5 Arynes Carbenes Nitrenes and Related Species By R. C. STORR 0 Robert Robinson Laboratories University of Liverpool Liverpool L69 3BX 1 Arynes Non-ortho-dehydroarenes have provided the highlights of aryne chemistry in 1975. MIND0/3 calculations published last year indicated that the lowest-energy situation for rn -benzyne corresponds to a bicycle[3,1,O]hexatriene-like structure (1) (1) (2) (3) rather than a relatively unperturbed hexagon,' and a claim for the generation of this species starting from a C-1-C-5-bonded precursor has now appeared.* Formation of the fulvene (5) by treatment of the dibromide (4) with three equivalents of KOBu' at -78 "Cin the presence of HNMe was rationalized by nucleophilic attack of the amine on the electrophilic carbon of (1) (Scheme l).However an alternative explanation (Scheme 2) for the formation of the fulvene (9,avoiding a rn-benzyne has already been ad~anced.~ Br Br NMe2 @ -(1) i,(-j, shift ' OI/NMe2 1.5 --+ (4) (5) Reagent i HNMe Scheme 1 + a" Ql Br + (===J Br --* pN"" *.-. 1 Br (4) (5) Scheme 2 1 M. J. S. Dewar and W. K. Li J. Amer. Chem. Soc. 1974,96,5569; R.C. Storr Ann. Reports (B),1974 71 154. 2 N. N. Washburn J. Amer. Chem. SOC.,1975,97 1615. 3 A. H. Amaro and K. Grohmann J. Amer. Chem. Soc. 1975,97,5946. 105 106 R. C.Stow MIND0/3 calculations have also indicated that two 1,4-dehydrobenzene struc- tures (2) and (3)correspond to energy minima.' Bergman has recently described a p-benzyne which could be identified with (2).4 Now evidence for another p-benzyne corresponding to the butalene structure (3) is claimed.Again the approach involves dehydrohalogenation of a precursor (6) in which the dehydrocarbons are already a-bonded. Evidence for the intermediate formation of (3) is the predominant but not exclusive formation of p-deuteriated dimethylaniline in the presence of DNMe2and I Ph isolation of the diphenylisobenzofuran adduct (7) which appears to require initial reaction between the isofuran and butalene.' Further examples of the unusual 1,3-addition of 1,8-dehydronaphthalene to dienes have appeared.6 Both addition-elimination and elimination-addition mechanisms are involved in the reactions of halogenobenzocinnolines with KNH2.A small amount of 2-aminobenzocinnoline formed from the 4-chloro-compound raises the possibility of a 1,3-dehydro intermediate although an addition-rearrangement- elimination sequence via (8) and (9) seems more pr~bable.~ H2N H (8) (9i Arynes are not involved in dediazoniation of arenediazonium salts in water' or trifluor~ethanol,~ although they do appear to be so in some cases of telesubstitution in heterolytic dediazoniation by halide ions in pyridinium polyhydrogen fluoride 4 R.R. Jones and R. G. Bergman J. Amer. Chem. SOC.,1972,94,660. 5 R. Breslow J. Napierski and T. C. Clarke J. Amer. Chem. SOC.,1975,97 6275. 6 J. Meinwald L. V. Dunkerton and G. W. Gruber J. Amet. Chem.SOC.,1975,97,681; see also M.Kato S. Takaoka and T. Miwa Bull. Chem. SOC.Japan 1975,48,932.7 G. E. Lewis R. H. Prager and R. H. M. Moss Austral.J. Chem. 1975,28,2057; see also p. 2459. 8 C. G. Swain J. E. Sheats and K. G. Harbison J. Amer. Chem. Soc. 1975,97,783. 9 P. Burri G. H. Wahl and H. Zollinger Helu. Chim. Acru 1974 57,2099. Arynes Carbenes Nitrenes and Related Species solution." Matrix photolysis of the diazolactone (10) results in the formation of benzyne with sufficient efficiency for the CECstretch (2085 cm-')to be observed for the first time." Benzyne has been produced in low to moderate yield in the ther- mal decomposition of diphenyliodonium acetate nitrosation of N-phenylphosphylamidates deoxygenation of benzenediazotoluene-p-sulphonateN-oxide,12 and in the photolysis of o-nitrobenzaldehyde N-acetyl-N-alkylhydrazones.l3 This last reaction involves the intermediate brmation of o-(N-ace tyl-N-a1 kyltriazeno) benzoic acids.Use of the highly hindered base lithium tetramethylpiperidide allows Diels-Alder trapping of benzyne generated by dehydrohalogenation of chl~robenzene.'~The regioselectivity of addition of metal amide to 2,3-dehydrotoluene varies with the cation; the selectivity for NH attack ortho to the methyl group is greatest for the most covalent LiNH2. l5Benzyne gives products arising from 2 +2 and 2 +4 addition to benzylidineaniline. l6 Further applications of the additions of enolate anions to arynes have appeared."J' With polyhalogeno-compounds benzofurans can be produced by successive arynic substitution and cyclization. l8 The reaction of benzyne with (dimethylaminomethy1)phenylsilanesresults in the formation of a novel rearrangement product (1 1) in addition to the expected Stevens rearrangement product.l9 Me +,Me Me I CHSiPh3 +PhN-CHSiPh, I Me (1 1) lo G. A. Olah and J. Welch J. Amer. Chem. Soc.,1975,97,208. 0.L. Chapman C. C. Chang J. Kolc N. R. Rosenquist and H. Tomioka J. Amer. Chem.Soc.,1975,97 6586. l2 J. I. G. Cadogan A. G. Rowley J. T. Sharp B. Sledzinski and N. H. Wilson J.C.S. Perkin I 1975,1072. l3 Y. Maki T. Furuta M. Kuzuya and M. Suuki J.C.S. Chem. Comm. 1975,616. l4 K. L.Shepard Tetrahedron Letters 1975,3371. R. Levine and E. R. Biehl J. Org. Chem. 1975,40,1835. l6 J. Nakayama H. Midorikawa and M. Yoshida Bull. Chem. Soc. Japan 1975,48 1063.I. Fleming and T. Mah J.C.S. Perkin I 1975,964;P.G.Sammes and T. W. Wallace ibid. p. 1377. P.Caubere and L. Lalloz J. Org. Chem. 1975,40 2853 2859. I9 Y. Sato T. Toyo'oka T. Aoyama and H. Shirai J.C.S. Chem. Comm. 1975,640. 108 R. C.Storr 2 Nitrenes A griori theoretical studies of the electronic structures of carbonylnitrenes XCON (X =H F Me and OMe) indicate a triplet ground state in all cases with a closely spaced pair of singlet states at higher energy (singlet-triplet separation CQ. 35 kcal for X =H).20Calculations have also been performed for the polar nitrene LiN.21 A critical review concludes that the evidence for phosphindenes R-P as reaction intermediates is largely ambiguous.22 The initial step in the deoxygenation of nitro-arenes with tervalent phosphorous reagents appears to be nucleophilic attack by P on oxygen of the NO group.A survey of such deoxygenating agents indicates that EtOP(NEt,) is particularly reactive but for overall convenience P(OEt) is the best general reagent.23 Disilanes have been used to deoxygenate nitro-arenes at 240 0C.24 Likely examples of aryl- and sulphonyl-nitrene formation by a-dehydration of hydroxylamines have appeared.25 Photolysis of N-arylsulphonyl-S,S-dimethylsulphoximides gives aryl radicals but no sulphonylnitrenes.26 Phenylnitrene has been suggested as an intermediate in the formation of azobenzene in the gas-phase pyrolysis of benzenesulphonyl azide (360"C 0.1 T~rr).~' No Curtius rearrangement occurs in the decomposition of arylsulphinyl azides.Products arising from these very labile azides appear to involve the polar nitrene (12) which does not 0 0 It .-II Ar-S-N Ar-S=NH + ArSO,NH I 0 iyOH II II Ar-S-N + Ar-S=N +-Yo 0 II 0 Ar-S-N=SR, II II Ar-S-N 0 (12) insert but reacts with water as shown and which gives an unexpected adduct with sulphoxides.28 The formation of both aziridines and indoles from oximes (13) and Grignard reagents is consistent with a vinylnitrene intermediate. Only the E-oxime reacts suggesting that a cyclic transition state is involved in the dehydrati~n.~' In both the direct photolysis and thermal decomposition of a selection of steroidal azides the product distribution can be satisfactorily explained by migration of that 2O J.F. Harrison and G. Shalhoub J. Amer. Chem. Soc. 1975,97 4172. 2l C. E. Dykstra P. K. Pearson and H. E. Schaefer J. Amer. Chem. Soc. 1975,97,2321. 22 U. Schmidt Angew. Chem. Intemat. Edn. 1975,14523. 23 M. A Armour J. I. G. Cadogan and D. S. B. Grace J.C.S. Perkin 11 1975 1185. z4 F.-P. Tsui T. M. Vogel and G. Zon J. Org. Chem. 1975,40 761. 25 K. T. Potts A. A. Kutz,and F. C. Nachod Tetrahedron 1975,31,2163,2171. 26 R. A. Abramovitch and T. Takaya J.C.S. Perkin I 1975 1806. 27 W. B. Renfrow and M. Devadoss J. Org. Chem. 1975,40,1525. 28 T. J. Maricich and V. L. Hoffman J. Amer. Chem. SOC.,1974,96,7770. z9 R. Bartnik and A. Laurent Bull. Soc. aim. France 1975 173. Arynes Carbenes Nitrenes and Related Species 109 1 R' PhC R'MgBr phwR1 $Ph + R2 N Ph H a-bond which best overlaps the azide n-system in concert with loss of nitr~gen.~' New systematic kinetic studies of the decomposition of ortho and para-substituted aryl azides confirm that the very large rate enhancement by certain ortho-substituents (PhN=N NO2 or COR) is a genuine neighbouring-group effect.The mechanism of this participation is still a matter for debate. A coupled azide fragmentation-pericyclic reaction in which incipient aromatic character in the developing ring assists loss of nitrogen seems the best description at pre~ent.~' It is a sobering thought that our knowledge of even one of the best known and cleanest 'nitrene' reactions is far from complete. Photolysis of azidobiphenyls gives carbazoles in high yield but by at least two routes one of which involves an intermediate which can be diverted by secondary amines.Mechanistic schemes such as Scheme 3 and Scheme 4 are the simplest that will account for the available data although one involving two intermediates (Scheme 5) is also possible (there is as yet no evidence for A'). The nature of the intermediates A and B is not yet proved but A ArN !% A carbazole ArN !!h A carbazole lk/ RZNH B azepine B k b azepine where k,,k >> k, k where k z k > k-2 Scheme 3 Scheme 4 ArN A' \"; car bazole kz B ",:,, k3 * azepine where k; > k Scheme 5 30 A. Pancrazi and Q. Khuong-Huu Tetrahedron 1975,31,2041,2049. 31 L. K. Dyall Austral. J. Chem. 1975,28,2147. 110 R.C.Storr could reasonably be the singlet nitrene and B the azirine (14) or conceivably an isomer such as the azacycloheptatrienylidene. In the past triplet biphenylnitrene has been advanced as the precursor to carbazole following detection of a transient species with the expected spectrum which apparently was converted into carbazole. Flash-photolytic studies now reveal that the rate of decay of the transient and the appearance of carbazole are significantly different.32 A flash-photolytic study of the formation of 2-substituted-3H-azepinesfrom aryl azides in the presence of secondary amines is basically in line with the generally accepted picture. Two intermediates are detected the first which does not absorb above 300 nm and which has a lifetime of ca.5 ms is believed to be the azirine (15) which is formed rapidly from the singlet nitrene. This species reacts with the amine to give a lhl-azepine (16) which ultimately tautomerizes to the 3H-isomer (17). (16) (17) Kinetic studies reveal that 1W-azepine formation is somewhat more complicated than just attack by the amine on (15) and may involve an isomeric species which is in equilibrium with (15).33 Formation of 2-diethylamino-3H-azepines by deoxygena- tion of nitro-arenes can be interpreted in much the same way there being good evidence for a common intermediate in nitro-arene deoxygenation and azide photo1 ysis. 34 Ring expansion of bicyclic aromatic nitrenes is rare.35 In the case of 0-substituted azides the azirines appear to be formed but open with nucleophiles e.g.amines to give 1,2-diamine~.~~ A procedure has now been developed however for intercept- ing the intermediate azirine with methoxide ion to give aziridines e. g. (18) which are sufficiently stable towards N-C-2 bond cleavage under the basic conditions for thermal rearrangement to azepine (19) to be effected. Immediate neutralization of 32 R. J. Sundberg D. W. Gillespie and B. A. DeGraff J. Amer. Chem. SOC.,1975,97 6193. 33 B. A.DeGraff D. W. Gillespie and R. J. Sundberg J. Amer. Chem. Soc. 1974,96,7491. 3.) T. de Boer J. I. G. Cadogan H. M. McWilliam and A. G. Rowley J.C.S.Perkin 11 1975 554. 35 See for example B. Iddon M. W. Pickering H. Suschitzky? and D. S. Taylor J.C.S.Perkin I 1975,1686; R. N. Carde and G. Jones ibid. p. 519.36 S.E. Carroll B. Nay E. F. V. Scriven and H. Suschitzky Synthesis 1975,710. Arynes Carbenes Nitrenes and Related Species the aziridine solution gives the normal 1,2-disubstituted bicycle (20) and so the mode of reaction can be contr~lled.~' Oxidative ring contraction of 2-aminopyrazolidin-3-ones(21)to /3 -1actams has been achieved by a novel procedure involving addition of aminating agent to the pyrazolidinone anion in the presence of ~xidant.~' A low reaction temperature R c''0 0 !I H,NN9-R'c:fi 0 (21) reveals syn selectivity between heterocyclic ring and vinyl substituent in the addition of phthalimido and related nitrenes to diene~.~' The first 1,4-diazaspiro[2,2 Jpentane (22) has been obtained by double addition of phthalimidonitrene to an a11ene.40 \ (22)Phth Addition of aryl nitrenes to alkenes is very rare and has only been observed for highly electrophilic pentafluorophenyl and related nitrenes full details of which have now a~peared.~' Addition of a sym-triazinylnitrene to a nitrile to give a triazolotriazine has been claimed.42 Thermal decomposition of 3-azidopyridazine N-oxide leads to cleavage of the pyridazine ring.43 A 1,4 H-shift in a vinylnitrene appears to be 37 J.Rigaudy C. Igier and J. Barcelo Tetrahedron Letters 1975 3845. 38 P. Y. Johnson N. R. Schmuff and C. E. Hatch Tetrahedron Letters 1975,4089. 39 R. S. Atkinson and J. R. Malpass J.C.S. Chem. Comm.. 1975 555. 40 R. S. Atkinson and J. R. Malpass Tetrahedron Letters 1975,4305. 41 R. A.Abramovitch S. R. Challand and Y. Yamada J. Org. Chem 1975,40,1541. 42 H. Yamada H. Shizuka. and K. Matsui. J. Org. Chem 1975,40 1351. 43 R. A. Abramovitch and I. Shinkai J.C.S. Chem. Comm. 1975,703. 112 R. C.Storr involved in the pyrolysis of b~t-3-enyl-2H-azirines.~~ Matrix photolysis of 2,2-diazidobiphenyl gives benzocinnoline a product notably absent from solution photolysis at room temperatu~e.~~ 3 Carbenes Reviews of the gas-phase reactions of carbene~,~~ of the chemistry of phosphonylcar- bene~,~' and of the photochemical ring expansion of cycloalkanones to oxacar- bene~~~ have appeared. There is further kinetic evidence for a sizeable energy separation (ca. 38 kJ mol-') between triplet and singlet meth~lene.~~ Equation (1)provides a successful correlation between carbene selectivity for the addition of free CXY to simple alkenes (Mcxyis a measure of selectivity relative to CCI,) and substituent parame- ters for both X and Y and so paves the way for a discussion of structure-selectivity relationship^.^' Further use has been made of the addition of crown ether to give free carbenes in butoxide-induced a- elimination^.^' The use of this technique reveals that PhCX carbenoids are less selective than the free carbenes in alkene additions.The presence of K'Cl- in the transition state leads to greater charge dispersal hence less positive charge localization on the alkene and therefore damped discrimination. The greater the internal stabilization of the carbene the greater its tendency to be free in the presence of KOBu'." The catalysis by trial kylamines of the two-phase (CHC1,-aq.NaOH) generation of CCl is attributed to transport of CCl into the organic phase as an ammonium ylide (23).The carbene is regenerated from the ylide by reaction with chloroform. Such :cc12(as) phase . + + -R3N-CC12(org) GR3kHC12 CCI3 -+ R3kHC12 ci+ :cci2 R3N (0%) (23) tertiary amine catalysts are superior to the normal phase-transfer catalysts for the two-phase generation of CBr2.52 p-Hydroxyethyltrialkylammoniumsalts as phase- transfer catalysts lead to production of highly selective CC12 in contrast to the normal catalysts. They also allow a low degree of asymmetric induction to be obtained in the carbene additions.53 44 A. Padwa and N.Kamigata J.C.S. Chem. Comm. 1975,789. 45 A. Yabe and K. Honda Tetrahedron Letters 1975,1079. 46 M. Jones Accounts Chem. Res. 1974,7,415. 47 M. Regitz Angew. Chem. Internat. Edn. 1975 14 222. 48 P. Yaks and R. 0.Loutfy Accounts Chem Res. 1975,8,209;see also P. Yates and J. C. L. Tam J.C.S. Chem. Comm. 1975,737,739. 49 H. M. Frey and G. J. Kennedy J.C.S Chem. Comm. 1975,233. 513 R. A. Moss and C. B. Mallon J. Amer. Chem. SOC. 1975,97 344. 51 R. A. Moss M. A. Joyce and F. G. Pilkiewin Tetrahedron Letters 1975 2425; see also ref. 70. 52 M. Makosza A. Kacprowin and M. Fedorysnki Tetrahedron Letters. 1975 21 19. s3 T. Hiyama H. Sawada M. Tsukanaka and H. Nozaki Tetrahedron Letters 1975 3013. Arynes Carbenes Nitrenes and Related Species Some uncertainty surrounds the reactive species in a-elimination routes to carbenes and the term carbenoid is generally used for any species (usually ill- defined) on the continuum from free carbene to a-halogenomethylmetal.In this connection a carbenoid of specific structure most probably (24) has been detected V,C1 c1-M:c ()'a (24) together with CCI and CC13M in the reaction of alkali-metal atoms with CC1 in an argon Also chlorocyclopropanation with LiCHCl, via nucleophilic addi- tion and 1,3-elimination can be observed in the favoured case of the 5,6-double bond of fulvenes; significantly chlorocarbene produced from MeLi and CH,Cl gives diff erent products. 55 The effect of added scavenger on the stereochemistry of carbene addition has been used to determine the multiplicity of a carbene at its inception where singlet-triplet equilibration makes the usual trap-dilution technique ineffective.Application to the benzocycloheptatrienylidene-2-naphthylcarbenerearrangement indicates that the latter is formed as a singlet.56 It is now clear that carbene formation is a general process in the photolysis of a1 kenes (Scheme 6).57Pyrolysis of the readily available Meldrum's acid derivatives t Scheme 6 (25) has been widely exploited as a route to carbenes and keten~.~~ 3-Aryl-3H-diazirines have some advantages over diazo-compounds as arylcarbene precursors. 0 R' R' R2R1<Ix ___) \ C=C=OR2/ or \ / C R2 0 (25) They are more stable thermally and decompose to the carbene either directly or via prior isomerization to the diazo-cornpour~d.~~ There is some evidence for the 54 D.A. Hutzenbuhler L. Andrews and F. A. Carey J. Amer. Chem. SOC.,1975,97 187. 55 A. Amaro and K. Grohmann J. Amer. Chem. SOC.,1975,97,3830. 56 K. E. Krajca and W. M. Jones Tetrahedron Letters 1975 3807. 57 S. S. Hixson J. Amer. Chem. Soc. 1975 97 1981;S. S. Hixson J. C. Tausta and J. Borovsky ibid.,p. 3230; T. R. Fields and P. J. Kropp ibid. p. 7559; Y. Inoue S. Takamuku and H. Sakurai J.C.S.Chem. Comm. 1975,577. 5s G. J. Baxter R. F. C. Brown and G. L. McMullen Austral. J. Chem. 1974 27 2605 and references therein; G. J. BaxterandR. F. C. Brown ibid. 1975,28,1551;R. F. C. Brown,F. W. Eastwood andG. L. McMullen J.C.S. Chem. Comm. 1975 328; G.J. Baxter R. F. C. Brown F. W. Eastwood and K. J. Harrington Tetrahedron Letters 1975,4283. 59 R. A. G. Smith and J. R.Knowles J.C.S.Petkin 11 1975 686. 114 R. C.Storr formation of the aromatic nucleophilic carbene (27) in the photolysis of the azirine (26).60Photolysis of the azirine (28)gives the nitrile ylide (29),which in the absence of dipolarophiles behaves as a carbene to give the intramolecular [2 +13cycloadduct (30). Interestingly this process proceeds with inversion of alkene stereochemistry making it the first case of 02s +m2a carbene addition.61 The first clear example of carbene formation by loss of SO from a sulphene has been claimed,6z and the first arylsulphinylcarbene a highly stereoselective cyclo- propanating agent has been obtained from phenyldiazomethyl s~lphoxide.~~ The 0 carbene (3 1)has been generated by base-induced 1,5-elimination from the corre- sponding p-i~dophenol.~~ Copper salts are commonly used as catalysts in the carbenic decomposition of diazo-compounds a1 though the mechanism of their action is not yet clear.It has now been shown that tetraphenylethylene can catalyse these decompositions in much the same way even giving cyclopropanation to an equivalcnt extent. The reaction is obviously complex but a one-electron oxidation of the diazoalkane is probably a key The mode of decomposition of the tosylhydrazone salts (32) depends largely on steric factors associated with ring size. For n =2 pyrazole formation occurs; this is disfavoured for n = 1,and carbenic products result except where R' and RZare aryl in which case 3H-1,2-benzodiazepines are formed by 1,7-dipolar cyclization of the intermediate diazo-compound.66 60 A.Padwa and J. K. Rasmussen J. Amer. Chem. SOC. 1975 97,5912. 61 A. Padwa and P. H. J. Carlsen J. Amer. Chem. SOC.,1975,97,3862. 62 B. E. Sarver M. Jones and A. M. van Leusen J. Amer. Chem. SOC.,1975,97,4771. 63 C. G. Venier H. J. Barager and M. A. Ward J. Amer. Chem. SOC.,1975 97 3238. P. Bartholmei and P. Boldt Angew. Chem. Internat. Edn. 1975 14 64. 65 C. T. Ho R. T. Conlin and P. P. Gaspar J. Amer. Chern. Soc. 1974,96,8109. 66 J. T. Sharp R. H. Findlay and P. B. Thorogood J.C.S. Perkin I 1975 102. Arynes Carbenes Nitrenes and Related Species R' dR2 n=I ~ @R2 Interest continues in the ketocarbene eoxiren eq~ilibrium.~~ The isomeric carbena-oxiran system (34) has now been suggested as a possible intermediate in the formation of ketens from the norbornadiene derivatives (33).No evidence for its (33) equilibration with an oxiren was found.68 The photochemical and acid-catalysed decomposition of a-bisdiazo-ketones appears to involve initial formation of cyclo- propenones rather than Wolff rearrangements. This is certainly the case for 1,3- bisdiazo-1,3-diphenylpropan-2-one.Thermal decomposition of the latter is entirely different and leads to the novel 2,5-dipheny1-3,4-dia~apentadienone.~~ Isopropylidenecarbene generated from the vinyl triflate with KOBu' is a free ~inglet,~' and much less electrophilic than when generated by alternative route^.^' Such vinylidenecarbenes provide a route to methylenecyclopropenes by low- temperature addition to alkyla~etylenes,~~ and they are also involved in the forma- tion of acetylenes from KOBu' and vinyl triflates bearing a The resonance-stabilized vinylidenecarbenes (35) are generated from 1-bromoalk- 1-ynes with alkoxide This type of carbene shows a marked tendency to insert + RCH=C=C c-) RCH-C-C-(35) 67 K.-P.Zeller Chem. Ber. 1975,108,3566. 68 R. W. Hoffmann and R. Schuttler Chem. Ber. 1975,108,844. 6q B. M. Trost and P. J. Whitman J. Amer. Chem. SOC.,1974,96,7421. 70 P.J. Stang and M. G. Mangum J. Amer. Chem. SOC. 1975,97 1459,6478. 71 T.Bally and E. Haselbach Helv. Chim. Am 1975,58 321.72 P.J. Stang and M. G. Mangum J. Amer. Chem. SOC.,1975,97,3854. 73 P.J. Stang J. Davis and D. P. Fox J.C.S. Chem. Comm. 1975 17. 74 C.D.Beard J. C. Craig and M. D. Solomon,J. Amer. Chem. SOC.,1974,% 7944; C.D. Beard and J. C. Craig ibid. p. 7950. 116 R. C. Storr into C-H bonds a to oxygen. For primary alcohols this specificity can be attributed to a favourable orientation of the vacant carbene orbital brought about by H-bonding through the nucleophilic C-1(36);for alkoxides complexation via the electrophilic C-3(37)can be envisaged.74375 C,C 3 ,' 1 HR Rearrangements.-Further calculations suppert tIie non-least-motion pathway for rearrangement of methylcarbene to ethylene. The process starts with the C-H a-bond electron density flowing towards the vacant p-orbital but finishes with the H becoming bonded to the orthogonal lone-pair orbital (38).76 Support for the (38) generally assumed involvement of the singlet state in carbene 1,2-shifts comes from the singlet- and triplet-sensitized photolysis of diazophenylethane.In the presence of an alkene the former gives a high yield of styrene and there is stereospecific cyclopropanation whereas the latter gives only a trace of styrene and there is non-stereospecific cycl~propanation.~~ Interest in the isomerization processes of arylcarbenes continues. For the pyridyl- carbene system studies with the picolylcarbenes where the methyl group interrupts the isomerization process by the formation of cyclobuta-[b]-and -[c]-pyridines reveal that the 2-carbene proceeds mainly and effectively irreversibly to the tolylnitrene whereas for the 3-and 4-carbenes the carbene centre tends to oscillate over the 3- 4- and 5-positions with slow leakage to the 2-po~ition.~' A phenylcarbene-cycloheptatrienylidene rearrangement and not a silacyclopropane intermediate is involved in the pyrolysis of phenyl trime th ylsilyldiazome thane.79 Evidence for the rearrangement of silylcarbenes to silaethylenes has appeared.80 The 75 T. B. Patrick and D. L. Schutzenhofer Tetrahedron Letters 1975 3259. 76 J. A. Altmann I. G. Csizmadia and K. Yates J. Amer. Chem. Soc. 1975,97 5217. 77 S. Yamamoto S. I. Murahashi and 1. Moritani Tetrahedron 1975,31,2663. 78 W.D.Crow A. N. Khan and M. N. Padden-Row Austral.J. Cltem.,1975; 28,1741; W.D.Crow and M. N. Padden-Row ibid. p. 1755;W.D.Crow A. N. Khan M. N. Padden-Row and D. S. Sutherland ibid. p. 1763. 79 T.J. Barton J. A. Kilgour R. R. Gallucci A. J. Rothschild J. Slutsky A. D. Wolf and M. Jones J. Arner. Chern. SOC.,1975.97 657. 8" W. Ando A. Sekiguchi J. Ogiwara and T. Migita J.C.S.Chern. Cornrn.,1975 145; R. L.Kreeger and H. Shechter Tetrahedron Letters 1975 2061; see however W.Ando A. Sekiguchi T. Migita S. Kammula M. Green and M. Jones J. Amer. Gem. SOC.,1975,97,3818. Arynes Carbenes Nitrenes and Related Species carbene (39) undergoes the novel rearrangement shown." 2,3-Homocycloheptatrienylidene (40) undergoes electrocyclic ring expansion to the (40) (41) allene (41);82formation of the cyclic allene (43) from (42) however involves a cyclopropylcar bene fragmentation.83 Insertions.-Although the formation of abstraction-recom bination products in the photosensitized decomposition of diazo-compounds can be explained in terms of insertion by the triplet carbene an alternative chemical-sensitization mechanism in which the sensitizer abstracts H from the substrate and the resulting radicals induce decomposition of the diazo-compound has to be considered. Such a mechanism has been ruled out by CIDNP studies for the irradiation of methyl diazoacetate in the presence of benzaldehyde. In this case the triplet carbene is produced by energy transfer from benzaldehyde to the diazo-compound and it readily abstracts H from hydrocarbons and ben~aldehyde.~~ Triplet vinylmethylene produced by photosen- sitized decomposition of diazopropene undergoes intermolecular insertion reac- tions in contrast to the singlet which gives only intramolecular products.CIDNP studies indicate that the reactions proceed via radical abstraction-rec~mbination.'~ The CIDNP signals observed in the insertion of singlet methoxycarbonylcarbene (produced by thermal decomposition of methyl diazoacetate) into ether C-0 bonds are consistent with formation of an ylide followed by a Stevens rearrangement via homolysis and recombination.86 Phase-transfer catalysis has been used to effect dihalogenocarbene C-H inser-tions in hydrocarbons and Intramolecular C-Br insertion of a carbene has provided the first naphthalene with a single carbon peri bridge.88 81 M.Christ1 and M. Lechner Angew. Chem. Internat. Edn. 1975 14,765. 82 M. Oda Y. It6 and Y. Kitahara Tetrahedron Letters 1975 2587. 83 W. R. Dolbier 0.T. Garza and B. H. Al-Sader J. Amer. Chem. SOC.,1975,97 5038. H. D. Roth and M. L. Manion J. Amer. Chem. SOC.,1975,97 779. x5 M. L. Manion and H. D. Roth J. Amer. Chem. SOC.,1975,97,6919. Hh H. Iwamura and Y. Imahashi Tetrahedron Letters 1975 1401. 87 S. H. Goh K. C. Chan T. S. Kam and H. L. Chong Austral. J. Chem. 1975 28 381. 88 R. J. Bailey and H. Shecter J. Amer. Chem. Soc. 1974 96 8116. 118 R. C.Storr Additions.-Papers concerning carbene additions have ranged from the purely physical organic studies of the kinetics of the gas-phase reaction of singlet CH2 with methylenecycl~propane~~ to numerous routine synthesis of cyclopropanes.Points of interest not mentioned elsewhere include the first example of the addition of an a-oxocarbene to an aromatic system to give a stable norcaradiene,” the irradiation of FCHI in the presence of alkenes as a route to monofluorocyclopropanes,9’ satisfactory copper-catalysed vinylcyclopropanation with diaz~rnethane,~~ and the use of zinc catalysts in cyclopropanation with diaz~methane.~~ The carbenoid reagent prepared from diethylzinc and CHBr in the presence of O2is a good reagent for monobromocyclopropanation.94 89 H. M. Frey G. E. Jackson R. A. Smith and R. Walsh J.C.S. Faraday I 1975,71 1991. 90 C. G. F. Bannerman J. I. G. Cadogan I. Gosney and N. H. Wilson J.C.S.Chem. Comm. 1975,618. 91 J. L. Hahnfeld and D. J. Burton Tetrahedron Letters 1975 1819. 92 R. G. Salomon M. F. Salomon and T. R. Heyne J. Org. Chem. 1975,40,756. 93 D. S. Crumrine T. J. Haberkamp and D. J. Suther J. Org. Chem. 1975,40 2274. 94 S. Miyano Y. Matsumoto and H. Hdshimoto J.C.S. Chem. Comm. 1975 364.