5 Arynes Carbenes Nitrenes and Related Species By M. S. BAlRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl 7RU 1 General The transition states for insertion of methylene and silylene into the C-H and Si-H bonds of methane and silane have been probed by ab initio methods at the 3-21 G level. Only the insertion of silylene into methane has a non-zero barrier.’ A6 initio studies of the insertion of methylene silylene and various fluorinated deriva- tives into hydrogen reveal a dramatic increase in the barrier height with increasing fluorine substitution.2 MNDO methods have been used to study the 1,2-rearrangement of substituted singlet methylcarbenes carbonylcarbenes methylnitrenes and carbonylnitrenes. The C-R bond of the migrating R-group prefers to be eclipsed to the vacant p-orbital of the carbene or nitrene and the migratory aptitudes of various groups are explained in terms of nucleophilicity and leaving group character of R.3 2 Arynes Generation of a 1,2-doubly labelled benzyne from phthalic anhydride at 830°C leads to biphenylene with a rearrangement of the label consistent with a 1,2- to 1,3-rearrangement either in the benzyne itself or in an intermediate C7H40.Forma-tion of the carbene (1) is one possible source of the ~crambling.~ The isolation of acridines from the reaction of benzyne with imines such as N-benzylideneaniline and with amidines such as N,N-dimethyl-N’-phenylformamidinehas been taken as unambiguous evidence for formation of an intermediate benzazetidine formed by [2 + 21 cycloaddition (Scheme l).5 Trapping of benzyne by 1,2-diketone monoketal enolates leads to benzocyc- lobutenes (2) in reasonable yield while thi-iranes react with benzyne in a stereos- pecific reaction which leads to phenyl vinyl sulphides ;thus cis-2,3-diphenylthi-irane is converted into cis-(pheny1thio)stilbene (79%).The reaction is thought to proceed by way of betaines (3).’ ’ M. S. Gordon and D. R. Gano J. Am. Chem. Soc. 1984 106 5421. ’ C. Sosa and H. B. Schlegel J. Am Chem. SOC.,1984 106 5847. G. Frenking and J. Schmidt Tetrahedron 1984,40 2123. M. Berry R. F. C. Brown F. W. Eastwood D. A. Gunawardana and C. Vogel Aust. J. Chem. 1984 37 1643. C. W. G. Fishwick R. C. Gupta and R. C. Storr J. Chem. SOC.,Perkin Trans.1 1984 2827. M.-C. Carre B. Gregoire and P. Caubere J. Org. Chem. 1984 49 2050. ’ J. Nakayama S. Takeue and M. Hoshino Tetrahedron Lett. 1984 25 2679. 79 0,H M. S. Baird + MezN>NPh -Q)-J,NMe2 Ph 1 H NMe, =+m+ Scheme 1 Substituted 3-amidobenzynes have been generated under neutral mild conditions by reaction of 2-trimethylsilyl-3-triflyl benzamides (4) with fluoride ion; the benzynes form [4 + 21 cycloadducts with dienes and undergo attack of nucleophiles to produce rn-disubstituted benzamides.* Benzynes derived from 3-halogenoaryloxazolines react with a-lithio-alkyl nitriles to give (5)in a reaction thought to involve addition directed by the oxazoline to produce (6) followed by cyclization to (7) and ring- opening.' The reaction of (8) with lead tetra-acetate provides the formal equivalent of 1,4-dibenzyne which is trapped by double addition to dienes in relatively good yield.This method is somewhat more tolerant of functionality in the diene than dehalogena- tion of tetrahalogenoarenes with an organolithium and with unsymmetrical dienes leads predominantly to one regioisomer e.g. (9) from 3-bromofuran." Thermal decomposition of ( 10) provides a convenient source of 3,4-didehy- dropyridine. This has been trapped by (11) to provide after decarboxylation a ' K. Shankaran and V. Snieckus Tetrahedron Left. 1984,25 2827. A. 1. Meyers and P. D. Pansegrau Tetrahedron Lett. 1984 25 2941 H. Hart and D. Ok Tetrahedron Lett. 1984 25 2073. 81 Arynes Carbenes Nitrenes and Related Species I N."*\\ N Brm Rr simple route to elliptycene (12) ;unfortunately the cycloaddition shows no regioselec- tivity and isoelliptycene is also obtained." Trapping of (13) with the pyridyne also provides a route to elliptycene though in this case isoelliptycene is the major product and overall yields are 10w.l~ Me Me I Me S02Ph Me MNDO calculations on all didehydro-derivatives of pyridazine pyrimidine and pyrazine suggest that these species generally resemble benzyne but there are some interesting differences. For example the two o-pyridazynes are predicted to be of very different energies and several of the species are expected to ring-open or lose nitrogen relatively easi~y.'~ 3Carbenes A re-examination of the reaction of singlet methylene with dialkyl ethers has been carried out using CD2N2.It is found that in addition to insertion of the carbene into various C-H bonds about 10% of the product consists of methyl alkyl ethers and dimethyl ether.This indicates that carbene attack takes place on the ether oxygen to produce ylides which are protonated by traces of methanol or water and then dea1k~lated.l~ The addition of difluorocarbene to propene has been examined by ab initio methods using the 3-21 G basis set. Two equal-energy transition structures have I' C. May and C. J. Moody J. Chem. SOC. Chem. Commun. 1984 926. 12 G. W. Gribble M. Saulnier M. P. Sibi and J. A. Obaza-Nutaitis J. Org. Chem. 1984 49 4518. l3 M. J. S. Dewar and D. R.Kuhn J. Am. Chem. SOC.,1984 106 5256. 14 G. A. Olah H. Doggweiler and J. D. Felberg J. Org. Chem. 1984 49 2116. 82 M. S. Baird been located with the fluorines approaching syn-or anti-to the methyl group; an activation energy 1.3 kcal mol-' lower than that for addition to ethene is f~und.'~ While calculations show that difluorocarbene does form a weakly bound complex with ethene as an intermediate in cyclopropanation no stable complex is predicted in the addition of dichlorocarbene.'6 Where complexes are found they are not likely to energy minima at room temperature and ether or hydrocarbon solvents should be better Lewis base co-ordinators than the alkenes. The conclusion that stable ncomplexes are not formed between reactive carbenes and alkenes requires a new explanation of negative activation energies and of entropy control of carbene selectivity.A model relating the position of the transition state in additions of :CX2 (X = F C1 Br) to alkenes to AH and -TAS has been presented and this shows that the AG maximum which corresponds to the transition state for reaction does not correspond to the AH maximum. In contrast to the .rr-complex model this one predicts that reactions of :CC12 :CClBr and :CBr2 will be unselective and diffusion- controlled at low temperature." The use of diazirines as sources of carbenes has continued to provide a wealth of interesting results. Fluorophenylcarbene generated from photolysis of the corre- sponding diazirine shows a relative reactivity very similar to that observed when the carbene is generated by an a-elimination from PhCHBrF in the presence of a crown ether.'* Chlorophenylcarbene generated by thermolysis of the diazirine is trapped with about equal efficiency by both vinyl ethers and a,p-unsaturated esters.The rate of nitrogen evolution is not markedly dependent on the alkene and cyclopropane formation is remarkably free of competing reactions such as formal C-H insertion. Reaction with dimethyl fumarate leads to a single cyclopropane but with dimethyl maleate the same (E) product is observed together with the two possible 2-diesters. The authors propose a mechanism involving formation of a nucleophilic ylide between the maleate (B) and the carbene followed by Michael addition of this to maleate to produce (14) and cyclization of this with elimination of (B).19 Phenylbromodiazirine reacts with a variety of primary and secondary amines at 25 "C to produce aminophenylcarbenes (19 probably through the corresponding aminophenyldiazirine.When R = H the product is an imine presumably formed by a 1,2-hydrogen shift but when R = R' = alkyl the product is (16) derived by a formal insertion of the carbene into the N-H bond of a second molecule of amine. The carbenes can also be trapped by intramolecular insertion into N-H or 0-H bonds when these are 5,6-related to the carbene centre; thus reaction of the diazirine with N-methylethanolamine leads to (17). Although the philicities of these species remain to be determined their behaviour in 1,2-shifts and insertions is typical of electrophilic carbenes.20 Irradiation of 3-chloro-3-methoxydiazirinein an argon matrix at 10 K leads to a photolabile product which is characterized as methoxychlorocarbene and which decomposes to acetyl chloride ketene and HCl ;this is in marked contrast to normal N.G. Rondan and K. N. Houk Tetrahedron Lett. 1984,25 5965. 16 K. N. Houk N. G. Rondan and J. Mareda J. Am. Chem. SOC., 1984 106 4291. I' K. N. Houk and N. G. Rondan J. Am. Chem. Soc. 1984 106,4293. 18 R. A. Moss and W. Lawrynowin 1. Org. Chem. 1984 49 3828. 19 M. P. Doyle J. W. Terpstra and C. H. Winter Tetrahedron Lett. 1984 25 901. 20 R. A. Moss D. P.Cox and H.Tomioka Tetrahedron Lett. 1984 25 1023. Arynes Carbenes Nitrenes and Related Species Ph 'C /R-N I R' PhCH(NRR') H N I Me (16) (17) thermal decomposition of the carbene.In a 3-methylpentane glass at 80 K (18) and (19) are formed.2' The photochemical or thermal decomposition of 3-chloro-3-benzyldiazirine leads to benzylchlorocarbene which either rearranges to E -and Z-chlorostyrenes or reacts with the environment. In the presence of acetic acid the main product is l-chloro-2- phenylethyl acetate and experiments with ['H,]acid show that some styrene is formed from the carbocation ;however cyclopropanation of 2,3-dimethylbut-2-ene shows that the carbene is formed even in the presence of the acid.22 Photolysis of the diazirine in the presence of alkenes leads to cyclopropanes as well as to the isomeric chlorostyrenes.An analysis of the E/Z ratio in the presence of alkene and the fact that the ratio falls in the presence of electron-withdrawing groups on the aromatic ring have been used to analyse the stereochemistry of the migration while the non-linear dependence of product distribution upon the alkene concentra- tion has been interpreted in terms of an intermediate in the addition of the carbene to the alkene.23 However a re-analysis of the kinetic equations presented in this paper has shown that in fact the results provide no evidence that there is a carbene-alkene complex!24 Singlet arylchlorocarbenes rearrange to E -and 2-p-chlorostyrenes but do not undergo phenyl migration -unlike 1,2-diphenylethyl- idene. In ethanol an additional product is the acetal(20; R = Et) which is derived from the product of insertion of the carbene into the 0-H bond.Ratios of alkenes to acetal increase as the p-aryl substituent changes from C1 to H to Me in support of a mechanism for hydrogen migration which involves considerable hydride charac- ter.25 The presence of ethanol also changes the E/Z ratio of Llkenes produced probably due to differences in the reactivity towards alcohols between the carbene conformations leading to each alkene. Relative yields of alkenes decrease at low temperature but increase sharply as the solid state is rea~hed.~' Trapping of ben- zylchlorocarbene by methanol which leads to (20; R = Me) has been shown to be termolecular with a frequency factor of 2 x lo5l2 mo1-2 s-' and an activation energy 21 R.S. Shendan and M. A. Kasselmayer,J. Am. Chem SOC.,1984 106 436. 22 M. T. H. Liu N. H. Chishti M. Tencer H. Tomioka and Y. Izawa Tetrahedron 1984,40 887. 23 H. Tomioka N. Hayashi Y. Izawa and M. T. H. Liu J. Chem. Soc. Chem. Commun. 1984 476; J. Amer. Chem SOC.,1984 106,454. 24 P. M. Warner Tetrahedron Lett. 1984 25 4211. 25 H. Tomioka N. Hayashi Y. Izawa and M. T. H. Liu Tetrahedron Lett. 1984. 25. 4413. 84 M. S. Baird of -4.5 kcal mol-'. The ratio of inter- to intra-molecular products does not vary linearly with methanol concentration and once again this has been interpreted in terms of a model in which a complex is formed reversibly between the carbene and two methanol molecules in the oligomer chain.26 Photolysis of (21) leads to a highly reactive carbene capable of C-H or 0-H bond insertion; the use of this diazirine in photo-affinity labelling is being examined." Ultra-violet excitation of diphenyldiazomethane leads to singlet diphenylcarbene from the excited state of the diazo-compound.Energy relaxation occurs through inter-system crossing to the triplet. The rate of this process has been measured using laser-induced fluorescence and is found to be strongly dependent on solvent polarity. The rate is higher in less polar solvents and there is a linear correlation between log(rate) and the empirical solvent polarity parameter ET(30).The singlet is more polar and is stabilized by interaction with the solvent; this infers that the rate of inter-system crossing increases as the S-T energy gap increases.28 The triplet carbene adds to ring-substituted styrenes as an ambiphile -reacting more rapidly with both electron-poor and electron-rich styrenes than with styrene itself presumably due to stabilization of a diradical intermediate by the aryl Diphenylcarbene also reacts with oxygen at a rate of 5 x lo9 M-'s-' at 300 K leading to an intermediate with an absorption maximum at ca.410nm which is believed to be the carbonyl oxide (22) the precise structure of which is unknown.30 CO,H I Ph,COO (22) The singlet carbene (23) inserts into 0-H bonds of alcohols to produce ethers whereas the triplet forms cyclopropanes in a non-stereospecific manner with alkenes and abstracts hydrogen from hydrocarbons and alcohols. Kinetic and product analysis shows that singlet-triplet crossing is slow compared with most bimolecular reactions of the triplet unlike the situation with fluorenylidene.In this case this is believed to be a consequence of a larger S-T gap.3' The chemistries and kinetics of six other diarylcarbenes in polycrystalline methanol at 77 K have been studied by means of e.s.r. and isotope effects for reaction with deuterated solvent. The singlet-triplet separation decreases in the order (24) diphenylcarbene (25) 1-naphthylcarbene fluorenylidene and (26).32 26 M. T. H. Liu and R. Subramanian J. Chem. SOC.,Chem. Commun. 1984 1062. 21 M. Nassal J. Am. Chem. SOC 1984 106 7540. 28 E. V. Sitzmann J. Langan and K. B. Eisenthal J. Am. Chem. SOC.,1984 106 1868. 29 H.Tomioka K. Ohno Y. Izawa R. A. Moss and R. C. Munjal Tetrahedron Lett. 1984 25 5415. 30 N. H. Werstiuk H. L. Casal and J. C. Scaiano Can. J. Chem. 1984 62 2391. 31 S. C. Lapin B.-E. Brauer and G. B. Schuster J. Am. Chem. SOC.,1984 106 2092. 32 B. B. Wright and M. S. Platz J. Am. Chem. SOC.,1984 106 4175. Arynes Carbenes Nitrenes and Related Species mw / / ? Carbene (27) generated by flash thermolysis of the tosyl hydrazone sodium salt in the gas phase is largely converted into 1- and 2-methylanthracenes through trapping of the corresponding anthrylcarbenes. In the condensed phase (27) can be trapped by addition to alkenes while (28) -an intermediate in the formation of the rearranged carbenes -may be trapped by [4 +21 cycloaddition.It is suggested that the singlet and triplet species (27) and cyclopropene (28) are in eq~ilibrium.~~ The use of excimer laser flash photolysis in studying the kinetics of carbene reactions has been reviewed.34 Laser flash photolysis of diphenyldiazomethane leads to triplet diphenylcarbene which may be characterized by an absorption at 314 nm. The carbene abstracts a hydrogen atom from cyclohexane toluene or cyclopentane and the observed activation energies for these processes are much lower than those extrapolated from matrix experiments. The results clearly show the radical nature of the carbene which is found to be much more reactive than the diphenylmethyl radical.35 The activation energy for reaction of triplet diphenylcarbene with methanol has been measured in various solvents; e.g.the figure in acetonitrile is 1.66 * 0.20 kcal mol-'. The results are inconsistent with earlier interpretations and may suggest a mechanism involving thermal triplet-singlet equilibrium followed by reaction of the latter with methanol.36 The singlet and triplet states of dimesitylcarbene show quite different chemistries the former reacting with cg. methanol and cyclohexa-l,3-diene while the latter 33 A. Hackenberger and H. Dun,Chem. Ber. 1984 117 2644. 34 D. Griller A. S. Nazran and J. C. Scaiano Acc. Chem. Res. 1984 17 283. 35 L. M. Hadel M. S. Platz and J. C. Scaiano J. Am. Chem. SOC.,1984 106 283. 36 D. Griller A. S. Nazran. and J. C. Scaiano. 1.Am. Chem. Soc. 1984 106 198. M. S.Baird dimerizes to alkenes or reacts with oxygen. The steric effect of the o-methyl groups has a considerable effect on this chemistry the triplet state being orders of magnitude less reactive to standard substrates than those of diphenylcarbene or fluorenylidene while the singlet shows all the normal reactivity expected of a diarylcarbene. Although singlet to triplet intersystem crossing is very efficient the reverse process is not and the sharp differences in the chemistries of the two species suggest a substantial free-energy separation between them.37 A series of carbenes (29) has been generated by photolysis of the corresponding diazo-compound in paraffin matrices at 93 K. When n = 9,11 or 12 persistent e.s.r. signals are observed but with n = 8 or 10 no signals are seen.The e.s.r. characteristics have been correlated with the shapes of the carbene~.~~ Laser flash photolysis of 9-diazofluorene leads to a transient species characterized as the triplet carbene which is believed to be in thermal equilibrium with the singlet -their energies being within a few kcals of each other. The singlet is frequently more reactive and tends to dominate the chemistry; the lifetime of this is s5 ns. In nitrile solvents a nitrile ylide is formed which reacts rapidly with electron-deficient alkene~.~~ Photolysis of 9-diazofluorene in trans-1,2-dichloroethene leads to >92% stereospecific addition; the stereoselectivity of addition to the cis-isomer is <86'/0. In the presence of styrene or butadiene the selectivity increases and absolute and relative yields of 9-( 2,2-dichloroethylidene)fluorene,a rearrangement product from a diradical intermediate fall.It is suggested that both singlet and triplet species are present and the former undergoes stereospecific cis-addition and the latter leads to mixtures of stereoisomeric products as well as to diradical rearrangement. The S-T interconversion is rapid compared to reactions with dichloroethene and methanol but not as rapid as triplet scavenging by styrene or butadiene. The effect of added hexafluorobenzene may be to form a carbenoid which mimics triplet fl~orenylidene.~' Irradiation of 2-halogeno- 1,3-diphenylindenyl anions causes loss of halide ion to produce an intermediate best described as the carbene (30) or the related allene form; this undergoes ready C-H insertion addition to electron-rich alkenes and halide exchange!l The heterocyclic carbene (31) has been generated thermally or photochemically from the corresponding diazo-compound ; it inserts into C-H bonds e.g.in cyclohexane in competition with hydrogen ab~traction.~~ Ph 37 A. S. Nazran and D. Griller J. Am. Chem. SOC.,1984 106 543. 38 R. Alt H. A. Staab H. P. Reisenauer and G. Maier Tetrahedron Lerr. 1984 25 633. 39 D. Griller L. Hadel A. S. Nazran M. S. Platz P. C. Wong T. G. Savino and J. C. Scaiano J. Am. Chem. SOC.,1984 106 2227. 40 P. P. Gaspar C.-T. Lin B. L. W. Dunbar D. P. Mack and P. Balasubramanian J. Am. Chem. SOC. 1984 106 2128. 41 L. M. Tolbert and S. Siddiqui J. Am.Chem. SOC.,1984 106 5538. 42 M. Nagarajan and H. Shechter J. Org. Chem. 1984 49 62. Arynes Carbenes Nitrenes and Related Species Pyrolysis of (32) leads to cyclopropenylidene which is stable in an argon matrix at 10 K and has a singlet ground-state with the aromatic ylide structure (33) making a considerable contribution. Irradiation of the carbene in a matrix leads to propyny- lidene (34).43 (32) (33) (34) (35) There has been continued interest in the chemistry of vinylmethylenes and in their relationship to cyclopropenes. Irradiation of the four diazo-compounds (35 ; R' or R2or R3or R" = Me others = H) in matrices at 10-15 K leads to the four corresponding carbenes which are shown by e.s.r. to have triplet ground-states. Photolysis of p-and rn-tolylcarbenes leads to methylcyclohepta- 1,2,4,6-tetraenes with no evidence for bicyclo[4.1 .O]hepta-2,4,6-trienes.The o-tolylcarbene leads largely to o-xylylene though a small amount of l-methylhepta-l,2,4,6-tetraene is formed; the latter is in turn photolysed to phenylmethylcarbene and thence to styrene.44 Carbon and deuterium labelling studies of the rearrangement of benzocyc- lobutene to styrene at high temperatures have also suggested the formation of intermediate o-tolylcarbene and methyl~ycloheptatetraene.~~ Extensive ab initio studies of the C3H4 energy-surface suggest that the allene to methylacetylene rear- rangement should proceed through vinylmethylene cyclopropene and prop-1 -eny- lidene. The 3A" states of trans and cis vinylmethylenes are shown to have the allylic structure (36) and to be isoenergetic and 46 kcal mol-' above the ground state of methyla~etylene."~ Thermolytic or photolytic ring-opening of a range of aryl- and heteroaryl-substituted cyclopropenes has been rep~rted"~ (for example see Scheme 2).Moreover 1,2-dichloro-3,3-dimethylcyclopropene undergoes ring-opening to car- bene (37; X = Y = C1) even at temperatures below ambient and can be trapped Ph Scheme 2 43 H. P. Reisenauer G. Maier A. Riemann and R. W. Hoffmann Angew. Chem. Znf. Edn. Engl. 1984 23 641. 44 0. L. Chapman R. J. McMahon and P. R. West J. Am. Chem. Soc. 1984 106 7973. 45 0. L. Chapman and U.-P. E. Tsou J. Am. Chem. SOC.,1984 106 7974; W. S. Trahanovsky and M. E. Scribner ibid.7976. 46 N. Honjou J. Pacansky and Y. Hoshimine J. Am. Chem. Soc. 1984 106 5361. 47 A. Padwa U. Chiacchio A. Compagnini A. Corsaro and G. Purrello J. Chem. SOC.,Perkin Trans. 1 1984 2671; A. Padwa M. J. Pulwer and R. J. Rosenthal J. Org. Chem. 1984,49 856; A. Padwa and G. D. Kennedy ibid. 4344; 3113; A. Padwa L. A. Cohen and H. L. Gingrich J. Am. Chem. !984, 106,1065. ' 1 8' Q M. S. Baird readily with alkenes,48 while 1 -bromo-2-chloro-3,3-dimethylcyclopropene (38) opens to both carbenes (37; X = Br Y = C1) and (37; X = C1 Y = Br) at 10 "Cand reacts with methyl lithium at -70 "C in the presence of alkenes to give products apparently derived by trapping of the carbene (39) or a related carbenoid. Labelling studies indicate that C-2 of the cyclopropene becomes the central allenic carbon of (39).49 Reaction of oligohalogenoprop- 1-enes with base provides a convenient source of the corresponding oligohalogenovinylcarbene,50 while reaction of 1,3,3-trichloroprop-1-yne with potassium t-butoxide leads to 3,3-dichloropropa-l,2-dien-1-ylidene;the precursor acetylene may be obtained in situ by dehydrohalogenation of 1,1,3,3- or 1,2,3,3-tetrachloroprop-l-enes or of 1,1,2,3,3-penta~hloropropane.~~ The carbene selectivity index for perchlorovinylcarbene has been found to be ca.0.38 whereas a figure of 0.34 f 0.1 is calculated from the Moss equation; however it is certainly an electrophilic carbene.'* Photolysis of the 5-nitro-3H-pyrazole (40) in furan leads to (41) (42),and (43). The furan (42) appears to be derived by cycloaddition of the nitrovinylcarbene (44) to furan while (41) is the result of ring closure of the carbene to the corresponding cyclopropene followed by cycloaddition.It is felt that the most likely origin of the aldehyde is a rearrangement of an intermediate cyclopropane derived from the carbene and furan. Photolysis of the pyrazole (45) leads to open-chain oximes which are explained in terms of oxygen transfer from the nitro-group to an intermediate carbene (46) followed by rea~~angement.'~ Ngo2 (45) 48 M. S. Baird S. R. Buxton and J. S. Whitley Tetrahedron Lett. 1984 25 1509. 49 M. S. Baird Tetrahedron Lett. 1984 25 4829. 50 W. Gothling S. Keyaniyan and A. deMeijere Tetrahedron Lett. 1984 25 4101. 51 S.Keyaniyan W. Gothling and A. deMeijere Tetrahedron Lett. 1984 25 4105. 52 R. Kostikov and A. de Meijere J. Chem. SOC.,Chem. Commun. 1984 1528. 53 M. Franck-Neumann and M. Miesch Tetrahedron Lett. 1984 25 2909. Arynes Carbenes Nitrenes and Related Species Photolysis of (47) in wet benzene in the presence of 2,3-dimethylbut-2-ene leads to the cyclopropene (48). This is thought to be the result of formation of (49) which cyclizes to the corresponding cyclopropene form and is then hydroly~ed.’~ There are also several reports of differently substituted vinylmethylenes obtained from photolysis of unsaturated epoxides undergoing ring-closure to cyclopropenes e.g. (50)leads to (51).” (50) There has also been continued interest in rearrangement of cycloalkyl carbenes.The effect of alkyl and phenyl substituents on the rearrangement of cyclopropylidenes (52) generated from the corresponding dibromides has been examined. When R’ and R6 are alkyl groups the formation of cyclopentadienes from the rearranged carbene (53) is suppressed and acyclic allenes pred~minate.~~ An analysis of the kinetics of decomposition of the diazo-compound (54) shows that there is a very low barrier to rearrangement of the carbene (55) to (56) in support of earlier work proposing this rearrangement,” while carbene (57) also undergoes a 1,2-alkyl shift leading eventually to dihydr~pentalenes.’~ Temperature and solvent effects in the trapping of the carbenes (55) and (56) by methanol are interpreted in terms of reversible formation of an ylide for the former but of a different mechanism for the latter.59 (54) (55) (56) (57) 54 S.Wolff and W. C. Agosta 1. Am. Chem. SOC. 1984 106 2363 55 A. O’Sullivan B. Frei and 0.Jeger Helu. Chim. Acta 1984 67 815; A. Siewinski B. Henggeler H. R. Wolf B. Frei and 0.Jeger ibid. 120; A. Pascual T. Nishio B. Frei and 0.Jeger hid 129. 56 K. H. Holm and L. Skattebdl Acta Chem Scand. 1984 38 783. 57 P. Warner and I.-S. Chu J. Org. Chem 1984 49 3666. 58 U. H. Brinker and K. Lothar Chem. Lett. 1984,45. 59 P. M. Warner and I.-S. Chu J. Am. Chem SOC. 1984 106 5366. 90 M.S. Baird A full account has also appeared of the use of intramolecular insertion of cyclopropylidenes into C-H bonds in the synthesis of ishwarane aAd ishwarone together with a discussion of insertions in several model systems.60 Intermolecular additions of cyclobutylidene generated by reaction of 1,l-dibromocyclobutane with methyl lithium or from diazocyclobutane have also been reported.61 Reaction of N,N-dialkylated pyruvamides (58) with diethyl (diazomethy1)phos- phonate (59) leads to either (60) or (61) by insertion of an intermediate methylenecar- bene (62) into a 5,6-related C-H bond or a 1,2-shift.Although the reactions were carried out in methanol no intramolecular insertion into the 0-H bond was observed. Moreover in the intermolecular insertion primary C-H bonds appeared to be the most reactive probably reflecting conformational factors leading to pre- ferred orientation of the amide with primary bonds syn to the diazoalkenyl-group of the intermediate.62 R 0 II TR Me-C-C-N A LRI (EtO),P(O)CHN (59) /p Me-CGC-C \ Me%l!l-R' R' 0 The phosphonate route has also been used to generate (63) which rearranges to cyclopentyne and is trapped in a 99% stereospecific [2 + 21 cycloaddition with cis-1-meth~xyprop-l-ene.~~ It is interesting to note that a cyclobutyne (64) is reported to undergo the reverse rearrangement to produce the methylene carbene (65).6" Mide formation is presumably involved in the formation of (66; X = Se Te) from Me,C=C and diphenyldiselenides and ditell~rides,6~ while a N-ylide (67) may well be involved in the transformation of azobenzenes to 2H-indazoles e.g.(68) in reasonable yield on reaction with the same carbene."6 [2H]-Labelling implicates S-ylides in the intramolecular reactions of o-alkylthiophenylcarbenesin the gas phase.67 (63) (64) (65) 60 R.M. Cory L. P. J. Burton D. M. T. Chan F. R. McLaren M. H. Rastall and R. M. Renneboog Can. J. Chem. 1984 62 1908. 61 U. H. Brinker and M. Boxberger Angew. Chem. Int. Edn. Engl. 1984 23,974. 62 J. C. Gilbert and B. K. Blackburn Tetrahedron Lett. 1984,25 4067. 63 J. C. Gilbert and M. E. Baze J. Am Chem. Soc. 1984 106 1885. 64 K.-D. Baumgart and G. Szeimies Tetrahedron Lett. 1984 25 737. 65 P. J. Stang K. A. Roberts and L. E. Lynch J. Org. Chem 1984 49 1653. 66 K. Krageloh G. H. Anderson and P. J. Stang J. Am. Chem. Soc. 1984 106 6015. 67 W. D. Crow and Y.T. Pang Aust. J. Chem. 1984 37 1903. Ayes Carbenes Nitrenes and Related Species Reaction of P-hydroxyselenides (69; R = R’ = alkyl) with thallium ethoxide and chloroform leads to rearranged ketones in a reaction thought to involve dichlorocarbene.68 When R = H the product is the epoxide (70),formed in a stereospecific reaction with inversion at the carbon-bearing selenium.69 Presumably both reactions involve initial ylide-formation between selenium and dichlorocarbene. In the same way optically pure P-ethanolamines are converted into epoxides by reaction with dichlororocarbene in a reaction which proceeds in high yield and with high enantiomeric excess.7o The effect of added ethanol on the efficiency and rate of trapping of dibromocarbene generated under phase-transfer conditions has been disc~ssed.~’ Reaction of the carbene under these conditions with trans-cyclo- octene leads to partial isomerization to the cis-isomer.This has been explained in terms of the formation of an intermediate between the carbene and the trans-alkene which can revert to ~is-alkene.~~ Other unusual reactions to be reported are the apparent 1,4-addition of dibromocarbene to 1,2-dimethylene~ycloheptane,~~ and the formation of (7 1) from quadricyclane and bis(ethoxycarbony1)carbene which has been formulated as an addition of a carbene to two u-bond~.~~ (69) X = Se (71) 4 Nitrenes Ab initio calculations of the reaction of hydrogen with the lowest singlet-state of :NH are rep~rted.~’ Similar calculations on the vinyl azide to 2H-azirine conversion indicate a very weak bond between vinyl nitrene and nitrogen parts of the molecule; a singlet planar nitrene is predicted to cyclize with no activation barrier to give the a~irine.~~ The photochemistry of phenylazide has been examined using laser flash photolysis in inert and nucleophilic solvents.Direct and triplet-sensitized irradiation produces J. L. Laboureur and A. Krief Tetrahedron Lett. 1984 25 2713. 69 J. L. Laboureur W. Durnont and A. Krief Tetrahedron Lett. 1984 25 4569. 70 L. Castedo J. L. Castro and R. Riguera Tetrahedron Lett. 1984 25 1205. 71 E. V. Dehrnlow and J. Wilkenloh J. Chem. Res. 1984 396. 72 E. V. Dehrnlow and R. Kramer Angew. Chern. Int. Edn. Engl. 1984 23 706. 73 L.W. Jenneskens F. J. J. DeKanter L. A. M. Turkenburg H.J. R. DeBoer W. H. DeWolf and F. Bickelhaupt Tetrahedron 1984 40,4401. 74 M. L. Tetef and M. Jones Tetrahedron Lett. 1984 25 161. 75 T. Fueno 0.Kajirnoto and V. Bonacic-Koutecky J. Am. Chem. SOC.,1984 106 4061. 76 T. Yamabe M. Kaminoyarna T. Minato K. Hori K. Isomura and H. Taniguchi Tetrahedron 1984 40.2095. M. S. Baird triplet phenyl nitrene but the reaction products depend dramatically on the con- centration of the azide and the intensity of the irradiation. In the direct irradiation a relatively long-lived singlet precursor of triplet nitrene is also observed; this species is probably the dehydroazepine (72). Formation of triplet nitrene from the transient species is relatively slow compared to other reactions it can undergo.77 Quantum yields for the disappearance of phenyl azide on photolysis in acetonitrile increase exponentially with log(concentration) and can reach three thousand.This provides evidence for a branching chain-reaction which is thought to result from reaction of a phenyl nitrene intermediate with ground-state phenylazide to form two nitrene~.~* An analysis of the photochemistry of 1-and 2-naphthylazides and 1-and 2-pyrenylazides in benzene and diethylamine has also been carried out using product analysis and laser flash spectroscopy. In each case two intermediates were character- ized and identified as the triplet nitrenes and ground-state singlet azirines. The key to the photochemistry seems to be the relative energies of the singlet nitrene and these two species.When the singlet-to-azirine separation is small the lifetime of the azirine is short and its reactions with nucleophiles do not compete efficiently with triplet formation. When the singlet-triplet separation is small inter-system crossing is reversible and singlet species have longer lifetimes than triplets leading to efficient trapping by nu~leophiles.~~ The carbamates (73; X = H Y = C1) and (73; X = C1 Y = H) react with sodium hydride in dimethylformamide at 70°C to give the same azo-compound (74). This has been explained in terms of the formation of the common anion (75) which ring-opens selectively to the more stable nitrene (76).80 c1a N-cozEt N c1n:):Et The bis-azide (77) undergoes thermal decomposition to (78) and (79).The latter apparently arises through loss of nitrogen from an intermediate ylide (80) as indicated followed by ring cleavage with loss of ethyne.81 77 A. K. Schrock and G. B. Schuster J. Am. Chem. Soc. 1984 106 5228. 78 C. H. L. Go and W. H. Waddell J. Am. Chem. Soc. 1984 106 715. 79 A. K. Schrock and G. B. Schuster J. Am. Chem. SOC.,1984 106 5234. R. K. Smalley and A. W. Stocker Tetrahedron Lett. 1984 25 1389. 81 C. J. Moody C. W. Rees and S. C. Tsoi J. Chem. Soc. Perkin Trans. I 1984 915. Arynes Carbenes Nitrenes and Related Species dN \/ When o-azidobiphenyl is photolysed in acetonitrile in the presence of tetracyanoethylene the major product is carbazole derived from the phenylnitrene but a considerable amount of (81) apparently derived by trapping of a 2-azacyc- loheptatrienylidene is also obtained.The formation of the carbene may be explained in terms of a rearrangement of phenylnitrene. Thermolysis of the product leads to carbazole apparently by cheletropic elimination of tetracyanoethylene followed by a rearrangement in the opposite sense -from carbene to nitrene.82 Laser flash photolysis has been used to examine the absolute rate of formation of (82) by intramolecular addition of the corresponding nitrene to the fluorene ring-sy~tem,~~ while spray pyrolysis of aryl 2-azidobenzoates leads to carbazoles in a reaction apparently involving spiro-intermediates (Scheme 3). Scheme 3 H The corresponding benzyl 2-azidobenzoates are converted into (83) in a reaction thought to involve an insertion of a nitrene into the C-0 bond.84 Photolysis of the acylazide (84) which is readily derived from 19,20-dihydromutilin leads to nitrene insertion into the C-H bond indicated despite the proximity of the methyl group at ~-9.~' 82 S.Murata T. Sugawara and H. Iwamura J. Chem. SOC.,Chem. Commun. 1984 1198. 83 S. Murata T. Sugawara N. Nakashima K. Yoshihara and H. Iwamura Tetrahedron Lett. 1984,25,1933. 84 M. G. Clancy M. M. Hesabi and 0.Meth-Cohn J. Chem. SOC.,Perkin Trans. 1 1984 429. 85 H. Berner H. Vyplel G. Schulz and P. Stuchlik Tetrahedron 1984 40,919. 94 M. S. Baird Photolysis of p-toluenesulphonyl azide in p-xylene leads primarily to products derived from insertion reactions of the corresponding nitrene.An intermediate is formed which decomposes both to the insertion products and to p-toluene sul- phonamide. Photolysis of the ground-state charge-transfer complex between the azide and aniline leads to the product of nitrene insertion into the solvent (among others) giving evidence for nitrene production from an excited charge-transfer complex.86 First-order rate constants for the thermolysis of a series of sulphonyl azides bearing nucleophilic neighbouring groups reveal no anchimeric assistance and are best interpreted in terms of rate-limiting formation of s~lphonylnitrenes.~’ Solution pyrolysis of 3-arylpropanesulphonyl azides in Freon 113 leads to 7-membered ring sultams e.g. (85) while in hydrocarbon solvents hydrogen-abstraction and solvent- insertion products are observed.An interesting rearrangement is seen with 3-(2,6- dichloropheny1)propane sulphonyl azide in which the product (85; X = C1) has the chlorines para-related.88 0 Y x N P On heating to 80-120 “C the sulphenamides (86) decompose to naphthalene and arenesulphenylnitrenes which can be trapped by alkenes in quantitative yield. This method is more efficient than the oxidation of arenesulphenamides although both routes apparently proceed through the same intermediate r~itrene.~~ The kinetics of thermal decomposition of benzenesulphinyl azide is first order and consistent with the formation of a dipolar sulphinylnitrene intermediate.” The highly non-selective nitrenes (87) are unusual in that they show no tendency to undergo intramolecular reactions and instead are presumably captured on almost 86 C.E. Hoyle R. S. Lenox P. A. Christie and R. A. Shoemaker 1. Org. Chem. 1983 48 2056. ” S. P. McMinus M. R. Smith R. A. Abramovitch and M. N. Offor 1. Org. Chem. 1984,49 683. 88 R. A. Abramovitch A 0. Kress S. P. McManus and M. R. Smith J. Org. Chem. 1984,49 3114. 89 R. S. Atkinson M. Lee and J. R. Malpass J. Chem Soe. Chem. Commun. 1984,919. 90 T. J. Maricich C. N. Angeletakis and R. Mjanger 1. Org. Chem. 1984 49 1928. Arynes Carbenes Nitrenes and Related Species every collision in intermolecular processes. The collision frequencies apparently overcome the normal entropic effect favouring intramolecular proces~es.~' N-Nitrenes (88) derived -by oxidation of the corresponding amines undergo intramolecular addition to the double bonds and the effect of various R and R' groups on the competiGve addition suggests a non-concerted reaction through a 7-membered transition-state with the nitrene behaving as an ele~trophile.~~ When the double bonds are replaced by a 2-arylethyl group the product is for example (89) apparently derived by addition of the nitrene to the benzene ring followed by ring-opening and hydrogen transfer.93 Me0 Although singlet ethoxycarbonylnitrene adds to cyclohexene reasonably efficiently it does not attack the annelated sulphone (go) or several related molecules under the same conditions.The effect is apparently not due to interception of the nitrene by the sulphone group itself but may reflect the unusually high ionization potentials of these corn pound^.^^ The nitrene derived by (Y -elimination adds to allylic ethers to form aziridines whereas the corresponding nitrenium ion (EtOCONH+) forms P-amino-alcohol derivative^.'^ 91 R.Breslow F. Herman and A. W. Schwabacher J. Am. Chem SOC.,1984 106 5359. 92 R. S. Atkinson J. R. Malpass K. L. Skinner and K. L. Woodthorpe J. Chem. SOC.,Perkin Trans. I 1984 1905. 93 R. S. Atkinson J. Fawcett N. A. Nawad D. R. Russell and L. J. S. Sherry J. Chem. Soc. Chem. Commun. 1984 1072. 94 R. A. Aitken J. I. G. Cadogan H. Farries I. Gosney M. H. Palmer I. Simpson and E. J. Tinley J. Chem SOC.,Chem Commun. 1984 791. 95 M. A.Loreto L. Pellacani P. A. Tardella and E. Toniato Tetrahedron Lett. 1984 25 4271. M. S. Baird 5 Silylenes Calculations using the 3-21G basis set show that abstraction of a hydrogen from methane by triplet silylene should be an endothermic reaction with a barrier of 32.6 kcal mol-' while abstraction of a hydrogen from silane should be nearly thermoneutral with a barrier of 15.9kcal mol-'. In contrast the abstraction of a hydrogen from silane by triplet methylene should be an exothermic process with a barrier of 9.1 kcal m01-l.~~ Laser flash photolysis of dodecamethylcyclohexasilane in a hydrocarbon solvent at 293 K leads to dimethylsilylene which has an absorption band at 350 nm that is quenched by the silylene scavenger triethylsilane and by methanol.No band is seen for the silylene at 450 nm as previously reported in matrices. The rate of quenching by methanol in dilute solution was 3.1 x lo7M-'s-'.~~ An analysis of the products from generation of the silylene at high temperature in the presence and absence of trapping agents has led to the conclusion that an equimolar equilibrium mixture of dimethylsilylene and HMeSi=CH2 is produced irrespective of the direction from which the equilibrium is appr~ached.~~ Potential energy surfaces for silylene insertion into a range of single bonds have been reported. The reaction with water is found to involve a complex with a fairly high rearrangement barrier while that with ammonia involves a very deep minimum with a rearrangement barrier of 38 kcal mol-' -suggesting that the addition complex may be a suitable candidate for spectroscopic dete~tion.~~ 96 M.S. Gordon J. Am. Chem. SOC.,1984 106 4054. 97 A. S. Nazran,J. A. Hawari D. Griller I. S. Alnaimi. and W. P. Weber J. Am. Chem. SOC.,1984,106,7267. 98 I. M. T. Davidson S. Ijadi-Maghsoodi T. J. Barton and N. Tillman J. Chem. SOC.,Chem. Commun. 1984 478. 99 K. Raghavachari J. Chandrasekhar M. S. Gordon and K. J. Dykema J. Am. Chem SOC,1984,106,5853.