5 Arynes Carbenes Nitrenes and Related Species By S. A. MATLIN Department of Chemistry The City University St. John Street London ECl V 4PB 1 Arynes It has been a quiet year for aryne chemistry. The action of dialkylamines on u-and p-difluorobenzenes gives rise to fluorobenzyne intermediates from which isomeric mixtures of dialkylaminofluorobenzenes are formed.' Nucleophilic attack on the aryne triple bond by a side chain carbanion a to a nitrile group is the basis of a new synthesis of benzoheterocycles,* as illustrated by the high-yield formation of N-methylisoindole (Scheme 1). EcMe a KH2-liq.NH; N-Me / CN Scheme 1 The formation of biphenylenes substituted on one ring has been accomplished by the gas phase pyrolysis of mixtures of precursors of appropriate ben~ynes.~ The mediocre yields are offset by the simplicity of this method.The cyclobuta[c]benzyne (1)has been generated from the corresponding dihalide and undergoes the expected trapping reaction with f~ran.~ Compound (1)does not appear to form a dimer in marked contrast to the behaviour of its isomer (2) reported last year.5 This presumably reflects the lower stability of (1) compared with (2). (1) (2) Competition between nucleophilic attack (route a) and [4 + 21 and [2 + 2Jcyclo-addition pathways (routes b and c respectively) is seen in the reactions of allylic Grignard reagents with benzyne (Scheme 2).6 ' M. F. Moreau-Hochu and P. Caubere Tetrahedron 1977,33 955. B. Jaques and R. G. Wallace Tetrahedron 1977,33,581. A.Martineau and D. C. DeJongh Canad. J. Chem. 1977,SS 34. R. L. Hillard and K. P. C. Vollhardt Angew. Chem. Internat. Edn. 1977 16 399. R. L. Hillard and K. P. C. Vollhardt J. Amer. Chem. SOC.,1976,98 3579; S. A. Matlin Ann. Reports (B) 1976,73,85. J. G. Duboudin B. Jousseaume and M. Pinet J.C.S. Chem. Comm,. 1977,454. 105 106 S. A. Matlin I R R Scheme 2 1-Naphthyne reacts with the amine (3)by a [4+ 21 route with subsequent loss of diethylamine giving phenanthrene. Other arynes behave analogously.' (3) The initial adduct of 2-vinylnaphthalene and benzyne undergoes an unusual concerted ene reaction' with a further molecule of benzyne (Scheme 3; R = H or D). Scheme 3 2 Nitrenes In contrast to previous ab initio studies new calculations (generalized valence bond and configuration interaction) predict the ground state of aminonitrene (H2NN) to be a singlet lying about 15 kcal mol-' below the triplet state.' S.Tanirnoto. Tetrahedron Letters 1977 2903. Y. Ittah I. Shahak J. Blum and J. Klein. Synthesis 1977 678. J. H. Davis and W. A. Goddard J. Amer. Chem. SOC.,1977,99,7111. Arynes Carbenes Nitrenes and Related Species The low-temperature addition of the nitrenes formed by the oxidation of N- aminophthalimide (4) and N-aminobenzoxazolone (5) to olefins conjugated to G;-NH? ao>o N 0 I NH (4) (5) ester vinyl or aryl functions is highly stereoselective." For example phthali- midonitrene (Z-N:) adds to methyl acrylate at -30 "C affording exclusively the cis-aziridine (6) which largely inverts to the more stable trans-isomer (7) on Z (6) (7) warming.This strong preference for formation of the cis-isomer appears to be due to a secondary orbital interaction between the ?r electrons of the group conjugated to the olefin and those of the carbonyl group in the N-acyl-N-nitrene. When phthalimidonitrene is generated in the presence of 0-diketones addition of the nitrene to the enol double bond is followed by opening of the azirane ring to give an amide (Scheme 4)'' Z Scheme 4 Arylnitrenes are known to equilibrate wifh the corresponding 7-azanor- caradienes which can be trapped by a variety of nucleophiles as azepines.12 The photolysis of CY -azido-a y-dienoic esters (8)leads to pyrrole f~rmation'~ via the azirines (9).Contrary to earlier reports the thermolysis of 2-penta- 1,3-dienyl- 2H-azirines (10) also affords pyrroles and not azepines.I4 However azepine formation has been cleverly induced in one instance." The azirine (11; R=H) cvclises to a pyrrole via S C-H insertion by the nitrene (12; R = H) and the corresponding methyl-substituted case (1 1; R = Me) gives a lo R. S. Atkinson and J. R. Malpass J.C.S. Perkin I 1977 2242. " H. Person K. Luanglath and A. Foucaud Tetrahedron Letters 1977 221. '* M. Masaki K. Fukui and J. Kita Bull. SOC.chim. France 1977 50 2013; B. Nay E. F. V. Scriven H. Suschitzky and Z. U. Khan Synthesis 1977,757;S. E. Carroll B. Nay E. F. V. Scriven H. Suschitzky and D. R. Thomas Tetrahedron Letters 1977 3175; S.E. Carroll B. Nay E. F. V. Scriven and H. Suschitzky ibid.. p. 943. l3 H. Hemetsberger I. Spira and W. Schonfelder J. Chem. Res. (S) 1977 247. ld K. Isomura T. Tanaka and H. Taniguchi Chem. Letters 1977 397. l5 K. Isomura H. Taguchi T. Tanaka and H. Taniguchi Chem. Letters 1977,401. 108 S. A. Matlin R' R' \ hv \ C=CH-CH=C-C02R3 -C=CH-CH-C-CO~R' R2' I N3 -'OoC R2' \/ (8) (9) RZ 1-20"C R' 0COZR3 -RZ I R'9C0,R' H I H OT&C(-J2Me C0,Me OTcK N (11) (12) pyridine. With a phenyl group S to the nitrene (12; R =Ph) attack on the aromatic ring affords an azepine. A new method of generating acylnitrenes involves the pyrolysis of N-acyl-S,S-diphenylsulphimides.'6 The singlet state of acylnitrenes is stabilized by the use of dichloromethane as solvent leading to enhancement of yields of C-H insertion products in both interm~lecular'~and intramolecular18 reactions.The latter generally proceed by attack at the y S or E C-H bonds and are useful in remote functionalization. l9 o-Methylbenzenesulphonylnitrenesgreatly prefer intermolecular reactions to intramolecular insertion into the side chain C-H bonds presumably because of the geometry of the sulphonylnitrene. In the case of the sulphonylazide (13) the intramolecular cyclization product (14) results exclusively from attack at the more accessible secondary hydrogen rather than the more reactive tertiary one.2o The ww SO2N.3 SOZ-N \ H (13) (14) N. Furakawa M. Fukumura T. Nishio and S.Oae J.C.S. Perkin I 1977 96. l7 H. Takeuchi N. Murata Y. Nakagawa T. Tsuchida and K. Koyama J.C.S. Perkin II 1977 80. l8 W. Lwowski and S. Linke Annalen 1977 8. l9 P. F. Alewood M. Benu J. Wong and A. J. Jones Canad. J. Chem. 1977,55,2510;M. R. Czamy B. W. Benson and T. A. Spencer J. Org. Chem. 1977,42 556. R. A. Abrarnovitch R. Chellathurai,W. D. Holcomb I. T. McMaster and D. P. Vanderpool J. Org. Chem. 1977,42,2920. Arynes Carbenes Nitrenes and Related Species 109 biaryl ketone derivative (15) cyclizes to a seven-membered ring sultam (16) on thermolysis.21 There is no evidence for the formation of spiro intermediates or azepines on thermolysis of biarylsulphonylazides,22 which contrasts with the be haviour of u-(arylt hio)arylni trenes .23 0 0 S02-N \ Evidence in favour of an exclusively singlet pathway for the formation of carbazole from 2-nitrenobiphenyl has been pre~ented.~~ Nitrenes derived from the amines (4) and (5) will react with allylic sulphides to form ylides which undergo 2,3-sigmatropic rearrangements (Scheme 5).25 At Ar Z Ar Z ‘R2 Scheme 5 Acylnitrenes have been reported to abstract oxygen atoms from N-nitrosoamines26 and alkoxycarbonylnitrenes will abstract oxygen from DMSO.” In the latter case the resulting nitrosoformate has been trapped by cycloaddition with thebaine.Insertion of alkoxycarbonylnitrene into the C-H bonds of trans -1,2-dichloro-cyclohexane takes place largely at the 4-position suggesting a controlling inter- action of the nitrene with a halogen atom.28 The reactions of azides and azirines in the presence of transition metal catalysts appear to involve metal-nitrene complexes.29 A number of such complexes are now becoming well ~haracterized.~’ 21 R.A. Abramovitch and D. P. Vanderpool J.C.S. Chem. Comm. 1977 18. 22 R. A. Abramovitch T. Chellathurai I. T. McMaster T. Takaya C. I. Azogu and D. P. Vanderpool J. Org. Chem. 1977 42 2914. 23 I. M. McRobbie 0.Meth-Cohn and H. Suschitzky. J. Chem. Res. (S) 1977 17. 24 J. M. Lindley I. M. McRobbie. and 0.Meth-Cohn J.C.S. Perkin I 1977 2194. ” R. S. Atkinson and S. B. Awad J.C.S. Perkin I 1977 346. 26 K. Nishiyama and J.-P. Anselme J. Org. Chem. 1977,42 2636. 27 G. W. Kirby J. W. M. Mackinnon and R. P. Sharma Tetrahedron Letters 1977,215.28 P. A. Tardella and L. Pellacani Tetrahedron Letters 1977,4451. 29 H. Alper J. E.Prickett and S. Wollowitz J. Amer. Chem. SOC.,1977 99 4330; I. Yamamoto H. Tokanou H. Uemura and H. Gotoh J.C.S. Perkin I 1977 1241. 30 0.R. Chambers M. E. Harman D. S. Rycroft D. W. A. Sharp and J. M. Winfield J. Chern. Res. (S), 1977 150; J. Chatt and J. R. Dilworth J. Indian Chem. Soc. 1977 54 13; F. Basolo ibid. p. 7. 110 S. A. Marlin 3 Carbenes A number of reviews containing references to carbene chemistry have been pub- lished.31 Uncertainty still remains concefning the triplet-singlet (3B1-'A1) energy separa- tion in methylene. A recent spectroscopic measurement of 19.5f0.7 kcal mol-' much higher than previous experimental values has provoked two new state-of- the-art configuration interaction Both place the energy separation in the region of 11kcal mol-' and it seems unlikely that the true value will turn out to be significantly greater than this.M~Bride~~ has emphasized the structural importance of bent bonds in carbenes. For diphenylcarbene INDO calculations predict the triplet to be lower than the singlet by 4 kcal mol-' with the two states interconverting rapidly.34 In CHBr the triplet is calculated to be only 1kcal mol-' below the singlet and in other halo- genocarbenes (CHCl CHF CCl, CF,) the singlet becomes more stable than the triplet by progressively larger amounts.35 There is now good evidence36 for homoallylic conjunction between the carbene and the cyclopropane orbitals in the bridged systems (17) and (18).Generation.-Halogen exchange37 in the dihalogenocarbenes derived from CHBrC12 and CHBr2C1 under phase transfer catalysis (PTC) conditions leads to adducts of all three possible carbenes CBr2 CBrCl and CC1,. This can be avoided3* by the use of dibenzo[l8]crown[6] as the catalyst which gives exclusively CBrCl adducts from CHBr2Cl. The use of solid or immobilized phase transfer catalysts for halogenocarbene generation is receiving increasing attention.39 When tertiary amines are used as catalysts selectivity in the product distributions upon addition of dichlorocarbene to polyenes suggests the intermediate formation of nitrogen ylide~.~' In the PTC addition of dihalogenocarbenes to allylic alcohols " M. D. Roth Accounts Chem.Res. 1977 10 85; W. M. Jones ibid. p. 353; S. Braslavsky and J. Heicklen Chem. Rev. 1977 77,473; R. A. Firestone Tetrahedron 1977 33 3009. 32 R. R. Lucchese and H. F. Schaefer J. Amer. Chem. Soc. 1977 99 6765; B. 0. Roos and P. M. Siegbahn ibid.,p. 7716. 33 J. M. McBride J. Amer. Chem. Soc. 1977 99 6760. 34 J. Metcalfe and E. A. Halevi J.C.S. Perkin IZ 1977 634. 35 C. W. Bauschlicher jun. H. F. Schaefer and P. S. Bagus J. Amer. Chem. Soc. 1977 99,7106. '' P. K. Freeman T. A. Harding R.S. Raghavan and D. G. Kuper J. Org. Chem. 1977 42 3882; K. Okumura and S.-I. Murahashi Tetrahedron Letters 1977 3281. 37 E. V. Dehmlow M. Lissel and J. Heider Tetrahedron 1977 33 363. 38 M. Fedorynski Synthesis 1977,783. 39 E. Chiellini and R. Solaro J.C.S.Chem. Comm. 1977,231; S. L. Regen J. Org. Chem. 1977,42 875; S. Julia and A. Ginebreda Synthesis 1977 682. 40 Y. Kimura K. Isagawa and Y. Otsuji Chem. Letters 1977,951. For another example of nitrogen ylide formation see K. Berg-Nielsen Acta Chem. Scand. 1977 B31 224. Arynes Carbenes Nitrenes and Related Species there is no evidence for any intermediate interaction of the carbene with the OH function.41 CFCl and CFBr have been generated by reaction of the corresponding fluorotri- halogenomethanes with butyl-lithium but attempts to produce monofluorocarbene were not SUCC~SS~U~.~~ Carbenes are formed by the metallation of cycloalkane epoxides with lithium dialkylamides and show a highly temperature-dependent behaviour low tempera- tures favouring transannular C-H insertion Reactions.-The reactivities of a wide range of electrophilic carbenes in cyclo- propenation of olefins have been correlated quantitatively with inductive and conjugative effects of the carbene substituents.An exceptional case is that of ethoxycarbonylcarbene for which a strong preference for C-H insertion over cyclopropanation leads to deviation between the observed and predicted reac- ti~ity.~~ Substituent effects in nucleophilic carbenes have also been The ratio of 1;2 to 1,4 addition in the reactions of dihalogenocarbenes with norbornadiene is highly dependent on the carbene substituents. Perturbation theory supports the contention that the dominant electronic effect which augments the relative rate of 1,4 addition is the reduction in electrophilic character of the carbene p orbital.46 The nucleophilic dimethoxycarbene also adds 1,4 to cyclic diene~.~~ Further studies of ketocarbene-oxirene equilibration on the pathway to Wol.ff rearrangement have been published.Zeller48 finds 13-16'/0 oxiren participation in the formation of keten on photolysis of the diazo-aldehyde (19). In the case of the isomeric '3C-labelled diazoketones (20) and (21) a marked difference in the extent of oxiren participation from the two precursors suggests a strong displacement of the equilibrium (Scheme 6) in favour of carbene (22). (22) Scheme 6 41 K. Kleveland L. Skattebol and L. V. Sydnes Acta Chem. Scund. 1977 B31,463. 42 D. J. Burton and J. L. Hahnfeld I. Org. Chem. 1977.42 828.43 R. K. Boeckman jun. Tetrahedron Leffers 1977,4281. 44 R. A. Moss,C. B. Mallon and C.-T. Ho J. Amer. Chem. SOC.,1977,99,4105. 4s R. W. Hoffmann B. Hagenbruch and D. M. Smith Chem. Ber. 1977,110.23; H. Dun S. Frolich and M.Kausch Tetrahedron Letters 1977 1767. 46 Y. Jean Tetrahedron Letters 1977,2689. 47 W. Lilienblum and R. W. Hoffmann Chem. Ber. 1977,110 3405. 48 K.-P. Zeller Angew. Chem. Internat. Edn. 1977 16 781; Tetrahedron Letters 1977 707. 112 S. A. Matlin There has been further confirmation that ring contraction of normal size rings by Wolff rearrangement does not involve oxiren parti~ipation.~~ Thiirens are also attracting considerable attention.” A new route to these intermediates involves thermolysis of bis-amine disulphides in the presence of acetylenes the isomeric thionocarbenes being trapped as 1,3 dipoles by cyclo- additions (Scheme 7).In some cases synthetically useful yields of thiophens are obtained.” RIC=CR2 1RIC=CR2 R!QR2 R’ R:QR2 R* R’ R1 Scheme 7 Cyclopropylidenes may collapse to alleness2 or insert into neighbouring C-H The latter reaction is characteristic of tetrasubstituted cyclopropylidenes and generally follows a pattern in which insertion takes place into the CY C-H bond of the group geminal to the bulkiest substituent. For example the carbene (23; R’ = Pr’ R2 = Me) yields the bicyclobutane (24). However the aryl derivatives (23; R’ =Me or Pr’ R2= Ar) undergo insertion in the opposite dire~tion,~~ affording the bicyclobutanes (25).I (23) (24) (25) Two pathways can be envisaged for the rearrangement of vinylcyclopropylidenes to cyclopentadienes (Scheme 8). The results of a labelling study (o=”C) are consistent with operation of path A in agreement with earlier work.’’ 49 U. Timm K.-P. Zeller and H. Meier Tetrahedron 1977 33,453. L. Benati P. C. Montevecchi and G. Zanardi J. Org. Chem. 1977,42 577; H. Buhl B. Seitz and H. Meier Tetrahedron,1977 33 449; T. Wooldridge and T. D. Roberts Tetrahedron Letters 1977 2643. ” F. M. Benitez and J. R. Grunwell Tetrahedron Letters 1977 3413. 52 Y. Okude T. Hiyama and H. Nozaki Tetrahedron Letters 1977 3829. ” R. M. Cory and F. R. McLaren J.C.S. Chem. Comm. 1977,587. s4 T. Shono I. Nishiguchi T. Komamura and K. Fujita Tetrahedron Letters 1977,4327.” K. H. Holm and L. Skattebol Tetrahedron Letters 1977 2347. Arynes Carbenes Nitrenes and Related Species b Me Li Br Br Jfl Scheme 8 There is continuing interest in the cyclopropylidene-vinylmethylene rearrange-n~ent.~~ Generalized valence bond and MIND0/3 calculations predict” a methyl- ene-like triplet ground state for the parent vinylcarbene. The irradiation of the sulphinyl-3H-pyrazole (26) leads to nitrogen extrusion and formation of a sulphinyl cyclopropene which is evidently capable of ring opening to a vinylcarbene at room temperature. Thus trapping with furan or cyclopentadiene in a dark reaction after photolysis is complete affords the cyclopropane (27; X = 0 or CH2)rather than a Diels-Alder adduct of the cyclopr~pene.~~ A (26) (27) The vinylcarbene formed by irradiation of the 3-diazo-pyrazole (28) undergoes C-H insertion reactions and also rearrangement via the vinylnitrene (29) giving azirine (30) and its decomposition N RH Ph 1 Ph‘ II Ph (29) (30) s6 J.A. Pincock and A. A. Moutsokapas Canad. J. Chem. 1977,55 979; M. Vincens A. Dussauge and M. Vidal Tetrahedron 1977 33 2937; A. Padwa R. Loza and D. Getman Tetrahedron Letters 1977 2847. s7 J. H. Davis W. A. Goddard and R. G. Bergman J. Amer. Chem. SOC.,1977 99 2427; J. A. Pincock and R. J. Boyd Canad. J. Chem. 1977,55,2482. M. Franck-Neumann and J.-J. Lohmann Angew. Chem. Infernat. Edn. 1977 16 323. 59 W. L. Magee and H. Shechter. J. Amer. Chem. SOC.,1977,99 633. 114 S.A. Mutlin The photochemistry of benzocyclopropenes (31)has been examined6' and the formation of products accounted for by competition between ring contraction ?f the vinylcarbene (32) to fulvalene and trapping reactions of the biradical (33). R' R' It has now been shown that thermal reversion of the fulvalene to the phenyl- carbene is possible and consequently the results of all previous attempts to study the mechanism of thermolysis of benzocyclopropenes by labelling studies must be treated with caution.61 The intervention of phenylcarbene-cycloheptatrienylidene interconversions results in the formation of indane derivatives (35; M=C or Si) from the p-substituted phenylcarbene precursors (34;M = C or Si).62 The sila-indane (35; M = Si) is also formed63 in the gas-phase pyrolysis of phenyltrimethylsilyldiazo-methane (36) via the carbene (37).Na' Me3M+H=N-N-\ -Ts (34) Me Me : (35); (36) (37) A new entry into the diphenylcarbene-phenylcycloheptatrienylidene involves the pyrolysis of the acetate (38) and similarly results in the formation of fluorene. The participation of bicycloheptatrienes such as (39) in these interconversions offers a further possible entry into the Thus dehydrohalogenation of (40) affords a mixture of the isomeric ethers (41)and (42) 6o H. Durr and A.-J. Ahr Tetrahedron Letters 1977 1991. 61 C. Wentrup E. Wentrup-Byrne and P. Muller J.C.S. Chem. Comm. 1977 211. 62 A. Sekiguchi and W. Ando Bull. Chem. SOC.Japan 1977,50 3067. 63 W. Ando A. Sekiguchi and A.J. Rothschild J. Amer. Chem. SOC.,1977 99 6995. 64 R. W. Hoffrnann R. Schuttler and I. H. Loof Chem. Ber. 1977,110,3410. 65 W. E. Billups and L. E. Reed Tetruhedron Letters 1977,2239. Arynes Carbenes Nitrenes and Related Species Ph YOAC 11 11 OBu' (42) The first synthesis of a benzothiete (44; X =S) involves photochemical Wolff rearrangement of the diazoketone (43; X = S). Interestingly the migration ability in (43; X = S 0 or NR) decreases from S to 0 to nitrogen functions the latter being unable to rearrange66 and whereas vapour-phase copyrolysis of (43; X = CO) with alcohols yields 2-carboalkoxybenzocyclobutenones (44; X = CO),irradiation in methanol gives the homophthalate (45).67 66 E. Voigt and 13. Meier Chem. Ber.1977 110 2242. h7 R.J. Spangler J. H. Kim,and M. P. Cava J. Org. aern. 1977,42 1697. 116 S. A. Matiin Whereas carbenes normally insert into the 0-H bonds of alcoholic solvents the irradiation of aryldiazomethanes in low-temperature alcoholic matrices results in insertion into the C-H bond a to the OH group. This reaction appears to follow an abstraction-recombination mechanism from the triplet carbene under condi- tions where the mobility of the latter is restricted.68 Intramolecular insertion of carbenes into C-H bonds continues to be applied to the synthesis of bridged polycyclic systems.69 In the case of (46) dehydro-halogenation results in insertion into the C-H bond (Y to nitrogen but the initial product undergoes a base-catatysed eliminati~n,~' yielding the styryl derivative (47).H CI CH ==CHPh fj !*+Qfi +$ CH,Ph CHPh I CH ,CH Ph I H (46) (47) The directing influences of methyl substituents on the course of C-H insertion reactions of carbenes obtained from 7,7-dibromonorcarane derivatives have been examined in detail.7' Depending on its relative orientation a methyl group may have a buttressing effect on the cyclopropylidene ring which facilitates ring closure or it may inductively have a direct activating influence on the adjacent C-H bond. In the carbene (48) the distance for intramolecular C-H insertion to give 2,4-methanoadamantane appears to be too great and only 1,2-shifts to give the olefins (49) and (50) are 1,2-H shifts to carbenic centres are known in general to show a strong pref- erence for migration of the hydrogen atom into the empty p orbital of the cz~rbene~~ and theoretical justification for this has been provided.The poor stereoselectivity in axial versus equatorial H-migration in the cyclohexylidene (51) has therefore H (51) H. Tornioka and Y. Izawa J. Amer. Chem. SOC.,1977,99,6128. " D. Farcasiu H. Bohm and P. von R. Schleyer J. Org. Chem. 1977,42,96. 70 R. F. Boswell and R. G. Bass J. Org. Chem. 1977 42 2342. 7' L. A. Paquette and R. T. Taylor J. Amer. Chem. Soc. 1977,99,5708. 72 T. Sasaki S. Eguchi and Y. Hirako J. Org. Chem. 1977,42,2981. 73 E. P. Kyba and C. W. Hudson J. Org. Chem. 1977.42 1935. 117 Arynes Carbenes Nitrenes and Related Species caused some concern.A theoretical reinvestigation by MIND0/3 and MNDO methods supports the argument that migration of either hydrogen can proceed through a transition state in which the H atom is aligned with the empty p ~rbital.’~ Further work on this problem is clearly required but it may well be that stereoselectivities previously observed with bridged systems reflect the influence of torsional interactions rather than the stereoelectronic demand of the migration itself.75 Reversal of the reaction by which an olefin is formed by 1,Z-migration to a carbene is relatively uncommon. The formation of the carbene (53) from the bridgehead olefin (52) has been authenticated by trapping reactions and is attri- buted to release of o train.'^ (52) (53) Direct photolysis of cycloalkenes such as (54) also gives rise to carbene inter- mediate~.~’ (54) Ylide formation and rearrangement in the reactions of carbenes with divalent sulphur compounds has been reviewed.78 An interesting intermolecular example is the rearrangement” of the ylide formed on decomposition of the penicillin-derived diazoketone (55).1 74 E. P. Kyba J. Amer. Chem. SOC. 1977,99,8330. 75 P. K. Freeman T. A. Hardy J. R. Balyeat and L. D. Wescott jun. J. Org. Chem. 1977,42 3356. 76 T. H. Chan and D. Massuda J. Amer. Chem. SOC. 1977,99,936. 77 Y. Inoue S. Takamuku and H. Sakurai J.C.S. Perkin I? 1977 1635. 78 W. Ando Accounts Chem. Res. 1977 10 179. See also C. Huynh V. Ratovelomanana and S. Julia Bull. SOC. chim. France 1977 710. 7q I. Ernest Tetrahedron 1977 33 547.118 S. A. Math Methylenecarbenes can be formed by the treatment of vinyl triflates (56; R3=H) with strong base.80 New considerably milder procedures involve" the fluoride-ion initiated decomposition of silylvinyl triflates (56;R3=SiMe3) and room temperature thermal decomposition of tosylazo-alkenes (57). Isopropylidenecarbene formed by K0Bu'-induced elimination from the triflate (56; R'= R2=Me; R3= H) behaves as an electrophilic singlet in cyclopropanation reactions.82 Rgso2cF3 Rx=Nso2Ar R R2 H (56) (57) The 14C-labelled adamantylacetylene (58) shows 25% scrambling of the label to the adjacent alkyne carbon atom on flash vacuum pyrolysis owing to reversible formation of the methylenecarbene (59) by migration of either the H or adamantyl groups.83 (58) (59a) (59b) Dimethylvinylidenecarbene best generated in the free states4 by phase transfer catalysed elimination from the bromide (60) undergoes mainly addition reactionss5 with the double bonds of unsaturated heterocycles such as 2,5-dihydrofuran.'O P. J. Stang and M. G.Mangum J. Amer. Chem. SOC.,1977,99 2597. " P.J. Stang and D. P. Fox J. Org. Chem. 1977,42 1667. 82 P.J. Stang J. R. Marsden M. G. Mangum and D. P. Fox J. Org. Chem. 1977 42,1802. For a theoretical study see J. H. Davis W. A. Goddard and L. B. Harding J. Amer. Chem. SOC.,1977,99 2919. R3 R. F. C. Brown,F. W. Eastwood and G. P. Jackman Austral. J. Chem. 1977,30 1757. T.B.Patrick and D. J. Schmidt J. Org. Chem. 1977,42 3354. " S.Landor V. Rogers. and H. R. Sood Tetrahedron 1977,33 73.