6 Arynes Carbenes Nitrenes and Related Species By R. C. STORR Robert Robinson Laboratories University of Liverpool L iverpool L 69 3BX 1 Arynes Generation.-Labelling studies have shown that the formation of benzyne from benzenediazonium acetate involves an E2 rather than Elcb elimination.' A major disadvantage in the 'one pot' formation of arynes from benzenediazonium acetates is that the diazonium compound can decompose via the normal radical route. As expected addition of alkenes which are good phenyl radical traps promotes benzyne formation by inhibiting the radical chain. This also probably explains the similar effect previously observed for tetracyclone.2 Acenaphthyne and benzyne have been implicated as intermediates in the formation of diphenylfluoroanthene and 1,4-diphenylnaphthalene from the phosphole ylides (1) and (2) respectively by reaction of the aryne with the liberated 1,2,5-triphenylphospholeoxide.Intramolecular mechanisms cannot be discounted for the above reactions however as added anthracene failed to intercept free benzyne in the latter case.3 Benzyne is produced in low yield (6%) by pyrolysis of benzene o-disulphonyl a~ide.~ The use of sodamide-containing complex bases in aryne chemistry has been re~iewed.~ Elimination-addition is largely responsible for cine substitution in the methoxydehalogenation of halogenobenzofurazans6 but different addition- elimination sequences are involved in both normal and cine substitution for thiometho~ydehalogenation.~The reaction of polyhalogenobenzenes with primary and secondary amines in the presence of NaNH2 or NaNH2-NaOBut proceeds mainly via arynes but in certain cases an S,Ar mechanism operates.8 3,4-Dehydrotoluene formed viu a sulphurane has been implicated in the reaction of p-tolylsulphoxide with p-tolyl-lithium,' and the new aryne 2,3- didehydrotriptycene has been generated." ' J.I. G. Cadogan C. D. Murray and J. T. Sharp J.C.S. Chem. Comm. 1974 133; J.C.S. Perkin 11 1974 1321. J. I. G. Cadogan C. D. Murray and J. T. Sharp J.C.S. Chem. Comm. 1974 901. J. I. G. Cadogan R. J. Scott and N. H. Wilson J.C.S. Chem. Comm. 1974.902. R. A. Abramovitch and G. N. Knaus J.C.S. Chem. Comm. 1974 238. ' P. Caubere Accounts Chem. Res. 1974 7 301. L. Di Nunno S. Florio and P.E. Todesco J.C.S. Perkin ZZ 1974 1171. L. Di Nunno S. Florio and P. E. Todesco Tetrahedron 1974 30 863. P. Caubere and L. Lalloz Bull. SOC.chim. France 1974 1983 1989 1996. B. K. Ackerman K. K. Andersen I. Karup-Nielsen N. B. Peynircioglu and S. A. Yeager J. Org. Chem. 1974 39 964. lo V. R. Skvarchenko and V. K. Shalaev Doklady Akad. Nauk S.S.S.R. 1974 216 110. 153 154 R.C.Storr I 1'1-(1) (2) (3) Thermal decomposition of benz[bd]iodolium iodide (3) in dimethyl acetylene- dicarboxylate gives some dimethyl phenanthrene-9,lO-dicarboxylate possibly via the 2,2'-dehydrobiphenyl biradica1.l ' Structure and Reactions-MIND0/3 calculations for the three didehydro- benzenes involving optimization of geometry lead to the prediction.that 1,2-didehydrobenzene has a ground singlet state a small 1,2-bond length and large 1,2,3 and 6,1,2 bond angles as expected. For 1,3-didehydrobenzene a singlet ground state with considerable bonding character between the dehydro-centres approximating to (4) and an energy similar to 1,2-benzyne is calculated and it is suggested that this species should be considered more seriously in reaction mechanisms. There are two energy minima for 1,4-didehydrobenzene corre- sponding to (5)and (6) the latter being of higher energy. Species (5)is most likely a singlet with a low singlet-triplet separation.' The small effect of pressure on the proportions of stepwise [2 + 21 and con- certed homo-Diels-Alder addition of tetrachlorobenzyne to norbornadiene runs contrary to the current idea that concerted reactions have smaller transition states.It is suggested that the [2 + 21 addition involves solvation of an inter- mediate zwitterion which causes a compensating contraction of the system.' The mode of addition of benzyne to ccg-unsaturated carbonyl compounds depends on the reactivity of the carbonyl group.14 Further examples of [2 + 21 additions of benzyne to dienes" and of ene reactions involving benzyne16 have '' T. Sato K. Shimizu and H. Moriya J.C.S. Perkin I 1974 1537. I' M. J. S. Dewar and W.-K. Li J. Amer. Chem. Soc. 1974 % 5569. l3 W. J. le Noble and R. Mukhtar J. Amer. Chem. Soc. 1974 96 6191. I4 A. T. Bowne and R. H. Levin Tetrahedron Letters 1974 2043. L. Lombard0 and D. Wege Tetrahedron 1974 30 3945; M.R. Decamp R. H. Levin and M. Jones Tetrahedron Letters 1974 3575. l6 G. Mehta and B. P. Singh Tetrahedron 1974 30,2409; H. H. Wasserman and L. S. Keller Tetrahedron Letters 1974 4355. Arynes Carbenes Nitrenes and Related Species appeared. A biradical intermediate (7) seems likely in the novel phenylation of camphene with benzyne.” The reaction of benzyne with pyridine N-oxides leads mainly to fl-hydroxyarylated pyridines (8) probably as shown.’ Benzyne reacts with CS2 to give products which can be rationalized in terms of the di- thiolium carbene (9).’g Addition of 1,2-dehydronaphthalenes to tetrahydro- benzo[b]furans provides a one-step synthesis of hexahydrochrysenes,20 and intermolecular addition of 1,2-dehydronaphthalene to enolate anions has been exploited.2 Further applications of intramolecular aryne cyclizations have appeared ;22 in the case of compound (10) the expected phenolic aryne cyclization was not observed but the benzodiazepine (1 1)was formed.23 1 17 G.Mehta and B. P. Singh Tetrahedron Letters 1974 4297. I8 R.A. Abramovitch and I. Shinkai J. Amer. Chem. SOC. 1974 96,5265. 19 J. Nakayania J.C.S. Chem. Comm. 1974 166. 20 W. Tochtermann G. Stubenrauch K. Reiff and U. Schumacher Chem. Ber. 1974 107 3340. 21 P. Caubere and M. S. Mourad Bull. SOC. chim. France 1974 1415. 22 S. Kano T. Yokomatsu N. Yamada K. Matsumoto S. Tokita and S. Shibuya, Chem. and Pharm. Bull. Japan 1974 22 1607; J. A. Skorcz J. T. Suh and R. L. Germershausen J. Heterocyclic Chem.1974 11 73. 23 R. J. Spangler D. C. Boop and J. H. Kim J. Org. Chem. 1974,39 1368. 156 R.C.Stow 2 Nitrenes Deoxygenation of nitroxyl radicals with triethyl phosphite has provided a new route to nitrenium ions.24 The dianisylnitrenium ion and its conjugate acid have been directly observed.25 Full details of the formation of triarylarsinimines from a variety of nitrene precursors have now appeared. Thermolysis of those azides which lead to electrophilic nitrenes and stabilized ylides copper-catalysed decomposition of 3-aryl-1,4,2-dioxazolidin-5-ones (12) (the value of copper in lowering the de- composition temperature is stressed) and a-elimination routes probably involve nitrenes.26 However lead tetra-acetate (LTA) oxidation of sulphonamides and amides in the presence of triphenylarsine give arsinimines uia diacetoxy-triphenylarsorane (13),the amides being unoxidized in the absence of triphenyl- arsine.The situation with N-aminophthalimide where rapid oxidation occurs in the absence of triphenylarsine is less clear. The same arsinimine acetate is formed by LTA oxidation and from heating of the aziridine (14)in the presence of acetic acid suggesting that at least some of this product in the LTA oxidation is formed by a non-arsorane route and involves a nitrene-lead complex nitrene or nitrenium ion. The last intermediate could explain the differing modes of reaction of ‘phthalimidonitrene’ with dimethoxybenzene when generated by LTA oxida- tion (nitrenium ion) and pyrolysis of aziridine (14) (free r~itrene)~~ (Ann.Reports (B),1973,70,186).A number of N-substituted 1-amino- 1,2,5-triphenylphospholes (15) have been produced by trapping of nitrene or nitrenoid species with 1,2,5-triphenylphosphole and by its reaction with azides which does not involve nitrenes.28 Nphth N-N S Ph -r( >Ph KYN\? N 0’ “0 I :N 24 J.I. G. Cadogan and A. G. Rowley J.C.S. Chem. Comm. 1974 179; J.C.S. Perkin 11 1974 1030. 25 U. Svanholm and V. D. Parker J. Amer. Chem. SOC.,1974 96 1234. 26 J. I. G. Cadogan and I. Gosney J.C.S. Perkin I 1974,460. ” J. I. G. Cadogan and I. Gosney J.C.S. Perkin I 1974 466. 28 J. I. G. Cadogan R. J. Scott R. D. Gee and 1. Gosney J.C.S. Perkin I 1974 1694. Arynes Carbenes Nitrenes and Related Species 157 Other workers assume that a nitrene is involved in the stereospecific formation of aziridines from LTA oxidation of N-aminophthalimide in the presence of alkenes.A transition state (16) involving lateral approach of an sp-hybridized singlet nitrene allows favourable interaction between the nitrene HOMO and olefin LUMO and vice uersa. The relative reactivities of different alkenes can be rationalized in terms of estimated FMO energies. Secondary interactions in this parallel-plane approach favour a syn orientation of the phthalimide and olefinic ester groups.29 Such a stereochemical preference is normally masked by rapid inversion at nitrogen in aziridines but has now been elegantly confirmed by carrying out the oxidation at low temperature where inversion is eliminated.30 A free nitrene (17) is thought to be involved in the oxidation of 4-amino-3,S-diphenyltriazole since the proportion of fragmentation to benzonitrile and intermolecular aziridine formation in the presence of olefins is the same with different oxidant^.^ N-Nitrenes have been suggested as intermediates in several other reactions.32 Photolysis of dioxazolidinones (12) and thiadioxazolidine S-oxides (18)appears to give acyl nitrene~~~ since the same reactivity pattern regarding C-H insertion is observed as with the corresponding acyl a~ides.~" There is evidence that in the photolysis of acyl azides in halogen-free solvents the photo-Curtius reaction and nitrene formation are independent and c~ncurrent.~~ Further evidence for the stabilization of singlet ethoxycarbonylnitrene by methylene chloride has been claimed.35 Detailed analysis of the thermolysis of methanesulphonyl azide in aromatic solvents with electron-withdrawing groups is consistent with initial formation of the singlet nitrene which collapses to the triplet in competition with addition to the solvent to give benzaziridines.These ring-open to give rn-sulphonamides whereas o-sulphonamides result from radical substitution by the triplet ~~itrene.~~ The trapping of azepines as the kinetically controlled products of such reactions supports the proposed intermediacy of benzaziridines. Sulphonyl azides give tetraphenyl-2-pyridone and tetraphenyl-2-pyridyl sulphonates on pyrolysis in the presence of tetracyclone most likely by 1,4- or 1,Zaddition of the nitrene followed by rearrangement.4 Detailed studies of the competition between carbazole and azepine formation in the photolysis of biphenylazides and deoxygenation of nitrosobiphenyls in the j9 H.Person C. Fayat F. Tounard and A. Foucaud Bull. Soc. chim. France 1974 635. .'' R. S. Atkinson and R. Martin J.C.S. Chem. Comm. 1974 386. " F. Schroppel and J. Sauer Tetrahedron Letters 1974 2945. 32 S. B. Matin J. C. Craig and R. P. K. Chan J. Org. Chem. 1974 39 2285; S. Mataka and J.-P. Anselme J.C.S. Chem. Comm. 1974 554; B. V. Ioffe and L. A. Kartsova Zhur. org. Khim. 1974 10 989. 33 E. E. Eibler and J. Sauer Tetrahedron Letters 1974 2565. '' E. Eibler and J. Sauer Tetrahedron Letters 1974 2569.35 R.C. Belloli M. A. Whitehead R. H. Wollenberg and V. A. LaBahn J. Org. Chem. 1974 39 2128. 36 R. A. Abramovitch G. N. Knaus and V. Uma J. Org. Chem. 1974 39 1101. '' R. A. Abramovitch T. D. Bailey T. Takaya and V. Uma J. Org. Chem. 1974 39 340. 158 R. C. Storr presence of diethylamine lead to the conclusion that the initially formed nitrene (19) can give rise to carbazoie via two intermediates. One of these can be inter- cepted with diethylamine and is assumed to be the azirine (20) the other could be the unstable species (21) or the triplet nitrene.38 9s I N: .. H 0- t 19) Thermolysis of 1-and 2-azidonaphthalenes gave dibenzo[ah]phenazine and other products typical of the triplet nitrenes. Photolysis of the azides in the presence of diethylamine in the hope of intercepting a singlet nitrene was successful for the 2- but not the 1-azide.The former gave 1-amino-2-NN-diethylamino-naphthalene in yields which were increased by the presence of singlet sensiti~ers.~’ The photolysis of 4-and 5-azidobenzo[b]thiophensis similar only the 5-azide giving products (4-amino-5-dialkylaminobenzo[b]thiophens)indicative of a singlet nitrene and an azirine intermediate.40 The first example of ring expansion via an azirine in the photolysis of bicyclic aromatic azides occurs with 6-azido- benzo[b]thiophens which give 8H-thieno[2,3-c]azepines in the presence of dieth~lamine.~ The thermal decomposition of 1-and 2-azidobenzylnaphthalenesgives almost exclusively aromatic C-H insertion products benzacridan and benzacridine (by oxidation).In contrast the tetralin analogues (22) give largely azepines the difference being attributed to the greater loss in resonance energy involved in the ring expansion of the intermediate aziridines in the case of the naphthalene^.^^ 38 R. J. Sundberg and R. W. Heintzelman J. Org. Chem. 1974 39 2546. ’’ S. E. Hilton E. F. V. Scriven and H. Suschitzky J.C.S. Chem. Comm. 1974 853. 40 B. Iddon H. Suschitzky and D. S. Taylor J.C.S. Perkin I 1974 579. 41 B. Iddon M. W. Pickering and H. Suschitzky J.C.S. Chem. Comm. 1974 759. 42 R. N. Carde and G. Jones J.C.S. Perkin I 1974 2066. Arynes Carbenes Nitrenes and Related Species HH The formation of thieno[3,2-b]quinoline (25) by thermal decomposition of 2- (2-azidobenzy1)thiophen (23) can be explained by insertion uia the aziridine (24) followed by oxidation.The thione (27) is also produced possibly via the 1,4- nitrene-thiophen adduct (26) formed either directly or from the aziridine (24).43 There has been little real evidence for nitrenes as intermediates in the gas- or liquid-phase thermolysis of alkyl azides which normally gives imines possibly by a mechanism involving anchimeric assistance. Intramolecular substitution has now been observed to compete with imine formation in the decomposition of a tertiary alkyl azide (28 ; R = Me or Ph). This competing insertion and the very narrow range of migratory aptitudes observed (Ph Me 1.9;o-PhC,H Me 2.0) argue strongly for a reactive nitrene in this case4 The photolysis of cyclopropyl azides gives a high yield of nitrile and olefin most likely via cheletropic fragmentation of a nitrene in contrast to thermolysis where ring-expansion to azetine can dominate and anchimeric assistance in the decomposition would seem to be involved.45 The formation of pyrazoles and pyrroles from thermolysis of 2-imino- and 2-vinyl-2H-azirines is rationalized as a further example of vinylnitrene formation from 2H-a~irines.~~ However activation parameters for the formation of azirines from vinyl azides argue against a free nitrene intermediate and indicate that anchimeric assistance is important.47 Similar conclusions are drawn for the thermal conversion of 2-azido-3-vinyl- 1,4-quinones (29) into indolequinones (30)."* A nitrene is also ruled out for the formation of 2-alkenyl-2,3-dihydro- indole-4,7-diones e.g.(32) from the photolysis (360nm) of 2-azido-l,4-quinones (31) in the presence of acyclic and cyclic dienes. The reaction is believed to involve addition of photoexcited azide followed by decomposition of the resulting " G. R. Cliff G. Jones and J. McK. Woollard J.C.S. Perkin I 1974 2072. 44 R. A. Abramovitch and E. P. Kyba J. Amer. Chem. Soc. 1974 % 480. 45 A. Hassner A. B. Levy E. E. McEntire,and J. E. Galle J. Org. Chem. 1974 39 585. 46 A. Padwa J. Smolanoff and A. Tremper Tetrahedron Letters 1974 29. '' G. L'abbe and G. Mathys J. Org. Chem. 1974,39 1779. 48 P. Germeraad and H. W. Moore J. Org. Chem. 1974 39 774. 160 R. C.Storr tria~oline.~~ Thermolysis of the acetoxy-azide (33) gives the iminoquinones (34) and (35).The former probably involves either direct acyl transfer to the nitrene or anchimerically assisted azide decomposition while the latter appears to require rearrangement of an azirine intermediate.50 The diazidonaphthalenes (36 ; R = OAcS0 or H5')give benzocyclobutanes via o-quinodimethanes. In each case the major isomer is that expected from orbital symmetry considerations. In addition the diacetoxy-compound (36; R = OAc) gives the quinoline (37) which suggests that a carbene Gnitrene skeletal interconversion may be occurring. O 49 P. Germeraad W. Weyler and H. W. Moore J. Org. Chem. 1974 39 781. D. S. Pearce M. S. Lee and H. W. Moore J. Org. Chem. 1974,39 1362.5L M. E. Peek C. W. Rees and R. C. Storr J.C.S. Perkin I 1974 1260. Arynes,Carbenes,Nitrenes,and Related Species A new route to nitrenes by thermal deoxysilylation of hydroxylamine deriva- tives e.g. (38) has been developed. Most of the products isolated from (38) which is more labile than phenyl azide parallel those expected from triplet phenylnitrene but in cyclohexene 2 % of aziridine is formed. It is not yet clear whether this last reaction which is most uncommon involves the nitrene.” /OSiMe3 Ph-N -+ Ph-N + Me,SiOSiMe, \ SiMe (38) NBr (39) Phosphorylnitrenes (39) have been generated from the dibromides (40)’ with zinc and by photolysis of phosphoryl a~ides.’~ They are extremely unselec- tive and undergo intermolecular reactions rather than intramolecular rearrange- ment and are therefore ideal for affinity labelling purposes.Intramolecular reactions do however occur on photolysis of the related 1-azidophosphetan oxides (41) in methanol. Ring-expansion may involve a nitrene or Curtius-like rearrangement and ring-opening to give phosphoramides may involve intra- molecular hydrogen transfer in an intermediate nitrene.’ ’ 52 F. P.Tsui T. M. Vogel and G. Zon J. Amer. Chem. SOC.,1974 96 7144. 53 A. Zwierzak and S. Zawadski Tetrahedron 1973 29 3899. 54 R. Breslow A. Feiring and F. Herman J. Amer. Chem. SOC.,1974 96,5937. 55 M. J. P. Harger J.C.S. Perkin I 1974 2604. 162 R. C. Storr 3 Carbenes The application of ab initio calculations to the understanding of the structure of methylene has been reviewed.56 Hydrogen abstraction is feasible through a least-motion approach for triplet methylene but through a non-least-motion route for the singlet species.Since the overall dynamics of the reaction favour the least motion approach hydrogen abstraction by singlet methylene is for- bidden.57 Analytic least-motion studies5 and new ab initio calculation^^^ for methylene dimerization agree with previous studies that the methylenes prefer to approach initially through a perpendicular configuration. Detailed trajectory calculations have given an insight into the dynamics of the insertion of singlet methylene into hydrogen. These reveal that although for appropriate initial conditions the reaction can follow the minimum energy path predicted in earlier static calculations the insertion can take place over a very wide range of initial conditions for some of which the dynamics may be very complicated.60 Cyclopentadienylidine (42) has been directly observed in the low-temperature matrix photolysis of diazocyclopentadiene.On warming it dimerizes or reacts with carbon monoxide present in the matrix to give the previously uncharacter- ized keten (43).6 Geometrical isomers of ground-state triplet vinyl methylene have been detected by e.s.r. spectroscopy after low-temperature photolysis of vinyl diazomethane.6 Matrix-isolated anthronylidene has been characterized as a ground-state triplet by emission63 and e.s.r. spectro~copy,~~ and the photo- electron spectrum of difl~oromethylene~~ has been recorded.There is further evidence for the reduction of methylene to a methylene radical- anion.66 1,l-Dicyclopropylethylene has been suggested a simple probe for the spin multiplicity of carbenes. Thus addition of typical singlet species gives the unrearranged cycloadduct while a typical triplet carbene gives a biradical intermediate which leads to cyclopropane-cleaved product^.^' A comprehensive review on alkoxycarbonylcarbenes has appeared.68 56 J. F. Harrison Accounts Chem. Res. 1974 7 378. 51 L. Salem J. Amer. Chem. SOC. 1974 % 3486. 58 S. Ehrenson J. Amer. Chem. SOC. 1974 % 3784. 59 P. Cremaschi and M. Simonetta J.C.S. Faraday II 1974 1801. 60 I. S. Y. Wang and M. Karplus J. Amer. Chem. SOC. 1973,95 8160. 61 M.S. Baird I. R. Dunkin and M. Poliakoff J.C.S. Chem. Comm. 1974 904. 62 R. S. Hutton M. L. Manion H. D. Roth and E. Wasserman J. Amer. Chem. SOC. 1974 % 4680; see also G. E. Palmer J. R. Bolton and D. R. Arnold ibid. p. 3708. 63 N. Filipescu J. W. Pavlik P. T. Frangopol and M. Frangopol Rev. Roumaine Chim. 1973 18 1959. 64 P. Devolder P. Goudmand and J. P. Grivet J. Chim. phys. 1974 71 899. 65 J. M. Dyke L. Golob N. Jonathon A. Morris and M. Okuda J.C.S. Faraday II 1974 1828. 66 G. D. Sargent C. M. Tetum and R. P. Scott J. Amer. Chem. SOC. 1974 96,1602. 67 N. Shimizu and S. Nishida J. Amer. Chem. SOC.,1974 96 6451. 68 A. P. Marchand and N. M. Brockway Chem. Rev. 1974 74 431. Arynes Carbenes Nitrenes and Related Species Generation.-Phase-transfer catalysis has now become standard practice in the generation of dichloro and other carbenes by a-eliminati~n.~~ Crown ethers have also been employed successfully as phase-transfer catalysts7' and in one of the most significant papers this year it has been shown that generation of phenylhalogenocarbenes from benzal halides with potassium t-butoxide in the presence of 18-crown-6-ether gives a species which shows the same selectivity as that from photolysis of the corresponding diazirine.It appears therefore that complexation of the K+ ion leaves a free carbene. In the absence of the crown ether elimination gives a species presumably a carbenoid such as (44) which shows a different ~electivity.~' In the short-wavelength photolysis of mercury bisdiazoacetate in the presence of olefins a substantial proportion of products arise from ethoxycarbonyl- methyne (45) which is formed in a doublet ground state and behaves essentially as a singlet carbene adding stereospecifically to give cyclopropyl radicals and inserting into C-H bonds to give alkyl radicals.72 Interception of this carbyne by metal complex formation has been reported.73 1,2-Ketocarbenes (46) are generated in the thermolysis of sodium o-bromo- phenoxide and trapped with added nucleophiles.There is no evidence for benzoxiren participation but benzothi-irens (47) appear to be intermediates in the same reaction of o-bromoben~enethiolates.~~ The stereospecificity observed in the photoinduced isomerization of cyclobutanone to tetrahydrofurylidine is inconsistent with the intervention of a biradical intermediate.75 An explanation has been advanced for the unique ability of cyclobutanones to undergo this rearrangement.Introduction of a 2-phenyl substituent causes C-C bond cleavage leading to an iminocarbene rather than the normal C-N cleavage in the gas-phase thermolysis of azirines (48). 69 See for example E. V. Dehmlow Angew. Chem. Internat. Edn. 1974 13 170; R. Mathias and P. Weyerstahl ibid. p. 132; T. Sasaki K. Kanematsu and Y. Yukimoto J. Org. Chem. 1974,39,455; T. B. Patrick Tetrahedron Letters 1974 1407; T. Sasaki S. Eguchi and T. Ogawa J. Org. Chem. 1974,39 1927. 70 M. Makosza and M. Ludwikow Angew. Chem. Internat. Edn. 1974 13 665. 71 R. A. Moss and F.G. Pilkiewicz J. Amer. Chem. Soc. 1974 96,5632. 12 0. P. Strausz G. J. A. Kennepohl F. X. Garneau T. Do Minh B. Kim S. Valenty, and P. S. Skell J. Amer. Chem. SOC.,1974 96 5723. 73 W. A. Herrmann Angew. Chem. Internat. Edn. 1974 13 812. 74 J. I. G. Cadogan J. T. Sharp and M. J. Trattles J.C.S. Chem. Comm. 1974 900. 75 G. Quinkert P. Jacobs and W.-D. Stohrer Angew. Chem. Internat. Edn. 1974 13 197; G. Quinkert and P. Jacobs Chem. Ber. 1974 107,2473. 76 W.-D. Stohrer G. Wiech and G. Quinkert Angew. Chem. Internat. Edn. 1974 13 199 200. 71 L. A. Wendling and R. G. Bergman J. Amer. Chem. Soc. 1974,96 308. 164 R.C. Storr Carbene/carbenoids are generated in good yield from primary vinyl triflates with KOBU'.~' Various organomercury reagents have been examined as carbene- transfer reagents.79 Dichlorocarbene appears to be formed from carbon tetra- chloride and antimony pentabutyl,80 and full details of its formation by vapour- phase decomposition of trifluoro(trichloromethy1)silanehave appeared.' Car-benes have been generated by deprotonation of benzodithiolium (49) and related (-50) salts.82 The carbene (50)has been intercepted in the photochemical rearrange- ment of tripty~ene,~~ and cyclopentadienylidene (42)produced by decarbonyla- tion of keten (43) is implicated in a minor pathway in the flash thermolysis of methyl ~alicylate.~~ Lithium dicyclohexylamide can be used as a convenient base for the in situ generation of carbenoids for reaction with carbonyl com- pounds.8s Kinetic studies reveal the complexity involved in the copper-catalysed decomposition of diazo-compounds.86 Rearrangements.-Estimated energies for phenylcarbene (5 I ) cyclohepta-trienylidene (52),and related species have been used in a detailed analysis of the rearrangements observed for aromatic carbenes and nitrenes.It is claimed that such a thermochemical approach allows a unifying explanation of the majority of available experimental data if the importance of chemical activation in the formation of the reactive species and hence its available energy for further transformations is taken into account. This review also casts doubts on the reported interconversion of aryl carbene and isomeric 1,3-biradicals (53) and argues strongly against the intermediacy of bicycloheptatriene intermediates in the rearrangement (51)G(52).87 However evidence for the trapping of such intermediates in related reactions has also appeared.* ' Other semi-empirical calculations for the phenylcarbene-cycloheptatrienylidene system also consider 78 P.J. Stang M. C. Mangum D. P. Fox and P. Haak J. Amer. Chem. SOC.,1974 96 4562. 79 D. Seyferth and R. A. Woodruff J. Organometallic Chem.. 1974 71 335. A. N. Nesmeyanov A. E. Borisov and N. G. Kizim Izvest. Akad. Nauk S.S.S.R. Ser. khim. 1974 1672. J. M. Birchall G. N. Gilmore and R. N. Haszeldine J.C.S. Perkin I 1974 2530. G. Scherowsky and J. Weiland Annalen 1974 403; G. Scherowsky Chem. Ber. 1974 107 1092; C. Th. Pedersen and 3. Mdler Tetrahedron 1974 30 553. 83 H. Iwamura and K.Yoshimura J. Amer. Chem. SOC.,1974 96 2652. 84 0. A. Mamer K. G. Rutherford and R. J. Seidewand Canad. J. Chem. 1974,52,1983. H. Taguchi H. Yamamoto and H. Nozaki J. Amer. Chem. SOC.,1974 96 3010. 85 86 D. Bethel1 and M. F. Eeles J.C.S.Perkin IZ 1974 704. *' C. Wentrup Tefrahedron 1974 30 1301. 88 T. T. Coburn and W. M. Jones J. Amer. Chem. SOC.,1974 96 5218; W. E. Billips L. P. Lin and W. Y. Chow ibid. p. 4026. Arynes Carbenes Nitrenes and Related Species the isomeric cycloheptatetraene (54). This non-planar allenic form is the most stable although the seven-membered ring systems behave only as (51) or (52). However the carbene (55) does behave as the allene (56),in which the larger ring more easily accommodates the allenic structure.The calculated effects of differing modes of annelation on the species (51) (52) and (54) are compatible with experimental observations. 89 CND0/2 and extended Hiickel calculations for the analogous phenylnitrene-azepinylidene interconversion indicate that the nitrene N moves out of the plane during the process. Recognition of this necessity for out-of-plane motion clarifies the experimental observations for certain aryl nitrenes and assuming a similar situation applies to aryl carbenes allows an explanation for new data concerning the proportions of styrene and benzocyclo- butanes from the different tolylcarb~ne~.~~ C-1 and C-2 labelled phenylacetylenes interconvert via benzylidene carbene at high temperature.” Ab initio calculations indicate that the singlet state for methylcarbene is almost degenerate with but slightly lower in energy than the triplet.The most favourable route for its rearrangement into ethylene is from the So--+ So states via a syn transition state (57).9’ The high exo endo hydrogen migration ratio observed for the carbene derived from the tosylhydrazone of ketone (58) is consistent with this analy~is.’~ Carbenes are usually assumed to react with very small or non-existent activation energies. However the two competing reactions of cyclopropyl carbene (59)(assuming both are carbene reac- tions) have measurably different activation energies.94 The fragmentation mode follows the stereochemistry of a concerted non-linear cheletropic extrusion.95 89 R.L. Tyner W. M. Jones Y. Ohm and J. R. Sabin J. Amer. Chem. SOC.,1974 96 3765. 90 R. Gleiter W. Rettig and C. Wentrup Helv. Chim. Acta. 1974 57 21 11. 91 R. F. C. Brown K. J. Harrington and G. L. McMullen J.C.S. Chem. Comm. 1974 123. 92 J. A. Altman 1. G. Csizmadia and K. Yates J. Amer. Chem. SOC.,1974 96 4196. 93 A. Nickon F. Huang R. Weglein K. Matsuo and H. Yagi J. Amer. Chem. SOC. 1974 96 5264. 94 S. S. Olin and R. M. Venable J.C.S. Chem. Comm. 1974 104. 95 S. S. Olin and R. M. Venable J.C.S. Chem. Comm. 1974 273. 166 R. C.Storr H ,c-c 0 H-H + (59) H -C rC-( CH Jn -C H =CH -CO,Et A Wolff rearrangement has been employed as the key ring-contraction step in the formation of a [7]-from an [8]-para~yclophane.’~ The diazo-compounds (60) undergo normal photo-induced Wolff rearrangement but copper-catalysed decomposition gives the product of a vinylogous Wolff rearrangement.97 p-Oxidocarbenoids eg.(61) readily formed from cyclic ketones and LiCHBr2 are useful intermediates in ring-enlargement reactions.98 2-Furylcarbene has been ruled out as an intermediate in the conversion of furan (62) into methylene- cyclobutenone and benzoic acid.99 In contrast to azidopyridine 1-oxides which undergo ring-contraction to 2-cyano-1-hydroxypyrroles on thermolysis 2- pyridyldiazomethane 1-oxides give 2-acylpyridines. O0 (60) R = H or Me n=lor2 Addition.-Non-linear cheletropic 1,2- and linear cheletropic 1,Caddition of singlet methylene to cisoid butadiene are both allowed although clear evidence for the latter mode has never been obtained.An MO study of this system has now accounted for this in terms of the greater closed-shell repulsion involved in 96 N. L. Allinger T. J. Walter and M. G. Newton J. Amer. Chem. SOC.,1974 96 4588. 97 A. B. Smith J.C.S. Chem. Comm.,1974 695. 98 H. Taguchi H. Yarnamoto and H. Nozaki J. Arner. Chem. SOC.,1974 96 6510. 99 W. S. Trahanovsky and M.-G. Park J. Org. Chem. 1974,39 1449. * O0 R. A. Abramovitch C. S. Menon M. Murata and E. M. Smith J.C.S. Chem. Comm. 1974 693. Arynes Carbenes Nitrenes and Related Species the 1,4-addition.'" Linear cheletropic homo- 1,4-addition of fluorocarbenes to norbornadiene has been observed. The nucleophilic carbene 2,3-diphenyl- cyclopropenylidene gives the product expected from 1,4-addition to tetracyclones.However addition of cycloheptatrienylidene is complex and gives benzocyclo- heptatrienes by a mechanism in which extrusion of carbon monoxide from an initial 1,4-adduct can be excluded. lo3 The 1,4-adducts from 2,6-di-t-butylcyclo- hexadienone carbene (63) and dienes arise via 1,2-addu~ts."~ Addition of cyclo- hexylidene carbene (64) to cyclopentadiene gives (65) by spontaneous rearrange- ment of the initial adduct uia a trimethylenemethane.105 .. Addition of 'carbenes' to olefins continues to be used widely as a route to cyclopropanes and an extensive review on the Simmons-Smith reaction has appeared. O6 Non-stereospecific addition of dimethoxycarbene to diethyl maleate and fumarate is explained in terms of a dipolar intermediate.'" Excep-tionally high enantioselectivity (up to 70 %) in the formation of cyclopropane-carboxylates from ethyl diazoacetate has been achieved using bis-[( + )-camphor-quinonedioximato]cobalt(~~)as catalyst."* The ready hydrolytic cleavage of 2-aminocyclopropane esters formed from carbenes and enamine esters provides the basis of an easy route from p-to y-keto-e~ters.''~ Labelling studies reveal that a carbenoid mechanism is involved in the formation of arylcyclopropanes in the acid-catalysed decomposition of aryldiazomethanes in the presence of olefins.lo The electrophilic character of cyclopentadienylidene is apparent in its reaction with alkynes to give spir0-2,4-heptatrienes,' and substituted styrenes to give spiroheptadienes.' l2 Neither singlet nor triplet carbenes such as (66) can assume the requisite geometry for intramolecular cycloaddition.113 Oxirans have been H.Fujimoto and R. Hoffmann J. Phys. Chem. 1974 78 1167. Io2 C. W. Jefford A. N. Kabengele J. Kovacs and U. Burger Tetrahedron Letters 1974 257; Helv. Chim. Acta 1974 57 104. lo' T. Mitsuhashi and W. M. Jones J.C.S. Chem. Comm. 1974 103. G. A. Nikiforov B. D. Sviridov and V. V. Ershov Bull. Acad. Sci. U.S.S.R. 1974 23 339. lo' M. S. Newman and M. C. Vander Zwan J. Org. Chem. 1974,39,761. '06 H. E. Simmons T. L. Cairns S. A. Vladuchick and C. M. Hoiness Org. Reactions 1973 20 1131. R. W. Hoffrnann W. Lilienblum and B. Dittrich Chem. Ber. 1974 107 3395. Y. Tatsuno A. Konishi A.Nakamura and S. Otsuka J.C.S. Chem. Comm. 1974,588. H. Bierangel J. M. Akkerman J. C. Lapierre Armande and U. K. Pandit Terruhedron Letters 1974 28 17. 'lo G. L. Closs and S. H. Goh J. Org. Chem. 1974 39 1717. H. Durr B. Ruge and B. Weiss Annalen 1974 1150. H. Durr and F. Werndorff Angew. Chem. Internat. Edn. 1974 13,483. 'I3 A. Viola S. Madhavan and R. J. Proberb J. Org. Chem. 1974 39 3154. 168 R. C. Stow isolated from the addition of halogenocarbenes to highly halogenated carbonyl compounds.' l4 Tetrachloroaziridines have similarly been obtained from carbonimidoyl dichlorides but with azo-arenes fragmentation of the initially formed diaziridine or ylide occurs.' .. Ph 5-Diazomethyl- 1,4-diphenyl- 1,2,3-triazole (67 ; R = H) decomposes readily in aromatic hydrocarbons to give tropylidenes as expected for a singlet carbene with high selectivity resulting from the stabilizing effect of the triazole ring.The a-phenyl derivative (67; R = Ph) gives products of the linear triplet carbene. This would be favoured by steric hindrance and independent conjugation of the orthogonal p-orbitals by the two substituents.' '' Detailed mechanistic investigation of the thermal decomposition of diphenyl diazomethane in the presence of butylamines shows that the main product N-(diphenylmethy1)butylamineis formed from the singlet carbene by nucleophilic attack of the amine followed by rearrangement of the resulting ylide. A minor hydrogen-abstraction recombination route involving the triplet carbene is detected by CIDNP but the major product from the triplet carbene is tetra- phenylethane.' '' Diacetyl- and dialkoxycarbonyl-carbenesdisplay complete selectivity for equatorial addition to 4-t-butylthiacyclohexane and high selectivity in intermolecular attack on a pair of dialkyl sulphides in contrast to toluene-p- sulphonyl- and ethoxycarbonyl-nitrenes which are non-selective.' Catalytic intramolecular cyclization of bis-a-diazo-ketones (68) has been achieved at high dilution and applied to a new synthesis of y-tropolone from glutaconic acid.' COCHN 'COAH Br Br D. Seyferth W. Tronich W. E. Smith and S. P. Hopper J. Organometallic Chem. 1974 67 341. D. Seyferth W. Tronich and H. Shih J. Org. Chem. 1974 39 158. 'I6 P. A. S. Smith and E. M.Bruckmann J. Org. Chem. 1974,39 1047. 'I7 D. Bethell J. Hayes and A. R. Newall J.C.S. Perkin II 1974 1307. 'I8 D. C. Appleton D. C. Bull J. McKenna J. M. McKenna and A. R. Walley J.C.S. Chem. Comm. 1974 140. l9 J. Font J. Valls and F. Serratosa Tetrahedron 1974 30 455. Arynes Carbenes Nitrenes and Related Species 169 Insertion.-Almost complete racemization is observed at the insertion centre in the reaction of triplet diphenylmethylene (from thermolysis of diphenyldiazo- methane) with optically active diastereoisomeric 1-methylheptyl cc-fluoro- phenylacetates. CIDNP observed for the -19F and -'H spectra of the products also shows no stereochemical preference and is consistent with recombination of radical pairs formed by hydrogen abstraction after many diffusive displacements.Significantly the slight predominance for insertion with retention may involve some reaction uia the singlet carbene.' 2o Insertion into cyclohexane of diphenyl- methylene from photolysis of diphenyldiazomethane has been observed con- trary to previous reports. ' ' Dihalogenocarbene insertion into unactivated cycloalkane C-H bonds has been achieved in respectable yields using phenyl- trihalogenomethyl mercury. '22 The products of intramolecular C-H insertion in 4-and 5-protoadamantylidenes indicate that when a carbene cannot approach with its vacant p-orbital along the C-H bond axis insertion can occur uia the most convenient triangular transition state. '' The cyclopropylidene derived from dibromide (69) undergoes intramolecular insertion into the primary rather than tertiary C-H bonds probably for steric reasons.124 Rare 1,3-intramolecular insertion of a silyl carbene occurs in the pyrolysis of phenyltrimethylsilyldiazo-methane.' 25 Various studies ofcarbene insertions into organosilanes,' 26 Group IV element- halogen bonds,' and polar X-H bonds (X = 0,s,or N)' s have been reported. 4 Silylenes Several papers concerning silylenes have appeared this year. Phenylmethyl-silylene has been produced by photolysis of the trisilane (70) and trapped with cyclohexene as the silacyclopropane.' 29 Photochemical extrusion of the central silylene fragment from the related 1,2,3-trisilacycloheptanesis highly stereo- specific configuration being retained at both Si-1 and Si-3 in the resulting disilacyclohexane.This process may therefore be a novel type of cheletropic rea~tion.'~' Silylene 31SiH2 produced by the nuclear recoil method from a 31Pprecursor gives the 1,4-adduct with b~tadiene.'~','~~ Addition of NO as ''O D. Bethel1 and K. McDonald J.C.S. Chem. Comm. 1974 467. ''I J. H. Boyer V. T. Ramakrishnan and K. G. Srinivasan Synthesis 1974 192. 12' D. Seyferth and Y. M. Cheng Synthesis 1974 114. ' D. Skare and Z. Majerski J.C.S. Chem. Comm. 1974 1000. '' D. P. G. Hamon and V. C. Trenerry Tetrahedron Letters 1974 137 1. I " W. Ando A. Sekiguchi T. Hagiwara and T. Migita J.C.S. Chem. Comm. 1974 372. H. Watnabe T. Nakano Y. Araki H. Matsumoto and Y. Nagai J. Organometallic '" Chem. 1974 69 389; W. Ando K. Konishi and T.Migita ibid. 67 C7-C9. R. N. Haszeldine A. E. Tipping and R. O'B. Watts J.C.S. Perkin I 1974 2391; M. Weidenbruch and C. Pierrard J. Organometallic Chem. 1974 71 C29. R. Paulissen E. Hayez A. J. Hubert and P. Teyssie Tetrahedron Letters 1974 607. 129 M. Ishikawa and M. Kumada J. Organometallic Chem. 1974 81 C3. H. Sakurai Y. Kobayashi and Y. Nakadaira J. Amer. Chem. SOC.,1974 96 2656. "I P. P. Gaspar R. J. Hwang and W. C. Eckelrnan J.C.S. Chem. Comm. 1974 242. I." 0. F. Zeck Y. Y. Su G. P. Gennaro and Y.-N. Tang J. Amer. Chem. SOC.,1974 96 5967. 170 R. C.Storr OM' RR R \/ 'E:{-R RISilR Me,SiSiMe,SiMe / 'OMe Si-R (70) R\ R Si R R /\ (71) (72) RR (73) scavenger indicates that the silylene is present as both triplet (80%) and singlet (20%) moderator studies suggesting a ground singlet state.132 The 1,4-addition of silylene produced by pyrolysis of disilane to trans,trans-hexa-2,4-dieneis non-stereospecific;this rules out a concerted 1P-addition but could be consistent with concerted 1,2-addition followed by rearrangement of vinylsilacyclopropane to silacyclopentene via a biradical intermediate.133 A vinylsilacyclopropane rearrangement involving C-C bond cleavage is probably involved in the formation of the silanonadiene (71) from methylmethoxysilylene and cyclo- octa- 1,3-diene.' 34 It has been suggested that 1,2-disilacyclobutene (72) which reacts with acetylenes uia its ring-opened form may be a key intermediate in the formation of 1,4-disilacyclohexa-2,5-dienes (73) from acetylenes and ~i1ylenes.l~ P.P. Gaspar and R. J. Hwang,J. Amer. Chem. SOC.,1974 % 6198. '34 M. E. Childs and W. P. Weber Tetrahedron Letters 1974 4033. ' 3s T. J. Barton and J. A. Kilgour J. Amer. Chem. Soc. 1974 96 71 50.