5 Arynes Carbenes Nitrenes and Related Species ~~~~~ ~ ~ ~ ~ By S. A. MATLIN Department of Chemistry The City University St. John Street London EClV 4PB 1 Arynes New calculations on the bonding in benzyne based on the previously reported matrix4.r. spectrum indicate a true cycloalkyne structure (1).Based on this model the i.r. spectrum of tetradeuteriobenzyne was predicted and awaits experimental verification. * 138.7 H 141.0 140.5 + H Y -134.4 H (1) distance in pm angles in degrees There is a growing interest in the immobilization of reactants by attaching them to the surface of solid supports,’ amongst the benefits of which is the prevention of intermolecular reactions. One of the most striking publications of the year in the field of aryne chemistry concerns the application of this ‘pseudodilution’ technique to the generation of immobilized ben~yne.~ The polymer-linked benzyne (2; R = carboxylated polystyrene resin) prepared by lead tetra-acetate oxidation of the corresponding aminotriazole did not give dimers but could be trapped up to 70 s after generation by addition of tetracyclone affording (3).The exceptionally long lifetime of the intermediate suggests that this immobilization technique is likely to prove a useful complement to matrix isolation for studying the properties of arynes. Ph ROCH2CH2 ROCH2CH2yy4 h Ph Ph (3) A benzyne-triosmium complex has been isolated as an intermediate in the formation of benzene from benzyl alcohol and triosmium do.decacarbonyl.* J.W. Laing and R. S. Berry J. Amer. Chem. SOC.,1976,98 660. * J. I. Crowley and H. Rapoport Accounts Chem. Res. 1976,9 135. P. Jayalekshmy and S. Mazur J. Amer. Chem. Soc. 1976,98,6710. K. A. &am and A. J. Deeming J.C.S. Gem. Comm. 1976,852. 85 S. A.Matlin Whereas the decomposition of N-nitrosoacetanilide alone in benzene which occurs by competing pathways to benzyne and to phenyl radicals affords up to 80% of biphenyl in the presence of tetracyclone equally high yields of the benzyne adduct 1,2,3,4-tetraphenylnaphthaleneare formed. Cadogan's group5 has demonstrated that the tetracyclone plays a dual role scavenging the phenyl radicals which otherwise initiate a chain radical phenylation of the solvent as well as acting as an arynop hile.Benzyne has been generated by the action of lithium tetramethylpiperidide on phenyl benzenesulphonate. The aryne formed was entirely derived from the phenolic portion of the sulphonate ester but yields of adducts were generally lower than when bromobenzene was used as a precursor.6 No benzyne adduct could be detected when o-dibromobenzene was treated with the strongly basic lithium diphenylphosphide in the presence of furan evidently owing to the instability of the expected product to base.' Cycloaddition of o-benzene to trans -olefins generally proceeds with about 70-80% retention of configuration which can be accounted for either in terms of competing concerted pathways or by a biradical mechanism. If the former explana- tion were correct it would be expected that a monosubstituted deuterium-labelled olefin would give more inversion since the steric effect would favour a ,,2,+,2 process.However it has now been shown that the addition of benzyne to E-deuterio-t-butylethylene gives mainly the product of retention (Scheme l),arguing strongly in favour of the biradical mechanism.8 75% 25% Scheme 1 Steric effects in the addition of 3-methylbenzyne to 2-substituted furans have been examined by Newman and Kannan.9 For a given furan [Scheme 2; R = Me But eHO(CH,),d or CO,Me] the same ratio of products was obtained from each benzyne precursor (X Y =NH2,C02H or F Br) demonstrating that a true benzyne intermediate is involved in each case. The product ratio showed very little sensitivity to polar or steric effects in the furan substituent.Cyclobuta[ 1,2-d]benzyne has been generated from the dihalide (4; n = 1). In the absence of trapping agents the yields of biphenylene-type dimers in the series (4; n = 1-3) increase with decreasing ring size implying that the stability of the benzyne increases with increasing ring strain." Whereas no evidence of ring rotation by any pathway has ever been observed in the [3,3]paracyclophanes the corresponding 5,6-dehydro-derivative (5) is capable J. I. G. Cadogan C. D. Murray and J. T. Sharp J.C.S.PerkinII 1976,583;E. A. Bell J. I. G. Cadogan P. W. Milburn C. D. Murray R. M. Paton and J. T. Sharp ibid. p. 588. I. Fleming and T. Mah J.C.S. Perkin I 1976 1577. D. G. Gillespie B. J. Walker and D.Stevens Tetrahedron Letters 1976 1905. 8 A. T. Browne T. A. Christopher and R. H. Levin Tetrahedron Letters 1976,4111. M. S. Newman and R. Kannan J. Org. Chem. 1976,41,3356. lo R. L. Hillard and K. P. C. Vollhardt J. Amer. Chem. Soc. 1976,98 3579. Arynes Carbenes Nitrenes and Related Species (. Me R y\ H + $. &$ X I R Scheme 2 of undergoing 90"kotation to the perpendicular orientation from which cycloaddi- tion affords the bridged benzobarralene (6) in 66% yield." (5) (6) For the first time a 1,4-dihydronaphthalene-1,4-imine has been isolated from the reaction of a benzyne with a pyrrole from which only 2-naphthylamine derivatives have previously been obtained.12 The adducts (7a; R' = R2= Me) and (7b; R' = But R2= H) of tetrachlorobenzyne and N-alkylpyrroles were of surprising thermal stability eliminating acetylene only on strong heating.In the case of (7b) the isoindole (8; R = But X = C1) could be isolated on thermolysis. A new synthetic route to a variety of polynuclear aromatic hydrocarbons employs the oxidative deamination of dihydroaromatic 1,4-imines generated by aryne addition to py~roles.'~ For example the adduct (9)of 1,2-dehydronaphthalene and (8; R = Me X = F) eliminates nitrosomethane on peracid oxidation to give 8,9,10,11- tetrafluorobenz[a]anthracene. Benzyne reacts with bisethynylsulphone to give exclusively homo-Diels-Alder products probably via zwitterionic intermediates. l4 l1 D. T. Longone and J. A. Gladysz Tetrahedron Letters 1976 4559.l2 M. Ahmed and J. M. Vernon J.C.S. Chem. Comm. 1976 462. l3 G. W. Gribble and R. W. Allen Tetrahedron Letters 1976 3673. l4 E. E. Nunn Tetrahedron Letters 1976,4199. S. A.Matlin Me (7) (8) (9) Studiesof the arynic condensations of ketone enolates contin~e.’~ In the conden- sation of benzyne with cup -unsaturated ketone enolates the indanone products (12) are formed in addition to the previously reported tetralones (13). The ratio of these two products which are formed via opening of the benzocyclobutenol (10) and recyclization of the intermediate (ll) is most sensitive to the nature of the R’ substituen t (Scheme 3). R3 R’CH=d-Y-CHR’ + PhBr Bu‘ONa R2 aR3 NaNH2) R2 &R2 \ 0:-Y O CHR ’ 0 CHR’ R3 R3 mR1 \ + 0 0 (12) (13) Scheme 3 2,3-Thiophyne generated for the first tirnel6 by the flash vacuum thermolysis of the anhydride (14) has been trapped via a variety of dienes to afford benzo[b]thiophens.9,lO-Dehydroanthracene is formed” in the photochemical decarbonylation of the bis-keten (15). 0 c II 0 15 M. Essiz G. Coudert G. Guillaumet and P. Caubere TetrahedronLetters 1976 3185. 16 M. G. Reinecke and J. G. Newsom J. Amer. Chem. SOC.,1976,98,3021. 0.L. Chapman C.-C. Chang and J. L. Kolc J. Amer. Chem. SOC.,1976,98 5703. Arynes Carbenes Nitrenes and Related Species 2 Nitrenes The formation of triplet-state nitrenes from azides has been reviewed." Electrochemical generation of a nitrene from TsNC12 has been achieved by constant-current electroreduction in acetonitrile-dioxan.l9 The thermal unimolecular fragmentation of the NO-bis(trimethy1-sily1)hydroxylamine(16)in cyclohexene afforded mainly (53%)aniline and only 2% of the azirane (17),together with other minor products consistent with the intermedi- acy of phenylnitrene.20 Attempts to trap the nitrene with stilbenes were largely unsuccessful but thermolysis in the presence of amine solvents gave high yields of products from the trapping of the ring-expanded azepines (Scheme 4). The alterna- tive process of ring-contraction of phenyl- and 2-pyridyl-nitrenes has been examined in detail with the aid of I3Clabels.2' Scheme 4 Ethoxycarbonylnitrene evidently in the triplet state can be obtained22 in high yield by the triplet-sensitized photolysis of the ylide (18).I -NCO,Et (18) Solvent effects on the behaviour of nitrenes have been noted by several groups. 1,4-Dioxan stabilizes the singlet state of ethoxycarbonylnitrene presumably by an interaction of the nitrene p-orbital with the lone pairs of electrons of both the ether oxygen^.^^ No comparable effect is seen with tetrahydrofuran or tetrahydropyran and nitrene insertion into the a-C-H bonds of cis- and trans-2,5-dimethylfuran H. Durr and H. Kober Topics Current Chem. 1976,66,89. l9 T. Fuchigarni T. Nonaka and K. Iwata J.C.S. Chem. Comm. 1976 951. 2o F. P. Tsui Y.H. Chang T. M. Vogel and G. Zon J. Org. Chem. 1976,41 3381. 21 C. ThBtaz and C. Wentrup J. Amer. Chem. Soc. 1976 98 1258; R. Harder and C.Wentrup ibid. p. 1259. 22 M. Nastasi H. Strub and J. Streith Tetrahedron Lerters 1976 4719. 23 H. Takeuchi K. Kinoshita S. M. Abdul-Hai M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin ZZ 1976,1201. 90 S.A.Math proceeds non-stereospecifically consistent with the intermediacy of an 0-N ~lide.~~contrast with the thermally generated ethoxycarbonylnitrene the In photochemically generated species in halogenated solvents such as dichloromethane shows an increased selectivity for insertion into the C-H bonds of deca1i1-1.~~ Similarly dichloromethane solvates and stabilizes pivaloylnitrene without markedly decreasing its reactivity. The singlet nitrene adds stereospecifically to olefins whilst additions of the triplet are stereoselective.26 The effect of temperature on the relative reactivity of the singlet and triplet states has also been empha~ized.~’ Thermally the singlet derived product (20; X =Me) from the azide (19; X=Me) is greatly favoured (98% in boiling PhBr).Direct photolysis of (19; X = Me) at room temperature affords 62% of (20; X =Me) the yield rising to 90% on irradiation in boiling PhCl. However in the sensitized photolysis of (19; X =Me) an increase in temperature favours increased yields of the triplet-derived product (2 1 ; X = CH). Similarly direct room-temperature photo- lysis of (19; X = N3) affords (20; X = N3),whereas triplet sensitization gives mainly azo-dimer and a low yield of (2 1;X = N). Irradiation of (19; X = N3)in a rigid matrix at 77 K affords (21; X= N) quantitatively.X X H In neither of the nitrenes (22) or (23) did reaction lead to any products derived from attack of the sulphur atom on the nitrene.28 In contrast lead tetra-acetate oxidation of the amine (24; R=phthalimido) in the presence of benzothiophen resulted in formation of some (25) and the cis-azo-compound (26) as the major product. Similar results were obtained with other sulphides but in the absence of a sulphide only the trans-isomer of (26) was obtained. The intermediacy of S-N ylides was invoked2’ to explain the formation of both (25) and (26) the cis stereochemistry of the latter being explained by steric effects in the intermediate (27). Reports appearing during the year suggest that investigation of metal-nitrene interactions may prove very fruitful.Thus aryl azides are cleanly reduced to aryl amines by aqueous vanadium dichloride although alkyl azides give complex product mixtures from the vanadium-co-ordinated nitrene interrnediate~.~’ With iron pen- tacarbonyl aryl azides afford ureas in good yield evidently via iron-nitrene com- 24 N. Torimoto T. Shingaki and T. Nagai Bull. Chem. SOC.Japan 1976,49 2572. 25 P. A. Tardella and L. Pellacani J. Org. Chem. 1976 41 2034. 26 G. R. Felt and W. Lwowski J. Org. Chem. 1976 41 96. 27 J. M. Lindley I. M. McRobbie 0.Meth-Cohn and H. Suschitzky TetrahedronLetters 1976 4513; A. Yabe and K. Hondo Bull. Chem. SOC.Japan 1976,49,2495. z8 1. M. McRobbie 0.Meth-Cohn and H. Suschitzky Tetrahedron Letters 1976 929. 29 D.W. Jones J.C.S. Perkin I 1976 1150. 3O T.-L. Ho M. Henninger and G. A. Olah Synthesis 1976,815. Arynes Carbenes Nitrenes and Related Species ~lexes,~’ in the reaction of 2-aryl and similar complexes also seem to be inv01ved~~ azirines with Fe,(CO),. The analogy with carbene-metal complexes is further strengthened by the that the products of reaction of 2-aryl azirenes with dicarbonylchlororhodium dimer are strongly dependent on the azirene :rhodium ratio (cf.the work cited in ref. 75). When this is 10:1 the indole (28) is obtained whereas with a ratio of 2 1 the diary1 pyrrole (29) is the principal product. 3 Carbenes Rearrangements and interconversions of carbenes and nitrenes have been reviewed. 34 The method of structural fragments has been applied to the assessment of singlet-triplet energy diff eren~es.~’ Ab initio MO calculation^^^ on the manifold of 31 A.F. M. Iqbal Helo. Chim. Acta. 1976,59 655. 32 H. Alper and J. E. Prickett J.C.S. Chem. Comm. 1976 191. 33 €3. Alper and J. E. Prickett J.C.S. Chem. Comm.,1976,483. 34 C. Wentrup Topics Current Chem. 1976,62 173; See also ref. 21. 35 J. F. Liebman P. Politzer and W. A. Sanders J. Arner. Gem. SOC 1976,98 51 15. 36 W. J. Hehre J. A. Pople W. A. Lathan L. Radom E. Wasserman and Z. R. Wasserman J. Amer. Chem. Soc. 1976,98,4378. S.A. Matlin C3H2 isomers indicate that cyclopropenylidene is the most stable singlet isomer whereas the most stable triplet species is prop-2-ynylidene for which a 1,3-biradical valence structure is evidently preferred.New ab initio calculations3' suggest vinyl- methylene to have a localized methylene-like triplet ground state but this seems to be inconsistent with e.s.r. evidence,38 which favours a 1,3-biradical structure. Triplet carbenes can be conveniently trapped with nitric oxide to give long-lived iminoxyl radicals which may be detected by e.~.r.~' Previous calculations on the relative stability of oxiren and formylcarbene have produced conflicting data. A new assessment employing a nonempirical SCF-MO study with full optimization of the geometrical parameters has yielded data which are likely to be the most reliable so far obtained and which in broad agreement with the experimentally observed behaviour of ketocarbenes places ground-state oxiren 49.3 kJ mo1-I higher in energy than ground-state formylmethylene.An activation energy of 30.5 kJ mol-1 was computed for the ring-opening of oxiren to ketocar- bene leading to an estimation of a very short lifetime ( < lo-* s)for the unsubstituted oxiren molecule and explaining why attempts to trap oxirens have so far been U~SUCC~SS~U~.~~ The intermediacy of thiiren in the photochemical decomposition of 1,2,3-thiadiazole (30) has been established by 13Clabelling. Interestingly thiiren lies only on the pathway to ethyl mercaptan formation and not thioketen forma- tioa41 Generation.-There has been a continuing development in the use of phase-transfer catalysis for the generation of carbene~.~* Several new methods of forming carbenes have been reported including the Hg(3P1) sensitized photolysis of trifluoroethylene to furnish difl~orovinylenecarbene,~~ and the reaction of sodium iodide in potassium carbonate with ethyl dichloroacetate to give chlorocarbene (Scheme 5)." 0 IIC1&C-aCH2CH3 -+C12CH- Cl-%-H IH I- Scheme 5 37 J.H. Davis W. A. Goddard and R. G. Bergman J. Amer. Chem. SOC.,1976,98,4015. 38 D. R. Arnold,R. W. Humphreys W. J. Leigh and G. E. Palmer,J. Amer. Chem. SOC.,1976,98,6225. 39 A. R. Forrester and J. S. Sadd.,J.C.S. Chem. Comm. 1976 631. 40 0.P. Strausz R. K. Gosavi A. S. Denes and I. G. Csizmadia J. Amer. Chem. SOC.,1976,98,4784. 4' J. Laureni A. Krantz and R. A. Hajdu J. Amer. Chem. Soc. 1976,98,7872. 42 E. V. Dehmlow Tetrahedron Letters 1976,91; E.V.Dehmlow and M. Lissel ibid. p. 1783; S. Kwon Y. Nishimura M. Ikeda and Y.Tamura Synthesis 1976 249; F. De Angelis A. Gombacorta and R. Nicoletti ibid. p. 789; T. Sasaki S. Eguchi M. Ohno and F. Nakata J. Org. Chem. 1976 41 2408. 43 R. J. Norstrom H. E. Gunning and 0.P. Strausz J. Amer. Chem. SOC.,1976 98 1454. 44 J. K. Makrandi and S. K. Grover Tetrahedron Letters 1976 3179. Arynes Carbenes Nitrenes and Related Species 93 The reaction of halide ions with epoxides gives a low concentration of 2-halogenoalkoxide ions sufficient to generate difluorocarbene from chlorodifluoromethane.45Another new method of forming this carbene involves the use of Wittig reagents as bifunctional reactants. The phosphonium ylide (31)serves first as a base to form the carbene from chlorodifluoromethane and then as a trapping agent providing a new synthesis of 1,l-difluoro-olefins (Scheme 6).46 (31) Ph36-CR'R2 +HCF2CI -+ Ph3P+-CHR1R2 +:CF2 -Ph3P+ F2C=CR'C2 (31) Scheme 6 Olofson and co-worker~~~ have described two new approaches to cyclopropanol synthesis.The first involves treatment of 2-chloroethyl chloromethyl ether with lithium tetramethylpiperidide in the presence of olefins furnishing good yields of 2-chloroethyl cyclopropyl ethers from which cyclopropanols can be obtained by the action of n-butyl-lithium. In the second method acyloxycarbenes of which only one example was previously known are generated by the action of hindered base on chloromethyl esters and add to olefins to give cyclopropyl esters in moderate yields.The Meldrum's acid derivatives (32) also decompose on pyrolysis to acyloxycar-benes and in the absence of trapping agents these rearrange to 1,2-diketones in high yield.48 PCO R2C0 0 (32) Carbenes are also formed in the thermal decomposition of arylidene-oxazolones and -isoxaz~lones~~ and in the photochemical fragmentation of cyclic carbonate^.'^ Reactions.-Sensitized photolysis of diazoacetone produces a triplet ketocarbene which reacts by a combination of H-abstration and intersystem crossing to the singlet. In partially chlorinated solvents chlorine abstraction by the singlet ketocar- bene occurs together with H-abstraction. The fact that chlorine abstraction is not seen in the direct photolysis of diazoacetone suggests that the singlet ketocarbene is not an intermediate in the Wolff rearrangement of diaz~acetone.~' Stereochemical aspects of the previously reported vinylogous Wolff rearrange- ment have been examined.52 Interest continues in the stereochemistry of 1,2-shifts 45 M.Kamel W. Kimpenhaus and J. Buddrus Chem. Ber. 1976 109,2351; W. Kimpenhaus and J. Buddrus ibid. p. 2370. 46 G. A. Wheaton and D. J. Burton Tetrahedron Letters 1976,895. 47 R. A. Olofson K. D. Lotts and G. N. Barber TetrahedronLetters,1976,3381,3779; G. N. Barber and R. A. Olofson ibid. p. 3783. 4a R. F. C. Brown F. W. Eastwood S. T. Lim and G. L. McMullen Austral J. Chem. 1976 29 1705. 49 C. Wentrup and W. Reichen Helv. Chim. Acta 1976,59,2615 2618. 50 G.W. Griffin R. L. Smith and A. Manmade J. Org. Chem. 1976,41 338. 5' H. D. Roth and M. L. Manion J. Amer. Chem. SOC.,1976 98 3392. 52 A. B. Smith B. H. Toder and S. J. Branca J. Amer. Chem. SOC.,1976,98,7456. S. A.Matlin to carbenic Examples of 1,2-shifts of groups other than hydrogen are seen in two reports on the thermolysis of tosylhydrazone sodium 6-Norpinanylidene (33) rearranges by an alkyl shift via (34) which furnishes (35) through a methylenecyclopropane rearrangement. It is not clear in this case whether the preference for alkyl over H migration is due to the requirement for the migrating group to be perpendicular to the carbene or simply due to avoidance of the anti-Bredt product of H-migration (36). Phenyl migration is preferred from the axial position in (37a) over the equatorial position in (37b) by a factor of 5 1 H-migration dominating in both cases.However the interpretation of results in this system is complicated by the presence of a certain degree of conformational flexibility. Not only the singlet but also the triplet carbenes generated from the isomers (38a) and (38b) undergo ring-expansion and fragmentation stereospecifically suggesting a singlet-triplet equilibrium with reaction rates favouring rearrangement and frag- mentation via the singlet Ph H N2 (384 (38b) Work on the arylcarbene-aromatic carbene rearrangement Resonance in the dialkynylcarbenes (39) results in formation of a series of isomeric tetra-alkynyl olefins on dimerization. Trapping of (39a) with olefins gives substi- tuted cyclopropanes without stereoselection which implies a triplet-state path~ay.~’ Rearrangements of cyclopropylidenes generated from the corresponding 1,l-dibromocyclopropanes have been used in ~ynthesis.~~ An examination of 53 E.P. Kyba and C. W. Hudson J. Amer. Chem. Soc. 1976,98,5696. 54 U. Langer and H. Musso Annalen 1976 1180; L. Seghers and H. Shechter TetruhedronLetters 1976 1943; for a study of competing phenyl and methyl migrations see J. J. Have] J. Org. Chem. 1976 41 1465. 55 R. R. Gallucci and M. Jones jun. J. Amer Chem. SOC.,1976,98 7704. For other examples of carbene fragmentations see T. L. Gilchrist and D. P. J. Pearson J.C.S. Perkin I 1976 1257; W. R. Dolbier jun. 0.T. Garza and B. H. Al-Sader Tetrahedron Letters.1976 887. 56 N. M. Lan and C. Wentrup Helu. aim. Acta 1976 59 2068; U. H. Brinker and W. M. Jones TetruhedronLetters 1976,577. 57 H. Hauptman Tetruhedron 1976,32 1293. 58 M. S. Baird and C. B. Reese Tetrahedron Letters 1976,2895;J.-C. Damiano J.-L. Luche and P. CrabbC ibid. p. 779. Arynes Carbenes Nitrenes and Related Species C4c/-\c \c e- C-CrC-CrC-R R-CEC-CEC-C R/ 'R R/ \R (39d (39b) (394 intramolecular insertion reactions of cyclopropylidenes into N-H bonds and into C-H bonds adjacent to nitrogen suggests that there is a general requirement for a 1-6 relationship between the carbene and the H atom.59 Pyrolysis of the salt (40 gives tetramethylallene episulphide via the ylide (41) whereas no evidence for ylide formation could be found in the case of the salt (42).f Na+ NNTS I The failure of the oxygen analogue to form an ylide in this case is presumably due to steric factors.60 .Ylide formation with oxygen is readily observed in other cases e.g. in the dichlorocarbene-induced deoxygenation of epoxides to olefins6' and in the carbene-induced cleavage of ethem6* Re~erdy~~ has carried out a detailed examination of the reactions of xanth-enylidenecarbene (43) which shows only weakly electrophilic character as a conse- quence of a resonance contribution by (43b). + (434 (43b) The typically electrophilic behaviour of most carbenes is contrasted by the nucleophilic character of dimethoxycarbene for which a p value of +2.0 has now been determined in its reactions with aryl is~cyanates.~~ The first mechanistically unequivocal demonstration of reaction stereochemistry for this carbene shows that 59 M.S. Baird and A. C. Kaura J.C.S. Chem. Comm. 1976 356. 6o A. G. Hortmann and A. Bhattacharjya J. Amer. Chem. SOC. 1976,98 7081. 61 I. Tabushi Y. Kuroda and Z. Yoshida Tetrahedron 1976 32 997. 62 G. K. Agopian D. W. Brown and M. Jones jun. Tetrahedron Letters 1976 2931; H. Iwamura Y. Imahashi K. Kushida K. Aoki and S. Satoh Bull. Chem. SOC. Japan 1976,49 1690. 63 G. Reverdy BullSoc. chim. France 1976 1131 1136 1141. 64 R. W. Hoffmann and M. Reiffen Chem. Ber. 1976,109,2565. S. A.Matlin its additions to p -deuteriostyrenes are stere~specific.~~ Addition reactions of dimethoxycarbene to heterodienes have also been reported.66 Further work on the 1,4-addition of dihalogenocarbenes to norbornadiene and related dienes has been published,67 and the question of homoallylic delocalization in 7-norbornenylidene has been examined.68 Metal-Carbene Complexes.-Several interesting developments in the field of metal- carbene chemistry have taken place during the year. Reviews have discussed gold-carbene complexes69 and the complexation and activation of diazenes and diazo-compounds by transition metals.70 The first examples of isolable alkyl dialkyl and alkylaryl metal-carbene complexes have been reported.71 Cyclopropanations of alkenes with diazoacetates are efficiently catalysed by rhodium(I1) carboxylate~,~~ as are additions of diazoacetates to carbodi-imides.The latter reaction is also catalysed by copper triflate.73 The importance of the choice of catalyst in determining reaction course is clearly illustrated in a study of the behaviour of the diazoketone (44). This can give either the product of cyclopropana-tion (45) or the furan-3(2H)-one (46) via a carbonyl ~lide,~~ palladium complexes favouring the former rhodium acetate the latter and copper complexes giving either product depending on the ligands present. N2 (44) (45) (46) W~lfman'~ has emphasized that not only the nature of the metal and its ligands but also the catalyst concentration has an important influence on reaction of diazocarbonyl compounds. Detailed studies of the behaviour of diazomalonate with various concentrations of soluble copper catalysts reveal that in the routes to cyclopropanation allylic C-H insertion and dimerization there are competing pathways for the formation of each of the products.It is particularly interesting that 65 R. A. Moss and J. K. Huselton J.C.S. Chem. Comm. 1976,950. e6 R. Hoffmann K. Steinbach and W. Lilienblum Chem. Ber. 1976 109,1759. 67 C. W. Jefford A. Delay T. W. Wallace and U. Burger Helu. Chim. Acta 1976,59,2355;C. W. Jefford J. Mareda J. C. E. Gehret T. Kabengele W. D. Graham and U. Burger J.Amer. Chem. SOC.,1976,98 2585; C. W. Jefford V. de 10s Heros and U. Burger Tetruhedron Letters 1976,703;P. M. Kwantes and G. W. Klumpp ibid. p. 707. R. A. Moss and C.-T. Ho Tetrahedron Letters 1976 1651 3397. 69 H. Schmidbaur Angew.Chem. Internut. Edn. 1976 15 728. 70 H. Albini and H. Kisch Topics Currenf Chem. 1976,65 105. 71 E. 0.Fischer R. G. Clough G. Besl and F. R. Kreissl Angew. Chem. Infernat. Edn. 1976,15,543; R. Aumann H. Wormann and C. Kruger ibid. p. 609. 7* A. J. Hubert A. F. Noels A. J. Aciaux and P. Teyssit Synthesis 1976 600. A. J. Hubert A. Feron R. Warin and P. Teyssit Tetrahedron Letters 1976 1317. 74 S. Bien A. Gillon and S. Kohen J.C.S. Perkin I 1976 489. For another example of carbonyl ylide formation see; T. Ibata T. Motoyama and M. Hamaguchi Bull. Chem. SOC. Japan 1976,49,2298. 75 D. S. Wulfman Tetrahedron 1976,32 123 1 ;D. S. Wulfman R. S. McDaniel jun. and B. W. Peace ibid. pp. 1241 1251; D. S. Wulfman R. G. McGibboney E. K.Steffen N. V. Thinh R. S.McDaniel jun. and B. W. Peace ibid. p. 1257. '3 Arynes Carbenes Nitrenes and Related Species a metallocyclobutane is considered a likely intermediate in cyclopropanation in view of recent results on the olefin metathesis reaction. This year has seen the emergence of convincing proof of the intermediacy of metal-carbene complexes in the olefin metathesis rea~tion.’~ Detailed examinations of the behaviour of deuteriated olefins argue strongly in favour of a mechanism involving exchange of alkylidene moieties between a metal-carbene complex and an olefin via a metallocyclobutane intermediate77 (Scheme 7) and this picture is entirely consistent with the observed stereochemistry of the rea~tion.’~ Scheme 7 The tungsten-complexed ethylidene fragment generated in the metathesis of but- 2-ene has been trapped by addition to ethyl acrylate affording the cyclopropane (47).In a reversal of this addition it was demonstrated that under metathesis conditions alkylcyclopropanes will undergo fragmentation to alk- 1-enes with selec- tive loss of a methylene unit consistent with insertion of tungsten into the three- membered ring to form a transient metallocyclobutane. By combining these two sets of results the first cross-metathesis between an olefin and a cyclopropane was obtained (Scheme 8).79 PhWC13-AIC13 + PCO,Et -b. C0,Et (47) Scheme 8 76 N. Calderon E. A. Ofstead and W. A. Judy Angew. Chem. Internat. Edn. 1976,15,401;M. F. Farona and V. W. Motz J.C.S. Chem. Comm. 1976,930;T. J.Katz S. J. Lee and N. Acton Tetrahednh Letters 1976 4247; T. J. Katz and N. Acton ibid. p. 4251. 77 T. J. Katz and R. Rothchild J. Amer. Chem. Soc. 1976,98,2519. R. H. Grubbs D. D. Carr C. Hoppin and P. L. Burk ibid.,p. 3478. 78 J. L. Bilhou J. M. Basset R. Mutin and W. F. Graydon J.C.S. Chem. Comm. 1976 970. 79 P. G. Gassman and T. H. Johnson J. Amer. Chem. SOC.,1976,98,6055 6057 6058.