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.