tetrahedron 1973 29 275. " S. Wolff and W. C. Agosta J.C.S. Chem. Comm. 1973 502. 48 D. Caine P. F. Brake J. F. De Bardeleben and J. B. Dawson J. Org. Chem. 1973,38 967. 49 E. J. Gardner R. H. Squire R. C. Elder and R. M. Wilson J. Amer. Chem. Sor. 1973 95 1693. 50 D. Bryce-Smith A. Gilbert and M. G. Johnson J. Chem. Soc. (0,1967 383. 51 T. J. King A. R. Forrester M. M. Ogilvy and R. H. Thomson J.C.S. Chem. Comm. 1973 844. Photochemistry 319 In this work the study of a-pyrone photochemistry has shown that keten (44) formation is the primary photochemical path and that lactone (45) formation is only a minor path~ay.’~ However prolonged irradiation of the a-pyrone using Pyrex-filtered light at 8 K eventually gives a high yield of the lactone (45).Subsequent irradiation of the lactone with quartz-filtered light leads to CO extrusion and the formation of cyclobutadiene (i.r. absorptions at 1240 650 570 cm-1).53 The simplicity of this spectrum suggests that cyclobutadiene is square planar with D, symmetry. Other workers have arrived at the same con~lusion.’~ This symmetry is further suggested by the synthesis and i.r. investigation of deuteriocyclobutadienes. The experiments demonstrated that while irradiation of isomeric monodeuterio-a-pyrones (46) gave different /?-lactones the same monodeuteriocyclobutadiene was obtained.’ ’ (44) (46) a; R’= H R2 = D (47) b; R’ = D R2= H (46) Benzyne has been isolated in an argon matrix from the prolonged irradiation of phthaloyl peroxide or benzocyclobutenedione.56 Irradiation (at 8 K) of the peroxide for shorter times produces the elusive benzopropiolactone (47).’ The photochemistry of N-oxides continues to produce interesting results.Ullman and his co-w~rkers~~.~~ have studied the photoconversion of the radical (48)into two new radicals (49) and (50). The ring-contraction [to (49)] encountered in this sequence is all the more interesting as a result of the photorearrangement of the related N-oxide (51) into ketone (52).60 Other workers6’*62 have reported novel ring-contractions in the photochemistry of 4-substituted 1,2,3-benzotria- zine 3-N-oxides (53) and 4-methylcinnoline 2-N-oxide.Irradiation of 4-amino- naphthotriazinone in acetonitrile leads to the formation of the relatively stable azetinone (54).63 The mechanism of conversion of oxazoles into isoxazoles has been studied6* and shown to involve the formation of isocyanides. 52 0. L. Chapman C. L. McIntosh and J. Pacansky. J. Amer. Chem. SOC.,1973,95 244. 53 0. L. Chapman C. L. McIntosh and J. Pacansky J. Amer. Chem. SOC.,1973,95 614. 54 A. Krantz C. Y. Lin and M. D. Newton J. Amer. Chem. SOC.,1973 95 2744. 55 0.L. Chapman D. De La Cruz R. Roth and J. Pacansky J. Amer. Chem. SOC.,1973 95 1337. 56 0. L. Chapman K. Mattes C.L. McIntosh J. Pacansky G. V. Calder and G. Orr J. Amer. Chem. SOC.,1973 95 6134. 57 0.L. Chapman C. L. McIntosh J. Pacansky G. V. Calder and G.Orr J. Amer. Chem. SOC.,1973 95 4061. ’’ E. F. Ullman L. Call and S. S. Tseng J. Amer. Chem. SOC.,1973 95 1677. 59 L. Call and E. F. Ullman Tetrahedron Letters 1973 961. 6o R. Felden 0. Meth-Cohn and H. Suschitzky J.C.S. Perkin I 1973 702. 61 W. M. Horspool J. R. Kershaw A. W. Murray and G. M. Stevenson J. Amer. Chem. SOC.,1973 95 2390. 62 W. M. Horspool J. R. Kershaw and A. W. Murray J.C.S. Chem. Comm. 1973 345. ‘’ N. Bashir and T. L. Gilchrist J.C.S. Perkin I 1973 868. 64 J. P. Ferris F. R. Antonucci and R. W. Trimmer J. Amer. Chem. SOC.,1973,95 919. W.M. Horspool ?-0 0 (49) I 0' (48) (50) 0- 'Ph Ph (53) R' = Et or Me RZ = H R' = Ph R2= H or C1 (51) (52) Interest in the photochemistry of thiocarbonyl compounds has continued during the past year.Thiophosgene adds photochemically to 2,3-dimethylbut- 2-ene to afford the thietan (55) in 51 % yield.65 The dithiolethione (56) is also photochemically reactive and irradiation (A > 420nm) in benzene gave the product (57).66 This is formed by the intramolecular addition of the photo- chemically excited thiocarbonyl group to the benzene ring. The excited thio- carbonyl group can be trapped intermolecularly by irradiation in the presence of 1,l -diphenylethylene when (58) is formed. Study of adamantanethione photo- chemistry has led to the proposal that there is significant intersystem crossing (54) (55) 65 H.Gotthardt Tetrahedron Letters 1973 1221. 66 P. de Mayo and H. Y. Ng Tetrahedron Letters 1973 1561 Photochemistry 321 to the triplet manifold from the second excited m*singlet ~tate.~’ The first example of a stable a-dithione (59) has been prepared by the photo-decarbonyla- tion of the corresponding diary1 dithiovinylene carbonate.68 Ar S (59) Ar = p-dimethylaminophenyl SxAr Several unrelated topics make up a final paragraph. Zimmerman et have published an account of a method for the determination of ultra-fast (in the picosecond range) singlet reaction rates. A report by Williams et suggests the use of a photosensitizer dye (Eosin or Rose Bengal) complexed upon a basic anion exchange resin (or Methylene Blue upon an acidic exchange resin).This method gets around the separation problems in dye-sensitized singlet oxygen reactions. The thermal population of electronicalIy excited states has been further studied and a report on the production of benzene triplets from the thermolysis of ‘Dewar’ benzenes has been p~blished.~’ An account of ‘anti- Stokes’ sensitization of the triplet Norrish Type I1 elimination of valerophenone by energy transfer from the biacetyl-tetramethyl 1,2-dioxetan is also of interest.72 Acetone has been used for the facile epimerization of unactivated tertiary centres such as the conversion of cis-decalin into trans-decalin. 67 A. H. Lawrence and P. de Mayo J. Amer. Chem. SOC.,1973,95,4084. 68 W. Kusters and P. de Mayo J.Amer. Chem. SOC.,1973,95 2383. 69 H. E. Zimmerman D. P. Werthemann and K. S. Kamm J. Amer. Chem. SOC.,1973 95 5094. ’O J. R. Williams G. Orton and L. R. Unger Tetrahedron Letters 1973 4603. 71 P. Lechtken R. Breslow A. H. Schmidt and N. J. Turro J. Amer. Chem. SOC.,1973 95 3025. 72 N. J. Turro and P. Lechtken Tetrahedron Letters 1973 565. 73 R. G. Salomon and J. K. Kochi Tetrahedron Letters 1973 4387.

ISSN:0069-3030    

DOI:10.1039/OC9737000310

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

11 Post Reactions of Polymers By L. P. ELLINGER British Petroleum Company Limited Group Research and Development Department Epsom Division Epsom. Surrey 1 Introduction By post reactions of polymers we mean reactions of the preformed polymer in contrast to reactions leading to the formation of polymers. The variety of syn-thetic and natural organic polymers is great; polymers may contain any of the substituents and structural features known in smaller molecules. In most respects the reactions of these groups in polymers are very similar to their reactions in smaller molecules. An extensive account about ten years ago’ has been followed by further advances along established lines and important exten- sions into new directions. The chemical modification especially of the natural polymers proteins,2 and carbohydrates is the subject of an extensive literature.Within the very limited space available an attempt is made to select recent progress particularly where the reaction shows features ascribable to a polymer rather than to a small molecule. Technologically polymers are used in a wide variety of applications. Their suitability for particular applications can often be improved by modification of their molecular weight molecular weight distribution crystallinity stereo- chemistry and structure and may include structural modification by chemical reactions. Polymers are often solid and less soluble than the corresponding smaller molecules; particular substituents groups and even whole molecules can be rendered less soluble by attachment to suitable polymeric molecules.The features of polymers which distinguish them from small molecules are mainly associated with their high molecular weight and in linear polymers at least with the frequently regular and repetitive arrangement of smaller units corresponding closely to individual small molecules throughout the extent of the polymer mole- cule only the terminal groups differing somewhat from the centre units. Owing to their high molecular weight and viscosity polymer reactions are often viscosity controlled the solubility and the rate of diffusion of reagents and the mobility of the smaller units all being slowed. Many polymer molecules such as the poly- olefins are partially amorphous and crystalline.The reactivity of the same groups ’ ‘Chemical Reactions of Polymers’ ed. E. M. Fettes Interscience New York-London- Sydney 1964. * G. E. Means and R. E. Feeney ‘Chemical Modifications of Proteins’ Holden-Day San Francisco-Cambridge-London-Amsterdam 1971. 322 Post Reactions of Polymers in the amorphous and in the crystalline domains may differ sharply mainly again owing to differences of accessibility to the reagent. In many polymers for instance in synthetic polymers made with reagents exerting steric control (Ziegler-Natta catalysts anionic initiators) and in most natural polymers the substituents are sited in a regular sequence with often a very high degree of steric regularity along the main structure. The reactivity of regularly sited substituents may be greatly enhanced by this regularity which promotes concerted reactions over a range not often feasible between small separate molecules.The regular recurrence of particular substituents along a molecular chain affects the kinetics of their reactions. Kinetic expressions have been derived for the probability of a reaction involving a particular pair of substituents depending on whether 0 1 or 2 of the adjacent pairs have already reacted and the results have been compared with the experimental results for certain polymer reaction^.^ Such effects are thought to account very largely for the great catalytic efficiency and specificity of enzyme action and of protein and nucleic acid syntheses and reproduction. In view of limitations of experience and space these cannot be considered here but the design of polymeric supports of increasing regularity for groups known to be catalytically active and the study of the catalytic behaviour of the products is discussed.The chemical modification of both synthetic and natural polymers with a view to improved technological performance includes grafting cross-linking and curing. Only grafting in which it is aimed to attach identical or different polymer chains to the original molecule is considered here. Close control over the modification of product properties may be achieved and there have been important recent advances. Finally the use of polymer molecules as solid supports in stepwise synthesis for catalytic groups or for compounds of pharmacological photochemical or antioxidant reactivity is described.2 Complex Formation Complex formation is the simplest type of post reaction. Some complex types have been known for many years and recent work has led to better understanding of their nature ;a few examples are discussed. Oxygen shifts the optical absorption of liquid nucleophilic organic compounds to longer wavelengths ;the effect is ascribed to donor-acceptor interaction. Light absorption in the range 285-340 nm by polypropene is greater in an oxygen than in a nitrogen atm~sphere.~ The extra absorption has been correlated with an e.s.r. ~ignal.~.~charge-transfer band has also been observed for poly-4- A methylpent-l-ene-oxygen.6 A similar red shift by up to 350nm of the absorp- tion spectrum of polystyrene appears reversibly in the presence of oxygen.It is ' E. A. Boucher J.C.S. Faraday I 1972,68 2281. ' K. Tsuji and T. Seiki J. Polymer Sci.,Part B Polymer Letters 1970,8 517. V. K. Milinchuk Vysokomol. Soedineniya 1965 7 1923. ' G. W. Wood and T. M. Kollman Chem. and Ind. 1972,424. 324 L. P.Ellinger postulated that on irradiation the complex disintegrates with singlet oxygen formation.' This accounts for the photodegradation of polystyrene in the presence of oxygen but not in its absence by light in the range 280-340 nm. The complexes of iodine with oxygen-containing polymers particularly with starches are wellknown. The effect of the degree of polymerization of the amylose has been studied upon the absorption maximum of its iodine complex; this shifts with increasing degree of polymerization from 500 to 620 nm ;the extinction coefficient increases sharply between P, = 25-65 and the optical rotatory dis- persion and the circular dichroism reach maxima in the range P, = 70-100.8 The kinetics of amylose-iodine complex formation have been studied by the stopped-flow technique' and by pulse radiolysis' on aqueous amylose-KI solutions the change of absorption at 600 nm being followed.The detailed kinetic pattern depends on both the relative and absolute concentrations of the com- ponents ; at low iodine concentrations the rate of growth by successive uptake of iodine molecules is faster than the rate of chain initiation which is thought to involve the formation of stable nuclei in the he lice^.^ The conductivities and optical properties of complexes have been determined in which one of the components is a polymer containing a recurring regularly disposed substituent.The syntheses of two polymers containing a polyethyl- eneimine backbone carrying either N-4-[4-(methylthio)phenoxy]butyryl(1) or N-4-[(1O-methyl-3-phenothiazinyl)]butyryl (2) substituents on the backbone / McS~OCH2CH2CH2CO-N\ /CH CH (1) \2 Me / aynCH,CH,CH2CO-N \ nitrogens have been described.' With the ethyleneimine repeating unit in the polymer backbone the repeat distance of the substituent is 0.64-0.66nm. This together with the substituent design (a nucleophilic aromatic ring-system linked through a flexible short aliphatic chain perpendicularly to the backbone) B.RAnby J. F. Rabek and Z.Joffe presented at the conference on 'Degradability of Polymers and Plastics' The Plastics Institute London 1973 preprint 3/7. B. Pfannemuller H. Mayenhofer and R. C. Schulz Makromol. Chem. 1969 121 147. J. C. Thompson and E. Hamori J. Phys. Chem. 1971,75,272. M. Gratzel A. Henglein M. Scheffler H. M. Bossler and R. C. Schulz Ber. Bunsenge- sellschaft phys. Chem. 1972 76 72. M. H. Litt and J. W. Summers J. Polymer Sci. Polymer Chem. Edn. 1973 11 1339 1379. Post Reactions of' Polymers gives good orbital overlap and allows complex formation with organic electron- acceptor molecules in such a way that they are neatly accommodated between the donor branches. 2 :1 Charge-transfer complexes of the polymers with dichlorodicyanobenzoquinone tetracyanoquinodimethane tetracyanoethylene and 2,4,5,7-tetranitrofluorenone have 3-50 times higher equilibrium constants than the corresponding charge-transfer complexes formed from p-methylthio- anisole or 10-methylphenothiazine and the absorption spectrum maxima of the polymer complexes exhibit some red shifts.Polymer (1) is crystalline; all but one of the polymer (1)and (2)acceptor complexes are amorphous. They are up to 200 times more conducting than the complexes made from the non-polymeric donors. The concentration of unpaired electrons (e.s.r.) are essentially indepen- dent of temperature most of the electrons being trapped. Elongation of one of the amorphous polymer complexes decreases resistivity suggesting that conductivity is parallel to the polymer backbone.The complexes are photoconducting. The charge-transfer-band extinction coefficient of the complex of tetracyano- quinodimethane with polyesters of trans-2,3-dicarboxyspirocyclopropane-1,9'-fluorene and ethylene glycol has been found to increase over a range of 1-4 times with increasing degree of polymerization reflecting most probably an increase in the equilibrium constant of complex formation. Thus conductivity optical absorption maximum and equilibrium constant for complex formation are all affected by the degree of polymerization of at least the donor species. In most polymerization reactions the polymer has little direct effect on the propagation reaction other than that ascribable to increasing viscosity ; it is however possible that polymers derived from highly polar monomers may interact as templates with polar monomers.If the polymer is sufficiently stereo- regular an increase of the propagation rate and a stereoregular product could ensue. Thus poly(methy1 methacrylate) is known to promote polymerization of its monomer. Comparison of the influence of conventional isotactic and syndio- tactic poly(methy1 methacrylate) and of a 1 1 stereo-complex of the latter two upon the bulk polymerization of methyl methacrylate has provided detailed evidence of such effects particularly at low conversion ;l compared with the conventional polymer isotactic poly(methy1 methacrylate) at 90 "C promotes the formation of the syndiotactic polymer with which it forms an acetone-insoluble stereo-complex ; no polymerization occurs under the same conditions in the absence of the polymer.Crown ethers have remarkable complexing power particularly for small metal cations ;crown-ether-substituted polystyrenes are even more efficient complexing agents. The reduced viscosity pattern of complexes based on the polymers resembles that ofpolyelectrolytes the reduced viscosity increasing as the complex concentration falls. l4 '* R. C. Schulz and H. Tanaka Pure Appl. Chem. 1972,30,249. l3 R. Buter Y. Y. Tan and G. Challa J. Polymer Sci. Polymer Chem. Edn. 1973 11 989. l4 S. Kopolov Z. Machacek U. Takaki and J. Smid J. Macromol. Sci. Chem. 1973,7 1015. 326 L. P.Ellinger 3 Reduction and Hydrogenation Lithium aluminium hydride has proved a powerful reducing agent replacing aliphatic chlorine essentially quantitatively by hydrogen.Lithium deuteride is similarly effective at introducing deuterium in the place of chlorine. l5 Various heterogeneous hydrogenation catalysts add hydrogen efficiently to aliphatic double bonds ; the reaction products of lithium trialkyls with nickel salts of organic acids are examples. l6 Co-ordination compounds of lithium have also been used as hydrogenation ~ata1ysts.l~ The complete removal of heterogeneous catalysts from polymers is often difficult but essential if the product is to be stable. An effective homogeneous non-catalytic hydrogenation technique is based on the use oftoluenesulphonyl hydrazide in DMF.Thermally discoloured poly(viny1 chloride) has been decolorized and substantially stabilized against further discoloration by treatment with this reagent ;I8 cis-polypentenamer could be hydrogenated more readily than the trans-isomer and without significant degradation of the polymer backbone;lg at a low degree of hydrogenation of a high trans-material an amorphous product was obtained ;complete hydrogena- tion yielded a product of 85-86 % crystallinity and a crystalline melting point of 130 "C. Polycyclohexa- 1,3-diene has also been hydrogenated. 4 Halogenation The surface of polyethylene film has been fluorinated by exposure to fluorine. Fluorination is thought to eliminate the weak boundary layer by cross-linking or at least increasing the molecular weight in the surface.20 Polyethylene crystal mats or monolayer single crystals became insoluble in xylene when fluorinated to a composition approaching C2F ;despite the essentially complete replacement of hydrogen by fluorine the appearance of the single crystal remained essentially unchanged only the a and b dimensions in the unit cell being somewhat expanded.Thus the crystal structure differs from the hexagonal symmetry of polytetra-fluorethylene.2 Poly(viny1 fluoride) film is much less readily fluorinated than polyethylene only ca. 62% of the hydrogen being replaced by fluorine,22 sug- gesting hindered-type substitution. There is a suggestion that fluorination at a C-H bond loosens the other C-H bond at the same carbon and then at an adjacent carbon but that complete fluorine substitution at the latter retards progress of the reaction.The fluorination of polyolefins is highly exothermic D. Braun and F. Weiss Angew. makromol. Chem. 1970 13 55 67. l6 Shell Oil Co. U.S.P. 3 752 767/1973. J. C. Falk Makromol. Chem. 1972 160 291. T. Nakagawa and M. Okawara J. Polymer Sci. Part A-I Polymer Chem. 1968 6 1795. l9 K. Sanui W. J. McKnight and R. W. Lenz J. Polymer Sci. Polymer Letters Edn. 1973 11 427. H. Schonhorn and R. H. Hansen J. Appl. Polymer Sci.,1968,12 123 1. " H. Schonhorn P. G. Gallagher J. P. Luongo and F. J. Padden jun. Macromolecules 1970 3 800. '' H. Shinohara M. Iwasaki S. Tsujimura K. Watanabe and S. Okazaki J. Polymer Sci. Part A-I Polymer Chem. 1972 10 2129.Post Reactions of Polymers 327 and fluorination of single crystals specifically at chain folds would require very carefully controlled conditions. However a slurry of single crystals has been ~hlorinated~~ or brominated at the chain folds under U.V. irradiation in dry-ice- acetone-cooled Freon 11 solution. The heat of fusion of the chlorinated material as prepared stays constant independently of halogen content but after melt crystallization it drops to a degree dependent on the extent of halogenation. Halogenation appears to result in a block structure consistent with halogenation occurring only at chain folds. With ‘nascent’ polyethylene halogenation also appears to occur preferentially in the non-crystalline region. N.m.r. and i.r.spectroscopic examinations of slurry-phase chlorinated (24.k 45.2 ‘x)high-density polyethylene indicate a hindered-type substitution unaffec- ted by the molecular weight degree of chlorination or residual crystallinity ;24 by contrast the chlorine distribution in chlorosulphonated polyethylene is random.25 Since the stereochemistry of the double bonds in polydienes and poly- alkenamers can be controlled to a high degree it should also be possible to direct the stereochemistry of halogen addition to these. Chlorine addition to the double bonds of high trans-and high cis-polyalkenamers has yielded partially crystalline poly- 1,2-dichloroalkenamers. Chlorine addition appears to proceed over long sequences of double bonds.26 5 Mechanical and Thermal Degradation Degradation can be brought about me~hanically,~’ and has been used for grafting.In high molecular weight polyacrylonitrile application of stress has been found to produce shifts proportionally in the i.r. absorption peaks at 1352 1247 and 1071 cm-’.28 Thermal degradation sets an upper limit to the temperature at which various polymers can be used and there has been much study of the mechanism and prevention of thermal degradation. It has also been used preparatively and for structural study. The preparation of carbon fibres from polymer fibres may involve separate pyrolytic and oxidative stages. The pyrolysis of polyacrylonitrile and of its copolymers with minor proportions of acrylates acrolein methyl vinyl ketone styrene and vinyl acetate has been studied systematically by differential thermal analysis.Polyacrylonitrile pyrolyses to a heterocyclic ladder polymer. The first three of the above comonomers can participate structurally and kinetically in the formation of the ladder structure whereas the latter two comonomers inter- rupt it.29 In air ladder-structure formation prevails up to 350°C the cyclized 23 I. R. Harrison and E. Baer J. Polymer Sci. Part B Polymer Letters 1971,9 843. 24 I. A. Abu-Isa and M. E. Myers jun. J. Polymer Sci. Polymer Chem. Edn. 1973 11 225. 25 E. G. Brarne jun. J.. Polymer Sci. Part A-I Polymer Chem. 1971 9 205. 16 G. Dall’Asta P.Meneghini I. W.Bassi and U. Gennaro Makromoi. Chem. 1973,165 83. 21 A. Casale L. R. Whitlock R. S. Porter and J.F. Julian Amer. Chem. SOC.,Div. Polymer Chem. Polymer Preprints 1971 12 496. 28 S. L. Dobretsov Vysokomol. Soedineniya (B) 1972 14 786. 29 N. Grassie and R. McGuchan European Polymer J. 1973 9 113. 328 L. P.Ellinger 1P-dihydropyridine ladder structures containing oxidizable methylene groups. As the temperature is raised further the dihydropyridine structure is either oxi- datively aromatized or oxidized to the corresponding 4-pyridone in which mole- cular motion is restrained by hydrogen b~nding.~' Porous carbon may be obtained by the complete dehydrochlorination of poly(viny1idene chloride) ;the quality of the product depends on the structure and pretreatment of the (co)polymer and on the pyrolysis conditions. The dehydrochlorination (at 150-190 "C) is subject to an induction period; up to about half the hydrogen chloride separates by an unzip reaction.31 Loss of the second hydrogen chloride with complete carbonization requires far more energetic conditions e.g.a temperature above 700 "C. The thermal degradability of PVC seriously impairs the performance of this polymer and is associated with progressive discoloration. The loss of hydrogen chloride32 by an unzipping elimination yields polyenic sequences. Initiation appears to be associated with 2-3 % of the chlorine of the PVC which alone reacts with triphenylaluminium and which is grafted cationically in the presence of certain aluminium compounds ;33 after replacement of this reactive chlorine by a stable substituent the polymer becomes appreciably more resistant to thermal degradation.Part only of the unzipping reaction appears to be pro- moted by HCl.34 From a study at 180 "C in [Me-14C]toluene and in [Me3H]- toluene (in absence of oxygen) in which both tritium and 14C were found to be incorporated in the polymer it was concluded that the initial stages of degradation involve a free-radical mechanism.35 The cause of the enhanced reactivity of part of the chlorine is obscure ;although tertiary chlorine which has been quantitatively introduced into model copolymers of vinyl chloride and 2-chloropropene does impart enhanced degradability and discoloration tertiary chlorine has not been detected spectroscopically," as it should be at a branching level of 5-6 branches per 1000 C atoms.Tertiary deuterium has not been detected by n.m.r. in PVC reduced with lithium deuteride but is easily discerned in the product obtained similarly from vinyl chloride-2-chloropropene copolymer. At least some chloromethyl branches have been detected.36 HCl elimination should be easier when the chlorine is attached to a carbon next to a tertiary rather than to a secondary CH group ;alternatively the reactive chlorine may be activated by a neighbouring double bond. The nature of the reactive chlorine is thus still unestablished. It has even been concluded from another comparison of PVC and vinyl chloride-2-chloropropenecopolymer that 0.1 mol % tertiary chlorine would suffice to account for the thermal decomposition rate of PVC.37 30 J. W. Johnson W.Potter P. G. Rose and G. Scott Brit. Polymer J. 1972 4 527. '*D. H. Davies D. H. Everett and D. J. Taylor Trans. Furuduy SOC.,1971 67 382. 32 A. Guyot and M. Bert J. Appl. Polymer Sci. 1973,17,753. 3J J. P. Kennedy and M. Ichikawa Amer. Chem. SOC.Div. Polymer Chem. Polymer Preprinfs 1973 14 677. J4 M. Carenza Yu.-V. Moiseev and G. Palma J. Appl. Polymer Sci. 1973 17 2685. 35 C. H. Bamford and D. F. Fenton Polymer 1969 10 63. 36 A. Rigo G. Palma and G. Talamini Mukromol. Chem. 1972 153 219. 3' A. R. Berens Amer. Chem. SOC., Div. Polymer Chem. Polymer Preprints 1973 14,671. Post Reactions of Polymers 329 Dehydrochlorination in PVC powder begins just above the glass transition temperature ( -70 0C).32Following some initial acceleration it proceeds with zeroth-order kinetics and an activation energy of 92 kJ mol- The changes in U.V.absorption suggest that some HCl remains dissolved in the polymer and that initially the reaction mechanism may be ionic diffusion controlled and partially reversible termination involving inter- or intra-molecular reactions.32 The kinetic patterns of decomposition by unzipping in solid polymers have been described and the derived equation is applicable to the thermal degradation of PVC.38 Techniques for the study of degradation kinetics and for the identification and even the elucidation of structural details of (co)polymers are being steadily im- proved.The importance of closely defined and reproducible conditions of decomposition has led to the refinement of resistive heater and of Curie-point pyrolysers which have then been coupled to a gas chromatograph or a mass spectrometer.Polyolefins are amongst the (co)polymers for which decomposition patterns have been determined.39 The diad distribution of copolymers has been elucidated for acrylonitrile-methyl methacrylate copolymer^.^^ Direct pyrolysis in the mass spectrometer yields characteristic patterns with poly-p-alanine~.~' 6 Photodegradation The photodegradation of polymers may but need not involve oxidation ; in some instances oxidation takes a progressively increasing share. U.V.irradiation with light of wavelength < 340 nm of PVC results in dis- coloration loss of hydrogen chloride and ~ross-linking.~~ In the absence of oxygen discoloration is preceded by bleaching.In PVC films photodegrada- tion is confined to a thin surface layer in which light-absorbing polyene sequences are formed. Initially dehydrochlorination is subject to an activation energy of 58-75 kJ mol-I and highly dependent on intensity. becoming independent of temperature and light intensity after about one hour. It is postulated that under the latter conditions HCI availability controls the reaction ;the absorbing species appears not to change in kind during this stage.43 With light of 185nm at -196 "C in uucuo an e.s.r. signal develops and has been ascribed to the breaking of a C-Cl bond.44 The initiation mechanism is obscure but may be associated with unsaturated sites or as in thermal degradation with a small proportion of C-CI sites more reactive than the rest.The development of polyenic sequences which may resemble that for thermal degradation is promoted by ferrocene '* J. D. Danforth and T. Takeuchu J. Polymer Sci. Polymer Chem. Edn. 1973 11 2083 209 1. 39 B. R. Northmore Brir. Polymer J. 1972 4 5 11. 40 Y. Yamamoto S. Tsuge and T. Takeuchu. Macromolecules 1972 5 325. 4' A. Leuderwald and H. Ringsdorf Angew. mnkrornol. Chem.. 1973 29/30 453. 42 C. A. Brighton G. C. Marks and J. L. Benton in 'Encyclopaedia of Polymer Science and Technology' ed. H. F. Mark N. G. Gaylord and N. M. Bikales Wiley-Interscience New York 1971 Vol. 14 p. 391. 4J W. H. Gibb and J. R. MacCallum European Polymer J. 1972 8 1223. 44 M. Yamamoto M. Yano and Y. Nishijima Reports Progr.Polymer Phys. Japan 1968 11 495. 330 L. P.Ellinger which is thought to react to yield a polymeric free radical and a ferrocinium cation :45 (C,H,),Fe + +CH2-CHClt + (C,H,)2Fe+ C1- + -CH2-~H+CH,-CHCl~3-,_ Carbonyl groups which are important chromophores in the context of polymer photodegradation can be introduced synthetically either into the main chain (olefinxarbon monoxide copolymers) or attached to the main chain (vinyl ketone copolymers) ; they are formed also through photo- or auto-oxidation. The main photodegradation mechanisms are (i) the Norrish I mechanism which results in free-radical products and (ii) the Norrish I1 mechanism which involves non-radical intermediates and products and is limited to carbonyl groups associated with a y-C-H ;46 sterically the carbonyl-containing segment must conform to a cyclic transition state involving a y-CH group (see Scheme 1).The quantum efficiencies at 25 "C for both mechanisms are listed in Table 1 for methyl ketones having second alkyl 0..... . .. . H // \ R~-CO-CR:-CR:-CHR 3 R~C ,CR: \ CRi-CRj Yo + RiHC-C + CR:=CRi \ R2 Scheme 1 Table 1 R' @l a2 c3 0.023 0.2 1 c 0.003 0.2 1 c7 0.002 0.21 c9 0.0003 0.22 poly(methy1 vinyl ketone) 0.04 0.025 poly(ethy1ene-carbon -0.025 monoxide) copolymer groups R' of various lengths in solution47 and for two carbonyl copolymer^.^^ It is notable that in the copolymers an appreciable quantum efficiency applies to 45 A. W. Birley and D. S. Brackman presented at a conference on 'Degradability of Polymers and Plastics' The Plastics Institute London 1973 preprint 1 /4.46 J. E. Guillet J. Dhanraj F. J. Golemba and G. H. Hartley 'Stabilization of Polymers and Stabiliser Processes' in Adv. Chem. Ser. Amer. Chem. Soc. Washington 1968 No. 85 p. 272. 47 F. J. Golemba and J. E. Guillet Macromolecules 1972 5 63. Post Reactions of Polymers 33 1 the Norrish I mechanism only where the carbonyl groups are attached to the polymer main chain. The quantum efficiencies of the Norrish I1 mechanism of carbonyl-containing polymer [poly(phenyl vinyl ketone)] and copolymer (styrene-phenyl vinyl ketone and methyl methacrylate-methyl vinyl ketone) films increase sharply above the glass transition to the value observed in solution.48 Below the glass transition temperature the quantum efficiencies for the copolymers decrease further with temperature reaching zero at about -150"C.At ambient temperature the non-propagating Norrish I1 reaction makes substantially the major contribution to the photodegradation of carbonyl- containing polymers. Using triplet quenchers triplet carbonyl lifetimes of -1 x lO-*s have been determined for solid and dissolved ethylene-carbon monoxide copolymer ;49 in this polymer in the absence of oxygen 45 % of the Norrish I1 reaction was thought to occur through the triplet state. The Norrish I reaction is subject to a greater activation energy (-21 kJ mol- for pentadecan-8-one 2 1.7 kJ mol- for ethylene-carbon monoxide copolymers) and is much more sensitive to viscosity than the Norrish I1 reaction (3.55 kJ mol- for pentadecan-8-0ne).~~*~'~ In smaller carbonyl compounds the quantum efficiency of the Norrish I reaction increases below -350 nm sharply with decreasing wavelength whereas that of the Norrish I1 reactions is almost unaffected ;this effect has not apparently been studied in polymers.The Norrish I reaction can initiate through its free-radical products subsequent auto-oxidation (Photo-oxidation p. 329). The Norrish I1 reaction is at least partially quenched by oxygen much more efficiently for methyl methacrylate- methyl vinyl ketone than for styrene-methyl vinyl ketone or ethylene carbon monoxide50n copolymer. Quenching affects mainly the triplet excited state of the carbonyl group which is thought to contribute appreciably to the Norrish I1 reaction only at low oxygen concentration^,^' the singlet excited carbonyl making the major contribution to the reaction under normal conditions.When quenching the triplet carbonyl oxygen is raised to the singlet excited state which may contribute to subsequent photo-oxidation. Owing to their sensitivity to sunlight carbonyl-containing polymers provide an approach to photodegradable plastics. It is usually not practicable to produce plastics in which photodegradability is built into the main component which has to be carefully and specifically designed for particular applications. But if the carbonyl- containing polymer can impart photodegradability to a base polymer it can then be used as effective additive.Styrene-methyl vinyl ketone or especially isopropenyl vinyl ketone copolymers appear to be more efficient in this respect than ethylene-carbon monoxide copolymer which correlates with the higher Norrish I quantum efficiency of the copolymers containing a pendant carbonyl 48 E. Dan and J. E. Guillet Macromolecules 1973 6 231. 49 M. Heskins and J. E. Guillet Macromolecules 1970 3 224. '' G. H. Hartley and J. E. Guillet Macromolecules 1968 1 (a) 165 (6) 413. 5' A. M. Trozzolo and F. H. Winslow Macromolecules 1968 1 98. 332 L. P.Ellinger the Norrish I reaction initiating auto-oxidation. It is not known to what extent triplet-carbonyl-photosensitized singlet oxygen contributes to the imparted photo-oxidation.Other types of polymers whose photodegradability in sunlight may be con- trolled are based on the reversible photochemical dimerization of cinnamic acid and its derivatives to the corresponding truxillic acid products. Examples include the poly(viny1 esters) of substituted cinnamylideneacetic acid5’ and polyamides made from diphenyl-a-truxillic or diphenyl-6-truxillic acid.53 7Oxidation In the absence of oxygen high-energy radiation causes loss of hydrogen cross- linking and some double-bond formation in polyolefins ; mercury-lamp U.V. light is sufficiently energetic for a similar effect in polystyrene. In chain-folded polyolefin domains the cross-linking appears to be mainly between neighbouring folds since the effect on solubility and stress-strain characteristics is smaller than can otherwise be readily explained.54 Fuming nitric acid oxidizes crystalline polyethylene at the chain folds with the formation of carboxylic acid groups.The molecular weights of the dicarboxylic acids produced reflect the fold lengths. This oxidation has been used for the preparation from crystalline polyethylene of chain-extendable C oo dicarboxylic acid. Selective nitric acid oxidation of crystalline polyethylene irradiated with up to 6 MJ kg-y-radiation yields dicarboxylic acids of various chain lengths. Gel-permeation chromatography indicates the absence of any cross-linked material in the product; double bonds due to the y-irradiation are also oxidized to carboxylic acid but are randomly distributed.These conclusions are further borne out by a comparison of the effect of y-irradiation upon polyethylenes of varying crystallinity. 56 Auto-oxidation and photo-oxidation are important degradative reactions limiting the useful life of plastics. Polypropene and poly-4-methylpent- 1-ene are particularly prone to oxidative degradation and much effort has been devoted to a better understanding of the degradation mechanisms and the formulation of efficient antioxidant systems. Recently the promotion of photo-oxidation has become an objective with a view to making photodegradable plastics (see p. 331). Access and absorption of light of the appropriate wavelengths is essential to photo-oxidation ; the nature of the oxidation reactions depends on oxygen availability at the site of oxidation.In polymers the solubility and diffusion rate of oxygen are usually lower in crystalline than in amorphous domains. In amorphous polyethylene the solubility is similar to that in alkanes ;it increases with branching. Oxygen is regarded as insoluble in and non-diffusing through the folded lamellar 52 H. Tanaka and Y.Sato J. Polymer Sci. Part A-I Polymer Chem. 1972 10 3279. 53 H. Takahashi M. Sakuragi M. Hasegawa and H. Takahashi J. Polymer Sci. Part A-I Polymer Chem. 1972 10 1399. 54 R. Salovey and A. Keller Bell Syst. Tech. J. 1961,40 1397 1409. 55 D. G. H. Ballard and J. V. Dawkins European Polymer J. 1973,9 21 1. 56 A. Keller G. N. Patel and H. Keller presented at the Polymer Phys. Gr. Biennial Meeting R. M. C.Shrivenham September 1973 Preprint; G. N. Patel and A. Keller J. Polymer Sci. Polymer Letters Edn. 1973 11 737. Post Reactions of Polymers 333 domains of p~lyethylene.~' In polyolefins the difference of density between amor- phous and crystalline domains decreases with increasing substituent size the structure of the crystalline domain becoming more open. Crystalline atactic polypropene has a folded lamellar structure ; the crystalline domain of poly-4- methylpent-1-ene is helical and below 50 "C slightly less dense than the amor- phous domain.58 The differences between the domains as regards oxygen solu- bility and diffusion would be expected to decrease with increasing substituent size but detailed information is not available. Like oxygen antioxidants appear to concentrate in the amorphous domains of p~lyethylene.'~ In polyolefins the size of the crystalline structures may itself be affected by oxidation.The size of spherulites in polypropene has been reported as increasing during oxidation at 190"C.This effect did not occur in the absence of oxygen nor in the presence of antioxidants.60 The scattering of light by crystallites increases with decreasing wavelength. Light of -350 nm penetrates only a few microns into polyethylene polypropene or poly(ethy1ene terephthalate) films. The extent of photo-oxidation in terms of carbonyl formation measured by attenuated total reflection i.r. spectroscopy and of surface damage seen by electron microscopy declines sharply from the surface into the interior of polypropene6' and poly(ethy1ene terephthalate) film;62 it is accompanied by changes in the surface which develops cracks under stress.The effect has been ascribed to the attenuation of the light of shorter wavelengths rather than to inadequate oxygen diffusion. Auto-~xidation~~ share the same free-radical propaga- and photo-~xidation~~ tion and termination mechanisms but differ in respect of initiation. In polyolefins transition metal-ions their ligands and anions may affect every one of these mechanisms. Some insight into the complicated effects observed is being gained grad~ally.~~.~~ Auto-oxidation by non-radical pathways has been suggested with organo- metallic compounds capable of complexing both with oxygen and with the sub- strate.This may possibly occur with natural polymers but at present there is no supporting evidence concerning synthetic polymer^.^' In polyolefins initiation of auto-oxidation comprises the following distinct steps the formation (i) of the propagating alkyl and hydroperoxyl radicals (ii) of 57 A. S. Michaels and H. J. Bixler J. Polymer Sci. 1961 50 393 413. 58 C. E. Wilkes and M. H. Lehr Macromol. Sci. (B) 1973,7,225. 59 D. A. Curson Proc. Roy. Microscop. SOC.,1972,7 96. 6o J. Balteniene and R. Baltenas Polim. Muter. Ikh Issled. Muter. Respub. Nauch Tekh. KonJ 12th 1971 14 (Chem. Abs. 1973,78 125 077). 6' P. Blais D. J. Carlson and D. M. Wiles J. Polymer Sci. Part A-I Polymer Chem. 1972 10 1077. '* P. Blais M. Day and D. M. Wiles J. Appl. Pol-vmer Sci.1973 17 1895. " L. Reich and S. S. Stivala 'Auto-oxidation of Hydrocarbons and Polyolefins' Dekker New York 1969; 'The Mechanism of Pyrolysis Oxidation and Burning of Organic Materials' ed. L. A. Wall Nat. Bur. Stand. Washington Special Publication No. 357 1972. 64 (a) K. Tsuji Adv. Polymer Sci.,1973 12 131; (b)0.Cicchetti ibid. 1970 7 70. " 0. Cicchetti and F. Gratani European Polymer J. 1972 8 561. 66 0. Cicchetti R. De Simone and F. Gratani European Polymer J. 1973 9 1205. 67 J. P. Vollman Accounts Chem. Res. 1968 1 136. 334 L. P.Ellinger hydroperoxide and possibly some peroxide and (iii) the homolysis of hydro- peroxide or peroxide to propagating free radicals. At ordinary temperatures the formation of hydroperoxide is very slow.The oxidation of 2,2,4-trimethylpentane appears to proceed by a bimolecular initiation step involving the formation of radical species RH + 0 -+ R-+ HO .68 The rate constants have also been determined and estimated for secondary and tertiary C-H groups. During the processing of antioxidant-free polyolefins carbonyl groups are formed progre~sively,~~ presumably from hydroperoxide. Photo-oxidation initiation mechanisms may include the following (1) Formation of reactive species following light absorption by a polymer- oxygen charge-transfer complex (p. 323). (2) Photolysis of pre-formed hydroperoxide or peroxide. (3) Photodegradation of pre-formed carbonyl groups by the Norrish I mechanism leading to reactive free-radical products (p.330). (4) Formation of singlet oxygen from ground-state oxygen molecules quenching pre-formed excited singlet carbonyl groups or other photosensitizer and reaction of the singlet oxygen with p~lymer.~.~' A problem is the low excitation energy (-109 kJ mol-') of singlet oxygen but various reactions observed with small unsaturated molecules (olefins 1,3-diene~)~ ' should occur with polymers even though efficiencies may be small. (5) Below a limiting concentration Ti'" compounds which are often present as polymerization catalyst residues have been shown to catalyse auto-oxidation of atactic and isotactic polypropene and of 2,4,6,8-tetramethylnonane7 becoming inhibitors at higher concentrations. The initiation is formulated66 ri = ki[RH] [O,] [Ti4+] As in smaller alkanes the propagation of auto-oxidation usually involves the two stages R'.+02+ R'O,. R'O,. + R2H + R102H + R2. the latter being rate-determining. In alkanes and alkenes the bond energy of the least stable C-H bond is closely related to the propagation rate constant7 and subject to an activation energy of 42-84 kJ mol-'. Similarly the longest wave- length of light capable of effecting photodegradation in terms of a 'damage index' corresponds often to the strength of the weakest C-H or C-C bond in the polymer molecule ;7 this may reflect on the propagation rather than on the initia- tion reaction. 68 T. G. Degtyareva L. N. Denisova and E. T. Denisov Kinetika i Kataliz 1972 13 1400. 69 D. Mellor A. Moir and G.Scott European Polymer J.1973,9,219; G. V. Hutson and G. Scott Chem. and Ind. 1972 725. 70 M. L. Kaplan and P. G. Kelleher J. Polymer Sci.,Pari B Polymer Letters 1971,9 565. " D. R. Kearns Chem. Ret.. 1971 71 395. 72 S. Korcek J. H. B. Chenier J. A. Howard and K. U. Ingold Canad.J. Chem. 1972,50 2285. 73 G. V. Stephenson B. C. Moses and W. S. Wilcox J. Polymer Sci.,1961 55 451. Post Reactions of Polymers 335 In solid polymers the first propagation stage is slowed by restricted oxygen access and by the high viscosity so that termination by recombination becomes relatively important. Detailed kinetic data are not available. Under typical photo-oxidation conditions (20 "C)the propagation reaction should be very much slower absolutely and relative to termination (being subject to a much lower activation energy) than under the higher-temperature conditions characteristic of auto-oxidation.The comparative ineffectiveness of antioxidants in preventing photo-oxidation has been explained in these terms.64b It may also be due to photo-sensitization by the antioxidant or products derived from the latter such as dienone peroxides.74 1,2-Epoxides and 1,3-peroxides between tertiary carbons are formed during the auto-oxidation of polypropene the peroxyl radicals abstracting a hydrogen either from the neighbouring carbon atom or from the weaker tertiary C-H bond. The hydroperoxides introduced into polyolefins are often thermally stable below 100 OC,but are photolysed at wavelengths <340 nm with quantum efficien- cies of 0.7-1 to yield two radicals.This degenerative chain-branching is likely to contribute more to photo-oxidation than to auto-oxidation. The decomposition of the hydroperoxides to products including free-radical species is greatly pro- moted by many transition-metal ions. Alkoxyl radicals are thus formed. The secondary alkoxyl radicals derived from small molecules are subject to dis- sociative propagation which yields an aldehyde and an alkyl radical that of tertiary alkoxyl radicals a ketone and an alkyl radical. These reactions are of interest as carbonyl-forming reactions ;they may be of limited importance only in polymers since introduction of hydroperoxide groups into polypropene by initiated auto-oxidation was found to be unaccompanied by keto-carbonyl and alcoholic hydroxy-group~,~~ presumably owing to the size and viscosity of the polymer molecules ;the matter is however controversial.Termination between two free-radical sites is favoured by the high viscosity of solid polymers assisting the cage effect. It is subject to a much lower activation energy than the propagation reaction. Transition-metal ions also interact with free-radical sites with electron transfer resulting in termination.66 Photo-oxida- tion and auto-oxidation must be regarded as complementary. As regards individual polymers the information regarding polyethylene is still rather incomplete ;owing to the lack of penetration of U.V. light some of the more important work has been on films. The main chain-breaking reaction fol- lows the formation of keto-groups and proceeds mainly by the Norrish I1 mechan- ism.A comparison of the photo-oxidation of branched and linear polyethylene has shown that whereas under daylight conditions branched polyethylene takes up oxygen much more rapidly than linear polyethylene it embrittles more slowly and '4 L. V. Samsonova V. I. Gol'denberg E. V. Bistritskaya G. A. Nikiforov and V. Ya. Shlyapintokh Mitt. Chem. Forschungsinst. Wirt. Oesterr. Oesterr. Kunststofinst 1972 26 242 (Chem. Ah. 1973 78 148 562). 75 c.R.Boss H. Jabloner and E. J. Vandenberg J. Polymer Sci. Part B Polymer Letters 1972 10 915. 336 L. P. Ellinger maintains appreciable elongation and complete solubility to much higher oxygen- uptake levels.76 While auto-oxidation occurs mainly in the amorphous region it is accompanied by some increase in lateral order ascribed to the breaking of interlamellar bonds.Chain breaking may be followed by further crystallization which in turn results in a progressive increase in fusion temperature. With polypropene the formation of hydroperoxide occurs rather easily owing to the weak tertiary C-H bonds; photolysis of this hydroperoxide leads directly to main-chain breakage through the intermediate t-alkoxyl radical (p. 335) with the formation of a penultimate carbonyl. The subsequent Norrish I1 reaction eliminates only a small segment replacing it by a double bond (Scheme 2). Main-chain breakage occurs thus at an earlier stage of the sequence than with poly- ethylene.1 0 FH2 II fCH 3 + CH,-C-CH Scheme 2 The photo-oxidation of polystyrene by light of wavelengths >280 nm is now believed to involve the absorption band of the polystyrene-oxygen charge- transfer complex which in solution and possibly also in films disintegrates upon photoabsorption with formation of singlet oxygen. To account for the yellowing during photo-oxidation it is postulated that the benzene ring is opened by reaction with singlet or triplet oxygen to yield without main-chain fracture the diene dialdehydes corresponding to hexa-2,4-diene- I ,6-dial which is known to be one of the photo-oxidation products of ben~ene.~ Photoisomerization with formation of fulvene and benzvalene groups may contribute to the yellowing.Aromatic rings absorb in the range 25@-280 nm and are raised to the singlet excited state. Following intersystem crossing to the triplet state several reactions are thought to be possible (a)fracture of the bond linking the aromatic ring to the main chain; (b)fracture of main-chain C-C or C-H bonds; (c) quenching by oxygen which is raised to the singlet excited state followed by reaction of singlet oxygen to form main-chain hydroperoxide or to abstract hydrogen from the main chain. 7h F. H. Winslow W. Matreyek E. P. Otocka and P. M. Muglia Helsinki IUPAC 1972 Preprint IV-38. Post Reactions of Polymers 8 Graft Copolymerization The grafting of a polymer backbone with branches formed by the polymerization of one or several monomers has been studied and technically exploited over many years.The grafts are usually selected to differ in their physical properties from those of the backbone and the products may have improved properties and a widened range of ~ompatibility.~~ The main problems in graft polymerization still concern the homogeneity of the product [its freedom from ungrafted backbone and branch (co)polymers] and the separation and identification of the copolymer. The value of much published work and claims for graft polymerization are often impaired by the method used for product isolation. In particular single-stage batch solvent extraction and precipitation unchecked by fractionation with determination of the homogeneity of fractions is not regarded as satisfactory in supporting quantitative claims regarding the grafting.Fractionation followed by degradation is an essential feature of many st~dies.~~,~' The attachment of the graft to the backbone can either be part of the initiation or of the termination step of graft growth; either can fail owing to transfer to monomer resulting in ungrafted polymer. Grafting can involve condensation step or addition polymerization. When the graft is initiated from the polymer backbone and proceeds by addition polymerization reactive sites must be created on the backbone by reagents which are either not themselves active initiators or which are completely consumed before the monomer is introduced. Quite efficient techniques involving free- radical cationic and anionic mechanisms have been reported recently.Radical- initiating sites have been attached to cellulose by converting it into amino- aromatic esters or ethers ;diazotization and reaction of the diazonium salt with ferric chloride created aromatic free-radical sites to which acrylic and methacrylic monomers and vinylpyrrolidone were grafted." Cellulose is activated by water to be grafted during initiation by acrylic and methacrylic esters without added initiator but some homopolymer is formed especially in the presence of carbon tetrachloride.8 Pairs of donor (styrene isoprene) and acceptor (maleic anhydride acrylates acrylonitrile) vinyl monomers often form alternating copolymers. The copoly- merization may be aided by complexing with an electrophilic inorganic or metal- organic additive (ZnC1 ,MgCI ,NiCl ,lithium halide or alkylaluminium halide) and may not (sometimes above a limiting temperature) require a free-radical initiator.According to Gaylord,82 termination involves either a unimolecular or a bimolecular step ;the bimolecular step proceeds through a carbene (involving a " V. Stannett J. Macromol. Sci. Chem. 1970 A4 1177. '13 J. P. Fischer Angew. makromol. Chem. 1973 33,35. Z. A. Rogovin J. Polymer Sci.,Part C Polymer Symposia 1972 37 221. C. I. Simionescu and S. Dumitriu J. Polymer Sci. Part C Polymer Symposia 1972,37 *' 187. M. Imoto K. Takemoto and T. Otsu Makromol. Chem. 1967 104,244. '' N.G. Gaylord J. Polymer Sci. Part C Polymer Symposia 1970 31,247. 338 L. P.Ellinger rearrangement of the propagating site) to yield either a double bond or by insertion a six-membered ring.If a polymer containing reactive C-H bonds (tertiary aliphatic C-H allylic C-H or aldehyde C-H) is added to the alternat- ing copolymerization grafting occurs presumably by termination involving insertion of the intermediate carbene into the reactive C-H group of the back- bone polymer. Grafting-site formation during initiation however cannot be excluded. Grafting on to polystyrene butadiene-acrylonitrile copolymers poly(buty1 acrylate) low-density polyethylene p~lypropene,~ or cellulose has been described. In this work batch extraction-precipitation techniques were used ;the amount of non-grafted copolymers and the characterization of the graft copolymers are not firmly established.An elegant cationic grafting technique has been developed84 in which polymers containing reactive tertiary aliphatic chlorine react in solution (ethyl chloride dichloroethane or chlorobenzene) with AlEt or AlEt,CI which are themselves not active initiators under the conditions. A cationic site is formed on the polymer P which is grafted by a cationically polymerizable monomer P-Cl + AIEt + P+ AIEt,Cl-P+ + M -+ PM+ % graft copolymer About 2-3 % ofthe chlorine contained in poly(viny1 chloride) could be replaced by short polyisobutene or polybutadiene grafts with a dramatic improvement of its thermal stability (p. 328),84 suggesting that the poor thermal stability is associated with the most reactive chlorine. Chlorinated butyl rubber and chlorinated ethylene-propene rubber have been grafted similarly with poly~tyrene.~’ If transfer to monomer to which cationic polymerization is prone can be limited graft copolymers essentially free from homopolymer should be obtainable.A few sufficiently large grafts would yield products approaching ABA block copolymers in structure and physical proper- tiesE5 A related reaction is the phenylation of PVC by triphenylaluminium with replacement of the reactive chlorine by a phenyl substituent also with marked improvement of thermal stability.33 Since anionic polymerization can often be conducted under conditions for which transfer and termination are largely eliminated ;anionic grafting appears attractive but it is subject to experimental difficulties attending the placing of initiating sites on the backbone polymer and the choice of solvent.Monoethyl malonate derivatives of cellulose have been metallated and grafted with acrylo- nitrile.86 Polyamides are quite readily metallated and grafted anionically by 83 N. G. Gaylord A. Takahashi S. Kikuchi and R. A. Guzzi J. Polymer Sci.,Part B Polymer Letters 1972 10 95. 84 N. G. Thame R. D. Lundberg and J. P. Kennedy J. Polymer Sci.,Part A-1 Polymer Chem. 1972 10 2507. J. P. Kennedy J. J. Charles and D. L. Davidsuri Amer. Chem. Soc. Diu.Polymer Chem. Polymer Preprints 1973 14 974; J. P. Kennedy and R. R. Smith ibid. p. 1069. 86 C. I. Simionescu and V. Rusan J. Polymer Sci.,Part C Polymer Symposia 1972 37 173. Post Reactions of Polymers 339 suitable monomers ; poly(ethy1ene oxide) grafts have then been attached to Nylon-6." An interesting possibility in graft copolymerization is that the backbone poly- mer may exert more influence -a kind of template effect -upon the polymeriza- tion of the graft than is provided by the initiating site.Polyvinylidene grafts attached to oriented amorphous poly(methy1 methacrylate) or crystalline polyethylene are both stated to yield heterogeneous films. The supermolecular structure of the polyvinylidene blocks in the polyethylene copolymer is stated to be crystalline and oriented whereas those in the poly(methy1 methacrylate) copolymers are described as amorphous and isotropic. The grafted copolymer is claimed to have increased strength and thermal stability compared with the backbone polymers." There is a great deal of work on radiation grafting.Isotropic low-density polyethylene film swollen with acrylonitrile is grafted upon y-irradiation in the amorphous domains ; on stretching the grafts concentrate in the interfibrillar spaces.89 Graft copolymerization of natural polymers (cellulose wool silk starch) with acrylic monomers or with cyclic monomers such as ethylene sulphide has been studied extensively. It improves crease resistance dimensional stability dye acceptance and resistance to degradation of fibres.g0 The grafting of hydro- phobic non-polar vinyl polymers with hydrophilic or polar monomers results in products which can be used in semi-permeable membranes of value in liquid separation processes and in improved adhesives and films.Better control over the number and size of grafts and over homopolymer in the product should lead to products closely related in structure and properties to block copolymers and liquid rubbers. 9 Polymers as Supports General.-Organic polymers are used increasingly as supports for instance in sequential syntheses (proteins nucleotides polyols oligosaccharides) or for groups which have catalytic photosensitizing antioxidant or pharmaceutical activity. A major advantage is the gain in ease and efficiency with which the supported species is separated. Activity specificity and selectivity are frequently greater for supported than unsupported species and with pharmaceuticals a depot effect providing more extended and uniform dosage is sought.Polymers supporting active groups may be made by polymerization of the appropriate monomers or the supported group may be introduced into pre- formed polymers. Some examples of the latter and some of the reactions in which the supported groups take part will be discussed. '' T. Yamaguchi J. Maezawa E. Sasaki and M. Kawamoto Kobunshi Kuguku 1973,30 331. '' A. I. Kurilenko L. P. Krul V. I. Gerasimov and V. N. Kalinin Dokfudy Akad. Nuuk S.S.S.R. 1973,209,648 (Chem. Abs. 1973,79 54008). 89 A. I. Kurilenko L. P. Krul V. I. Gerasimov and N. F. Bakeev. Doklady AkQd. Nuuk S.S.S.R. 1973 209 144 (Chem. Abs. 1973,79 19 275). 90 'Block and Graft Copolymerisation' ed. R. J. Ceresa Wiley 1973 Vol. 1. 340 L.P.Ellinger Polymeric Catalysts.-Various polymers including especially commercially available macroreticular styrene-divinylbenzene copolymer beads have been used as supports ; certain ion-exchange resins have proved suitable. Inorganic polymers are however often preferred since they may provide better thermal stability and control over pore size more stable bonding to active groups and greater ease of separation. Metals catalytically active in hydrogenations hydroformylations and cyclo- oligomerizations have been attached to chloromethylated ‘Amberlite’ ion- exchange re~in,’~.’~ to cross-linked chloromethylated styrene-divinylbenzene copolymer,’ 1,93-’8 or to polystyrene ring-br~minated’~ by Br,-FeBr through complexing diphenylphosphine or imidazole groups.” The extent of cross-linking” and the bead size96 of supports based on styrene-divinylbenzene copolymer may be varied.The ligand may be attached to halogen-containing polymers before the metal is introduced. The diphenylphosphino-group may be attached using lithi~rn’~ or potassium diphenylphosphine,’ or by lithiation of aromatic bromine groups followed by reaction with Ph,PCl. The metal may then be attached from its complex (palladium rhodium) halide (cobalt nickel) carbonyl or other derivative.’ 1,997100 Rhodium has also been attached by equilibration of the phosphine-substituted polymer with metal bearing a tri- phenylphosphine ligand,93-’s provided the latter is sufficiently readily displaced. Ligand and metal together e.g.RhCl(C0){ Ph2PCH,CH2Si(OEt),} or RhH(C0){ Ph,PCH,CH,Si(OEt),) ,,have been attached to halogen-substituted polymer by displacement of the halogen.” Palladium-(0) and -(II) and platinum- (11) have also been complexed to ‘Amberlite XAD-4’ supported diphenylphos- phine ligand~.~~.~~ Up to 20 % titanocene (TiCp,Cl,) groups have been attached to chloromethylated styrene-divinylbenzene copolymers by a three-step synthesis.’* Imidazole has been attached directly and following condensation with iron rneso-tetraphenylporphyrin and reaction with carbon monoxide a product regarded as a model for deoxymyoglobin was ~btained.’~ Polystyrene reacts with chromium hexacarbonyl to yield a product in which 32 % of the benzene rings carry Cr(CO) groups ;the reaction is not accompanied by degradation so that from a narrow molecular weight distribution polystyrene a product ofnarrow molecular weight distribution could be obtained.Copolymers 91 K.G. Allum R. D. Hancock S. McKenzie and R. C. Pitkethly ‘Proceedings of the 5th International Congress on Catalysis Miami Beach 1972’ p. 477. 92 H. Bruner and J. C. Bailar jun. horg. Chem. 1973 12 1465. 93 R. H. Grubbs and L. C. Kroll J. Amer. Chem. SOC. 1971,93 3062. 94 J. P. Collman L. S. Hegedus M. P. Cooke J. R. Norton G. Dolcetti and D. N. Marquardt J. Amer. Chem. SOC. 1972,94 1789. ” R. H. Grubbs L. C. Kroll and E. M. Sweet J. Macromol. Sci. Chem. 1973 A7 1047. 96 M. Capka P. Svoboda and J. Hetfejs Coll. Czech. Chem. Comm. 1973,38 1242. 97 J. P. Collman and C.A. Reed J. Amer. Chem. SOC. 1973,95,2048. ’* R. H. Grubbs C. Gibbons L. C. Kroll W. D. Bonds jun. and C. H. Brubaker jun. J. Amer. Chem. SOC. 1973 95 2373. 99 N. Higara S. Takahashi and Y. Nonaka Japan Kokai 1973,13,487 (Chem.Abs. 1973. 78 148 481). loo M. Kraus Chem. listy 1972 66 1281 (Chem. Abs. 1973 78 72 607). Post Reactions of PoIymers 341 carrying the same group were obtained also by free-radical copolymerization of styrenechromium tricarbonyl with styrene or methyl acrylate. lo' Poly-2-vinylpyridine has been used as complexing polymeric support for cobalt@) iron-@) and -@I) nickel(II),lo2 and rhodium. O3 Cations or anions containing catalytically active metals have been supported directly on ion-ex- change resin. Thus tungstate anions have been supported on the quaternary ammonium ion-exchange resin 'Amberlite-IRA 400'.O4 Polymerically supported metal catalysts of a different type are those of the lower alkali metals especially of lithium. They can be made by the reaction usually in an ether solvent of an alkyl derivative of the metal (butyl- or pentyl- lithium) with a polymer containing a sufficiently reactive hydrogen e.g.styrene-p-benzylstyrene c~polymer,'~~ or a reactive halogen ;lo6 with butyl-lithium in tetramethylethylenediamine even the aromatic rings of polystyrenelo7 or poly(pheny1ene oxide)"' are lithiated. The metallated polymer is an anionic initiator and a metallating agent. Polymers as Catalysts.-Catalytically active polymers include ion-exchange resins which are used technically in esterifications and dehydrations.Most of the transition-metal catalysts which have been supported on polymers are -without such support -homogeneous catalysts for hydrogenation hydro- formylation cyclo-oligomerization acetoxylation silylation oxidation or polymerization reactions. Many supported 'homogeneous' hydrogenation and hydroformylation catalysts retain their activity sati~factorily,~~ but others for instance Pdo and Pd" supported on cross-linked polystyrene through di- phenylphosphine ligands lose their catalytic activity for the dimerizing addition of trimethylsilanol to butadiene to yield l-trimethylsiloxy-octa-2,7-dienequite quickly. The palladium complex becomes soluble presumably owing to a competition between ligand and product for the metal.96 Triphenylphosphine also removes the metal from its support. In hydrogenations advantages over the corresponding unsupported catalysts (Ru Rh Pd or Pt) include greater activity and improved specificity. The greater activity is ascribed to the inability of the supported catalytic sites to associate as readily as in the absence of The greater activity ofsupported reduced titanocene dichloride as hydrogenation catalysts for olefins and acetylenes is also ascribed to this.99 The supported catalysts show selectivity for the smaller olefin molecule in dependence on the pore size of the cross-linked support. The effective pore size lo' C. U. Pittman jun. P. L. Grube 0.E. Ayers S. P. McManus M. D. Rausch and G. A. Moser J.Polymer Sci. Part A-I Polymer Chem. 1972 10 379. lo* H.-G. Biedermann E. Griessl and K. Wichmann Mukromol. Chem. 1973,172,49. Io3 L. D. Rollmann Inorg. Chim. Acta 1972 6 137. '04 G. G. Allan and A. N. Neogi J. Catalysis 1970 19 256. '05 M. Kikuchi and T. Kakurai Kobunshi Kagaku 1972 29 911 (Chem.Abs. 1973 78 85 012). '06 M. L. Hallensleben Angew. makromol. Chem. 1973,31 147. lo' A. T. Bullock G. C. Cameron and P. M. Smith Polymer 1973,14,525. lo* A. J. Chalk and A. S. Hay. J. Polymer Sci. Part A-I Polymer Chem. 1969 7,691. 342 L. P.Ellinger in terms of improved selectivity is at 740 pm only 5-10 % of that measured by other techniques. Polar solvents enhance this selectivity effect more than non- polar solvents through reducing effective pore size.95 Activation of supported palladium hydrogenation catalyst by alcohols has been noted.92 Polymer-supported rhodium hydroformylation catalysts have been described.' ' A polymer-supported nickel catalyst has oligomerized phenylacetylene to a mixture containing mainly 1,2,4- and 1,3,5-triphenylbenzenes." The epoxidation of maleic acid on unsupported tungstate and on tungstate supported on a quaternary ion-exchange resin has been compared ;'O4 on the supported catalyst the kinetics appear to be those for a diffusion-controlled catalyst-lined pore model.The possibility of controlling the electronic and steric environment of the co-ordinated catalyst site particularly effectively in supported catalyst both through the polymer backbone and when the metal is attached by more than one ligand through control of the distance between and the relative position of the ligands has been recognized," but has not yet been studied in depth.Certain polymers supporting an alkali metal are in suitable solvents such as aliphatic ethers or aliphatic ditertiary amines initiators of anionic polymerization. They can initiate without transfer so that the newly forming polymer is linked to the supporting polymer. 'O5 Starting from suitably metallated supports graft or block copolymers of quite highly specified structure and composition are at least potentially accessible (p. 341). Esterolytic enzyme function has been related to the imidazole group of histi- dine. Imidazole and other small molecules containing the imidazole group have the same activity but at a much lower level.The factors responsible for the much greater activity of imidazole groups in at least some polymers have been studied extensively' O9 on amongst others poly-4(5)-vinylimidazole,poly-2(N)-methyl-5-vinylimidazole poly-5(6)-vinylbenzimidazole poly-N-vinylimidazole poly-2-methyl-N-vinylimidazole polyethyleneimine backbone containing a lateral N-dodecyl groups supporting imidazole rings ' ' and amongst related polymers poly-3-vinyl- 1,2,4-triazole. Copolymers have been made with vinyl alcohol p-vinylphenol acrylic- maleic- or vinyl-sulphonic acid vinylimidazolium salts,' ''vl ' and vinylpyrroli- done113 designed to introduce into the imidazole polymer substituents of a different charge type and to study the effect of this upon the pattern of the catalysis kinetics.These polymers were made by free-radical polymerization under conditions which do not impart stereoregularity ;amongst related polymers this '09 C. G. Overberger and J. C. Salamone Accounfs Chem. Res. 1969 2 217. H. C. Kiefer W. I. Congdon I. S. Scarpa and I. M. Kletz Proc. Nut. Acud. Sci.U.S.A. 1972,69,2155. J. C. Salamone B. Snider S. C. Israel P. Taylor and D. Raia Amer. Chem. SOC.,Div. Polymer Chem. Polymer Preprints 1972 13 271. l1 ' C. G. Overberger and T. J. Pacansky Amer. Chem. SOC.,Div. Polymer Chem. Polymer Preprints 1973 14 766. K. Uehara Y. Kaji M. Tanaka and N. Murata Kobunshi Kugaku 1973,30,165 (Chem. Ah. 1973 79 5761). Post Reactions of Polymers has been achieved with poly-1-vinyluracil.' l4 The effect of stereoregularity upon catalytic efficiency and pattern appears not to have been studied yet but should be interesting.The catalytic behaviour of derivatives of imidazole is explained by its ampholy- tic character. Three species differing in basicity and connected by two equilibria are involved (see Scheme 3).l1 Their relative proportions and their contributions g1 Scheme 3 to esterolytic reactions depend on the pH and on the substrate. To study these contributions the esterolyses of phenyl acetate substrates of different charge type have been studied the most important being compounds (3)-(6).'09 The less hydrophilic C8 and CI3acyl ester analogues of these have also been used as sub- Me Me Me Me I I I I c=o c=o c=o C=O I I I I +\I$?$ \ \ ' h(CH,) I-NO2 c0,-SO3-Na+ (3) PNPA (4) NABA (5) NABS (6) ANTI strates.'I2 The polymers are insoluble in either pure ethanol or water; binary mixtures of water and (a minor proportion of) methanol ethanol or propanol are used as solvents.The effect of the fraction of the catalytically active charge species upon the rate of esterolysis has been studied in detail for both monomeric and polymeric species. For small molecules this activity increases essentially proportionally with the active imidazole fraction but for the polymeric species much more complex patterns are observed. For instance for PNPA esterolysis by poly-1-vinylimida- zole the catalytic rate constant is below that for imidazole up to a certain fraction but at higher fractions esterolysis by the polymer becomes much faster than that by imida~ole.'~~ Esterolysis rates of NABA and NABS with the polymer pass through maxima at intermediate fractions at which the rates are very much faster than with imidazole.For other imidazole-group-containingpolymers and co- polymers patterns of the relationship of esterolytic rate to fraction of any particular effective charge group type are found which bear some resemblance I14 H. Kaye and S.-H. Chang J. Macromol. Sci. Chem. 1973 AT 1127. "' C. G. Overberger and M. Morimoto J. Amer. Chem. Soc. 1971,93 3222. 344 L. P.Ellinger to either of these patterns. Esterolysis by the polymers proceeds under most conditions much more rapidly than with imidazole ; 1012-fold acceleration has been claimed in one instance.' lo Analysis of these patterns ascribes the enhancement of catalytic activity to the following effects which contribute variously according to conditions (a) Co-operative effects -nucleophilic interactions between neighbouring imidazole groups in the polymer.This effect falls sharply when the fraction of the effective species falls below a certain level. (b) Electrostatic effects -polar interactions involving charged groups in the polymer ;this becomes important at high and low pH. (c) Apolar hydrophobic interactions which are related to the solubility of the polymers in certain alkanol-water mixtures although they are insoluble in either pure solvent component.With polyvinylbenzimidazoles the esterolytic activity depends critically upon the solvent composition and rate enhancements of several thousand times over monomeric imidazole have been observed especially with esters having long alkyl groups. This is ascribed to the extent of coiling of the polymer. ''571 l6 The esterolyses proceed in two steps the acylation of the imidazole by the substrate followed by the rather slower deacylation (Scheme 4).'''9' l7 Some f-CH,-CH+ NABS ,KHz -cH- deacylation) Scheme 4 separation of the kinetics of these reactions has been achieved by studying the initial rates to much lower conversions.' Highly conjugated mainly polyheterocyclic polymers catalyse a variety of reactions. This is ascribed to the rather high concentration of delocalized electrons.* Polymer Supports in Synthesis.-Progress in the technique and its application to peptide synthesis' have been discussed. '2o Chloromethylated lightly cross- linked styrene-divinylbenzene copolymer is still the most important support ; beads are commercially available but product reproducibility could be better ;' they are unsuitable as stationary phase in columns. A new synthesis of an alternative support -cross-linked 4-hydroxy-3-nitropolystyrene-starts with the copolymerization of 4-methoxystyrene with (a little) divinylbenzene. Following C. G. Overberger M. Morimoto I. Cho and J. C. Salamone J. Amer. Chem. Soc. 1971,93,3228. 'I7 Y. Okamoto and C. G. Overberger J. Polymer Sci.,Part A-I Polymer Chem.1972,10 3387. "IJ D. R. Cooper A. M. G. Law and B. J. Tighe Brit. Polymer J. 1973,5 163. 'l9 'Progress in Peptide Research' ed. S. Lande Gordon and Breach New York-London- Paris 1972 Vol. 2. lZo R. B. Merrifield in ref. 119 paper 14. Post Reactions of Polymers demethylation of BBr the product is nitrated.I2' This support can cause undue racemization during peptide synthesis so that care over the selection of amino- acid blocking groups is mandatory. Polymeric supports have found increasing use in polysaccharide synthesis. Poly-p-(prop- l-en-3-ol-l-y1)styrene (7) has been synthesized from chloromethyl- ated styrene by a four-step synthesis and condensed with appropriately substituted CH =CHCH,OH (7) glycosyl halides. 22 In another approach p-vinylbenzoyl derivatives of suitably protected glucopyranoses have been prepared and polymerized or copolymerized with styrene.'23 6-Nitrovanillin has been attached through an ether linkage to chloromethylated styrene-divinylbenzene copolymer.After selective reduction of the aldehyde group by sodium borohydride the alcohol was condensed with a pro- tected p-nitrobenzoyl-substitutedglucopyranosyl bromide. The p-nitrobenzoyl group was removed by sodium ethoxide-ethanol-dioxan after each step. This polymeric support is photosensitive and releases the saccharide upon irradiation ( >320 nm dioxan) ;the recovered resin has typical aldehyde absorption. After hydrogenation isomaltose and glucose could be separated from the carbohydrate component.l2 Chloromethylated styrene-divinylbenzene copolymer has also been used in the stepwise synthesis of ethers.'24 The polymer (8) based on a p-(8) bromostyrene-divinylbenzene-styrene copolymer has been used as macroporous insoluble carrier in nucleotide synthesis.' Chloromethylated cross-linked polystyrene beads react with the sodium salt of 4-hydroxymethyl-2,2-dimethyl-1,3-dioxolan.Acidic hydrolysis yields a supported vicinal diol. Under acetal- fprming conditions this reacted with one of the aldehyde groups of the terephthal- dehyde and isophthaldehyde leaving the other aldehyde free for suitable reactions. 126 12' R. E. Williams J. Polymer Sci.,Part A-I Polymer Chem. 1972 10 2123. lz2 J. M. Frechet and C. Schuerch J. Amer. Chem. SOC.,1971,93,492.'*' R. D. Guthrie A. D. Jenkins and J. Stehlkek J. Chem. SOC.(0,1971 2690. lZ4 U. Zehavi and A. Patchornik J. Amer. Chem. SOC.,1973 95 5673. lZ5 R. Glaser U. Sequin and C. Tamm Helu. Chim. Acta 1973,56 654. C. C. Leznoff and J. Y.Wong Canad. J. Chem. 1973,51 3756. 346 L. P.Ellinger Polymer Supports for Enzymes and Pharmaceuticals.-The advantages of polymeric supports in these cases are similar to those described for otherwise homogeneous active catalyst. With enzymes they facilitate not only the study of their function and their use as chemical and analytical but are beginning to be used in large-scale technical applications. With pharmaceuticals the main advantages sought are long-term and depot effects.129 Complex formation for instance with polyvinylpyrrolidone has found applications but is very limited in scope.Local anaesthetics have been supported on polyamides -presumably through hydrogen bonds. '30 Grafting of deferox- amine B used for its iron-chelating ability on to polyacrolein has been effected without loss of activity.I3' The linking of enzymes and of pharmaceuticals to a polymeric support or its precursor presents problems. Reaction with the appropriate acid chloride is generally too unspecific and is accompanied by racemization and loss of activity. Acid derivatives which may react more specifically under mild conditions include esters with 1-hydroxy-5-methoxybenzotriazole, N-hydroxysuccinimide and 2,4,5-trichlorophenol. Monomeric and polymeric esters derived from acrylic methacrylic N-vinylcarbamic and isopropenylcarbamic acid have been examined in model reactions with nucleophiles.129 Generally the monomeric species which can be polymerized and copolymerized are more reactive and give better yields under mild conditions than the corresponding polymers but the latter retain their specificity. Polymeric supports should prove of particular value with pharmaceuticals providing radiation protection. 29 Polymeric Support of Other Active Molecules-With a rapidly extending scope only examples can be given. The attachment of 3,5-di-t-butylphenol groups to low-density and high-density polyethylene polypropene or polyoxymethylene is effected through the 4-diazo-oxide derivative of the phenol. This is readily access- ible and decomposes under milling conditions to yield a carbene sufficiently reactive to insert into a C-H bond on the polymer ba~kb0ne.l~~ It is claimed that in low-density polyethylene the retarding effect of the polymer-attached antioxidant groups is greater than that produced by an equivalent quantity of di-t-butylphenol alone.Polymers containing nitroxyl groups include poly-(4-methacryloylamino-and 4-methacryloyl-2,2,6,6-tetramethylpiperidine 1-oxyl),'33 prepared by oxidation '" D. Warburton K. Balasingham P.Dunnill and M. D. Lilly Biochem. Biophys. Acta 1972 284 278. G. P. Roger and J. P. Andrews J. Macromol. Sci. Chem. 1973 A7 1167. lz9 H. G. Batz G. Franzmann and H. Ringsdorf Macromol. Chem. 1973 172 27. I3O G. Bauer and E. Ullmann Arch.Pharm. 1973,306 86. ''I R.Kamirez and J. E. Andrade J. Macromol. Sci. Chem. 1973 A7 1035. '" M. L. Kaplan P.G. Kelleher G. H. Bebbington and R.L. Hartless J. Polymer Sci. Polymer Letters Edn. 1973 11 357. '33 T. Kurosaki K. W. Lee and M. Okawara J. Polymer Sci. Part A-1 Polymer Chem. 1972,10 3295. Post Reactions of Polymers with hydrogen peroxide-sodium tungstate-H,edta according to Rozantsev from the corresponding piperidine polymer. Polystyrene substituted in the 4-position by imidazoline 3-oxide 1-oxyl has been prepared in fair yield by condensation in DMF of poly(p-formylstyrene) with 2,3-bis(hydroxylamino)-2,3-dimethylbutanefollowed by oxidation of the poly-[4-(1',3'-dihydroxy-4',4,5',5'-tetramethyltetrahydroimidazol-2-yl)phenyleth-ylene] with lead dioxide in DMF.,The product gives a blue solution in DMF or THF. '34 Lithiation of polystyrene followed by reaction with 2-methyl-2-nitroso- propane yields poly-(2-t-butylnitrosophenylethylene),which is oxidized by silver oxide in toluene to the corresponding nitroxyl radical. The e.s.r. spectrum of the polymer is being studied the nitroxyl group serving as a spin label. lo' Rose Bengal has been supported on chloromethy€ated styrene-divinylbenzene copolymer and the product used to study photo-oxidations involving singlet oxygen as the reactive species.' 35 Poly-p-styryldiphenylalkylidenephosphoranes have been prepared from poly- styrene containing a nuclear diphenylphosphine substituent (p. 340). They react with carbonyl compounds as Wittig reagents.'36 Carbodi-imide groups supported on cross-linked polystyrene (R) RC,H,CH,N=C=NCHMe ,have been used in the Moffat oxidation of alcohols by DMS0.13' N-Chloro-nyIon~'~~ oxidize alcohols and sulphides. Poly-(5-t-butyl-3-vinyl-1,2-benzoquinone) has been made by polymerization of a protected monomer precursor and removal of the protecting group. The product is analogous to 3,5- di-t-butyl- 1,2-benzoquinone in oxidizing primary amines to ketones.' 39 A variety of quinonoid redox polymers have been made and investigated. 140 crPP'-Tribromocumene is the main product obtained from cumene with N-bromopolymaleimide. '41 134 Y. Miura K. Nakai and M. Kinoshita Makromol. Chem. 1973 172 233. 135 E.C. Blossey D. C. Neckers A. L. Thayer and A.P. Schaap J. Amer. Chem. SOC. 1973,95 5820. 13' S. V. McKinley and J. W. Rakhys jun. J.C.S. Chem. Comm. 1972 134. 37 N. M. Weinshenker and C. M. Shen Tetrahedron Letters 1972 3285. '* H. Schuttenberg G. Klump U. Kaczmar S. R. Turner and R. C. Schulz J. Macromol. Sci.,Chem. 1973 A7 1085. W. H. Daly and D. C. Kaufman Amer. Chem. SOC.,Div. Polymer Chem. Polymer Preprints 1973 14 1187. I4O G. Manecke H.-J. Kretzschmar and W. Hubner J. Macromol. Sci. Chem. 1973 A7 1181. 14' C.Yaroslavsky A. Patchornik and E. Katchalski Tetrahedron Letters 1970,42 3629.

ISSN:0069-3030    

DOI:10.1039/OC9737000322

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

12 Aliphatic Compounds Part (i) Hydrocarbons By R. S. ATKINSON Department of Chemistry The University Leicester LEI 7RH 1 Acetylenes Recent reviews of acetylene chemistry include coverage of addition reactions of tertiary phosphorus compounds with electrophilic olefins and acetylenes,' the chemistry of a-acetylenic ketones,2 1,3-dipolar cycloadditions to alkyne~,~ thermal rearrangement of acetylenes and cyclization of a,@-linear diacetylenes; and catalytic semihydrogenation of the triple bond.5 A book on naturally occur- ring acetylenes has also appeared.6 A promising alternative to the Lindlar catalyst for reduction of acetylenes to cis-olefins uses a particular nickel (P-2) catalyst prepared simply by reduction of nickel@) acetate with sodium borohydride in ethanol.The high cis:trans ratios (30 :1)ofolefins obtained areincreasedfurther by thepresence~fethylenediamine.~ The enormous synthetic potential of organoboron chemistry continues to unfold.8 Trialkylboranes react with alkali-metal acetylides to give alkali-metal 1-alkynyltrialkylborates. Addition of iodine induces the transfer of one of the R groups on boron to the adjacent carbon and high yields of acetylene (1) are ~btained.~ Providing the starting organoborane is readily available secondary alkyl and aryl groups may be transferred giving acetylenes which cannot be prepared by nucleophilic displacement using alkali-metal acetylides. ' M. A. Shaw and R. S. Ward Topics Phosphorus Chem. 1972,7 1. R. L. Bol'shedvorskaya and L. I. Vereshchagin Uspekhi Khim.1973 42 51 1. J. Bastide J. Hamelin F. Texier and Y. Vo. Quang Bull. SOC. chim. France 1973,2555. W. D. Huntsman Intra-Sci. Chem. Reports 1972,6 151. E. N. Marvel1 and T. Li Synthesis 1973 457. 'Naturally Occurring Acetylenes' F. Bohlmann T. Burkhardt and C. Zdero Academic Press London 1973. C. A. Brown and V. K. Ahuja J.C.S. Chem. Comm. 1973 553. * P. I. Paetzold and H. Grundke Synthesis 1973 635. A. Suzuki N. Miyaura S. Abiko M. Itoh H. C. Brown J. A. Sinclair and M. M. Midland J. Amer. Chem. SOC.,1973,95 3080. 348 Aliphatic Compounds-Part (i) Hydro carbons 349 Terminal alkynes are converted into ketones by alkylation or protonation of lithium alkynyltrialkylborates followed by oxidation. Alcohol toluene-p-sul- phonates can be used for the alkylation step protonation with methanesul- phonic acid gave products with R2 = H after oxidation (Scheme l)." R' I R:B + LiCrCRZ d'g'yme Li[R:BCrCRZ] 7RiBC=CR2R3 1)' R1COCHR2R3 Reagents i R3X;ii H,O,.Scheme 1 Conjugated enynes e.g. (2) are formed from alkenylboranes (3) by treatment with lithium acetylides followed by iodine-sodium hydroxide. The reaction HC=C(CH,),CH,OSiMe (ti) , ZBH LiC-CPr I Pr Pr & C C \ 'I C I,-THF (tii,B< /H H /c=c\ (CH,),CH,OSiMe 'NaOH H /c=c\ (CH,),CH,OSiMe A. Pelter C. R. Harrison and D. Kirkpatrick J.C.S. Chem. Comm. 1973 544. 350 R.S. Atkinson gives the trans-enyne which can be converted into a cis-trans-diene by cis- hydrogenation of the triple bond using known methods and illustrated by synthe- sis of the insect pheromone bombykol(4).l1 For trans-trans-dienes an alternative method has been devised which involves stepwise addition of two acetylene units (Scheme 2). Oxidative work-up provides erg-unsaturated ketones.' ' R2 R2 H R2 H R' \ /H /c=c\ / /c=c\H H H%\//C=C\ R' / /c=c\H \ /H /c=c\ //CCH2R' H 0 +"\OMe Reagents i CIC-CR'; ii HC=CR2; iii NaOMe; iv H202; v H +. Scheme 2 A stereospecific route to vinyl halides from acetylenes uses catechol esters of trans-alkenylboronic acids (5) prepared by hydroboration of alkynes with catecholborane. Treatment with two molar equivalents of bromine in methylene chloride followed by base gives cis-alkenyl bromides (6).This inversion of configuration on bromination is in contrast to the trans-alkenyl iodide (7)obtained by addition of sodium hydroxide-iodine to trans-alkenylboronic acids (8). Substituted pyridines can be prepared in reasonable yield by the reaction of two moles of acetylene with one mole of nitrile in the presence of a catalytic amount of a n-cyclopentadienylcobaltcomplex. The mechanism is thought to involve a cobaltacyclopentadiene intermediate (Scheme 3). Acetylene itself is of limited value in the Diels-Alder reaction owing to its sluggish reaction and the difficulty associated with its handling. Vinylene thiono- carbonate(9)and 2-phenyl-A4-1,3-dioxolen (10)have been suggested16 as acetylene ' E. Negishi G. Lew and T. Yoshida J.C.S. Chem.Comm. 1973 874. E. Negishi and T. Yoshida J.C.S. Chem. Comm. 1973 606. l3 H. C. Brown T. Hamaoka and N. Ravindran J. Amer. Chem. Soc. 1973,95,6456. l4 H. C. Brown T. Hamaoka and N. Ravindran J. Amer. Chem. SOC.,1973,95 5786. Y. Wakatsuki and H. Yamazaki Tetrahedron Letters 1973 3383. l6 W. K. Anderson and R. H. Dewey J. Amer. Chem. SOC.,1973,95 7161. Aliphatic Compounds-Part (i) Hydrocarbons 351 R H R Br \/ i Br,-CH,CI, \ / RC=CH -+ ii N~OM~-M~OH’ /c=c\ H /c=c\B,o H H (n-C5H5) PPh, \/ HC-CH + (n-C,H,)Co(Ph3P) -+ A HH il (n-C,H,) PPhj \/ (n-C,H,)Co( PPh3) + R = Me Ph PhCH, or MeO,CCH Reagents i C,H,; ii RCN. Scheme 3 synthons for use according to Scheme 4. (9) and (10) were obtained by reverse Diels-Alder fragmentation of their furan adducts.The butyl-lithium-induced cycloreversion of dioxolans to olefins (Scheme 4) is not generally applicable” but a new method for conversion of thionocarbonates into alkenes uses bis(cyc1o- octa-l,5-diene)nickel. Thionocarbonates of erythro- and threo-4-methylpentane- 2,3-diol (11) and (12) are converted stereospecifically into cis-and trans-4- methylpent-2-ene respectively on stirring in dimethylformamide. A nickel-containing intermediate is inyolved. ’* Synthesis of the marasmic acid skeleton (13) has been accomplished by addition of dimethyl acetylenedicarboxylate (DMAD) to the diene (14) followed by stereo- specific cyclopropanation. l9 l7 J. N. Hines M. J. Peagram E. J. Thomas and G. H. Whitham J.C.S. Perkin I 1973 2332.M. F. Semmelhack and R. D. Stauffer Tetrahedron Letters 1973 2667. l9 S. R. Wilson and R. B. Turner J. Org. Chem. 1973,38 2870. R. S. Atkinson 0 0 Reagents i [)=S; ii (RO),P; iii. C0>Ph; iv BuLi; v HCrCH. 0 H Ni(cod),. DMF. 7H 65 “C. 45 h ,c=s ___,Me H Me Me *Me Me Ni(cod) . Me DMF 65 “C.45 h ____) Me H Me C0,Me Me02C&e Me 95 % Me0,C / Me (14) (13) The highly strained cyclo-octa-1,5-diyne (15) has been isolated. It is stable in crystalline form at 0 “Cwith exclusion of air and X-ray analysis shows that the molecule deviates only slightly from planarity.” 2o E. Kloster-Jensen and J. Win Angew. Chem. Internat. Edn. 1973 12 671. A lip hat ic Compounds-Part (i) Hydrocarbons A potentially useful acetylene is (16),prepared from methyi vinyl ketone by the route shown in Scheme 5.21 H,C=CH H2C=CH Me \c=o , I 11 Me / U 0 II p" J HCGCC Me HCGCCH Me Reagents i (CH,OH),-H +;ii distil; iii 0,; iv HC=CMgBr; v CrO,.Scheme 5 Among mechanistic investigations involving acetylenes is an analysis of the silver(1)-catalysed propargyl-allenyl ester rearrangement of (17) to (I 8).22 By means of 8Oand 14Clabelling and using optically active and diastereoisomeric Ar yo y3 slow. R2 Scheme 6 '' E. F. Hahn J. Org. Chem. 1973,38 2092. 22 H. Schlossarczyk W. Sieber M. Hesse H.-J. Hansen and H. Schmid Helu. Chim. Acta 1973 56 875. 354 R.S. Atkinson esters a [3s,3s]-sigmatropic rearrangenient is shown to be involved (Scheme 6).The role played by the silver(1) ion is n-complex formation with the triple bond in (17) and the double bond which is not involved in the transition state in (18). Thus this rearrangement is of a type which has been recently described as a charge-induced sigmatropic rearrangement and is different from other transforma- tions of strained molecules induced by silver ions. Thermal Claisen rearrangement of 2,6-dichlorophenyl propargyl ether (19) gives the benzofuran (20) and the benzopyran (21) as the major products. These and other minor products are derived from a radical C-Cl homolysis after the initial [3s,3s]-sigmatropic rearrangement (Scheme 7).2 (21) Scheme 7 By choosing conditions benzopyrans or benzofurans can be obtained by pyrolysis of aryl propargyl ethers containing a free cc-po~ition.~ 2,6-Dimethyl-substituted-phenyl propargyl ethers (213)give tricyc10[3,2,1,0~~~]- oct-3-en-8-one derivatives (23) on heating.24 The corresponding methylene derivatives have been prepared and their thermolysis has been studied.25 In the case of the dideuteriated compound (24),acetylenes (25)and (26) are obtained 23 N.Sarcevic J. Zsindely and H. Schmid Helo. Chim. Ada 1973,56 1457. 24 J. Zsindely and H. Schmid Helo. Chim. Am 1968 51 1510. ” P. Gilgen J. Zsindely and H. Schmid Helo. Chim. Acta 1973 56 681. Aliphatic Compounds-Part (i) Hydrocarbons by two competitive reverse Diels-Alder reactions followed by aromatization via [3s,3s]-sigmatropic rearrangements (Scheme 8).The latter are considered to be concerted although many other semibenzene-benzene rearrangements proceed uia radical pathways. RZ A o* DYD Reagents i Ph,P=CH Scheme 8 Treatment of aliphatic nitroso-amides with base yields unstable diazotates. These react intramolecularly with triple bonds with a substituent effect that suggests a nucleophilic attack.26 The mechanism proposed and supported by deuterium labelling (Scheme 9) is strongly reminiscent of the Favorskii with the cyclopropanone opening depending upon the carbanion-stabilizing ability of the substituent R. Using optically active pent-1-yne-3-diazotate (27 ;R =Et) the ester (28) was isolated with 88 inversion of configuration which eliminates the planar oxyallyl cation (29)as the precursor of the cyclopropanone.The latter may z6 W. Kirmse A. Engelmann and J. Hesse J. Amer. Chem. SOC.,1973 95 625; Chem. Ber. 1973 106 3073; W. Kirmse and A. Engelmann ibid. p. 3086. R. S. Atkinson LR-AH -0 OR OR (29) (31) \ MeCHRC0,Me + RCH,CH,CO,Me (28) Reagents i NaOMe-MeOH; ii MeOH. Scheme 9 arise by backside displacement of nitrogen either within the methyleneoxadiazo- line anion (30) giving the cyclopropanone enolate (31) or by using the n-electrons in the methyleneoxadiazoline (32). Metallation of isobutene with butyl-lithium in the presence of tetramethylethyl- enediamine gave the methylallyl anion (33)and the trimethylenemethane dianion (34). In spite of the introduction of a second negative charge this lithiation step occurs faster than the first.This has been ascribed to aromatic character of six electrons in four parallel p-~rbitals.~’ Dimetallation ofinternal acetylenes occurs by abstraction of both protons from the same carbon. 1.r. data in several solvents support the view that these dianions exist in either the sesquiacetylenic (35) or allenic form (36) according to the conditions. Thus a change to more co-ordinating solvents (e.g.HMPT) produces a shift in the position of the absorption band from -1800 to 2050 cm-1.28 Li / 4 \ Li R’ 27 J. Klein and A. Medlik J.C.S. Chem. Comm. 1973 275. 2a J. Klein and J. Y. Becker J.C.S. Chem. Comm. 1973 576. Aliphatic Compounh-Part (i) Hydrocarbons 2 Alkarres Cyclo-octane exists predominantly in the boat-chair conformations although the crown family (Figure 1) may become more important with the introduction of substituents or heteroatoms into the ring.A detailed study of cyclo-octane using boat-chair family 11 crown family Figure 1 13Cn.m.r. and of cis-1,2-hz-[2H,,]cyclo-octane using 'H n.m.r. at low tempera- tures has shown that cyclo-octane itself exists to a small extent (6% at room temperature) in the crown-family conformati~n.~~ Intermolecular hydrogen exchange and alkylation of alkanes with their parent carbenium ion salts have been studied under stable ion conditions in superacid media. The reactions involve electrophilic attack by the carbenium ion on the C-H or C-C bonds oiu triangular three-centre-bonded carbonium ion transition states.30 3 Allenes Recent progress in the chemistry of a1lenes3l and the synthesis absolute con- figuration and optical purity of chiral allene~~~ has been reviewed.Allene reacts with various amines or carbon acids in the presence of catalytic amountsof palladium(0) or rhodium(1) complexes to give high yields of butadienes. Mono(dieny1)- or bis(dieny1)-derivatives could be obtained by varying the reaction temperature or mole ratio of reactants (Scheme 10). The products formed the corresponding mono- and bis-Diels-Alder ad duct^.^^ 29 F. A. L. Anet and V.J. Basus J. Amer. Chem. SOC.,1973,9S 4424. 3o G. A. Olah Y.K. Mo and J. A. Olah J. Amer. Chem. SOC.,1973,9S 4939; G. A. Olah J. R. DeMember and J.Shen ibid. p. 4952; G. A. Olah Y. Halpern J. Shen and Y. K. Mo ibid. p. 4960. 31 T. Okamoto Bull. Inst. Chem. Res. Kyoto Univ. 1972 SO 450. 32 R. Rossi and P. Diversi Synthesis 1973 25. 33 D. R.Coulson J. Org. Chem. 1973 38 1483. 358 S.A t k inson CH =C=CH R2 R3 = CN CO,Et COMe Reagents i R'R'NH; ii CH,=C=CH,; iii CH,R3R4. Scheme 10 The salt (37)can beobtained from the readily available 1,3-diamino-1,3-dichloro-ally1 cation (38)by treatment with dimethylamine. This salt and the diethoxy- analogue (39) prepared by alkylation of NN-dimethyl-P-ethoxy-P-dimethyl-amino-acrylamide with triethyloxonium tetrafluoroborate are the protonated forms of allenetetramine (40) and dialkoxydiamino-allene respectively. The corresponding allenes are obtained as distillable liquids on treatment with butyl lithium or sodium amide in liquid ammonia but react readily with electrophiles to give substituted 1,1,3,3-tetrakis(dialkylamino)allylcations e.g.(41) and (42).34 Gt Me,N NMe EtO BF4- Me,N NMe NMe2 Me2N NMe H H H (37) (39) NMe, Me,N \ /NMe i. PhOCN c10,-Me,N /c=c=c TEiz NMe, \ Me,N NMe CN NMe2 Me,N NMe H. G. Viehe Z. Janousek R. Gompper and D. Lach Angew. Chern. Znrernat. Edn. 1973 12 566. Aliphatic Compounds-Part (i) Hydrocarbons A series of racemic allenes were partially hydroborated with the chiral (+)-tetra-3-pinanyldiborane (43). In every case examined the recovered allene was enriched in the (R)-enantiomer. The model used to explain this result also correctly predicts the configuration of chiral alcohols produced from simple olefins.For the case of alkyl substituted allenes both vinylborane (44) and allylborane (45) RHC CHR I1 C !HR major/CHzR '--:,minor RHcyH (44)/HB (43) CH,B' \ (45) are produced the optical purity of the recovered allene would have been higher but for the minor allylborane formation which is predicted by the model to remove the (R)-enantiomer preferentially. The mechanism of allene dimerization continues to attract attention. If the allene dimerization within (46) takes place uia a biradical route then it would give rise to (47),a species which can also be generated by pyrolysis of (48) (49) or (50). An analysis of the gas-phase thermolysis products of (46) (48) (49) and (50) at -110-120°C shows that (47)is a common intermediate but is generated from (46) with excess vibrational energy.36 A slightly different interpretation of the results of thermolysis of (46) (48) and (50) is given in terms of two conformations (51) and (52) of the reactive 2,3-dimethylene-l,4-cyclohexadiylbiradical (47) a'CH k.; CH (51) 35 W.R.Moore H. W. Anderson and S. D. Clark J. Amer. Chem. SOC.,1973,95,835. 36 W. R.Roth M. Heiber and G. Erker Angew. Chem. Inrernar. Edn. 1973,12,504; W. R. Roth and G. Erker ibid. p. 503; W. R. Roth and G. Erker ibid. p. 505. 360 R. S. Atkinson which react before achieving conformational equilibrium. 37 Thermolysis of (46) in solution gives mainly dimerization products of (47) and CIDNP signals from the dimers are observable.The contrasting behaviour of(47) in the gas phase and in solution may be the result of two different spin states.36 Addition of electron-rich olefins to electron-deficient allenes has been studied with the prospect that these allenes might resemble keten in their reactivity. From the reaction of the enamine (53) (2 equivalents) and the allene precursor (54) the cyclobutane (55) is isolated in which the two methyl singlets at C-4 coalesce to a singlet in the n.m.r. spectrum at 76 "C in CDC1,. An equilibrium between (55) and the zwitterion (56) with rapid rotation around the C-3-C-4 bond explains this phenomenon. Prolonged heating of (55) results in complete conversion into (57) which requires C-1 -C-4 bond rotation.38 Me /NMe NMe CN \ /c=c\ + Me,CHC=C(CN) + I Me H c1 Me Me Me Me + Me ,CN 11 lr1r Me,N Me (57) Me Treatment of 2,3-dipropargylnaphthalenewith potassium t-butoxide in t-butyl alcohol under carefully controlled conditions allows the isolation of the crystalline diallene (58).A solution of(58) in oxygen-saturated methanol yields the crystalline peroxide (59). The o-quinodimethane (60) is presumed to be an inter- mediate and can be trapped by dimethyl fumarate or maleate. No analogous peroxide was isolable starting from (61).,' \I (61) (59) (60) 37 W. Grimme and H.-J. Rother Angew. Chem. Internat. Edn. 1973 12 505. 38 R. Gompper and D. Lach Angew. Chem. Internat. Edn. 1973,12 567. 39 C. M. Bowes D. F. Montecalvo and F.Sondheimer Tetrahedron Letters 1973 3181. Aliphatic Compounds-Part (i) Hydrocarbons 361 4 Olefins Recent reviews involving olefins include the stereochemistry of biogenetic-like olefin cyclizations ;40 aromatic substitution of olefins by palladium salts ;41 homogeneously catalysed dimerization of olefins ;42 addition reactions of butadi- ene catalysed by palladium complexes ;43 structure of 1,3-diene hydrocarbons and their reaction with electrophilic reagents;44 conjugate addition reactions of organocopper reagents ;45 protection of carbon-carbon multiple bonds ;46 syntheses of seven- and five-membered rings from ally1 cations ;47 and the stereo- chemistry of double bonds by n.m.r. spectro~copy.~~ A volume covering open- chain and cyclic polyenes and enynes has a~peared.~’ A stereospecific synthesis of trisubstituted olefins involves the addition of organocopper reagents to acetylenes in the presence of alkyl iodides.This method has been extended to the synthesis of allylic alcohols as illustrated for the case of (62)” Me EtCuMgBr I. MeC=CH. \ c=c/H ~ iiCH,OCH,CH,CI / \ 1 Et CH20 Br I CH,CH,Cl Me \ /H c=c Et /\ CH20H 70 % (62) overall Quaternary ammonium salts containing large alkyl residues function as phase-transfer catalysts by ion-pair formation in the aqueous phase with anionic reactants and their subsequent delivery to the waiting substrate in the organic phase. Using the dichloromethane-water two-phase system and tetrabutyl- ammonium iodide (TBAI) Wittig olefination of aromatic aldehydes with non- stabilized ylides is feasible.In aqueous solution only degradation of the phos- phonium salt to phosphine oxide occurs. Omission of the TBAI is possible since the phosphonium salts themselves can function as phase-transfer catalysts5 40 K. E. Harding Bioorg. Chem. 1973 2 248. 41 I. Moritani and Y. Fujiwara Synthesis 1973 524. ” J. Hetflejs and J. Langova Chem. listy 1973 67 590. 43 J. Tsuji Accounts Chem. Res. 1973 6 8. 44 V. S.Aksenov Sovrem. Probl. Org. Khim. 1971 33. 45 G. H. Posner Org. Reactions 1972 19 1. 46 D. W. Young in ‘Protective Groups in Organic Chemistry’ ed. J. F. W. McOmie Plenum Press London 1973. 47 H. M. R. Hoffmann Angew. Chem. Internat. Edn. 1973,12 819. 48 G.J. Martin and M. L. Martin Progr. N.M.R. Specrroscopy 1972 8 163. 49 ‘Methods of Organic Chemistry’ (Houben-Weyl) Vol. 5 Part Id ‘Open-chain and cyclic polyenes enynes’ 4th edn. E. Mueller Thieme Stuttgart. J. F. Normant G. Cahiez C. Chuit and J. Villieras Tetrahedron Letters 1973 2407. 5‘ G. Mark1 and A. Merz Synthesis 1973 295. R.S. Atkinson 2,3-Bis(bromomethyl)buta-1,3-diene (63) has been prepared by zinc-copper couple debromination of the tetrabromide (64) in ether-hexamethylphosphor-triamide and found to be stable in solution. Both (63)and its Diels-Alder or other addition products are potentially valuable in synthesis. The heterocycles (65) (66),and (67)are readily obtainable and the reactive allylic bromines in (63)may be displaced by a variety of nucleophiles.Additionally a 1,3-diene may be regenerated after Diels-Alder addition i.e. (63)-+ (68).52 H2cc:: H2czx (65)(66) X = S0 Br H2C H2C (67) X = NR /MeO,CCrCCO,Me C02Me Br C0,Me H2C x H,C’ x X (71) X = CN (69) X = CN (72) X = C02Me (70) X = C0,Me Improved routes have been reported to the potentially useful 2,3-dicyano-and 2,3-dimethoxycarbonyl-butadienes(69) and (70) by thermal ring opening of the corresponding cyclobutenes (71) and (72). Diels-Alder cycloadditions of (69) occur readily with electron-rich olefins with an inverse of the normal electron demand. Iron pentacarbonyl has been found to reduce enol acetates vinyl chlorides and ap-unsaturated aldehydes to the corresponding olefins.The procedure simply involves heating the substrate and iron pentacarbonyl under reflux in dibutyl ether. A radical mechanism is impli~ated.~~ A method of generating allyl-and methallyl-lithium which avoids troublesome coupling reactions uses butyl-lithium-tetramethylethylenediaminetreatment of propene or i~obutene.~2-Methoxypropene and other useful vinyl ethers are 52 Y. Gaoni Tetrahedron Letters 1973 2361 ; S. Sad& and Y. Gaoni ibid. p. 2365. D. Bellus and C. D. Weis Tetrahedron Letters 1973 999. s4 S. J. Nelson G. Detre and M. Tanabe Tetrahedron Letters 1973 447. 55 S. Akiyama and J. Hooz Tetrahedron Letters 1973,4115. Aliphatic Compounds-Part (i) Hydrocarbons conveniently prepared by succinic anhydride-benzoic acid-mediated removal of methanol from the corresponding dimethylacetal~.~~ N-p-Tolylvinylmethylketenimine(73) has been prepared from the amide (74) and in contrast to vinylketens was fairly stable at room temperature and even distillable.(73) exhibits ambident character in cycloadditions ;thus with electron- deficient dienophiles e.g. dicyanostyrene the adduct (75) is formed regiospecific- ally and can be hydrolysed by mild acid to the ketone (76). An electron-rich H,C=CH CH,=CH \ i Ph,PBr,; \ CHCONHAr ii,E1,N C=C=NAr / / Me. Me (74) (73) diqophile however prefers to react across the [C=C(aromatic) and C=N]- diene system. N-Diethylaminophenylacetyleneand (73) in acetonitrile gave a quantitative yield of the quinoline (77).57 CHMe (77) Addition of the acid chloride of monoethyl fumarate to the piperidine (78) yielded a crystalline adduct (79) in 70 % yield in which four contiguous centres are created at one blow in an intramolecular Diels-Alder addition.Their configurations are assigned by n.m.r. with the assumption of endo-addition and using the fact that base epimerizes two of these centres. Pentacyclic aza- or diaza-steroid skeletons can conveniently be constructed from (80).58 Intramolecular ene reactions have been used for the synthesis of substituted pyrrolidines exemplified by the conversion of (81) into (82). In general the 56 M. S. Newman and M. C. Vander Zwan J. Org. Chem. 1973,38,2910. '' E. Sonveaux and L. Ghosez J. Amer. Chem. SOC.,1973,955417. 58 H. W. Gschwend Hefv.Chirn. Acfu 1973,56 1763; H. W. Gschwend A. 0.Lee and H. P. Meier J. Org. Chem. 1973 38 2169. R.S. Atkinson _7 (78) (79) Ph” C0,Me reaction is highly stereoselective and it has been used in a key step for synthesis of the naturally occurring p-acorenol (83) (Scheme 1 l).59 m& C0,Et 280 “C _____ + 3 days. 19% several steps OH (83) Scheme 11 ’’ W. Oppolzer E. Pfenninger and K. Keller Helv. Chirn. Acta 1973 56 1807 W. Oppolzer ibid. p. 1812. Aliphatic Compounds-Part (i) Hydrocarbons Members of the PGC series of prostaglandins are of considerable interest; they are intermediates on the deactivation pathway of A prostaglandins (PGA’s) to B prostaglandins (PGB’s) (Scheme 12) in mammalian systems.0 PGA PGC PGB Scheme 12 The reaction of the lactol (84) with two equivalents of tri-irondodecacarbonyl gives the stable conjugated diene complex (85). Oxidation ofthe derived hydroxy- carboxylic acid (86) with Collins’ reagent gave the tetrahydropyranyl (THP) derivative of PGC (87) in which the iron is removed under conditions mild enough to avoid isomerization of the sensitive cyclopentene double bond.60 OH CO,H pyridine = O eCozH ‘OTHP The nickel hydrogenation catalyst for acetylenes referred to earlier is also highly sensitive to the environment of the double bond permitting selective reduction of less hindered olefins in the presence of more hindered ones. Thus oct-1-ene can be 6o E. J. Corey and G. Moinet J. Amer. Chem. SOC.,1973,95 7185.366 R. S. Atkinson reduced in the presence of cyclohexene. Hydrogenolysis of benzylic and allylic alcohols and ethers did not occur with this catalyst.6' An increasing number of cycloeliminations corresponding to reverse anionic 1,3-cycloadditions are being uncovered. The lithiated triazolidine (88) is obtained (73%) from trans-trans-1,3-diphenyl-2-aza-allyl-lithium (89) and azobenzene at temperatures below 20 "C. Above 60 "C cycloreversion back to the starting components occurs and the 2-aza-ally1 anion is trapped by trans-stilbene to give the pyrrolidine (90).62 Ph Ph PhN=NPh H Ph ' ' 20°C i. trons-stilbene +A Ph-H H.J-XH => H-AN,!j H 60°C Ph Li Ph Ph H Ph Some rationalization of the regioselectivity (ortho :meta) and reactivity obtaining in the Diels-Alder addition of monosubstituted olefins and dienes is possible using experimentally available frontier-orbital energies and calculated orbital coefficient^.^^ Acid catalysis has a profound effect on the Diels-Alder reaction and large rate accelerations increased regioselectivity and stereo- selectivity occur.The predominant frontier-orbital interaction in the reaction with 'normal electron demand' is the diene HO and dienophile LU*. Rate accelera- tions are the result of lowering of the energy of the dienophile LU in the protonated form. Stereoselectivity (endo :exo ratio) is controlled by secondary orbital interactions between the diene HO and the dienophile LU which are augmented in the protonated case as a result of the larger coefficient at the carbonyl carbon (Figure 2).64 acrolein protonated acrolein case case Figure 2 6' C.A. Brown and V. K. Ahuja J. Org. Chem. 1973,38,2226. 62 T. Kauffmann A. Busch K. Habersaat and B. Scheerer Tetrahedron Letters 1973 4047. 63 K. N. Houk J. Amer. Chem. SOC.,1973,% 4092. 64 K. N. Houk and R. W. Strozier J. Amer. Chem. SOC.,1973,95,4094. * HO highest occupied LU lowest unoccupied. Aliphatic Compounds-Part (i) Hydrocarbons A similar frontier-orbital treatment has also been applied to regioselectivity in concerted cycloadditions of 1,3-dipoles to olefins and the conclusion experi- mentally verifiable is that unidirectional addition of monosubstituted dipolaro- philes should no longer be observed when the latter are highly ele~tron-deficient.~~ Primary carbon-14 kinetic isotope effects in the 1,3-dipolar addition of N-a-diphenylnitrone and styrene are interpreted as being consistent with a concerted rather than a biradical mechanism.66 The stereochemistry of hydrogen and formyl-group addition to alkenes (the ‘0x0’ process) has been determined using (E)-and (2)-3-methyl-pent-2-ene and a hydridocarbonyltris(tripheny1phosphine)rhodium catalyst.Analysis of the dia- stereoisomeric composition of the major product (91) from each isomer shows overwhelming cis-addition of CHO and H allowing for the small amount of isomerization of the initial olefins by the catalyst which is shown to occur by deuteri~formylation.~’ Me Me Me I Co H, [(Ph,P),RhH(CO)] CH,CH,CHCHCHO I1 CH,CH,C=CHCH Methoxymercuration of ethylene proceeds stereospecifically by trans-addition as is shown using cis- and trans-dideuterioethylene.threo- and erythro-1,2-dideuterio-2-methoxyethylmercuricchloride assignments were made on the basis of solvent effects upon the vicinal proton-proton coupling constants.68 A number of penta- and hexa-substituted butadienes have been prepared by substitution of one or both hydrogens in (E,E)-1,2,3,4-tetrachlorobuta-1,3-diene (92). In (93) the benzylic hydrogens which show an AB system at 34°C are diastereotopic as a result of the non-planarity of the butadiene. Coalescence at a higher temperature (58 “C)is the result of accelerated twisting about the central C-C bond which interconverts the magnetic environments of HAand H,.Although the substituents which are introduced are ‘outside’ in the intermediate planar transoid conformation yet they exert significant effects upon the twisting barrier. 65 J. Sims and K. N. Houk J. Amer. Chem. SOC.,1973,95 5798. 66 B. M. Benjamin and C. J. Collins J. Amer. Chem. Sou. 1973,95,6145. 67 A. Stefani G. Consiglio C. Botteghi and P. Pino J. Amer. Chem. SOC.,1973,95,6504. 68 T. Ibusuki and Y. Saito J. Organometallic Chem. 1973 56 103. 69 G. Kobrich B. Kolb A. Mannschreck and R. A. Misra Chem. Ber. 1973,106 1601.

ISSN:0069-3030    

DOI:10.1039/OC9737000348

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

12 Aliphatic Compounds Part (ii) Other Aliphatic Compounds By E. W. COLVIN Chemistry Department University of Glasgow Glasgow G72800 Owing to the virtually exponential growth in the chemical literature the selection of material for this Annual Report is even more subjective than hitherto. The author has attempted to discern and delineate important areas of activity rather than to present a disconnected set of isolated topics. These areas of activity include the following cyclic per-anhydrides and the chemically related a-lactones ;the tetrahedral intermediate in carbonyl substitu- tion reactions and the possible importance of its conformation ; the precise mechanism of asymmetric induction ;enol and enolate anion studies ;the Favor- skii and homo-Favorskii rearrangements ;and the anchimeric intermediacy and the structure of b-halogenoalkyl radicals a controversial topic reminiscent of the classical-non-classical carbonium ion dialogue.1 Carboxylic Acids and Derivatives The intrinsic acidities of some carboxylic acids have been determined' from gas- phase studies. The significant differences in the ionization constants of the diastereoisomeric forms of 2,3-dicarboxylic acids have been discussed in terms of proximity effects between the functional groups2 Exclusive 0-protonation was observed3 in a 'H n.m.r. study of fumaric and maleic acids in strong acids ; diacid esters including oxalic acid derivatives appear to be diprotonated in super-acid media.4 In contrast to acyclic saturated anhydrides protonated (FS0,H-SbF,-SO,) cyclic unsaturated analogues5 do not cleave below 0 "C ;the 0-protonated species formed undergo rapid inter- molecular proton exchange with solvent or excess anhydride even at the lowest accessible temperatures.Continuing interest is apparent in the study of the decomposition of cyclic per- anhydrides. Such species are prepared by the reaction of the corresponding ' R. Yamdagni and P. Kebarle J. Amer. Chem. SOC.,1973 95 4050. N. Purdie and M. B. Tomson J. Amer. Chem. SOC.,1973,95,48. ' J. W. Larsen and P. A. Bouis J. Org. Chem. 1973,38 1415. G. A. Olah and P. W. Westerman J. Org. Chem. 1973,38 1986. ' G. A. Olah Y. K. Mo and J. L. Grant J. Org. Chem. 1973,38 3207 368 Aiiphatic Compounds-Part (ii) Other Aliphatic Compounds diacids or anhydrides with methanesulphonic acid and 98 % H,O ,or with less risk,6 Na,O,.The ketonic products formed in the vapour-phase thermolysis of malonyl peroxides are taken as further evidence7 of the intermediacy of a-lactones (Scheme 1). 1 0 0 1 0.0 + 0+co+co Scheme 1 Adam* has succeeded in isolating a relatively stable or-lactone bis(trifluor0- methy1)acetolactone(1)(Scheme 2) ;although stable at -20 "C this compound a gas at ambient temperatures has t$40cof 8 h. The solvolysis of di-n-butyl malonyl peroxide has been st~died.~ Reagents i 98 % H,O,-MeS0,H. scheme 2 While the propenolide (2) is postulated" as an intermediate in the decomposi- tion of phenyl maleoyl peroxide benzpropiolactone (3) has been synthesized' (Scheme 3) by low-temperature photolysis of phthaloyl peroxide ;this lactone (3) has been proposed as an intermediate (or by-product) in the formation of benzyne from benzenediazonium 2-carboxylate.The preparation' and reactions of P-peroxy-P-propiolactols,cyclic analogues of the a-hydroperoxy Baeyer-Villiger intermediate have been described (Scheme 4). A. H. Alberts H. Wynberg and J. Strating Synthetic Comm. 1973 3 297. M. M. Martin F. T. Hammer and E. Zador J. Org. Chem. 1973,38 3422. W. Adam J.-C. Liu and 0.Rodriguez J. Org. Chem. 1973,38,2269. ' W. Adam and R. Rucktaschel J. Org. Chem. 1972,37,4128. lo M. M. Martin and J. M. King J. Org. Chem. 1973 38 1588. l1 0. L. Chapman C. L. McIntosh J. Pacansky G. V. Calder and G. Orr J.Amer. Chem. SOC.,1973,95,4061. l2 D. H. Gibson H. L. Wilson and J. T. Joseph Tetrahedron Letters 1973 1289. 370 E. W. Colvin Ph hv Scheme 3 / liV Bu'CO H Reagents i H,O,-H'; ii Ph,P;iii H'; iv A or hv. Scheme 4 The powerfully dienophilic nitrosocarbonyl compounds (4) formed by periodate oxidation of hydroxamic acids,I3 can be trapped reversibly with 9,lO-dimethylanthracene; they react efficiently with dienes to form dihydro-1,2- oxazines (Scheme 5). + Reagent i Et,N 10,-; ii u. Scheme 5 '' G. W. Kirby and J. G. Sweeny,J.C.S. Chem. Comm. 1973,704. A Iiphatic Compounds-Part (ii) 0ther Aliphatic Compounds 37 1 Aqueous nitrosation ' of primary a-carbonyl diazo-compounds yields a-carbonyl nitrile oxides (Scheme 6) which undergo rapid 1,3-dipolar addition to olefins.Scheme 6 The migration of a carboxylate group has been detected' in the benzilic-acid-type rearrangement shown (Scheme 7) ;at pH < 10 ester hydrolysis occurs with subsequent rearrangement while at pH > 11.5 the intact ester group migrates with hydrolysis as a second step. n AA C0,Et c0,- +OH 0 ;ccoc YLW2 \* A co2-0 Scheme 7 The relative migratory aptitude of the ethoxycarbonyl residue has been deter-mined for the pinacol rearrangement of 2,3-dihydroxy-esters (Scheme 8) when the order Ph > C0,Et -Et > Me -H was obtained.16 OH OH C0,Et II I Ph-C-C-CO,Et Ph-C-COR id R Scheme 8 A gas-phase electron-diffraction study' of oxalyl chloride has revealed that the molecule exists as a mixture of trans-and gauche- rather than trans-and cis- conformers.The microwave spectrum' of formimide supports the assignment of an asymmetric cis-trans planar conformation (5) in the gas phase. The tetrahedral intermediate involved in carbonyl-substitution reactions has been the subject of considerable attention. Studies have been described of l4 H. Dahn B. Favre and J.-P.Leresche Helv. Chim. Acra 1973 56 457. l5 H. Rode-Gowal and H. Dahn Helv. Chim. Acta 1973 56 2070. J. Kagan D. A. Agdeppa and S. P. Singh Helv. Chim. Acta 1972,55 2252; J. Kagan and D. A. Agdeppa ibid. p. 2255. K. Hagen and K. Hedberg J. Amer. Chem. SOC.,1973 95 1003. W. E. Steinmetz J. Amer. Chem. SOC.,1973 95 2777. 372 E.W. Colvin substituent influence^'^ on the lactonization of coumarinic acids and of the kinetic isotope effects2' exhibited by [methoxy-'*O]methyl formate. Based on his earlier studies2 'of the ozonolysis of acetals Deslongchamps22 has considered the possible importance of the conformation of this tetrahedral intermediate. An investigation of the hydrolysis of the cation (6) via the intermediate (7) revealed that the ratio of ester to amide produced varied with the size of R (Scheme 9); this suggests that the two possible conformations (6a) and (6b) lead to intermediates (7) of different conformations which lead in turn to different products for stereo-electronic reasons. He proposes that specific cleavage of a C-0 or C-N bond is allowed only if the other two heteroatoms (0or N) of the tetrahedral intermediate have each a lone-pair orbital orientated antiperi- planar to the bond to be cleaved.The basic hydrolysis of salicoylpyrrole (8) is a rapid reaction (Scheme lo) and intramolecular catalysis depends23 on a pK that is 1.4 units higher than the pK of (8);this higher value is probably the pK of the phenol in the tetrahedral intermediate (9). A further of the ambident nature of the amide group has been reported but last year's controversy over the site of amide protonation has largely abated. Aliphatic amides react2' with anhydrous HF and BF to give stable amide hydrofluoroborates (10). l9 R. Hershfield and G. L. Schmir J. Amer. Chem. SOC.,1973,95 7359. *' C. B. Sawyer and J. F. Kirsch J. Amer.Chem. SOC.,1973 95 7375. * P. Deslongchamps and C. Moreau Canad.J. Chem. 197 1,49,2463 ;P. Deslongchamps C. Moreau D. Frehel and P. Atlani ibid. 1972 50 3402. 22 P. Deslongchamps P. Atlani D. Frkhel and A. Malaval Cunud. J. Chem. 1972 50 3405; P. Deslongchamps C. Lebreux and R.Taillefer ibid. 1973 51 1665. 23 F. M. Menger and J. A. Donohue J. Amer. Chem. SOC.,1973,95,432. 24 J. L. Wong and D. 0.Helton J.C.S. Chem. Comm. 1973 352. S. S. Hecht and E. S. Rothman J. Org. Chem. 1973,38 395. 373 Aliphatic Compounds-Part (ii) Other Aliphatic Compounds 1!11 [iil n N N N U (9) 1L Scheme 10 /OH R'-C BF,-\+ N-R~ I R3 2 Nitriles The role of HCN in the prebiotic evolution of biomolecules such as the purines continues to evoke interest.The thermodynamic and kinetic parameters for the formation of glyconitrile (formaldehyde cyanohydrin) have been determined ;26 since the formation of purines requires free HCN,and that of sugars requires free formaldehyde these parameters are of interest in terms of the possibility of simultaneous prebiotic formation of purines and sugars. Full descriptive detailsz7 have been reported for the synthesis of the HCN tetramer diaminomaleonitrile (1 l) by reduction of diiminosuccinonitrile (12); the reverse oxidative transformation of (11) into (12) has been studied,'* as has the chemical reactivity29 of (12). The effects of exposing dilute aqueous solutions of simple nitriles to low doses of ionizing radiations have been d~cumented.~' 26 G.Schlesinger and S. L. Miller J. Amer. Chem. SOC.,1973,95 3729. '' 0.W. Webster D. R. Hartter R. W. Begland W. A. Sheppard and A. Cairncross J. Org. Chem. 1972 37 4133. 28 J. P. Ferris and T. J. Ryan J. Org. Chem. 1973,38 3302. 29 R. W. Begland and D. R. Hartter J. Org. Chem. 1972,37,4136. 30 I. DraganiC Z. DraganiC Lj. PetkoviC and A. NikoliC J. Amer. Chem. SOC.,1973,95 7193. 374 E. W.Colvin The synthesis and reactions of methyl and ethyl nitrosocyanamide (13) have been reported31 as part of a general study on the carcinogenicity of N-nitroso- compounds. The microwave spectrum of cyanogen isocyanate (14) shows32 that although the molecule is bent there is considerable tendency toward linearity with LCNC = 140". A similar of nitrosyl cyanide (15) showed that this molecule is also bent with LCNO = 114" 43' and more strikingly LNCN = 172"31' f3".R-N-CNI NO NC-NCO NC-NO (13) (14) (15) 3 Ketonesand Aldehydes Ab initio (STO-3G) calculation^^^ on simple aldehydes and ketones have shown that a chiral centre induces a hybridization change in a vicinally unsaturated system ;the carbonyl n-electron cloud becomes dissymmetric the electron density being greater on one diastereotopic face than the other. This suggests that orbital factors may be at least partly responsible for asymmetric induction ; indeed neglecting steric factors results similar to those predicted by the rules of Cram Cornforth and Prelog were obtained. Ashby3' has presented a new slant on the stereochemistry of addition of Grignard reagents to carbonyls ;stereochemical control is derived from a balance between steric approach control and a 'compression effect' of the complexed carbonyl in the six-centred transition state against for example the 2,6-diequa- torial hydrogens in cyclohexanone.E.s.r. studies36 have shown that 2-alkanonyl radicals e.g. (16) are stabilized by the contributing allylic structure e.g. (17) to the extent of cu. 15 %. The kinetic results and lack of rearrangement observed in the solvolysis of 3-oxotosy!ates (Scheme 11) can be explained by proposing3' 1,4carbonyl participation. 31 S. S. Mirvish D. L. Nagel and J. Sams J. Org. Chem. 1973,38 1325. 32 W. H. Hocking and M. C. L. Gerry J.C.S. Chem. Comm. 1973,47. 33 R.Dickinson G. W. Kirby J. G. Sweeny and J. K. Tyler J.C.S. Chem. Comm. 1973 241. 34 N. T. Anh 0.Eisenstein J.-M. Lefour and M.-E. T. H. Dau J. Amer. Chem. SOC.,1973 95 6146. '' J. Laemmle E. C. Ashby and P. V. Roling J. Org. Chem. 1973,38 2526. 36 D. M. Camaioni H. F. Walter and D. W. Pratt J. Arner. Chem. SOC.,1973,95 4057. J7 P. Hodgson and S. Warren J.C.S. Chem. Comm. 1973 756. Aliphatic Compounds-Part (ii)Other Aliphatic Compounds -(6 (16) (17) CF,CO,H OCOCF Scheme 11 A detailed of Woodward's predictive U.V. rules for enones has been published. While trialkylated enols (18)can be readily isolated the dialkylated analogues3' are considerably more nucleophilic and less basic ;as a consequence intramole- cular alkylation occurs producing the furanone (19) (Scheme 12).H 0Q H (19) I"+ t Scheme 12 The rates of reaction of a variety of halogenating agents with ketone enols have been studied.40 The poor correspondence between the rates and orientation of enolization with orientation of acetoxylation in the reaction of lead tetra-acetate 38 A. Bienvenue J. Amer. Chem. SOC.,1973,% 1345. 39 H. M. R. Hoffmann and E. A. Schmidt Angew. Chem. Internat. Edn. 1973,12,239. 40 N. C. Den0 and R. Fishbein J. Amer. Chem. Soc. 1973,95 7445. 376 E. W.Colvin with unsymmetrical ketones is taken as evidence4' that enolization may not be the rate-determining step. The suggestion that the direct reaction of bromine with unenolized ketone may be a major pathway in ketone halogenation has been strongly refuted4* in a study in which no measurable dependence of the rate of bromine consumption on bromine concentration was detected.The orientation of halogenation of unsymmetrical ketones with halogen in carbon tetrachloride depends on the halogen used indi~ating~~" that halogen or halide is involved in the enolization step ;the application436 of this observation to the chlorination of ketals has been studied as a facile route to chloromethyl ketones. Swain's44 product study of the orientation of alkaline halogenation of butan-2- one has received kinetic confirmation4' that C-1 and C-3 are attacked equally rapidly. Despite the relative inertness of the monosubstitution products formed in the halogenation of ketones to further halogenation a,a'-disubstitution is frequently observed ;evidence has been presented46 for the following mechanism (Scheme 13).R = H or Ac. Scheme 13 Activation parameter^^^ determined for the bimolecular substitution reactions of a-halogenoketones support the conclusion that only those a-halogenoketones which are stereochemically disposed for carbonyl conjugation or bridging in the transition state undergo substitution by a mechanism significantly different from that operating in the corresponding reactions of alkyl halides. House4* has delineated the parameters required to optimize either C-or 0-acylation of enolate ions. A CND0/2 study4' of the orientation of base-induced ketone alkylations provided satisfactory predictions for all but alkyl-cycloalkyl cases.Speculation continues on the mechanistic ambiguities of the Favorskii re- arrangement. The ratio of diastereoisomeric esters (20) and (21) produced on 41 S. Moon and H. Bohm J. Org. Chem. 1972,37,4338. 42 J. W. Thorpe and J. Warkentin Canad. J. Chem. 1972 50 3229; R. A. Cox and J. Warkentin ibid. p. 3233; R. A. Cox J. W. Thorpe and J. Warkentin ibid. p. 3239; R. A. Cox and J. Warkentin ibid. p. 3242. 43 (a) Y. Jasor M. Gaudry and A. Marquet Bull. SOC.chim. France 1973 2732; (6) ibid. p. 2735. 44 C. G. Swain and R. P. Dunlap J. Amer. Chem. SOC.,1972,94 7204. 45 A. C. Knipe and B. G. Cox J. Org. Chem. 1973,38 3429. 46 K. E. Teo and E. W. Warnhoff J. Amer. Chem. SOC.,1973,95 2728. 47 J. W. Thorpe and J. Warkentin Canad.J. Chem. 1973,51,927. 48 H. 0. House R. A. Auerbach M. Gall and W. P. Peet J. Org. Chem. 1973,38 514. 49 M.-E. T. H. Dau M. Fetizon and N. T. Anh Tetrahedron Letters 1973,851,855. Aliphatic Compounh-Part (ii) Other Aliphatic Compoundr rearrangement of 2-bromo-4-methyl-4-phenylcyclohexanone varies with meth- oxide ion concentration ;Bordwell’ proposes as part-explanation an equilibra- tion between the two cyclopropanones via the oxyallyl zwitterion as shown (Scheme 14). H 7 Ph Ph’ 1 1 Ph’ C02Me Ph-Q..C02Me Scheme 14 A vinylogous Favorskii rearrangement accountss1 for the formation of 5-chloropenta-2,4-dienoic acid by alkalipe hydrolysis of 5,5,5-trichloropent-3-en-2-one (Scheme 15). c1 \ Cl,CCH=CHCOCH,-+ C=CH-CH \ c1/ 1 /c=o CH J-OR c1 c1 -\ \ CHCH=CHCHCO,R +-C=CH-CHCH,CO,R Cl/ \ c1/ c1 \ C=CH-CH=CHCO,R / H Scheme 15 Treatment of the isomeric propargyl derivatives (22) and (23) with sodium methoxide gave the same ester (24) whereas the methyl analogue (25) of (23) gave the branched ester (26).Kirmse” suggests that intramolecular addition of the intermediate diazotate to the triple bond initiates a Favorskii reaction 50 F. G. Bordwell and J. G. Strong J. Org. Chem. 1973,38 579. 51 A. Takeda and S. Tsuboi J. Org. Chem.. 1973,38 1709. 52 W. Kirmse A. Engelman and J. Heese,J.. Amer. Chem. SOC.,1973 95 625. 378 E. W.Colvin (Scheme 16); if R can stabilize an adjacent negative charge as with phenyl the linear ester is produced otherwise one gets branched-chain products.The possible major intermediacy of an oxyallyl zwitterion was excluded by use of chiral substrates when inversion and not racemization was observed. PhCr CCH,N(NO)CONH (22) PhCH,CH,CO,Me HCrCCH(Ph)N(NO)CONH (24) ’i (23) HCrCCH( Me)N(NO)CO,Me -Me,CHCO Me (25) (26) -0-N ’*’;?R-0 I J r’ RCH,CH,CO,Me RCHC0,Me I Me Scheme 16 While the mechanism of the homo-Favorskii reaction exemplified in Scheme 17 is still unknown the possibility of a keten such as (27) being involved has been excluded by two groups,’ 334 although (27) if independently generated will indeed cyclize’’ to the product (28). 4 Alcohols Two proposed oxidation intermediates have been trapped or detected The cyclic manganate ester (29),assumed to be formed in the oxidation of olefins to ’ R.H. Bisceglia and C. J. Cheer J.C.S. Chem. Comm. 1973 165. 54 S. Wolff and W. C. Agosta J.C.S. Chem. Comm. 1973 771. 55 S. W. Baldwin and E. H. Page J.C.S. Chem. Comm. 1972 1337. Aliphatic Compounds-Part (ii) Other Aliphatic Compounds m (27) Scheme 17 diols (Scheme 18) has now been detected spectrophotometrically by two gro~ps.~~*~' R = Me or Ph (29) Scheme 18 While the [2,3]-sigmatropic rearrangement in the proposed mechanistic sequence (Scheme 19) for selenium dioxide oxidation of olefins to allylic alcohols has received some support,58 evidence substantiating the initial ene reaction has been lacking. Such evidence has now been provided by the trapping5' of the selenic acid (30) as the lactone (31).Chiral 1-deuterio-alcohols have been subjected to considerable study. An enzyme exchange method has been reported6' for the preparative-scale synthesis of both (1R)-and (1s)-monodeuteriopropanol.The chirally complexed deuteride reagent (32)affords predominantly6 'the (S)enantiomer on reducing an aldehyde. An n.m.r. method for the determination of enantiomeric purity6' and of the absolute configuration62 of such compounds has been described. 56 D. G. Lee and J. R. Brownridge J. Amer. Chem. SOC.,1973,95 3033. 57 K. W. Wiberg C. J. Deutsch and J. R&k J. Amer. Chem. Suc. 1973,95 3034. K. B. Sharpless and R. F. Laver J. Amer. Chem. Suc. 1972,94,7154. 59 D. Arigoni A. Vasella K.B. Sharpless and H. P. Jensen J. Arner. Chem. SOC.,1973 95 7917. 6o H. Gunther F. Biller M. Kellner and H. Simon Angew. Chem. Internat. Edn. 1973,12 146. 61 C. J. Reich G. R. Sullivan and H. S. Mosher Tetruhedrun Letters 1973 1505. 62 H. Gerlach and B. Zagalak J.C.S. Chem. Cumm. 1973 274. 380 E. W. Colvin H 0’ (30) f X Y = OH OAc or OR (31) Scheme 19 LiA1D.J OR*Iz (32) R*H = ( +)-(2S,3R)-4-dimethylamino-3-methyl-1,2-diphenylbutan-2-ol A gas-chromotographic modification6 of Horeau’s method for the determina- tion of absolute configuration of secondary alcohols has been reported. Brewster’s optical-rotation rules have been modified64 to allow their application to tertiary alcohols. It has been confirmed that the gas-phase acidity order6’ for simple alcohols is the reverse of the solution order solvent assuming a major role in determining relative acidities.5 Amines and Derivatives In a continuing study of amine gas-phase basicities it has been found66 that proton-bridged 1’2-diaminoethane shows a large strain energy inferring that the N...H+...N bond tends to be linear. The utility of the induced Cotton effect for chirality determination of alcohols has found extension as was predicted to simple amines ad cyclic 1,2-amino-alcohols. 67 63 C. J. W. Brooks and J. D. Gilbert J.C.S. Chem. Comm. 1973 194. 64 R. M. Carman Austral. J. Chem. 1973 26 879. 65 R. T. McIver J. A. Scott and J. M. Riveros J. Amer. Chem. SOC.,1973,95 2706. 66 R. Yamdagni and P.Kebarle J. Amer. Chem. SOC.,1973,95 3504. 67 G. N. Mitchell and F. I. Carroll J. Amer. Chem. SOC.,1973 95 7912. Aliphatic Compounds-Part (ii) Other Aliphatic Compounds 38 1 Semi-empirical MO calculations6* on nitrenium ions have shown that for R,N+ singlet-triplet interconversion should occur readily as the energy separation is small ; the exclusive triplet reactivity69 of piperidine nitrenium ion can be rationalized on this basis. The preparation7' of several stable acyl t-butyl nitroxyl radicals has been described (Scheme 20). I Bu' Bu' R = OEt Ph p-PhC,H, or Me(CH,), Scheme 20 INDO evidence has been presented'l for the conformation of the NO group in symmetrical nitroxyl radicals ; the results obtained are consistent with a non-planar rapidly inverting NO group the minimum-energy conformation having an out-of-plane angle of 35".Ab initio SCF computations on the interconversion of ammonium oxide with hydroxylamine have revealed72 a non-least motion path bearing some resem- blance to the allowed motion for a [ 1,3]-sigmatropic shift ; the interconversion is opposed by a considerable activation energy apparently derived from a weaken- ing of the NO bond in the transition state. The out-of-plane rotational mechanism for oxime anion syn-anti isomerization seems unlikely. Peroxyacetyl nitrate (PAN) the simplest member of a class of compounds formed in photochemical smog has had its chemical reactivity investigated. PAN reacts with primary amines and ammonia to give amide~,~~ and it oxidizes aldehydes to The syntheses of t-butyl and methyl peroxynitrate (Scheme 21) have been reported ;76 although alkyl peroxynitrates have not been detected ROOH + N,O -+ ROONO + HNO R = Bu' or Me Scheme 21 68 G.F. Koser J.C.S. Chem. Comm. 1973 461. 69 P. G. Gassman and G. D. Hartman J. Amer. Chem. SOC.,1973,95 449. 70 M.. J. Perkins and P. Ward J.C.S. Chem. Comm. 1973 883. " A. Rassat and P. Rey Tetrahedron 1973 29 1599. '* C. Trindle and D. D. Shillady J. Amer. Chem. SOC.,1973 95 703. 73 E. J. Grubbs D. R. Parker and W. D. Jones Tetrahedron Letters 1973 3279. l4 P. H. Wendschuh H. Fuhr J. S. Gaffney and J. N. Pitts J.C.S. Chem. Comm. 1973 74. 75 P. H. Wendschuh C. T. Pate and J. N. Pitts Tetrahedron Letters 1973 2931. 76 E.F. J. Duynstee J. G. H. M. Housmans J. Vleugels and W. Voskuil Tetrahedron Letters 1973 2275. 382 E. W. Coluin so far in such smog the derived alkyl peroxy-radicals undoubtedly play a large part in its production. 6 Halides Controversy continues over the structure of P-halogenoalkyl radicals. There seems little doubt of their intermediacy in the transition state for the photo- bromination of alkyl bromides :Ske1177 proposes the pathway shown in Scheme 22 to explain an anchimeric assistance of ca. lo3 when compared to the photo- bromination of alkanes; the activation energy is lowered and the activation entropy is more negative as required by the bridging mechanism. # Scheme 22 Explanations of the apparently increased stability of these radicals as compared to simple alkyl radicals have involved either bridged structures (33) or radicals existing in a preferred conformation (34) allowing hyperconjugative stabilization by halogen.Based on kinetic and racemization data it has been suggested78 that in the 2-bromoethyl radical the Br atom is either centred between the two extreme positions or it moves between them with a frequency in excess of 101's-'. However CIDNP studies have inferred7' that the ground state for this radical has non-equivalent methylene groups suggesting (34) although (33) may well be involved in the transition state leading to it; interconversion of methylene groups by bromine migration cannot occur at a rate greater than the rate of diffusion from the cage.E.s.r. spectroscopy f~rther'~ evidence that the 2-chloroethyl radical has the unsymmetrical structure (34). The first e.s.r. spectrum assignable to an or-iodo-radical(35) has been reported.82 The cyclization of the 3-iodopropyl radical to cyclopropane probably proceeds 77 K. J. Shea D. C. Lewis and P. S. Skell J. Amer. Chem. SOC.,1973,95 7768. '* P. S. Skell R. R. Pavlis D. C. Lewis and K. J. Shea J. Amer. Chem. Soc. 1973 95 6735. '' J. H. Hargis and P. B. Shevlin J.C.S. Chem. Comm. 1973 179. 8o J. Cooper A. Hudson and R. A. Jackson Tetrahedron Letters 1973 831. 81 K. S. Chen I. H. Elson and J. K. Kochi J. Amer. Chem. SOC.,1973 95 5341. 82 G. W. Neilson and M. C. R. Symons J.C.S. Chem. Comm. 1973 717. Aliphatic Compounds-Part (ii) Other Aliphatic Compounds via a unimolecular carbon radical displacement ; the unimolecular alternative of a symmetrically bridged limiting structure is considered' unlikely.I~HCONH (35) Earlier attempts to form six-membered cyclic halonium ions from 1,Sdihalides gave exclusive rearrangement to five-membezed rings ;an alkylative route has been rep~rted'~ to such species which are produced in very high proportion from 1 ,Sdi-iodopentane and as major products from 1,5-dibromopentane (Scheme 23). f 65 35 Reagents i MeF-SbF,-SO,. Scheme 23 The preparation of the four-membered cyclic bromonium ion (36) has been described,' as has the preparation86 of dimethylbromonium hexafluoroanti- monate (37) which is stable at ambient temperatures.FCH (MeBrMe)+SbF,-FCH (37) (36) I3Cn.m.r. studies8' on cyclic halonium ions have been described ;temperature-dependent 13C chemical shifts can be used as indicators of the equilibrium8' between cyclic halonium ions and acyclic carbonium ions. 13' R. F. Drury and L. Kaplan J. Amer. Chem. SOC.,1973,95 2217. '* P. E. Peterson B. R. Bonaua and P. M. Henrichs J. Amer. Chem. SOC.,1973 95 2222. 85 J. H. Exner L. D. Kershner and T. E. Evans J.C.S. Chem. Comm. 1973 361. 86 G. A. Olah and J. J. Svoboda Synthesis 1973 203. " B. R. Bonaua and P. E. Peterson J. Org. Chem. 1973 38 1010. P. M. Henrichs and P. E. Peterson J. Amer. Chem. SOC.,1973 95 7449. 384 E. W. Colvin 7 Sulphur The first example of optical activity due to isotopic dissymmetry of carbon has been achieved in the synthesis*’ of the sulphoxide (38),which has = +0.71.”CH,Ph 0-S-13CH2Ph While earlier H n.m.r. evidence suggested that protonation of sulphoxides occurs on sulphur 13Cn.m.r. resultsg0 show protonation on oxygen in accord with i.r. and acid-base equilibrium studies. The similarity of sulphonyl sulphur and carbonyl carbon as electrophilic centres has been exemplified” in a study of their relative reactivity towards a number of nucleophiles. 8 Miscellaneous A number of theoretical st~dies~~*’~*’~ on asymmetric induction have been reported. Optical resolution by differential complexation has been achieved with the designg4 of chiral host molecules whose resolution involves only stability differences in solution of diastereoisomeric complexes.Enzyme-analogue chiral polymersg5 have been described as has their use as resolving agents. Hudson96 has expounded a perturbation theory of reactivity as an alternative to transition-state theory ; based on the general hypothesis that the initial per- turbation determines the course of a reaction such apparently diverse concepts as the symmetry rules for cyclic processes and the theory of hard and soft acids and bases can be derived. A photoelectron-spectroscopic studyg7 of the MO sequence in diazo-com- pounds has revealed that the highest occupied level is the non-bonding b2(n) orbital. An LCAO-MO-SCF calculation on the Wolff rearrangement (Scheme 24) has indicated’* that the postulated oxiren (39) and formyl carbene (40) have almost identical energies both ca.70 kcal mol- less stable than keten ;a related study suggests9’ that keten tends to protonate on the b-carbon. 89 K. K. Andersen S. Colonna and C. J. M. Stirling J.C.S. Chem. Comm. 1973 645. 90 G. Gatti A. Levi V. Lucchini G. Modena and G. Scorrano J.C.S. Chem. Comm. 1973,251. 91 J. L. Kice and E. Legan J. Amer. Chem. SOC.,1973,95 3912. 92 L. Salem J. Amer. Chem. Soc. 1973,95 94. 93 G. R. Franzen and G. Binsch J. Amer. Chem. SOC.,1973 95 175; R. D. Norris and G. Binsch ibid. p. 182; G. Binsch ibid. p. 190. 94 R. C. Helgeson K. Koga J. M. Timko and D. J. Cram J. Amer. Chem. SOC.,1973 95 3021 ; R. C. Helgeson J. M. Timko and D. J. Cram ibid. p. 3023. 95 G. Wulff A. Sarhan and K. Zabrocki Tetrahedron Letters 1973 4329.96 R. F. Hudson Angew. Chem. Internat. Edn. 1973 12 36. 97 E. Heilbronner and H.-D. Martin Chem. Ber. 1973 106 3376. 98 A. C. Hopkinson J.C.S. Perkin II 1973 794. 99 A. C. Hopkinson J.C.S. Perkin II 1973 795. Aliphatic Compounds-Part (ii)Other Aliphatic Compounds R',k2= H (40) R' )=c=o R' R2 AR2 Scheme 24 Reviews have been published on slow proton-transfer reactions,' O0 isocyan-ates,"' phosgene,'" and the structures and chemistry of the macrolide anti- biotics.'O3 A vigorous warning has been sounded in a variety of publications on the extreme carcinogenic hazards of handling bis(chloromethy1) ether. loo R. E. Barnett Accounts Chem. Res. 1973,6,41. S.Ozaki Chem..Rev. 1972 72,457. lo' H. Babad and A.G. Zeiler Chem. Rev. 1973,73 75. Io3 W. Keller-Schierlein,Forrschr. Chem. org. Nuturstofe 1973 30,3 13.

ISSN:0069-3030    

DOI:10.1039/OC9737000368

出版商:RSC    

年代:1973

数据来源: RSC

摘要:

13 Aromatic Compounds By J. W. BARTON School of Chemistry University of Bristol. CantockS Close Bristol BS8 1 TS 1 General Hiickel and non-Huckel systems have been discussed in a short review on antiaromatic effects.' A recent ab initio valence bond calculation of the benzene structure gives energy expectation values for the Kekule and Dewar forms which are closer than those resulting from semi-empirical treatments ; it also appears that charge-separated forms are more important than was thought hitherto.2 The flexibility of aromatic rings has been discussed. On the basis of accrued theoretical and experimental evidence it is suggested that mono- and poly- nuclear hydrocarbons are capable of 5-20' deviations from planarity in the ground state.3 The effects of substituents on rotation about the phenykarbon bond in substituted toluenes have been e~amined.~ X-Ray crystal analyses of peri-substituted naphthalenes have shown the 1,8-dimethyl derivative to be planar non-bonded interactions of the methyl groups being reduced mainly by bond-angle distortion at the junctions between the methyl groups and the nu- cle~s.~ In contrast there is little angle distortion in 1,8-bisdimethylaminonaph-thalene which is twisted about the C-4a-C-8a axis so that the dimethylamino- groups lie on either side of the mean plane of the nucleus;6 the barrier to change from one such conformation to its mirror image is at least 7.5 kcal mol- Values found for the dipole moments of 2,3,4,5-tetra- and 2,3,4,5,6-penta-fluorotoluenes are higher than expected possibly owing to hyperconjugation in the methyl groups.' Attempts to use 3Cn.m.r.data as criteria of aromaticity in potentially aromatic systems have given conflicting results ;'" ring-current effects play a ' R. Breslow Accounts Chem. Res. 1973 6 393. J. M. Norbeck and G. A. Gallup J. Amer. Chem. SOC. 1973 95,4460. H. Wynberg W. C. Niewport and H. T. Jonkman Tetrahedron Letters 1973 4623. J. E. Anderson H. Pearson and D. I. Rawson J.C.S. Chem. Comm. 1973,95. ' D. Bright I. E. Maxwell and J. de Boer J.C.S. Perkin 11 1973 2101. E. Einspahr J. B. Robert R. E. Marsh and J. D. Roberts Actu Cryst. 1973 B29 161 1. ' R. W. Alder and J. E. Anderson J.C.S. Perkin 11 1973,2086. * H. H. Huang J.C.S. Chem. Comm. 1973,723. (a) A. J.Jones P. J. Garratt and K. P. C. Vollhardt Angew. Chem. Internat. Edn. 1973 12 241; (b) A. V. Kemp-Jones A. J. Jones M. Sakai C. P. Beeman and S. Masamune Cu~d J. Chem. 1973 51 767; (c) H. Gunther H. Schmickler. H. Konigshofen K. Recker and E. Vogel Angew. Chem. Internat. Edn. 1973 12 243; (4E. Wenkert E. W. Hagaman L. A. Paquette R. E. Wingard and R. K. Russell J.C.S. Chem. Comm. 1973 135; (e) R. H. Levin and J. D. Roberts Tetrahedron Letters 1973 135; cf) H. Gunther H. Schmickler U. H. Brinker K. Nachtkamp J. Wassen and E. Vogel Angew. Chem. Internat. Edn. 1973 12 760. 386 Aromatic Compounds minor role in determining the 13Cresonance frequency compared with stereo- chemical factors such as ring size and ring train.^^'^'^ Rearrangements involving intramolecular hydrogen transfer in the mass spectra of aromatics have been reviewed;" further studies on benzyl C7H7+ and tolyl C7Hs+,cations' lad and the tolyl radical cation C,Hs'+,' le in the gas phase have been reported.While C7H7+ ions from various precursors usually isomerize to a common structure within 10- 's,it is possible to form benzyl and tolyl cations which are stable for this interval.'lc'd Active carbon functions as an efficient catalyst in the racemization of 1,l'- binaphthyl.' Syntheses of chiral crown ethers derived from resolved 2,2'-dihydroxy-1,l'-binaphthylhave been described ;'3n these show chiral recognition when complexing or-phenylethylammonium hexafluorophosphate. ' The o,m'-bridged biphenyl (1) racemizes rapidly at 60 "C;I4 conformational isomers of 1,6-bis-( 1-cyano- 1-methylethyl)triptycene have been prepared which are found to be stable against thermal (tt = 115 min at 200 "C)and photochemical inter- conversion.' Further studies of the synthesis resolution and absolute con- figurations of optically active 9,10-dihydro-9,10-ethenoanthracenes'6and trip- tycenes17 have been reported; the failure of c.d.spectral analysis to assign the correct absolute configurations to certain derivatives of these systems' is attributed to error in the calculation of transition rnoment~.'~ lo J. T. Bursey M. M. Bursey and D. G. I. Kingston Chem. Rev. 1973 73 191. I' (a)R. C. Dunbar J. Amer. Chem. SOC.,1973,95,472; (b)R. C. Dunbar and E. W. Fu ibid p. 2716; (c)J. Winkler and F.W. McLafferty ibid. p. 7533; (4M. K. Hoffmann and J. C. Wallace ibid. p. 5064; (e) K. Levson F. W. McLafferty and D. M. Jerina ibid. p. 6332. R. E. Pincock W. M. Johnson K. R. Wilson and J. H. Farmer J. Amer. Chem. SOC. 1973,95,6477. I' (a) E. P. Kyba M. G. Siegel L. R. Sousa G. D. Y. Sogah and D. J. Cram J. Amer. Chem. SOC.,1973 95 2691; (b) E. P. Kyba K. Koga L. R. Sousa M. G. Siegel and D. J. Cram ibid.,p. 2692. l4 E. W. Warnhoff and P. Reynolds-Warnhoff Canad. J. Chem. 1973 51 2338. l5 H. Iwamura J.C.S. Chem. Comm. 1973,232. l6 Y. Sakata H. Tatemitsu F. Ogura and M. Nakagawa J.C.S. Chem. Comm. 1973 22; H. Tatemitsu F. Ogura and M. Nakagawa Bull. Chem. SOC.Japan 1973,46,915. M. Kuritani Y. Sakata F. Ogura and M. Nakagawa Bull. Chem. SOC.Japan 1973 46 605; Y.Sakata F. Ogura and M. Nakagawa ibid. p. 611; F. Ogura and M. Nakagawa ibid. p. 651; Y. Shimizu T. Naito F. Ogura and M. Nakagawa ibid. p. 1520. J. Tanaka C. Katayama F. Ogura H. Tatemitsu and M. Nakagawa J.C.S. Chem. Comm. 1973,21. l9 s.F. Mason J.C.S. Chem. Comm. 1973 239; A. M. F. Hezemans and M. P. Groen- wege Tetrahedron 1973 29 1223. 388 J. W. Barton Homo- and Pseudo-aromaticity.-An alternative MO-based model has been put forward to account for the unusual stability of homoaromatic systems.20 For example with the homotropylium ion it involves treating the external rnethylene group together with its two CH neighbours as a cyclopropane ring. There is evidence of homoaromatic stabilization of the dianion (2) from 2,4-diphenyl- bicyclo[3,2,1]oct-6-en-3-one.21Attempts to use polarography to assess homo- aromatic stabilization of the radical anions from cis-bicyclo[6,1,O]nona-2,4,6-triene and its 2,3-benzo-derivative were unsu~cessful.~ There has been conflict of opinion over the suggestion that there is homoaromatic stabilization in the pentaene (3) which could exist as a 67t (or lOn)-electron homoaromatic system (4);23a*b n.m.r.studies of this and related bridged annulenes provide strong evidence that there is not.23c Bishomocyclopropenium cations such as (5) generated by additions to hexameth~lDewarbenzene,~~" are further protonated in strong acids at low temperature to give the stable symmetrical dication (6),24b,cwhich passes irreversibly into the hexamethylbenzenium cation at 40"C.Me (5) X = C1or Br (6) Further studies of the protonation of cis-bicyclo[6,l,0]nona-2,4,6-triene and some derivatives by 'C n.m.r. spectroscopy have been reported. In superacidic media there appears to be initial formation of transoid 1,3-bishomotropylium ions such as (7) which subsequently undergo conformational inversion of a 2o W. J. Hehre J. Amer. Chem. SOC., 1973 95 5808. " G. B. Trimitsis E. W. Crowe G. Slomp and T. L. Helle J. Amer. Chem. SOC.,1973 95 4333. 22 L. A. Anderson M. J. Broadhurst and L. A. Paquette J. Amer. Chem. SOC.,1973 95 2 198. 23 (a)L. A. Paquette R. E. Wingard and R. K. Russell J. Amer. Chem. SOC., 1972 94 4739; (b) G. P. Ceasar J. Green L. A. Paquette and R. E. Wingard Tetrahedron Letters 1973 1721; (c) E.Vogel U. H. Brinker K. Nachtkamp J. Wassen and K. Mullen Angew. Chem. Internat. Edn. 1973 12 758. 24 (a) H. Hogeveen and P. W. Kwant Tetrahedron Letters 1973 423; (b) H. Hogeveen and P. W. Kwant ibid. p. 1665; (c) H. T. Jonkman and W. C. Niewport ibid. p. 1671. Aromatic Compounds methylene bridge to give their cisoid counterpart^.^^ Isomerization of the bridged ion (8) to give 2-deuterioindene takes place by a process which is thought to involve the bishomotropylium ion (9) resulting from interaction of the cationic D D centre at C-9 with the C-7-C-8 ene function rather than the other ion which would result from C-9 interaction with the diene bridge.26 The term pseudoaromatic (cf. ref. 1) has been applied to the dication (lo) generated from 1,4-dichlorobicyclo[2,2,2]octanein superacidic media at -78 “C.The ion which is isoelectronic with the cyclobutadiene dication exhibits unusual stability and gives rise to unrearranged products on quenching.” Benzene Isomersand Benzene Oxides.-Prismane (12) has been obtained in low yield by photolysis of the azo-compound (11). It is a colourless liquid stable at room temperature but rearranging at 90°C to give benzene (t+ = 11 h).28 Photolysis of 1,2,4,5-tetrakis(trimethylsilyl)benzenehas given the 1,2,3,5-isomer together with fulvenes and benzvalenes ; no prismanes or Dewarbenzenes were obtained.*’ The first examples of this type of isomerization in the naphthalene ” L. A. Paquette M. J. Broadhurst P. Warner G. A. Olah and G.Liang J. Amer. Chem. SOC.,1973,95 3386. 26 D. C. Sanders and H. Schecter J. Amer. Chem. Soc. 1973,95 6858. 27 G. A. Olah G. Liang P. von R. Schleyer E. M. Engler M. J. S. Dewar and R.C. Bingham J. Amer. Chem. SOC.,1973,95 6829. T. J. Katz and N. Acton J. Amer. Chem. SOC.,1973 95 2738. 29 R. West M. Furne and M. N. M. Rao Tetrahedron Letters 1973 91 1. 390 J. W. Barton series have been reported with peri-di-t-butyl derivatives ; 1,3,6,8-tetra-t-butyl-naphthalene affords a photostationary state in which the Dewar isomer (13) is present in 94% yield.30 There is further evidence that the bicyclopropenyl re- arrangement proceeds via Dewarbenzene rather than prismane intermediates. Thermal rearrangement of (14) at 45 "Cgives rise to the benzene derivatives (16) and (17) the former being formed via the Dewar isomer (15).31 The presence of Me phPxph Ph Ph Ph Ph (17) (15) was demonstrated spectroscopically and when silver(1) catalysis was em- ployed it could be isolated ;a reaction sequence initiated by retrocarbene cleavage of one of the cyclopropene rings is suggested.This rearrangement is markedly influenced by the donor or acceptor properties of the bridge substituents ;under silver(1) catalysis the corresponding monocyanobicyclopropenyl shows no tendency to rearrange whereas the monomethoxy-derivative gives almost exclusively the Dewar isomer corresponding to (15) in a few minutes at room temperature. Thermolysis of the tetramethylhomofulvene (18) gives pentamethyl- benzene exclusively ;the main products from the corresponding endo-and exo-pentamethyl compounds are the three isomeric ethyltetramethylbenzenes formed by a radical chain process.32 30 W.L. Mandella and R. W. Franck J. Amer. Chem. Suc. 1973 95 971. 31 R. Weiss and S. Andrae Angew. Chem. Internat. Edn. 1973 12 150 152. 32 R. Criegee D. Schonleber R. Huber C. Schweickhardt R. Wolf and R. Ramirez Chem. Ber. 1973,106 857. Aromatic Compounds 39 1 The synthesis of anti-benzene dioxide (19) has been reported; unlike the syn- isomer it is thermally stable and does not equilibrate with 1,4-dio~ocin.~~ Com-parison of the vicinal coupling constant J4.5in the n.m.r. spectrum of (19) with that of the syn-isomer and the corresponding one in the recently isolated anti- biotic (20)34indicates that the latter should be assigned the syn-benzene dioxide structure.Nucleophilic substitutions of benzene trioxide (21 ;X = 0)have been studied,35 certain of which have led to a synthesis of the tri-imine (21 ;X = NH). Me Me (18) Kinetic studies of the dienol-benzene rearrangement36 and of the aromatization of indane 8,g-oxide (22)3’have been reported. In the latter it has been shown that 33 E. Vogel H. J. Altenbach and E. Schmidbauer Angew. Chem. Internal. Edn. 1973 12 838. 34 D. B. Borders P. Shu and J. E. Lancaster J. Amer. Chem. SOC.,1972 94 2540. 35 R. Schwesinger and H. Prinzbach Angew. Chem. Internat. Edn. 1973 12 989; R. Schwesinger H. Fritz and H. Prinzbach ibid. pp. 993 994. 36 V. P. Vitullo and M.J. Cashen Tetrahedron Letters 1973 4823. 37 G.J. Kasperek P. Y. Bruice T. C. Bruice H. Yagi and D. M. Jerina J. Amer. Chem. SOC.,1973 95 1673 6041. 392 J. W. Barton 5-hydroxyindane the main product formed in acidic solutions results from allylic hydration of and subsequent elimination from the cation (23) whereas in neutral and alkaline solutions the observed product 4-hydroxyindane is formed mainly through an 'oxygen walk' process uia indane 4,8-oxide (24). The formation of rearranged arene oxides has also been observed on halogen elimination from the bridged structures (25).38 X B$k? X (25) X = CN or C0,Me 2 Benzene and its Derivatives Application of the perturbation theory treatment of chemical reactivity to aro- matic substitution has been disc~ssed.~' Recent reviews have dealt with anodic aromatic ~ubstitution,~' the reactivity and stability of arenediazonium ions,4' and the Zinin reduction of nitroarenes ;42 accounts of the acyl~xylation~~ and arninati~n~~ of aromatics and of carbocations derived from aromatic have been included in reviews of a wider nature.The ambident nature of the triphenylmethyl cation has been established in certain hydrogen-transfer reactions.46 Doubts as to the correctness of the accepted value for the rate of protodetritiation of benzene at 70 "C(9.5 x s-') com-monly used as a standard for comparing partial rate factors were unjustified as shown by a recent study;47 a quantitative correlation between hydrogen exchange rates and charge distribution in the benzenium ion has been estab- li~hed.~* Criticism has been levelled at the way in which the Hammett equation has been employed for structure-reactivity correlations in aromatic substitution reaction^.^' A second example of steric hindrance to acid-catalysed hydrogen exchange has been encountered with the ortho-positions in tetraphenylmethane.'' It is found that rn-phenylenediamines undergo uncatalysed deuterium exchange 38 F.G. Klarner and E. Vogel Angew. Chem. Internat. Edn. 1973 12 840. 39 R. F. Hudson Angew. Chem. Internat. Edn. 1973 12 36. 40 L. Eberson and K. Nyberg Accounts Chem. Res. 1973 6 106. 41 H. Zollinger Accounts Chem. Res. 1973 6 335. 42 H. K. Porter Org. Reactions 1973 20 455. 43 D. J. Rawlinson and G.Sosnovsky Synthesis 1973 567. 44 F. Minisci Synthesis 1973 1. " G. A. Olah Angew. Chem. Internat. Edn. 1973 12 173. 46 G. A. Olah and J. J. Svoboda J. Amer. Chem. SOC. 1973,95 3794. 4' H. V. Ansell and R. Taylor J.C.S. Chem. Comm. 1973,952. 48 H. V. Ansell J. Le Guen and R. Taylor Tetrahedron Letters 1973 13. 49 C. D. Johnson and K. Schofield J. Amer. Chem. SOC.,1973,95 270. 50 H. V. Ansell and R. Taylor J.C.S. Chem. Comm. 1973 936. Aromatic Compounds at positions ortho and para to the amino-groups whereas o-and p-phenylene- diamines do not ; a mechanism involving direct attack by a deuterium cation is suggested.' Further studies of the protonation of aromatic hydrocarbon^,'^" fluoroaro-ma tic^,^^^ phenols and phenol ethers52c in superacidic media have been reported ; the reaction of methoxybenzenes with alkylfluoroantimonates in sulphuryl chlorofluoride at -70 "C has given dialkylaryloxonium salts which are stable at that temperature but which are transformed into ring-alkylated alkoxy- benzenes on warming to O"C.53The methylation of anisole with ['H,]methyl chloroformate and silver hexafluoroantimonate at 30 "C gives a mixture of o- rn- and p-methyl and -[2H3]methyl anisoles consistent with a principal process involving formation of the methyl-[2H,]methyl-phenyloxonium ion (26) and its subsequent intermolecular reaction with ani~ole.'~ Methoxycarbenium hexafluoroantimonate (27) proves to be an effective methylating agent for aro- matics;" arylations by means of aryl cations have also been reported.56 Gas- phase methylenation of aromatic substrates occurs in the presence of methoxy- methyl cations produced by the fragmentation of dimethyl ether ;" activated aromatics are rapidly alkylated by t-butyl trifluoroacetate in trifluoroacetic acid 51 J.C. Grivas J. Org. Chem. 1973 38 1204. '' (a) G. A. Olah and Y. K. Mo J. Amer. Chem. SOC., 1973,95,6827;(6) G. A. Olah and Y. K. Mo J. Org. Chem. 1973,38,3212; (c)G. A. Olah and Y. K. Mo J. Amer. Chem. SOC.,1972 94 5341 ; J. Org. Chem. 1973 38 2212. 53 G. A. Olah and E. G. Melby J. Amer. Chem. SOC.,1973 95,4971. 54 P. Beak J. T. Adams P. D. Klein P. A. Szczepanik D. A. Simpson and S. G. Smith J. Amer. Chem. SOC.,1973 95 6027. 55 G. A. Olah and J. J. Svoboda Synthesis 1973 52. 56 H.Bottcher H. G. C. Becker V. L. Ivanov and M. G. Kusmin Chimia (Switz.),1973 27 437; P. Burri and H. Zollinger Helu. Chim. Acfa 1973 56 2204; N. Kamigata R. Hisada H. Minata and M. Kobayashi Bull. Chem. SOC.Japan 1973 46 1016. 57 R. C. Dunbar J. Shen E. Melby and G. A. Olah J. Amer. Chem. SOC.,1973,95,7200. 394 J. W. Barton at room tem~erature.~~ confor-A detailed investigation of the synthe~is,~~" mati~n,~'~ and electrophilic substitution59c of cyclopropylbenzenes has been reported. In 1-methylcyclopropylbenzene the symmetrical conformation (28) is preferred owing to steric requirements thus the 1-methylcylopropyl group is unfavourably placed electronically and is less effective in stabilizing positive charge in the transition states of electrophilic substitutions than is a cyclopropyl group.H. Titanium(rv)-catalysed electrophilic bromination and formylation of 2-carbonyl-substituted phenols shows a marked preference for attack at the vacant position ortho to the hydroxyl function possibly owing to the formation of cyclic titanium complexes.60 Bridged ions such as (29) have been postulated as inter- mediates in electrophilic nitrations ;61 variations in the rates and isomer distri- butions for the competitive nitration of benzene and toluene in several organic solvents have been investigated.62 Nitrations of aromatics with dinitrogen tetr~xide,~~ titanium(1v) nitrate,64 and nitronium triflu~romethanesulphonate~~ have been studied and polyalkylbenzenes have been selectively mononitrated using methyl nitrate-boron trifluoride in nitromethane.66 Following studies of the ipso-nitration of o-xylene discussed in last year's Report (p.570),it is found that ipso-nitration of toluene giving rise to the diastereomers of (30) occurs to the extent of 3-4 % in acetic anhydride at -30 0C.67Similarly the action of acetyl nitrate on benzo[a]cyclopropa[c]cycloheptenegives up to 60%of the stereoisomers of (31) one of which has been shown to undergo nitro-group migration in acidic media.68 In 2,3-and 3,4-dimethylbenzonitrilessome ips0 attack takes place at the carbon atom bearing a methyl group meta to the cyano substituent giving 58 U. Svanholm and V. D. Parker J. C.S. Perkin Z 1973 562. 59 (a) W. Kurtz and F. Effenberger Chem. Ber.1973 106 511 560; (b) P. Fischer W. Kurtz and F. Effenberger ibid. p. 549; (c) W. Kurtz P. Fischer and F. Effenberger ibid. p. 525. 6o T. M. Cresp M. V. Sargent J. A. Elix and D. P. H. Murphy J.C.S. Perkin Z 1973 340. 61 F. Bernardi and W. J. Hehre J. Amer. Chem. SOC.,1973,95 3078. 62 S. Sekiguchi A. Hirose S. Kato and K. Matsui Bull. Chem. Sue. Japan 1973 46 646. 63 G. R. Underwood R. S. Silverman and A. Vanderwalde J.C.S. Perkin Z 1973 1177. 64 D. W. Amos D. A. Baines and G. W. Flewett Tetrahedron Letters 1973 3191. 65 C. L. Coon W. G. Blucher and M. E. Hill J. Org. Chem. 1973 38 4243. 66 G. A. Olah and H. C. Lin Synthesis 1973 488. 6' A. Fischer and J. N. Ramsay J.C.S. Perkin I 1973 237. 68 R. C. Hahn and M. B. Groen J. Amer. Chem. SOC.,1973,95 6128.Aromatic Compounds 395 rise to adducts such as (32),69" whereas 3,4-dimethylanisole gives (33 ;R = OAc) and the ketal(33 ;R = OMe) resulting from attack para to the metho~y-group.~~' Isomer distributions in electrophilic cyanation have been determined for a range of aromatic compounds and compared with those from other direct cyanation reactions anodic photo etc7' Friedel-Crafts-type s~lphonylation~ and aminati~n~~ have been further studied ;certain aromatics are arylaminated to give diphenylamine derivatives by N-arylhydroxylamines in a reaction which has the characteristics of an electrophilic substitution and possibly involves attack by nitrenium ions.73 Oxidative substitution reactions to receive attention include those promoted by Ce'v,74 Pd" ,'' and CO'",~~ salts.There is e.s.r. evidence that radical cations are readily formed under thallation condition^,^' and mechanistic studies of the reaction of benzene with cobalt(II1) trifluoro- acetate indicate the formation of radical cations which subsequently react with nu~leophiles.~ Rate studies of the base-catalysed deuteriation of 1,3,5-trinitrobenzene in NN-dimethylformamide4euterium oxide mixtures show that there is competi- tion between carbanion formation leading to proton exchange and Meisenheimer complex formati~n.~~ Meisenheimer complexes as intermediates in nucleophilic substitution continue to attract attention ;79 in particular several studies of the more stable spiro 0-C-O8' and 0-C-N" types have been reported.The formation of a gem-di(alky1thio) analogue of a Meisenheimer complex has been observed in solution,82 as has the formation of both cis-and trans-forms of 69 (a) A. Fischer and C. C. Greig J.C.S. Chem. Comm. 1973 300; (b) A. Fischer and D. R. A. Leonard ibid. p. 396. 70 S. Nilsson Acra Chem. Scand. 1973 27 329. " G. A. Olah S. Kobayashi and J. Nishimura J. Amer. Chem. Soc. 1973 95 564. 72 J. W. Strand and P. Kovacic J. Amer. Chem. SOC.,1973 95 2977. 73 K. Shudo and T. Okamoto Tetrahedron Letters 1973 1839. 74 M. E. Kurz E. S. Steele and R. L. Vecchio J.C.S. Chem. Comm. 1973 109. 75 G. G. Arzoumanidis and F. C. Rauch J. Ore. Chem. 1973,38 4443; L. Eberson and L. Gomez-Gonzales Acta Chem. Scand. 1973 27 1162 1249 1255. 76 J. K.Kochi R. T. Tang and T. Bernath J. Amer. Chem. SOC.,1973,95 71 14. 77 I. H. Elson and J. K. Kochi J. Amer. Chem. Soc. 1973,95 5060. 78 E. Buncel and E. A. Symons J. Org. Chem. 1973 38 1201. '' C. A. Fyfe M. Cocivera and S. W. H. Damji J.C.S. Chem. Comm. 1973 743; M. R. Crampton and H. A. Khan J.C.S. Perkin fi 1973 710 1103. M. R. Crampton J.C.S. Perkin fi 1973 2157. (a)C. F. Bernasconi R. H. De Rossi and C. L. Gehriger J. Org. Chem. 1973,38 500 2838; (6)S. Sekiguchi T. Itagaki T. Hirose K. Matsui and K. Sekine Tetrahedron 1973,29 3527; (c)S. Sekiguchi and T. Shiojima Bull. Chem. SOC.Japan 1973,46 693. 82 G. Biggi and F. Pietra J.C.S. Chem. Comm. 1973 229. 396 J. W.Barton complex (34)by addition of sulphite ion to 1,3,5-trinitroben~ene.~~ Nucleophilic substitutions of hexacyanobenzene including Meisenheimer complex formation have been described;84 the compound reacts with water to give the highly acidic pentacyanophenol (pK = -2.9).Full details of copper-catalysed sub- stitutions of aryl halides by phthalimide ion have been published,* and indirect nucleophilic displacements with carbanions on aryl halides have been achieved in the presence of tetrakis(triphenylphosphine)nickel(O)(Scheme 1);86 in a related LiCH,COPh PhBr + Ni(PPh,) + PhNi(PPh,),Br [PhNi(PPh,),CH,COPh] + PhCOCH,Ph Scheme 1 reaction replacement of halogen by methyl has been brought about by methyl- tris(triphenylphosphine)rhodium(~).~’ Amine copper(1) perchlorates are effective in promoting homolytic cleavage of aryldiazonium salts in neutral media ;’* rate studies of the homolytic phenyl- ation of 4-substituted pyridines in acidic and non-acidic media indicate that the phenyl radical has some nucleophilic ~haracter.~~ Irradiation of chlorobenzene at 2537 has given a high-energy species n-chlorobenzene (39,which behaves as a biradical.” The thermal isomerization reactions of phenylcarbene dis- cussed in last year’s Report (p.572) have now been published in full.9’ The end product of these rearrangements fulveneallene (37) has been obtained in pre- parative quantities by the vacuum thermolysis of phthalide at 7G750oC.92a,b 83 C. F. Bernasconi and R. G. Bergstrom J. Amer. Chem. SOC.,1973 95 3603; M. J. Strauss and S. P. B. Taylor ibid. p. 3813. 84 K. Friedrich and S.Oeckl Chem. Ber. 1973 106 2361 3796 3803. R. G. R. Bacon and A. Karim. J.C.S. Perkin I 1973 272. 86 M. F. Semmelhack R. D. Stauffer and T. D. Rogerson Tetrahedron Letters 1973 4519. M. F. Semmelhack and L. Ryono Tetrahedron Letters 1973 2967. A. H. Lewin and R. J. Michl J. Org. Chem. 1973,38 1126. 89 A. Clerici F. Minisci and 0.Porta Gazzetta 1973 103 171. 90 M. A. Fox W. C. Nicholls and D. M. Lemal J. Amer. Chem. SOC.,1973,95 8164. 91 W. D. Crow and M. N. Paddon-Row Austral. J. Chem. 1973 26 1705. 92 (a) U. E. Wiersum and T. Nieuwenhuis Tetrahedron Letters 1973 258 1 ;(b)C. Wentrup and P. Muller ibid. p. 2915. Aromatic Compounds 397 It is suggested that an extrusion process leads to the carbene (36) which subse- quently undergoes a Wolff-type rearrangement.Ethynylcyclopentadiene toluene ‘0 (37) and benzene are also formed in minor quantities ; the thermolysis of benzocyclo-propene gives similar products.92b The formation of benzocyclobutene and styrene in the vacuum thermolysis of methyl p-tolylacetate indicates the inter- mediacy of p-tolylcarbene ;93 the corresponding reaction of p-ethyltoluene may be mechanistically similar. although only styrene could be isolated in this case. There is recent evidence that benzonitrile oxide (38a) which is known to undergo f-Ph-CEN-0 ++ Ph-C-N=O (384 (38b) 1,3-~ycloadditions with dipolarophiles can also behave as phenylnitrosocarbene (38b) and attack olefinic bonds.94 7-Ethylcyclohepta- 1,3,5-trienes have been obtained by ring expansion of alkylbenzenes with the carbenoid reagent from diethylzinc and i~doform.~~ Cyclopentadienylcarbenes from the photolysis of diazocyclopentadienes react with arenes to form derivatives of the spironorcaradiene-spirocycloheptatriene system (39a) d(39b) the dynamics of which have been investigated ; the corresponding reactions of carbenes from diazocyclohexadienones (40) with 93 W.J. Baron and M. R. Decamp Tetrahedron Letters 1973 4225. 94 G. Lo Vecchio G. Grassi F. Risitano and F. Foti Tetrahedron Letters 1973 3777. ” S. Miyano and H. Hashimoto J.C.S. Chem. Comm. 1973 216; Bull. Chem. SOC. Japan 1973 46 3257. 398 J. W.Barton benzene give biphenyl derivatives (4l)dire~tly.’~ The action of phthalimidonitrene on 1,3-dimethoxybenzene in benzene solution gives the 3H-azepine (42) but if acetic acid is present the insertion product (43) is formed at the expense of (42).” OH VHNY YNf=Jle OMe QoMe OMe (43) Whereas arenes are unreactive in photoadditions to acrylonitrile it is found that irradiation of acrylonitrile-arene-zinc chloride complexes brings about the formation of 1,2-cycloadducts to a much greater e~tent.’~ The photoaddition of cyclopentene to benzene gives the endo-cycloadduct (44),together with a 1 (44) smaller amount of the corresponding exo-isomer ;99a in parallel reactions of 0-,rn- and p-xylenes a methyl group is oriented specifically at C-1 ;99b further studies of the photoaddition of other 1,3-dienes to benzene have also been re- port ed.’O0 96 H. Durr and H. Kober Chem. Ber. 1973 106 1565. 97 D. W. Jones J.C.S. Chem. Comm. 1973 67. 98 M. Ohashi A. Yoshino K. Yamazaki and T. Yonezawa Tetrahedron Letters 1973 3395. 99 (a)V. Y. Merritt J. Cornelisse and R. Srinivasan J. Amer. Chem. SOC.,1973,95 8250; (b) V. Y. Merritt J. Cornelisse and R. Srinivasan ibid. p. 6197. loo N. C. Yang and J. Libman Tetrahedron Letters 1973 1409. Aromatic Compounds Molecular Rearrangements.-Rearrangemen ts which have received further study include the Fischer-Hepp,"' 0-'O2 and N-Clai~en,"~ and the Smiles full details of the rearrangement of acetophenones to methyl arylacetates by thallium(w) nitrate have been published. 'O5 The reversibility of certain acid- catalysed Fries rearrangements has been demonstrated '06' and several other studies of the reaction lo6-and of its photo-induced counterpart 106e~'07a,b have been reported ; CIDNP evidence points to the operation of a radical-pair mechanism in the photorearrangement of p-cresyl p-chlorobenzoate.'07b There have been further examples of the photo-Fries-type rearrangement of N-acylaryl-amines' O8 and of the photorearrangement of azoxyarenes to o-hydroxyazo- compounds.'O9 In this connection the rearrangement of o-nitrobenzanilide to the o-hydroxyazobenzenecarboxylic acid (46) may be mentioned ; the process % a'''" d N H P h +hv ' NO N=NPh I 0- (45) (46) is probably initiated by abstraction of the amide N-proton by the excited nitro- group leading to the azoxy-derivative (45) which undergoes further photo- rearrangement to give the product.'lo The formation and rearrangement of ylides such as (47)is a useful method for the ortho-methylation of primary aromatic amines (Scheme 2):' 'la a mechanis- tically similar sequence has been employed to introduce formyl groups ortho to a methyl substituent.' lb Preferential ortho attack by sulphonyl oxygen has been observed in the rearrangement of 0-(arenesulphony1)phenylhydroxyl-amines ;l '' the effects of temperature solvent etc.,are consistent with heterolysis Io1 T. D. B. Morgan D. L. H. Williams and J. A. Wilson J.C.S. Perkin II 1973 473. lo* N. Sardvic T. Zsindely and H. Schmid Helu. Chim. Acta 1973 56 1457. M. Schmid H. J. Hansen and H. Schmid Helu.Chim. Acra 1973,56 105; H. Scheurer T. Zsindely and H. Schmid ibid p. 478. Io4 N. W. Gilman P. Levitan and L. H. Sternbach J. Org. Chem. 1973 38 373. lo' A. McKillop B. P. Swann and E. C. Taylor. J. Amer. Chem. SOC. 1973 95 3340. '06 (a) F. Effenberger H. Klenk and P. L. Reiter Angew. Chem. Inrernat. Edn. 1973 12 775; (b)J. R. Norelle J. Org. Chem. 1973 38 1924; (c)R. Martin J. M. Betoux and G. Coton Bull. SOC. chim. France 1973 1438 1442; (d)R. Martin ibid. p. 3087; (e) H. T. J. Chan and J. A. Elix Austral. J. Chem. 1973 26 1442. 107 (a) A. S. Kende J. L. Belletire and E. L. Hume Terrahedron Letters 1973 2935; (6) W. Adam J. A. De Sanabia and H. Fischer J. Org. Chem. 1973 38 2571. lo' Y. Katsuhara H. Murayama Y. Shigemitsu and Y. Odaira Tetrahedron Letters 1973 1323.Io9 R. H. Squire and H. H. Jaffe J. Amer. Chem. SOC. 1973 95 8188; D. J. W. Goon N. G. Murray J. P. Schoch and N. J. Bunce Cunad. J. Chem. 1973,51 3827. 'lo B. C. Gunn and M. F. G. Stevens J.C.S. Chem. Comm. 1972 835. 'I1 (a)P. G. Gassman and G. Gruetzmacher J. Amer. Chem. Soc. 1973 95 588; (6)C. Huyn S. Julia R. Lorne and D. Michelot Bull. SOC.chim. France 1972 4057. D. Gutsche and A. Heesing Chem. Ber. 1973 106 2379. 400 J. W. Barton Bu'OCI NaOMePhNH -PhNHCl % PhNHS+Me2C1--+ (47)1 Scheme 2 to give an intimate arylnitrenium-tosylate ion pair which recombines rapidly. Periodate oxidation of o-hydroxymethylphenols having at least one bulky ring substituent gives isolable monomeric spiroepoxycyclohexa-2,4-dienones.''3a When o-hydroxydiarylcarbinolsare oxidized in the same way the resulting epox- ides (48) undergo spontaneous rearrangement to catechol acetals (49).'36 A R' R2 = H alkyl or halogen (49) R3 = H or Ph similar spiro intermediate is possibly involved in the conversion of o-hydroxy- aldehydes and -ketones into o-hydroxy-anilides by reaction of their sodium salts with chloramine (Scheme 3).lI4 Ylides such as (50; R = CH,Ph) derived from 1 R=HorMe Scheme 3 'I3 (a) H.Becker T. Bremholt and E. Adler Tetrahedron Lerrers 1972 4205; (6) H. Becker and T. Bremholt ibid. 1973 197. 'I4 R. A. Crochet and P. Kovacic J.C.S. Chem. Comm. 1973 716. Aromatic Compounds 401 o-dimethylaminophenol are isolable whereas the corresponding p-isomers are unstable above 0°C.On heating (50; R = CH,Ph) gives mainly the ether (51 ;R = CH,Ph) formed by an intramolecular 1,4-sigmatropic rearrangement ; the corresponding ally1 derivative (50; R = CH,CH=CH,) gives in addition to (51 ;R = CH,CH=CH,) the phenol (52) formed in a sequence of 2,3-and 3,3-rearrangements.' l5 Biaryk-The UHman synthesis of biaryls has been reviewed.' l6 Oxidative coupling of phenols has received further study,"' as have coupling reactions of other aromatics in the presence of PbIVll l8 Pd",' ' and Vv' 2o salts and by anodic oxidation.' The mechanism of biaryl formation by the thermolysis of benzoyl peroxide in hexafluorobenzene has been investigated ;'22 pentafluorobiphenyl is formed in a reaction which may not involve free CF radicals when a mixture of benzene and hexafluorobenzene is photoly~ed.'~~ It may be mentioned here that photocyclizations of o-methoxybenzanilides such as (53),to phenanthridones (53) OMe take place by elimination of methanol ;'24 oxidative photocyclization with retention of the o-methoxy-group which is the usual reaction with cis-stilbenes and related compounds was not observed.I" S. Mageswaran W. D. Ollis I. 0.Sutherland and Y. Thebtaranonth J.C.S. Chem. Comm. 1973 651,653 654. M. Goshaev 0.S. Ostroshchenko and A. S. Sadykov Russ. Chem. Rev. 1972 41 2198 (English translation p. 1046). M. A. Schwartz B. F. Rose and B. Vishuvajjala. J. Amer. Chem. Soc. 1973 95 612; S. M. Kupchan and A. J. Liepa ibid. p. 4062; P. D. McDonald and G. A. Hamilton ibid.p. 7752. 'I8 R. 0.C. Norman. C. B. Thomas and J. S. Willson J.C.S. Perkin I 1973. 325. '" H. Yoshimoto and H. Itatani Bull. Chem. Sac. Japan 1973 46 2490; J. Org. Chem. 1973 38 76. Izo S. M. Kupchan A. J. Liepa V. Kameswaran and R. F. Bryan J. Amer. Chem. Suc. 1973 95,6861. K. Nyberg Acta Chem. Scand. 1973 27 503. lz2 R. Bolton and J. P. B. Sandall J.C.S. Chem. Comm. 1973 286. D. Bryce-Smith A. Gilbert and P. J. Twitchett J.C.S. Chem. Comm. 1973 457. lZ4 Y. Kanaoka and K. Itoh J.C.S. Chem. Comm. 1973 647. 402 J. W. Barton Arynes and Aryne Precursors.-The synthesis and chemistry of 1,Qdehydrobenz-enes has been reviewed.'25 Irradiation of phthaloyl peroxide in an argon matrix at 8 K results in decarboxylation to give benzopropiolactone (54) and the keto- keten (59 the two being interconvertible photochemically.' 26a Prolonged irradiation of (54) gives benzyne the i.r.spectrum of which has now been recorded. Irradiation of benzocyclobutenedione under similar conditions also gives benzyne and when acetone is included in the matrix as an internal filter there appears an i.r. spectral band at 1838 cm-' which is tentatively assigned to benzocyclopropenone (56).lZ6* When a matrix of free benzyne was allowed to \ -% @o + co \ warm triphenylene was the isolable product while with a matrix containing furan the known [4 + 21 cycloadduct was obtained. Biphenylenes from aryne dimerization are formed in vacuum pyrolyses of lH-2,3-benzoxazin- 1-ones.I2 + Iz5 R. G. Bergman Accounts Chem.Res. 1973 6 25. (a)0.L. Chapman C. L. McIntosh J. Pacansky G. V. Calder and G. Orr J. Arner. Chem. Soc. 1973.95,4061; (b)0.L. Chapman K. Mattes C. L. McIntosh J. Pacansky G. V. Calder and G. Orr ibid. p. 61 34. lz7 M. P. David and J. F. W. McOmie Tetrahedron Letters 1973 1361. Aromatic Compounds In the reaction of ortho-deuteriated benzenediazonium acetate with anthracene which gives both radical- and aryne-derived products there is extensive deuterium removal before the diazonium ions decompose ;the finding lends support to the previous suggestion that the betaine (57)is an intermediate.' 28 The cleavage of ethers by benzyne has been re~0rted.I~' Substituted naph- thalenes have been obtained from [2 + 41 cycloadditions of benzynes with dienolate anions (Scheme 4),' 30 while condensations of bromoarenes' with diethyl 0-M+ R' R2 OH R' R2 R' R' R' R' = H R2 = OH R' = OMe R2 = OH R' = OMe RZ = Me Scheme 4 malonate in the presence of sodamide have given monoesters of homophthalic acid (59)together with homophthalimides.' ' In the latter reaction it is probable that benzocyclobutenones (58) result from interaction of an aryne with diethyl malonate and that these undergo ring-opening to (59) under the strongly basic R (58) R = H or OMe conditions.The reaction of tetrachlorobenzyne with 1,2,3,4-tetramethoxy-naphthalene gives rise to the semibullvalene (60),together with the expected 1,6adduct;'32 2,3-dehydrobiphenylene (61) generated by the thermal decompo- sition of biphenylene-2-diazonium-3-carboxylate, gives [2 + 41 cycloadducts with tetracyclone and with benzene (1% yield) but apparently shows no tendency to dimerize.' 33 12* P.C. Buxton and H. Heaney J.C.S. Chem. Comm. 1973 545. G. D. Richmond and W. Spendel Tetrahedron Letters 1973 4557. I3O P. G. Sammes and T. W. Wallace J.C.S. Chem. Comm. 1973 524. 13' M. Guyot and D. Molho Tetrahedron Letters 1973 3433. '32 F. Serratosa and P. Sola Tezrahedron Letters 1973 821. E. N. Losey and E. LeGoff J. Org. Chern. 1973 38 3812. 404 J. W.Barton Quinone Methides Dimethides and Related Compounds.-Further studies of benzo-' 340 and na~hth0-I~~~ a-quinone methides and their dimers have been reported ; formation of the thione methide (62) on irradiation of 3H-benzo[b]- thiophen-2-one has been demonstrated by isolation of the adduct with N-phenylmaleimide in high yield.' SCF MO calculations for a number of quinone dimethides give no support to earlier predictions of appreciable electron delocalization in such non-benzenoid systems.' 36 Free o-quinone dimethide has been prepared by irradiation of 1,4-dihydrophthalazine in a matrix at -196 "C;on warming to -150 "C it is converted into the known spiro-dimer.' 37 Extrusion of carbon dioxide from isochroman-3-one (63) at 540 "C has given high yields of benzocyclobutene by ring closure of the o-quinone dimethide intermediate.'38 Base-catalysed re- arrangement of o-dipropargylbenzene (64) which involves the diallene (65) and presumably naphthalene-2,3-quinone dimethide (66) gives rise to the per- oxide (67) as well as to a mixture of the dimers of (66).It is suggested that (66) is oxidized to (67) by triplet oxygen ;the reaction of 2,3-dipropargylnaphthalene gives analogous products.'39 Further examples of [4 + 21 cycloadditions of o-quinone dimethides generated by photoenoli~ation~~~ and by eliminations from dihalides,' 41 have been reported. Sodium borohydride reduction of 134 (a) M. S. Chauhan F. M. Dean S. McDonald and M. L. Robinson J.C.S. Perkin I 1973,359; (6)M. S. Chauhan F. M. Dean D. Matkin and M. L. Robinson ibid. p. 120. 13' G. Jacqmin J. Nasielski G. Billy and M. Remy Tetrahedron Letters 1973 3655. 136 G. J. Gleicher D. D. Newkirk and J. C. Arnold J. Amer. Chem. SOC.,1973 95 2526. 13' C.R. Flynn and J. Michl J. Amer. Chem. Soc. 1973 95 5802. 13* R. J. Spangler and J. H. Kim Synthesis 1973 107. '39 C. M. Bowes D. F. Montecalvo and F. Sondheimer Tetrahedron Letters 1973 3181. I4O A. K. C. Chu and M. F. Tchir J.C.S. Chem. Comm. 1973 619; B. J. Arnold S. M. Mellows and P. G. Sammes J.C.S. Perkin I 1973 1266; M. Pfau E. W. Sarvkr and N. D. Heindel Bull. SOC.chim. France 1973 183. 14' J. M. Holland and D. W. Jones J.C.S. Perkin I 1973 927; J. F. W. McOmie and D. H. Perry Synthesis 1973 416. Aromatic Compounds 2,3-dimethyl-5,6-bisacetoxymethyl-l,6benzoquinone has given the spiroquinone (70) together with dur0quin0ne.l~~ It is likely that initial reduction gives the KOBu' -78 "C C3 quinol (68) which eliminates acetic acid spontaneously to form the dimethide (69);the latter then dimerizes or is further reduced to give the observed products.CH,OCOMe MeMe$ CH20COMe OH (68) 0 Me Me Me 0 The semibenzene derivative (72) is formed together with N-diphenylmethyl- N-phenylcarbamate when the bidentate anion (71) reacts with ethyl chloro- formate (Scheme 5).'43 Reaction of the acetate (73)with organo-lithium and -magnesium compounds gives mixtures of ethers (74)and conjugate addition products (75);when the organometallic reagent is tertiary or benzylic (75)may be formed almost ex~lusively.'~~ 14* A. J. Lin and A. C. Sartorelli J. Org. Chem. 1973 38 813. 143 J. G. Smith and G. E. F. Simpson Terrahedron Letters 1973 1947. 144 B. Miller J. Amer.Chem. Soc.. 1973. 95 8458. 406 J. W. Barton C0,Et C0,Et C0,Et I I I CIC0,Et p& H C02Et C0,Et FCO,Et IC0,Et - Ph2C-NPh C0,Et I (71) Et0,C CO2Et p6ph (72) Scheme 5 Quinones.-Black complexes formed when terminal acetylenes are irradiated in the presence of iron pentacarbonyl can be oxidatively degraded to mixtures of 2,s-and 2,6-disubstituted p-benzoquinones in good ~ie1d.l~' A synthesis of the diterpenoid quinone miltirone (76) has been described'46 and various poly- cyclic quinones have been prepared by condensations of o-dialdehydes with cyclic P-diket~nes,'~'also by direct oxidation of hydrocarbons with ceric ammonium nitrate.'48 An X-ray crystal analysis has been carried out on o-benzoquinone ; the ring adopts a slight boat conformation with the oxygen atoms displaced by 0.036 8 on either side of the mean plane of the ring.'49 13C and 'H n.m.r.i45 R. Victor R. Ben-Shoshan and S. Sarel Tetrahedron Letters 1973 421 1. 14' D. Nasipuri and A. K. Mitra J.C.S. Perkin I 1973 285. 14' A. Verine and Y.Lepage Bull. SOC.chim. France 1973. 11 54; M. Peyrot and Y. Lepage ibid. p. 2856. 14' T. L. Ho T. W. Hall and C. M. Wong Synthesis 1973 206. '49 A. L. McDonald and J. Trotter J.C.S. Perkin II 1973 476. Aromatic Compounds spectra of a-benzoquinone and its methyl derivatives have been analysed ; double-resonance experiments have shown that the previous assignments of (76) olefinic carbon resonances should be inverted.' 50 The photoreduction of p-quinones in the presence of hydrogen donors has been studied using CIDNP techniques.' '' Dimeric species formed in the autoxidation of quinone and hydro- quinone have been identified by e.s.r.spectro~copy.'~~ Quinones react with trimethylsilyl cyanide to give monosilyl ethers of cyano-hydrins which have been used to afford selective protection to one carbonyl group during reactions of the other with alkyl-lithium and -magnesium com- pounds (Scheme 6).'53 Nuclear alkylation of quinones has been effected with R = Me Bu",or Ph M = Li or MgBr Scheme 6 organoboranes.' 54 Further studies of the cyclo-oligomerization of quinones are reported' and the cyclotrimerization of 1,4-anthraquinone has been used as the starting point for a new synthesis of the ten-ring benzenoid hydrocarbon decastarphene[3,3,3].' 56 The kinetics of 0-Claisen rearrangements of allyl- oxynaphthoquinones show that different mechanisms operate in protic and aprotic solvents ;'57 reactions of certain quinone epoxides have been described.I5* 150 R.Hollenstein and W. von Philipsborn Helv. Chim. ACTU,1973 56 320. 151 K. Maruyama T. Otsuki A. Takuwa and S. Arakawa Bull. Chem. SOC.Japan 1973,46,2470. P. Ashworth and W. T. Dixon J.C.S. Perkin II 1973 2128. 15' D. A. Evans J. M. Hoffman,and L. K. Truesdale J. Amer. Chem. SOC.,1973,95,3822. G. W. Kabalka Tetrahedron 1973 29 1159. 155 H. E. Hogberg Acta Chem. Scand. 1973,27,2559 2591. 156 H. Brockman and H. Laatsch Chem. Ber. 1973,106,2058. 15' J. A. Miller and C. M.Scrimgeour J.C.S. Perkin II 1973 1 137. G. Read and V. M. Ruiz J.C.S. Perkin I 1973 235 368. 408 J. W.Barton The dione (79) is one of the products of photolysis of 2,6-di-t-butyl-1,4-benzoquinone in benzene. It is suggested that (79) is formed by closure of the initially formed biradical (77) to the strained cyclobutane derivative (78) and 0 (77) (78) (79) subsequent rearrangement.' 59 The photoaddition of p-benzoquinone to cyclo- octatetraene has been reinvestigated using argon laser excitation.'60 In non- acidic solvents the spiro-1,4-adduct (80)is formed accompanied by the peroxide (81) if oxygen is present. When (80)is warmed in acetic acid it rearranges to a mixture of (82) and (83) these being the sole products if the irradiation is carried out in this solvent.Chemical and structural evidence shows conclusively that the anion { Fe(CN),-[C,H,(NH),]) -contains o-benzoquinone di-imine (84) stabilized by co-ordination to iron(n).' '" T. J. King A. R. Forrester M. 0.Ogilvy and R.H. Thompson J.C.S. Chem. Comm. 1973 844. I6O E. J. Gardner R. H. Squire R. C. Elder and R. M. Wilson J. Amer. Chem. SOC.,1973 95 1693. G. G. Christoph and V. L. Goedken J. Amer. Chem. SOC.,1973 95 3869. Aromatic Compounds Cyc1ophanes.-The synthesis of [2,2]cyclophanes has been reviewed. ' 62 A new general method of cyclophane synthesis by the photochemical extrusion of sulphur bridges on irradiation in a trialkyl phosphite solvent e.g. (85)-+ (86) has been announced independently by two groups.' 63 [7]Paracyclophane (89) Li + N-N-TS the smallest of this class of compounds to be prepared results from rearrangement of the carbene (88)which has been generated by flash pyrolysis of the correspond- ingtoluenesulphonylhydrazonesalt (87).164[8]Paracyclophanesare now available from thermal cycloadditions of buta-1,3-dienes to dispiro[2,2,2,2]deca-4,9-diene (90).'65 An X-ray analysis of [8]paracyclophane-4-carboxylic acid has shown the aromatic ring to be boat-shaped nuclear atoms C-1 and C-4 attached to the chain being 9" out of the plane of the other four.'66 The [2,n]paracyclophanes (91; n = 3-10) have been synthesized.Reaction of bromine with the ethylenic bond of(91;n = 3)is predominantly by cis addition the proportion of trans addition increasing with n in the series.Oxidative photo- cyclizations of (91; n 2 7) give the bridged phenanthrenes (92).'67 As expected 162 F. Vogtle and P. Neumann Synthesis 1973 85. 163 V. Boekelheide I. D. Reingold and M. Tuttle J.C.S. Chem. Comm. 1973 406; J. Bruhin and W. Jenny Tetrahedron Letters 1973 1215; J. Bruhin W. Kneubuhler and W. Jenny Chimia (Switz.) 1973 27 277. 164 A. D. Wolf V. V. Kane R. H. Levin and M. Jones J. Amer. Chem. Soc. 1973 95 1680. 165 T. Tsuji and S. Nishida J. Amer. Chem. SOC.,1973 95 7519. 166 M. G. Newton T. J. Walter and N. L. Allinger J. Amer. Chem. SOC., 1973,95 5652. S. E. Potter and I. 0.Sutherland J.C.S. Chem. Comm. 1973 520. 410 J. W. Barton the naphthalenophanediene (93) readily undergoes photocyclization to coro- nene.168 Full details of the synthesis and an X-ray crystallographic study of the highly crowded [2,2,2] (1,3,5)cyclophane-1,9,17-triene(94) have been reported ;'69 n-n interactions in this and related compounds have been studied by photo- electron spectroscopy.' 70 A novel synthesis of the triply bridged cyclophane (95) @ / \ \ / 17 has been achieved by the construction of a third bridge across [2,2]paracyclophane making use of the unusual property of an acetyl group at position 4 to direct the entry of a chloromethyl group into the closest (13) position in the opposite ring.' 7' Further studies of the synthesis of triple-,' 72a~bquadruple-,' 72b and quintuple-' layered cyclophanes have been reported ; catalytic trimerization of cyclododeca-1,7-diyne in the presence of dimesitylcobalt has given the com- pletely bridged 'percyclophane-4' (96).' 74 (94) 16' J.R. Davy and J. A. Reiss J.C.S. Chem. Comm. 1973 806. 169 V. Boekelheide and R. A. Hollins J. Amer. Chem. SOC.,1973 95 3201. I7O R. Boschi and W. Schmidt Angew. Chem. Inrernat. Edn. 1973 12 402. 17' E. A. Truesdale and D. J. Cram J. Amer. Chem. SOC.,1973,95 5825. 172 (a) N. Kannen T. Umemoto T. Otsubo and S. Misumi Tetrahedron Letters 1973 4537; (b) T. Umemoto T. Otsubo Y. Sakata and S. Misumi ibid.,p. 593. 173 T. Otsubo S. Mizogami Y.Sakata and S. Misumi Terrahedron Letters 1973 2457. 174 R. D. Stephens J. Org. Chem. 1973 38 2260. Aromatic Compounds 41 1 The absolute configuration of l-oxo[2,2]metacyclophane has been deter- mined' 75 and further studies of conformational mobility in mixed para-' 76a3b and metapara-' 76c cyclophanes have been reported.The donor-acceptor cyclo-phanes (97) capable of internal charge-transfer interaction have been synthesized. Surprisingly the interaction is least in (97a) where the nuclei are closest and centrally superimposed and greatest in (97c). A synthesis of the internal quin- hydrone (98) has been described ;17* no rapid change of oxidation states involving OMe (97) a;n=m= 5 b;n=m=6 c;n=4,m=8 hydrogen transfer between the two aromatic rings could be detected in solutions of (98). Transannular coupling takes place during the hydrogenation of [2,2]-metacyclophane in acetic acid the reaction giving hexadecahydropyrene (99) stereospecifically.'79 Electrophilic formylation unlike nitration proceeds with- out transannular coupling to give 4-formy1[2,2]metacyclophane (100 ; R = CHO),'8o while homolytic benzoyloxylation in acetonitrile has given low yields 175 K.Mislow M. Brzechffa H. W. Gschwend and R. T. Puckett J. Amer. Chem. SOC. 1973,95 621. 176 (a)M. Sakamoto and M. Oki Tetrahedron Letters 1973 3989; Bull. Chem. SOC.Japan 1973,46 270; (b)J. F. Haley and P. M. Keehn Tetrahedron Letters 1973 4017 4019; (c) S. A. Sherrod and R. L. da Costa ibid. p. 2083. L. G. Schroff A. J. A. van der Weerdt D. J. H. Staalman J. W. Vehoeven and Th J. 177 de Boer Tetrahedron Letters 1973 1649. W. Rebafka and H. A. Staab Angew. Chem. Internat.Edn. 1973 12 776. 179 E. Langer and H. Lehner Tetrahedron Letters 1973 1143; Monatsh. 1973 104 1154 1484. A. Maquestian Y. Van Haverbeke R. Flammang M. Flammang-Barbieux and N. Clerbois Tetrahedron Letters 1973 3259. 412 J. W.Barton ofa mixture of the 4-benzoate (100; R = OCOPh) the 8-cyanomethyl compound pyrene and 4,5-dihydropyrene.l 81 3 Non-benzene Systems Three- and Four-membered Rings.-Measurement of pKR+ values for a series of diphenylcyclopropenium salts Ph,C 'X BF,- has given an order of stability R,N >> cyclo-C,H > OEt > Pr" -SMe > Ph > H indicating that conjuga- tive effects predominate in stabilizing these cations although to a smaller extent than with most other cations.' 82 For certain symmetrical cyclopropenium cations C +X ,which are unstable in aqueous solution the relationship between pKR+ and skeletal deformation frequency has been used to compare stabilities.' 83 Nucleophilic displacements on 1,2,3-tris(N-methylanilino)cyclopropenium per-chlorate have given the cyclopropen-one (101 ; X = 0)and -thione (101 ; X = S).'84 Salts of tris-(alky1thio)- and tris(phenylthi0)-cyclopropenium cations (102a) have been prepared ; their n.m.r.spectra suggest that 2p3d interaction Ph as in form (102b) is important in stabilizing these structure^.'^^ The previously known reaction of cyclopropenyl cations with cyclopropenes leading to benzene derivatives is paralleled by their reaction with 1-azirines which gives rise to pyridines but a different mechanistic pathway is suggested for the latter (Scheme 7).86 Attempted bimolecular reduction of cyclopropenium salts to bicyclo- propenyls with sodium dithionite has given instead the sulphones (103 ; R = H or Ph; X = SO,). On photolysis these yielded the benzene derivatives (105; R = H or Ph) in high yield probably via (104;R = H or Ph ;X = SO,) and thence a prismane intermediate;IR7 the thioketone (103; R = Ph; X = C=S) formed from triphenylcyclopropenium bromide and dimethylsulphonium methylide behaves in the same way.'88 IB' T. Sato and K. Nishiyama J.C.S. Chem. Comm. 1973 220. R. C. Kerber and C. M. Hsu J. Amer. Chem. SOC.,1973 95 3239. lB3Z. Yoshida H. Ogoshi and S. Hirota Tetrahedron Letters 1973 869. Z. Yoshida H. Konishi Y. Tawara K. Nishikawa and H.Ogoshi Tetrahedron Letters 1973 261 9. IB5 Z. Yoshida S. Miki and S. Yoneda Tetrahedron Letters 1973 4731. R. E. Moerck and M. A. Battiste Tetrahedron Letters 1973 4421. Is' R. Weiss C. Schlierf and H. Kolbl Tetrahedron Letters 1973 4827. B. M. Trost R. C. Atkins and L. Hoffman J. Amer. Chem. SOC.,1973 95 1285. Aromatic Compounds 413 R3 = Ph H or C02Et Ph R2 Ph )-4+R3-)ppVR3 R' Ph R' Ph R2 Br-Br-R' phrl~Rz -+ Ph*' R3 Ph N' RZ Ph R' H Scheme 7 X-Ray crystallographic analyses of diphenylcyclopropenone h~drate'~~",~ and of 2,3-diphenyl-4,4-dicyanotriafulvene1 89b have been reported. The former is nearly planar but the phenyl groups are slightly twisted in the same sense with respect to the three-membered ring ; the results suggest a considerable contribution from the dipolar form.The triafulvene (107) has been obtained from the reaction of di-p-tolylcyclopropenonewith bis(trifluoromethyl)keten presumably via the cycloadduct (106). It appears that the strong negative induc- tive effects of the trifluoromethyl groups favour an important contribution from the 'aromatic' dipolar form (107b) greater than in other triafulvenes studied previously.' (a) H. Tsukada H. Shimanouchi and Y. Sasada Tetrahedron Letters 1973 2455; (6) H. L. Ammon J. Amer. Chem. SOC.,1973 95 7093. I9O I. Agranat and M. R. Pick Tetrahedron Letters 1973 4079. 414 J. W.Barton The chemistry of benzocyclopropenes has been reviewed.' 91 Reaction of the adduct (108) of dichlorocarbene and 1,4-dihydronaphthalene with potassium t-butoxide has given the parent naphtho[b]cyclopropene (109) some reactions of m ( 107a) (107b) which have been described.' 920 It readily undergoes catalytic hydrogenation to a mixture of benzocycloheptene (86 %) and 2-methylnaphthalene (14 %) ; uncatalysed methanolysis is very slow but in the presence of silver(1) ions 2- methoxymethylnaphthalene is formed quantitatively in less than one minute at 25 "C.Using enthalpy changes found for the latter reaction an estimated strain energy of 65-47 kcal mol- ' was obtained in good agreement with a value of 67.8 kcal mol- from the heat of combustion ; the corresponding reaction of benzocyclopropene gave a value of 68 kcal mol- '. An X-ray crystallographic analysis of (109) while showing the 1,2- and 2,3-bonds to be short gave no evi- dence for bond localization in the direction of either limiting structure (l09a) or (1Wb).' 92b MO calculations of the electron distributions in benzocyclopropene its cation anion and radical have been carried out ;193 significant stabilization of the ions by charge delocalization is predicted the radical having a stability intermediate between the cation and anion.There is evidence that the benzo- cyclopropenium ion is formed when benzocyclopropene reacts with triphenyl- methyl fluoroborate in acetonitrile ;194n it is also suggested that the halogen- exchange reaction of 7,7-dichloro-2,5-diphenylbenzocyclopropene with silver fluoride takes place via benzocyclopropenium cations.'94b 19' B. Halton Chem. Rev. 1973 73 113. 19* (a) W. E. Billups and W. Y. Chow J. Amer. Chem. SOC.,1973 95 4099; (6) W. E. Billups W. Y.Chow K. H. Leavell E. S. Lewis J. L. Margrave R. L. Sass J. J. Shieh P. G. Werness and J. L. Wood ibid. p. 7878. 193 B. Halton and M. P. Halton Tetrahedron 1973 29 1717. 194 (a) P. Muller Helv. Chim. Acta 1973 56 500; (b) P. Miiller J.C.S. Chem. Comm. 1973,895. 195 (a) 0.L. Chapman D. De La Cruz R. Roth and J. Pacansky J. Amer. Chem. Soc. 1973 95 1337; (6) G. Maier and B. Hoppe Tetrahedron Letters 1973 861; (c) 0. L. Chapman C. L. McIntosh and J. Pacansky J. Amer. Chem. SOC.,1973 95 617; (d)A. Krantz C. Y. Lin and M. D. Newton ibid. p. 2744. Aromatic Compounds 415 Further evidence of structure of matrix-isolated cyclobutadiene has been reported analysis of its i.r.spectrum provides support for the assignment of a square geometry to the molecule. There is loss of chirality in the adducts when cyclobutadienyliron tricarbonyl complexes are oxidatively decomposed in the presence of trapping agents thus demonstrating the intermediacy of free cyclobuta- diene.'96 The hindered cyclobutadienes (111; R = H or C02Me) have been obtained by low-temperature photolysis of the diazo-compounds (110; R = H,C02Me);197"(111 ; R = H) is also formed by photolysis of the two isomeric tri-t-butylcyclopentadienones.' 976 From its n.m.r. spectrum the paramagnetic contribution to the induced ring current of (111 ;R = H) is estimated to be 1.04 p.p.m.Evidence from photoelectron spectroscopy suggests that the stability of donor-acceptor cyclobutadienes such as (112)may be due to strong second-order bond fixation rather than to the contribution of zwitterionic structures.' 98 The results of a preliminary X-ray crystallographic study of squaric acid have been reported ;19' Meerwein methylation of the squaric acid derivative (113) has given (114) the salt of a stable cyclobutenium dication.200 New syntheses of cyclobutene-20'" and benzocyclobutene-20 la*' diones have been described including (115) which is a benzologue of squaric acid and is relatively acidic Me Me Me Me Me Me 2BFi (114) L96 E. K. G. Schmidt Angew. Chem. Internat. Edn. 1973 12 777; R. H. Grubbs and R. A. Grey J. Amer. Chem.SOC.,1973 95 5765. 19' (a) S. Masamune N. Nakamura M. Suda and H. Ona J. Amer. Chem. SOC.,1973 95 8481; (b)G. Maier and A. Alzerreca Angew. Chem. Internat. Edn. 1973 12 1015. 19* R. Gompper F. Holsboer W. Schmidt and G. Seybold J. Amer. Chem. SOC.,1973 95,8479. 199 D. Semmingsen Tetrahedron Letters 1973 807. zoo S. Hunig and H. Putter Angew. Chem. Internat. Edn. 1973 12 149. (a) T. Kowar and E. LeGoff Synthesis 1973 212; (b)J. F. W. McOmie and D. H. Perry J.C.S. Chem. Comm. 1973 248. 416 J. W.Barton (pK = 4.48; pK2 = 8.05).2016 Flow pyrolysis of (116) in nitrogen at 250°C gives small yields of the thienocyclobutadiene (1 17 ;X = S) which like the furan analogue (1 17;X = 0)discussed in last year's Report (p. 584) is highly reactive and dimerizes in solution to the bisthienocyclo-octatetraene(1 18).202 Partial catalytic reduction of (117; X = 0) gives (119) which shows properties of a rather reactive 3,4-dialkylfuran ; the striking difference in properties between these two compounds lends support to the view that (117; X = 0) should be regarded as a truly antiaromatic system.203 HO Homo Five-and Seven-membered Rings.-The antiaromatic cyclopentadienyl cation has been generated at -78 0C;204 the e.p.r.spectrum shows it to have a triplet ground state as predicted. Sulphonium cyclopentadienylides are formed by the photolysis of diazocyclopentadiene in dialkyl sulphides ;'OS preparations of iodonium cyclopentadienylidesZo6 and of the tripolar mesomeric salt (1 20)207 + (121a) (121b) have been reported.From bond-length data obtained in an X-ray crystallo- graphic analysis of triphenylphosphonium cyclopentadienylide (121)' and from the phosphorus photoelectron spectrum the relative contributions of forms '02 K. P. C. Vollhardt and R. G. Bergman J. Amer. Chem. Soc. 1973,95 7538. '03 R. G. Bergman and K. P. C. Vollhardt J.C.S. Chem. Comm. 1973 214. '04 M. Saunders R. Berger A. Jaffe J. M. McBride J. O'Neill R. Breslow J. M. Hoffman C. Perchonock E. Wasserman R. S. Hutton and V. J. Kuck J. Amer. Chem. Sor. 1973,95,3017. '05 W. Ando J. Suzuki Y. Saiki and T. Migita J.C.S. Chem. Comm. 1973 365. '06 K. Friedrich and W. Amann Tetrahedron Letters 1973 3689. '07 Z. Yoshida S. Yoneda T. Yato and M. Hazama Tetrahedron Letrers 1973 873.Aromatic Compounds 41 7 (121a) and (121 b) are estimated to be 20 and 80% respectively,208 in good agree- ment with previous MO calculations. Reactions of (121) with electrophiles and dienophiles have been Vacuum thermolysis of a-coumarone (122) at 750 "C has given fulvene (5.5 %) benzene (22 %) and a trimer of the presumed quinone methide intermediate (123).92b The bromination of 6,6-diphenylfulvene (124) a; R = Me b; R = Ph has been re-investigated ;21 nucleophilic displacement reactions of certain 1- and 6-substituted fulvenes have been reported,2' as has the cycloaddition of 6,6-disubstituted fulvenes to dienes2' The isobenzofulvenes(l24) have been obtained as transient intermediates ;21 trapping with reagents such as N-phenylmaleimide gave 1,3-cycloadducts and (124a) gave the 1,8-cycloadduct with tropone.The quaternary salt (125) undergoes Hofmann elimination at 20 "C to give a mixture of the pentalene dimers (126) and (127) which on photolysis in a matrix at -196 "C are converted into monomeric pentalene ; the latter conversion is reversed by warming the matrix to 20 0C.2140 Fission of the dimer (126) has also QMe "6 'O* H. L. Ammon G. L. Wheeler and P. H. Watts J. Amer. Chem. Soc. 1973 95 6158. *09 Z. Yoshida S. Yoneda and Y. Murata J. Org. Chem. 1973 38 3537. 'lo P. T. Cheng P. A. Gwinner S. C. Nyberg R. k.Stanforth and P. Yates Tetrahedron 1973 29 2699; K. Hafner and F. Schmidt Tetrahedron Letters 1973 5101. 2'1 K. Hafner and F. Schmidt Tetrahedron Letters 1973 5105; D.Copocasale L. Di Nunno S. Florio and F. Naso J.C.S. Perkin 11 1973 2078. 'I2 K. N. Houk and L. J. Luskus J. Org. Chem. 1973,38 3836. 'I3 H. Tanida T. Irie and K. Tori Bull. Chem. SOC.Japan 1972 45 1999; P. L. Watson and R. N. Warrender Austral. J. Chem. 1973 26 1725. *I4 (a)K. Hafner R. Donges E. Goedecke and R. Kaiser Angew. Chem. Internat. Edn. 1973 12 337; (6) W. Weidemuller and K. Hafner ibid. p. 925. 418 J. W. Barton been brought about by iron enneacarbonyl at 50 “C,when the pentalene complex (128) was isolated ;’14’ a comparable bis(trimethylgermyldicarbony1ruthenium) complex has recently been obtained by dehydrogenative transannular cyclization I1 (CO),Fe Fe(CO) \/ C JI 0 of cyclo-octatetraene.21 1,3,5-Tri-t-butylpentalene (129) which is too sterically hindered to dimerize readily has been isolated ;preliminary analysis of the 3C and ‘H n.m.r.spectra rules out the existence of a localized bond system in (129) and indicates the presence of a paramagnetic ring current.’16 Convenient preparations of tropylium trifluoroacetate and other salts from cycloheptatriene have been described.2 The dissociation equilibrium of tropy- lium isothiocyanate has been determined,218 and the reaction of tropylium ion with cycloheptatriene has been in~estigated.~” Further reactions of tropo-thione (130) have been reported,’” including the formation of 8,8-dicyanohepta-fulvene (131; R = CN) from the reaction with tetracyanoethylene;220b there appears to be very little contribution from the 6n-electron dipolar form towards the structure of (130).220dDibenzo[c,e]tropone (132) has been synthesized ;221 as expected it behaves as a reactive 6z-electron component in thermal [6 + 41-cycloaddition reactions.Using 13Cn.m.r. spectroscopy it has been shown that ’I5 A. Brookes J. Howard S. A. R. Knox F. G. A. Stone and P. Woodward J.C.S. Chem. Comm. 1973 587. ’I6 K. Hafner and H. U. Suss Angew. Chem. Internat. Edn. 1973 12 575. *I7 J. V. Crivello Syntheric Cornrn. 1973 3 9. ’18 H. Kessler and A. Walter Angew. Chem. Internat. Edn. 1973 12 773. ’ S. It& A. Mori I. Saito K. Sakan H. Ishiyama and K. Sasaki Tetrahedron Letters 1973,2737. ’” (a)T. Machiguchi M. Hoshino S. Ebine and Y. Kitahara J.C.S. Chem. Comm. 1973 196; (6)T. Machiguchi K.Okuma M. Hoshino and Y. Kitahara Tetrahedron Letters 1973 201 1 ;(c) T. Machiguchi Y. Yamamoto M. Hoshino and Y. Kitahara ibid. p. 2627; (4T. Machiguchi T. Hoshi J. Yoshino and Y. Kitahara ibid. p. 3873. ’” C. E. Hudson and N. L. Bauld J. Amer. Chem. Soc. 1973,95 3822. Aromatic Compounds 419 tropolone acetate undergoes rapid degenerate rearrangement at low tempera- tures ;222 there is spectroscopic evidence of Meisenheimer-type complex (1 33) formation in the reaction of 2-methoxy-5-nitrotropone with sodi~rnrnethoxide.~~~ Syntheses of heptafulvene (131 ;R = H) and pentaheptafulvene have been des- ~ribed,~’~ as has a synthesis of the higher homologue 8,8’-biheptafulvenyl ;225 S 0 iron carbonyl complexes of (131 ; R = H) have also been isolated.226 Stable phenazulene derivatives (1 35) have been obtained from cycloadditions of 8,9-dicyanosesquifulvalene (134) and the method has been extended to prepare acepleiadylene derivatives.” The 8-cycloheptatrienylheptafulvenylcation (136) has been generated ;228 it is one of the most stable hydrocarbon cations yet known. Treatment of either 1,2- or R’ & -R2 02yJo OMe OMe R’-R2 = -(CH2)4-(133) R’ = Ph R2 = H ”* S. Masamune A. V. Kemp-Jones J. Green D. L. Rabenstein M. Yasunami K. Takase and T. Nozoe J.C.S. Chem. Comm. 1973,283. 223 T. Abe and T. Asao Tetrahedron Letters 1973 1327. 224 M. Nuenschwander and W. K. Schenk Chimia (Switz.) 1972 26 194. 225 S. Kuroda M. Oda and Y. Kitahara Angew. Chem. Inrernar.Edn. 1973 12 76. 226 R. C. Kerber and D. J. Ehntholt J. Amer. Chem. SOC.,1973 95 2927. 227 H. Prinzbach and H. W. Schneider Angew. Chem. Internat. Edn. 1973 12 1007; H. Prinzbach L. Knothe and H. W. Schneider ibid. p. 1009. ’” I. Fleming J.C.S. Perkin I 1973 1019. 420 J. W.Barton 3,4-benzocyclohepta-1,3,5-trienewith potassium amide in liquid ammonia affords a red-brown solution of benzocycloheptatriene mon~anion.~~’ The H n.m.r. spectrum of this ion shows it to be strongly paratropic and the proton coupling constants indicate first-order bond fixation as in (137),which will tend to decrease the antiaromaticity of the seven-membered ring. H The copper(1)-catalysed decomposition of diazo-ketones derived from dihydro- cinnamic acids forms the basis of a recent azulene synthesis (Scheme 8).230A novel ferrocene-catalysed photoalkoxycarbonylation of azulene takes place in R’,RZ = H or Me Scheme 8 carbon tetrachloride-alcohol solutions ;23 the reaction is unaffected by dissolved oxygen or by radical scavengers and possibly takes place via an azulene-iron complex.Reactions of diethyl azulene-1,3-dicarboxylateswith Grignard re-agents proceed by nuclear rather than by carbonyl addition the products giving mixtures of 2- 4- and 6-substituted azulenes on deh~drogenation.~~~ Annu1enes.-The anions (1 38)and (1 39)have been generated at low temperature and characterized by ‘Hn.m.r. spectroscopy.233 Both ions appear to be more or less planar and the spectrum of (139)suggests that there is substantial polarization 229 S.W. Staley and A. W. Orvedai J. Amer. Chem. SOC.,1973 95 3382. 230 L. T. Scott J.C.S. Chem. Comm. 1973 882. 231 T. Ishigami T. Akiyami H. Watanabe T. Kato and A. Sugimori J.C.S. Chem. Comm. 1973 871. 232 N. Abe T. Morita and K. Takase Tetrahedron Letters 1973 3883 4739. 233 S. W. Staley and G. M. Cramer J. Amer. Chem. SOC.,1973,95 5051 ;S. W. Staley and W. G. Kingsley ibid. p. 5805. Aromatic Compounds 42 1 of electron density out of the cyclopropyl ring by spirocyclopropyl conjugation ; thus the eight-membered ring of (139) supports a diamagnetic ring-current whereas that of (138) is an atropic (4n +1) system. Recent e.s.r. data on the benzocyclo-octatetraene radical anion are best interpreted in terms of a planar or near-planar structure with strong n-n interaction in the eight-membered ring.234 Reduction of [18]annulene with potassium at -110 "C gives an equi- librium mixture of two conformers of the dianion probably (140) and (141).Both CGG \ \\ \ species exhibit n-bond localization and are non-planar having six intra- and twelve extra-annular protons.23 Charged Huckel (4n +2)-electron species which have been studied include the [16]annulenediyl dication (142)236 and the [17lannulenyl anion (143),237 generation of the former being the first example of formation of a dication of this type from a [Qnlannulene; cations from oxygen- bridged annulenes have also been ~haracterized.~ 234 J. R. Dodd Tetrahedron Letters 1973 3943. 235 J. F. M.Oth E. P. Woo and F. Sondheimer J. Amer. Chem. SOC.,1973 95 7337. 236 J. F. M. Oth D. M. Smith U. Prange and G. Schroder Angew. Chem. Znternat. Edn. 1973 12 327. 237 G. Schroder G. Plinke D. M. Smith and J. F. M. Oth Angew. Chem. Znternat. Edn. 1973 12 325. 238 H. Ogawa I. Tabushi H. Kato and Y. Taniguchi Tetrahedron Letters 1973 5065; H. Ogawa and M. Kubo Tetrahedron 1973 29 809. 422 .I. W. Burton Syntheses of the bridged [14]annulene (1#)239 and of the [18]annuleno- phenanthrene (145)240 have been reported ;in both cases the macrocyclic system G sustains a diamagnetic ring-current. It appears that the upper limit for diatropic character in [4n + 2lannulenes is higher than has been predicted ;it is exhibited by 3,15,18,30-tetra-t-buty1-1,16-bisdehydro[30]annulene, the synthesis of which was described recently.241 Fluxional behaviour (146a) S(146b) has been observed with certain 1,6-methano[ 101ann~lenes.~~~ Generation of the arylcarbene (147) by heating the sodium salt of the corres- ponding toluene-p-sulphonylhydrazone in diglyme at 135 "C results in re-arrangement to the l0n-electron aromatic carbene (148),which dimerizes to give a mixture of (149) and (150); similar results are obtained if (148) is generated The mode of formation of (149) and (150) is unknown it possibly involves electrocyclic ring-closure of undecafulvene intermediates.The chemistry 239 W. Flitsch and H. Peeters Chem. Ber. 1973 106 1731. 240 U. Meissner B. Meissner and H. A. Staab Angew. Chem. Internat.Edn. 1973,12,916. 241 M. Iyoda and M. Nakagawa Tetrahedron Letters 1973 4743. 242 H. Giinther H. Schmickler W. Bremser F. A. Straube and E. Vogel Angew. Chem. Internat. Edn. 1973 12 570. 243 (a) P. H. Gebert R. W. King R. A. LaBar and W. M. Jones J. Amer. Chem. SOC. 1973 95 2357; (b)R. A. LaBar and W. M. Jones ibid. p. 2359. Aromatic Compounds 423 a:> //--‘ A/ of carbene (151) has also been investigated ;in the absence of acceptors it dimerizes to the fulvene (152).243b An X-ray crystallographic analysis of the tropolone analogue (153) discussed in last year’s Report (p. 590),has been carried The molecule shows near- mirror symmetry perturbed only by the hydroxy-group; it appears that there is more delocalization in the eleven-membered ring than in the corresponding tropone analogue.Experiments are in progress to obtain information as to possible quinonoid character in the annulenedione (154) the synthesis of which has been 4 Polycyclic Compounds Using MO methods a ‘Kekule Index’ placing valence bond structures quantita- tively in their relative order of importance has been compiled for a number of 244 D. W. J. Cruickshank G. Filippini and 0.S. Mills Angew. Chem. internat. Edn. 1973 12 855. 245 K. Yamamoto and F. Sondheimer Angew. Chem. internat. Edn. 1973 12 68. 424 J. W.Barton polycyclic benzenoid and non-benzenoid aromatic hydrocarbons.246 Photo- cyclizations of 2-vinylbiphenyl 4-~inylphenanthrene,~~~" and 2-styrylphenanth- rene247b have been studied ;the last-named cyclizes exclusively at position 3 of the phenanthrene nucleus.Syntheses of arene oxides of polycyclic benzenoid hydrocarbons have been reported ;2480 the 9,lO-oxide of phenanthrene isomerizes to dibenzo[b,d]oxepin (155) on photolysi~.~~~~ The structure of lamellicolic anhydride a heptaketide from Verticilliurn larnellicola has been established as (156).249 Surprising selectivity is observed in the formation of naphthalene derivatives from P-he~a-ketones,~~' for example treatment of (157) with silica gel at pH 8 gave (158) whereas cyclization with aqueous potassium hydroxide gave the resorcinol (159) which could be further cyclized to (160) on treatment with potassium carbonate or trifluoroacetic acid. Ph(COCH,),COPh "O2 &COPh (1 57) HO Ph 4 (158) aq.KOH I 246 A. Graovac I. Gutman M. Randid and N. Trinajstic J. Amer. Chem. SOC.,1973 95 6267. 247 (a)S. W. Horgan D. D. Morgan and M. Orchin J. Org. Chem. 1973 38 3801; (6) G. Snatzke and K. Kunde Chem. Ber. 1973,106 1341. 248 (a)S. H. Goh R. G. Harvey H. Yagi and D. M. Jerina J. Amer. Chem. SOC.,1973 95 242 243; (6)N. E. Brightwell and G. W. Griffin J.C.S. Chem. Comm. 1973 37. 249 N. J. McCorkindale A. McRitchie and S. A. Hutchinson J.C.S. Chem. Comm. 1973 108. 250 P. J. Wittek and T. M. Harris J. Amer. Chem. SOC.,1973 95 6865. A romatic Compounds (163) R' RZ = H,Me Ph or alkoxycarbonyl ( 165) (164) R' R2 = C0,Me R' = C02Et,R2 = H R' = Ph R2 = H Highly substituted naphthalenes (164) have been obtained from thermal reactions of the diyne (161) with alkyne~.~~'" It is suggested that (16l)first isomerizes to the benzocyclobutadiene (162) followed by cycloaddition and rearrangement of the adduct (163).When tris(triphenylphosphine)rhodium(I)chloride is included in the reaction mixture addition takes place in a linear mode to give the naphtha- lenes (165).25lb Various pericyclic naphthalenes have been synthesized and their derived radical anions examined.' The protonation of naphthalenes in super- acidic media has been studied ;253 rates of normal and photo-induced hydrogen- deuterium exchange of naphthalene in sulphuric acid solutions have been deter- mined.254 The reversibility of intramolecular Friedel-Crafts acylation at the a-position in the naphthalene nucleus of (166) has been demonstrated.255 2,2'-Binaphthyl undergoes a high-temperature diene addition with maleic anhydride at positions 1 and 3' rather than giving a 1,l'-adduct which might be expected from bond-order consideration^.'^^ Photochemical reactions of naphthalene continue to evoke interest.Primary and secondary products arising from [2 + 2]photocycloadditions to the nuclei of l-'57aand 2-257b naphthonitriles and of methyl 2-naphth0ate~~~' have been 251 (a) E. Muller and A. Huth Tetrahedron Letters 1972 4359; (b) H. Straub A. Huth and E. Muller Synthesis 1973 783. 252 S. F. Nelsen and J. P. Gillespie J. Amer. Chem. SOC.,1973 95 1874 2940; J. Org. Chem. 1973 38 3592. 253 G. A. Olah G. D. Mateescu and Y.K. Mo J. Amer. Chem. SOC. 1973 95 1865. 254 C. G. Stevens and S. J. Strickler. J. Amer. Chem. SOC.,1973 95,3918 3922. 255 I. Agranat and D. Avnir J.C.S. Chem. Comm. 1973 362. 256 E. Clar and F. Clar Tetrahedron 1973 29 3267. 257 (a)C. Pac T. Sugioka K. Mizuno and H. Sakurai Bull. Chem. SOC. Japan 1973,46 238; (6) K. Mizuno C. Pac and H. Sakurai J.C.S. Chem. Comm. 1973 219; (c) G. Sugowdz P. J. Collin and W. H. F. Sasse Austral. J. Chem. 1973 26 147. 426 J. w.Burton characterized ;displacement of fluorine in photosubstitution reactions of fluoro- nitronaphthalenes has been reported.258 1-Acenaphthenium ions have been observed spectroscopically.259 The dianion of acenaphthylene (167) a 14n-electron system undergoes protonation to give (168) quantitatively;260" however the main product of carbonation is the di- carboxylic acid (169) formed together with a smaller amount of the acenaphthene derivative (170).260b In contrast the dianion from pyracyclene (171) reacts at C-1 and C-2 exclusively.261 CO,H (169) ( 1 70) (171) 258 J.G. Lammers and J. Lugtenburg Tetrahedron Letters 1973 1777. 259 G. A. Olah G. Liang and P. Westerman J. Amer. Chem. SOC.,1973 95 3698. 260 (a)C. V. Ristagno and R. G. Lawler Tetrahedron Letters 1973 159; (b)T. S. Cantrell ibid.,p. 1803. 261 B. M. Trost D. Buhner and G. M. Bright Tetrahedron Letters 1973 2787. Aromatic Compounds 427 Full details of a study of the electrochemical potentials of cyclobutadieno- naphthoquinone derivatives have been published.262 The formation of phenan-throcyclobutenes by [2 + 2]photocycloaddition of olefins to phenanthrene has been further investigated.263 Phenanthro[lJcyclobutadiene (1 72) has been gener- ated and characterized as its iron tricarbonyl complex ;its reactions with dienes and with tetracyanoethylene have been reported.264 The bridged benzocyclo- butadiene derivative (173) which may be regarded as a biphenylene analogue (172) (1 73) possessing an oxepin ring has been synthesi~ed;'~~ solutions of (173) are stable for a few hours at -78 "C,but it rapidly forms a linear [2 + 21 dimer at room temperature.Thermolysis of the 1-diazoindene-benzyne cycloadduct (1 74) gives high yields of 2H-cyclopent[ j,k]fluorene (175); fluoradene derivatives may be obtained in the same way from 9-diazofluorene.266 240 "C ___, g \ N / Nucleophilic attack on 9-benzoylanthracene and related derivatives by cyanide ion in dimethylformamide under mild oxidizing conditions gives 9,lO-dicyano- anthracene and thence the 2,3,9,1O-tetra~yano-derivative.~~' Photonucleophilic substitution by ammonia has been observed with 1- and 2-methoxyanthra-9,lO- quinone2680 and with sodium anthra-9,10-quinone-2-sulphonate ;268b on present 262 R.Breslow D. R. Murayama S. Murahashi and R. Grubbs J. Amer. Chem. Soc. 1973,95,6688. 263 G. Kaupp Angew. Chem. Znternat. Edn. 1973 12 765. 264 T. Miyamoto and Y.Odaira Tetrahedron Letters 1973 43; T. Miyamoto S. Tanaka and Y.Odaira J.C.S. Perkin I 1973 138. 265 M. P. Cava and K.T. Buck J. Amer. Chem. Soc. 1973,95 5805. 266 C. Tuchscherer M. Bruch and D. Rewicki Tetrahedron Letters 1973 865. 267 N. A. Goeckner and H. R. Snyder J. Org. Chem. 1973,38 481. 268 (a)J. Griffiths and C. Hawkins J.C.S. Chem. Comm. 1973 11 1 ;(6)G. G. Wubbels D. M. Tollefsen R. S. Meridith and L. A. Herewaldt J. Amer. Chem. Soc. 1973 95 3820. 428 J. W.Barton evidence a radical mechanism appears unlikely in the case of the methoxyl displacement. Photochemical addition of cycloheptatriene to anthracene gives the [6 + 41 and [4 + 41 cycloadducts (176) and (177) the former predominating; ( 176) (1 77) the corresponding thermal reaction affords only a [2 + 41 adduct in low yield.269 Other studies involving exciplex formation between anthracenes and secondary amine~,~"" 1,3-diene~,~~'~ have been reported the last- and aromati~s~~~'*~ named being those where the participants were held together by a trimethylene chain.In the case of 1-(9-anthry1)-3-( 1-naphthy1)propane the adduct (178) was formed in a reversible process ;270d intramolecular [4 + 41 photocycloadditions have also been observed with 1,l'-and 2,2'-linked anthra~enes.~~' Full details of syntheses leading to azupyrene (179) have been published.272 The ketone (180) has been synthesized and its spectra in acidic solution indicate formation of a delocalized cation with substantial contribution from (181).273 26q T. Sasaki K. Kanematsu and K. Hayakawa J. Amer. Chem. SOC.,1973,95 5632. 270 (a) N. C. Yang and J. Libman J. Amer.Chem. SOC.,1973 95 5783; (b)J. Saltiel and D. E. Townsend ibid. p. 6141; (c) M. Itoh T. Mimura H. Usui and T. Okamoto ibid. p. 4388; (d) E. A. Chandross and A. H. Schiebel ibid. p. 61 I. 2" F. C. De Schryver M. D. Brackeleire S. Toppet and M. Van Schoor Tetrahedron Letters 1973 1253. 272 A. G. Anderson G. M. Masada and A. F. Montana J. Org. Chem. 1973,38 1439; A. G. Anderson A. F. Montana A. A. McDonald and G. M. Masada ibid. p. 1445. 273 N. Abe T. Morita and K. Takase Tetrahedron Letters 1973 4755. Aromatic Compounds The tetrafluoroborate salt of cation (182) has been prepared ;attempts to convert it into a dication or into the 16n-electron system (183) have so far proved un- ~uccessfu1.~~~ Several studiesofthechemistry ofphenalenes have been reported.275 The reaction of phenalene with dichloromethane and n-butyl-lithium gives rise to the naphthobicyclobutane (184) together with a smaller amount of the pleia- dene (185);2750,b the latter is fairly stable thermally but (184) isomerizes at 150 "C to the cyclobutene (186).275b Irradiation at 2537 8 brings about rearrangement of (184) to (185) as does catalysis by silver(1) ions;275' other isomerizations of (184) promoted by transition metals have been E.s.r.spectral studies of the radical anions of pleiadenes and related compounds including (184) and (185) have been reported.252 Cycloaddition of benzyne to acenaphthy- lene has given the cyclobutene (187) which is converted into the 'biradicaloid' pleiadene (188) on low-temperature photolysis in a matrix.276 Similar prepara- tions from 1,2- and 2,3-naphthalynes gave benzologues of (188) of which only the angularly annelated derivative (189) survived briefly in solution at room temperature.(187) (188) 274 J. Beeby and P. J. Garratt J. Org. Chem. 1973 38 3051. 275 (a) I. Murata and K. Nakasuji Tetrahedron Letters 1973 47; (b) R. M. Pagni and C. R. Watson ibid. p. 59; (c) I. Murata and K. Nakasuji ibid. p. 1591; (d)I. Murata K. Nakasuji and H. Kurne ibid. p. 3405; (e) T. T. Coburn and W. M. Jones ibid. p. 3903. 276 J. Kolc and J. Michl J. Amer. Chem. SOC.,1973 95 7391. 430 J. W.Barton The subject of circulenes was mentioned in last year’s Report (p. 596). Another example of this class of compound the thiacoronene (191) has been obtained by oxidative cyclization of the bridged [18lannulene (190).277 277 J.Lawson R. DuVernet and V. Boekelheide J. Amer. Chem. SOC.,1973,95 956.

ISSN:0069-3030    

DOI:10.1039/OC9737000386

出版商:RSC    

年代:1973

数据来源: RSC