3 Reaction Mechanisms Part (ii) Orbital Symmetry Correlations and Pericyclic Reactions By R. GRlGG Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Theoretical Aspects Alternative theoretical treatments of pericyclic processes continue to appear and a number of pericyclic processes have been discussed in terms of the principle of least motion.2 A number of new concepts of importance in pericyclic processes such as the relationship between symmetry topology and ar~maticity,~ and bond stretch isomerism and polytopal rearrangement^,^ have also been dis- cussed. The problem of those ‘forbidden’ processes which occur comparatively easily has been discussed’ and it is concluded that a majority of the observed ‘forbidden’ processes are concerted if they require the opening of only one C-C bond.It is suggested that substituents which introduce low-lying excited singlet states can dramatically enhance the rates of forbidden reactions by increasing the anharmonicities of the potential curves. A series of stimulating papers formulat- ing a new theoretical approach to thermal and photochemical cycloaddition reactions has appeared6 in which the manifold problems of predicting rates stereo- chemistry and solvent effects are tackled. A very important difference between this approach and previous theoretical treatments is the implication that non- stereospecificity(e.g.in thermal 2 + 2 reactions) might be the outcome of several well-defined and competing concerted processes rather than an indication of the intervention of a partially or totally non-discriminating intermediate such as a biradical.The energy separation of the interacting MO’s of the cycloaddends is noted as the key factor which determines the extent by which a concerted reaction will be favoured over a non-concerted reaction. Thus thermal cycloadditions are classified according to a donor-acceptor scheme giving a spectrum varying from cycloaddends with similar electron-donor and -acceptor abilities undergoing non-polar addition through systems with diverging electronic character to the W. J. van der Hart J. J. C. Mulder and L. J. Oosterhoff J. Amer. Chem. SOC.,1972 94 5724; W. A. Goddard ibid. p. 793; R. J. Buenker S. D. Peyerimhoff and K. Hsu ibid. 1971 93 5005; R. G. Pearson ibid.1972,94 8287. 0. S. Tee and K. Yates J. Amer. Chem. SOC.,1972 94 3074. M. J. Goldstein and R. Hoffmann J. Amer. Chem. SOC.,1971 93 6193. W.-D. Stohrer and R. Hoffmann J. Amer. Chem. SOC.,1972,94 779 1661. W Schmidt Tetrahedron Letters 1972 581 ; Heh. Chim. Acta 1971 54 862. N. D. Epiotis J. Amer. Chem. SOC. 1972 94 1924 1935 1941 1946. 120 Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations extreme near-ionic cycloaddition. It is suggested that when the MO interactions (HOMO/LUMO) between cycloaddends are strong a concerted reaction will be favoured whereas a weak interaction will favour the two-step mechanism. Since HOMO/LUMO interactions are weakest towards the extreme non-polar end of the spectrum for both 2 + 2 and 4 + 2 cycloadditions it is in this region that two-step cycloadditions are expected.A similar reactivity spectrum is discussed for photochemical reactions. Perturbation theory has been applied to the lowest triplet state of conjugated hydrocarbons7 and the rules for ground-state aromaticity are found to be reversed in the lowest triplet state such that 4n rings display 'aromatic' character whereas 4n + 2 systems display 'antiaromaticity'. A major contribution to rationalizing the anomalies in the mechanics of photochemical processes has appeared.8 Following a previous pioneering paperg concerned only with electro- cyclic processes Dougherty employs PMO theory and classifies photochemical reactions into three general types (i) X-type reactions which occur entirely on an excited-state surface and result in luminescent products ; (ii) N-type reactions which start from an excited state and proceed to a non-bonding (intermediate) ground state; (iii) G-type reactions which start on an excited surface and proceed directly to a bonding ground-state configuration.Photochemical pericyclic reactions are G-type processes and result in ground-state products because of a breakdown of the Born-Oppenheimer approximation.' A suggestion for a new class of pericyclic reactions dyotropic processes (Gk. dyo; two) has been made." Dyotropic processes are defined as processes in which two a-bonds simultaneously migrate intramolecularly. This can lead to an interchange of bonds [e.g. (1)-P (2)] as observed in many vicinal trans-dibromosteroids.' ' However no direct positional interchange need occur since the photoracemization (3) + (4)12is also classified as a dyotropic process.X Bi (3) (4) N. C. Baird J. Amer. Chem. SOC.,1972 94,4941. R. C. Dougherty J. Amer. Chem. SOC.,1971,93 7187. ' W. Th. Am. van der Lugt and L. J. Oosterhoff J. Amer. Chem. Soc. 1969,91 6042. lo M. T. Reetz Angew. Chem. Infernat. Edn. 1972 11 129 130. P. L. Barili G. Bellucci G. Berti F. Marioni A. Marsili and I. Morelli J.C.S. Perkin IZ 1972 59. D. G. Farnum and G. R. Carlson J. Amer. Chem. SOC.,1970,92 6700. 122 R.Grigg A number of reviews covering both arornaticl3 and related heterocyclic systems14 and their valence isomers have appeared. The theory of cycloaddition reactions has also been reviewed15 and two new books on pericyclic processes have appeared." A curly arrow symbolism employing three types of arrow has been proposed which allows both electron shifts and stereospecificities of peri-cyclic processes to be depicted and provides a simple predictive approach at the same time.' 2 Electrocyclic Reactions Both CND0/2 and MIND0/2 studies18 concur with Dewar's view [see Ann.Reports (B),1971,68 1441 that cyclopropyl radicals should undergo disrotatory opening to the allyl radical and in a pyramidal radical one disrotatory mode is favoured over the other [i.e.(5)]. A further example of steric control in the con- rotatory opening of a cyclopropyl carbene to an allene has been provided [(6)-D (7)I.l' Both thermal and solvolytic" opening of cyclopropyl derivatives have attracted much attention.Thermal ring-opening of cyclopropyl chlorides leads to stereospecific recapture of the chloride ion by the allyl cation on the same face of the molecule [e.g. (8)-(9)].2' The concerted solvolysis of exo-substituted bicyclo[2,l,O]pentanes (10) to cyclopentenes is clearly sterically impossible. Studies show that exo (10) into endo (11) conversion occurs first l3 (a) E. E. van Tamelen Accounts Chem. Res. 1972 5 186; (6) S. Masamune and N. Darby ibid. p. 272; (c)L. T. Scott and M. Jones Chem. Rev. 1972,72 181. l4 A. G. Anastassiou Accounts Chem. Res. 1972 5 281. Is W. C. Herndon Chem. Rev. 1972 72 157. l6 T. L. Gilchrist and R. C. Storr 'Organic Reactions and Orbital Symmetry' Cambridge Univ.Press 1972; R. E. Lehr and A. P. Marchand 'Orbital Symmetry. A Problem Solving Approach' Academic Press 1972. C. Kaneko Tetrahedron 1972 28 4915. G. Boche and G. Szeirnies Angew. Chem. Internat. Edn. 1971 10 91 1,912. l9 W. R. Moore and R. D. Bach J. Amer. Chem. Soc. 1972,94 3148. 2o P. von R. Schleyer W. F. Sliwinski G. W. Van Dine U. Schollkopf J. Paust and K. Fellenberger J. Amer. Chem. SOC.,1972 94 125; W. F. Sliwinski T. M. Su and P. von R. Schleyer ibid. p. 133. *l I. Fleming and E. J. Thomas Tetrahedron 1972 28,4989. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations 123 thus removing the steric constraints and is followed by a rapid solvolysis to ( 12).22 The exo-endo isomerization could conceivably involve either 1,4 or 1,5 ( =4,5)bond fission generating a biradical intermediate.Studies on deuteri- ated derivatives demonstrate that 1,6bond cleavage is occurring.23 Recent calculation^^^ indicate that the bicyclopentanes (13 ;R = F or OR’)can undergo thermal ring-flip by an ‘allowed’ path. Electrocyclic opening of cyclopropyl anions to allyl anions has been achieved but the mode ofopening was not determined 25*26 except in one case [( 14)-+(IS)] where the molecule is constrained to the unfavourable disrotatory mode.” It proved possible in some cases to trap the allyl anions by cycloaddition to aromatic olefins (16)-+ (17)26and a related cycloreversion has also been achieved ( 18) -+( 19).2 H Ph PhCH=CH ---A CN H-H Ph H Ph C0,Me C0,Me Phfico2Me + PhAMe Ph Nx=N Me Ph C0,Me (18) (19) ” K.Fellenberg U. Schollkopf C. A. Bahn and P. von R. Schleyer Tetrahedron Letters 1972 359; J. J. Tufariello A. C. Bayer and J. J. Spadero ibid. p. 363. 23 J. J. Tufariello and A. C. Bayer Tetrahedron Letters 1972 3551 24 D. B. Chesnut S. Ferguson L. D. Smith and N. A. Porter Tetrahedron Letters 1972 3713. 25 R. Huisgen and P. Eberhard J. Amer. Chem. Soc. 1972,94 1346. ‘* G. Boche and D. Martens Angew. Chem. Infernat. Edn. 1972 11 724. l7 M. E. Londrigan and J. E. Mulvaney J. Org. Chem. 1972,37,2823. 28 P. Eberhard and R. Huisgen J. Amer. Chem. Soc. 1972,94 1345. 124 R.Grigg Pyrolysis at 280 “C of cis-3,4-dimethylcyclobutenegives ca. 0.005 % of trans,-trans-2,4-hexadiene the product of a ‘forbidden’ disrotatory opening.From this and a consideration of steric effects it is concluded that the conrotatory transition state is 15 kcal mol-I lower in energy than the disrotatory Studies on electrocyclic reactions in the vitamin D series and of benzene valence isomers which are sterically constrained to occur by ‘forbidden’ paths have provided useful kinetic data.30.3 Thus the antiaromatic transition state for the re-arrangement (20) +(21) is markedly stabilized by unsymmetrical substitution on the two central carbons.31 2-Pyridones’ generally thought to only undergo 4 + 4 photocycloadditions have now been found to undergo disrotatory photocycliza- tions to the bicyclic isomers (22) +(23).32 Photocyclization ofazine monoxides (24)-+ (25) occurs from the excited singlet state.33 The radical-anions prepared from the benzocyclobutenes (26 ;R’= H R2 = Ph or R’= Ph R2= H) undergo stereospecific conrotatory opening to the xylylene radical-anions (27) and the corresponding dianions formed by further reduction can be trapped with dimethyldichl~rosilane.~~ .I X A I R’ R‘ R2 I K’ (24) (25) 29 J.I. Brauman and W. C. Archie J. Amer. Chem. Soc. 1972 94 4262. 30 A. M. Bloothoofd-Kruisbeek and J. Lugtenburg Rec. Trav. chim. 1972,91 1364. 31 R. Breslow J. Napierski and A. H. Schmidt J. Amer. Chern. SOC.,1972 94 5906; D. M. Lemal and L. H. Dunlap ibid. p. 6562. 32 H. Furrer Chem. Ber. 1972 105 2780; R. C. De Selms and W. R. Schleigh Tetra-hedron Letters 1972 3563. 33 W.M. Williams and W. R. Dolbier J. Amer. Chem. SOC.,1972 94 3955. 34 N. L. Bauld C.-S. Chang and F. R. Farr J. Amer. Chem. Soc. 1972,94 7164. 125 Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations Ph Photoequilibration of cyclohexa-1,3-dienes with hexa-1,3,5-trienes has been studied,35 and the effects of ground-state conformational control on the direc- tion of conrotatory opening have been disc~ssed.~~ Structural and charge effects on the cycloheptatriene-norcaradiene3' and cyclo-octatriene-bicyclo[4,2,0]-he~adiene~~ equilibria have been reported Calculations on the related hetero- cyclic equilibria (28) (29) (X = 0,NR or S) predict that protonation on the heteroatom will displace the equilibrium towards the bicyclic form (29).39a N.m.r.studies show that the azepines (30) exist predominately in the monocyclic form whereas the diazepines prefer the bicyclic structure (31).39b R,2 R5 R4 A further facet of the mechanisms of rearrangement of bicyclo[6,1 ,O]nonatrienes [see Ann. Reports (B),1971,68 1491 has been uncovered by studies on the esters (32a and b) which are thermally converted into an 86 14 mixture of(33) and (34).40 At 100 "C epimerization (32a) +(32b) without rearrangement occurred and an analogous epimerization of (32; R' = Me R2 = C0,Me) to (32; R' = CO,Me R2 = Me) was observed at 160 "C. A biradical mechanism involving breaking of the 1,9-bond is favoured for this reaction. It has also proved possible to prepare (35) and study its Cope rearrangement to (36)41 at temperatures below 0°C.The reverse process (36) +(35)has been suggested to intervene in the conversion of some bicyclo[6,l,0]nonatrienes into the corresponding dihydr~indenes.~~ 35 W. G. Dauben J. Rabinowitz N. D. Vietmeyer and P. H. Wendschuk J. Amer. Chem. SOC.,1972,94 4285. 36 C. W. Spangler and R. P. Hennis J.C.S. Chem. Comm. 1972 24. 37 J. Daub and W. Betz Tetrahedron Letters 1972 3451; E. Vogel W. Wiedemann H. D. Roth J. Eimer and H. Gunther Annafen 1972,759 1. 38 F. A. Cotton and G. Daganello J. Amer. Chem. SOC.,1972 94 2142. 39 (a) W.-D. Stohrer and R. Hoffmann Angew. Chem. Internat. Edn. 1972 11 825; (b) A. Steigel J. Sauer D. A. Kleier and G. Binsch J. Amer. Chem. Soc. 1972 94 2770. 40 M. B. Sohn M. Jones and B. Fairless J.Amer. Chem. SOC.,1972 94 4774. 41 L. A. Paquette and M. J. Epstein J. Amer. Chem. SOC.,1972 94 5936. 42 P. Radlick and W. Fenical J. Amer. Chem. SOC.,1969 91 1560; A. G. Anastassiou and R. C. Griffith ibid. 1971,93 3083. I26 R. Grigg The related aza-system has been studied and was found to give the trans-dihydro- ind01e~~ in accord with prediction and not the cis-product as previously reported. H Et0,C (32) a; R’ = D R2 = C0,Et (33) (34) b;R’ = CO,Et R2 = D Ph Ph Ph (35) (36) A number of thermal44 and phot~chernical~~~~~ electrocyclic reactions in both ani~nic~~.~’ species have been reported. An intriguing example and ~ationic~~ [(37)-+(38)] is thought to arise via photochemical conrotatory opening of the protonated species to a heptatrienyl cation (39) followed by a thermal conrota- tory closure utilizing a pentadienyl cation moiety.46 Electrocyclic reactions followed by elimination reactions [e.g.(40 +(41)-+(42)] have synthetic poten- tial in aromatic and heterocyclic chemistry.47 bFSY;H,,9-R (I R (38) R (37) R = H or Me (39) $ =y$ +)$ /N-OH \ N-OH \N (40) (41) (42) 43 A. G. Anastassiou R. L. Elliot and A. Lichtenfield Tetrahedron Letters 1972 4569. 44 R. B. Bates S. Brenner and C. M. Cole J. Amer. Chem. SOC.,1972 94 2130; R. B. Bates S. Brenner and B. I. Mayall ibid. p. 4765. 45 H. Kloosterziel and G. M. Gorter-La Roy J.C.S. Chem. Comm. 1972 352. 46 R. Noyori Y. Ohnishi and M. Kato J. Amer. Chem. SOC.,1972 94 5105. 47 P. Scheiss H.L. Chia and P. Ringele Tetrahedron Letters 1972 313; C. Jutz and R. M. Wagner Angew. Chem. Internat. Edn. 1972,11 315. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations 127 3 Cycloaddition Reactions The plenary lectures of a symposium on cycloadditions have been published?’ MIND0/2 calculations and considerations of transition-state geometry suggest that in general transition states of cycloaddition reactions are likely to be non- symmetric. For N + M cycloadditions the likelihood of a non-symmetric transition state will increase as N and M in~rease.~’ This asymmetry in the transi- tion state can be turned to a predictive advantage since the preferred reaction paths can be predicted by considering formation of the most stable (hypothetical) biradical intermediate” as is well known for the Diels-Alder reaction.A new criterion the rate ratio for em-addition to norbornene (k,) and 7,7-dimethyl- norbornene (k2),for distinguishing between cyclic and non-cyclic addition processes has been proposed. Processes with cyclic transition states have k,/k2= 480-1820 whereas for non-cyclic processes k,/k < 58.51 The lifetime of singlet oxygen (‘Ag) contrary to popular belief is solvent dependents2 and a new source of singlet oxygen potassium perchromate has been rep~rted.’~ The pyrolysis of the cyclobutane (43) is non-stereospecific and leads to cis-and trans-2-butene and cis-and trans-dideuteri~ethylene.’~ In contrast the thermal cleavage of the bicyclo[2,2,0]hexane (44) to the corresponding hexa-1,5-diene is stereospecific and apparently a concerted ,2 + ,2 pro~ess.’~ The gas-phase isomerization of the methyl bicyclo[2,1 ,O]pentenes (45a and b) gives mixtures of the corresponding cyclopentadienes (46a and b) uia ,2 + ,2 proce~ses.’~ How-ever the previous liquid-phase work which reported (45a) as giving (46b)” has been repeated and the product shown to be (46a).58 Clearly double-labelling experiments are required in.this series. Thermal decomposition of the dioxetan (47)efficiently and selectively yields triplet-state acetone which is not generated from acetone singlet precursor^.^^ Me Mb**D R’ :xGD qR2Rd2 ‘D (44) (45) a; R’ = Me R2 = H (46) (43) b; R’ = H RZ = Me 48 ‘Cycloaddition Reactions’ ed. R. Gompper Butterworths London 1972.49 J. W. Mclver J. Amer. Chem. SOC.,1972 94 4782. G. L. Goe J. Org. Chem. 1972 37 2434. 51 H. C. Brown and K.-T. Liu J. Amer. Chem. SOC.,1971 93 7335. 52 P. B. Merkel and D. R. Kearns J. Amer. Chem. SOC.,1972 94 1030; P. B. Merkel R. Nilsson and D. R. Kearns ibid. p. 1030; C. S.Foote E. R. Peterson and K.-W. Lee ibid. p. 1032. ” J. W. Peters J. N. Pitts I. Rosenthal and H. Fuhr J. Amer. Chem. SOC.,1972,94,4348. ’4 R. Srinivasan and J. N. C. Hsu. J.C.S. Chem. Comm. 1972 1213. 55 M. J. Goldstein and M. S. Benzon J. Amer. Chem. SOC.,1972 94 51 19. ’’ J. E. Baldwin and G. D. Andrews J. Amer. Chem. SOC.,1972,94 1775. ” J. E. Baldwin and A. H. Andrist Chem. Comm. 1970 1561. ” S. McLean D. M. Findlay and G. I. Dmitrienko J.Amer. Chem. SOC.,1972,94 1380. 59 N. J. Turro and P. Lechtken J. Amer. Chem. SOC.,1972 94 2886. 128 R.Grigg MeuMe -% [CH3COCH3]3+ [CH3COCH,]' Me Me -50% -1% (47) Keten cycloadditions and cycloreversions of cyclobutanones" continue to attract attention. MO treatments indicate that stabilization through the inter- action of the ketenophile n-system with the carbonyl n-bond plays a dominant role in the orthogonal .2 + ,2 approach of the two reactants and is also respon- sible for directing addition to the C=C as opposed to the carbonyl group of the keten.61 8-Oxoheptafulvene (48) behaves as a typical keten in cycloadditions and with cyclopentadiene gives (49).62 The keteneimmonium cation (50) generated at -60 "C reacts with olefins and dienes by what is thought to be a concerted ,2 + ,2 cycloaddition giving on work-up the corresponding cyclo- butanones (51) in high yield.6 Studies on keten-allene cycl~additions~~~~~ using optically active allenes suggest that the reaction is a concerted .2 + ,2 process in which either allene or keten may be utilized ~uprafacially.~~ A study of secondary deuterium isotope effects in allene-olefin cycloadditions leads to the conclusion that the thermal ,2 + .2 processes proceed stepwise whereas the ,2 + .4 cycloadditions are concerted.66 Photocycloaddition of certain olefins to the nitrile group of benzonitrile is followed by electrocyclic opening of the intermediate 1-azetine (52)+ (53).67 Photoelectron spectroscopy provides an explanation for the failure of hypo- strophene (54) to photocyclize to (55).The reaction is rendered symmetry 'for-bidden' by effective through-bond coupling of an exceptionally high-lying a-level with the two 7c-orbitals.68 The kinetics of the hydroboration of alkenes have been studied and deuterium isotope effects are consistent with a concerted four- centred addition of B-H to the double bond.69 However it is suggested that oczo-% / (48) (49) 6o K. W. Egger and A. T. Cocks J.C.S. Perkin ZZ 1972,2 11 ;J. Metcalfe H. A. J. Carless and E. K. C. Lee J. Amer. Chem. SOC.,1972,94 7235. 61 R. Sustmann A. Ansmann and F. Vahrenholt J. Amer. Chem. SOC.,1972,94 8099. 62 T. Asao N. Morita and Y. Kitahara J. Amer. Chem. SOC.,1972.94 3655. 63 J. Marchand-Brynaert and L.Ghosez J. Amer. Chem. SOC.,1972,94 2870. 64 W. Weyler L. R. Byrd M. C. Caserio and W. H. Moore J. Amer. Chem. SOC.,1972 94 1027. 65 M. Bertrand J.-L. Gras and J. Gore Tetrahedron Letters 1972 1189 2499. 66 S.-H. Dai and W. R. Dolbier J. Amer. Chem. SOC.,1972 94 3946. 6' T. S. Cantrell J. Amer. Chem. SOC.,1972 94 5929. 68 W. Schmidt and B. T. Wilkins Tetrahedron 1972 28 5649. 69 D. J. Pasto B. Lepeska and T.-C. Cheng J. Amer. Chem. SOC.,1972,94 6083; D. J. Pasto B. Lepeska and V. Balasubramaniyan ibid. p. 6090. Reaction Mechanisms-Part (ii) Orbital symmetry Correlations 1 29 (CH,),C=C=&Me BF,-H,O (50) Me Me (51) (52) (53) R = Me or OMe the energy barrier to this ,2 + @2cycloaddition is overcome by prior formation of a n-c~mplex.~~ Frontier orbital effects on the Diels-Alder reaction [see Ann.Reports (B),1971 68,1551 have been placed on a semiquantitative basis.71 The case for the import- ance of van der Waals forces in determining the preferred endo orientation of the methyl group in the Diels-Alder adducts of the methyl-substituted dienophiles (56; X = CN CHO or C0,Me) has been re-argued with supplementary results for 2 + 2 + 2 cycloadditions to norbornadiene which also show the same orienta- tional ~electivity.~~ Synthesis and kinetic study of the thermal reactions of the optically active butadiene dimer 4-vinylcyclohexene (57) revealed that racemiza- tion and deuterium exchange was occurring implicating fission to a biradical and demonstrating that a concerted Cope rearrangement was not involved.The data obtained suggest that Diels-Alder dimerization of butadiene is concerted per- haps two-stage but not The thermal intramolecular cycloaddition of (58) gives (59a) the product of a 4 + 2 cycloaddition rather than (59b) the 2 + 2 product.74 This study shows that the 4+ 2 process is kinetically favoured by at least 4kcal mol-'. Electron-donor solvents (dioxan o-xylene) stabilize the reactants in a Diels-Alder reaction whereas more electronegative solvents lo P. R. Jones J. Org. Chem. 1972 37 1886. R. Sustmann and R. Schubert Angew. Chem. Internat. Edn. 1972,11 839. 72 Y. Kobuke T. Sugimoto J. Furukawa and T. Fueno J. Amer. Chem. SOC.,1972,94 3633. 73 W. von E. Doering M. Franck-Neumann D. Hasselmann and R.L. Kaye J. Amer. Chem. SOC.,1972,94 3833. l4 A. Krantz J. Amer. Chem. SOC.,1972,94,4020. 130 R.Grigg (CHCl, C,H,Cl) stabilize the transition state. The solvent effect on the enthalpy of activation is a consequence of these two A dramatic solvent effect in Diels-Alder additions to the cyclobutene double bond in (60)is ascribed to inter- action of the hydroxy-group with the double bond. The rate in DMSO is 470 times that in chloroform. Intramolecular hydrogen-bonding of the hydroxy-group to the double bond in chloroform solvent results in decreased electron density in the double bond and hence discourages addition of electron-deficient diene~.~~ Systems related to (60)may provide a probe for separating the effects of steric and electronic influences on Diels-Alder reaction^.'^ Most current =4participants in K4+ .2 cycloadditions require activated olefins as partners.A series of papers77 introduces and exploits a new class of dienophiles which react easily with un- activated olefins. a-Chloronitrones (61) are the precursors and they undergo a facile silver-ion-induced addition to unactivated olefins to give products (63) considered to arise from addition of N-alkyl-N-vinylnitrosonium ions (62). New sulphur dienophiles include disulphur monoxide,78 ~ulphines,~ and N-sulphonylamines.8o Acetylenic dienophiles react with thiophens to give benzene derivatives via non-isolable intermediates8 However in the presence of AlCl a reaction (59) a; R’ = D R2 = H (60) b;R1 = H,R2 = D ’’ P. Haberfield and A.K. Ray .I. Org. Chem. 1972 37 3093. ’‘ I. W. McCay M. N. Paddon-Row and R. N. Warrener Tetrahedron Letters 1972 1401 ; M. N. Paddon-Row and R. N. Warrener ibid. p. 1405; M. N. Paddon-Row ibid. p. 1409. ’’ U. M. Kempe. T. K. Das Gupta K. Blatt P. Gygax D. Felix and A. Eschenmoser Helv. Chim. Acta 1972 55 2187; T. K. Das Gupta D. Felix U. M. Kempe and A. Eschenmoser ibid. p. 2198; P. Gygax T. K. Das Gupta and A. Eschenmoser ibid. p. 2205. 78 R. M. Dodson V. Srinivasan K. S. Sharma and R. F. Sauers J. Org. Chem. 1972 37 2367. ’’ B. Zwanenburg L. Thijs J. B. Broens and J. Strating Rec. Trav. chim. 1972,91,443. E. M. Burgess and W. M. Williams J. Amer. Chem. Soc. 1972 94 4386. R. Helder and H. Wynberg Tetrahedron Letters 1972 605; H. J.Kuhn and K. Gollnick ibid. p. 1909. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations 131 DcH2vcH2D Me Me occurs at room temperature (64) +(65),82 suggesting that the other cycloaddi- tions may be more complicated than at first thought. Good evidence for the intermediacy of oxyallyl cations (66) in the ironcarbonyl- initiated cycloadditions (67) -+ (68) has been provided.83 Similar ions can be generated from 2-dimethylamino-4-methylene-l,3-dioxolans.84Tetrahydro-furan and its alkyl derivatives are deprotonated by n-butyl-lithium in hexane and undergo a .4 + .2 cycloreversion (69) +(70).85 The related radical cleavage (71) -+ (72)is non-concerted.86 Further stereospecific examples of the addition of aza-ally1 anions to olefins have been reported [e.g.(73)+(74)].8’ 0-R-R (70) (71) (72) Ph H %ph -PhlJ; J,:,A H Ph N Ph Ph H (73) (74) H.Wynberg and R.Helder Tetrahedron Letters 1972 3647. 83 R. Noyori Y. Hayakawa M. Funakura H. Takaya S. Murai R.4. Kobayashi and S. Tsutsumi J. Amer. Chem. SOC.,1972 94 7202. H. M. R. Hoffmann K. E. Clemens and R. H. Smithers J. Amer. Chem. SOC.,1972 94 3940. 85 R. B. Bates L. M. Kroposki and D. E. Potter J. Org. Chem. 1972 37 560. 86 W. R. Dolbier I. Nishiguchi and J. M. Riemann J. Amer. Chem. SOC.,1972,94 3642. 87 T. Kauffmann and E. Koppelmann Angew. Chem. Internat. Edn. 1972 11 290; T. Kauffmann K. Habersaat and E. Koppelmann ibid. p. 291. 132 R.Grigg MO calculations provide an explanation for the unexpected resistance of (75)88 and (76)89to the retro-Diels-Alder elimination of C202and N20,respectively.The thermal decomposition of (77) gives (78) and not (79) as the primary product and is therefore not a retrohomo-Diels-Alder rea~tion.~' The photochemical Diels-Alder reaction (e.g. 80-8 l),if concerted could involve a ,4 + ,2 or ,4 + .2 cycloaddition. Results in this area indicate that the selectivity of the process is controlled by substrate structure and that it would be unwise to depend on orbital symmetry control in exploiting this reacti~n.~ (80) (81) Further theoretical and experimental studies on 1,3-dipolar cycloadditions have been reported but again steric effects are not satisfactorily encompas~ed.~~ The question of regioselectivity in 1,3-dipolar cycloadditions has been discussed and the case for a biradical mechanism argued at length.93 The synthetic poten- tial of 1,3-dipolar species generated in situ and trapped intramolecularly [e.g.(82)+(83) or (84)] has been well ill~strated.~~ 1,3-Dipolar cycloreversions are implicated in the thermal decomposition of pyrazolinesg5 and triaz~les.~~ 88 R. C. Haddon Tetrahedron Letters 1972 3897. 89 J. P. Snyder L. Lee V. T. Bandurco C. Y. Yu and R. J. Boyd J. Amer. Chem. SOC. 1972,94 3260. 90 E. L. Ellred and J. C. Hinshaw Tetrahedron Letters 1972 387. 91 D. A. Seeley J. Amer. Chem. SOC.,1972,94,4378; A. Padwa L. Brodsky and S. Clough ibid. p. 6767; C. W. Alexander and J. Grimshaw J.C.S. Perkin I 1972 1374. 92 R. Sustmann and H.Trill Angew. Chem. Internat. Edn. 1972 11 838; J. Bastide N. El Ghandour and 0.Henri-Rousseau Tetrahedron Letters 1972 4225. 93 R. A. Firestone J. Org. Chem. 1972 37 2181. 94 W. Oppolzer Tetrahedron Letters 1972 1707. 95 D. H. White P. B. Condit and R. G. Bergman J. Amer. Chem. SOC.,1972 94 1348; R. A. Keppel and R. G. Bergman ibid. p. 1350; D. E. Eaton R. G. Bergman and G. S. Hammond ibid. p. 1351. 96 L. H. Zalkow and R. H. Hill Tetrahedron Letters 1972 2819. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations The question of whether singlet oxygen reacts with olefins by a concerted ene mechanism (85)*(86)or by a perepoxide route (85)-+(87) continues to cause controversy. The azide-trapping experiments which favoured the perepoxide route involved a misinterpretation of the results.It appears that azide radicals were responsible for the products and that azide ions strongly quench singlet oxygen.97 The concerted ene mechanism continues to attract most support.98 Asymmetric induction in the ene reaction of (-)-methyl glyoxylate with pent-l- ene has been studied.99 Optical yields were found to depend on temperature solvent and catalyst. The allylic Grignard reagent (88) undergoes an intra- molecular ene-type reaction giving predominantly the cis-product (89).loo 0 0 II r4o 0 97 C. S. Foote T. T. Fujimoto and Y. C. Chang Tetrahedron Letters 1972 45; K. Gollnick D. Haisch and G. Schade J. Amer. Chem. SOC.,1972 94 1747; N. Hasty P. B. Merkel P. Radlick and D. R. Kearns Tetrahedron Letters 1972 49.98 C. S. Foote and R. W. Denny J. Amer. Chem. SOC.,1971,93 5162 5168; A. Nickon V. T. Chuang P. J. L. Daniels R. W. Denny J. B. Di Giorgio J. Tsunetsugu H. G. Vilhuber and E. Werstiuk ibid. 1972 94 5517. 99 0.Achmatowicz and B. Szechner J. Org. Chem. 1972 37 964. loo H. Felkin J. D. Umpleby E. Hagaman and E. Wenkert Tetrahedron Letters 1972 2285. 134 R.Grigg Thermal elimination (retro-ene) reactions of esters,"' carbonates,"' thiol-acetate~,"~and NN-dialkyl carbonate^"^ (90)+(91) have been studied and all appear to be concerted processes although bond-breaking and bond-making may well be non-synchronous. Pyrolysis of adamantane sulphonate esters (92)gives (93)and (94). The latter product is thought to be an example of a general reaction envolving a seven-membered transition state (95).'05 Y (90) X =2=0,Y=R X =Z =0,Y =OR X = S,Y =R,Z =0 X=Z=O,Y=NR 0 0 (92) (93) 2 3 (94) The 4 +4 photodimerization of anthracene is a two-step singlet process involving a biradical intermediate.'06 4 Sigmatropic Reactions Optically active (96)was subjected to thermal rearrangement and the deuterium- scrambled product (96;D at *positions) separated from the skeletally rearranged products. From a study of the rate constants for racemization and for deuterium scrambling it was concluded that a major amount and possibly all of the degener- ate rearrangement proceeds with antarafacial participation of the allylic unit. lo' The bicyclohexenes(97) rearrange at 30-50 "Cto (98),apparently by concerted suprafacial 1,3-shifts with inversion at the migrating centre.lo* lo' A. Tinkelenberg E. C. Kooyman and R. Louw Rec. Trav. chim. 1972 91 3; R. Taylor J.C.S. Perkin 11 1972 165; H. Kwart and J. Slutsky J.C.S. Chem. Comm. 1972 1182. Io2 D. B. Bigley and C. M. Wren J.C.S. Perkin 11 1972 926 1744. P. C. Oele A. Tinkelenberg and R. Louw Tetrahedron Letters 1972 2375. H. Kwart and J. Slutsky J.C.S. Chem. Comm. 1972,552; N. J. Daly and F. Ziolkowski ibid. p. 911. J. Boyd and K. H. Overton J.C.S. Perkin I 1972 2533. '06 G. Kaupp Angew. Chem. Internal. Edn. 1972 11 313 718. lo' J. E. Baldwin and R. H. Fleming J. Amer. Chem. Soc. 1972,94 2140. Io8 F. Scheidt and W. Kirmse J.C.S. Chem. Comm. 1972 716. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations Examples of 1,3-migration of silicon have been reported.log A number of examples of thermal 1,7-antarafaciaI H shifts involving olefinic side-chains of substituted benzenes have been uncovered,' lo and a thermal 1,7-migration with inversion at the migrating centre occurs in the bicyclononatrienes (99)3(100).l1 Thermal rearrangement of (101) gives a mixture of (102 ; 91 %) and (103 ; 9 %).These arise by competitive homo-1,7-H shift (i.e.64s + n2s+ %2J and homo-1,5-H shift with the former being favoured kinetically [and leading to (102)].112 Me (97) R = OMe or N (98) (99) X = Me Y = CN X = CN.Y = Me Studies on the photochemistry of by-unsaturated ketones have shown that 1,3-acyl shifts result from the singlet state whereas the triplet-state ketones undergo oxa-di-n-methane rearrangements.Several attempts have been made to rationalize this divergent behaviour,' and steric conjugative and conforma- tional effects on l,3-acyl migrations have also been discussed. l l4 A second higher-energy degenerate rearrangement of hexa- 1,5-diene has been uncovered using tetradeuteriated material and is tentatively assigned the 'boat' Io9 H. Kwart and J. Slutsky J. Amer. Chem. SOC.,1972,94 2515. 'lo H. Heimgartner H.-J. Hansen and H. Schmidt Helv. Chim. Acta 1972 55 1385; R. Hug,H.-J. Hansen and H. Schmidt ibid. p. 1828. ''I F. G. Klarner Angew. Chem. Internat. Edn. 1972 11 832. "* J. C. Gilbert K. R. Smith G. W. Klumpp and M. Schakel Tetrahedron Lerters 1972 125.D. I. Schuster G. R. Underwood and T. P. Knudsen J. Amer. Chem. SOC.,1971 93 4305; K. N. Houk D. J. Northington and R. E. Duke ibid. 1972 94 6233. H. Sato N. Furutachi and K. Nakanishi J. Amer. Chem. SOC.,1972,94,2150. 136 R. Grigg transition state. A mathematical analysis of possible 1,3-and 3,3-sigmatropic rearrangements in hexa- 1,5-diene is also given and a variety of transition-state geometries considered for each case. The analysis emphasizes that appropriate experiments can make as yet unrealized mechanistic distinctions with complete rigour. ' ' CND0/2 and extended Hiickel calculations indicate that of the possible C,Hi systems the 9-barbararyl cation (104) enjoys unusual stabilization. This is due to strong conjugative interaction of the central p-orbitals with the cyclo- propane rings as depicted in (105)' l6 The unexpected retention of configuration in the solvolysis (106)j (107) revealed on further study that Cope rearrange- ment of the bridgehead carbonium ion (108) was intervening.'" A study of the Cope rearrangement (109)+(110)has provided evidence that a conformational change (111) +(1 12) precedes and follows the Cope rearrangement.'* A number of studies on steric effects in Cope rearrangements have been reported. l9 EtCO H v 4 4 I OCOEt D. D nD L \ r " / D n n (109) (110) l1 M. J. Goldstein and M. S. Benzon J. Amer. Chem. SOC.,1972 94 7147 7149. S. Yoneda S. Winstein and Z.-i. Yoshida Bull. Chem. Sot Japan 1972 45 2510; R.Hoffmann W.-D. Stohrer and M. J. Goldstein ibid. p. 2513. R. Breslow and J. M. Hoffman J. Amer. Chem. SOC.,1972 94 21 11. 118 H. Gunther J. B. Pawliczek J. Ulmen and W. Grimme Angew Chem. Internat. Edn. 1972 11 517. 'I9 C. J. Dixie and I. 0.Sutherland J.C.S. Chem. Comm. 1972,646; T. Sasaki S. Eguchi, and M. Ohno J. Org. Chem. 1972,37,466; C. Ullenius P. W. Ford and J. E. Baldwin J. Amer. Chem. SOC.,1972 94 5910. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations R' (113) a R' = R2 = H b; R' = H R2 = Me In particular the reIative rates of Cope rearrangement via a boat transition state to the corresponding cyclo-octa-1,5-diene of (113a) and (113b) are 181 OOO 1.'" Rearrangements in diallyl or ally1 propargyl ammonium ylides occur pre- dominantly by a concerted 2,3-sigmatropic shift but minor amounts of 1,Zshift products are sometimes obtained presumably involving a radical-pair mechan- ism.' 21 Ylides generated by intra- and inter-molecular reactions of heteroatoms with carbenes undergo 2,3-sigmatropic shifts [e.g.(114)+ (115)].'22 Deprotona- tion of dihydropyrans (116)generates the anions (117) which undergo rearrange- ment possibly via a 1,6sigmatropic shift to (1 18).123 C0,Me Ix.] X '''cozMe'z+ -3 qC0,Me (114) X = SR OR NR2 C1 or Br C(CO Me) (1 15) 5 Cheletropic Reactions MO calculations on cheletropic reactions of diazirine and vinylene carbonates have appeared.124 A theoretical treatment of five-co-ordinate phosphorus '" J. A. Berson and P. B. Dervan J.Amer. Chem. Soc. 1972,94 7597; J. A. Berson P. B. Dervan and J. A. Jenkins ibid. p. 7599. 12' R. W. Jemison T. Laird and W. D. Ollis J.C.S. Chem. Comm. 1972 556; T. Laird and W. D. Ollis ibid. p. 557; V. Rautenstrauch Helv. Chim. Acta 1972 55 2233; S. Julia C. Huynk and D. Michelot Tetrahedron Letters 1972 3587. W. Ando S. Kondo K. Nakayama K. Ichibori H. Kohoda H. Yamato I. Imai S. Nakaido and T. Migita J. Amer. Chem. SOC.,1972 94 3870; J. E. Baldwin and J. A. Walker J.C.S. Chem. Comm. 1972 354; K. Kondo and I. Ojima ibid. p. 860; M. Yoshimoto S. Ishihara E. Nakayama E. Shoji H. Kuwano and N. Soma Tetrahedon Lerters 1972 4387. V. Rautenstrauch Helv. Chim. Acta 1972 55 594. 24 J. Fleischhauer and H. D. Scharf Tetrahedron Letters 1972 11 19; J. P. Snyder R.J. Boyd and M. A. Whitehead ibid. p. 4347. 138 .R.Grigg suggests that the cheletropic process (119) will involve axial-axial or equatorial- equatorial departure of the R substituents from the trigonal-bypyramidal phosphorus.’ 25 Cycloheptatrienylcarbene(120) undergoes cheletropic loss of acetylene from its norcaradiene valence tautomer to give benzene.’ 26 Numerous examples of a preparatively useful fragmentation-cheletropic sequence involving N-aminoaziridine monohydrazones have been reported e.g. (121)+(122).127 A combined disrotatory cyclizationsheletropic process has been devised for the synthesis of polypyrrole macrocycles related to corrins [e.g.(123)+(124)].128 N R’ Ill N I R2 Me kS (124) 125 R. Hoffmann J.M. Howell and E. L. Muetterties J. Amer. Chem. SOC.,1972,94,3047. lZ6 H. E. Zimmerman and L. R. Sousa J. Amer. Chem. SOC.,1972,94 834. ”’ D. Felix R. K. Muller U. Horn R. Joos J. Schreiber and A. Eschenmoser Hefv. Chim. Acra 1972 55 1276. lZ8 M. J. Broadhurst R. Grigg and A. W. Johnson J.C.S. Perkin I 1972. 1124 21 11. Reaction Mechanisms-Part (ii) Orbital Symmetry Correlations A number of examples are given and rate enhancements resulting from incorpora- tion of metal ions are discussed. A study of the rates of quinone dehydrogenations of cyclohexa-l&dienes has led to the suggestion that these may be concerted hydrogen-transfer processes. 12’ lz9 F. Stoos and J. Rocek J. Amer. Chem. SOC.,1972,94,2719.