5 Arynes Carbenes Nitrenes and Related Species By D. W. KNIGHT Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Arynes There has been relatively little activity in this area during the past year. The mechanism by which aryl halides undergo dehydrohalogenation by hydroxide ions has been shown to involve a benzyne intermediate.' An alternative pathway via @so-hydroxydehalogenation,appears to operate only in the presence of copper salts. Thermolysis of titanocene (1) at 80-100°C gives rise to the titanium-benzyne complex (2) which subsequently can react with molecular nitrogen to give anilines.' The intermediacy of the benzyne (2) is supported by the nature of the products arising from titanocenes having substituted phenyl groups. Full details have been given3 for the preparation of perchlorobenzyne (3)(from C&16 and Bu"Li) and for its subsequent trapping with benzene leading to benzobarrelene after dechlorina- tion.Perchlorobenzyne gives much higher yields of addition products than benzyne itself in this case and perhaps could find use in place of the latter elsewhere provided that the intermediate from (3)could be successfully dechlorinated as in this instance. 3,6-Disubstituted 1,2,4,5-tetrabromobenzenesserve as biaryne equivalents giving adducts such as (4) in good yield on reaction with butyl-lithium and f~ran.~ This procedure which is an improvement of a similar reaction reported some time ago by Wittig is apparently a stepwise process. Various pyrolysis experiments with 3,4-dialkyl-hexa-1,5-diyn-3-eneshave pro- vided further evidence that 1,4-dehydrobenzene exists as a diradical species.' The formation of 9,lO-phenanthrynes (5)by pyrolysis (at 700-900°C) of the corresponding diacid anhydrides at very low pressure has been reported toget her with some comparison of their reactivities relative to other arynes6 The transient M.Zoratti and J. F. Buqnett J. Org. Chem.. 1980,45,1769. 1776. E.G. Berkovich V. B. Shur M. E. Vol'pin B. Lorenz S. Rummel and M. Wahren Chem. Ber. 1980 113,70. N.J. Hales H. Heaney J. H. Hollinshead and P. Singh Org. Synth. 1979 59 71. H.Hart C.Lai G. Nwokogu S. Sharmouilian,A. Teuerstein and C. Zlotogorski J. Am. Chem. SOC. 1980,102,6649. T. P.Lockhart C. B. Mallon and R. G. Bergman J. Am. Chem.Soc. 1980,102 5976. H.-F.Grutzmacher and U. Straetmans Tetrahedron 1980,36 807. D. W.Knight @/J(-J \ R' / R 2 2 &J\ - (4) R \'I (6) B (5) aryne (6) has been obtained by dehydrobromination of a monobromo-derivative of the corresponding 127r antiaromatic system; (6) can be trapped with for example 1,3-diphenyli~obenzofuran.~ 2 Nitrenes There now seems to be little doubt about the intermediacy of azacycloheptatetraenes and cyclic carbodi-imides (7) in the reactions of phenylnitrenes and pyridylnitrenes. Further evidence for the participation of (7) rather than the corresponding carbene (8)in the 2-pyridylnitrene + 2-pyridylnitrene equilibration is the direct observation of (7) by matrix-isolation techniq~es.~*~ Similarly direct observation as well as detailed product analysis and labelling studies support the formation of carbodi-imide (9) during the thermolysis of both tetrazolo[ 1,5-~]quinazolines and tetrazolo[l,5-a]quinoxalines.10 However it also is clear that these are not the exclusive pathways by which such nitrenes react.Flow pyrolysis of azidopentafluorobenzene at 330°C gives rise to a dimer the structure of which suggests the involvement of a monoaza-carbene analogous to (8).11 Moreover photolysis of 6-azidobenzothiazoles at ambient temperatures in the presence of secondary amines gives products which are consistent with the intermediacy of azirines (10) rather than the corresponding cycloheptatetraene;12 possibly the former is trapped by the amine before it can ring-expand to the latter.During infrared studies of the photolysis of naphthyl azides in argon matrices at 12 K both azirine and aza-cycloheptatetraene species have been ob~erved.'~ A full report of studies on the reactions of pyrazolenitrenes (11) has been p~blished.'~ Briefly the singlet state of nitrene (11)attacks the 2-nitrogen to give ' H. N. C. Wong and F. Sondheimer Tetrahedron Lett. 1980,21 983. * C.Wentrup and H.-W. Winter J. Am. Chem. SOC.,1980 102 6159. For a review of some aspects of this work see 0.L. Chapman Pure Appl. Chem. 1979,51 331. For a useful review of matrix-isolation techniques see I. R. Dunkin Chem. SOC.Rev. 1980,9,1. lo C. Wentrup C. Thitaz E. Tagliaferri H. J. Linder B. Kitschke H.-W. Winter and H. P. Reisenauer Angew.Chem. Znt. Ed. Engl. 1980 19,566. " R. E. Banks N. D. Venayak and T. A. Hamor J. Chem. SOC.,Chem. Commun. 1980,900. '* P.T. Gallagher B. Iddon and H. Suschitzky J. Chem. SOC.,Perkin Trans. I 1980,2362. l3 I. R.Dunkin and P. C. P. Thomson J. Chem. SOC.,Chem. Commun. 1980 499. l4 J. M. Lindley I. M. McRobbie 0.Meth-Cohn and H. Suschitzky J. Chem.SOC.,Perkin Trans. 1 1980 982.For some related work see P. C. Hayes G. Jones C. Keates I. Kladko and P. Radley J. Chem. Res. (S) 1980,288;P.C.Hayes and G. Jones J. Chem. SOC.,Chem. Commun. 1980,844;A.Yabe Bull. Chem. SOC.Jpn. 1980,53 2933. Arynes Carbenes Nitrenes and Related Species Me h4e)r-I N ON>.b S N N R\ ylide (12) whereas the corresponding triplet of (11)attacks the 5-methyl group or becomes protonated.By varying the substituent in (1l) the latter's multiplicity can be altered. Thus when R the para-substituent of (1l) is electron-withdrawing more singlet-derived products are formed whereas when it is electron-donating only triplet-type products are obtained. A first example of intramolecular ring-expansion of a benzene to an azepine involves pyrolysis of benzyl azidoformates (Scheme 1).l5 The initially formed azepine dimerizes under the reaction conditions. Arylnitrenes derived from for example 2-azidophenyl phenyl sulphides are known to react via intramolecular ipso-attack leading to five-membered spiro-intermediates. Much the same process but with novel six-membered spiro-intermediates (13) has been evoked to explain why pyrolysis of aryl 2-azidobenzoates leads to carbazoles when R'= M [see (13)j and to acridines or acridones when R' # H.16 Scheme 1 NR // MeO-c 3 R' R2 (14) R = CN or S02Me (13) A useful review has appeared on the rearrangements of N-phenylcarb-imidoylnitrenes." The preparation of two new preparatively useful imidoyl- nitrenes (14) has been reported." Both react stereospecifically with olefins to give aziridines in good yield.It has been provenlg that photolysis of 1-azatriptycene gives rise to nitrene (15); an alternative cleavage leading to a phenylcarbene does not occur to any significant extent. Some evidence for the intermediacy of vinylnitrenes (17) in the thermal rearrange- ment of substituted phenylazirines to indoles comes from the observation that Is 0.Meth-Cohn and S. Rhouati J. Chem. SOC.,Chem. Commun. 1980,1161. l6 M. G. Clancy M. M. Hesabi and 0.Meth-Cohn J. Chem. SOC.,Chem. Commun. 1980 1112. " C. W. Rees PureAppl. Chem. 1979,51 1243. *' W. Lwowski and 0.S. Rao Tetrahedron Lett. 1980,21,727. l9 T. Sugawara and H. Iwamura J. Am. Chem. Soc. 1980,102,7134. \ \ thermal racemization of the phenylazirine (16) is much faster than its rearrangement to an indole.20 There still exists some doubt about the existence of simple alkyl-nitrenes. Calcula- tions suggest*l that alkyl-nitrenes are stable in the absence of collisions and that the methylnitrene (triplet ground state) -+ methylenimine rearrangement is endother- mic by 18 kcal mol-’ with an activation energy of 48 kcal mol-l.Ethoxycarbonylnitrene is usually rather unselective in its reactions; however it has been found to add to vinyl chlorides to give the expected aziridines in 1748% yield.22 There is some evidence to suggest that this is due to co-ordination between the nitrene and the chlorine atom. Some sources of fl~oronitrene~~ have been recorded. and of amino-nitrene~~~ 3 Carbenes Singlet carbenes are normally considered to possess a ‘bent’ structure. It has now been s~ggested’~.~~ that when the substituents are less electronegative than carbon (e.g.Li B) then the carbene has a linear structure as such substituents have a vacant p-orbital which can stabilize the carbene by two-electron three-centre overlap. The electronegativity of its substituents not only determines the stability of a carbene,26 but is also probably the crucial factor influencing the multiplicity of the ground state of the species,27 electronegative substituents favouring singlet ground states whereas electropositive atoms or groups favour triplet ground states.A review of the selectivities of carbenes in cyclopropanation reactions has appeared,” containing a useful summary of the electrophilicities of many carbenes. The first practical example of the use of the classical Simmons-Smith procedure for methylcyclopropanation has been claimed,29 in which 1,2-bis(trimethylsiloxy)cyclo-hexene is allowed to react with 1,l-di-iodoethane to give the expected product (18) in 76% yield. Carbenoids can also be generated from some aldehydes and ketones by deoxygenation using zinc-copper couple and either dichlorodimethylsilane or 20 K.Isomura G.-I. Ayabe S. Hatano and H. Taniguchi J. Chem. SOC.,Chem. Commun. 1980 1252; see also K.Isomura S. Noguchi M. Saruwatari S. Hatano and H. Taniguchi Tetruhedron Lett. 1980 ’’ J. 21 3879. Demuynck D. J. Fox Y. Yamaguchi and H. F. Schaefer 111 J. Am. Chem. SOC.,1980,102,6204. See also A. Mavridis and J. F. Harrison ibid. p. 7651. ’’ L. Pellacani F. Persia and P. A. Tardella Tetrahedron Lett. 1980 21,4967. 23 D.L. Klopotek B. G. Holrock P. Kovacic and M. B. Jones J. Org. Chem. 1980,45,1665. 24 E.Fahr and K. H. Koch Liebigs Ann. Chem. 1980,219. ” W. W. Schoeller J. Chem. SOC.,Chem. Commun. 1980,124. 26 L.Pauling J. Chem. SOC.,Chem.Commun. 1980 688. ’’ J. F. Harrison R. C. Liedtke and J. F. Liebman J. Am. Chem. SOC.,1979,101,7162. R.A. Moss Acc. Chem. Res. 1980 13 58. 29 S. Lewicka and W. H. Okamura Synth. Commun. 1980 10,415. Arynes Carbenes Nitrenes and Related Species OSiMe cl\ PMe *OSiMe c1 chlorotrimethylsilane.30Thermolysis of the cyclopropene (19)leads to the vinylcar- bene (20) which adds to olefins to give vinylcyclopropanes in reasonable yield.3' Further evidence has been found to suggest that direct irradiation of some diazo-compounds can give rise to excited singlet carbene~.~~ Thus pyrolysis or sensitized irradiation of (21) in the presence of cis-2-butene gives the diene (22) and a mixture of the cis- and trans-isomers of the cyclopropane (23); these are products which could reasonably be expected to come from the singlet and triplet carbenes derived from (2l),respectively.However direct irradiation results only in the formation of (22) and the cis-isomer of the dimethylcyclopropane (23) suggestive of the generation of another presumably excited singlet-state carbene in addition to that which leads to (22). The possible involvement of an excited diazo species cannot be completely discounted. (21) (22) (23) Calculations indicate that singlet cyclopropylcarbene rearranges to cyclobutene via initial electrophilic attack of the empty p-orbital of the carbene on an electron-rich bond of the cy~lopropane.~~ Other theoretical work on the nature of the transition states in the additions of various electrophilic ambiphilic and nucleophilic carbenes to ethylene has been reported;34 values for the HOMO and LUMO levels of the carbenes studied are quoted.Competition experiments between a range of dihalogeno-carbenes and mixtures of olefins at various temperatures suggest that variation of the substituents in the order F 3C1 +Br results in the activation enthalpies and activation entropies being changed in the same dire~tion.~~ It has been reported36 that the energy level of the singlet state of dibromocarbene is some 8 kcal mol-' below that of its triplet state. Difluorocarbene can be obtained at relatively low temperatures (-25 "Cor lower) by reactions between the metal complex [(CF3)2Cd.glyme] and acyl halides.37 Contrary to previous reports the addition of dichlorocarbene to cycloalkenols does appear to be significantly influenced by the hydroxyl group as a greater proportion of syn-addition by the carbene OCCU~S.~' 30 C.L. Smith J. Arnett and J. Ezike J. Chem. SOC.,Chem. Commun. 1980 653. 3' W. Weber and A. de Meijere Angew. Chem. Int. Ed. Engl. 1980,19 138. 32 G. R. Chambers and M. Jones jun. J. Am. Chem. SOC., 1980,102,4516. 33 W. W. Schoeller J. Org. Chem. 1980 45 2161. 34 N. G. Rondan K. N. Houk and R. A. Moss J. Am. Chem. SOC.,1980,102 1770. B. Giese,W.-B. Lee and J. Meister Leibigs Ann. Chem. 1980 725. 36 C. W. Bauschlicher jun. J. Am. Chem. SOC.,1980 102 5492. 37 L. J. Krause and J. A. Morrison J. Chem. SOC.,Chem. Commun. 1980 671. R. H. Ellison J. Org. Chem. 1980,45 2509.72 D. W.Knight Carbenoids (24) generated from [m.n.llpropellanes show a distinct se!ectivity towards insertion into neighbouring axial C -H bonds in the order cyclopentyl > norcaranyl 4.1-tetralyl > cyclohexyl > cyclohexenyl; presumably both the relative proximity and the nucleophilicity of the C-H bonds are the major controlling A novel example of a ‘tandem’ vinylcyclopropylidene-cyclopentadiene rearrangement has been used to prepare the triene (27) from the bis-dibromo- cyclopropane (25) presumably by way of the cyclopentadiene (26).40 p<”’ .. Br4 Br (25) (26) (27) A review of the chemistry of arylcarbenes and arylnitrenes in the gas phase has been published as part of a larger work on reactive intermediate^.^^ The triplet states of diphenylcarbene fluorenylidene and phenylcarbene have been monitored in matrices by e.s.ra4* This work serves to emphasize that especially in the case of the first two species such intermediates have long lifetimes in matrices and therefore that any reactivity studies at these low temperatures must be allowed to proceed for a sufficient length of time to ensure that all of the products that are isolated have arisen from carbenes that have reacted at the temperature of the matrix and not above that temperature.Phenylcarbenes generated by photolysis of aryl-diazomethanes in matrices of t-butyl alcohol (at -196 “C) give olefinic products presumably by dimerization of triplet species whereas carbenes that have been derived from a-diazocarbonyl compounds under the same conditions do not give olefinic products suggesting that only the singlet-state carbenes are Photolytically generated phenylcarbene reacts with liquid 2-chloropropane at low temperatures predominantly by insertion into the C-Cl bond whereas in a solid matrix the carbene inserts into the primary C-H bonds of the propane almost excl~sively.~~ This change in reactivity could be due to the occurrence in the C-Cl insertion reaction of an intermediate halonium ylide species which can be stabilized by solvation in the liquid phase but not in the solid phase.The stable salt [Cp(CO)2FeCHPh]+PF6- serves as a good source of phenylcarbene for the efficient and largely stereoselective phenylcyclopropanation of a range of 01efins.~~ Diphenylcarbene generated by U.V.laser irradiation of diphenyldiazomethane shows a markedly different reactivity and gives rise to different products to those formed when the carbene is generated by conventional photolytic means. The 39 L. A. Paquette E. Chamot and A. R. Browne J. Am. Chem. SOC.,1980 102 637; L.A. Paquette A. R. Browne E. Chamot and J. F. Blount ibid. p. 643. 40 U. H. Brinker and I. Fleischhauer Angew. Chem. Int. Ed. Engl. 1980,19,304. 41 C. Wentrup ‘The Behaviour of Arylcarbenes & Nitrenes in the Gas-phase’ in ‘Reactive Intermediates’ ed. R. A. Abramovitch Plenum New York Vol. 1 1980. 42 C.-T. Lin and P. P. Gaspar Tetrahedron Lett. 1980 21 3553. 43 H.Tomioka T. Miwa S. Suzuki and Y. Izawa Bull. Chem. SOC.Jpn. 1980,53,753. 44 H.Tomioka S.Suzuki. and Y. Izawa Chem. Lett. 1980,293. 4s M. Brookhart M. B. Humphrey H. J. Kratzer and G. 0.Nelson J. Am. Chem. Soc. 1980,102,7802. Arynes Carbenes Nitrenes and Related Species reasons for this are not as yet clear.46 Laser flash photolysis of diazofluorene at room temperature has been shown to give fluorenylidene in the singlet state; this is short-lived relative to the triplet state into which it converts ~nidirectionally.~~ This type of laser experiment looks to be a useful way to measure the rates of reactions of both singlet and triplet carbenes. In contrast to this are some reactions between aryl-carbenes and 1,l-dimethylallene where it is assumed that singlet carbenes add to the more substituted double-bond of the allene while triplet species add to the less substituted double-bond to give the thermodynamically preferred isopropyl- idenecyclopropanes.The results48 indicate that many monoaryl-carbenes react by way of the singlet state even though initially generated in the triplet state. In addition electron-donating substituents seem to stabilize the singlet species (cf.refs. 14 and 27). An investigation into the chemistry of benzocyclobutenylidene (28) has produced some odd results.49 In the gas phase (28)mainly undergoes dimerization while intermolecular additions together with some insertion reactions occur in solution. Additions to olefins are stereoselective implying the presence of a singlet species but oddly such additions display the largest positive p value yet observed for this type of reaction; i.e.(28)appears to be nucleophilic in contrast to the related phenylcarbenes. Absolute rate constants have been measured directly for the first time for the addition of a singlet carbene (PhCCl) to alkyl-olefins in ~olution.~~ The results show a regular rate decrease with decreasing substrate alkylation which is an understandable finding in view of the electrophilicity of the carbene. Cycloheptatrienylidene (29)can be obtained by pyrolysis (at 450OC) of 7-acetoxynorbornadiene or 7-aceto~ycycloheptatriene;~~ the carbene dimerizes under these conditions but at higher temperatures (>600 "C) it rearranges to fulveneallene (30). The carbene (29)has also been obtained from the reaction of benzene with arc-generated carbon atoms at -195 Cycloheptatrienylidene(29)undergoes a OC5* formal [4 +21reaction with anthracene; the precise mechanism by which this occurs is unclear.53 Thermally generated 1-naphthylcarbene gives cyclobuta[de]naphthalene (31)by intramolecular C-H insertion apparently before any rearrangements involving for example benzocycloheptatrienylidene species occur.54 Remarkably however 2-46 N.J. Turro,M. Aikawa J. A. Butcher jun. and G. W. Griffin J. Am. Chem. SOC., 1980 102 5127. See also P. P. Gaspar B. L. Whitsel M. Jones jun. and J. B. Lambert ibid. p. 6108. 4' J. J. Zupancic and G. B. Schuster J. Am. Chem. SOC.,1980,102,5958. 48 X. Creary J. Am. Chem. Soc. 1980,102 1611. 49 H. Durr H. Nickels L. A. Pacala and M. Jones jun. J. Org. Chem.1980 45 973. so N. J. Turro J. A. Butcher jun. R. A. Moss W. Guo R. C. Munjal and M. Fedorynski J. Am. Chem. SOC.,1980,102,7576. s1 R. W. Hoffmann I. H. Loof,and C. Wentrup Liebigs Ann. CFem. 1980 1198. s2 K. A. Biesiada C. T. Koch and P. B. Shevlin J. Am. Chem. SOC., 1980,102,2098. 53 K. Saito Y. Omura andT. Mukai Chem. Lett. 1980 349. 54 J. Becker and C. Wentrup J. Chem. SOC.,Chem. Commun. 1980 190. D. W.Knight naphthylcarbene gives the same product (31) presumably by equilibration to 1-naphthylcarbene via the cyclo heptatrienylidene (32). Some experiments involving the pyrolysis of various 4-phenylbut-3-en-1 -ynes have further delineated the mechanism of the azulene-naphthalene rearrange-ment.55aa@-O& 1 .-. \/ \ (31) (32) (33) (34) pc e p-c Me2C=C=C=C=C=C (35) (34) (37) Ab initio calculations suggest that cyclopentadienylidenecarbene (33) should be highly electrophilic because there is a significant contribution from the resonance structure (34) which contains an aromatic cyclopentadienyl Similar arguments lead to the conclusion that cyclopropenylidenecarbene [(35) C) (36)] should behave as a nucleophilic (or ambiphilic) alkylidenecarbene.Stang and Ladika have gone one better than last year by succeeding in preparing the 1,2,3,4-hexatetraenyl- idenecarbene (37),57 using the same approach (i.e. y-elimination from l-ethynyl- vinyl triflates) as for the lower homologues. Calculations suggest that the sequence of events in the photochemical reaction between carbon suboxide and ethylene involves the intermediacy of carbonylcar- ber~e,~* as outlined in Scheme 2.The initial reaction of carbon suboxide proceeds via its second excited state. The next higher homologue of carbonylcarbene can be stabilized by formation of the complex (38) using pentacarbonylchromium.59 Scheme 2 The first (spectroscopic) characterization of an electrophilic transition-metal- methylene complex has been reported.60 The complex (39) shows non-equivalent carbene protons in its ‘H n.m.r. spectrum and the freelenergy of activation for Fe-carbene bond rotation was found to be 10.4*0.1 kcal mol-1 by variable- temperature studies. The complex (39) is capable of effecting cyclopropanation of olefins (cf. ref. 45). The role of carbenes in olefin metathesis continues to attract interest.The bridging methylcarbene complexes (40; M =Fe or Ru) react with alkynes under ultraviolet irradiation to give intermediates that are reminiscent of vinylcarbene-metal com-plexes and which can be further allowed to react with carbon monoxide to give actual ” J. Becker C. Wentrup E. Katz and K.-P.Zeller J. Am. Chem. SOC.,1980 102 5110. ”Y.Apeloig R. Schrieber and P. J. Stang Tetrahedron Lett. 1980 21,411. ’’P.J. Stang and M. Ladika J. Am. Chem. SOC.,1980,102 5406; see also L. T. Scott and G. J. DeCicco J. Org. Chern. 1980,45,4055. ”T. Minato Y. Osamura S. Yamabe and K. Fukui J. Am. Chem. SOC.,1980,102,581. 59 H.Berke and P. Harter Angew. Chem. Znt. Ed. Engl. 1980,19,225. ‘O M. Brookhart J. R. Tucker T. C.Flood and J. Jensen J. Am. Chem. SOC.,1980 102,1203; for some calculations relevant to the structure of such complexes see R. J. Goddard R. Hoffmann and E. D. Jemmis ibid. p. 7667. Arynes Carbenes Nitrenes and Related Species 0 + II C II C H II / II :.co \ OC-Cr-CO /\ H /I Ph Ph oc co vinylcarbene complexes (41; M = Fe).61 This overall acetylene ‘insertion’ reaction provides a mechanism for polymerization of alkynes using transition metals. A careful analysis of the products that formed in some crossed olefin-metathesis reactions shows that carbene complexes RCH=M (M = metal) are preferred to H2C=M as chain carriers and that the latter species are very reactive towards C-1 of the terminal olefin.62 A very useful review of intramolecular insertion reactions of a-diazocarbonyl compounds has recently appeared.63 Epoxy-diazo-ketones (42) rearrange to diones (43) (isolated as their dimethyl acetals) on treatment with copper in methanol; a cyclic oxonium ylide may be an ix~termediate.~~ (42) (43) (44) The Wolff rearrangement continues to attract a considerable amount of interest.It has been briefly reported that vinylene thioxocarbonates (44) can serve as precursors of a-keto-carbenes as The initial cis-conformation of the carbene could favour the formation of oxirens i.e. the elusive species which are probably intermediates in the Wolff rearrangement. Various experiments in which a-keto-carbenes have been trapped by olefins during the Wolff rearrangement of some a-diazo-ketones strongly support the idea that singlet keto-carbenes are in equilibrium during the reaction and that the position of equilibrium depends upon the nature of the substituents.66 Similar experiments with dialkyl a-diazo-ketones 61 A.F. Dyke S. A. R. Knox,P. J. Naish and G. E. Taylor J. Chem. SOC. Chem. Commun. 1980 803; A. F.Dyke S. A. R. Knox P. J. Naish and A. G. Orpen ibid. p. 441.See also J. Levisalles H. Rudler F. Dahan and Y. Jeannin J. Organomet. Chem. 1980,188 193. 62 L. Benne K. J. Ivin and J. J. Rooney J. Chem. SOC. Chem. Commun. 1980 834. 63 S.D. Burke and P. A. Grieco Org. React. 1979,26 361. 64 L.Thijs and B. Zwanenburg Tetrahedron 1980,36 2145. ” M.Torres A. Clement and 0.P. Strausz J. Org. Chem. 1980 45 2271. H.Tomioka H. Okuno S. Kondo and Y. Izawa J. Am. Chem. SOC. 1980 102 7123. See also H.Tomioka H. Okuno and Y. Izawa J. Org. Chem. 1980,45,5278. D. W.Knight lend further weight to the notion of equilibrating keto-carbenes and also suggest the intermediacy of an oxiren as the epoxidation of an unsymmetrical dialkyl-acetylene leads to the same ratio of enone products as that produced by Wolff rearrangement of the corresponding dia~o-ketone.~~ Calculations indicate that oxirens would be rather short-lived species as expected.68 The photochemical Wolff rearrangement of monothiolo-esters (45)of malonic acid results in a remarkably selective migration of the sulphur group to give the keten (46) which can be trapped by a variety of reagent^.^' The reason behind this could be participation of a cyclic sulphonium ylide (cfiref.64). 0 N2<c02MeC-SR 44c-c II 0 (45) (46) Me0/\H (47) Carbenes with electron-withdrawing substituents usually have triplet ground states and hence it was surprising that methoxycarbonylcarbene showed what is clearly a singlet ground state. MIND0/3 calc~lations,~~ however have led to a formulation of a 'closed' structure (47)for such carbenes where the singlet state is favoured by donation of a non-bonding pair of electrons from the carbonyl oxygen into the vacant p-orbital of the carbene. In solution an 'open' structure may predominate with solvent molecules acting as electron donors. Work on the selectivity of insertion of (47)into C-H bonds further shows it to possess a singlet ground state whereas the corresponding diester :C(CO,Et), displays both singlet and triplet reactivity when generated by photolysis of diethyl diaz~malonate.~' What is still not clear is the role if any played by excited diazo-ester species in the overall insertion reaction (cf.ref.32). Cyclopropanation of olefins with diazo-esters catalysed by rhodium(I1) carboxyl- ates appears to proceed via a carbenoid mechanism [as with catalysis by copper(^)] and to be a good synthetic procedure especially with isolated disubstituted olefins whereas the use of palladium(I1) carboxylates is much less effe~tive.~ The cyclopro- panation of electron-poor olefins (e.g. ap-unsaturated nitriles and esters) by ethyl diazoacetate and a-diazoacetophenone is viable when [Mo(CO),] is used as the catalyst.73 An extensive study of the reactions between pyrroles and ethyl diazoace- tate catalysed by various copper salts leads to the conclusion that they are best described as electrophilic substitutions rather than insertions involving homopyrrole intermediate^.^^ 67 R.A. Cormier Tetrahedron Lett. 1980 21 2021. 68 K.Tanaka and M. Yoshimine J. Am. Chem. SOC.,1980,102,7655; for a review see M. Torres E. M. Lown H. E. Gunning and 0.P. Straw Pure Appl. Chem. 1980,52 1623. 69 V. Georgian S. K. Boyer andB. Edwards J. Org. Chem. 1980 45 1686. 'O R.Noyori and M. Yamakawa TetrahedronLett. 1980,21,2851.See also K.S. Kim and H. F. Schaefer 111 J. Am. Chem. SOC.,1980 102 5389. 71 H. Tomioka M. Itoh S. Yamakawa and Y.Izawa J. Chem. SOC.,Perkin Trans. 2 1980 603; H. Tomioka H.Okuno and Y. Izawa ibid. p. 1636. 72 A. J. Anciaux A. J. Hubert A. F. Noels N. Petiniot and P. Teyssit J. Org. Chem. 1980,45,695; A.J. Anciaux A. Demonceau A. J. Hubert A. F. Noels N. Petiniot and P. Teyssib J. Chem. Soc. Chem. Commun. 1980,765. 73 M. P. Doyle and J. G. Davidson J. Org. Chem. 1980 45 1538. 74 B. E. Maryanofi J. Org. Chem. 1979,444410. Arynes Carbenes Nitrenes and Related Species 4 Silylenes The volume of work reported in this area during the past year merits a separate section. Calculations show that methylsilylene MeSiH has a singlet ground state the lowet! triplet state being 19 kcal mol-' higher.75 By contrast silylmethylene H3SiCH has a triplet .Fround state with a lowest singlet state that is 25 kcal mol-' higher.Although MeSiH and silaethylene H2C=SiH2 are of approximately the same energy their interconversion via a [1,2]-hydrogen shift is difficult owing to a large energy barrier of ca 40 kcal mol-'. Silylenes (R2Si:) are often prepared by pyrolysis of 7-silanorbornadienes; by studying the thermal stability of a series of such precursors the formation of the silylenes has been-found to be a two-step process via the diradical (48).76 Germylenes (R2Ge:) can be similarly prepared again by a two-step me~hanism.~' Calculations predict that such germylenes (with R=H F or Me) have singlet ground states with 10 64 and 14kcalmol-' respectively between the singlet and lowest triplet Insertion reactions of silylenes sometimes require higher than expected activation energies.This has been-explained by assuming that such insertions by ground-state singlet silylenes are forbidden and thus the activation energy partly consists of the energy needed to promote the silylene to a triplet Dimethylsilylene requires approximately zero activation energy to insert into Si-H bonds whereas insertion into the H-Cl bond requires ca 28 kcal mol-1.80 (48) Dimethylsilylene generated at 0 "C by photolysis of dodecamethylcyclo-hexasilane adds to oxetan to give two products (50) and (51) both of which probably arise from the ylide (49)." Much the same type of ylide intermediates have been postulated to account for the products obtained by reactions between dimethyl- silylene and @-unsaturated epoxidesS2 or ally1 Competition experiments in which Me2Si has been allowed to react with pairs of alcohols show that the silylene has a much greater selectivity of insertion into the 0-H bonds when ether is used as solvent.This suggests the involvement of a less reactive silylene probably due to some interaction with the ether oxygens4 [cf. (49)]. Dimethylsilylene reacts with olefins to give silirans which can be trapped by methanolysis. The initial reaction has now been shown to be stereospecific as is the 75 J. D. Goddard Y. Yoshioka and H. F. Schaefer 111 J. Am. Chem. SOC.,1980 102 7644. 76 B. Mayer and W. P. Neumann Tetrahedron Lett. 1980,21,4887. 77 W. P. Neumann and M. Schriewer Tetrahedron Lett. 1980,21 3273. 78 J.-C.Barthelat B. S. Roch G. Trinquier and J. Satgt J. Am. Chem. Soc. 1980 102 4080. 79 T. N. Bell K. A. Perkins and P. G. Perkins J. Chem. SOC.,Chem. Commun. 1980 1046. 8o I. M. T. Davidson F. T. Lawrence and N. A. Ostah J. Chem. SOC.,Chem. Commun. 1980,859. T.-Y. Yang Gu and W. P. Weber J. Am. Chem. SOC.,1980,102 1641. '* D. Tzeng and W. P. Weber J. Am. Chem. Soc. 1980,102 1451. 83 V. J. Tortorelli and M. Jones jun. J. Chem. SOC.,Chem. Commun. 1980 785. 84 K. P. Steele and W. P. Weber J. Am. Chem. Soc. 1980 102 6095. D. W.Knight addition of methanol but it is still not proven to be cis or trans although the former seems highly likely.85 In the gas phase difluorosilylene appears to behave similarly to a carbene adding to butadiene for example to give a siliran.However its reactions in the condensed phase follow a rather different path which could involve diradicals such as (52; n = 2,3 . . .).86 Trapping experiments in which difluorosilylene is co-condensed at -196"C with for example propene suggest that the initial stages of the overall sequence involve formation of a siliran perhaps in an excited state which can either ring-open to give the diradical(53) or relax to the ground The main product of the reaction is a regular polymer which seems to support the idea of radical intermediates. 85 V. J. Tortorelli and M. Jones jun. J. Am. Chem. SOC.,1980,102 1425. 86 T. Hwang Y. Pai and C. Liu J. Am. Chem. SOC.,1980 102,7519; T. Hwang and C. Liu ibid. p. 385. W. F. Reynolds J. C. Thompson and A.P. G. Wright Can. J. Chem. 1980,58,419,425,436.