4 Reaction Mechanisms Part (i) Orbital Symmetry Correlations and Pericyclic Reactions By D.W. JONES Department of Organic Chemistry The University Leeds LS2 WT 1 General and Theoretical Aspects New books,’ general review articles,2 useful mnemonic^,^ and numerous papers on both the fundamental and synthetic aspects of pericyclic reactions testify to a continued high level of interest in this area. Stimulus for further activity has undoubtedly been provided by Dewar’s MINDOj3 calculations which e.g. predict that the prototype Diels-Alder addition between butadiene and ethylene proceeds via a very unsymmetrical biradical transition state (TS)(1) in which one new o-bond is almost completely formed while the other has hardly formed at aIL4 Both the boat-like Cope rearrangement of hexa-l,5-diene and the conversion of (2) into (3) could be regarded as involving TS’s related to the biradical (4).A. J. Bellamy ‘An Introduction to Conservation of Orbital Symmetry A Programmed Text’ Longman London 1974; G. B. Gill and M. R. Willis ‘Pericyclic Reactions’ Chapman and Hall London 1974; W. B. Smith ‘Molecular Orbital Methods in Organic Chemistry HMO and PMO’ Marcel Dekker New York 1974 p. 127. (a) N. D. Epiotis ‘The Theory of Pericyclic Reactions’ Angew. Chem. Internat. Edn. 1974 13 751; J. B. Hendrickson ‘The Variety of Thermal Pericyclic Reactions’ ibid. p. 47; R. Gleiter ‘Effects of Through-bond Coupling’ ibid. p. 696; (6)T. Kauff-mann ‘1.3-Anionic Cycloadditions of Organolithiums’ ibid. p. 627. R. H. Wollenberg and R.Belloli Chem. in Britain 1974 10 95; F. G. Holliman ibid. p. 361; J. Mathieu Bull SOC.chim. France 1973 807; A. Rassat Compt. rend. 1972 274 730. M. J. S. Dewar A. C. Griffin and S. Kirschner J. Amer. Chem. SOC. 1974 96 6225. 95 D. W.Jones Yet heating (2)gives none of the isomer (5) expected from a boat-like species (4). The TS's for the boat-like Cope rearrangement and the isomerization (2)-+ (3) are clearly distinct. The concept of orbital isomerism5acan be used to rationalize this ob~ervation.~'The TS for the (2)into (3) conversion is akin to the through-space coupled biradical (6) in which the bonding combination + G2) is the HOMO. On the other hand the TS for the boat-like Cope reaction is related to the biradical (7)in which through-bond coupling places -t,b2) at lower energy; (6)and (7)are therefore lumomers [HOMO (6)converts into LUMO (7)] and their interconversion is forbidden and must proceed via the higher-energy biradical (4) as TS.The biradicaloids (8) and (9) formed respectively from two ethylenes and cyclobutane are also lumomers whose interconversion is for-bidden ;there is thereforeopportunity for loss of configuration in the dimerization of substituted ethylenes by rotation about the C,-Cb bond.5c Forbidden reac-tions will try to cross the biradical barrier at a point corresponding to the most stable form of the biradical. Symmetrical disrotatory opening of cyclobutene would lead to the antiaromatic (0 0)-biradical (10). MIND0/3 suggests that this is avoided; the TS corresponds to rotation of one methylene group by 45" while the other has moved little.Thus the TS is more closely allied to the orthogonal methylene-ally1 biradical (11) where destabilizing interaction of the allyl and carbon 2p orbitals is avoided. As the twist angle increases beyond 45" bond formation in the allyl radical moiety leads to a decrease in energy and the HOMO-LUMO crossing occurs after the TSSd H .. ,H 9 HX $ H-D 'D C H -.. H -H D H H (5) (6) (71 (8) (9) (10) (1 1) (12) The TS for the .2 + ,2 cycloreversion of dioxetan (12) to formaldehyde is calculated to lie ca. 65 kcal mol-' above (12) and the TS for cleavage via a biradical some 45 kcal mol-' above (12). These compare poorly with observed activation energies (21-29 kcal mol-in substituted cases).Calculation of the (a) M. J. S. Dewar S. Kirschner and H. W. Kollmar J. Amer. Chem. Soc. 1974 96 5240; (b) M. J. S. Dewar S. Kirschner H. W. Kollmar and L. E. Wade ibid. p. 5241; (c) M. J. S. Dewar and S. Kirschner ibid. p. 5246; (d) M.J. S. Dewar and S. Kirschner ibid. p. 5244 6809; (e) M. J. S. Dewar and S. Kirschner ibid. p. 7578; v) M. J. S. Dewar S. Kirschner and H. W. Kollmar ibid. p. 7579; (g) R. S. Case M. J. S. Dewar S. Kirschner R. Pettit and W. Slegeir ibid. p. 7581 ;(h)W. H. Richard-son F. C. Montgomery M. B. Yelvington and H. E. O'Neal ibid. p. 7525. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 97 triplet energy surface shows it to intersect the singlet surface at 38.3 kcal mol-’.It is t herefore suggested that the cleavage of dioxetans involves intersystem crossing to the triplet surface which is pursued with formation of triplet-excited carbonyl compound and consequent light emi~sion.’~ Because the triplet surface for Dewar benzene intersects the singlet surface after the TS for opening to benzene only a small amount of triplet benzene is formed.’/ Circumstances appear to be more auspicious for conversion of (13) into (1 4) which gives rise to light emission in the presence of 9,10-dibromoanthracene,5” An extended biradical mechanism for dioxetan cleavage is favoured by others.5h (13) (14) An ab initio study of the methylenecyclopropane rearrangement6 implicates the orthogonal methylene-allylic biradical (15) which lies 8.8 kcal mo1-l below the TS and is suggested asa target for experimental detection.This result contrasts with the conclusion of an experimental study that the biradical is a TS in the racemization and geometrical isomerization of cyclopropanes. While thermo- chemical arguments consistently predict biradical intermediates quantum mechanics yield a biradical TS when no secondary stabilization of the radical is possible and a biradical intermediate when one odd electron is stabili~ed.~ The singlet orthogonal trimethylenemethane (16)reacts with olefins by addition 3 (15) (16) at C-1 and C-4; addition at C-3 and C-4 would leave an orthogonal n-system between C-1 and C-2. In contrast the additions of the related triplet are orienta- tionally and stereochemically non-specific.’ Doubts that the Stevens rearrangement proceeds exclusively via biradicals have been raised.The process detected by CIDNP may not be the only biradical process involved,’ nor indeed the only process involved for the concerted- forbidden pathway is predicted to have a very low activation energy (ca. 4 kcal mol -’).’ The CIDNP enhancements accompanying rearrangement are much less than the theoretically estimated values. l1 W. J. Hehre L. Salem and M. R. Willcott J. Amer. Chem. Soc. 1974 96,4328. ’ W. von E. Doering and K. Sachdev J. Amer. Chem. SOC.,1974 96 I 168. a J. A. Berson C. D. Duncan and L. R. Corwin J. Amer. Chem. SOC.,1974 96 6175; J. A. Berson L. R. Corwin and J. H. Davis ibid. p. 6177. F. Gerhart and L.Wilde Tetrahedron Letters 1974,475. lo M. J. S. Dewar and C. A. Ramsden J.C.S. Perkin I 1974 1839. ’ * G. L. Closs Sheffield Stereochemistry Symposium 1974. D. W.Jones For reactions observed in the mass spectrometer the extra activation energy requirement of a concerted-forbidden reaction results in kinetic energy release and the accompanying metastable peaks are broad (flat-topped or dished). Thus forbidden 1 ,Zhydrogen elimination from protonated formaldehyde to give the formyl cation and the decomposition of C3H7+to the ally1 cation give broad metastables. But for the allowed 1,l-hydrogen eliminations in the frag- mentation of CzH5+to the vinylium cation and the conversion of protonated benzene into the phenyl cation the metastables are normal.I2 2 Electrocyck Reactions Ab initio SCF and CI calculations support the qualitative predictions of orbital symmetry theory for the cyclopropyl-ally1 interconversions (17) C(18).In the forbidden processes as well as in the allowed interconversion of the anions ring-opening fails to coincide with methylene rotation.' In agreement with preferred conrotatory opening of cyclopropyl anions (19) opens to an ally1 CN anion some lo4 times more rapidly than (2O).I4 Attention has been drawnI5 to the first example'$ ofan electrocyclic reaction of an an-ally1 anion (21) -+(22) and a possible example of the disrotatory opening of a cyclopropyl radical is provided17 by pyrolysis of (23) in ethylbenzene to give (24). The bicyclobutane FN '' D.H. Williams and G Hvistendahl J. Amer. Chem. SOC.,1974 96 6753 6755. l3 P. Merlet S. D. Peyerimhoff R. J. Buenker and S. Shih J. Amer. Chem. SOC.,1974 96 959 and cited references. M. Newcomb and W. T. Ford J. Amer. Chem. SOC.,1974 96 2968. H. Quast and C. A. Weise-Velez Angew. Chem. Internat. Edn. 1974 13 342. *' A. T. Bottini and R. E. Olsen J. Amer. Chem. SOC.,1962 84 195. I' A. Barmetler C. Ruchardt R. Sustmann S. Sustmann and R. Verhulsdonk Tetra-hedron Letters 1974 4389. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (25) opens thermally to (26) and photochemically to (27); it is now shown'* that the related radical anion resembles excited (25) in opening to the radical anion of pleiadiene (27). It is pointed out that both excited (25) and the radical anion have one electron in the same frontier orbital ;the ring-opening is regarded as a rare ,,2* +u2aprocess (28).The cation (29) gives (30) by conrotatory closure1g and the anion (31) closes to (32) in a disrotatory way.** (25) (26) (27) (33) (34) (35) The six-election photocyclization of (33) to (34) is followed by 1,4-sigmatropic hydrogen shift to (35).21 Related photocyclization of the amides (36) affords intermediates(37) which go to products (38) by a 1,hhift of the methoxy or acyl group R.22 J. R. Dodd R. F. Winton R. M. Pagni C. R. Watson andJ. Bloor J. Amer. Chem. Sac. 1974 96 7846. l9 C. W. Shoppee and B. J. A. Cooke J.C.S. Perkin I 1974 189. 2o C. W. Shoppee and G. N. Henderson J.C.S. Chem. Cumm.1974 561. *'A. G. Schultz and M. B. De Tar J. Amer. Chem. SOC.,1974 96 296. 22 I. Ninomiya T. Kiguchi and T. Naito J.C.S. Chem. Cumm. 1974 81. 100 D. W. Jones Full papers have appearedz3 dealing with the generation of the enol(39) either by irradiation of o-methylbenzaldehyde or by thermal opening of benzocyclo-butanol. The preference of the hydroxy-group for an outside position shown in these reactions extends to the opening of (40) to the enol (41).24 The cyclo- butene (42) opens by the conrotatory path that places both acetylene moieties in internal positions and the product (43) is the first stable benzo~yclobutene.~~ The effect of substituents on the cycloheptatriene-norcaradiene equilibrium has been studied.26 Homoporphyrins undergo 18~-electron electrocyclic reac- tions and 1,17-sigmatropy analogous to the Berson-Willcott rearrangement of cycloheptatrienes.3 Cycloaddition and Cheletropic Reactions The usually observed reaction of carbenes with dienes is 1,Zaddition via a non-linear approach (44).The alternative 1P-addition involving a linear approach (45)has now been realized with nucleophilic carbenes. Difluorocarbene is thought to be more nucleophilic than dichlorocarbene owing to better 2p2p than 3p2p orbital overlap and consequently more important donation of the halogen lone pairs into the carbon p-orbital; addition of the former to norbornadiene gives both the 1,2-and 1,4-adducts (46) and (47).'* Similarly the nucleophilic carbene 23 B. J. Arnold S. M. Mellows P. G. Sammes and T.W. Wallace J.C.S. Perkin I 1974 401; B. J. Arnold P.G. Sammes and T. W. Wallace ibid. p. 409 415. 24 C. W. Jefford A. F. Boschung and C. G. Rimbault Tetrahedron Letters 1974 3387. 25 H. Straub Angew. Chem. Internat. Edn. 1974 13 405. 26 F. G. Klarner Tetrahedron Letters 1974 19; W. Betz and J. Daub Chem. Ber. 1974 107 2095. 27 H.J. Callot and T. Tschamber Tetrahedron Letters 1974 3155. 28 C. W. Jefford n'Tanda Kabengele J. Koyacs and U. Burger Tetrahedron Letters 1974 257; Helv. Chim. Acta 1974,57 104. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations (48) is believed to add 1,4 to the electron-deficient diene (49) to give after de- carbonylation the product (50). The reaction of (51) with (49) is more complex but may also involve initial 1,4-additi0n.~~ Reaction of (51) with the triene (52) may proceed via the 1,4-adduct (53) which rearranges to (54) by a u2 + u2 + u2s R2 0-/A &yo (55) R' (56) R' = Me R2= H (57) R' = Br RZ= C0,Me Me The INDO-MO technique suggests that 1,3-interaction in the 2-oxidoallyl cation (55)is so strong that it is best described as a monohomocyclopropenone.3 Species related to (55) add 1,4 to pyrroles to give e.g.(56) and (57) which offer access to tropane alkaloids ;32 the 2-methoxyallyl cation will even add to benzene giving after acid work-up the ketone (58).33 The betaine (59) incorporates an oxidoallyl cation moiety and undergoes [4 + 21-addition to 2,3-dimethyl-butadiene to give (60).3" Betaines of the type (61) likewise add as oxidoallyls 29 T.Mitsuhashi and W. M. Jones J.C.S. Chem. Comm. 1974 103. 30 K. Saito Y. Yamqshita andT. Mukai J.C.S. Chem. Comm. 1974 58. 31 B. K. Carpenter J.C.S. Perkin ZZ 1974 1. 32 G. Fierz R. Chidgey and H. M. R. Hoffmann Angew. Chem. Znternat. Edn. 1974 13 410; R. Noyori Y. Baba and Y. Hayakawa J. Amer. Chem. SOC.,1974 96 3336. 33 H. M. R. Hoffmann and A. E. Hill Angew. Chem. Znternat. Edn. 1974 13 136. 34 (a)K. L. Mok and M. J. Nye J.C.S. Chem. Comm. 1974,608; (b)N. Dennis B. Ibrahim and A. R. Katritzky ibid. p. 500;N. Dennis A. R. Katritzky T. Matsuo S. K. Parton and Y. Takeuchi J.C.S. Perkin I 1974 746. 102 I). w.Jones to dienes but with olefins they behave as 4n-electron oxidopentadienyl cations giving adducts of the type (62); the reversible dimerization of (61) is therefore a .4 + .2 addition.34b ((i@,? pqo no N N R xx A number of studies have been concerned with retro-cy~loaddition.~~ In agreement with a concerted ,,2s + ,2 + (r2,process (63) loses nitrogen stereo- specifically to give cis,cis-octa-2,6-diene ; nitrogen loss is however much slower than for (64)in which the 'bent-bond' of the bridge ensures better overlap with the breaking C-N bonds.35" Such overlap would be expected to increase with the angle 8 in (65); the rates of deamination and photoelectron spectra of (65 ; n = 0 and n = 1) support this Full papers have appeared dealing with the synthesis of semibullvalenes from diazasnoutenes by reaction of the type (66)-+ (67).35c The activation energy for the related loss of nitrous oxide from azoxy-compounds is much greater.However the process is thought to be concerted and shows the same response to the presence of a 'bent-bond'; (68) loses nitrous oxide lo6 times more rapidly than (69).35dA similar large rate difference is observed for the decomposition of (70) and (71) to butadiene and sulphur dioxide; (70)decomposes at ca. 0 "C and (71) requires heating to 120°C. The effect of pressure on fhe relative amounts of [2 + 2} and [2 + 43 adducts formed in the dimerization of chloroprene and in the addition of tetrachloro-benzyne to norbornadiene has been studied.36 Only for the first process is the [2 + 41 reaction strongly favoured by pressure increase. In the benzyne addition 3s (a)J.A. Berson E. W. Petrillo and P. Bickhart J. Amer. Chem. SOC.,1974 96 636; J. A. Berson S. S. Oh E. W. Petrillo and P. Bickhart Tetrahedron 1974 30 1639; (6) H. Schmidt A. Schweig B. M. Trost H. B. Neubold and 9. H. Scudder J. Amer. p. 7454 7465; (4H. Olsen and J. P. Snyder ibid. p. 7839; (e) F. Jung M Molin R. Chem. SOC.,1974 96 622; (c) D. R. James G. H. Birnberg and L. A. Paquette ibid. Van Den Elzen and T. Durst ibid. p. 935. 36 (a) W. J. le Noble and R. Mukhtar J. Amer. Chem. Suc. 1974 96 6191; (b) W. G. Dauben and A. P. Kosikowski ibid. p. 3664; J. Rimmelin and G. Jenner Tetrahedron 1974,30 3081; K. Seguchi A. Sera and K. Maruyama Bull. Chem. SOC.Japan 1974 47 2242. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations the [2 + 21 adduct is thought to arise via the zwitterion (72),solvation of which is associated with a large volume decrease.Thus only those two-step additions proceeding via non-polar intermediates can be distinguished from concerted processes by the effect of The effect of pressure on several Diels- Alder additions supports a concerted mechanism.36b C1 H a++JSAr (77) (75) R' = H R2= R,X = CH (76) R' = R,R2 = H,X = 0,NRZ The conformation and the ethanol trapping of zwitterion intermediates in the reaction of tetracyanoethylene with en01 ethers has been discussed. Most zwitterions arise in U-conformations like (73) in which a donor-acceptor inter- action exists.37 Whilst quadricyclane (74; X = CHI)undergoes .2 + ,2 + ,2 addition to acetylenes (RC-CR) to give adducts of type (73 3-heteroquadri-cyclanes (74 ;X = 0or NR)give adducts of type (76)by initial.opening to dipolar intermediates( The 13-anionic cycloaddition of organolithiums to olefins hasbeen reviewed2b and used in a neat construction of a five-ring compound (78)-+ (79),by addition to ethyl a~rylate.~' The addition of the lithium derivative (80)to trans-azobenzene is a clear-cut two-step process; the intermediate (81) can be trapped by quenching and gives (82) on heating.40 Ph Ph 37 R. Huisgen R. Schug and G. Steiner Angew. Chem. Internat. Edn. 1974 13 80 81. 38 E. Haselbach and H. D. Martin Heiu. Chim. Acta 1974 57 472; H. D. Martin C. Heller E. Haselbach and 2.Lanyjova ibid. p. 465. 39 J. P. Marino and W. B. Mesbergen J.Amer. Chem. SOC.,1974 96,4050. 40 W. Bannwarth R. Eidenschink and T. Kauffmann Angew. Chem. Internut. Edn. 1974 13,468. 104 D. W.Jones Several studies emphasize the role of steric factors in determining endo/exo ratios in the Diels-Alder rea~tion.~' In additions to cyclopentadiene the domi- nant steric factor appears to involve a methylene hydrogen of the diene and an exo-substituent on the dien~phile.~'" Destabilization of the endo-TS in additions to 1,2-diphenylbutadiene may be associated with non-coplanarity of the 2-phenyl group with the diene In additions to hexachlorocyclopenta- diene the stereochemistry of the olefin may be lost and with unsymmetrical olefins the bulkier substituent takes the endo-p~sition.~" More subtle factors influencing stereochemical course41d and reaction rate4'" have also been dis- cerned.Domino-Diels-Alder reactions have been explored with a view to the synthesis of dodecahedrane ;e.g. (83) gives (84) and (85) with dimethyl acetylene- dicarb~xylate.~'A neat synthesis of ( & )-patchouli alcohol involves an internal addition (86) -P (87) which requires base catalysis ; this could involve activation of the diene system in (86) by the nearby ionized hydro~y-group.~~ Me ,H & (84) X = C0,Me;Y = H (87) (85) X = H;Y = C0,Me The isomerization of simple cyclo-octatetraenes e.g. (88) -+ (89) involves [4+ 21 addition in the bicyclic valence tautomer (90). This gives (91) which by reverse [4 + 2) addition in the manner shown (91; arrows) gives (89) after valence tautomerism ;an alternative mechanism involving 1,5-shift of the bond a in (90)would give 1,3-dirnethylcyclo-octatetraene and so cannot operate here.44 41 (a)J.M. Mellor and C. F. Webb J.C.S. Perkin 11 1974 17,26; B. C. C. Cantello J. M. Mellor and C. F. Webb ibid. p. 22; D. Bellus K. von Bredow H. Sauter and C. D. Weis Helv. Chim. Actu 1973 56 3004; S. Tsuboi Y. Ishiguro and A. Takeda Bull. Chem. SOC.Japan 1974 47 1673 ; D. Bellus H. C. Mez and G. Rihs J.C.S. Perkin 11 1974 884; (b) P. C. Jain Y. N. Mukerjee and N. Anand J. Amer. Chem. SOC.,1974 96,2996; (c) V. Mark J. Org. Chem. 1974,39,3 179,3 181 ; (6)W. Oppolzer Tetrahedron Letters 1974 1001 ; K. Mackenzie ibid. p. 1203; (e) C. K. Bradsher W. A. Porter and T.G. Wallis J. Org. Chem. 1974 39 1 172. 42 L. A. Paquette and M. J. Wyvratt J. Amer. Chem. SOC.,1974 96 4671 ; D. McNeil B. R. Vogt J. J. Sudol S. Theodoropulos and E. Hedaya ibid. p. 4673. '' F. Naf and G. Ohloff Helv. Chim. Actu 1974 57 1868. 44 L. A. Paquette and M. Oku J. Amer. Chem. SOC.,1974 96 1219. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 105 Examination of the acrolein dimerization using the frontier-orbital method with orbital coefficients calculated by an ab initio SCF procedure allows pre- diction of the observed regio-isomer (92); use of HMO coefficients is unsatis- fa~tory.~'8,8-Dirnethylisobenzofulvene (93) and tropone give the [6 + 41 adduct (94) rather than the isomer (95) theoretically ~redicted.~~" It has now been shown that (95) is indeed the kinetically controlled product which is readily converted into (94) by Cope rearrangement.46b Since 6,6-dimethylfulvene gives an adduct like (94) with tropone the possibility exists that it too is formed via a kinetically controlled isomer of type (95).OCHO 0 H (94) (95) 0 0 R2 HH' Me Pr" 0 0 6 (96) (99) (97) R = Me RZ = Pr" (98) R = Pr" R2 = Me Although originating from the triplet state the photo-rearrangement of (96) to (97)and (98) is regarded as a ,2 + ,2 process cf. (99);a biradical intermediate is not consistent with the retention of optical activity observed.47n Optical 45 P. V. Alston and D. D. Shillady J. Org. Chern. 1974 39 3402. 46 (a) M. N. Paddon-Row Austral.J. Chern. 1974 27 299; (b) M. N. Paddon-Row and R. N. Warrener Tetrahedron Letters 1974 3797. 47 (a) D. I. Schuster and B. M. Resnick J. Amer. Chern. Soc. 1974 96 6223; (6) R. L. Coffin R. S. Givens and R. C. Carlson ibid. p. 7554; (c) H. E. Zimrnerman J. D. Robins R. D. McKelvey C. J. Samuel and L. R. Sousa ibid. p. 1974 4630. 106 D.W.Jones activity is also retained in the sensitized oxa-di-z-methane rearrangement (100)-(101) which can likewise be regarded as a =2 + u2 though the .2 + u2a+ .2 description favoured for the di-n-methane rearrangement47C is also possible. A possible =4 + =2 photoaddition has been and it has been pointed out that the 4 + 2 photo-reaction is feasible for a donor-acceptor partner~hip.~~ ( 102) The secondary isotope effect for the addition of [a-'H]styrene to dimethylketen (kdkD = 0.8) indicates important bonding to Ca in the TS ;"' the result contrasts with that for the addition of the same olefin to diphenylketen (kdkD = 1.12).Sob The addition of t-butylcyanoketen to norbornadiene gives the .2 + .2 + .2 adduct (102) as well as the expected cyclobutanone.sl" Ketenimmonium cations + (Me,C=C==NMe,) add in a similar way to cisoid dienes e.g.(103) from cyclo- ~entadiene,~~~ adducts of the more but with transoid dienes and 01efins~'~ familiar type (104) are formed. 4 Sigmatropic Reactions Unlike the normal 1,2-shift in a carbonium ion degenerate rearrangement of the bicyclobutyl carbonium ion (105) should proceed with inversion at the migrating group.The inversion TS (106) permits better overlap of the C-4 orbital with a filled Walsh orbital of the cyclopropane ring than is present in the retention TS (107) between the same C-4 orbital and the double bond. The process is suggested 48 H. Hart T. Miyashi D. N.Buchanan and S. Sasson J. Amer. Chem. Soc. 1974 96 4857. 49 N. D. Epiotis and R.L. Yates J. Urg. Chem. 1974 39 3150. 50 (a) N. S. isaacs and B. G. Hatcher J.C.S. Chem. Comm. 1974 593; (b) J. E. Baldwin and S. Kapecki J. Amer. Chem. SOC.,1970 92 4874. 51 (a)P. R. Brook and K. Hunt J.C.S. Chem. Comm. 1974,989; (b)J. Marchand-Brynaert and L. Ghosez Tetrahedron Letters 1974 377; (c) A. Sidani J. Marchand-Brynaert, and L. Ghosez Angew. Chem. Internat. Edn. 1974 13 267. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations 107 as an extreme example of subjacent orbital contr01.'~ The photoconversion of cyclobutanones to or-oxocarbenes has been investigated in detail ;53 (108) gives (109) stereospecifically indicating a direct 1,2-~hift.'~' aH H Ally1 vinyl ether anions are thought to rearrange to oxyanions of the type (1 10) by cleavage to vinyl anion and enone and recombination.Since the process is stereospecific cleavage to configurationally unstable vinyl radicals is ~nlikely.'~ The anions (111;X = CN or COMe) undergo the indicated 2,3-shift,"" and a related rearrangement (112) is thought to intervene in the base-catalysed re- arrangement of catechol monoallyl ethers.' 5b Allylic alcohols react with di- methylformamide diethyl acetal via carbenes (1 13) which undergo 2,3-shift to #+unsaturated amide~.'~' The use of allylic sulphoxides as synthetic inter- mediates via 2,3-shift and phosphite desulphurization has been reviewed,56a and several related processes are described.56* 52 A.J. P. Devaquet and W. J. Hehre J. Amer. Chem. SOC.,1974,96 3644. 53 (a)G.Quinkert K. H. Kaiser and W.-D. Stohrer Angew. Chem. Internat. Edn. 1974 198;W.-D. Stohrer G.Wiech and G. Quinkert ibid. p. 199 200; (6) G. Quinkert P. Jacobs and W. D. Stohrer ibid. p. 197. 54 V. Rautenstrauch G.Buchi and H. Wuest J. Amer. Chem. SOC.,1974 96 2576. 55 (a)B. Cazes and S. Julia Tetrahedron Lerrers 1974,2077;A. F. Thomas and R. Dubini Helv. Chim. Acra 1974,57,2084;(b)W. D.Ollis R. Somanathan and I.0.Sutherland J.C.S. Chem. Comm. f974 494; (c) G.Buchi M. Cushman and H. Wuest J. Amer. Chem. SOC.,1974 96 5563. 56 (a) D.A. Evans and G. C. Andrews Accounts Chem. Res. 1974,7 147;(6) G.Buchi and R. M. Friedinger J. Amer. Chem. Soc. 1974,96 3332;P. T. Lansbury and J. E. Rhodes J.C.S. Chem. Comm. 1974 21; J. G.Miller W. Kurz K. G. Untch and G. Stork J. Amer. Chem. SOC.,1974,96 6774. 108 D. W.Jones The intramolecular 1’3-benzyl shift in the conversion of (114) into (1 15) involves 295% inversion at the benzyl carbon.57 There is retention of optical activity in the rearrangement of (116) to (117) consistent with the indicated antarafacial shift.58u When the migrating bond does not initially lie in the nodal plane of the z-bond an antarafacial shift is not ~bserved.’~’ The acid-catalysed rearrangement of (1 18) gives inter alia the 1,3-shift product (119).’“ CH,Ph 0 Ph [l]Ph H4H Me+H7-Me Me I D-C-H Me Ph (1 14) Full papers have appeared dealing with the circumambulation of polymethyl derivatives of the cation (120).59uSince electron-releasing groups at C-6 speed 1A-shift build-up of positive charge at C-6 in the TSis indicated cf.(121). This Mobius aromatic TS becomes the ground state for the aluminium chloride complexes ( 122) of various acylpentamethylcyclopentadienes. These undergo rapid degenerate rearrangement by 1,5-shifts involving bridged TS’s related to ’’ J. W. Lown M. H. Akhtar and R. S. McDaniel J. Org. Chem. 1974 39 1998; J. W. Lown and M.H. Akhtar Tetrahedron Letters 1974 179. 58 (a)H. Bertrand J. L. Gras and G. Gil Tetrahedron Letters 1974 37. (6) J. A. Berson T. Miyashi and G. Jones J. Amer. Chem. SOC.,1974 96 3468; cf. J. E. Baldwin and R. H. Fleming ibid. 1973 95 5256 5261. (c) B. Miller ibid. 1974 96 7155. 59 (a) R. F. Childs M. Sakai B. D. Parrington and S. Winstein J. Amer. Chem. Soc. 1974 96,6403; R. F. Childs and S. Winstein ibid. p. 6409; (b) R. F. Childs and M. Zeya ibid. p. 6418; (c) W. 3. Hehre ibid. p. 5207. Reaction Mechanisms-Part (i) Orbital Symmetry Correlations I09 (120).59b The rapid rearrangement of the cations (120) may be due to their antiaromatic character ;related rearrangement is not observed for the aromatic homo tropylium cation. 9c The 1,fi-shift involved in the Berson-Willcott rearrangement of (123) to give e.g.(124) proceeds with inuersion suggesting the dominance of least-motion over orbital-symmetry control in this instance ;60 this result has implications for related rearrangements shown to proceed with inversion in accord with orbital-symmetry control. Me C02Me \ ,JI Deuterium labelling studies show that hexa- 1,s-diene undergoes two stereo- chemically distinct Cope rearrangements in preference to either 1,3-sigmatropy or homolytic dissociation. The easier process involves either a chair or a 'twist' (125) TS and the less easy either the boat or a 'plane' (126) TS6'" In contrast the ''0-scrambling ofacetyl peroxide involves competing 1'3- and 3,3-sigmatropy and at higher temperatures dissociation overtakes both these processes.61 b.The rearrangement of (127) to (128) viu a necessarily boat-like TS requires a temperature some 50-75 "C higher than required for the chair (or twist) re- arrangement of hexa- 1,5-diene.6'' Semibullvalene shows the lowest known barrier to Cope rearrangement (AG* = 5.5 & 0.1 kcal mol-' at -140 oC).61d The fragmentation of (129; R = Ph) proceeds in a concerted manner. The estimated activation energy for the reaction of (129; R = H) is only 2-3 kcal mol-' greater than for the related six-electron process in ethyl vinyl ether.61e The process (129; arrows) can be viewed as 1,7-sigmatropy in which o-bonds replace two of the three participating double bonds; a similar reaction 6o F.-G. Klarner Angew.Chem. Internat. Edn. 1974 13 268; CJ R. B. Woodward and R. Hoffmann 'The Conservation of Orbital Symmetry' Academic Press 1970 p. 124. 61 (a) M. J. Goldstein and M. R. De Camp J. Amer. Chem. SOC.,1974 96 7356; (b) M. J. Goldstein and W. A. Haiby ibid. p. 7358; (c) J. J. Gajewski L. K. Hoffman and C. N. Shih ibid. p. 3705; (d)A. K. Cheng F. A. L. Anet J. Mioduski and J. Meinwald ibid. p. 2887-;(e) A. Viola S. Madhavan R. J. Proverb B. L. Yates and J. Larrahondo J.C.S. Chem. Comm. 1974 842. 110 1).W.Jones H in which only one double bond is replaced by a a-bond has been suggested (130 ; arrows).62 5 Miscellany D-labelling and asymmetric induction in the ene-reaction of 8-pinene with maleic anhydride (131; arrows) establish attack on the side of the molecule remote from the gem-dimethyl group as well as the endo nature of the TS.63" The attack of benzyne on 8-pinene occurs from the same direction.63b The kinetics of the reverse ene-reactions (132; arrows) agree with a concerted mech- anism,64" but for X = NPh a radical mechanism The reaction (133 ; arrows) has been used to prepare thiobenzaldehyde and thi~acrolein.~~~ R SiMe (132) X = CH, NH or 0;R = CH=CH (133) X = S;R = Ph or CH=CH Further examples of dyotropic reactions [(134) and (135); arrows] have been discussed.6s The first process may be made allowed by use of low-energy vacant orbitals of silicon.The second is an allowed @2,+ ,2 + ,2 process in which at the TS,the migration of the trimethylsilyl group is more advanced than that of the ally1 group.62 J. S. Hastings H. G. Heller H. Tucker and K. Smith J.C.S. Chem. Comm. 1974 348. 63 (a)R. K. Hill J. W. Morgan R. V. Shetty and M. E. Synerholm J. Amer. Chem. SOC. 1974 96,4201; (6)V. Garsky D. F. Koster and R. T. Arnold ibid. p. 4207. 64 (a) K. W. Egger and P. Vitins J. Amer. Chem. SOC.,t974 % 2714; J.C.S. Perkin II 1974 1289 1292; (b)K. W. Egger and P. Vitins Helv. Chim. Acta 1974 57 214; (c) H. G. Giles R. A. Marty and P. de Mayo J.C.S. Chem. Comm. 1974 409. 65 M.T. Reetz M. Kliment and M. Plachky Angew. Chem. Internat. Edn. 1974 13,813; M. T. Reetz ibid. p. 402.