5 Arynes Carbenes Nitrenes and Related Species By D. W. KNIGHT Department of Chemistry The University Nottingham NG7 2RD 1 Arynes Ab initiu calculations have established that 0- rn- and p-benzynes have singlet ground states although some doubts still exist in the case of p-benzyne.' The total energies of the isomers increase in the order o < rn <p with the equilibrium structures of the latter two isomers showing considerable diradical character in contrast to the aryne nature of o-benzyne. Although the bicyclic structure of p-benzyne may represent an energy minimum this is far above the monocyclic structure. An analysis of the products arising from the thermolysis of hexa-1,5-diyn- 3-enes e.g. (l) suggests that the reaction proceeds via 1,4-dehydrobenzyne diradi- cals which rearrange to 1,3-dehydrobenzenes by [1,2] shifts of the trimethylsilyl groups presumably reflecting the greater stability of rn -benzynes.* Treatment of the indene derivative (2) with a strong base at 0°C gives a large number of products whose formation can be rationalized by assuming the intermediacy of 2-chloro-l,3- dehydronaphthalene in both the bicyclic (diradical) and tricyclic form^.^ u-Benzyne can be generated photochemically from the u -nitrobenzaldehyde derivative (3),4 and a full report has appeared on the generation of polymer-linked benzyne.' Ab initiu calculations suggest that the high electrophilicity of benzyne is due to a significant lowering of its LUMO energy level relative to the LUMO of linear but-2-yne while the HOMO energies of the two molecules are approximately the same.6 The synthetic utility of this electrophilicity continues to be exploited.Thus a J. 0.Noel1 and M. D. Newton J. Amer. Chem. SOC., 1979,101 51. * G.C. Johnson J. J. Stofko Jr. T. P. Lockhart D. W. Brown and R. G. Bergman J. Org. Chem. 1979,44 4215. ' W.E.Billups J. D. Buynak and D. Butler J. Org. Chem. 1979 44,4218. Y. Maki T. Furuta and M. Suzuki J.C.S. Perkin I 1979,553. S. Mazur and P. Jayalekshmy,J. Amer. Chem. SOC., 1979,101,677;c6 J. Rebek Jr. and J. E. Trend ibid. p. 737. N. G.Rondan L. N. Domelsmith K. N. Houk A. T. Brown and R. H. Levin Tetrahedron Letters 1979 3237. D. W.Knight Japanese group have reported a potentially useful route to polycyclic alkaloids (Scheme 1)7 while a new route to pyrrolo[3,4-~]pyridines probably proceeds via a pyridyne intermediate (Scheme 2).* Scheme 1 Scheme 2 Phthalide enolates add to benzyne to provide a simple if not very efficient route to anthraquinones (Scheme 3)9 and in a similar type of reaction enolates of cyclic cup -unsaturated ketones condense with benzyne (Scheme 4) affording ultimately the tricyclic alcohols e.g.(4) which are useful as precursors of benzocyclo-octenones.10 0- Scheme 3 0, +-0/o:--+Q \ HO Scheme 4 Some evidence of the existence of the heteroaryne species (5) is the isolation of (6) its (formal) Diels-Alder adduct with anthracene." H. Iida Y. Yuasa and C. Kibayashi J. Org. Chem. 1979,44 1074; cf ibid.,p.1236. I. Ahmed G. W. H. Cheeseman and B. Jaques Tetrahedron 1979,35,1145; see also M. Mallet and G. Qutguiner ibid. p. 1625. P. G. Sammes and D. J. Dodsworth J.C.S. Chem. Comm. 1979,33. lo M. Essiz G. Guillaumet J.-J. Brunet and P. Caubkre J.C.S. Chem. Comm. 1979,276; see also M. L. Viriot J. Chem. Res. (S) 1979,324;H. Iida Y. Yuasa and C. Kibayashi J. Org. Chem. 1979,44,3985. l1 M. G. Reinecke and H. H. Ballard Tetrahedron Letters 1979,4981. 87 Arynes Carbenes Nitrenes and Related Species m&\ \ (6) 2 Nitrenes A useful review has appeared concerning advances in the synthetic applications of aryl and heteroaryl nitrenes.'* Chapman et al. have summarized recent studies on the low-temperature photochemistry of C6H5N specie^;'^ it now seems clear that under these conditions triplet phenylnitrene and triplet 2-pyridylmethylene inter- convert via l-aza-l,2,4,6-~ycloheptatetraene Ab initio calculations predict that an energetically favourable pathway in the reduction of ethylene by trans-di-imide proceeds by way of ethylamin~nitrene.'~ The reaction between cyanamide and iodobenzene diacetate produces cyano- nitrene which can be trapped by a variety of reagents." The nitrene (7) obtained by oxidation of the corresponding arylsulphenamide adds to electron-rich olefins to give aziridines in 38-64% yield.16 The stereoselec- tivities of the additions imply that the singlet and triplet states of (7) are in equilibrium and that the addition of triplet (7) to olefins is reversible.A milder method for the generation of phthalimidonitrene is by thermolysis in refluxing benzene of the sulphimide (8).17 The nitrene so produced is a relatively stable singlet species which adds to olefins stereoselectively to give aziridines.OS-N N-N=SMe OZN \ (7) A study of the reactions between alcohols and ethoxycarbonylnitrene generated thermally or photolytically from ethyl azidoformate indicates that only the singlet species is capable of inserting into the O-H bonds of alcohols.18 The photolysis of pyridine N-oxides leading to formyl-pyrroles (Scheme 9,gives considerably improved yields when performed in the presence of copper(I1) ~a1ts.l~ l2 B. Iddon 0.Meth-Cohn E. F. V. Scriven H. Suschitzky and P. T. Gallagher Angew. Chem. Infernat. Edn.1979,18,900. l3 0.L. Chapman R. S. Sheridan and J.-P. LeRoux Rec. Trav. chim. 1979,98,334;0.L. Chapman and R. S. Sheridan J. Amer. Chem. SOC. 1979,101,3690. l4 D. J. Pasto and D. M.Chipman J. Amer. Chem. SOC. 1979,101,2290. J. E. G. Kemp D. Ellis and M. D. Closier Tetrahedron Letters 1979 3781. l6 R. S. Atkinson and B. D. Judkins J.C.S. Chem. Comm. 1979,832 833. " M. Edwards T. L. Gilchrist C. J. Harris and C. W. Rees J. Chem. Res. (S) 1979 114. '* N. Torimoto T. Shingaki and T. Nagai J. Org. Chem. 1979 44 2585; see also L. D'Epifanio L. Pellacani and P. A. Tardella ibid. p. 3605. l9 F. Bellamy and J. Streith J. Chem. Res. (S) 1979 18. D. W. Knight I 0- Scheme 5 A highly practical route to pentaco-ordinate phosphoranes consists of treatment of a suitable azide with a phosphine*' (Scheme 6).(::+ PR'R2R3 + (EPR'R2R3 -* Z=OorNH Scheme 6 Previous studies with imidoylnitrenes (9; R = H) revealed that a major reaction pathway was attack by the nitrene at the ortho-position of the aromatic ring followed by re-aromatization. It has now been found that when the ortho-positions are blocked e.g. (9; R=Me) a rather different set of rearrangements occurs via 3aH-benzimidazoles (lo) leading to cyclopenta[d]pyrimidines by sequential [1,5]vinyl- imidoyl- and hydride-shifts (Scheme 7).*l Also isolated from thermally generated (9; R = Me) were low yields of benzimidazoles e.g. (1l) which could have arisen from (10) by extremely rare but thermally allowed [1,9] alkyl shifts. Scheme 7 J.I. G. Cadogan I. Gosney E. Henry T. Naisby B. Nay N. J. Stewart and N. J. Tweddle J.C.S. Chem. Comm.,1979 189; J. I. G. Cadogan N. J. Stewart and N. J. Tweddle ibid.,p. 191. T. L. Gilchrist C. J. Moody and C. W. Rees J.C.S. Perkin I,1979,1871;T. L. Gilchrist P. F. Gordon,D. F. Pipe and C. W. Rees ibid.,p. 2303. 89 Arynes Carbenes Nitrenes and Related Species Some evidence that intramolecular insertion of an arylnitrene into thiophenes proceeds via a ring-opened form is the isolation of one such compound (12) (see Scheme 8);22however it is still uncertain that (12) is a true intermediate and not a by-product. Scheme 8 A second example of a bis-nitrene metal complex C~S-[MO(NP~),(S,CNE~~)~], has been reported. X-Ray analysis at -130 "C reveals that the two phenylnitrene ligands are not identical one being nearly linear while the second is bent at nitrogen (LMoNC= 139.4 ") with a longer Mo-N bond than the former.23 Phenylphosphinidine (PhP:) thermally generated from (PhP)5 inserts into Ge-X bonds (X = H C N 0,or P) to provide a route to germylph~sphines.~~ 3 Carbenes The stable isolable salt (13) serves as a useful source of methylene for the stereospecific conversion of olefins into cyclopropanes in good yields2' The formation of cyclopropanes from carbenes and olefins when thought of in Frontier Orbital terms is generally carried out with an electrophilic carbene (e.g.:CC12) where the dominant orbital interaction is between the LUMO of the carbene and the HOMO of the olefin and less commonly using a nucleophilic carbene e.g.zC(OM~)~. This spectrum of reactivity has been summarized,26 and it has also been demonstrated that methoxychlorocarbene (14) is an ambiphilic species in that although its additions to electron-rich olefins may be characterized as electrophilic (14) is a nucleophilic species in the reactions with electron-poor ole fin^.^' The preparation of (14) by thermolysis of diazirine (15) both in solution and in the gas phase involves a direct two-bond cleavage to the exclusion of the alternative pathway via one-bond rupture to give a diazo intermediate; a process which operates when various other substituents (e.g.Ph or alkyl) are present.28 co + C1 OMe YMe I Cp-Fe -CH2SMe2 N=N 's' I '* G. Jones C.Keates I. Kladko and P. Radley Tetrahedron Letters 1979,1445; see also G.Jones and W. H. McKinley J.C.S. Perkin I 1979 599. 23 B. L. Haymore E. A. Maatta and R. A. D. Wentworth J. Amer. Chem. SOC. 1979,101,2063. 24 J. Escudit C. Couret and J. Satgt Rec. Trav. chim. 1979,98,461. '' S. Brandt and P. Helquist J. Amer. Chem. SOC. 1979 101 6473. 26 R. A. Moss and R. C. Munjal Tetrahedron Letters 1979,4721. " R. A. Moss M. Fedorynski and W.-C. Shieh J. Amer. Chem. SOC.,1979,101,4736; N. P. Smith and I. D. R. Stevens J.C.S. Perkin ZI 1979 1298. ''N. P. Smith and I. D. R. Stevens J.C.S. Perkin ZI 1979 213. D. W.Knight The generation of dibromocarbene from bromoform under phase-transfer condi- tions is successful when cetyltrimethylammonium bromide is used as catalyst whereas benzyltriethylammonium chloride favours the formation of -CBr3 only.29 A careful choice of reagents is also necessary for the generation of bromo-chlorocarbene from HCBr2Cl by phase-transfer methods.30 Dihalogeno-carbenes generated in this way can be added sequentially to all of the olefinic bonds in bullvalene and cyclo-octatetraene to give (16) and (17) re~pectively.~’ The preparation and chemistry of 1,2,2-trifluoroethylidenehas been investigated; reactions with alkanes lead mainly to C-H insertion products with the usual order of reactivity (3’ >2’> 1’) being Halogenocarbenes thermally generated from chloroform add to pyrrole and some methyl-pyrroles to give surprisingly high yields of chloro-pyridine~.~~ Interest in Simmons-Smith-type reactions continues.Conventional conditions using gem -dihalides and zinc or copper-based catalysts are usually only successful when applied to electron-rich olefins. It has now been found that by employing a nickel catalyst i.e. [Ni(PPh3)4] in the presence of sodium iodide and zinc electron- poor olefins (e.g. vinyl ketones or acrylates) can be efficiently converted into cycl~propanes.~~ When copper powder is used in these reactions the addition is highly stereoselective indicating the intermediacy of organo-coppers rather than free carbene~.~~ Indeed ab initio calculation^^^ support this view and suggest that the true intermediates in the Simmons-Smith reaction may have considerable metallocarbenium character. Such species’ have been directly observed using 13C n.m.r.37 Furthermore in the reaction between dibromocyclopropane (18) and methyl-lithium products are found which attest to the intermediacy of (19)as well as of cyclopropylidene (20).38 29 M.S. Baird A. G. W. Baxter B. R. J. Devlin and R. J. G. Searle J.C.S. Chem. Comm. 1979 210. 30 E. V. Dehmlow and M. Slopianka Annalen 1979 1465. 31 E. V. Dehmlow and M. Lissel Annalen 1979 181. 32 R. N. Haszeldine R. Rowland J. G. Speight and A. E. Tipping J.C.S. Perkin I 1979 1943; R. N. Haszeldine C. Parkinson P. J. Robinson and W. J. Williams J.C.S. Perkin IZ 1979,954. 33 R. E. Busby M. Iqbal M. A. Khan J. Parrick and C. J. G. Shaw J.C.S. Perkin I 1979 1578. 34 H. Kanai and N. Hiraki Chem. Letters 1979 761. 35 N. Kawabata I.Kamemura and M. Naka J. Amer. Chem. SOC.,1979 101 2139; N. Kawabata M. Tanimoto and S. Fujiwara Tetrahedron 1979 35 1919. 36 T. Clark and P. von R. Schleyer J.C.S. Chem. Comm. 1979,883; Tetrahedron Letters 1979,4963. 37 D. Seebach H. Siegel K.Miillen and K. Hiltbrunner Angew. Chem. Internat. Edn. 1979,18,784;H. Siegel K. Hiltbrunner and D. Seebach ibid. p. 785. 38 M. S. Baird and A. G. W. Baxter J.C.S. Perkin I 1979 2317. Arynes Carbenes Nitrenes and Related Species Br%k Br CONRz Br* Li CONRZ CONR Cyclopropylidenes usually undergo ring-opening reactions to give allenes unless these would be highly strained or unless insertion pathways are readily available. An example of the latter phenomenon is the formation of (22) and (23) from the anti-3-vinylbicyclo[4.1.0]heptane (21),39by insertion into the C-3-H and C-5-H bonds respectively.Similar treatment of the syn isomer of (21) at low temperatures results in addition to the vinyl group. The dual pathway in the reactions of cyclopropylidene (24)to give allenes (25) and insertion products (26)has been investigated. A Hammett plot gives p = -0.5 indicating little build-up of charge at the benzylic carbon during insertion assuming that changes in the para -substituent have little effect on the rate of formation of a11ene.40 Vinylcyclopropylidenes usually undergo [1,3] shifts to give 3-cyclo-pentenylidenes; it has now been suggested that the cis-dienylcyclopropylidene (27) reacts by a novel [1,5] shift to give (28) ultimately isolated as cycloheptatriene (29).41Alternative mechanistic rationales (e.g.insertion into vinylic C-H or into the terminal double bond) seem to be energetically unfavourable.39 M. S. Baird J.C.S. Perkin I 1979 1020; see also R. M. Cory L. P. J. Burton and R. G. Pecherle Synth. Comm. 1979,9,735;M. S. Baird P. Sadler J. Hatem J.-P.Zahra and B. Waegell J.C.S. Chem. Comm. 1979,452. 40 M.S. Baird J.C.S. Chem. Comm. 1979,776. 41 U.H. Brinker and I. Fleischhauer Angew. Chem. Internar. Edn.,1979 18 396. D. W.Knight MIND0/3 calculations indicate that the initial step in the transformation of cyclobutylidene into methylenecyclopropane involves electrophilic attack by the empty p-orbital of the carbene carbon onto the methylene group at C-3 to produce a bicyclobutane-type intermediate a formulation which represents a ‘non-classical’ ~arbene.~* 2-Vinylcyclobutylidenes (30) in contrast to vinylcyclopropylidenes (cf.ref. 41) do not undergo carbene-carbene rearrangements but instead give mainly allyl- idenecyclopropanes (3 1)by C-2-C-3 bond cleavage.43 Isopropylidenechlorocarbene when generated in the presence of olefins under- goes mainly intramolecular rearrangement whereas under similar conditions cyclopropylidenechlorocarbeneadds to the alkenes although lower selectivities are found than those theoretically predicted. These observations can be explained by assuming that the latter carbene is stabilized by interaction between the cyclopropyl a-bonds and the empty p-orbital of the carbene carbon (32) and that this stability is lost when the incoming olefin causes the molecule to adopt a less crowded twisted conformation (33).44 The greater stability and longevity of (32) contrasts with the cyclobutyl analogue (34) which undergoes intramolecular rearrangement and cyclopropanation reactions in approximately equal amo~nts.~’ The aluminium-carbon bond energy in the vinylidene complex Al-C=CH2 has been calculated to be ca.20 kcal mol-’ showing the profound influence that a metal can have on such reactive intermediate^.^^ A novel thermal rearrangement termed the ‘a-alkynone cyclization’ involves the formation of 2-cyclopentenones (37) from a-alkynones (39 probably via alkylidenecarbenes (36).47 A general route to alkylidenes is by a facile loss of nitrogen from diazoalkenes these being prepared by Wadsworth-Emmons reactions between dimethyl diazomethylphosphonate and aldehydes or alkyl aryl ketones.48 Ab initio calculations on the C4H2system have shown that the two arrangements butatrienylidene (38) and ethynylvinylidene (39) are of almost the same energy and 42 W.W. Schoeller J. Amer. Chem. SOC., 1979,101,4811. 43 U. H. Brinker and L. Konig J. Amer. Chem. SOC., 1979,101,4738. 44 R. A. Moss M. Vezza W. Guo R. C. Munjal K. N. Houk andN. G. Rondan J. Amcr. Chem. SOC., 1979 101,5088. 4s R. A. Moss M. E. Fantina and R. C. Munjal Tetrahedron Letters 1979 1277. 46 M. Trenary M. E. Casida B. R. Brooks and H. F. Schaefer 111 J. Amer. Chem. SOC., 1979,101,1638. 47 M. Karpf and A. S. Dreiding Helv. Chim.Acta. 1979 62 852. 48 J. C. Gilbert U. Weerasooriya and D. Giamalva Tetrahedron Letters 1979,4619. Arynes Carbenes Nitrenes and Related Species that the energy difference between them and the stable structure buta-1,3-diene is similar to the difference between vinylidene and acetylene.49 A brief report has appeared on the generation of the 4,4-dimethyl derivative of (38) by base-induced y-elimination of triflate from (40).” An authoritative review has appeared on [1,2]-hydrogen shifts involvingcarbenes and nitrene~.’~ A detailed study of such [1,2] rearrangements in non-rigid cyclo-hexylidenes e.g. (41),amplifies previous work in leading to the conclusion that for such conformations an axial hydrogen is more likely to migrate as its bonding orbital is nearly parallel with the empty p-orbital of the carbene and perpendicular to the filled non-bonding ~rbital.’~ In contrast to earlier findings with methylene the energy gap between the singlet and triplet ground states in simple alkylcarbenes (42) appears53to be considerably less than 20 kcal mol-’.Ar R-C IH (41) (42) R =But or Pr’ (43) (44) PadwaS4 has discussed the probable intermediacy and subsequent reactions of vinylcarbenes (44) in photochemical reactions of cyclopropenes (43). Photo-chemically generated vinylsulphinylcarbene (45) undergoes a novel rearrangement to give vinyl-sulphines (46) which can be trapped or isolated as the corresponding ap-unsaturated ketone.” 1,l-Dichloroallyl-lithium serves as a useful source of chlorovinylcarbene (47) which reacts with simple olefins to give 1-chloro-1 -vinyl-cyclopropanes in 1842%yield.56 0 t s-0 9 CI (47) (45) 49 C.E. Dykstra C. A. Parsons and C. L. Oates J. Amer. Chem. SOC.,1979,101 1962. ’O P. J. Stang and T. E. Fisk J. Amer. Chem. SOC., 1979,101,4772; Synthesis 1979,438. ” H. F. Schaefer 111 Accounts Chem. Res. 1979,12,288. 52 L. S. Press and H. Shechter J. Amer. Chem. Soc. 1979 101,509. 53 K.-T. Chang and H. Shechter J. Amer. Chem. SOC., 1979,101,5082. ”A. Padwa Accounts Chem. Res. 1979,12 310; A. Padwa T. J. Blacklock and R. Loza Tetrahedron Letters 1979 219 1617. ”M. Franck-Neumann and J. L. Lohmann Tetrahedron Letters 1979 2397. 56 R. A. Moss and R. C. Munjal Synthesis 1979 425.D. W. Knight In a rather odd reaction 1-chloro-2-alkyl-cyclohexenesreact with organo- lithiums to give bicyclo[4.1.O]heptanes (Scheme 9) probably via the carbene (48) and the cyclopropene (49).” Reactions of (presumably) singlet pyrrolylidene (50) with substituted benzenes proceed via two pathways depending on the electronic properties of the substituent (Scheme 1O);58 not surprisingly triplet (50)gives only the substitution products (52) without the participation of spironorcaradiene (51). Z = CN or NOz (52) Z = OMe or Me Scheme 10 The thermal rearrangement of benzocyclopropane to ethynylcyclopentadiene (Scheme 11)is well known but the final product has always eluded proper charac- terization. This has now been achieved with the naphthalene analogue (53),obtained from naphth0[2,3]cyclopropane.~~The preparation of benzvalene via cyclo-pentadienylcarbene could proceed by a conventional cyclopropanation reaction or by an overall [1,4] addition (Scheme 12).Compelling evidence for the latter pathway is the isolation of only (55)from the rearrangement of the 1-methyl analogue (54).60 Full details for the preparation of p-tolylcarbene have been published together with some useful notes on the generation of carbenes.6l Arylcarbenes photo- 57 P. G. Gassman J. J. Valcho and G. s.Proehl J. Amer. Chem. SOC.,1979 101 231. ” M. Nagarajan and H. Shechter J. Amer. Chem. SOC.,1979,101,2198. ’’ C. Wentrup. E. Wentrup-Byrne P. Muller and J. Becker Tetrahedron Letters 1979,4249. 6o U. Burger and G.Gandillon Tetrahedron Letters 1979 4281. 61 C. M. Dougherty and R. A. Olofson Org. Synth. 1978,58 37. Arynes Carbenes Nitrenes and Related Species (53) Scheme 11 4d ;dl _[1,21 ’@ [1,4] 5 41 2 5 Scheme 12 chemically generated in rigid matrices of alkanes preferentially insert into primary C-H bonds in contrast to the propensity for reaction with secondary and tertiary C-H bonds in liquid alkanes at room temperature. This effect is probably due to the steric constraints imposed by the rigid matrix.62 These constraints can also cause the reactivity of arylcarbenes to be dependent on the precursor. Thus determinations of singlet/triplet ratios by an analysis of the products arising from carbenes generated in Thermally matrices at low temperatures (e.g.-196 “C) can be ~nreliable.~~ generated a-substituted arylcarbenes such as (56) mainly undergo intramolecular insertion reactions to give five- and six-membered rings (Scheme 13);64a-alkoxy-arylcarbenes also react by a formal double proton abstraction to give the cor- responding vinyl although labelling studies reveal that this reaction does not occur via a geminal abstraction followed by a [1,2] hydride shift. X = CH2 NR 0,or S Scheme 13 It is known that the bivalent carbon in p-tolylcarbene is capable of migration to give styrene and benzocyclobutene. Good evidence has been found for the occur- rence of a similar process in icosahedral carbaboranes;66 it is interesting to note that other analogies between benzene and carbaboranes have been made in the past.62 H. Tomioka J. Amer. Chem. SOC., 1979,101,256. 63 H.Tomioka G. W. Griffin and K. Nishiyama J. Amer. Chem. SOC.,1979,101,6009. 64 W.D.Crow and H. McNab Austral. J. Chem. 1979,32 89,99 123. ” W.D.Crow and H. McNab Austral. J. Chem. 1979 32,111. 66 S.Chari G. K. Agopian and M. Jones Jr. J. Amer. Chem. SOC., 1979,101,6125. D. W. Knight Further progress towards understanding the role of copper in catalysing the decomposition of diazodiphenylmethane has been made.67 It seems that copper(I1) is involved in a chain reaction with radical cations as carriers whereas copper(1) reacts via unknown complexes. The mechanism of the reactions between carbenes and metal carbonyls to give ketens has remained obscure for many years.It now seems that this occurs not by addition of carbon monoxide to a metal-carbene complex but rather by addition of carbene across the metal-CO bond.68 More evidence has been found to support the idea that olefin metathesis reactions proceed via metal-carbenoid complexe~.~~ The specific formation of vinylcyclopropanes (58) can be achieved under mild conditions from diazo-ketones (57) by intramolecular carbenoid insertion reactions; good yields are obtained only for small rings (n = 2,3 or 4).70 Thermolysis of compounds (58) leads to cis-fused cyclopentenes. Surprisingly the effects of conformation on the Wolff rearrangement of a-diazo-ketones to ketens have not until recently been investigated. It has now been shown7' that under aprotic thermal or photochemical conditions an S-2 conformation with the migrating group trans to the nitrogen is necessary for keten formation suggesting that the Wolff rearrangement is a concerted reaction rather than one involving ketocarbenes (Scheme 14).Set against this conclusion are some further studies'* on the photo-Wolff reaction of a-diazo-ketones using 13C labelling which indicate the occurrence of a carbene-carbene rearrangement via an oxeten inter- mediate in line with previous work. 0 S-Z Mainly S-E Bu' Scheme 14 It has long been known that ketocarbenes (59; R = H) react by insertion into the endo-C-H bond at C-5. When this position is blocked by an acetate group i.e. (59; 67 D. Bethell K. L. Handoo S. A.Fairhurst and L. H. Sutcliffe J.C.S.Perkin ZZ 1979,707,D. Bethell M. F. Eeles and K. L. Handoo ibid. p. 714. 68 W. A. Herrmann J. Plank M. L. Ziegler and K. Weidenhammer J. Amer. Chem. SOC.,1979,101,3133. 69 R. H. Grubbs and C. R. Hoppin J. Amer. Chem. SOC.,1979,101,1499; F. N. Tebbe G.W. Parshall and D. W. Ovenall ibid. p. 5074; see also C. O'Donohoe J. K. A. Clarke and J. J. Rooney J.C.S. Chem. Comm. 1979,648. 70 T. Hudlicky J. P. Sheth V. Gee and D. Barnvos Tetrahedron Letters 1979,4889. 71 F. Kaplan and M. L. Mitchell Tetrahedron Letters 1979 759. 72 K.-P. Zeller Chem. Ber. 1979,112 678; Annalen 1979 2036; Monatsh. 1979,110 393. 97 Arynes Carbenes Nitrenes and Related Species R =OAc) insertion into the C-R bond is replaced by reaction with the carbonyl group of the acetate followed by rearrangement to (60).73When R is OMe the carbene inserts into a C-H bond of the methoxy-group to give (61).a-1minocarbene (62) when photochemically generated in methanol largely undergoes rearrangement to the ketenimine (63)before being trapped by the solvent (Scheme 15).74 The intermediate (63) can be trapped by other reagents; for example with benzylideneanilide when the p -1actam imide (64) can be isolated. N2 Ph2NpH & Ph2NyeH __* MeOH Ph2NYcH20Me NCN NCN NCN (62) ca. 25% 'I Ph Ph2N OMe 'CScSNCN MeOH Yt'ph I +P~~NANCN Ph2N NCN H ca. 75% (64) (63) Scheme 15 The reaction of carbene (65) with alcohols leads to a number of products the most notable of which is the methoxysilane (66); its formation may involve a silene i~~termediate.~' A A MeOH Me3Si C02Et __* Me2Si C02Me PhAP(OMe), I II (65) Me0 0 (66) (67) Photolytically generated phosphorylcarbenes (67) insert into the hydroxy-group of simple alcohols at 27 "C but into the C-H bonds at -196 "C As the sensitized photolytic generation of (67) at 27°C leads to an increase in products of C-H insertion these results may be rationalized by assuming that 0-H insertion products arise via the singlet carbene (cf.ref.18) whereas C-H insertion products involve the intermediacy of the triplet carbene. The triplet carbene probably reacts by proton abstraction followed by combination of the resulting radicals.76 73 P. Yates and G. F. Hambly Canad. J. Chem. 1979,57 1668. 74 B.Arnold and M.Regitz Angew. Chem. Internat. Edn. 1979,18,320. 75 W.Ando A. Sekiguchi T. Hagiwara T. Migita V. Chowdhry F. H. Westheimer S. L. Kammula M. Green and M. Jones Jr. J. Amer. Chem. SOC.,1979,101 6393. 76 H. Tomioka T. Inagaki S. Nakamura and Y. Izawa J.C.S. Perkin I 1979 130. 98 D. W. Knight Further reports have appeared on the reactions of thiophen with various diazo- compounds in the presence of rhodium(I1) (Scheme 16).77 The remarkably stable ylide (68) serves as a good source of bis-methoxycarbonyl-carbene (69) which can be used to prepare aryl malonates from electron-rich aryl~.'~ The conversion of (68) into (69) requires the presence of Cu" or Rh" catalysts as simple thermolysis of the ylide leads to (70). Carbenoids derived from diazomalonates by Cu"-induced decomposition react with 1,l -dicyclopropylethylene to give only the addition product; this suggests ihat the singlet rather than the triplet carbenoid is involved as the latter would be expected to give rise to rearrangement HH iii 1 0 AMeO,CACO,Me 1 (69) Me0,C C02Me (70) (68) Reagents i N,CHCO,Me; ii PhCOCHN,; iii N,C(CO,Me),; iv A; v Cu" Scheme 16 [2,3]-Sigmatropic rearrangements of dithiocarbenes (7 l),generated from the corresponding tosylhydrazone by base-induced elimination give only the trans product (72) probably due to the intermediate adopting a 'folded envelope' con- formation in which the substituent R occupies a pseudo-equatorial position." Theoretical calculations indicate that in contrast to cyclopropylidene which is considerably less stable than its neutral acyclic isomers (i.e.allene propyne) silacyclopropylidene (73) is much more stable than 2-sila-allene and 2-silapr0pyne.~~ Also a singlet ground state is predicted for (73) again in contrast to cyclo- propylidene which has a triplet ground state. The elusive dimethylsilylene (74) has been detected by U.V. and i.r. spectroscopy in hydrocarbon glasses at -196 "C; the 77 R. J. Gillespie and A. E. A. Porter J.C.S. Perkin I 1979 2624; R. J. Gillespie J. Murray-Rust P. Murray-Rust and A. E. A. Porter J.C.S. Chem. Comm. 1979,366. 78 R. J. Gillespie and A. E. A. Porter J.C.S. Chem. Comm. 1979 50. 79 M. E.Alonso and M. G6mez. Tetrahedron Letters 1979,2763. 80 T. Nakai and K. Mikami Chem.Letters 1979 1081. J.-C. Barthelat G. Trinquier and G. Bertrand J. Amer. Chem. SOC.,1979 101,3785. Arynes Carbenes Nitrenes and Related Species species appears to be stable under these conditions and can be trapped by various reagents to give ca. 60% yields of the expected products.82 Dimethylsilylene (74) can also be generated by photolysis of (Me2Si), and it adds to a-diketones to give silacyclopent-4-enes such as (75).83Further support for the existence of silylenes is the isolation of (77)from the co-thermolysis of (76)with anthracene; presumably the silylenes dimerize to disilenes prior to reaction with the anthracene. If the latter is replaced by 2,3-dimethylbuta-1,3-diene,then the 1-silacyclopentene (79) is iso- lated the silylene probably being trapped as the silacyclopropane (78) before it can dimeri~e.~~ ,SIRR'R2Si I 'R2 R 1.R' \/ (77) R2 82 T.J. Drahnak J. Michl and R. West J. Amer. Chem. Soc. 1979 101 5427. 83 W.Ando and M. Ikeno J.C.S. Chem. Comm. 1979,655. 84 Y.Nakadaira T. Kobayashi,T. Otsuka and H. Sakurai J. Amer. Chem. Soc. 1979,101,486; H.Sakurai Y. Nakadaira and T. Kobayashi ibid. p. 487.