5 Arynes Carbenes Nitrenes and Related Species By S. A. MATLIN Chemistry Department The City University St John Street London EC1 V 4PB 1 Arynes The use of complex bases for the generation of arynes has been reviewed by Caubkre.' Diels-Alder reactions of benzyne with 1-vinylcyclobutene or with 1,2-dimethyl- enecyclobutane lead to tricyclic adducts (Scheme 1)which can be dehydrogenated with DDQ to give naphtho[a]cyclobutene or naphtho[b]cyclobutene respectively. Similar reactions of 2,3-didehydronaphthalenewith the dienes provide synthetic routes to anthrocyclobutenes.' Scheme 1 Arynic condensations have been used in several new syntheses of heterocyclic systems. Condensation of benzyne with diazocyclopentadienes gives 3H-indazole- (3-~piro)-cyclopentadienes.~ Treatment of N-phenyl 2-chlorobenzylamines or of N-phenyl 3-bromobenzylamines with potassium amide in liquid ammonia affords mixtures of dihydrophenanthridines and dihydrobenzazetes (Scheme 2).4 1 Ra/ph R \ Scheme 2 ' P.Caubere Topics Current Chem. 1978,73,49. * R. P. Thummel W. E. Cravey and W. Nutakul J. Org. Chem. 1978 43,2473. H. Dunand A. Hackenberger Synthesis 1978 594. K. Krohn D. Carboo and U. Puttfarcken Annulen 1978,608. 79 S. A. Madin 2-Isopentoxy-l,3-benzothioleshave been prepared by aprotic diazotization of substituted anthranilic acids in the presence of carbon disulphide and isopentyl alcohol.' The dithioles (1) are useful intermediates in the synthesis of diben- zotetrathiafulvalenes.(1) The initial adduct formed by reaction of the 1,4-dithiin- 1,l -dioxide (2) with benzyne extrudes sulphur dioxide and phenylacetylene to furnish the benzo[bJthio- phen (3). This appears to be the first example of the thioethylene moiety reacting as a 1,3-dip0le.~ Following an earlier report of the generation of 4,5 -didehydrotropone its reac- tions with cyclic dienes have now been described.' The first example of the trapping of 4,5-didehydropyrimidine with a cyclic diene has also been reported (Scheme 3).' Scheme 3 2 Nitrenes A theoretical study of CHNO isomers has been carried out by Poppinger and Radom.' CH3C(0)N ClC(O)N and perpendicular H,NC(O)N were found not to correspond to true energy minima on their respective potential energy surfaces and to collapse without activation to the corresponding isocyanates.However planar H,NC(O)N emerged as a stable structure 4 kcal mol-' below the perpendicular geometry with HOC(0)N and FC(0)N also having energy minima. Direct observation of the N-nitrene (4a) has been made by i.r. and electron spectroscopy,lo which provide evidence for considerable double bond character (4b) in the ground state. A new route to N-dibenzylaminonitrene (5) involves deoxy- genation of N-nitrosodibenzylamine with phenacyl bromides in the presence of silver hexafluoroantimonate.' ' J. Nakayama E. Seki and M. Hoshino J.C.S. Perkin I 1978 468. K. Kobayashi and K. Mutai Tetrahedron Letters 1978 905. T.Nakazawa Y.Niimoto and I. Murata Tetrahedron Letters 1978 569.* D. Christophe R. Promel and M. Maeck Tetrahedron Letters 1978,4435, D.Poppinger and L. Radom J. Amer. Chem. SOC.,1978,100,3674. lo W. D. Hinsberg and P. B. Dervan J. Amer. Chem. SOC.,1978,100,1608. I' K. Nishiyama and J.-P. Anselme J. Org. Chem. 1978,43,2045. Arynes Carbenes Nitrenes and Related Species \T ~ N=N N-N Spin delocalization in 4-substituted triplet phenylnitrenes has been examined by e.s.r. and by INDO calculations. The latter indicated an essentially constant spin density (ca.1.83)on the nitrene nitrogen the e.s.r. data being interpreted in terms of the influence of substituents on the delocalization of the remaining spin. l2 A new procedure for the generation of acylnitrenes reminiscent of the Lossen rearrangement involves the pyrolysis of the readily prepared N,O-bistrimethylsilyl hydroxamic acids (6)with elimination of hexamethyldisiloxane.l3 Another method of nitrene generation which involves N-0 bond cleavage is the reaction of oxinses with Grignard reagents. A cyclic elimination mechanism (Scheme 4) is implied by the observation that the intermediate vinylnitrene cyclizes regio- specifically onto the C atom syn to the 3bxime OH group. 31 Direct deamination of primary amines has been acc~mplished'~ by treatment with hydroxylamine 0-sulphonic acid and base. This Combination of reagents results in the formation of hydrazine intermediates and also in the generation of nitrene. The latter oxidises the hydrazines to diazenes which readily lose nitrogen (Scheme 5).NH,OSO,H :NH -N RNH2 ARNHNH2 -RN=NH 2RH OH-Scheme 5 Carbethoxynitrene is formed initially in the singlet state by a-elimination from ethyl p-nitrobenzenesulphonyloxycarbamate under phase transfer conditions and has been trapped by addition and insertion reactions with olefins.I6 Competition J. H. Hall J. M. Fargher and M. R. Gisler J. Amer. Chem. Soc. 1978 100 2029. l3 F. D. King S. Pike and D. R. M. Walton J.C.S. Chem. Comm. 1978 351. l4 G. Alvernhe and A. Laurent J. Chem. Res. (S),1978 28. lS G. A. Doldouras and J. Kollonitsch J. Amer. Chem. SOC.,1978 100,341. '' M. Seno T. Namba andH. Kise J. Org. Chem. 1978 43 3345. 82 S. A. Math experiments on the selectivity of singlet nitrene insertions into the tertiary secon- dary and primary C-cH bonds of hydrocarbons provide further evidence for the stabilization of carbethoxynitrene by solvents such as dichloromethane and 1,4- dioxan." The observed changes in product ratios with increasing concentration of the stabilizing solvent can be accounted for by the formation of bulky solvent- nitrene complexes whkh discriminate in favour of less hindered positions.Unlike dichloromethane in which singlet stabilization is counterbalanced by inter-system crossing 1,4-dioxan also stabilizes the singlet nitrene in addition reactions with olefins the solvent-nitrene complex favouring addition over insertion reactions and giving improved selectivity in the additions.I8 Azabicyclohexanes and/or pyridines are formed in high yields by the thermolysis of 2-allyl-3-phenyl-2H-azirines(7; R1= Ph) the suggested mechanism involving initial rearrangement to vinylnitrenes and recycli~ation.'~ The isomeric 2-allyl-2- phenyl derivatives (7; R2= Ph) undergo competitive cyclization of the nitrene onto the aromatic ring affording indoles.I I R3 R3 R3 R3 p'= (7) Ph R3 0LJR4 I H The photolysis of aroyl azides in the presence of diketene leads to l-aroyl-4- hydroxy-3-pyrrolin-2-ones(8) via intermediate aziridines.*' Bis-aziridines have been obtained by cycloaddition reactions of aziridinonitrenes.2' (8) 2-Substituted azepines are formed by a singlet pathway when phenyl azide is irradiated in solutions containing nucleophiles such as amines. Attempts to use alcohols as the nucleophiles have hitherto met with little success but it has now been '' P.Cassagrande L. Pellacani and P. A. Tardella J. Org. Chem. 1978 43 2725; H.Takeuchi Y. Kasamatsu M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin 11 1978 780. H. Takeuchi T. Igura M. Mitani T. Tsuchida and K. Koyama J.C.S. Perkin II 1978,783. l9 A. Padwa and P. H. J. Carlsen J. Org. Chem. 1978,43,2029;Tetrahedron Letters 1978,433. *' T. Kato Y. Suzuki and M. Sato Chem. Letters 1978 697. 21 L.Hoesch N. Egger and A. S. Dreiding Helu. Chim. Actu 1978,61 795. 83 Arynes Carbenes Nitrenes and Related Species shown that the carbonyl function in 2-azidobenzoyl esters and amides promotes 2-alkoxy-3H-azepine formation on irradiation in alcoiio1s.22 The previous assump- tion that the reaction involves cyclization of an intermediate phenylnitrene to an azirine followed by addition of nucleophile and ring expansion (Scheme 6) has now Scheme 6 been challenged.Chapman and Le ROUX*~ have observed an intermediate in the photolysis of phenyl azide in argon at 8 K which has an intense absorption at 1895cm-'. The same intermediate could be generated by irradiation of the triazene (9) and was suggested to be l-aza-l,2,4,6-~ycloheptatetraene (10). It is this cyclic ketenimine formed either via singlet phenylnitrene or directly from singlet excited phenyl azide which is claimed to add nucleophiles to form 2-substituted azepinesZ3 and to lie on the pathway between isomeric phenylnitrenes and pyridylmethylene~.~~ Irradiation of the azidopyrimidine derivatives (1 1 ;R =Me) in alkylamines gives the 1,3,5-triazepines (12;R =Me X = NR'R') and similarly the azide (11;R = CN) affords (12; R = CN X =OR') on irradiation in alcohols.25 However the azide (1 1; R =H) behaves differently,26 photolysis in alkylamines leading to the 6-alkylamino- 5-aminouracils (13).The differences were attributed to substituent-dependent behaviour of the intermediate (14)formed by addition of the nucleophile to the azirine (15). '* R.Purvis R. K. Srnalley W. A. Strachan and H. Suschitzky,J.C.S. Perkin I 1978 191. 23 0.L.Chapman and J.-P. Le Roux,J. Amer. Chem. Soc. 1978,100,282. 24 0.L.Chapman R. S. Sheridan and J.-P. LeRoux,J. Amer. Chem. Soc. 1978,100,6245. 25 S.Senda K. Hirota and T. Asao Tetrahedron Letters 1978 1531.26 S.Sendo K. Hirota T. Asao and K. Murahashi J. Amer. Chem. SOC.,1978,100,7661. S. A. Math 0 0 Me Me (14) (15) Studies continue on intramolecular insertion pathways for substituted aromatic nitrene~.~’The azidophenyl thienyl sulphides (16) yield pyrrolo[2,1 -b]benzo- thiazoles (17) with elimination of sulphur on thermolysis cyclization of the inter- mediate nitrene being followed by ring opening and reclosure (Scheme 7).28 -1 Scheme 7 Further studies of the manganese dioxide oxidation of aryl 1,2-diaminoirnidazoles (18; R=H or Ar) now provide evidence for formation of the N-nitrene (19)as a minor intermediate the major product-forming pathway being via the C-nitrene (201.~~ Ar Ar Ar ,&H2 R&: RN I R&HI!? NH, I I NH2 (18) (19) (20) A nitrene complex MO~O,(NH)[S~P(OE~)~]~ has been isolated from the reaction of MoO~[S~P(OE~)~]~ with HN3and was shown by X-ray crystallography to contain a nitrene group bridging the two molybdenum Whereas the 2-ketovinyl azirine (21) affords a high yield of the oxazepine (22) on thermolysis decomposition of (21) in the presence of MO(CO)~ gives a mixture of products (22)-(25) which result from an intermediate molybdenum-nitrene ’’ R.N. Carde,G. Jones W. H. McKinley,andC.Pn0e.J.C.S.PerkinI 1978,1211; R.A. Abramovitch C. I. Axogu I. T. McMaster and D. P. Vanderpool J. Org. Chem. 1978,43 1218. J. M. Lindley. 0.Meth-Cohn and H. Suschitzky,J.C.S. Perkin I 1978 1198. 2q M. Nakajima R. Hisada and J.-P. Anselme J.Org. Chem. 1978,43 2693. 30 A.W. Edelblut. B. L. Haymore and R. A. D. Wentworth J. Amer. Chem. Soc. 1978,100,2250. Arynes Carbenes Nitrenes and Related Species complex. Interestingly reaction of (21) with Fe2(C0)9 gives as well as the products (22)-(25) an additional product (26) arising from attack of the metallonitrene on the carbonyl gr~up.~' Ph Ph Ph Ph I 'Ph Ph COPh Ph Ph 1 1 3 Carbenes A review has been published of the synthesis of carbocyclic spiro compounds via cycloaddition routes including carbene additions to exocyclic double bonds.32 The results of theoretical and experimental assessments of the triplet-singlet energy separation in methylene are gradually converging. The latest configuration interaction (CI) calculation^^^ indicate an upper limit of 10.6 kcal mol-' for the separation compared with a new experimentally determined value34 of around 8.1 kcal mol-' .Like methylene diphenylmethylene has a triplet ground state. New e.s.r.studies of the triplet-singlet energy separation in diarylmethylenes isolated at low tempera- 31 F. Bellamy J.C.S. Chem. Comm. 1978,998; Tetrahedron Letters 1978,4577. 32 A. P. Krapcho Synthesis 1978.77. 33 C. W. Bauschlicher jun. and I. Shavitt J. Amer. Chem. Soc.,1978,100,739. 34 R.K.Lengel and R. N. Zare. J. Amer. Chem. Soc. 1978,100,7495. 86 S. A. Matlin tures in a rigid glass show an unexpectedly large stabilization when the p and p' positions are substituted by strong electron-withdrawing and electron-releasing groups re~pectively.~~ This is attributed to merostabilization in which charge- separated resonance structures [e.g.(27)Jpermit increased delocalization of the unpaired electron in the v orbital onto the positions ortho to the substituents. \ \ 0-1 0-+m+,o-Me,N N\ CI calculations indicate an allenic structure HC=C=N for cyanomethylene in agreement with earlier i.r. and U.V. studies. This result contrasts with restricted open-shell SCF theory which predicts a bent triplet carbene ground state. The discrepancy is explained by an inability of the restricted open-shell SCF theory to describe reliably the energy changes involved in simultaneous breaking of one bond and formation of an adjacent bond owing to an overemphasis of ionic character in bond breaking.36 MIND0/3 calculations on silylated carbenes have also been made.Trimethylsilylmethylene is predicted to be a bent triplet the result again conflicting with earlier Theoretical studies of singlet methylene 1,2-addition to ethylene and 1,4-addition to butadiene have also been reported.38 Generation.-Carbene transfer reactions of organomercury compounds have been reviewed.39 Optimization studies4' of dichlorocarbene addition to olefins by phase transfer catalysis (FTC) indicate the following conditions to be the best 4-molar excess each of chloroform and 50% aqueous sodium hydroxide over alkene 1mol% catalyst mixing at 0-5 "C stirring at >800 rev min-' for 1-2 h at room temperature then heating for 2-4 h at 50 "C. An improved electrochemical method41 for the generation of dichlorocarbene utilizes galvanostatic reduction of CCl and CHClj at lead cathodes in CHC13/Bu4NBr or CH2C12/Bu4NBr at -5 "C.The formation of carbenes by thermal decomposition of tosylhydrazone monoanions generally requires temperatures in excess of 130 "C. It has now been 35 D. R. Arnold and R. W. R. Humphreys J.C.S. Chem. Comm. 1978,181. 36 J. I. Harrison A. Dendramis and G. E. Lepoi J. Amer. Chem. Suc. 1978,100,4352. 37 R.Noyori M. Yamakawa and W. Ando Bull. Chem. SOC.Japan 1978,51,811. 38 B.Zurawski and W. Kutzelnigg J. Amer. Chem. SOC.,1978,100,2654: W.W.Schoeller and C. W. Kern ibid,p. 7548. 39 R. C. Larock Anpew. Chem. Internal. Edn. 1978,17,27. 40 E.V.Dehmlow and M. Lissel J. Chem. Res.(S),1978,310. O1 H. P.Fritz and W. Kornrumpf Annalen 1978 1416. Arynes Carbenes Nitrenes and Related Species shown that the anions of trisylhydrazones (2,4,6-tri-isopropylphenylsulphonyl-hydrazones) decompose to carbenes under significantly milder conditions.42 Carbenes are often formed during photochemical fragmentations and rear- rangements of 01efins.~~ Irradiation of cyclic olefins at 185nm leads to carbene intermediates by both C-C and C-H bond migration^.^^ Trimethylsilylcarbene can be generated from chloromethyl trimethylsilane by the use of the hindered base lithium 2,2,6,6-teramethylpiperidide in a hydrocarbon solvent and will cyclopropanate olefins. An analogous reaction was also obtained with chloromethyl trimethyl~tannane.~’ Evidence has been reported for the existence of free bi~(pheny1thio)carbene~~ and for the generation of dimethyl-aminocyanocarbene in the thermolysis of dimethyl- amin~malononitrile.~’ Reactions.-The reactivity of dichlorocarbene and related species continues to receive intensive scrutiny.Ausloos and Lia~*~ have carried out further measure- ments of the proton affinity of :CCl and reaffirm that its value is slightly below that of NH,. Giese and Mei~ter~~ have compared the selectivities of addition of dihalogeno- carbenes to 2-methylbut-2-ene and 2-methylpropene at different temperatures. A strong temperature dependence of the selectivities was observed with an isoselective temperature at about 330K. Below this temperature the selectivity order was :CF2> :CCI2> :CBr2 whereas above it the order was reversed.These results point to the caution which must be exercised in drawing conclusions about the stability or reactivity of individual carbenes on the basis of selectivity comparisons at one temperature. The partitioning between different reaction pathways for a given carbene is also temperature de~endent.~’ Diphenylcarbene reacts with 2-methyl- propene only by addition at 25”C but allylic C-H insertion becomes the near- exclusive process at -196 “C. The observation of rearranged alkene products (e.g. Scheme 8)during halogeno- carbene additions to olefins has now been re-interpreted in terms of ready intercon- version between the singlet and triplet states of the carbene the former being responsible for stereospecific cyclopropanation and the latter for rearrangement via diradi~als.’~ c1 Br CC1,H % + :CBr -C1 Br Br Scheme 8 *’ A.R. Chamberlin and F. T. Bond J. Org. Chem. 1978,43,154. 43 G. Kaup Angew. Chem. Internat. Edn. 1978,17 150. R. Srinivasan and K. H. Brown J. Amer. Chem. SOC.,1978,100,4602; Tetrahedron Letters,1978,3645. ‘’ R. A. Olofson D. H. Hoskin and K. D. Lotts Tetrahedron Letters 1978 1677. 46 M. Nitsche I).Seebach. and A. K. Beck Chem. Ber. 1978,111,3644. 47 L. De Vries J. Amer. Chem. SOC.,1978,100,926. ‘13 P. Ausloos and S. G. Lias J. Amer. Chem. Soc. 1978,100,4594. 49 B. Giese and J. Meister Angew. Chem. Internat. Edn. 1978,17 595. R. A. Moss and J. K. Huselton J. Amer. Chem. Soc. 1978,100,1314; R. A.Moss and M. A. Joyce ibid,p. 4475. 51 J. B. Larnbert K.Kobayashi and P. H. Mueller Tetrahedron Letters 1978,4253.; M. Jones jun.. P. P. Gaspar and J. B. Lambert ibid,p. 4257. 88 S.A. Math Insertion of dichlorocarbene into the C(2)-H bond of 2-substituted 1,3-diox- olans (28) affords the 2,2-disubstituted dioxolans (29). Fur 2-aryl derivatives (28; no:cc1* n 0 YH +Ox0 R R CCI,H (28) (29) R =m-or p-substituted phenyl) a good Hammett correlation with u+was obtained yielding p =-0.63. For 2-alkyl derivatives (28; R =alkyl) use of the modified Taft equation [Equation (l)]gave a good correlation with p =-0.73 and 6 =0. The p values indicate a largely concerted process with some positive charge development at C-2 in the transition state and it is apparent that the steric effect of a 2-alkyl substituent is of little imp~rtance.~~ gem-Dichloroaziridines have been synthesized by PTC addition of dichlorocar-bene to imine~.’~ This contrasts with the behaviour of diarylcarbenes with the imine (30) which give the product (31) of N-H insertion rather than a~irine.~~ :CF2 inserts directly into the N-H bond of bis(pentafluoropheny1) amine leading to an isolable difluoromethylamino compound (32).55 Ph Cu(acac) \ ArzC=N2 + C=N-CHAr2 Ph2C=NH -/ Ph (30) (31) :CF, (CzF5hNH -(C~FS)~NCF~H (32) Dichlorocarbene will efficiently abstract oxygen from many sulph~xides’~ and causes cleavage of a-hydroxyketoximes to ketone and nitrile products (Scheme 9).57 R \ / :cc1 \ C-C d C=O+RCrN /AH MOH / Scheme 9 Contrary to some earlier expectations it has been that alkylchlorocar- benes (methyl ethyl t-butyl) and cyclopropylchlorocarbene can be efficiently 52 K.Steinbeck and J. Klein J. Chem. Res. (S),1978 396; K. Steinbeck Tetrahedron Letters 1978 1103. 53 M. K. Meilahn D. K. Olsen W. J. Brittain and R. T. Anders J. Org. Chem. 1978,43 1346. 54 K. N. Mehrotra and G. Prasad Tetrahedron Letters 1978,4179. ’’R. Koppang J. Fluorine Chem. 1978,11,19. 56 H.S.D. Soya and W. P. Weber Tetrahedron Letters 1978 1969. 57 J. N. Shah J. P. Mehta and G. M. Shah J. Org. Chem. 1978,43,2078. R. A. Moss and R. C. Munjal J.C.S. Chem. Comm. 1978,775; R. A. Moss and M. E. Fantina J. Amer. Chem. SOC.,O1978,100,6788. Arynes Carbenes Nitrenes and Related Species trapped by addition to olefins.Only in the case of isopropylchlorocarbene did the alternative intramolecular 1,2-H shift leading to 1-chloro-2-methylpropene compete effectively. Phenylsulphinylcarbene is unusually stable and! shows a selectivity similar to that of methylchlorocarbene. The stability has been attributed to a powerful electron- donating ability of the PhS(0) substituent under conditions of high electron demand.59 Small amounts of asymmetric induction are observed in the cuprous chloride- catalysed additions of chiral carboalkoxycarbenes to olefins6' Substituted methyl- enecyclopropanes are formed by the addition of carboethoxycarbene to 1,l-dimethylallene.61 Rhodium(I1) carboxylates are particularly effective catalysts for the formation of cyclopropanecarboxylatesfrom ethyl diazoacetate and acetylenes.62 In the presence of rhodium(I1) acetate dimethyl diazomalonate reacts with thiophen to give the ylide (33; R=H) whose structure was confirmed by X-ray crystallography.The dichlorothiophenium ylide (33;R =C1) is remarkably stable C0,Me 00 and can be stored as a crystalline solid without special precautions. In the presence of Rh" acetate or copper acetylacetonate it serves as a useful precursor to bis- methoxycarbonylcarbene for cyclopropanation of ~lefins.~~ Carbonylcarbene :C=C=O formed by gas-phase photolysis of carbon suboxide reacts with the cyclic ethers tetrahydrofuran and oxetan to give products arising from both C-0 insertion and deoxygenation of the ether.64 Both types of reaction can be explained in terms of a common intermediate the ylide (34; n =0 or 1).The (34) reaction of ethyl diazoacetate with unsymmetrical ketones leads to highly selective formation of the least substituted enol ether consistent with the expected steric and 59 C.G. Venier and M. A. Ward Tetrahedron Letters 1978 3215. M P. E. Krieger and J. A. Landgrebe J. Org. Chem. 1978,43,4447. 61 X. Creary J. Org. Chem. 1978,43 1777. 62 N. Petiniot A. J. Anciaux A. F. Noels A. J. Hubert and P. TeyssiC Tetrahedron htters 1978 1239. 63 R. J. Gillespie J. Murray-Rust P. Murray-Rust and A. E. A. Porter J.C.S. Chem. Comm. 1978,83;J. Cuffe R. J. Gillespie and A. E. A. Porter ibid p. 641. T. R. Forbus P. A. Birdsong and P.B. Shevlin J. Amer. Chem. Soc. 1978,100,6425. 90 S. A. Matfin electronic effects for proton abstraction in the presumed carbonyl ylide intermediate (35)? R'R'CH \ &\ ,CO,Et R'R~CH \c/o\ CH ,CO ,Et C + I\ R3Ha LC\H R3HC H (35) Cyclopropanation of cycloheptatriene has been studied with diazomalonate and diazoacetate esters. In the case of the latter carbene precursor the products of addition to all three double bonds could be isolated.66 Bis-carbomethoxycarbene adds in the singlet state to cyclobutene to give the bicyclopentane (36),whereas under triplet conditions the vinylcyclopropanes (37) and (38) are also obtained. The latter are evidently formed via the diradical(39) and it has been argued that a similar mechanism is involved in vinylcyclopropane formation from methylene and cyclobutene .67 Me0,C C0,Me hulPh2CO + n -Meo2cm b 2 Me0,C C0,Me + MeO& C0,Me (36) (37) .-There is a growing interest in carbenes which exhibit nucleophilic character as a result of electron donation by a substituent into the vacant p-orbital of the carbene.As a model for the insertion reactions of such carbenes a theoretical study of the addition of hydrogen to cyclopropylidene has been carried out.68 As in the case of :CH2+ Hz, the reaction follows a pathway in which the H2a-orbital interacts with the carbene p-orbital (electrophilic phase) followed by population of the H20"-orbital by the carbene lone pair and rotation of the HCH plane to give the final tetrahedral arrangement (nucleophilic phase).A high activation energy (41 kcal mol-') is predicted with the electrophilic phase being rate-determining. Thioxanthenylidene (40a) shows evidence of nucleophilic character (40b) in its reactions with fumaric and maleic esters the former undergoing stereospecific cyclopropanation whereas the latter affords the olefin (41).69 By contrast a re- examination of xanthenylidene (42) for which nucleophilic or weak electrophilic " J. A. Landgrebe and H. Iranmanesh J. Org. Chem. 1978,43,1244. 66 B. Decock-le-Reverend M. Durand and R. Merenyi Bull. SOC. Chim. France 1978,369. '' M. E. Hendrick and M. Jones jun. Tetrahedron Letters 1978,4249. '* H. Kollmar J. Amer. Chem. Soc. 1978,100,2660. 69 T. B. Patrick M. A. Dorton and J. G.Dolan J. Org Chem. 1978,43 3303. Arynes Carbenes Nitreries and Related Species (42) (41) properties had earlier been claimed is now more indicative of typical electrophilic rea~tivity.~’ As expected methylchlorocarbene shows evidence of both electrophilic and nucleophilic character in its reactions with 01efins.~’ 1,3-Bis(dimethylamino)vinylcarbene(44a) generated by base attack on the perchlorate (43; R =Ph or OEt) has nucleophilic properties as a consequence of the allylic resonance (44b) and has been trapped as a dipole in a variety of Me,N*NMe H C=O H H H 8-. Me2N-YNMe2 I R (43) (444 (44b) Star~g~~ has reviewed the properties of vinylidenecarbenes and their formation from vinyl triflates and tosylazoalkenes. The unsaturated carbenes derived from these precursors exhibit mildly electrophilic character giving stereospecific olefin cyclopropanation consistent with the theoretical prediction74 of a ground state singlet.The free carbene (47) is a common intermediate generated by elimination from either of the isomeric vinyl triflates (45) or (46) and rearranges to l-phenyl- propyne by 1,2-phenyl migration.75 For vinylidene itself a low barrier of 8.6 kcal mol-’ has been calculated 74n for rearrangement to acetylene. Reversal of this rearrangement accounts for the scrambling of labels in HC_”CD observed on pyr~lysis.’~ 70 G. W. Jones K. T. Chang R. Munjal and H. Shechter J. Amer. Chem. Soc. 1978,100,2922. 71 N. P. Smith and I. D. R. Stevens Tetrahedron Letters 1978 1931 ;R.A. Moss and W.-C. Shieh ibid p. 1935. 72 R. Gompper and R. Sobotta Angew. Chem. Internat. Edn. 1978,17,762. 73 P. J. Stang Accounts Chem. Res. 1978,11 107; Chem. Rev. 1978,78,383. 74 (a)C. E. Dykestra and H. F. Schaefer,J. Amer. Chem. SOC., 1978,100,1378;(b)J. W. Kenney J. Simons G. D. Purvis and R. J. Bartlett ibid p. 6930. ” P. J. Stang D. P. Fox,C. J. Collins and C. R. Watson jun. J. Org. Chem. 1978,43,364. 76 R. F. C. Brown F. W. Eastwood and G. P. Jackman Austral. J. Chem. 1978,31,579. S. A. Matlin Me ‘&c Me*CECPh Ph/ (47) Cycloalkylides generated by flash vacuum pyrolysis of the leldrum A acid derivatives (48; n = 1-3) give products derived from rearrangement to bicy- clopropenes as well as cy~loalkynes.~~ Although ring closure to cyclopropenes is an important reaction for vinyl- carbene~,’~ intermolecular trapping by olefins to give vinylcyclopropanes is also readily Several studies have been reported of the reversal of the intramolecular process in which photolysis of phenyl-substituted cyclopropenes leads to vinylcarbenes.The latter may recyclize by an alternative mode affording indenes” or in the case of vinylcyclopropenes cyclopentadienes.81 The thiovinylcarbene (49) undergoes migration and dimerization reactions apparently without cyclopropene formation. Trapping with ethyl dimethylallyl sulphide leads uia rearrangement of the ylide (50) to the thioketal(51) of artemisi- aketone.82 Cycloheptatrienylidene has been generated by the fluoride ion induced desilyl- ation of the cation (52; R=H)83 as well as by decomposition of the anion of tropolone to~ylhydrazone.~~ In the absence of trapping reagents it undergoes dimerization but this process can be inhibited by the presence of 2,7-substituents which favour rearrangement to the arylcarbene (53).The reactions of the cyclopropanated cycloheptatrienylidenes (54)-(56) provide an interesting contrast each showing a unique rearrangement path~ay.~’ ’’ G. J. Baxter and R. F. C. Brown Austral. J. Chem. 1978,31 327. W. Welter A. Hartmann and M. Regitz Chem. Ber. 1978,111,3068. M. Franck-Neumann and C. Dietrich-Buchecker Tetrahedron 1978,34,2797. J. 0.Stoffer and J. T.Bohanon J.C.S. PerkinfZ 1978,692;M. I. Komendantov,R. R. Bekmukhamentov and I. N. Domnin Tetrahedron 1978,34,2743.A. Padwa T. J. Blacklock D. Getman N. Hatanaka and R. Loza J. Org. Chem. 1978,43,1481. M. Franck-Neumann and J. J. Lohmann Tetrahedron Lerters 1978,3729. 83 M. Reiffen and R. W. Hoffmann Tetrahedron Letters 1978 1107. IM C. Mayor and W. M. Jones J. Org. Chem. 1978,43,4498. M. Oda Y. Ito and Y. Kitahara Tetrahedron Letters 1978 977. Arynes Carbenes Nitrenes and Related Species 3S/Et -I-SEt (49) SiMe S. A. Matlin A new SCF of the rearrangement of cyclopropylidene to allene contradicts earlier calculations predicting barrier heights of 18 and 19 kcal. mol-' respectively for the singlet and triplet processes with the triplet carbene being more stable than the singlet by 8.4 kcal. mol-'. The related facile vinylcyclopropylidene-cyclo-pentenylidene rearrangement has also been examined using MIND0/3.In the singlet state the rearrangement is pictured*' as involving initial w-complex forma- tion between the double bond and the p-orbital of the carbene with synchronous opening of the three-membered ring leading to a non-classical carbene inter- mediate. For cyclopropylidenes the alternative reaction pathway to allene formation is intramolecular C-H insertion leading to bicyclobutanes.88 This process has hitherto not been observed for cycloprop-2-ylidene carbinols but evidence for the formation of intermediate bicyclo[ 1.1.O]butan-2-olates (58) has now been presen- ted.89 Reaction of the dibromocyclopropanes (57) with MeLi gave after work-up the aldehydes (59) and/or alcohols (60).R2 R* K R2 O * MeLi* R' R2&- "- Br Br D D D (57) R' R' R2 ,MeLi DH Intramolecular 1,2-migrations to carbenes are a useful method of olefin forma- tion. Adamantene (62) has now been generated by this type of reaction following pyrolysis of the anion (61). Biadamantyl isomers were obtained amongst the products and their formation is presumed to occur via radical intermediates generated from the strained olefin (62). Their isolation from this reaction raises questions about the nature of the processes involved in previous attempts at adamantene formation when no such products were ob~erved.~' In the deuterium-labelled 1-aryl-2-diazopropanes (63),kinetic isotope effects for H(D) migration in the thermally generated carbene provide evidence for a pull-push N2 II ArCHD-C-CH 86 D.J. Pasto M. Haley and D. M. Chipman J. Amer. Chem. SOC.,1978 100 5272. W. W. Schoeller and U. H. Brinker J. Amer. Chem. SOC.,1978,100,6012. " S. A. Matlin Ann. Reports (B),1977,74 105. 89 N. 0.Nilsen L. K. Sydnes and L. Skattebol J.C.S. Chem. Comm. 1978,128. 90 D. J. Martella M. Jones jun. and P. von R. Schleyer J. Amer. Chem. SOC.,1978 100 2896. Arynes Carbenes Nitrenes and Related Species mechanism.” Electrophilic attack on the C-H bond by the carbene p-orbital is accompanied by backside nucleophilic attack of the carbene electron pair assisting the H-transfer. According to SCFcalculations the preferred pathway for rearrangement of triplet formaldehyde to hydroxycarbene is a concerted process (1,2-H shift from C to 0) whereas for alkyl migration in ketones a diradical process involving dissociation- recombination is energetically favoured (Scheme lo).’’ Scheme 10 Some evidence consistent with this picture emerges from a study of the photorear- rangement of 1-alkoxytriptycenes (64).In alcoholic solvents the ketals (66) are formed by trapping of the oxacarbenes (65).However in inert solvents the aldehyde (67) and/or ketones (68) become major products and their formation can be rationalized by a homolysis of the 0-R bond in (65) to give acyl and alkyl radicals which may either recombine or abstract hydrogen atoms from the hv OR -I + I 91 D. T. T. Su and E. R.Thornton J. Amer. Chem.SOC.,1978,100 1872. 92 J. A. Altmann I. G. Csizmadia M. A. Robb,K. Yates and P. Yates J. Amer. Chem. SOC.,1978,100 1653. 93 H.Iwamura and H. Tukada Tetrahedron Letters 1978,3451.