3 Reaction Mechanisms Part (iii) By 6. G. ODELL University Chemical Laboratory Lensfield Road Cambridge. CB2 1EW SOPHISTICATED stereochemical and kinetic arguments continue to be used to show whether reactions are concerted or not and hence whether the principles of orbital symmetry conservation’ are applicable. Orbital Symmetry Correlations.-The tendency to maintain bonding governs the complex motions of molecules in the course of reactions.2 This will direct the nuclear motions which may or may not be the least-motion ones. The non- least-motion paths will often however be discriminated against by ‘extrasym- metric factor^',^ and thus will often have higher activation energies than non- concerted processes but not always. Concerted processes have been studied theoretically by mapping analysis4 and by permutation symmetry.’ Dewar’s book on molecular orbital theory6 contains a good analysis of pericyclic processes.A method for obtaining approxi- mate activation energies for four-centre thermally forbidden reactions based upon estimation of the energy of the avoided crossing point in the state correla- tion diagram has been de~cribed.~ Cycloaddition Reactions.4rbital symmetry theory in cycloaddition reactions has been reviewed8 and the criteria for deciding whether polar cycloadditions are one- or two-step processes discussed.’ Dilling has reviewed the photochemical cycloadditions of conjugated polyenes. The mechanisms of many [2 + 21 cycloadditions continue to arouse interest. MIND0/2 calculations on the dimerisation of ethylene lead to a pref- erence for a biradical process but leave open the possibility of a [,2 + .2,] ’ R.B. Woodward and R. Hoffmann ‘The Conservation of Orbital Symmetry’ Verlag Chemie Gmbh Weinheim-Bergstr. 1970. ’ R. Hoffmann and R. B. Woodward Science 1970 167 825. ’ J. A. Berson and S. S. Oh J. Amer. Chem. SOC.,1970 92 1086. ‘C. Trindle J. Amer. Chem. SOC.,1970 92 3251 3255. J. J. C. Mulder and L. J. Oosterhof Chem. Comm. 1970 305 307. ’ M. J. S. Dewar ‘Molecular Orbital Theory of Organic Chemistry’ McGraw-Hill New York 1969. ’ R. A. Jackson J. Chem. SOC.(B) 1970. 58. J. J. Vollmer and K. L. Servis J. Chem. Educ. 1970,47 491. ’ R. Gommper Angew. Chem. Internat. Edn. 1969,8 312. W. L. Dilling Chem. Rev. 1969 69 845.154 B. G.Ode11 symmetry-allowed process." The thermal dimerisation of cis,trans-cyclo-octa-1,3-diene gives (1)as the major product suggesting that a predominant portion of the reaction occurs by the symmetry-allowed process.I2 Pyrolysis of bicyclo[2,2,0]-hexanedicarboxylicesters(2)also gives products compatible with orbital symmetry theory but a biradical process is preferred since both the cis,endo-and cis'exo-isomers give the same product ration and the concerted transition state would be extreinely congested in the former case. Pyrolytic decomposition of bicyclo- [4,2,0]octane may be in part ~0ncerted.I~ Thermolysis of 1,1,2,2-tetramethyl- cyclobutane yields isobutylene ;the recovered deuteriated molecule (3)was not **= -,.or +,-(5) epimerised or racemised." These results were discussed in terms of rotations in 1,4-butanediyl intermediates.Bartlett's Centenary Lecture16" on the mechanisms of cycloadditions presents much recent work. If a diradical or dipolar intermediate in a two-step [2 +21 cycloaddition is formed in an extended conformation (4)it is likely to have a sufficient lifetime for rotation about the terminal C-C bonds with resulting configurational loss to occur before closure to a four-membered ring. The addition of cis-anethole (5) to tetracyanoethylene (TCNE) proceeds with greater loss of configuration in acetonitrile which favours formation of an extended dipole than in benzene.16" The diradical intermediate formed in the addition of a diene to an olefin can close to a cyclobutane or a cyclohexene.It has been found however that [2 +21 addition of difluorodichloroethylenes to cyclo- pentadiene is a two-step process while the competing [4+21 reaction is M. J. S. Dewar and E. Haselbach J. Amer. Chem. SOC.,1970 92 590. C. L. Osborn D. J. Trecker A. Padwa W. Koehn and J. Masaracchia Tetrahedron Letters 1970 4653. I3 L. A. Paquette and J. A. Schwartz J. Amer. Chem. SOC.,1970 92 3215. l4 J. E. Baldwin and P. W. Ford J. Amer. Chem. SOC., 1969 91 7192. J. A. Berson D. C. Tompkins and G. Jones jun. J. Amer. Chem. SOC..1970,92 5799. l6 (a) P. D. Bartlett Quart. Rev. 1970 24 473; (6) R. Wheland and P. D. Bartlett J. Amer. Chem. SOC.,1970,92. 3822. Reaction Mechanisms-Part (iii) 155 ~0ncerted.l~~ Interesting thermal [TC + 01 processes such as those of (6),are probably examples of trapping of biradical intermediates by double bonds.' Photolysis of methoxycyclobutanones [e,g. (711 in methanol is a stereo-specific reaction yielding olefins cyclopropanes and cyclic acetals. A diradical intermediate formed by or-cleavage can give rise to all these products.18" Such a diradical(8) has been trapped by butadiene to form 3-vinylcyclohexanone.'8b 1)40-3+ -p + VoMe Me0 Me0 Me0 Me0 (7) 0 The mechanisms of the formation of cyclobutanes in the sensitized photolysis of isomeric diethyl deca-2,8-diene-l ,lO-oates,' the photochemical reaction of styrene with tetramethylethylene,20 and the butadiene to bicyclobutane phote isomerisation2' have also been studied.Reaction between singlet oxygen and olefins can lead to 1,2-dioxetans. With l,Zdiethoxyethylenes cis stereospecificity is maintained ; the adduct with tetramethoxyethylene is remarkably stable (tt = 102 min at 56 oC).22The thermolysis of 1,Zdioxetans leads to ketonic species in an excited state. It is not yet clear whether formation22 and fragrnentati~n~~ of dioxetans are concerted or stepwise processes. Photo-oxygenation of olefins to allylic hydroperoxides is not a concerted ene-reaction; perepoxides [e.g. (9) from indene] are probable intermediate^.^^ OOH OOH \ L. A. Paquette and L. M. Leichter J. Amer. Chem. SOC.,1970,92,1765; D. T. Longone and D. M. Stehouwer Tetrahedron Letters 1970 1017. (a) N. J. Turro and D. M. McDaniel J.Amer. Chem. SOC.,1970,92,5727;(b)P. Dowd A. Gold and P. Sachdev ibid. 1970 92 5724. l9 J. R. SchefTerand B. A. Boire Tetrahedron Letters 1970 4741. 'O 0.L. Chapman and R. D. Lura J. Amer. Chem. SOC.,1970,92,6352. 21 W. G. Dauben and J. S. Ritscher J. Amer. Chem. SOC.,1970 92 2925. 22 P. D. Bartlett and A. P. Schaap J. Amer. Chem. SOC.,1970 92 3223; S. Mazur and C. S. Foote ibid. p. 3225. 23 H. E. O'Neal and W. H. Richardson J. Amer. Chem. $oc. 1970 92 6553. 24 W. Fenical D. R. Kearns and P. Radlick J. Amer. Chem. SOC.,1969,91. 7771. 156 B. G. Odell Olefins react with azodicarbonyl compounds to give diazetidines in two steps and dihydro-oxadiazines in a concerted Diels-Alder process.25 Here high polarity or polarisability of the olefin favours the [2 + 21 process; ethyl vinyl ether yields four-membered rings while 1,2-dimethoxyethylene gives 1,4 add~cts.~~ 0 (10) In contrast the reaction of dibenzoyl-diimide with diphenylketen gives an azomethine imine (lo) a 1,3-adduct which reacts further with dipolarophiles.26 Cycloaddition reactions of allenes have been reviewed in a timely Codimerisation of allene and tetradeuterioallene shows an intermolecular deuterium isotope effect (kJkD)of 1.013.Therefore there is little disturbance at the labelled site in the rate-determining step. However the intramolecular iso- tope effect in the dimerisation of 1,l-dideuterioallene is 1-14. It follows that there is more than one energy barrier in the dimerisation of a11ene.28 This is consistent with the formation of a 2,2'-biallyl diradical which closes to 1,2- dimethylenecyclobutane.This diradical has been observed by e.~.r.~~ and has been the subject of semi-empirical molecular orbital calculation^.^^ The highly strained allene cyclohexa-1,2-diene which is generated from the dibromo- carbene adduct of cyclopentene oiu an untrappable carbenoid yields tetramers at low temperatures but the cis-and trans-dimers (11) in refluxing ether.31 The change in product distribution with temperature is also interpreted as evidence for an intermediate 2,2'-biallyl diradical. Cyclohexa- 1,2-diene can also be trapped in a [2 + 21 reaction with styrene.31 The reaction of benzyne with allenes has also been studied.32 Tetramethoxyallene reacts with TCNE in a two-step process involving the reversible closure of the dipolar intermediate (12).33 (1 1) (12) 25 E.Koerner von Gustorf D. V. White B. Kim D. Hess and J. Leitch J. Org. Chem. 1970 35 1155; cf. ref. 9. 26 J. Markert and E. Fahr Tetrahedron Letters 1970 769. " T. F. Rutledge 'Acetylenes and Allenes; Additions Cyclisation and Polymerisation Reactions' Reinhold New York 1969. 28 W. R. Dolbier jun. and S. H. Dai J. Amer. Chem. SOC., '' P. Dowd J. Amer. Chem. SOC.,1970,92 1066. 1970,92 1774. 30 B. G. Odell R. Hoffmann and A. Imamura J. Chem. SOC.(B) 1970 1675. 3' W. R. Moore and W. R. Moser J. Amer. Chem. SOC.,1970 92 5469; W. R. Moore and W. R. Moser J. Org. Chem. 1970,35 908. 32 H. H. Wasserman and L. S. Keller Chem. Comm. 1970 1483.33 R. W. Hoffmann and W. Schafer Angew. Chem. Internat. Edn. 1970,9 733. Reaction Mechanisms-Part (iii) I57 Gommper has suggested that the addition of ketens to olefins may proceed by way of the dipolar intermediate (13) which he shows can account for the experi- mental facts.34 This is an alternative to the allowed concerted [,2 + .2,] process with the olefin as suprafacial component and the keten as antarafacial.' Addition of ketens to cyclic olefins shows a large selectivity for production of bicyclic systems in which the larger (L) of the keten substituents is enda3' These reactions are interpreted in terms of a concerted process as depicted in Scheme 1 Scheme 1 for the case of addition to cyclopentadiene The smaller substituent (S)preferenti-ally takes up the less hindered orientation.That the addition of cis-mono-olefins to ketens is faster than that of the trans-isomers is further evidence for a crossed [,2 + ,2,) process.36 The mechanisms of addition of ketens to imine~,~" azodicarbonyl and carbodiimides have also been studied. The reaction with carbodiirnide~~~~ proceeds in a two-step manner the intermediate being trapped by water or sulphur dioxide. Pyrolysis of p-lactams is stereospecific and is probably a reverse [,2 + ,2,] process where HNCO is the antarafacial component.38 Reactions between chlorosulphonyl isocyanate and double bonds could also be concerted but interpretation of results in this field is often complicated by rearrangements of the primary ad duct^.^^ Cyclodimerisations of acetylenes and allenes which proceed via vinyl cations (also potentially concerted') have been re~iewed.~' Molecular orbital symmetry conservation in transition-metal catalysis has been re~iewed.~' Van der L~gt~~ has shown theoretically that both ds and d" transition metals can interact with excited electronic configurations in the transi- tion states of [2 + 21 cycloadditions in such a way as to lower the activation energy for the process.The metals however do not change a forbidden process 34 H. U. Wagner and R. Gommper Tetrahedron Letters 1970 2819. 35 See W. T. Brady and R. Roe jun. J. Amer. Chem. SOC.,1970,92,4618 for many further references. 36 N. S. Isaacs and P. F. Stanbury Chem. Comm. 1970 1061; H. M. Frey and N.S. Isaacs J. Chem. SOC.(B),1970 830; T. DoMinh and 0.Strausz J. Amer. Chem. Soc. 1970 92 1766. 37 (a) W. T. Brady and L. Smith Tetrahedron Letrers 1970 2963; (b) J. Decazes J. L. Luche and H. B. Kagan ibid. p. 3665; (c) R. C. Kerber and T. J. Ryan ibid. p. 703; (4W. T. Brady and E. D. Dorsey J. Org. Chem. 1970,35,2732. 38 L. A. Paquette M. J. Wyvratt and G. R. Allen jun. J. Amer. Chem. Soc. 1970 92 1763. 39 See T. J. Barton R. Rogido and J. C. Clandy Tetrahedron Letters 1970 2081; E. J. Moriconi J. G. White R. W. Franck J. Jansing J. F. Kelly R. A. Salamone and Y.Shimakawa ibid. p. 27. 40 K. Griesbaum Angew. Chem. Internat. Edn. 1969 8 933. 41 F. D. Mango Adu. Catalysis 1969 20 291. 42 W. T. A. M. van der Lugt Tetrahedron Letters 1970 2281.158 B. G. Ode11 into an allowed one. Cubane (14)is isomerised by Rh' catalysts to syn-tricyclu- octadiene (15) while interaction with Ag' leads to cuneane (16),a wedge-shaped (16) (14) (15) molecule. Cuneane itself rearranges in the presence of Rh' to semibullvalene. 43 The above type of [,2 + ,2,] isomerisations of compounds with strained o-bonds is general for homo- and bishomo-cubane derivative^.^^ Some further evidence for metal-containing intermediates in rhodium-catalysed processes has been presen ted.43*44 The role of secondary orbital interactions' in stabilising the endo-transition states of [4 + 21 cycloadditions in the absence of great steric demands has received much support. Cyclopentadiene gives more than 97 o/ em-adduct (17) with 2,5-dimethyl-3,4-diphenylcyclopentadienone while cyclopentene which has similar steric demands but lacks the second double bond gives approximately equal amounts of em- and endo-adduct~.~~ It is argued that dominant orbital interactions and not steric effects or angular dependence of overlap account for the stereochemical course of this process.The adduct (17) was found to equili- brate with (18) in a Woodward-Katz rearrangement. The volume of activation for the Diels-Alder reaction of maleic anhydride with dienes has been studied. It was concluded that the transition state probably has a smaller volume than the adduct produced a surprising result also indicating stabilisation of the endo-transition state by secondary orbital interaction^.^^ The stereoselectivities of the reactions of cyclic dienes with cyclopropenes~7" cycl0butenes,4~~ and alkylated dien~philes~~' have also been studied.Dienes capable of assuming a cisoid conformation react faster with hexachlorocyclopentadiene than those 43 L. Cassar P. E. Eaton and J. Halpern J. Amer. Chem. SOC.,1970 92 3515 6366; L. Cassar and J. Halpern Chem. Comm. 1970 1082. 44 W. G. Dauben M. G. Buzzolini C. H. Schallhorn D. L. Whalen and K. J. Palmer Tetrahedron Letters 1970 787; see L. A. Paquette G. R. Allen jun. and R. P. Henzel J. Amer. Chem. Soc. 1970.92 7002 for further references. 45 K. N. Houk Tetrahedron Letters 1970 2621. 46 R. A. Grieger and C. A. Eckert J. Amer. Chem. SOC.,1970,92 2918. " (a) M. A. Battiste and C.T. Sprouse jun. Tetrahedron Letters 1970 4661 ;(b) C. M. Anderson I. W. McCay and R. N. Warrener ibid. p. 2735; (c) Y.Kobuke T. Fueno and J. Furukawa J. Amer. Chem. Soc. 1970,92 6548. Reaction Mechanisms-Part (iii) which may assume non-cisoid conformations. A stabilising 2-4‘ endo interaction in the transition state (19) probably in addition to 3-3’ is suggested.48 The mechanism of the Lewis-acid-catalysed [4 + 21 cycloaddition has been clarified. 2-Phenylcyclohex-2-enonereacts with butadiene in a manner that is a variant of Friedel-Crafts alkylation ; the intermediate (20) closes to both (21) and (Z).“’ The rearrangement of basketene (23) to Nenizescu’s hydrocarbon (24)has been shown to proceed by a retro-Diels-Alder reaction followed by a Cope rearrange- ment.” The [2 + 2 + 21 cycloaddition of TCNE to (23) reported last year’’ (23) (25) (24) must therefore be reinterpreted as a trapping of the intermediate diene (25).Deuterium-labelling evidence has been given for the intermediate formation of (26) in the rearrangement of bicyclo[4,2,0]decatetraene to cis-9,lO-dihydro- na~hthalene.’~ Schmidt has presented further evidence for the synchronous nature of the polar [4+ 21 Gycloaddition of amidomethylium cations (27) to 01efins.’~ The ratio of diastereomeric oxazines such as (28) formed in the regio- and stereo- specific cis-addition is not consistent with initial electrophilic attack of the cation on the ~lefin.’~ An interesting competition between 1,4-and 1,5-dipolar cyclo- addition has also been ~bserved.’~ Oxazoles react with diphenylcyclopropenone to produce y-pyrones ; the intermediates (29) extrude nitriles in a retrohomo-Diels-Alder proces~.~ Further (26) (27) (28) 48 C.G. Cardenas Chem. Comm. 1970 134. 49 H. W. Thompson and D. G. Melillo J. Amer. Chem. SOC.,1970,92 3218. 50 H. H. Westberg E. N. Cain and S. Masamune J. Amer. Chem. SOC.,1969,91 7512. 51 B. G. Odell Ann. Reports (B) 1969 66 143. 52 R. T. Seidner N. Nakatsuka and S. Masamune Canad. J. Chem. 1970,48 187. 53 R. R. Schmidt and R. Machat Angew. Chem. Internat. Edn. 1970,9 31 1. 54 S. Farid Chem. Comm. 1970 303. 55 R. Grigg and J. L. Jackson J. Chem. SOC.(0,1970 552. 160 B. G. Ode11 examples of the [2 + 2 + 21 cycloaddition have been observed in the addition of TCNE to barbaralane and dihydrobullvalene to give (30).56 A related nitrogen extrusion process has also been reported.' (29) (30) Fire~tone~~ has continued to provoke discussion of the mechanism of 1,3- dipolar cycloadditions.When diradical intermediates in these reactions are written with paired electrons in Lewis structures it is found that the calculated losses in bond energies are greater than experimental activation energies." Firestone has shown that the use of Linnett structures (without close electron pairing) leads to bond energy losses that are in better agreement with experiment. He points out however that good Linnett structures can also be written for the transition states of concerted processes. The fact that acetylenic dipolarophiles exhibit about the same reactivity as their olefinic counterparts even when an aromatic system is being formed and when a concerted transition state would be expected to possess some of the stabilisation of the product is quoted as evidence for the diradical nature of the pro~ess.'~ However the concensus of opinion favours a concerted [,4 + ,2,] process in 1,3-dipolar cycloadditions.Kinetic secondary deuterium isotope effects in the reaction of allene6OU and styrene60b with the carbonyl ylide derived from tetracyanoethylene oxide have been studied. 1,l-Dideuterioallene gives an inverse intramolecular isotope effect kdkD = 0.97 per deuterium; inverse isotope effects are also observed for a-and P-deuteriostyrenes. Dolbier argues that both these results point to a concerted cycloaddition6'" while it has been suggested that the styrene results do not rule out a two-step process where destruction of the intermediate is rate-limiting.60b Huisgen has presented full reports of the dipolar cycloaddition reactions of azlactones and related mesoionic 1,3-dip0les.~' Azlactone (3 1) reacts as its tautomer (32) a view supported by the fact that (33) is stable to dipolarophiles.Examples of cycloadditions of allylic anions which are isoelectronic with many 1,3-dipoles have been described for the addition of indenyl and 54 H. P. Loffler T. Martini H. Musso and G. Schroder Chem. Ber. 1970 103 2109. 57 L. A. Paquette J. Amer. Chem. SOC.,1970 92 576.5; R. Askani Tetrahedron Letters 1970,3349.58 R. A. Firestone J. Chem. SOC.(A) 1970 1570. 59 R. Huisgen J. Org. Chem. 1968 33 2291. 6o (a)W. R. Dolbierjun. and S. H. Dai Tetrahedron Letters 1970,4645; (6)W. F. Bayne and E. I. Snyder ibid. p. 2263. 61 See R. Knorr R. Huisgen and G. K. Staudinger Chem. Ber. 1970 103 2639; H. Gotthardt R. Huisgen and H. 0. Bayer J. Amer. Chem. Soc. 1970 92 4340. Reaction Mechanisms-Part (iii) 161 cyclopentadienyl anions to benzene.62 The related addition of (34)to stilbene to yield (35)has also been observed.63 11 0 (341 (35) (36) 2,2-Dimethylcyclopent-4-ene-1,3-dione (36)is a poor dienophile but a normally active dipolarophile. It has been argued therefore that secondary orbital inter- actions which are important in Diels-Alder reactions are not important in 1,3-dipolar cycloadditions.They are probably governed by steric control of reactant ~rientation.~~ Azomethine carbonyl and thiocarbonyl ylides all of which are 1,3-dipoles have been studied. The disrotatory photochemical opening of epoxides at low temperature yields carbonyl ylides. cus-and trans-Stilbene epoxides give isomeric red ylides for which fragmentation to benzaldehyde and phenylcarbene is preferred to thermal reclosure which would lead to epoxide is~merization.~~ The bicyclic oxiran (37) on photolysis or thermolysis gives ylide (38) which is I Ph Ph (37) (38) (39) fairly stable towards reclosure (a forbidden process; t+ = 8 min at 22 "C). It reacts with dipolarophiles with cis-stereo~pecificity.~~ Thiocarbonyl ylides (39)are formed with retention of stereochemistry in the retro-[4 + 2,] decompo-sition of 1,3,4-thiadiazolines.They undergo allowed conrotatory thermal '' W. T. Ford R. Radue and J. A. Walker Chem. Comm. 1970,966. '' T. Kauffmann H. Berg and E. Kopelmann Angew. Chem. Internat. Edn. 1970,9,380. 64 W. C. Agosta and A. B. Smith tert. J. Org. Chem. 1970,35 3856. 65 T. Do-Minh A. M. Trozzolo and G. W. Griffin J. Amer. Chem. SOC.,1970,92 1402. 162 B. G. Ode11 closure to episulphides and retain their stereochemistry in cycloadditions with dipolarophiles.66 The two-fold extrusion principle which has been applied to the synthesis of hindered olefid7 is obviously related to these reactions. While aziridines often react in cycloadditions via C-C bond cleavage to give azomethine ylides aziridinium salts undergo C-N rupture.68 TCNE adds to bicyclo[6,1,0]nonatriene to give (40) which contains a nine- membered ring.The stereochemistry of the ring junction has not yet been deter- mined but it is suggested6’ that this product may be formed by way of cis,cis trans,cis-cyclononatetraene whose all-cis-isomer undergoes [4+ 21 addition to the potent dienophile N-phenylpyra~olinedione.~’ It has been claimed that this dienophile gives (41) with oxonin either by a [2 + 2 + 21 process or by [8 + 21 addition followed by electrocyclic ring-clo~ure.~’ The [6 + 41 cycloadduct of tropone and cyclopentadiene is converted thermally into four isomeric [4+ 21 adducts probably via dissociation-re~ombination.~Woodward and Houk have reported72 on the structure and interrelations of the cycloadducts formed from 2,5-dimethyl-3,4-diphenylcyclopentadienoneand both tropone and cyclohepta- triene.In both cases em-[6 + 41 adducts and endo-[4 + 21 products [(42)and Ph H Me Ph (43)respectively] are formed in agreement with the guiding influence of secondary orbital interactions. In the case of tropone an [8 + 21 adduct is also produ~ed.~’ 66 R. M. Kellogg and S. Wassenaar Tetrahedron Letters 1970 1987; R. M. Kellogg S. Wassenaar and J. Buter ibid. p. 4689. 67 D. H. R. Barton and B. J. Willis Chem. Comm. 1970 1225; D. H. R. Barton E. H. Smith and B. J. Willis Chem. Comm. 1970 1226. 68 D. R. Crist and N. J. Leonard Angew. Chem. Internat. Edn.1969 8 962. 69 W. H. Okamura and T. W. Osborn J. Amer. Chem. SOC.,1970,92 1061 ; C. S. Baxter and P. J. Garratt ibid. 1970 92 1062. 70 A. G. Anastassiou and R. P. Cellura Tetrahedron Letters 1970 91 1 ; Chem. Comm. 1970 484. 71 S. ItB K. Sakan and Y. Fujise Tetrahedron Letters 1970 2873. I?. K. N. Houk and R. B. Woodward J. Amer. Chem. Soc. 1970,92,4143,4145. Reaction Mechanisms-Part (iii) 163 1,6-Dimethylenecyclohepta-2,4-diene also gives [8+ 21 adducts with dieno- phile~.~~ The dimer of cycloheptatriene (44)is probably formed by successive [6 + 41 and [4 + 21 additions.74 Diphenylnitrilimine reacts as a 4n component in a novel but predictable [,6 + .4,] cycloaddition reaction with tropone to yield (45);the major product however arises from [,4 + ,2,] addition.75 Dimethylfulvene has been found to act as a 67c system in reactions with dia~omethane~~” and with tr~pone.~~’ In the latter case the initial adduct (46)undergoes rapid [1,5] hydrogen migration and can then participate in a further exo-[,6 + .4,] addition with tropone as (44) (47) (48) the 6n component.The term ‘perispecific’ has been suggested to describe a process which follows only one of the symmetry-allowed pathways available.76b Concerted [,6 + ,2,] processes are forbidden. Further examples of [6 + 21 cycloadditions continue to be reported. The reaction of alkoxycarbonyl-azepines with azodicarboxylate esters7 7c requires further study in order to check assignments and the interrelation of different modes of addition.Askani has discovered the stereospecific addition of homofulvenes to chlorosulphonyl is~cyanate,~~ which should proceed with inversion of configuration at C-6 if it is a [,2 + ,4s+ ,2,] process (47). The remarkable photochemical racemisation of the thermally stable molecule (48)is a [,2 + .2 + u2a + .2,] cy~loaddition.~~ Sigmatropic Reactions-MIND0/2 calculations correctly predict the chair-like transition state for the Cope rearrangement to be more stable than the boat by 6.6 kcal mol-’ as compared to the experimental value of 5-7. An equatorial ‘3 G. C. Farrant and R. Feldmann Tetrahedron Letters 1970 4979. 74 K. Takatsuki I. Murata and Y.Kitahara Bull. Chem. SOC.Japan 1970 43 966. 75 K. N. Houk and C. R. Watts Tetrahedron Letters 1970 4025.76 (a)K. N. Houk and L. J. Luskus Tetrahedron Letters 1970,4029; (b)K. N. Houk L. J. Luskus and N. S. Bhacca J. Amer. Chem. SOC.,1970,92 6392. 77 (a) E. J. Moriconi C. F. Hummel and J. F. Kelly Tetrahedron Letters 1969 5325; (b) A. S. Kende and J. Y.-C. Chu ibid. 1970 4837; (c) W. S. Murphy and J. P. McCarthy Chem. Comm. 1970 1129. ” R. Askani Angew. Chem. Internat. Edn. 1970 9 167. ‘9 D. G. Farnum and G. R. Carlson J. Amer. Chem. SOC.,1970,92 6700. 164 B. G. Ode11 methyl substituent on the chair is preferred by 1.5 kcal mol-’ to an axial; calculations predict a value of 2.0.s0 The rapid degenerate [3,3] rearrangements of homotropylidenes such as (49) have been studied by n.m.r. The more stable transoid conformation inverts to the cisoid which isomerises by way of a bis- homobenzene-type transition state (50).8’ Thermal rearrangements of (51) are initiated by a [3,3] sigmatropic shift to give (52)s2“and not by initial opening of the cyclobutene ring.82b (52) The sigmatropic shifts encountered in dienone-phenol rearrangements can be explained in terms of orbital symmetry theory by the rationalisation that n-protonation of the carbonyl group leads to little perturbation of the dienone ring and to normal [3,3] and [1,5] shifts while n-protonation results in shifts which are allowed for the hexadienyl Thermal rearrangements of allylic imino-esters of caprolactam (53) proceed with migration to C-3 (54) via the enamine ta~tomer.~~ A related thio-Claisen rearrangement has also been observeds5 and the synthetic scope and utility of the thio-Claisen rearrangement has been extended.s6 The degenerate rearrangement of (55) has been rationalised in terms of the intermediacy of cis,trans,cis-cyclo-octatrienerather than an antara,antara-Cope rearrangement.87 (2)-Q? (53) H (54) 8o A.Brown M. J. S. Dewar and W. Schoeller J. Amer. Chem. SOC.,1970,92 5516. L. Birladeanu D. L. Harris and S. Winstein J. Amer. Chem. Soc. 1970 92 6387. 82 (a) E. Vedejs Tetrahedron Letters 1970 4963; (b) L. A. Paquette and J. C. Stowell ibid. p. 2259. 83 B. Miller J. Amer. Chem. SOC.,1970 92,432 6246 6252. 84 D. St. C. Black and A. M. Wade Chem. Comm. 1970,871. *’ B. W. Bycroft and W. Landon Chem. Comm. 1970 168. E. J. Corey and J.I. Shulman J. Amer. Chem. SOC.,1970,92 5522. FJ’ J. E. Baldwin and M. S. Kaplan Chem. Comm. 1970 1560. Reaction Mechan isms-Par t (iii) 165 &JeDpeDmD D (55) [3,3] Sigmatropic rearrangement has been shown to be important in the oxygen scrambling process for diacetyl peroxide while [1,3] shifts are probable in the reactions of t-butyl peresters." It is to be expected that similar processes will be found for other acetoxyl migrations. The [1,3] sigmatropic rearrangement of (56; R' = Me R2 = H) proceeds with inversion of configuration at the migrating centre to yield (57;R' = Me R2= H) while the epimer (56; R' = H R2 = Me) gives the same product." In the latter case a biradical pathway is followed (56) (57) (58) since the interaction between the methyl group and the ring in the concerted transition state (58) would be too severe to allow bonding to be maintained.In the [1,3] allylic rearrangements of phenyl ally1 sulphides an intermolecular 'antipolar' process sometimes competes with a unimolecular noncatalysed one." The thermal [1,5] hydrogen shift of (S)-trans-3-methyl-7-deuterio-octa-4,6-diene (59) proceeds (as indicated in Scheme 2) by way of a suprafacial transition state which is at least 8 kcal mol-' lower in energy than the antarafacial one which would generate antipodal product^.^ ' [1,5] Carbon shifts in cyclopentadiene derivatives have been the subject of several ~tudies.'~ On heating (60)undergoes a suprafacial [1,5] carbon migration with retention of configuration followed by a [1,5] hydrogen shift.92 Silicon and germanium have been found to undergo [1,5] sigmatropic rearrangements more readily than hydrogen.l-trimethyl-silylindene rearranges to 2-trimethylsilylisoindene,which can be trapped with TCNE.'3 Other metallic elements may well be found to undergo similar M. J. Goldstein and H. A. Judson J. Amer. Chem. SOC.,1970,92,4119,4120. 89 J. A. Berson and G. L. Nelson J. Amer. Chem. SOC.,1970,92 1096. 90 H. Kwart and N. Johnson J. Amer. Chem. SOC.,1970,92,6064. ' W. R. Roth J. Konig and K. Stein Chem. Ber. 1970 103 426. 92 M. A. M. Boersma J. W. de Haan H. Kloosterziel and L. J. M. van de Ven Chem. Comm. 1970 1168 and references cited therein. 93 A. J. Ashe tert Tetrahedron Letters 1970 2105; R. B.Larrabee and B. F. Dowden ibid. p. 915; A. Davison and P. E. Rakita Inorg. Chem. 1970 9 289. 166 B. G. Ode11 rearrangements. Examples of stereospecific [1,5] hydrogen shifts in terpenes which proceed with induction of chirality at the migration terminus have been given.94 Me Me I I wMe 1 Me -+ Ph flgh-Ph Ph Ph Ph The photochemical rearrangements of homofulvenes to spiroheptadienes [of which (61)- (62) is an example] are stereochemically consistent with a first excited state [1,5] carbon shift with inversion followed by a thermal [1,5] shift with retention of configuration ;95u other interpretations have been suggested however.956 Equilibration of cis,cis-di-o-propenylbenzenewith its cis,trans-isomer has been shown96 to proceed by way of [1,7] sigmatropic hydrogen shifts (presumably v4 G.Ohloff Angew Chem. Internat. Edn. 1970 9 743. 95 (a)N. K. Hamer and M. Stubbs Chem. Comm. 1970 1013; (6) T. Tabata and H. Hart Tetrahedron Letters 1969 4929; H. E. Zimmerman D. F. Juers J. M. McCall and B. Schrbder J. Amer. Chem. Soc. 1970 92 3474. 96 H. J. Hansen and H. Schmid Chimia (Switz.) 1970 24 89; H. Heimgartner H. J. Hansen and H. Schmid Helv. Chim. Acta 1970 53 173. Reaction Mechanisms-Part (iii) antarafacial) as shown in Scheme 3. Similar rearrangements of hexatriene derivatives have also been studied. 97 The photochemical rearrangement of 2-vinylcyclopentadiene to 6-methylful~ene~~ is the first example of a [1,7] suprafacial shift other than those of cycloheptatrienes.The sigmatropic [5, 5,] rearrangement (63) +(64) has now been reported in detail. It is an analogue of R' OH the Claisen rearrangement and follows first-order kinetics ; a ten-membered cyclic transition state is pr~bable.~' Further [3,2] sigmatropic rearrangements have been studied for allylic ether anions,'"" phosphonium ylides,'OOb and the all-carbon system (65) [which produces some (66)].loot In most cases the radical dissociation-recombination mechanism competes with the concerted process ; this competition is more important at higher temperatures."' Transfer of chirality from N to C in the rearrangement of (67)to (68) points to a concerted rearrangement. Decomposi-tion of cinnamylbenzyldiazene does not however follow the [3,2] sigmatropic pathway but gives products of radical recombination.lo2 Schollkopf has reviewed the mechanism of [1,2] sigmatropic rearrangements in anionic ~ysterns.~'~" He notes that the anionic [1,4] shift"3b is a symmetry- allowed process.A systematic study of the thermal rearrangements of 2-alkoxy-pyridine N-oxides to N-alkoxypyridones (for which it had previously been shown 97 P. Courtot and R. Rumin Tetrahedron Letters 1970 1849. 98 L. J. M. van de Ven J. L. M. Keulemans-Lebbink J. W. de Haan and H. Kloosterziel Chem. Comm. 1970 1509. 99 G. Frater and H. Schmid Heiv. Chirn. Acta 1970 53 269. O0 (a)V. Rautenstrauch Chem. Comm. 1970,4;(b)J. E. Baldwin and M. C. H. Armstrong ibid. p. 631 ;(c) J. E. Baldwin and F. J. Urban ibid. p. 165. lo' M. Moriwaki S.Sawada and Y. Inouye Chem. Comm. 1970,419. Io2 W. D. Ollis I. 0.Sutherland and Y.Thebtaranonth Chem. Comm. 1970 1199. (a) U. Schollkopf Angew. Chem. Internat. Edn. 1970 9 763. (b) H. Felkin and A. Tambute Tetrahedron Letters 1969 821. 168 B. G. Odell that migrating ally1 groups were not invertedL0&) has given conclusive proof that it is a concerted [ls,4s] process (except for the case of benzhydryl migration where a radical pair process intervene^)."^^ (69) (70) [1,6] Hydrogen shifts have been observed in pentadienyl anions the rearrange- ment of (69) to (70) being typical. Cyclic anions in which antarafacial migrations would be sterically impossible are thermally stable but are isomerised photo- chemically by a [ls,6s] pro~ess."~ Electrocyclic Reactions.-Salem has pointed out that there is no preferred motion for reclosure of diradicals having degenerate molecular orbitals of opposite symrnetry.'O6 Silver-ion-assisted solvolysis of halogenocarbene adducts of cyclic olefins in hydroxylic solvents can lead to trans allylic ethers or alcohols.107 Compound (71) leads to the single diastereomer (72) by loss of the em-9-bromine with dis- rotatory opening of the three-membered ring to give a trans,trans-allylic cation which is captured by methanol on the same side as that from which the bromide left.' The observed stereochemistry is also that expected on electronic grounds if a free cation is not involved.It has been calculated that the cyclobutyl cation should open to a homoallyl cation in a disrotatory sense and this has received experimental confirmation.' O8 Photochemical cyclisation of azoxy-compounds to oxadia~irans'~~~ and the related 471 photochemical closure of enolate anions to epo~ides'~~' have been observed.The cyclopropyl anion (73) opens thermally in the allowed conrotatory sense to give the mono-trans-cyclononatetraenide anion.' lo Thermal isomerisation of bicyclopentenes to cyclopentadienes which had previously been accepted as a two-step process has been shown for the case of Io4 (a) J. E. Litster and H. Tieckelman J. Amer. Chem. SOC. 1968 90 4361; (b) U. Schollkopf and I. Hoppe. Tetrahedron Letters 1970 4527. '05 R. B. Bates S. Brenner W. H. Deines D. A. McCombs and D. E. Potter J. Amer. Chem.SOC.,1970,92,6345. lob L. Salem Chem. Comm. 1970 981; Bull. SOC. chim. France 1970 3161. lo7 (a) C. B. Reese and A. Shaw Chem. Comm. 1970 1365; (6) D. Duffin and J. K. Sutherland ibid. p. 626. Io8 K. B. Wiberg and G. Szeimies J. Amer. Chem. SOC.,1970 92 571; C. D. Poulter E. C. Friedrich and S. Winstein ibid. p. 4274. Io9 (a)F. D. Green and S. S. Hecht J. Org. Chem. 1970,35,2482;(b) E. E. van Tamelen J. Schwartz and J. I. Brauman J. Amer. Chem. SOC. 1970 92 5798. G. Boche D. Martens and W. Danzer Angew. Chem. Internat. Edn. 1969 8 984. Reaction Mechanisms-Part (iii) (74) (74) to be a thermally allowed [,2 + ,2,] cycloaddition."'" Previous studies had indicated that the 5-methylene group retains its integrity during such isomerisations.' '' This result will doubtless prompt more thorough considera- tion of non-least-motion processes before drawing the conclusion that reactions are not concerted The remote double bond in systems such as (75) has little electronic effect on the transition state for the thermally forbidden disrotatory cyclobutane opening relative to corresponding dihydro-compounds.' Direct kinetic evidence has been presented that cis-trans isomerisation of simple dienes involves the inter- mediate formation of cyclobutenes.' l3 The rates of electrocyclic closure of sterically hindered dienes have been correlated with their ground-state conforma- tions as determined by X-ray crystallography.' l4 The concept of 1,Sdipolar cyclisation (76)--* (77) has been presented as a rationale of many heterocycle-forming reactions.' ' Bicyclo[5,l,0]octadienyl anion (78)and cyclo-octatrienyl anion do not undergo thermal interconversion ;'l6 conrotation is forbidden for the 871 electron system.A review of cyclodecapentaene chemistry contains a feast of pericyclic pro- cesses.' ' Hoffmann has suggested that the major structural feature which stabilises norcaradienes with respect to their seven-membered valence-tautomers is the presence of a low-lying acceptor orbital in the group at the 7-position."* If it is in the correct orientation (79) it will interact with one of the filled Walsh 'I' (a) J. E. Baldwin and A. H. Andrist Chem. Comm. 1970 1561; (6) J. E. Baldwin R. K. Pinschmidt jun. and A. H. Andrist J. Amer. Chem. SOC.,1970 92 5249.H. M. Frey J. Metcalf and J. M. Brown J. Chem. SOC. (B) 1970 1586. 'I3 H. M. Frey A. M. Lamont and R. Walsh Chem. Comm. 1970 1583. G. A. Doorakian H. H. Freedman R. F. Bryan and H. P. Weber J. Amer. Chem. SOC.,1970 92 399. ' ' H. Reimlinger Chem. Ber. 1970 103 1900. ' l6 H. Kloosterziel and E. Zwanenburg Rec. Trav. chim. 1969,88 1373. " T. L. Burkoth and E. E. van Tamelen in 'Nonbenzenoid Aromatics' ed. J. P. Snyder, Academic Press 1969 p. 63. ' I' R. Hoffmann Tetrahedron Letters 1970 2907. 170 B. G. Ode11 cyclopropane orbitals of the bicyclic form so as to effect a net transfer of electrons from the ring to the acceptor orbital making the 1,6-bond stronger. Examples of this effect have been discussed in the cycloheptatriene oxepin and azepine series.' The first example of a monocyclic azepine reacting as its azanorcaradi- ene tautomer has been given.12' The valence tautomerism of bromocyclo- octatetraenes and their isomerisation to bromostyrenes have been studied in detail.' 21 The predicted' electrocyclisation of meso-cyclodecahexaene (80) to naphthalene has been observed.'22 Cheletropic and other Pericyclic Processes.-Decomposition of N-arylazoaziri-dines (81) to olefins and azides is a stereospecific process,122 as is the oxidative fragmentation of N-arylaziridines where the N-oxide may be an intermediate.'23 Deoxygenation of epoxides and oxetans by atomic carbon'24 and oxidative loss of nitrogen from 1 -aminoaziridines' 25 probably proceed by radical mechanisms.Pyrolytic elimination of dialkoxycarbenes from acetals of norbornadienone (82) which could be concerted in the linear sense appears to be a two-step process.126 Thermal loss of a carbene from tropone ethylene acetal probably takes the concerted non-linear pathway from the norcaradiene tautomer.'" This process competes with a [ 1,7] sigmatropic oxygen migration. Thermal stereospecific dimerisation of sulphur dioxide from 3-thiabicyclo- [3,l,O]hexane 3,3-dioxides and the related epoxysulpholanes e.g. (83)-+(84),is a fully concerted CU2 + u2 + 62] retrogression. As would therefore be expected 119 H. Gunther Tetrahedron Letters 1970 5 173. 120 H. Prinzbach D. Stusche and R. Kitzing Angew. Chem. Internat. Edn. 1970 9 377; see L. A. Paquette in ref. 11 7 p. 249 for a discussion of oxepin and azepine chemistry.121 R. Huisgen and W. E. Konz J. Amer. Chem. SOC.,1970,92,4102,and following papers. 122 M. H. Akhtar and A. C. Oehschlager Tetrahedron 1970 26,3245. 123 H. W. Heine J. D. Myers and E. T. Peltzer tert. Angew. Chem. Internat. Edn. 1970 9 374. 124 J. H. Plonka and P. S. Skell Chem. Comm. 1970 1108. 125 L. A. Carpino and R. K. Kirkley J. Amer. Chem. SOC.,1970 92 1784. 126 R. W. Hoffmann and R. Hirsch Tetrahedron Letters 1970 4819. 127 K. Fukunaga T. Mukai Y. Akasaki and R. Suzuki Tetrahedron Letters 1970 2975. Reaction Mechanisms-Part (iii) (85) is relatively stable; the conczrted pathway here would lead to the very strained tr~ns,tr~ns-cyclohepta-1,4-diene.'~~ Sulphur monoxide extruded from (86)by way of (87) has been trapped by reaction with dienes.'" Fragmentation of the isomeric sulphones (88) and (89) gives sulphur dioxide and cyclo-octa- 1,3,5-triene.Compound (88j which can fragment in a concerted linear manner decomposes at 100"C while (89) for which only the non-linear mode is available for concerted decomposition requires 250 "C. The latter may however follow a multistep pathway. The difference of activation energies between the linear and non-linear processes has been roughly estimated as 10kcal mol-'.'30 Cheletropic loss of triphenylphosphine from the pentacovalent phosphorane (90) can be photochemically or thermally initiated and produces syn-tricyclo- octadiene.I3' D Thermal fragmentation of (91j leads to ['H ,]benzene by symmetry-allowed loss of HD while (92) yields a mixture of deuteriobenzenes by a radical mech- anism.Attempts to observe allowed stereospecific hydrogen transfers from these molecules to various hydrogen acceptors were frustrated by the incursion of radical processes.' 32 W. L. Mock J. Amer. Chem. SOC.,1970,92,6918. Y. L. Chow J. N. S. Tam J. E. Blier and H. H. Szmant Chem. Comm. 1970 1604. I3O W. L. Mock J. Amer. Chem. SOC., 1970,92 3807. 13' T. J. Katz and E. W. Turnblom J. Amer. Chern. SOC.,1970 92 6701. 32 I. Fleming and E. Wildsmith Chem. Comm. 1970 223. 172 B. G. OdeN Carbenes-The structures of carbenes and the stereochemistry of their addition to 01efins'~~ have been reviewed. Further and the reactions of diazoalkane~'~~ theoretical evidence for the non-linearity of the triplet ground state of methylene has been pre~ented.'~' Calculations on the dimerisation of methylene~'~~ show that the non-least-motion approach where a a-lone-pair of one molecule impinges on an unoccupied n-orbital of the other is preferred.Reversibility of singlet-triplet interconversion of diphenylmethylene has been invoked to explain the dependence of product distribution on the concentration of added isopropanol in the decomposition of diphenyldiazomethane in aceto- nitrile.' 37 Diphenylmethylene is produced in the photolysis of tetraphenyl- methane.'38 The ratio of diastereomeric cyclopropanes formed in the reaction of phenyl- carbene with olefins has been found to depend upon the olefin concentration a result which has wide implications on mechanistic proposals based upon such ratios.'39 The reaction between carbon atoms (IDor IS)and many classes of carbonyl compounds at 77 K results in the production of free singlet carbenes and carbon m~noxide.'~' In most cases the carbenes could not be trapped but underwent intramolecular rearrangements to valence-satisfied products.However dichloro- carbene (from phosgene) and methoxycarbene (from methyl formate) were found to add in a tereospecific manner to olefins. Dimethylcarbene produced from acetone rearranges to propylene. Study of the intramolecular deuterium iso- tope effect for this process has lead to the prediction that kJkD values greater than 1.4 in related reactions are indicative of the presence of complexed carbenes.140 In contrast to the above results it was found that 3P carbon atoms insert into the C-H bonds of a~et0ne.I~' Evidence has been presented that quadricyclanyli- dene (93) decomposes with the chemical formation of carbon atoms.142 Insertion of dichlorocarbene into C-H bonds usually leads to complex product mixtures. A preparatively useful example has been found in the exclusive insertion at the bridgehead of adamantane.'43 Fluorochlorocarbene is produced by thermolysis of PhHgCC1,F. '44 Decomposition of trichloromethyl-lithium probably gives uncomplexed dichlorocarbene. '45 133 G. L. Closs Topics Stereochem. 1968 3 193. 134 G. W. Cowell and A. Ledwith Quart. Rev. 1970,24 119. lJ5 C. F.Bender and H. F. Schaefer tert. J. Amer. Chem. Soc. 1970,92,4984. 13' R. Hoffman R. Gleiter and F. B. Mallory J. Amer. Chem. SOC.,1970 92 1460; H. Kollmar Tetrahedron Letters 1970 3337. 13' D. Bethell G. Stevens and P. Tickle Chem. Comm. 1970 792. 13' T. D. Walsh and D. R. Powers Tetrahedron Letters 1970 3855. 139 M. Schlosser and G. Heinz Chem. Ber. 1970 103 3543. 140 P. S. Skell and J. H. Plonka J. Amer. Chem. Soc. 1970 92 836 2160; Tetrahedron Letters 1970 2603 4557. 14' P. S. Skell J. H. Plonka and L. S.Wood Chem. Comm. 1970 710. 142 P. B. Shelvin and A. P. Wolf Tetrahedron Letters 1970 3987. 143 I. Tabushi Z. Yoshida and N. Takahashi J. Amer. Chem. Soc. 1970,92,6670. 144 D. Seyferth and K. V. Darragh J. Org. Chem. 1970,35 1297. G. Kobrich H.Biittner and E. Wagner Angew. Chem. Internat. Edn. 1970,9 169. Reaction Mechanisms-Part (iii) The mechanism of the intramolecular rearrangement of vinylcarbenes to cyclopropenes has received further and the first example of the trapping CN dc" /Curan (93) yCN (94) of such a carbene by an olefinic double bond as exemplified by reaction of (94) with furan to give (95) has been observed. 147 Methylchlorocarbene in addition to rearranging to vinyl chloride has been shown to be moderately selective in its singlet reactions with olefins; the order of resonance stabilisation of singlet carbenes is suggested14* to be FCCl > CC1 > PhCCl > MeCC1. The mechanism of addition of halogenocarbenes to bicyclic olefins has been reviewed.14' Phenylcarbene produced by pyrolysis of benzyl fluoride undergoes ring contraction to fulveneallene (96) and expansion to cycloheptatrienylidene (97) as shown in Scheme 4.l5' The latter process is reversible and results in the Scheme4 interconversion of 0-,m-,and p-tolylcarbenes.In agreement with this hypothesis 1-methylcycloheptatrienylidenegives some styrene which is presumably formed through the intermediacy of phenylmethylcarbene. '51 The reversible rearrangement of acylcarbenes (98) to oxirens (99) accompanies the Wolff rearrangement of diazoketones.' 52 Peracid oxidation of acetylenes also leads to these interconverting species.' H. Diirr Chem. Ber. 1970 103 369. 14' M. Franck-Neumann and C. Buchecker Angew. Chem. Internat. Edn.1970 9 526. R. A. Moss and A. Mamantov J. Amer. Chem. SOC..1970,92,6951. C. W. Jefford Chimia (Switz.) 1970 24 357. I5O P. Schissel M. E. Kent D. J. McAdoo and E. Hedaya J. Amer. Chem. SOC.,1970 92 2147. W. J. Baron M. Jones jun. and P. P. Gaspar J. Amer. Chem. SOC.,1970 92 4739; J. A. Meyers R.C. Joines and W. M. Jones ibid. p. 4740. 15* D. E. Thornton R. K. Gosavi and 0. P. Strausz J. Amer. Chem. Soc. 1970 92 1768; G. Frater and 0.P. Strausz ibid. p. 6654. J. Ciabattoni R. A. Campbell C. A. Renner and P. W. Concannon J. Amer. Chem. SOC.,1970 92 3826. 174 B. G. Ode11 Ethoxycarbonyltrimethylsilylcarbeneinserts into C-H bonds and undergoes cis-addition to double bonds in contrast to the carbon analogue which stabilises itself almost entirely by intramolecular paths.' 54 Addition of dicyanocarbene to cyclo-octatetraene to give (loo) a rare example of 1,4 carbene addition to a diene system arises from the triplet which gives a stabilised diradical intermediate.' Bicyclo[3,3,l]non-2-en-9-ylidene (101) reacts intramolecularly in a way that suggests that there is an initial stabilising interaction between the electron-deficient centre and the double bond although addition to the double bond is sterically imp0ssib1e.l~~ The carbenoid formed in a Corey-Winter alkene synthesis was found to be trapped by intramolecular insertion into an 0-H bond.I5' Loss of bromide ion from 3-bromobicyclo- [3,2,l]octa-2,6-dienyl anion probably gives the homoconjugated carbene (102)' 58 Calculations' 59 and observations' 6o on the cyclopropylidene to allene isomer- isation have been reported.The carbene (free or complexed) probably opens in aconrotatory or monorotatory manner. Some tetrasubstitutedcyclopropylidenes where conrotatory opening is hindered undergo C-H insertion to give bicyclo- butanes.16' Nitrenes.-A review on nitrenes in organic synthesis has appeared.16' Phenylnitrene and not ring-expansion products is formed in the photolysis of 1-phenyliminopyridinium ylides. '62 Intermolecular electrophilic aromatic substitution by arylnitrenes is very rare. It has now been observed that activated benzene rings are attacked in the ortho-and para-positions by electrophilic phenylnitrenes which bear an electron-withdrawing substituent at the para-position (e.g.p-cyanophenylnitrene).' 63 Intramolecular attack of arylnitrenes on aromatic rings is in contrast commonly observed and it is interesting to note that o-biphenylnitrene can be diverted from carbazole formation by diethylamine.' 64 Nitrenes formed by the action of tervalent phosphorus compounds on aryl nitrophenyl sulphides undergo 54 U. Schollkopf D. Hoppe N. Rieber and V. Jacobi Annalen 1969 730 1. L55 A. G. Anastassiou R. P. Cellura and E. Ciganek Tetrahedron Letters 1970 5267. 56 M. H. Fisch and H. D. Pierce jun. Chem. Comm. 1970 503. 15' D. Horton and C. G. Tindall jun. J. Org. Chem. 1970,35 3558. 15' R. G. Bergmann and V. J. Rajadhyaksha J. Amer. Chem. SOC.,1970,92,2163. 159 M. J. S. Dewar. E. Haselbach and M. Shanshal J. Amer. Chem.SOC.,1970,92 3505. I6O W. R. Moore and J. B. Hill Tetrahedron Letters 1970 4343 4553 and refs. cited therein. R. K. Smalley and H. Suschitzky Chem. and Ind. 1970 1338. 162 V. Snieckus and G. Kan Chem. Comm. 1970 172. 16' R. A. Abramovitch and E. F. V. Scriven Chem. Comm. 1970 787. 164 R. J. Sundberg M. Brenner S. R. Suter and B. P. Das Tetrahedron Letters 1970 2715. Reaction Mechanisms-Part (iii) intramolecular insertion. Spiro-intermediates such as (103) have been invoked to explain the rearrangements observed.' 65 (103) 1 N-Azido-amines decompose by way of N-nitrenes. '66 The isoelectronic alkoxynitrenes RON are generated by oxidation of 0-alkylhydroxylamines ; they give N-alkoxyaziridines on reaction with olefins.'67 The adducts formed between singlet methanesulphonylnitrene and nucleo- philic aromatic compounds such as toluene and anisole ring-open to give sul- phonamides. When a deactivated aromatic ring is the substrate some or all of the nitrene becomes demoted to the triplet ground state which gives typical radical substitution products. Nitrobenzene which is too electron-deficient to give any singlet-derived products reacts only with the triplet. In this case the presence of molecular oxygen prevents any substitution by reacting rapidly with the triplet nitrene. ' * 165 J. I. G. Cadogan and S. Kulik Chem. Comm. 1970 233 792; J. I. G. Cadogan S. Kulik and C. Thomson ibid. p. 436. 166 G. Koga and J.-P. Anselme J. Org. Chem. 1970,35,960. 167 S. J.Brois J. Amer. Chem. SOC.,1970 92 1079. '60 R. A. Abramovitch G. N. Knaus and V. Uma J. Amer. Chem. SOC.,1969,91 7532.