5 Arynes Carbenes Nitrenes and Related Species By M. S. BAIRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl7RU 1 Arynes An extensive review of heterarynes has appeared.' Benzyne is efficiently trapped by phenoxide or thiophenoxide leading to diaryl ethers and diaryl sulphides.* Trapping of substituted benzynes by [2 + 21-cycloaddition to 1,l-dimethoxyethene is a highly regioselective process; a non- synchronous mechanism is consistent with the steric and inductive effects of the substituents. The reaction has been applied as a key step in the synthesis of (A)-t axodione 3,4-dimet hoxy-5 -(1-me thyle t hyl)benzyne adding to the a1 kene to give initially (11.~ [4 + 21-Cycloaddition of methyleneisoquinoline derivatives and benzyne leads to compounds containing the phenanthrene skeleton; e.g.(2) +(3). This provides a potentially valuable route to alkaloids of the dehydroaporphine and aporphine type; for example compound (3) is norcepharadione B.4 Addition of benzyne to furans by a similar process is remarkably efficient; thus (4) is obtained from the Me0 Me0 (2) (1) (4) ' M. G. Reinecke Tetrahedron 1982,38,427. R. B. Bates and K. D. Janda J. Org. Chem. 1982 47,4374. R. V. Stevens and G. S. Bisacchi J. Org. Chem. 1982,47 2393,2396. L. Castedo E. Guitian J. M.Saa. and R. Suau Tetrahedron Lett. 1982 23 457. M. S.Baird corresponding tetrafuran in 84% yield.5 Arynes are also trapped in a 1,3-dipolar cycloaddition with 9-diazofluorenes and 10-diazo-anthrones leading to indazoles (5),which are further transformed into (6) on heating.6 Li (7) Treatment of N-pivaloyl-rn -fluoroaniline with excess n-butyl-lithium at -20 "C leads cleanly to (7) by intramolecular trapping of the intermediate benzyne.The lithium derivative (7) may then be further functionalized by reaction with a range of electrophiles.' 2 Carbenes The interaction of carbenes and carbenoids with neighbouring heteroatoms has been reviewed,8 while a review of lithium halocarbenoids contains a number of examples of typical reactions of carbenes.' Ab initio molecular orbital theory has been used to assess the effect of dimerization on the electronic and structural characteristics of a prototype carbenoid (CH,LiF). The structure found for the monomer is similar to that obtained earlier; the CH2LiF units in the dimer have essentially the same geometries as those in the monomer with relatively strong carbon-lithium bonds but the lithium-fluorine distance is increased.It is concluded that the CH,Li' F- ion-pair nature that is found for the monomer remains unaffected by dimerization and that calculations on the monomer are a valid method for investigating carbenoids and related species. lo However dissociation to singlet methylene is thermodynamically much more favourable from the dimer than from the monomer. Transient triplet carbenes have been converted into relatively long-lived imi- noxyls by reaction with nitric oxide and have then been detected indirectly by e.s.r. in a process that is formally analogous to spin-trapping of radicals by nitrones and nitroso-compounds.Triplet nitrenes could not be trapped in this way.'' Cycloadditionsof singlet carbenes to alkenes depend on the electrophilic proper- ties (p/n interaction) and nucleophilic properties (u/n*interaction) of the carbene. Because of the energy level involved the p/n interaction is dominant for dichlorocarbene and dibromocarbene and substituents which raise the energy of H. Hart and Y. Takehira J. Org. Chem. 1982,47,4370. W. Burgert M. Grosse and D. Rewicki Chem. Ber. 1982 115 309; K. Hirakawa Y. Minami and S. Hayashi J. Chem. SOC.,Perkin Trans. 1 1982 577. 'R. D. Clark and J. M. Caroon J. Org. Chem. 1982,47,2804. K. G. Taylor Tetrahedron 1982 38 2751. H. Siegel Top.Curr. Chem. 1982,106 55. in C. Rohde T. Clark E. Kaufmann and P. von R. Schleyer J. Chem. Soc. Chem. Commun. 1982,882. " A R. Forrester and J. S. Sadd J. Chem. SOC.,Perkin Trans. 2 1982 1273. Arynes Carbenes Nitrenes and Related Species the .rr-orbital should lower the activation energy for cycloaddition. Competition experiments show however that Me MeO and Ph substituents increase the enthalpy of activation. This can be shown not to be a steric effect and indicates that the nucleophilic U/T* interaction plays a dominant role in the addition; it is explained in terms of the existence of an intermediate in the carbene addition as in Scheme 1. For rapid equilibrium between (8)and (9)and (10) the experimental k2 ,A Scheme 1 Mixp= AHo+ AH:,where AHois the reaction enthalpy for complex formation; the substituents raise the U/T* gap and therefore A two-step model for addition of phenylchlorocarbene has also been proposed (see ref.58).The rate of addition of dichlorocarbene to styrenes (11) is increased by electron-donating substituents (X) but relative rates are independent of temperature. By contrast the relative rates of addition to (12) are temperature-dependent. The electronic effect of the substituents in (11) is an entropy effect and does not depend on temperature. A steric effect is operating in the case of (12); this controls activation enthalpies and therefore does depend on temperat~re.'~ The isoselective tem- perature for addition of dichlorocarbene to alkenes has been calculated from isokinetic temperatures and activation enthalpies as 368 K; this is in good agree- ment with the experimental value of 360 f 10 K.14 Me Me Differential frontier-orbital theory allows the prediction of the relative reactivity of a carbene with an alkene based only on the knowledge of the ionization potential of the alkene and the orbital coefficients of its HOMO.A scale of relative selectivity for carbenes compared to dichlorocarbene is in good agreement with that derived from other methods." The selectivity of phase-transfer-generated dichlorocarbene towards mono- bis- or tris-addition to non-conjugated polyenes has been examined in the presence of a number of catalysts. Earlier claims that the presence of a hydroxyl group in l2 B.Giese W.-B. Lee and C. Neumann Angew. Chem. Inf. Ed. Engl. 1982,21 310. l3 B. Giese and C. Neumann Tetrahedron Lett. 1982 23 3557. l4 B. Giese and W.-B. Lee Tetrahedron Len. 1982,23 3561. '' W. W. Schoeller N. Aktekin and H. Friege Angew. Chem. Int. Ed. Engl. 1982 21,932. M. S. Baird the catalyst leads to greater selectivity towards mono-attack are not borne out but the selectivity is found to depend on the nature of the alkene and on the nature and concentration of the catalyst. Generally the more hydrophilic catalysts lead to higher selectivity to the mono-adduct with tetramethylammonium chloride being most selective.16 The generation of the carbene from chloroform and solid sodium hydroxide does not actually require a catalyst provided that efficient stirring and ultrasonic irradiation are used; in this way very good yields of adducts with alkenes can be isolated although only in small-scale reaction^.^' Addition of dichlorocarbene to 1,2-bismethylenecycloheptanegives the 1,4- adduct (13) as well as the 1,2-adduct (ratio 1:99) -the first example of inter- molecular 1,4-addition by a singlet carbene.'' Quantum-mechanical calculations (13) at the MNDO level have been carried out for 1,4-addition of singlet carbenes to butadiene and for 1,2-addition to ethene.For a range of carbenes there is a linear relation between E for the two processes but in all cases the barrier to 1,4-addition is greater than that for 1,2-addition. The carbene :C(NH2) is noticeably off the line having a lower barrier to 1,2-addition than :C(OH)* but a higher barrier for 1,4-addition.Geometries of the transition states for typical electrophilic and nucleophilic carbenes are similar for the 1,4-process but not the 1,2-process when :C(NH& approaches for maximum nucleophilic interaction of the carbene (c-approach). This indicates that nucleophilicity in carbenes is of greater advantage to 1,2- than to 1,4-addition and suggests that neither electrophilic nor nucleophilic singlets promote synchronous 1,4-addition.19 Homo-1,4-addition of difluorocarbene to the 2-methoxy- and 2-carbomethoxy- 10-norbornadienes (14; X = Me0 or Me02C) proceeds to the exclusion of 1,2- addition whereas (14 X = H) gives 9% of 1,2-adduct; the relative rates of reaction indicate an electrophilic addition of the carbene.Difluoro- dichloro- and dibromo-carbenes all add to (14; X = H) exclusively from the endo-face to give 1,2- or 1,4-adducts. The homo-cheletropic reaction is increasingly sensitive to the bulk of the carbene and is rationalized in terms of linear approach of the carbene in the mirror plane bisecting the ring (15) so that both lobes of the p-orbital overlap the HOMO. The ligands must invert during the addition; difluorocarbene has the shortest carbon-halogen distance and van der Waals radii and hence leads to most homo-1,4-addition.20" A minor product from the reaction of unsubstituted trinor- bornadiene with dichlorocarbene has been identified as (16). This has been explained in terms of a stepwise addition from the exo-face to produce (17) which may either ring-close to a cyclopropane or rearrange to (16).20b l6 E.V. Dehmlow and M. Prashad J. Chern. Res. (S) 1982 354. S. L. Regen and A. Singh J. Org. Chem. 1982,41 1587. l8 L. A. M. Turkenburg W. H. de Wolf and F. Bickelhaupt Tetrahedron Lett. 1982 23,769. l9 W. W. Schoeller and N. Aktekin J. Chem. Soc. Chem. Commun. 1982 20. (a)C. W. Jefford and P. T. Huy Tetrahedron Lett. 1982,23,391; (b) C. W. Jefford G. Bernardinelli J.-C. Rosier and J. A. Zuber Helv. Chim. Acta 1982 55 1467. Arynes Carbenes Nitrenes and Related Species Dichlorocarbene that has been generated thermally from phenyl(bromodi- chloromethy1)mercury reacts selectively with aldehydes in the presence of dimethyl acetylenedicarboxylate to give a carbonyl ylide (18) which is then trapped by the acetylene to give after dehydrochlorination (19).2' Photolysis of di-t-butyldiazomethane that has been deposited on a caesium iodide window at 14 K leads to (20) (21) and (22).The first two are readily explained in terms of expected carbene reactions whereas the latter may originate from migration of a hydrogen atom (via a tunnelling mechanism) in a triplet di-t- butylcarbene. In addition to these products a moderately intense band is observed at 1290 em-' which disappears above ca. 100 K. This was found to be due to a primary photoproduct and when the irradiation was carried out in a 2-methyl- tetrahydrofuran matrix at 20 K a stable e.s.r. spectrum due to triplet di-t-butylcar- bene was seen. The triplet is either the ground state or is close to it in energy; the zero-field splitting parameters indicated a bond angle about the central carbon of 143" close to that in triplet methylene and in the carbene (F3C),:.22a It is inter- esting to note the prediction of a singlet ground state for di-t-butylcarbene.22b A detailed examination is reported of the products derived from the carbene (23; R = Ph) generated under a range of conditions.Pyrolysis of (24; M = Na) at 160-200°C in triglyme leads to (25) (26) and (27) but not to (28) or (29). At 80-120 "C less (25) is obtained but (28) (29) and (30) are observed together with an increased amount of (26). Pyrolysis of (24; M = Li) gives similar products but in differing ratios; results for (24; M = Na) at 80-120 "C and for (24; M = Li) at 80-200 "C correspond to competitive carbenic and cationic decomposition of p-tosylhydrazonates.Thermolysis of (2-phenyl)-prop-2-yldiazirineor (2-pheny1)prop-2-ylidiazomethane at 180 "C or their photosensitized decomposition lead to identical mixtures of (25)-(27); this implies that the carbene (23; R = Ph) 21 H. S. Gill and J. A. Landgrebe Tetrahedron Lett. 1982 23 5099; for carbonyl ylides derived from a-diazo-ketones see A. Giilon D. Ovadia M. Kapon and S. Bien Tetrahedron. 1982 38 1477. 22 (a)J. E. Gano R. H. Wettach M. S. Platz and V. P. Senthilnathan J. Am. Chem. Soc. 1982,104 2326; (6) P. H. Mueller N. G. Rondan K.N. Houk J. F. Harrison D. Hooper B. H. Willen and J. F. Liebman ibid. 1981,103 5049. 88 M.S. Baird ~ Me Ph-CMe2 -CH =N-N-S02C7 H7 IMe R-hH M+ Me Me)_QhMe H (23) (24) (25) (26) generated either as singlet or triplet rearranges and inserts via the singlet species. The product ratios indicate a 9 1 preference for phenyl- rather than methyl- migration under these conditions. Photolysis of a diazirine or a diazo-compound in basic environments leads to different product ratios and to the formation of an aromatic substitution product (31); this appears to involve an electronically excited carbene for which the phenyl/methyl migratory order is about 2 :l.23 Product analysis from reactions of (23; R = cyclopropyl) that has been generated by thermolysis indicates that the principal process is insertion into a C-H bond of a methyl group but that some insertion into the tertiary hydrogen-carbon bond of the cyclopropyl group does occur.Minor products are derived from migration of methyl or cyclopropyl groups in the ratio 1:2.8; this ratio is explained by delocaliz- ation of the a-bonds of the cyclopropane on interacting with the electrophilic carbene but it is much smaller than the migratory preference of phenyl over methyl (9:1).Carbene (32) also undergoes insertion into the C-H bonds of the 2-methyl groups but the major product is derived by a 1,2-shift of the alkenyl group; only a trace of methyl migration is seen (migratory ratio ca. 85 :1).It is not clear how the alkene migrates e.g. via a dipolar species (33) or a bicyclobutane or even that the product is certainly derived from a carbene intermediate.Migratory aptitudes of the alkyl groups in (23; R = Et or Pri) which are found to be Me > Et > Pr are opposite to those predicted on the basis of inductive electron release and the ease of insertion into C-H bonds is primary > secondary > tertiary -again the reverse of the expected order. When two alkenes can be produced by a rearrangement in which there are steric preferences the less strained product predominates. The transition states leading to rearrangement insertion and E/Z isomerism are 23 A. R. Kraska K.-T.Chang S.-J. Chang C. G. Moseley and H. Shechter Tetrahedron Lett. 1982 23,1627. Arynes Carbenes Nitrenes and Related Species apparently controlled by steric factors in which there is preferred attack on the smaller alkyl group in the order of Scheme 2.24 Scheme 2 When 1,l-di-iodoneopentane is passed through a hot tube that contains methyl- lithium-coated Pyrex chips 1,l -dimethylcyclopropane and 2-methylbut-2-ene are formed in nearly quantitative yield.The product ratio indicates that the intermediate t-butylcarbene is the same as that produced from thermal or photosensitized decomposition of t-butyldiazomethane. The ratios are however different from those in the direct irradiation of the diazo-compound and the authors suggest that this is the unusual reaction. This is clearly of importance in relation to the'two papers discussed immediately It is known that a perpendicular alignment of a migrating group Z as in (34) is preferred in a 1,2-shift to a singlet carbene relative to the vacant p-orbital.n However when the group Z does not have axial symmetry it may adopt a variety of orientations. For a migrating phenyl-group two extremes [(35) and (36)] may be visualized; these have been termed M-face and M-edge arrangements. Pyrolysis of the sodium salts of tosylhydrazones of two epimeric ketones was used to produce the epimeric carbenes (37; R' = H R2= Ph) and (37; R' = Ph R2= H); both led to (38; R' = H R2= Ph) with no detectable formation of (38; R' = Ph R2= H) i.e. a H/Ph migratory aptitude ratio of >lo00 for each carbene. This selectivity is exceptionally high compared to the corresponding value in non-rigid carbenes. For (37; R' = H R2= Ph) this may be due to the favourable alignment of the hydrogen but for (37; R' = Ph R2= H) the phenyl group is better aligned.It is suggested that the hydrogen at C-7 interferes with the ortho-hydrogen on the phenyl group if it adopts the M-face orientation. Since the M-edge orientation appears relatively feasible it seems that migration of a phenyl group may occur by a preferential M-face geometry.26 24 A. R. Kraska L. I. Cherney and H. Shechter TetrahedronLett. 1982 23 2163. 25 M. Fukushima M. Jones and U. H. Brinker Tetrahedron Len. 1982,23 3211. 26 A. Nickon and J. K. Bronfenbrenner J. Am. Chem. Soc. 1982,104 2022. M. S. Baird The carbene (39; R = Me) has been reported to lead to the corresponding cy~lobutadiene.~' However a more recent paper reports that both (39; R = Me) and (39; R = Bu') also fragment to di-t-butylacetylene and the corresponding alkyl t-butylacetylenecarboxylate.In both cases the ratio of cyclobutadiene to alkynes is ca.70 30 but is markedly dependent in temperature and solvent. By analogy with the Wolff rearrangement the singlet carbene should rearrange and the triplet should fragment; in confirmation of this the ratio of products was changed to about 20 70 by adding benzophenone sensitizer.28 2,3-Di-(tri-methyl- silyl)cycloprop-2-en-l-yl carbene also fragments giving ethyne and di-(trimethyl- ~i1yl)ethyne.~' The cycloalkylcarbene (40) has been generated photolytically at -78 "Cand found to lead to indene as the major product (15%) together with 7-ethynyl- bicyclo[2.2.l]hepta-2,4-diene,(4l),and ethynylcycloheptatriene.Labelling studies rule out intramolecular addition of the carbene (40) to the alkene bond and the most likely mechanism leading to the first three products involves a 1,2-alkyl shift to produce (42) followed by a retro-Diels-Alder reaction a rearrangement or a hydrogen abstraction. The cycloheptatriene may be derived through an alternative 1,2-alkyl shift. in (40) although a direct fragmentation to 7-ethynyl-bicyclo[4.1.0]hepta-2,4-dienealso seems likely.30 Two labelling studies in rearrangements of cyclopropylidenes are reported. Car- bene (43) generated by the reaction of the corresponding dibromide with methyl- supports a sequence involving a carbene-carbene rearrangement leading to (44) which then (.)labelling12C1-t-butyl-3-methylpyrrole;lithium rearranges to *' S.Masamume N. Nakamura M. Suda and H. Ona J. Am. Chem. SOC.,1973,95,8481. P. Eisenbarth and M. Regitz Angew. Chem. Int. Ed. Engl. 1982 21,913. 29 G. Maier M. Hoppe. H. P. Reisenauer and C. Kruger Angew. Chem. fnt. Ed. Engl. 1982,21,437. 3" J. Stapersma I. D. C. Rood. and G. W. Klumpp Tetrahedron 1982 38 3051; see also ibid. p. 191. Arynes Carbenes Nitrenes and Related Species undergoes a 1,2-shift of hydrogen3’ Treatment of the dibromocarbene adduct of homobenzvalene with methyl-lithium leads to 5-ethynylcyclohexa-l,3-diene.In this case ’H and 12C labelling indicate a rearrangement as in Scheme 3.32In related Scheme 3 reactions the cyclopropylidenes (45;XiY = CC12 or-O-CH2CH2-O-) are reported to undergo efficient insertion into a 5,6-related carbon-hydrogen bond to produce the compounds (46) [the corresponding reaction of the parent system (X = Y = H) proceeds only in low yield]33 and (47) undergoes a similar reaction when n = 5 or 6 but can lead to high yields of allenes when n is larger; the latter reaction leads to chiral allenes in the presence of (-)-~parteine.~~ Decomposition of cyclopropyl- diazonium salts in the presence of sodium methoxide leads to products that are apparently derived from cyclopropylidenes; thus one product from em -9-bicyclo[6.1 .O]nonane-9-diazonium ion is cyclonona- 1,2-diene.The carbene bicyclo[6.1 .O]non-2-en-9-ylidene rearranges to the allene cyclonona-l,2,4-triene with no evidence of insertion into C-H bonds (which is the reported reaction when the carbene is derived from 9,9-dibromobicyclo[6.l.0]non-2-ene) or rearrangement to bridged carbenes (as reported in lower hom~logues).~~ Decomposition of (48) under strongly alkaline conditions is also reported to lead to cyclopropylidene- derived products such as (49).36 This product is also obtained together with (50; X = H Y = Br) and (51) when (50; X = Y = Br) is heated with methyl-lithi~m.~’ 31 J.Arct and L. Skattebd Tetrahedron Lett. 1982,23 113. 32 M. Christ1 and M. Lechner Chem. Ber. 1982 115 1. 33 A. R. Allan and M. S. Baird J. Chem. Res. (S) 1982 290. 34 M. Nakazaki K. Yarnamoto M. Maeda 0.Sato and T. Tsutsui J. Org. Chem. 1982.47 1435. ” W. Kirmse and G. Hellwig Chem. Ber. 1982 115 2744. 36 H. Jendralla Chem.Ber. 1982,115,220; see also H. Jendralla and W. Pflaurnbaum ibid. p. 229. 37 H. Jendralla and W. Pflaumbaurn Chem. Ber. 1982 115 210. 92 M. S. Baird Valence-bond and molecular-orbital theories have been used to show that triplet cyclopropylidene can rearrange to singlet (orthogonal) a11ene.38 Although cyclopro- pylidenes have been studied extensively cyclobutylidenes are less well known. However the reaction of various 1,1-dihalogenocyclobutaneswith methyl-lithium at 0 to -78 "C leads to methylenecyclopropane and cyclobutene in a ratio similar to that obtained from cyclobutylidene that is generated by the Bamford-Stevens reaction. 39 The regioselectivity of insertion into C-H bonds at positions A and B in the cycloalkylidenes (52) is found to be highly dependent on small changes in geometry and electron distrib~tion.~' The related cycloalkylidene (53) undergoes addition to the alkene rather than insertion and produces an exceptionally strained [3.l.l]pr0pellane.~*a! -AlkyI-a! -trimethylsilylcarbenes (54) have been generated by the reaction of the dibromides (R'R2C=CR3CH2CBr2SiMe3) with methyl-lithium.These carbenes also undergo intramolecular addition to the alkene bond to produce 1-trimethylsilylbicyclo[l. l.O]butanes (55) rather than undergoing a 1,2-shift of hydrogen.42 "ZC% SiMe, Bb R2 (53) (54) (55) Photolysis of phenylcyclopropyldiazomethane in a matrix of 2-methyltetra-hydrofuran at 10-25 K does not produce the ex. spectrum of phenylcyclo- propylcarbene; only a weak spectrum is observed on similar treatment of phenylcyclobutyldiazomethane.However intense spectra for triplet carbene are observed for phenylcyclopentyl- and phenylcyclohexyl-carbenes even at 77 K; phenyl-( 1-benzylcyclopropy1)- and phenyl-( 1-benzylcyclobuty1)-carbenesalso give e.s.r. spectra. By extrapolation it is concluded that the triplet state of phenylcyclo- propylcarbene is the ground state or is within a few kcal of the ground state. Zero-field splitting parameters indicate that the cyclopropyl ring is much less effective than a phenyl group in delocalizing the spin density of the ~arbene.~~ Irradiation of phenyldiazomethane at >478 nm in an argon matrix at 10 K leads to phenylmethylene. Further irradiation at shorter wavelength leads to a second product which is also obtained by irradiation of phenyldiazirine in an argon matrix or by the thermolysis of phenyldiazomethane followed by trapping in a matrix.3R Y.-N. Chiu J. Am. Chem. SOC.,1982,104,6937. 39 U. H. Brinker and G. Schenker J. Chem. SOC.,Chem. Commun. 1982,679. " S. Hirsl-Starcevic and Z. Majerski J. Org. Chem. 1982,47 2520. 4' V. Vinkovic and Z. Majerski J. Am. Chem. SOC.,1982 104,4027. 42 M. S. Baird. S. R. Buxton. and M. Mitra Tetrahedron Lett. 1982 23 2701. " R. L. Barcus E. C. Palik and M. S. Platz Terrahedron Lett. 1982 23 1323. Arynes Carbenes Nitrenes and Related Species 93 Deuterium labelling has been used to show that this product is cyclohepta-1,2,4,6- tet ~aene.~~ Photolysis of PhC(N2)C02Me in styrene at 0 "C gives the corresponding 1,2- diarylcyclopropanes in 10 :1 &/trans ratio; in contrast irradiation in a frozen styrene matrix at -78 or -190 "C reduces the ratio to 2.8 :1and 1.5 :1 respectively.A similar change is observed for PhCHN or 4-BrC6H4CHN2 but for photolysis of phenyldiazomethane in para-substituted styrenes the reverse result is obtained. These findings are not mirrored in solution at -78 "C and are also dependent on the nature of the matrix. The reaction may involve stepwise addition of a triplet carbene to produce a diradical; when such species are generated independently by photolysis of 1,3-diarylpyrazolines there is also a marked change in degree of retention of configuration in the solid phase. Orientation of the diradicals in the matrix may be dependent on the relative size of host and guest molecules leading to differences in stereochemical Photolysis of PhCHN in ether leads to PhCH,OEt PhCH(Et)OEt PhCH,CH(Me)OEt and Ph(CH,),OEt.Product distri- butions are not altered by a triplet quencher but all products are quenched with differing efficiencies by a singlet quencher. The effect of concentration of singlet quencher suggests that the first two products which arise from an 0-ylide come from singlet carbene but that the last two arise from both singlet and triplet reactions. As the temperature is decreased the changes in product ratios indicate either a gain of triplet reaction over singlet or solvation effects. The selectivity between the final two products increases as the temperature is reduced but dramatically decreases at the phase change to a solid -indicating both temperature and matrix effects.In the matrix the characteristic reactions of singlet carbene are suppressed and C-H insertion by triplet carbene results.46 Photolysis of ArCHN in simple alcohols at ambient temperature leads to products of arylcarbene insertion into 0-H bonds and small amounts of C-H insertion. At lower temperatures more C-H insertion is observed; attempted sensitized decomposition of the diazo- compound does not lead to an increase in this process presumably due to rapid triplet-singlet equilibration. The OH/CH insertion ratio is also increased at ambient temperature by electron-donating substituents on the aromatic ring -the transition state for OH-insertion involving electron deficiency at the benzylic carbon.At low temperature the ratio is increased by both electron-donating and electron-with- drawing sub~tituents.~' Hammett plots for the selectivity of insertion or addition (ki/k,) for reaction of 'aryl carbenes' with ethyl vinyl ether and for the stereochemical selectivity in the addition process (kciS/ktrans) show that the p values are highly dependent on the mode of generation. The free carbene appears to be involved when photolytic generation from diazo-compounds is used but in the thermal process a ground-state diazo-compound seems to be masquerading as a ~arbene.~~ 44 P. R. West 0. L. Chapman and J.-P. Le Roux,J. Am. Chem. SOC.,1982 104 1779; for trapping of cyclohepta-1,2,4,6-tetraenesee J.W. Harris and W. M. Jones ibid. p. 7329,and for reactions of a naphthalene-fused analogue see M. Balci W. R. Winchester and W. M. Jones J. Org. Chem. 1982 47,5180. 45 H. Tomioka Y. Ozaki Y. Koyabu and Y. Izawa Tetrahedron Lett. 1982,23 1917. 46 H. Tomioka S. Suzuki and Y. Izawa Bull. Chem. SOC.Jpn. 1982,55,492. 47 H.Tomioka S. Suzuki and Y. Izawa J. Am. Chem. Soc. 1982 104,3156. 48 H.Tomioka S. Suzuki and Y. Izawa J. Am. Chem. Soc. 1982,104 1047. 94 M. S. Baird A number of naphthylcarbenes (56) have been generated by pyrolysis of the correspondingmethoxytrimethylsilanes. When Y = Z = H the carbene inserts into the C(8)-H bond to produce a cyclobutarene (57; X = Y = H); however for (56; Z = H Y = Me) two cyclobutarenes (57; Y = Me X = H) and (57; Y = H X = Me) are obtained (ratio 11:l) suggesting an equilibration between (6-methyl- nap ht h-1-y1)carbene and (7-methylnaph th- 1-yl)carbene.Moreover (nap hth-2-y1)carbene appears to rearrange to (56; Y = 2 = H) and isomeric (phenan- threny1)carbenes also eq~ilibrate.~~" The bridged naphthylcarbenes (58; X Y = H Me or -OCH2CH20-) undergo a 1,2-shift of the X group to produce an alkene rather than a 1,2-shift of the labelled carbon-carbon bond which would produce the skeleton present in (57). The keto-carbene (58; XY = 0)does not undergo a Wolff rearrangement but can be trapped by alcohols alkanes and electron-deficient alkene~.~~' Excited triplet diphenylcarbene has been studied using picosecond lasers and unlike the low-lying triplet it reacts with methan01.~' The pseudo-first-order rates for abstraction of hydrogen from toluene by triplet diphenylcarbene in a matrix at 77-106 K a reaction which proceeds by quantum- mechanical tunnelling of hydrogen atoms have been determined using e.s.r.Barrier heights for the process are in good agreement with a theoretical model and the calculated activation barrier for classical abstraction of hydrogen of 8.7 kcal mol-' is consistent with a relatively low rate of triplet abstraction of hydrogen atoms in s~lution.~' Decay of the e.s.r. signal of Ph2C in protonic and perdeuteriated organic matrices corresponds to sites of very different reactivity. Although the formation of cyclopropane in alkene matrices could occur by tunnelling of heavy atoms it is more likely that the carbene is generated directly over the double bond in suitable matrix sites and reacts with minimal motion in the Irradiation of diazofluorene has been carried out in acetonitrile on a picosecond time-scale at very low temperature and has led to a re-assignment of the structures of transient species.Two main products are observed; one gives rise to an absorption at 470 nm; as this decays the second appears absorbing at 400 nm. The latter is probably due to the formation of a nitrile ~lide;~* the former is now assigned to 49 (a) T. A. Engler and H. Shechter Tetrahedron Lett. 1982 23 2715; (b) S.-J. Chang B. K. Ravi Shankar and H. Shechter J. Org. Chem. 1982,47,4226. '"Y. Wang E. V. Sitzmann F. Novak C. Dupuy and K.B. kisenthal J. Am. Chem. SOC.,1982 104 3238. " B. B. Wright V.P. Senthilnathan M. S. Platz and C. W. McCurdy Tetrahedron Lett. 1982.23 833; M. S. Platz V. P. Senthilnathan B. B. Wright and C. W. McCurdy J. Am. Chem.SOC.,1982,104,6494. 52 D. Griller C. R.Montgomery J. C. Scaiano M. S. Platz and L. Hadel J. Am. Chem. SOC.,1982 104.6813. Arynes Carbenes Nitrenes and Related Species triplet fluorenylidene. Results suggest that there is a second species preceding the one that produces the 400nm peak and that this is singlet fl~orenylidene.~~ Fluorenylidene adds to aliphatic ketones to give carbonyl ylides which may be detected spectroscopically in solution. In the absence of quenchers these can undergo ring-closure to oxiranes but the ylides are quenched with rate constants of about lo7dm3 mol-’ s-’ by electron-deficient alkenes or by oxygen.54 The car- bene (59; X = Si) generated by photolysis of the corresponding diazo-compound has been observed by e.s.r.spectroscopy in a matrix at 40K and the triplet has been shown to be the ground state; reaction with cis-2-butene gives no cyclopro- pane but only products derived by abstraction of hydrogen. The carbene (59; X = C) reacts with cis-butene apparently by hydrogen abstraction followed by dimerization.” Photoinduced loss of halide from the anion (60; X = Br or C1) leads to 1,3-diphenylisoindenylidene (61),which again undergoes abstraction of hydrogen atoms rather than addition to alkenes; e.g. with dihydropyran (62) is formed. In a most unusual reaction the carbene is trapped by ethyl vinyl ether to produce (63) apparently through an initial cyclo-addition followed by loss of ethanol to produce the alkene at AB.56 X @x Me’ ‘Me Ph (59) (60) Ph Ph Ph WB\ Ph (62) (63) 4H-1,2,3-Triazolylidenes(64) isomerize to a-diazonitriles which in turn lead to carbenes Z-C-CN (64a).Benzene is trapped by (64) in a substitution reaction leading to (65) but with (64a) addition and ring-expansion or substitution are ob~erved.~’ 7 53 B.-E. Brauer. P. E. Grasse K. J. Kaufmann and G. B. Schuster J. Am. Chem. Soc. 1982. 104,6814. ” P. C. Wong D. Griller and J. C. Scaiano J. Am. Chem. Soc. 1982 104 6631. ” A. Sekiguchi W. Ando T. Sugawara H. Iwamura and M. T. H. Liu Tetrahedron Lett.1982,23,4095. 56 L. M. Tolbert and S. Siddiqui J. Am. Chem.SOC.,1982,104,4273. ” H. K.-W. Hui and H. Shechter Tetrahedron Len. 1982 23 5115. M. S.Baird Absolute rate-constants and activation parameters are reported for the addition of phenylchlorocarbene to alkenes. The values are dependent on the experimental system but a precision of >5% and reproducibility of >lo% are obtained. The Arrhenius plot for addition to tetramethylethylene shows a pronounced curvature and as the temperature decreases from 301 to 213 K the value of koW increases; Eib" is found to be -1.7 f 0.5 kcalmol-'. At temperatures below 213 K koW decreases and Eib" approaches the value for diffusion control. The results are interpreted in terms of a kinetic model for addition represented by two steps A + PhCCl kl PhCCl/A k-I k2 PhCCl/A -+ cyclopropane A structure for the intermediate that is consistent with the data would involve interaction of the vacant p-orbital of the carbene with the 7r-orbital of the alkene in the form of a loose 7r-complex.More reactive alkenes would lead to looser complexes. The kinetics are also consistent however with an intermediate 'proxim- ity pair' of reactants in a solvent cage.58 These conclusions should be compared to the two-step model arrived at for addition of dichlorocarbene (see ref. 12). A Hammett plot for the reaction of phenylchlorocarbene with styrenes indicates that it reacts as an electrophile; the p values obtained are markedly dependent on temperat~re.'~ Laser flash photolysis of arylchlorodiazirines leads to singlet arylchlorocarbenes which are trapped by alkenes in a first-order process.However when trapped by methanol the reactions are not first-order with respect to alcohol. Solvent and concentration effects indicate that OH bonds that are involved in hydrogen-bonding in methanol oligomers are more reactive towards carbenes than those in unassoci-ated molecules possibly due to a decrease in the bond dissociation energy. For t-butyl alcohol the reverse result was obtained apparently due to a steric effect.60 Flash vacuum pyrolysis of N-allyl-substituted 2-phenyl- 1,3,4-oxadiazolin-5- ones e.g. (66),can be explained by initial decarboxylation to (67),which undergoes a 3,3-sigmatropic shift to a diazoalkene PhC(N2)CH2CH=CH2.Loss of nitrogen leads to an allylphenylcarbene which undergoes migration of hydrogen or a vinyl group or insertion into C-H bonds. 2-Phenyl-4-(prop-2-yn-l-~!)-1,3,4-oxadiazolin-5-one leads to l-phenylbut-3-en-l-yne apparently via Ph-C-CH=C=CH2 and l-phenyl-3-methylene~yclopropene.~' This route is also useful in generating benzylp hen ylcarbene. 58 N. J. Turro G. F. Lehr J. A. Butcher R. A. Moss,and W. Guo J. Am. Chem. SOC.,1982,104 1754. 59 W. Bruck and H. Durr Tetrahedron Len. 1982 23 2175. 6o D. Griller M.T. H. Liu and J. C. Scaiano J. Am. Chem. SOC.,1982,104 5549. '' A. Padwa T. Caruso S. Nahm and A. Rodriguez J. Am. Chem. SOC.,1982 104,2865. Arynes Carbenes Nitrenes and Related Species Pyrolysis of vinyldiazomethanes (68; R' R2 = alkyl and aryl) gives mixtures of 3H-pyrazoles and cyclopropenes.By analysing the kinetics and the distribution of products in each case the rate of formation of vinylcarbene and therefore the relative stabilities of these intermediates has been assessed. It is found that the carbene from (68; R' = Me R2= Ph) is more stable than that from (68; R' = Ph RZ= Me). The results can be related to those from singlet carbenes derived by photolysis of unsymmetrical cycloprbpenes.62 Direct irradiation of 1,l -diphenylallenes Ph2C=C=CHR produces the corre- sponding indenes (69) 3,3-diphenylcyclopropenes,and 1,l -diphenylprop-2-ynes as primary products. The reaction occurs via a singlet excited state and is thought to involve a 1,2-shift of hydrogen to produce a.vinylmethylene diradical which can equilibrate with a vinylcarbene PhzC=CH-C-R.63 Pyrolysis of (70) in a flow system at 800°C leads to the carbene (71) after a shift of deuterium and loss of nitrogen.The carbene rearranges to penta- 1,3-diene and the deuterium label appears at C-2 indicating that this process occurs by a 1,4-shift of hydrogen.64 D The cyclic vinylcarbene (72) has been generated by pyrolytic or photolytic decomposition of alkali-metal salts of the corresponding tosylhydrazone and leads to a complex mixture of C,H and C8Hl products. The former which include (73) 5-ethynylcyclohexa-l,3-diene,and semibullvalene are ascribed to insertion reac- tions and rearrangement of a singlet carbene; the latter which include bicyclo[3.2.lloctadiene (74) and (73 arise through abstraction of hydrogen by a triplet carbene or by a closely related species.65 Substituent effects in the addition of photolytically generated (72) and (76) to styrenes correlate well with Hammett a constants and produce p values of +0.25 and +0.68 indicating nucleophilic character in the carbenes. Calculations indicate that the HOMO'S of (72) and (76) are close to that of vinylmethylene and higher than that of nucleophilic dimethoxy- carbene; they further show that steric effects play a part in the nucleophilic approach of (76) to styrenes.66 (72) (73) (74) (75) (76) 62 J. A. Pincock and N. C. Mathur J. Org. Chem. 1982 47 3699; for cyclopropene photolysis see A. Padwa M. Akiba C. S. Chou and L. Cohen ibid.,p.183. " M. G. Steinmetz R. T. Mayes and J.-C. Yang J. Am. Chem. SOC.,1982,104,3518. 64 J. D. Perez and G. I. Yranzo J. Org. Chem. 1982 47 2221. P. K. Freeman and K. E. Swenson J. Org. Chem. 1982,47 2033. 66 S.-I. Murahashi K. Okumura T. Naota and S. Nagese J. Am. Chem. Soc. 1982 104 2466. M. S.Baird Generation of (77) by pyrolysis of the lithium salt of the corresponding tosyl- hydrazone leads to indene and 7-ethynylcycloheptatriene; the related carbene (78) leads only to indene. The reactions of (77) are discussed in terms of a number of carbene-carbene rearrangements but it is interesting that unlike (72) nohydrogen-abstraction products are formed; this may reflect the involvement of allenic inter- mediates e.g. (79). Further calculations suggest that cyclohexa-1,2-diene has a triplet ground state but that cyclohepta-1,2-diene is a singlet.67 The carbene (80) has also been examined.68 Irradiation of diphenylcyclopropenethion in methanol leads to products which are apparently derived by trapping of Ph-C-C(Ph)=C=S either as the diradical PhC=C(Ph)-C=S or after ring-closure to (81).69 Calculations of the energies of conjugated cyclic carbenes (82) using various theoretical models are rep~rted.~' Ultraviolet photolysis of microcrystalline 5-diazoimidazole-4-carboxamidegives two S = 1species which have been detected by e.s.r.One of these has been characterized as the carbene (83).71 0 Generation of the carbene (84)in methacrylonitrile leads to some addition to the alkene bond.However the major product is (85) and control studies suggest that this is derived by reversible formation of the nitrile ylide (86) in competition with irreversible addition to the alkene. In support of this the carbene is trapped by acetonitrile in the presence of N-phenylmaleimide to produce (87).72 The base-promoted reaction of dimethyl diazomethylphosphonate (Me0)2POCHN2 with aldehydes or aryl ketones leads predominantly to alkynes. The results are consistent with the intermediacy of diazoethenes e.g. ArC(R)=CN2 from ArCOR which undergo unimolecular decomposition at -78 "C to form nitrogen and alkylidenecarbenes ArC(R)=C:.73 Treatment of (88) under similar 67 P. K. Freeman and K. E. Swenson J. Org. Chem. 1982,47,2040. D. 0.Farnum M. Ghandi S. Raghu and T.Reitz J. Org. Chem. 1982,47,2598. 69 S. Singh M. M. Bhadbhade K. Venkatesan and V. Ramamurthy J. Org. Chem. 1982,47 3550. 70 M. Kausch and H. Durr J. Chem. Res. (S),1982 2. 71 H. B. Ambroz B. T. Golding T. J. Kemp and V. S. Shukla J. Chem. Soc. Chem. Commun. 1982 414. 72 A. S. Kende P. Hebeisen P. J. Sanfilippo and B. H. Toder J. Am. Chem. Soc. 1982,104,4244. 73 J. C. Gilbert and U. Weerasooriya J. Org. Chem. 1982. 47 1837. Arynes Carbenes Nitrenes and Related Species N' %N Me 0 Ill C Me A MeA conditions leads to a methylenecarbene which undergoes intramolecular addition to the alkene to produce (89); this in turn undergoes ready ring-opening to a di-t-butylmethylcyclopentadipe.Derivatives involving the ring system (89) have also been obtained from intramolecular addition of methylenecarbenes that were derived by the reaction of 1,l-dibromides with methyl-lithi~rn.~~ Irradiation of the diradical (90) in a glass at 77K leads to 6-methylhept-5-en-1-yne as the major monomeric product.This process may formally be explained by ring-closure to produce 6,6-dimethylbicyclo[3.1 .O]hex-1 -ene and cheletropic elimination of a methylenecarbene -the reverse of the proces described above. The carbene could then undergo a 1,2-shift of hydrogen; however the typical barrier for the latter process is ca. 8.6 kcal mol-' and such a process would be immeasurably slow at 77 K.75 The methylenecarbene-alkyne rearrangement is reversed in the pyrolysis of ynones; this leads to carbenes such as (91) which undergo insertion into a 5,6-related C-H bond to produce cycl~pentenones.~~ 0 Neither oxirenes nor acylcarbenes can be detected directly on irradiation of diazo-ketones in an argon matrix at 10 K.This applies even to those diazo-ketones which do not undergo a Wolff rearrangement under standard condition~.~' Copper-catalysed decomposition of a diazo-ketone in the presence of an alkene has provided the cyclopropyl ketone (92) an intermediate in a synthesis of (*)-~pirolaurenone.~* The copper-sulphate-assisted decomposition of diaz~methyl-ketones~~ and the decomposition of a-diazo-& keto-esters (93) in the presence of rhodium acetate 74 R. F.Salinaro and J. A. Berson Tetrahedron Lett. 1982 23 1447 1451; M. Rule R. F.Salinaro D.R. Pratt and J. A. Berson J. Am. Chem. SOC.,1982,104,2223. 75 S. P. Schmidt A. R. Pinhas J. H. Hammons and J. A. Berson J. Am. Chem. SOC.,1982 104 6822. 76 M. Karpf J. Huguet and A. S. Dreiding Helv. Chim. Acra 1982 65 13. 77 G. Maier H. P. Reisenauer andT. Sayrac Chem. Ber. 1982,115 2192. '13 A. Murai K.Kato and T. Masamune Tetrahedron Lett. 1982 23 2887; for a more simple but related reaction see F.Bohlmann and W. Rotard Liebigs Ann. Chem. 1982 1220. 79 E. Wenkert L. L. Davis B. L. Mylari M.'F. Solomon R. R. da Silva S. Shulman R. J. Warnet P. Ceccherelli M. Curini and R. Pellicciari J. Org. Chem. 1982,47 3242. 100 M. S. Baird lead to good yields of cyclopentanones by insertion into the C(5)-H bond." In a similar reaction intramolecular addition of a carbene from another a-diazo$ -keto- ester has been used to obtain (94) a key step in a synthesis of cycloeudesmol.81 All of these reactions underline the preference for formation of five-membered rings in both addition and insertion reactions.0 Br-The regioselectivity of cyclopropanation of 1,3-dienes by ethyl diazoacetate has been examined in the presence of a range of catalysts and has been used to define a 'metal carbene regioselectivity index'.82 The reaction of l,l-dichloro-4-methy1- penta- 1,3-diene with 1-menthyldiazoacetate in the presence of a chiral catalyst gave (95)as a cis trans-mixture but the enantiomeric excesses for the two isomers were only 3 1and 51YO respectively. However addition to 2-methyl-5,5,5-trichloropent-2-ene followed by de hydrochlorination of the intermediate (trichloroethy1)cyclopro-pane gave largely cis-(95) with an enantiomeric excess of over 90% of the opposite enantiomer to that obtained from the diene.83 The rhodium-catalysed reaction of diazo-esters leads to a carbene which undergoes selective insertion into the HO bond of unsaturated and acetylenic alcohols.Depending on the catalyst counter- anion and the nature of the ester some addition to the double or triple bond can be Copper-catalysed decomposition of MeCOC(N2)C02R and (MeC0)2CN2 in the presence of vinyl ethers leads to 4-(alkoxycarbony1)- and 4-acyl-2,3-dihydrofurans respectively. Substituent effects indicate a non-synchronous stereospecific addition of a metal-carbene intermediate to the alkene viz.(96). This leads to cyclopropanes 8o D. F. Taber and E. H. Petty J. Org. Chem. 1982,47,4808. E. Y. Chen Tetrahedron Lett. 1982,23,4769. 132 M.P. Doyle R. L. Dorow W. H. Tamblyn and W. E. Buhro Tetrahedron Lett. 1982 23 2261. 83 T. Aratani Y. Yotleyoshi and T. Nagase Tetrahedron Lett. 1982 23 685. A. F. Noels A. Demonceau N. Petiniot A. J. Hubert and P. Teyssie Tetrahedron 1982 38 2733. Arynes Carbenes Nitrenes and Related Species dihydrofurans and products of apparent insertion of a carbene into allylic Thermolysis of dimethyl diazomalonate in the presence of benzaldehyde leads to diastereomeric dioxolanes (97) together with (98). The reaction is presumed to occur by initial formation of di(methoxycarbonyl)carbene and kinetic measure- ments provide no evidence for induced decomposition of the diazo-compound by benzaldehyde.The yield of oxirane was dramatically increased by decomposition of the diazo-compound in the presence of copper. The oxirane is+no_t a precursor of the dioxolanes and an intermediate carbonyl ylide PhCH=O-CH(C02Me)2 which can lead to either product is consistent with the kinetics and with the trapping with dimethyl fumarate that leads to (99).86 n qO,Me Oxiranylidene (100) is predicted to be a stable observable species; its lowest energy unimolecular decomposition is to methylene and carbon monoxide. By contrast formylmethylene and hydroxyvinylidene are predicted to rearrange to ketene and hydroxyacetylene respectively without activation energy.87 Oxirene intermediates that are formed by oxidation of acetylenes may be further oxidized to diketones or may rearrange without the involvement of keto-carbenes; in consequence keto-carbene-keto-carbeneinterconversions should proceed via an intermediate which is not an oxirene.88 The ambiphilic behaviour of methoxychlorocarbene has been demonstrated in its reactions with substituted styrenes; thus the reactivities of 4-methoxy- and 4-nitro-styrenes are 1.50 f 0.03 and 1.27 f 0.02 respectively relative to styrene itself.89 Phenoxychlorocarbene generated by thermolysis of 3-chloro-3-phenoxydiazirine also shows an ambiphilic reactivity pattern with a series of alkene~.~' This carbene may also be generated by reaction of a,a-dichloroanisole with sodium hydroxide under phase-transfer conditions; it reacts with styrenes by addition and the selectivity is found to increase at lower temperatures.In this study the carbene behaves as a nucleophilic species.'' The reaction of bromo(pheny1)diazirine with methoxide ion gives methoxy(pheny1)diazirine as an unstable species which generates methoxyphenyl- carbene on photolysis. The latter can be trapped by ethanol as PhCH(0Me)OEt and undergoes addition to alkyl-substituted alkenes albeit in low yield.92 " M. E. Alonso A. Morales and A. W. Chitty J. Org. Chem. 1982,47 3747. 86 P. de March and R. Huisgen J. Am. Chem. Soc. 1982 104 4952; R. Huisgen and P. de March ibid. p. 4953. '' W. J. Bourna R. H. Nobes L. Radorn and C. E. Woodward J. Org. Chem. 1982,47,1869. " Y.Ogata Y. Sawaki and T. Ohno J. Am. Chem. SOC.,1982,104 216. 89 R. A. Moss W. Guo and K. Krogh-Jespersen Tetrahedron Lett. 1982,23 15. 90 R. A. Moss L. A. Perez J. Wlostowska W. Guo and K. Krogh-Jespersen J. Org. Chem. 1982,47 4177. '' W. Bruck and H. Durr Angew. Chem. Int. Ed. Engl. 1982,21,916. 92 J. Wlostowska R. A. Moss W. Guo and M. J. Chang J. Cltem. SOC.,Chem. Commun. 1982,432. 102 M. S. Baird Vacuum pyrolysis of the aryl trimethylsilyl ketones (101; R' = H R2= SiMe3) leads to (101; R' = SiMe, R2= H) and trimethylsilyloxybenzocyclobutene.Both products apparently arise uia an intermediate siloxycarbene (102).93 Oxadiazoline (103) decomposes thermally in carbon tetrachloride with first-order kinetics. The reaction apparently proceeds by initial loss of nitrogen to form a carbonyl ylide; this fragments to dicyclopropyl ketone and (acetoxymethy1)carbene.The carbene either undergoes a 1,2-acyl transfer to produce biacetyl or abstraction of a chlorine atom from solvent to give the 1-acetoxy-1-chloroethyl radical.94 3 Nitrenes Photolysis of several polycyclic azides has been used to generate strained cyclic imines as intermediates; for example bicyclo[3.2.l]oct-l-yl azide leads to (104) which may be trapped by methan01.~~ Under particular conditions quantum yields as high as three hundred can be observed in the photodecomposition of phenyl azide; this is consistent with a chain reaction in which phenylnitrene reacts with phenyl azide to produce 1,4-diphenyltetra-azadiene Ph-N=N-N=N-Ph which then decomposes into two phenylnitrenes.Direct reaction of the nitrene and azide to produce two nitrenes and nitrcgen is also possible.96 Photolysis or thermolysis of the azide in acetic acid leads to complex product mixtures including (105) (106) and various products of nucleophilic aromatic substitution. Evidence from reactions in the presence of various addends indicates a reaction leading to 1-azacyclohepta-1,2,4,6-tetraene formed either via a singlet nitrene or a singlet excited phenyl azide; the reaction between nitrene and acetic acid leads to (107) and thence to the aromatic substitution Thermal decomposition of PhN3 passing the products into thioanisole leads to 2-[(phenylthio)methyl]aniline; initial attack of & HN&)\/ aNYMe '0 .-OAc b (104) (105) (106) (107) 93 C.Shih and J. S. Swenton J. Org. Chem. 1982,47 2668. 94 M. Bekhazi and J. Warkentin J. Org. Chem. 1982 41 4870. 95 K. B. Becker and C. A. Gabutti Tetrahedron Lett. 1982 23 1883; T. Sasaki S. Eguchi and T. Okano ibid. p. 4969; H. Quast and B. Seiferling Liebigs Ann.Chem. 1982 1553. 96 W. H. Waddell and C. L. Go J. Am. Chem. Soc. 1982,104,5804. ''H.Takeuchi and K. Koyama J. Chem. Soc. Perkin Trans. I 1982 1269; for a related decomposition of ethyl azidoformate in trifluoroacetic acid see J. Chem. Soc. Chem. Commun. 1982,226. Arynes Carbenes Nitrenes and Related Species singlet phenylnitrene on sulphur followed by a Sommelet-Hauser-type rearrange- ment is thought to explain the The reaction of ethoxycarbonylnitrene with enamines also leads to initial formation of an ~lide.~'~.Pyrolysis of the azide (108; X = N2)leads to a carbazole produced by apparent insertion of nitrene into the C-H bond at A; in contrast photolysis leads to an apparent insertion at the C-H bond at B. Deoxygenation of (108; X = 0)gives the carbazole confirming that formation of the latter is a low-energy process that is favoured by singlet nitrenes. Formation of phenothiazine is also feasible with the triplet species.99 fi ' NX s A (108) Photolysis of NN'- diarylbenzoquinonedi-imine"'-dioxides (109) and N-aryl- benzoquinoneimine N -oxides leads mainly to azo-arenes formed by the dimeriz- ation of triplet arylnitrenes;lOO similar photolysis of N-t-alkyl-benzoquinoneimine N-oxides leads to alkylnitrenes which generally abstract hydrogen to produce amines.lo0 C02Et m:OIFt Me Thermal decomposition of vinyl azides (110) leads to isoquinolines benzazepines or aziridines.When R is vinyl preferential reaction occurs at the R group and (111) is formed in good yield. When R is allyl the result is a much more complex mixture of products. lo' An examination of the activation parameters for thermal isomeriz- ation of 5-methylisoxazole leads to the conclusion that the rate-determining step is R1QR3 R2 Scheme 4 98 (a) L. Benati M. Grossi P. C. Montevecchi and P. Spagnolo J. Chem. SOC.,Chem. Commun. 1982 763; (b) L. Pellacani P. Pulcini and P. A. Tardella J. Org. Chem. 1982,47 5023. 99 D. G. Hawkins and 0.Meth-Cohn J.Chem. Res. (S),1982 105. loo A. R. Forrester M. M. Ogilvy and R. H. Thornson J. Chem. SOC.,Perkin Trans. I 1982 2023; P. J. Baldry. A. R. Forrester M. M. Ogilvy and R. H. Thornson ibid.. p. 2027; see also ibid. p. 2035. I"' D. M. B. Hickey C. J. Moody and C:W. Rees J. Chem. Soc. Chem. Commun. 1982 1419. 104 M. S. Baird not a simple ring-opening but rather a rearrangement to a 3-acyl-l-azirine which can then equilibrate with the ring-opened vinylnitrene (Scheme 4). lo* Oxidation of 3-amino-2-(arylalkyl)quinazolin-4(3H)-ones leads to N-nitrenes (112) which react intramolecularly with methoxy-substituted benzene rings. The substitution pattern required for this to occur suggests a reaction occurring through a seven-membered transition state involving electron donation from the aromatic ring to the vacant p-orbital of the nitrene.lo3 The synthesis direct spectroscopic observation and kinetics of decomposition of the persistent 1,l-diazenes N- (2,2,6,6-tetramethylpiperidyl)nitrene and N-(2,2,5,5-tetramethylpyrrolidyl)-nitrene have been rep~rted."~ Oxidation of 1-nitroso-1-alkylhydrazineswith a variety of reagents leads to products apparently derived by coupling of radical fragments that ar? produced by the extrusion of nitrogen from N-nitrene intermedi- ates viz.PhCH2 and NO from the PhCH2N(NO)-N radical;lo5 however oxida- tion of hydrazines with benzeneseleninic acid to produce tetrazenes has been found by trapping experiments to be uniikely to involve N-aminonitrenes. lo6 Oxidation of 2,4-dinitrobenzenesulphenamidewith lead tetra-acetate in the presence of (2)-1-phenylpropene leads to a ca.3 :1 mixture of cis-and trans-aziridines apparently by addition of the ArSN radical. The ratio of products is unaffected by traps for triplet nitrenes such as 1,l-diphenylethene 2-phenylpropene or oxygen although the alkenes do react rapidly. The ratio is changed by the addition of a trap for singlet nitrenes i.e. ally1 aryl sulphide. It seems that two intermediates are involved in the reaction; one is the singlet nitrene but the other is not the triplet."' 4 Silylenes Flash vacuum pyrolysis of (113; R = Me) at 680°C leads to MeOSiMe3 and the silacyclobutane (114). This can be explained by loss of the former from (113; R = Me) to produce a-cyclopropyl-a -methylsilylene (115) which ring-expands to J.D. Perkz G. I. Yranzo and D. A. Wunderlin J. Org. Chem. 1982,47,982. lo3 R. S. Atkinson J. R. Malpass and K. L. Woodthorpe J. Chem. SOC.,Perkin Trans. 1 1982 2407. W. D. Hmsberg P. G. Schultz and P. B. Dervan J. Am. Chem. SOC.,1982,104 766; P. G. Schultz and P. B. Dervan. ibid. p. 6660. K. Kano C. A. Kelly and J.-P. Anselme Tetrahedron Lett. 1982,23 1427. T. G. Back and R. G. Kerr Cuk. J. Chem. 1982,60,2711. lo' R. S. Atkinson B. D. Judkins and N. Khan J. Chem. SOC.,Perkin Trans. I 1982,2491. Arynes Carbenes Nitrenes and Related Species 1-methyl-1-silacyclobuteneby a silylene-to-silene rearrangement; this then ring- opens to 2-methyl-2-silabuta-1,3-dieneand undergoes head-to-tail dimerization.Co-pyrolysis of (113; R = Me) with 2,3-dimethylbutadiene produces (116) by trapping of the silylene. It is interesting to note that cyclopropylmethylsilylene seems to undergo an alkyl shift to the silacyclobutene rather than a hydrogen shift. Flash vacuum pyrolysis of (117) leads to MeSiH,C=CH among other products; this may arise uia a -vinyl-a- methylsilylene ring-closure to 1-methyl-1-silacyc-lopropene rearrangement to 3-methyl-3-silacycloprop-l-ene,and then further rearrangement to the alkyne."' Pyrolysis of (118) at 540°C in the presence of methanol leads to (119); this may be explained once again by formation of a silyl- ene a-cyclopropyl-a-phenylsilylene,which ring-expands to 1-phenyl-1-silacyclo-butene ring-opens to 2-phenyl-2-silabuta- 1,3-diene and is trapped by Me Me&- C1 ISi Ph IH&=CH-Si-OMe SiMe I Me 1 Me Pyrolysis of (113; R = SiMe,) leads to complex products including vinyltrimethyl- silane and 1-trimethylsilylprop-1-yne.An intermediate a-trimethylsilyl-a-cyclo-propylsilylene can again be trapped by 2,3-dimethylbutadiene; the authors speculate that in the absence of a trap this silylene may again ring-expand to produce 1-trimethylsilyl-1-silacyclobutene,and that a shift of a trimethylsilil group may then occur to give (120). This second silylene (120) might then fragment to H,C=Si :and vinyltrimethylsilane. It is interesting that the silylenes Me3Si-SiCH2CH=CH2 (12 1) and Me3Si-Si-CH=CH-CH3 (122) also lead to vinyltrimethylsilane and trimethyl- silylprop-1-yne in similar ratios.The similarity of the products from the three silylenes suggests that in the absence of traps they merge onto the same energy surface. When (121) was generated in the presence of 2,3-dimethylbuta-1,3-diene one of the products was (1231 apparently produced by trapping (124)."'Generation of the silylene Me3SiSiMe2SiMe in the presence of the butadiene trapping agent leads to (125); this is the first clear evidence of a silylene4silene rearrangement the species actually trapped apparently being Me2Si=Si(Me)SiMe3.' l1 log T. J. Barton G.T:Burns W. F. Goure and W. D. Wulff J. Am. Chem. SOC.,1982,104 1149. W. Ando Y.. Hamada and A. Sekiguchi J. Chem. SOC.,Chem. Commun. 1982,787. 'lo S. A. Burns G. T. Burns and T. J. Barton J. Am. Chem. SOC.,1982,104,6140.''I H. Sakurai H. Sakaba and Y. Nakadaira J. Am. Chem. SOC.,1982,104,6156. 106 M. S. Baird SiMe (125) Contradictions in the literature concerning the ease of rearrangement of MeSi(H)=CH to dimethylsilylene have been reconciled in terms of the kinetics of competing processes involved in the different conditions for silene generation." Ab initio calculations on the insertion of silylenes into 0-Hbonds indicate that H20-SiH2 is a stable complex formed from singlet silylene and water and has a significant barrier to rearrangement to more stable silanol; the carbon analogue is highly unstable and has been predicted to rearrange without a barrier.'13 R. Walsh J. Chem. SOC.,Chem. Commun. 1982 1415. n3 K. Raghavachari J. Chandrasekhar and M.J. Frisch J. Am. Chem. SOC.,1982,104,3779.