5 Arynes Carbenes Nitrenes and Related Species By M. S. BAlRD School of Chemistry University of Newcastle upon Tyne Newcastle upon Tyne NEl7RU 1 Arynes Calculations on 0-,rn- and p-benzynes using MNDO UMNDO and MNDO/CI methods agree well with earlier results. The 0-and rn-species are predicted to be similar in energy whereas the p-form should be considerably less stable. Both rn-and p-species are predicted to occur in two isomeric forms a bicyclohexatriene and a phenylene biradical with the latter of lower energy in each case. The bicyclic and biradical species should be separated by appreciable barriers (9 and 3 kcal mol-' respectively). The degenerate rearrangement of hex-3-ene- 1,5-diyne is found to involve the intermediacy of p-benzyne.' Benzynes are obtained on treatment of the readily available 6-chloropentafulvenes with strong base.Thus reaction of the di-t-butyl derivative (1) with lithium piperidide leads to I -piperidyl-3,5-di-t-butylbenzene, and the intermediate benzyne (2) may be trapped by [4 + 21 cycloaddition to benzofuran.2 I I (1) (2) (3) (4) The oxazoline (3; X = C1 Y = H) reacts with organolithium reagents to give selectively the benzyne (4). This was reported to react further by addition of the organolithium to produce (3;X = alkyl Y = Li). However it is now found that with n-butyl-lithium most products are derived from the alternative addition to produce (3; X = Li Y = Bu"). This is thought to be due to the kinetic effect of complexation of the organolithium to the oxazoline group.3 Reaction of the bromides (5; X = Br Y = H) and (5; X = H Y = Br) with the complex base t-butoxide ion-amide ion leads to the benzynes (6) and (7) respectively which are identified as their furan adducts.The regioselectivity of the former reaction is explained in terms of the lower bond length and angle deformations in the benzyne pr~duced.~ ' M. J. S. Dewar G. P. Ford and C. H. Reynolds J. Am. Chem. Soc. 1983 105 3162. ' K. Hafner H.-P. Krimmer and B. Stowasser Angew Chem. Int. Ed. EngL 1983 22 490. ' A. I. Meyers and P. D. Pansegrau Tetrahedron Letr. 1983 24 4935. B. Halton and C. J. Randall J. Am. Chem. Soc. 1983 105. 6310. 101 102 M. S. Baird Treatment of 1,2,4,5-tetrabromobenzenewith two equivalents of butyl-lithium in the presence of furan leads to the formation of (8),5 and 1,4-dilithio-tetrahalogenoarenes may be prepared from the corresponding hexahalides at low temperature and undergo elimination to give arynes at higher temperatures6 2 Carbenes STO-3G calculations show that triplet di-t-butylcarbene is ca.25 kcal mol- ' more stable than singlet. Because of steric repulsions between the alkyl groups the central CCC angles are 132.5' (singlet) and 142" (triplet). The normal alkyl-group stabiliz- ation of the singlet is overridden by destabilization due to sterically forced increase in this central angle. The value for the triplet is very close to the experimental figure.' Ab initio calculations on the lowest closed-shell singlet states of six isomers of C2H2S show that (9) (So),(10) (So),and (I 1) (S,) are the least stable species with stability decreasing in the order given.* Theoretical studies have also been reported CO,H (12) (13) on the ground triplet electronic state of cyanocarbene; these suggest a near degeneracy between the bent carbene and a linear allene-related geometry.' Calcula- tions relating to the reactions of atomic carbon with water and ammonia include an examination of hydroxycarbene and aminocarbene ;the barrier for rearrangement of the former species to methanal is found to be 38.9 and 41.3 kcal mol-' respectively for the singlet and triplet forms."." In practice the reaction of arc-generated carbon atoms with ammonia leads to NH-insertion and H-abstraction.The former leads to formaldimine and hydrogen cyanide and the latter to methylene which reacts with ammonia to produce methylamine.Hydrolysis of non-volatile residues leads to amino-acids." The reaction of atomic carbon with methanal has also provided evidence for the involvement of the first excited singlet state (I B,) of methylene. As predicted this appears to add to alkenes in a non-stereospecific manner.I3 H. Hart N. Raju M. A. Meador and D. L. Ward J. Org. Chem. 1983.48 4357. H. Hart and G. C. Nwokogu Tetrahedron Lett. 1983 24 5721. ' P. H. Mueller N. G. Rondan K. N. Houk J. E. Gano and M. S. Platz Terrahedron Lett. 1983,24,485. R. K. Gosavi and 0. P. Strausz Can. J. Chem. 1983 61 2596. K. S. Kim H. F. Schaefer L. Radom J. A. Pople and J. S. Binkley J. Am. Chem. SOC.,1983 105 4148.lo S. N. Ahmed M. L. McKee and P. B. Shevlin J. Am. Chem. SOC.,1983 105 3942. I' D. W. McPherson M. L. McKee and P. B. Shevlin J. Am. Chem. SOC.,1983 105 6493. l2 P. B. Shevlin D. W. McPherson and P. Melius J. Am. Chem. Soc, 1983 105 488. l3 S. N. Ahmed and P. B. Shevlin J. Am. Chem. Soc.. 1983 105 6488. Arynes Carbenes Nitrenes and Related Species Diazirines have been shown in recent years to be a very effective source of carbenes. It is now reported that exchange reactions of halogenodiazirines (12; X = C1 or Br) allow ready access to a variety of substituted derivatives e.g. (12; X = CN) with tetra-n-butylammonium cyanide and (1 2; X = N,) with tetra-n-butylammonium azide. The former is a source of cyanophenylcarbene which may be trapped by alkenes while thermolysis of the latter leads to benzonitrile either through azido- phenylcarbene or (12; X = N:).I4 The diazirine (13) is a highly photolabile carbene- generating label which is readily fixable to biochemical agent~.'~ Normally stable lithio-derivatives of diphenyl thioacetals decompose cleanly to carbenes when another negative charge is present nearby in the same molecule.The derived carbenes can be highly selective and their reactions depend on the nature of the second anionic site and its relationship relative to the carbene centre. Thus (14) is converted into (15) at 0°C via a 1,2-hydrogen shift and (16) leads to (17).16 Crown ether- catalysed dehydrobromination of o-(bromomethy1ene)adamantane with potassium t-butoxide provides a convenient and efficient source of adamantylidenecarbene which can be trapped by alkenes.In competition experiments the carbene is very similar to isopropylidenecarbene and it appears to be an unencumbered electrophilic singlet. Plots of their relative reactivities versus the HOMO levels of the alkenes both show non-linearity in keeping with considerable steric retardation." I (14) (15) (17) Interest in absolute rate measurements for carbene reactions has continued. Photolysis of a series of 3-aryl-3-chlorodiazirinesin 3-methylpentane matrices at 77 K led to arylchlorocarbenes which showed U.V. spectra similar to those observed when the carbenes were generated by laser flash photolysis in iso-octane at 300 K. When the laser excitation was carried out in the presence of a series of alkenes the decay of the transient absorption followed pseudo-first-order kinetics.Analysis of the kinetic data gave good Hammett correlations p = +I.4-1.6 with o+-constants of the aryl substituent ; electron-donating substituents in the carbene retard the additions. Moreover although each carbene shows the expected electrophilic selec- tivity the carbenes all show a comparable range of selectivities towards the series of alkenes.18 Absolute rates of reaction of fluoro- bromo- and chloro-phenyl carbenes again generated by laser flash photolysis indicate that with each alkene the reactivity order is Br > C1 > F; this is the reverse of the carbene stability order based on expected halogen lone-pair resonance stabilization of an adjacent singlet carbene p-orbital.In addition the selectivities of the carbenes towards a series of alkenes as measured by the rate spread in each case follows the inverse order i.e. F > CI > Br. This provides direct evidence of the inverse relationship between l4 D. P.Cox R. A. Moss and J. Terpinski J. Am. Chem. SOC.,1983 105 6513. Is M. Nassal Liebigs Ann. Chem. 1983 1510. l6 T. Cohen and L.-C. Yu J. Am. Chem. Suc. 1983 105 281 I. I' T. Sasaki S. Eguchi M. Tanida F. Nakata and T. Esaki 1. Org. Chem. 1983 48 1579. IR R. A. Moss L. A. Perez N. J. Turro 1. R. Could and N. P. Hacker Tetrahedron Lett. 1983. 24 685. 104 M. S. Baird absolute reactivity and selectivity such a 'normal' relationship has been assumed for some time.'' It is interesting to note that the reactions of halogenocarbynes with alkenes produce a very similar pattern of absolute rate versus selectivity.20 An analysis of the effect of a two-step mechanism for carbene addition involving a carbene-alkene complex on the relationship between absolute rates as determined by laser flash photolysis and relative rates as determined by product ratios has been provided.It is concluded that the widespread occurrence of valid correlations means that complex formation may not be a general phenomenon or that when it does occur complexation is either much faster or much slower than the product- forming step.2' The absolute rates of reaction of phenylchlorocarbene and p-anisylchlorocarbene with carboxylic acids have been found to be very high compared with rates of reaction with alkenes; rates are relatively insensitive to the acid strength suggesting an early transition state.The high reactivity must be associated with the HO-bond since ethyl acetate is not an effective quencher.22 In cycloaddition reactions of methyl-substituted alkenes with halogenocarbenes the steric repulsion between the methyl groups and the carbene substituents appears to be negligible. However when a series of alkenes (18)-(20) is examined each set of alkenes gives a different correlation for the relative reactivities of.dibromo- and dichloro-carbenes i.e. no common Skell-Moss linear correlation exists between the reactivities of the two carbenes towards alkenes if the number and bulkiness of the substituents is changed.23 R R R )=: L P= '* Me (18) (19) (20) (21) Phenoxychlorocarbene derived by thermolysis of phenoxychlorodiazirine acts as an ambiphile in addition reactions with styrene derivatives.This is consistent with frontier orbital predictions and with its ambiphilic behaviour towards sub- stituted ethenes; however it is in contrast to the reported nucleophilic behaviour of the carbene towards styrenes when it is generated from a,a-dichloroanisole and base under phase-transfer conditions. It is suggested that this latter process actually involves attack of the dichlorophenoxymethyl anion on the styrene rather than the ~arbene.~~ Addition of carbenes (2 1) to the unsymmetrical alkenes 2-methylpropene and 3,3-dimethylbut-1-ene leads in all cases to predominantly E -rather than 2-products with increased stereoselectivity for the second alkene and when R is changed from ethyl to t-butyl.The highest stereoselectivity observed was about 10 1 corresponding to an energy difference of ca. 1 kcal mol-' at -20 "C.Model studies of the vinylidene- ethylene reaction by MNDO and ab initio methods and of more complex examples using MNDO indicate that attack of the carbenes on 2-methylpropene leads to the '' D. P. Cox I. R. Could N. P. Hacker R. A. Moss and N.J. Turro Tetrahedron Lett. 1983 24 5313. 20 B. P. Ruzsicska A. Jodhan H. K. J. Choi 0. P. Strausz and T. N.Bell J. Am. Chem. Soc. 1983 105 2489. " M. S. Platz Tetrahedron Lett. 1983 24 4763. 22 D. Griller M. T. H. Liu C.R. Montgomery J. C. Scaiano and P. C. Wong J. Org. Chem. 1983 48 1359. 23 B. Giese W. Bung Lee and C. Stiehl Tetrahedron Lett. 1983 24 881. 24 R. A. Moss and L. A. Perez Tetrahedron Lett. 1983,24,2719;see W. Bruck and H. Durr Angew. Chem. Int. Ed. Engl. 1982 21 916. Arynes Carbenes Nitrenes and Related Species favoured transition state (22) whereas with 3,3-dimethylbut- 1-ene the transition state resembles the perpendicular form (23).25 An ab initio study of the reaction of the carbenoid (24) with ethylene leads to a transition state (25) in which the methylene group is in a plane nearly parallel to that of the alkene. The alignment allows the LUMO to interact in an electrophilic sense with the double-bond HOMO on one side and with the fluorine lone pair on the other.This transition state is quite similar to that for free halogenocarbene cycloadditions except that the carbene fragment is less strongly bound.26 Phenyl(tri- bromomethy1)mercury reacts with either fumaronitrile or styrene to give high yields of cyclopropanes but in both cases the reaction is not stereospecific even though a triplet reaction does not appear to be involved. Kinetic analysis indicates that two distinct intermediates are involved and that the loss of stereospecificity is directly proportional to the concentration of the mercury deri~ative.~’ A detailed analysis of ionization potentials electron affinities and .rr-orbital shapes of 2-substituted bicyclo[2.2. llheptadienes has allowed an interpretation of the reac- tivities and selectivities of these species in carbene cycloadditions.A 2-substituent not only affects the 2,3-.rr-bond but also influences the 5,6-bond by through-space interaction. Orbital energy changes and polarization induced by the substituents provide a rationale of variations in 1,2-and homo- 1,4-addition processes and confirm the electrophilic nature of both reactions.28 The main product from the reaction of dilithiopentalene with dichloromethane and methyl-lithium is (26). This is thought to arise by addition of chlorocarbene followed by loss of chloride ion to produce (27) which undergoes 1,4-addition to give (28). Reaction of (29) with methyl-lithium leads to (30) in moderate yield.” (29) (30) ’’ Y Apeloig M. Karni P. J. Stang and D.P. Fox J. Am. Chem. SOC.,1983 105 4781. 26 J. Mareda N. G. Rondan K. N. Houk T. Clark and P. von R. Schleyer J. Am. Chem. SOC.,1983 105 6997. 27 J. B. Lambert E. G. Larson and R. J. Bosch Tetrahedron Lett. 1983 24 3799. 28 K. N. Houk N. G. Rondan M. N. Paddon-Row C. W. Jefford P. T. Huy P. D. Barrow and K. D. Jordan J. Am. Chem. SOC.,1983 105 5563. 29 U. Burger and B. Bianco Helc. Chim. Acta. 1983 66 60. 106 M. S. Baird Intramolecular addition of the keto-carbenes derived from (3 1) and (32) occurs at the bonds indi~ated.~'?~' Related reactions have been used in syntheses of sarkomy- cin and of cyclopentanoid terpenic acids,32 and copper-catalysed addition of the carbene derived from (33) gave a bicyclo[4.1 .O]heptane which is a key intermediate in a synthesis of sirenin;'3 a rather more exotic 6,7-addition is reported in the case of the carbene (:4) when the product is (35).34 A range of esters (36) undergo rhodium-catalysed cyclization to cyclopentenes (37) with relatively high (ca.85 15) diastereoselection in favour of the isomer shown.Furthermore the major and minor diastereoisomers are separable chromatographi- cally providing a simple route to cyclopentane derivatives of high optical purity. The reaction is explained in terms of a transition state such as (38) with the transferred hydrogen occupying one position in a chair six-membered ring and assuming an insertion which proceeds with retention of c~nfiguration.~~ R (36) (37) (38) Singlet excitation of the oxirane (39) leads to the carbonyl ylide (40) together with two carbenes (41) and (42).The former rearranges to a cyclopropenc but the latter gives (43),the product of apparent carbene insertion into the C-C bond of (41).36 Photolysis of (44) in acetonitrile leads to cleavage of the oxirane to (45) and (46) as the main singlet processes. The former carbene rearranges to a cyclopropene but 30 T. Hudlicky D. B. Reddy S. V. Govindan T. Kulp B. Still and J. P. Sheth J. Org. Chem. 1983,48,3422. 3' R. J. Sundberg and T. Nishiguchi Tetrahedron Left. 1983 24 4773. 32 S. V. Govindan T. Hudlicky and F. J. Koszyk J. Org. Chem. 1983 48 3581; R. P. Short J.-M. Revol B. C. Ranu and T. Hudlicky ibid. p. 4453. 33 T. Mandai K. Hara M. Kawada and J. Nokarni Tetrahedron Lett.1983 24 1517. 34 J. A. Marshall J. C. Peterson and L. Lebioda J. Am. Chem. SOC.,1983 105 6515. 35 D. F. Taber and K. Rarnan J. Am. Chem. Soc. 1983 105 5935. 36 N. Bischofberger B. Frei and 0. Jeger Helu. Chim. Acta 1983 66 1638. 107 Arynes Carbenes Nitrenes and Related Species )= (43) OH (44) ) OH OH (45) the latter again inserts this time into the adjacent carbinol C-H bond to give the en01.~’ Treatment of dibromides (47; X = Br) with methyl-lithium leads to cyclopropyl- idenes which undergo efficient insertion into the 5,6-related C-H bonds of the R group; no insertion into 3,4-related bonds is observed possibly owing to co-ordina- tion of the intermediate lithio-bromide (47; X = Li) to the acetal oxygen^.^^ The carbene (48) generated by thermolysis of the !ithium salt of the corresponding tosyl hydrazone inserts exclusively into the C -H bond indicated.39 The photolysis of several a-diazo-amides in mixed solvent systems gave unusually large OH CH insertion selectivity ratios of 103-104 1 far higher than those observed for photolysis of diazoacetate esters.Photolysis of the amides in t-butyl alcohol or 2,3-dimethylbutane led to OH-and tertiary-CH insertion products respec- tively and no primary CH-insertion product was isolated in either case indicating a very discriminating intermediate. The effect of the carboxamide group is thought to be electronic in origin and is consistent with an increased electrophilic character of the intermediates in respect of OH-insertion and more discriminative behaviour in respect of CH-in~ertion.~’ 37 G.de Weck N. Nakamura K. Tsutsumi H. R. Wolf B. Frei and 0.Jeger Helu. Chim. Acta 1983 66 2236. 3n J. Arct L. Skattebbl and Y. Stenstrom Acta Chem. Scand. (B),1983 37 681. 39 C. A. Andruskiewicz and R. K. Murray J. Org. Chem. 1983 48 1926. 40 J. Wydila and E. R. Thornton Tetrahedron Lett. 1983 24 233. 108 M. S. Baird Lithium alkoxides of alkyl allyl and benzyl alcohols react with chloroform in the presence of lithium t-butoxide to give dichloromethylcarbinols by insertion of dichlorocarbene into the a-C-H bond of the alkoxide (Scheme I). X / R’ :C \ R‘ X Scheme 1 Potassium alkoxides react in an analogous manner with benzal chloride and potassium t-butoxide by insertion of phenylchlorocarbene into the a-C-H bond followed by cyclization of the resulting 2-chloro-2-phenethyl alkoxide to the stereoisomeric oxiranes (Scheme 2).41 Dimethylvinylidenecarbene also inserts regioselectively into the a -C-H bond of alkoxides giving allenylcarbinols e.g.(49) in moderate yield.42 Scheme 2 Rhodium-catalysed addition of diazo-compounds to (50) leads to ring expansion to (51) in high yield apparently through the ylide (52).43 Similarly (53) reacts with diphenyldiazomethane to produce a 1 :2 adduct (54) apparently via another S-vlide.44 Treatment of (55) with chloroform and base under phase-transfer conditions gave (56)in addition to the product of apparent 1,2-dichlorocarbene addition to the double bond.The first product is analogous to one earlier reported between (55) and diphenylpropenylidene and various routes to it are possible including 1,Ccarbene addition or intermediate formation of the carbonyl ylide (57).45Thermal decomposi- tion of several phenyltrihalogenomethylmercury compounds in the presence of 41 T. Harada E. Akiba and A. Oku J. Am. Chem. Soc. 1983 105 2771. 42 T. Harada Y. Nozaki and A. Oku Tetrahedron Lett. 1983 24 5665. 43 W. D. Crow I. Gosney and R. A. Ormiston J. Chem. SOC.,Chem. Commun. 1983 643. 44 D. M. McKinnon Can. J. Chem. 1983 61 1161. 45 H. Hart and J. W. Raggon Tetrahedron Lerr. 1983 24 4891. Arynes Carbenes Nitrenes and Related Species substituted benzaldehydes and dimethyl acetylenedicarboxylate gave 2-halogeno-5- arylfuran-3,4-dicarboxylatesby selective attack of the resulting carbene on the aldehyde followed by trapping of the carbonyl ylide by the acetylene and elimination of HCl.46 Thermolysis of phenyl(bromodichloromethy1)mercury in the presence of benzophenone leads to Ph,CClCOCl as the only major product.An intermediate carbonyl ylide could not be trapped by addition of dimethyl acetylenedicarboxylate. It is suggested that the difference in behaviour between benzophenone and benzal- dehyde is due to the adoption of a different geometry in the ylide derived from benzophenone because of endqendo-interaction of chlorine and the aromatic rings (58) leading to rapid closure to (59) which rearranges to the observed produ~t.~’ PhAvA;h phAQA;h 0 c1 c1 (55) (56) I Ph 0 c1 %% Ph CI (57) (58) (59) The formation of carbon monoxide from the reaction of dibromocarbene with various aldehydes is extremely general contrary to earlier reports ;when carried out on benzaldehyde labelled at the aldehyde carbon the label is retained indicating that the reaction involves deoxygenation rather than decarbonylation.Variations in the yield of carbon monoxide with the structure of aliphatic aldehydes coupled with earlier results lead to the mechanism shown in Scheme 3 in which steric and conformational effects on the rate of step b play an important part in determining the yield. For aromatic compounds there is a linear correlation between carbon monoxide production and increasing electron release by the substituent,.indicating a dominance of electronic factors.48 alkenes lb \ Br-0-C-Br C+ -co Br t \c’ ‘ ‘Br \C/ ‘Br /\Br Scheme 3 46 H. S. Gill and J. A. Landgrebe J. Org. Chem. 1983 48 1051. 47 C. W. Martin H. S. Gill and J. A. Landgrebe J. Org. Chem. 1983 48 1898. 48 Z. Huan J. A. Landgrebe and K. Peterson Tetrahedron Lett. 1983 24 2829; J. Org. Chem. 1983 48 4519. 110 M. S. Baird Thermolysis of 2-methoxy-A3- 1,3,4-0xadiazoline involves loss of nitrogen to form a carbonyl ylide which mainly fragments to a carbonyl compound and a carbene the latter of which can be trapped by alkene~.~~ Thermolysis of (60) also leads to a carbonyl ylidt which can be trapped by cycloaddition to acetone but also undergoes fragmentation either to methyl acetate and 1 -methylethylidene or to acetone and 1-methoxyethylidene.Both carbenes react with acetone to generate carbonyl ylides and also insert into C-H(D) bonds of the ketone. The carbonyl ylides themselves react with acetone by 1,3-~ycloaddition.~~ Reaction of cyclopentadienylidene with oxygen in a low-temperature matrix leads to an intermediate which has been formulated as the carbonyl oxide (61) on the basis of i.r. data.5' Laser photolysis of 1-naphthyldiazomethane in acetonitrile has been postulated to produce the nitrile ylide (62). Photolysis in acetonitrile gives the same absorption spectrum and the transient species in each case reacts with acrylonitrile at the same rate; the products are (63; X = CN Y = H) and (63; X = H Y = CN).Photolysis of the diazirine (64) in acetonitrile-acrylonitrile leads to both carbene- and nitrile- ylide-derived products.52 eo/o ... MkoMe NO h+Me Me N=N NP H \Me Y // (62) Np = I-naphthyl (63) (64) It is widely believed that singlet arylcarbenes insert into 0-H bonds and add stereospecifically to alkenes but that the triplet ground states are efficient hydrogen- abstracting agents and add non-stereospecifically to alkenes. Rapid quenching of optical absorption spectra due to triplet carbenes by methanol has been explained in terms of a thermal equilibrium between triplet and singlet states; however it is also possible that the triplet reacts directly with methanol. It is now reported that the triplet state of dimesitylcarbene does not convert into the singlet state and that the two carbenes do indeed show quite different chemistries.The triplet species generated from the corresponding diazo-compound does not react with its precursor to give an azine but instead undergoes dimerization; the lifetime of the carbene is essentially unaffected by the addition of methanol although the quantum yield of the triplet is reduced owing to scavenging of the singlet carbene precursor.53 Photoly- 49 M. Bekhazi and J. Warkentin Can. J. Chern. 1983 61 619. 50 M. Bekhazi and J. Warkentin J. Am. Chem. SOC.,1983 105 1289. '' G. A. Bell and I. R. Dunkin J. Chem. SOC.,Chem. Commun. 1983 1213. 52 R. L. Barcus B. B. Wright M. S. Platz and J.C. Scaiano Tetrahedron Left. 1983 24 3955. 53 A. S. Nazran and D. Griller J. Chem. Soc. Chem. Cornrnun. 1983 850. Arynes Carbenes Nitrenes and Related Species sis of dimesityldiazomethane in a glass leads to a strong e.p.r. signal due to dimesitylcarbene. The signal changes on annealing the glass and it is suggested that the rigid environment prevents the carbene from adopting its preferred near-linear minimum-energy ge~metry.'~ Photolysis of diphenyldiazomethane in (RS)-butan-2-01 between ambient tem- perature and -1 15 "C gives (65) as the only volatile product. Photolysis in polycrystal- line (S)-butan-2-01 at -196 "C also gives the ether together with products of carbene insertion into each non-equivalent C-H bond of the alcohol. Insertion into the tertiary C-H bond gives a single enantiomer.At -196 "C an intense and relatively long lived e.s.r. signal is seen for triplet diphenylcarbene. It is felt to be unlikely that all the tertiary alcohol is derived from the singlet carbene and a substantial amount must be formed from the triplet uia the radical pair (66). Although each component is achiral the radical pair is chiral and the solid environment allows coupling in only one sense.55 Photoacoustic calorimetry has been used to determine the heat of reaction for formation of triplet diphenylmethylene from diphenyldiazomethane; the value obtained -12 f2 kcal mol-' is in contrast to the endothermic heat of reaction of ca. 33 kcal mol-' for triplet methylene from diazomethane. The heat of reaction for photolysis of diphenyldiazomethane in ethanol which leads to (67) is -54 f 2 kcal mol-I leading to a heat of reaction of -47 * 2 kcal mol-' for formation of the ether from singlet diphenyl~nethylene.~~ Photolysis of 2-biphenyldiazomethane at 77 K leads to a single set of triplet carbene resonances in the e.s.r.spectrum assigned to an unresolved superposition of signals due to syn- and anti-rotamers of 2-biphenylmethylene. Kinetic studies indicate that the triplet carbene decays faster in ether than in inert matrices and an abstraction-recombination mechanism is predicted to be the major decay route in an ether glass.57 Photolysis of diazoindene generates indenylidene (68) which reacts with simple alkenes by addition or by insertion into an allylic C-H bond.The stereochemistry of the addition has been investigated as a function of dilution of the alkenes with octafluorocyclobutane or 2,3-dimethylbuta- 1,3-diene. In the presence of 97 mol% of the fluoro-compound ca. 47% of the adduct obtained with cis-butene was derived from triplet indenylidene but in the presence of 25 mol% of the diene the triplet reactions were largely eliminated.58 Irradiation of 9-diazofluorene with a pulsed laser on a picosecond or nanosecond time-scale either at ambient temperature or at 10 K in a glassy matrix leads to fluorenylidene which exists as a rapidly equilibrating singlet-triplet mixture. Analysis of kinetic data indicates that the singlet 54 A. S. Nazran E. J. Gabe Y. Le Page D. J. Northcott J. M.Park and D. Griller J. Am. Chem. Soc. 1983 105 2912. 55 J. Zayas and M. S. Platz Tetrahedron Lett. 1983 24 3689. Sh J. D. Simon and K. S. Peters J. Am. Chem. Soc. 1983 105 5156. 5' E. C. Palik and M. S. Platz J. Org. Chem. 1983 48 963. 58 R. A. Moss and C. M. Young J. Am. Chem. SOC.,1983 105 5859. 112 M. S. Baird is only 1.1 kcal mol-' above the triplet ground state and rates of reaction with penta- 1,3-diene suggest that this small energy gap may be due to increased energy of the triplet caused by angle strain.59 N2 PhKCH-Ph I Me Photolysis of (69) at low temperature leads to an e.s.r. signal due to the triplet ground state of the corresponding carbene. The only kinetically important process at low temperature in a glass is hydrogen abstraction from the matrix.The triplet carbene is long lived in polycrystals but at 103-1 19 K 1,2-hydrogen shifts do occur in polyfluorinated matrices albeit only slowly; the mechanism of this process is not known. The singlet carbene undergoes 1,2-shifts without appreciable product isotope effects when in a matrix but more appreciable effects are seen in fluid solution.60 Thermolysis of the diazirine (70; X = MeO) in the presence of tetramethylethylene leads to 83% intramolecular trapping of the carbene by way of a 1,2-hydrogen shift. When X = Me the intramolecular contribution is reduced to 64'70,and with X = Ph the carbene is exclusively trapped by intermolecular addition to the alkene. Apparently the methoxy-group is better able to stabilize the positive charge developed on carbon in the transition state of the 1,2-hydrogen shift.61 Reaction of the trihalides (71 X = Br C1) with n-butyl-lithium leads to good yields of the alkenes (72) with the 2-configuration.The reasons for the high stereoselectivity are not immediately apparent; however examination of the alternative transition states in a 1,2-hydrogen shift to an intermediate carbene centre (73) and (74) respectively indicate the possibility of dominant lone pair to silicon repulsion or of hyperconjuga- tion of the lone pair to low-lying C-0 a*-orbital favouring the former confor- mation.62 X The carbene (75) generated from the corresponding diazo-compound ring- expands to cyclopentyne which is trapped in a [2 + 21 cycloaddition with 2,3- dihydrofuran ;labelling studies confirm the intervention of a symmetrical intermedi- SQ P.B. Grasse B.-E. Brauer J. J. Zupancic K. J. Kaufmann and G. B. Schuster J. Am. Chem. SOC.,1983 105 6833. 00 H. Tomioka N. Hayashi Y. Izawa V. Senthilnathan and M. S. Platz J. Am. Chem. SOC.,1983 105,5053. " M. T. H. Liu and M. Tencer Tetrahedron Lett. 1983 24 5713. 62 M. C. Pirrung and J. R. Hwu Tetrahedron Leu. 1983 24 565. Arynes Carbenes Nitrenes and Related Species ate.63 However cyclopentyne generated from dibromomethylenecyclobutane and alkyl-lithium undergoes stereospecific 1,2-addition to alkenes and preferred 1,2- addition to buta- lY3-diene; this has been interpreted in terms of an antisymmetrica! singlet ground state (76).64 Flash vacuum pyrolysis of 5-adamantylidene-2,2-dimethyl- 1,3-dioxane-4,6-dione leads to (77) by an unusual rearrangement of an intermediate carbene which can be formulated as in (78); presumably the rearrange- ment occurs because lY2-alkyl shifts to an anti-Bredt alkene or homoadamantene are di~favoured.~~ Labelling studies have shown that the rearrangement of (79) to (80) occurs by a hydrogen rather than an acyl-group migration and no evidence has been obtained for the rapid reversibility of the process.66 (78) Dicyclopropylcarbene can be generated by thermolysis of 5,5-dicyclopropyl-2- methoxy-2-methyl-A3- 1,3,4-0xadiazoline at 80 "C.The carbene undergoes ring expansion to 1-cyclopropylcyclobutene but will also abstract a chlorine from carbon tetrachloride and insert efficiently into the C-H bond of chloroform; it also adds to tetrachloroethylene albeit in low yield.67 Treatment of (8 1) with alkyl-lithium reagents gave a complex product mixture which included (82) and (83).These are thought to arise through rearrangement of the carbene (84) to (85) followed by trapping with ether or intramolecular insertion to produce a bicyclo[ 1.1 .O]butane followed by trapping with water on work-up.68 In the presence of tetramethylethylene the cyclopropylidene (84) is trapped by cycloaddition. However with 1,l -diethoxyethylene or di-t-butyl maleate the ap- parently ambiphilic cyclobutylidene (85) is trapped.69 Treatment of the dibromo- cyclobutane (86) with an alkyl-lithium in the presence of diphenylisobenzofuran 63 J.C. Gilbert and M. E. Baze J. Am. Chem. SOC.,1983 105 664. 64 L. Fitjer and S. Modaressi Tetrahedron Lett. 1983 24 5495. 65 J. Scharp and U. E. Wiersum J. Chem. Soc. Chem. Commun. 1983 629. 66 M. Koller M. Karpf and A. S. Dreiding Helu. Chim. Acra 1983 66 2760. 67 M. Bekhazi P. A. Risbood and J. Warkentin J. Am. Chem. Soc. 1983 105 5675. 68 M. Bertrand A. Tubul and C. Ghiglione J. Chem. Res. (S) 1983 250. 69 A. Tubul A. Meou and M. Bertrand Tefrahedron Lerr. 1983 24 4199. 114 M. S. Baird Br produces (87); this apparently originates by a 1,2-alkyl shift in an intermediate carbene to produce the strained alkene (88) followed by [4 + 23 cy~loaddition.~’ Cleavage of (89; R = H) with sodium methoxide produces a vinylcyclopropylidene by deprotonation of an intermediate diazonium salt.The carbene leads to penta- 1,2,4- triene cyclopentadiene and 4-methoxycyclopentene the latter two products by trapping of the rearranged ‘foiled’ carbene (90). In the presence of methyl vinyl ether and methanol (90) may be trapped as (91) and (92). Protonation of the foiled carbene leads to (93) as a major species which is then trapped by methanol to give 4-methoxycyclopentene; this reaction is dependent on the stereochemistry of the initial vinylcyclopropane (89; R = D) leading to (94) and this rules out the inter- mediacy of a planar intermediate.7’ Vinylcyclopropylidene rearrangements in a number of species derived by treatment of mono- and bis-dibromocarbene adducts of hexatrienes with methyl-lithium have been the subject of a careful labelling study.72 R-Tq-CoNH,R NO @ b moMe (89) (90) (91) H D Pyrolysis of the sodium salt of 2-vinylcyclobutanone tosylhydrazone gives a complex of products including 2-vinylmethylenecyclopropane,1-vinylcyclobutene and methylenecyclopentene ; however no products appear to be derived from cyclohex-3-en-1-ylidene the product expected from the cyclobutane analogue of a vinylcyclopropylidene rearrar~gement.~~ However flash vaccum pyrolysis of the sodium salt of the tosylhydrazone corresponding to (95) leads to (96) (97) and (98).The first two products may be explained by a cyclobutylidene-methylenecyc-70 C. W. Jefford J. C. Rosier J. A. Zuber 0. Kennard and W. B.T. Cruse Tetrahedron Leff.,1983 24 181. ” W. Kirmse P. V. Chiem and P. G. Henning J. Am. Chem. SOC. 1983 105 1695. 72 I. Fleischhauer and U. H. Brinker Tetrahedron Lett. 1983 24 3205. 73 U. H. Brinker and L. Konig Chem. Ber. 1983 116 882. Arynes Carbenes Nitrenes and Related Species lopropane rearrangement and a vinyl migration respectively; the final product is thought to result from a vinylcyclobutylidene-cyclohexenylidenerearrangement to (99) followed by a 1,2-alkyl shift.74 (95) Although the reaction of 1,1 -dihalogenocyclopropanes with methyl-lithium is one of the simplest routes to cyclopropylidenes the corresponding reaction of several I 1,2-trihalogenocyclopropanesleads instead to a 1,2-dehaIogenation to produce cyclopropenes; in the case of (100) the derived cyclopropene undergoes ring expansion under the reaction conditions to generate the vinylcarbene (101).75 Vinyl- carbenes are also generated from the thermolysis or photolysis of allenes.Photolysis of cyclonona-1,2-diene in pentane leads to (102). Two carbenes (103) and (104) appear to be likely intermediates in this rearrangement and indeed when generated independently both lead to some cyclopropane. However other products are also formed and it seems unlikely that either is a true intermediate in the photochemical process. A concerted mechanism involving a 1,2-hydrogen shift and 1,3-bonding (a formal a2a + 1r2a process) may occur.76 Irradiation of tetraphenylallene leads to 1,2,3-triphenyIindene and products derived from it in a process initiated by a singlet reaction ; irradiation of triphenylallene leads to 1,3,3-triphenylcyclopropene diphenylindenes and 1,3,3-triphenyIpropyne.Vinylcarbenes are implicated in these reactions.77 Flow pyrolysis of the allene (I 05) gave 2-methylfuran ; this rearrangement may involve initial 1,2-hydrogen shifts to produce the vinylcarbenes (106) or ( 107).78 Further examples of cyclopropenes as vinylcarbene sources are also reported. Photolysis of the spiro-alkene (108) using far-u.v. radiation leads to (109) as the major product together with ethynylcyclopentane and 1-vinylcyclopentene. Ther- molysis leads largely to the acetylene and gives no allene. The formation of (109) 74 U. H. Brinker and L. Konig Chem. Ber. 1983 116 894.75 M. S. Baird and W. Nethercott Tetrahedron Lett. 1983 24 605. 76 T. J. Stierman and R. P. Johnson J. Am. Chem. Soc. 1983 105 2492. 77 M. W. Klett and R. P. Johnson Tetrahedron Lett 1983 24 2523. 78 W. D. Huntsman and T.-K. Yin 1. Org. Chem.. 1983 48,3813. 116 M. S. Baird in the photochemical process is explained by ring-opening of the excited spiran to the S1 excited vinylcarbene ( I 10) followed by radiationless decay to proximate maxima on the ground-state energy surface corresponding to transition states in the thermal isomerization. The least favoured thermal transition state (that to allene) offers the closest approach to the S excited state and is therefore photochemically preferred. A considerable proportion of the photoproduct appears however to derive from the Sovinylcarbene and the thermal reaction shows that the conversion of So into S vinylcarbenes does not occur.79 Photolysis of (1 11 ; R = H) and (1 12) gave unsaturated carbenes which do not cyclize to cyclopropenes.When the former compound is photolysed in furan the carbene (1 13; R = H) is trapped as a 34 1 mixture of endo-and exo-1,2-adducts; photolysis of (1 12) leads to the acetylene (1 14).80Photolysis of (I 1 I) in vinyl ether or cyclopentadiene leads to the trapping of two formal carbene species (1 13) and (1 IS> depending on the nature of R. Thus with vinyl ether and R = Me exclusive trapping of (I 15) occurs whereas with R = C0,Me it is (1 13) which is exclusively trapped.” R Irradiation of (116) (R’ R2 = H; R’ R2 = CH2-CH2; or R1,R2 = CH=CH) does not lead to ketenes unless carried out in an inert matrix.Under these conditions (1 16; R’ R’ = Hj gives the corresponding triplet carbene as the sole primary product and this can be characterized spectroscopically. The triplet is the ground state and can be trapped as (1 17) and (I 18) respectively by oxygen and carbon monoxide. Longer irradiation leads to the ketene (I 19) and the yield of this product is inversely proportional to the concentration of oxygen or carbon monoxide in the matrix. The triplet has a high barrier to Wolff rearrangement and it is suggested that activation 79 M. G. Steinmetz Y.-P. Yen and G. K. Poch J. Chem. SOC.,Chem. Commun. 1983 1504. 80 M. Franck-Neumann P. Geoffroy and J.J. Lohmann Tetrahedron Lett. 1983 24 1775. M. Franck-Neumann and P. Geoffroy Tetrahedron Lett. 1983 24 1779. Arynes Carbenes Nitrenes and Related Species R1 RZ // of the ground state To to T and intersystem crossing of the latter to S”’ carbene occur followed by rapid rearrangement.82 A comparative study of the conformational control in the Wolff rearrangement has been carried out using low-temperature photolysis of either matrix-isolated Q -diazoketones or vinylene thioxocarbonate (1 22). Under these conditions equili- brium between the s-E (123) and s-Z (124) conformers is hindered. Results indicate that the conformational control in the rearrangement is not per se a proof of a concerted mechanism and that a ketocarbene intermediate is most rea~onable.’~ 3 Nitrenes An ab initio study of the chemical reactions of singlet and triplet :NH is reported.84 Photolysis of 1-azidobicyclo[2.2.llheptane in an argon matrix at 10 K leads pre- dominantly to (1 25) which reacts with methanol to give (126) even at 100 K.85Similar strained imines are obtained on photolysis of 1-azidobicyclo[2.2.2.]octanes and 1-azidoadeamantanes.86 ( 125) (126) ( 127) Thermolysis of phenyl azide with sulphides (R1SCH2R2)leads to 2-substituted anilines (127) by Sommelet-Hauser rearrangement of intermediate N-phenylsul- phimides arising from phenylnitrene attack on sulphur. However with ethyl phenyl sulphide the products are PhSNHPh and ethylene which are formed by cycloelimina- tion in an intermediate N-phenylsulphimide.With acyclic benzyl sulphides the intermediate sulphimides undergo a Stevens rearrangement whereas cyclic benzyl sulphides give largely Sommelet-Hauser-derived product^.^' R. A. Hayes T. C. Hess R. J. McMahon and 0. L. Chapman J. Am. Chem. SOC.,1983 105 7786. M. Torres J. Ribo A. Clement and 0. P. Strausz Can. J. Chem. 1983 61 996. x4 T. Fueno V. Bonacic-Koutecky and J. Koutecky J. Am. Chem. SOC.,1983 105 5547. XS R. S. Sheridan and G. A. Ganzer J. Am. Chem. SOC.,1983 105 6158. nt I. R. Dunkin C. J. Shields H. Quast and B. Seiferling Tefrahedron Lett. 1983 24 3887. H’ L. Renati. P. C. Montevecchi and P. Spagnolo J. Chem. Soc. Perkin Trans. I 1983 771. 118 M. S. Baird Photorearrangement of I -azatriptycene (1 28) at 77 K leads to an absorption spectrum attributed to the nitrene (129) which is also obtained from the correspond- ing azide and one attributed to the azanorcaradiene (130); the latter could be a key intermediate in a number of reactions.Analysis of the kinetics of the process and of chemical and spectroscopic data from the two precursors suggested the involve- ment of two conformers of the nitrene (I 3 1) and (1 32).88In order to prove this the conformationally fixed azides (133) and (1 34) were synthesized and indeed were found to lead to different nitrenes and to quite different reaction products.89 (131) X = H Y = N (132) X = N Y = H Flash vacuum pyrolysis of o-azidodiphenylmethane leads to acridan and to acridine whereas at lower temperature in solution the main product is 10H-azepinoindole.Since the singlet nitrene is thought to be favoured by high tem- perature it does not appear that the acridan can be derived from triplet nitrene. It is suggested that the reactions occur via the singlet which leads to (135). Cleavage of bond a to form a biradical is a relatively easy process and occurs at low temperature producing the azepine. Cleavage of bond b is relatively more difficult and only occurs at higher temperatures; this leads to acridan and acridine.” Ther-molysis of (136) leads to (137) through a nitrene intermediate; the insertion process provides a key step in the synthesis of the bacterial coenzyme methoxatin.” Me0,C Me0,C N N RO / NHAc RO / NHAc (135) ( 136) (137) 88 T.Sugawara N. Nakashima K. Yoshihara and H. Iwamura J Am. Chem. SOC.,1983 105 858. 89 S. Murata T. Sugawara and H. Iwamura J. Am. Chem. SOC.,1983 105 3723. 90 M. G. Hicks and G. Jones J. Chem. SOC.,Chem. Commun. 1983 1277. 9‘ A. R. MacKenzie C. J. Moody and C. W. Rees J. Chem. SOC.,Chem. Cornmun. 1983 1372. Arynes Carbenes Nitrenes and Related Species On heating in benzene (1 38) leads to (1 39). This is explained in terms of parallel decomposition pathways involving loss of benzonitrile to produce (140) and loss of sulphur dioxide to give the vinylnitrene (141); cycloaddition of (140) and (141) would then lead to the product. Some support for this mechanism is provided by the trapping of both species. However an alternative reaction pathway involving the sulphene (140) and (142) could not be ruled out.92 PhXi "k= =& R-R The reaction of ethoxycarbonylnitrene with 1,4-di-t-butyl- or 1,rl-di-isopropyl-benzene gave the heterocycles (143) and (144) together with dialkyla~epines.~~ C0,Me K I ,Me C0,Me (143) R = Pr'or But (144) 4 Silylenes Thermal decomposition of (145) has been shown to lead not only to silene (146) but also to methylsilylene and to silylene itself.All three species are trapped by cycloaddition to buta- 1,3-diene. The formation of methylsilylene is intriguing and from an examination of the temperature dependence of the reaction products it appears that this species is derived by rearrangement of the ~ilene.~~ Heats of formation of 1-methylsilaethylene and dimethylsilylene have been found to be 18 and 46 kcal mol-' respectively using ion cyclotron double-resonance spectroscopy.The difference in favour of the sila-alkene contradicts earlier experimental and theoretical A matrix study of the dimethylsilylene to sila-alkene rearrange- ment suggests that the previously observed formation of products characteristic of Y2 B. F. Bonini G. Maccagnani G. Mazzanti P. Pedrini B. H. M. Lammerink and B. Zwanenburg J. Chem. SOC.,Perkin Trans. 1 1983 2097. 93 T. Kurnagai K. Satake K. Kidoura and T. Mukai Tetrahedron Lett. 1983 24 2275. 94 R.T. Conlin and R.S. Gill J. Am. Chem. SOC. 1983 105 618. 95 C. F. Pau W. J. Pietro and W. J. Hehre J. Am. Chem. SOC.,1983 105 16. 120 M. S. Baird the silylene when the sila-alkene is warmed to 100-120 K may be due to rearrange- ment in the act of trapping the intermediate^.^^ Flash vacuum pyrolysis of (1 47) led to trimethylmethoxysilane trimethylvinyl- silane and (148).The latter two products are consistent with isomerization of trimethylsilylvinylsilylene to 1-trimethylsilyl-I -silacyclopropene and a subsequent 172-trimethylsilyl shift to produce a silacyclopropanylidene (l49) which can lose silicon to give the vinylsilane or insert into a C-H bond of a methyl group and give a product which can then rearrange to (148). In the presence of 2,3-dimethyl- butadiene the first formed silylene is readily trapped ; however a minor product from this reaction is (150). The latter may be rationalized in terms of trapping of the silacyclopropanylidene (149) by the diene to produce (151) which loses vinyltrimethylsilane to generate a third silylene (152) which again is trapped by the added diene.97 SiMe, I Me,Si-Si I-Me,SiASiH, ElH2 CH,=Si H OMe L/ (145) ( 146) (147) ( 148) pgMe MeDizMe SiMe Me Me SiMe Me Me Flash vacuum pyrolysis of methoxydisilanes has also been used to generate 1- 2- and 3-propenylsilylenes each of which gives siletene products although by different pathways.Thus (1 53) leads to (I 54; R = H) apparently through the bicyclic species (1 59 and (156) leads to the same product by a C-H bond insertion. The facility with which this occurs has led the authors to question the report that formation of (157) from (158) represents a 1,4-hydrogen migration and to suggest instead an insertion to form a cyclobutene followed by ring-opening to the diene.98 R 96 C.A. Arrington R. West and J. Michl J. Am. Chem. Soc. 1983 105 6176. 97 T. J. Barton and G. T. Burns Tetrahedron Lett. 1983 24 159. 98 G. T. Burns and T. J. Barton J. Am. Chem. Snc. 1983 105 2006. Arynes Carbenes Nitrenes and Related Species Thermolysis of (1 59) in benzene in a flow system at 450 "C leads to the disilacyc- lobutane (160) and methoxytrimethylsilane. The cyclobutane is readily explained in terms of an insertion of an intermediate silylene (161) into a y-C-H bond of a methyl group. However when (159) was copyrolysed with alcohols (160) was obtained together with (162); the latter is formally derived by addition of the alcohol to an intermediate silaethylene (163) itself formed by a silylene to silaethylene rearrangement.The intervention of the silaethylene was confirmed by trapping with benzophenone to give (164). It is interesting to note the preference for trimethylsilyl rather than hydrogen migration to the silylene. Thermolysis of the disilacyclobutane (160) in the presence of trapping agents is consistent with the formation of the disilabutanylidene (I 65).99 (Me,Si),Si -CH,Si Me Me,Si-Si-CH(SiMe,) ORI (Me,Si),Si=CHSiMe (161) (162) (163) A :Si\/SiMe2 I Ph2C=C HSi Me SiMe (164) (165) Thermolysis of (166) leads to dimesitylsilylene in a reaction similar to the thermoly- sis or photolysis of epoxides. The silylene may be trapped in various reactions for example as (167) on reaction with 1,1,3,3-tetramethylindane-2-thione, and undergoes intramolecular reaction to produce (168).'0° Mesity1 I Me3Si- Si- SiMe, I Mesityl Mesityl YY A.Sekiguchi and W. Ando Tetrahedron Lett. 1983 24 2791. I00 W. Ando Y. Hamada and A. Sekiguchi J. Chem. SOC. Chem. Commun. 1983 952; W. Ando Y. Hamada A. Sekiguchi and K. Ueno Tetrahedron Lett. 1983 24 4033.