4 Reaction Mechanisms Part (i) Pericyclic Reactions By R. J. BUSHBY School of Chemistry The University Leeds LS2 9JT 1 Cycloaddition Reactions The problem that has perhaps stimulated most interest in the past year is that of facial selectivity. A bold general solution to this problem was recently proposed by Hehre.' This involves mapping electrostatic potentials on the faces of the reacting molecules using an H+ or an H- ion as a probe. For a normal Diels-Alder reaction Hehre suggests there will be a match between the more electron-rich (nucleophilic) face of the diene and the more electron-poor (electrophilic) face of the dienophile as shown in diagram (1). Examples have been cited which follow this rule,'-3 but ELECTROPHILICITY H I Low NUCLEOPHILICITY ' S.D. Kahn and W. J. Hehre Tetrahedron Lett. 1986 27 6041; J. Am. Chem. SOC.,1987 109 663 and references therein. ' R. M. Ortuno M. Ballesteros J. Corbera F. Sanchez-Ferrando and J. Font Tetrahedron 1988,44 1711. M.J. Fischer W. J. Hehre S. D. Kahn and L. E. Overman J. Am. Chem. SOC.,1988 110 4625. 39 R. J. Bushby more have been found which do In the reaction between the sulphoxide (2) and N-phenylmaleimide,3 the failure to obtain the predicted syn-endo transition state is attributed to the local electrostatic interaction between sulphoxide and maleimide oxygens shown in (3). This illustrates one obvious drawback of using small structureless ions as probes for surface reactivity but Hehre's whole approach i.e. in using an electrostatic index for reactions which in other senses are FMO controlled is open to que~tion.~ -0S+ I 0-(2) In many cases of course steric factors dominate.For example in 27r + 47r additions to the allene-enes (4; 2 = H or ButMe,SiO-) the bulk of the substituents X and Y directs facial selectivity." Steric factors also seem to be important in additions of 1,3-dipoles to the bicyclo[3.2.0]-systems (5). These adopt the boat-like conformation shown and the degree of folding of the three-atom bridge can affect the ease of syn Another factor invoked to explain facial selectivity in 27r + 47r cycloaddition reactions of cyclobutene derivatives is out-of-plane bending of the vinyl hydrogens. Distortions of the type shown in formula (6) favour syn attack and those in formula (7) anti atta~k.~,~ The equivalent 'orbital tilting' explana- tion of Paquette and Gleiter has been invoked to explain the effect of substituents X on the facial selectivity in the reaction of compound (8) with (2)-1,2-bis(phenylsul- X R D.P. Curran and S. A. Gothe Tetrahedron 1988 44 3945. J. C. Lopez E. Lameignerc and G. Lukacs J. Chem. Soc. Chern. Comrnun. 1988 706; R. Tripathy R. W. Franck and K. D. Onan J. Am. Chem. Soc. 1988 110 3257. M. Burdisso R. Gandolfi P. Pevarello and A. Rastelli J. Chem. SOC.,Perkin Trans. 2 1988 753. ' M. Burdisso R. Gandolfi M. Lucchi and A. Rastelli J. Org. Chem. 1988 53 2123. H. Landen B. Margraf and H.-D. Martin Tetrahedron Lett. 1988 29 6593; H. Landen B. Margraf H.-D. Martin and A.Steigel ibid. p. 6597; H. Hake H. Landen H.-D. Martin,B. Mayer and A. Steigel ibid p. 6601. W.-S. Chung N. J. Turro S. Srivastava H. Li and W. J. le Noble J. Am. Chern. Soc. 1988 110 7882. lo J. B. Macaulay and A. G. Fallis J. Am. Chem. SOC.,1988 110 4074. H. J. Reich E. K. Eisenhart W. L. Whipple and M. J. Kelly J. Am. Chem. SOC.,1988 110 6432. Reaction Mechanisms -Purr (i) Pericyclic Reactions X I phonyl)ethylene.12 Addition syn to the -CH2-bridge is normally preferred but this preference is modulated by some substituents particularly those with a strong +M effect such as Me2N- and MeO-. In another intriguing study adamantane derivatives of type (9) have been used.' These eliminate the complications that normally arise from steric factors.Both in the thermal 27r + 47r reaction of the thione (9; X = S) with 2,3-dimethylbutadiene and in the photochemical 277 + 27r reaction of the ketone (9; X = 0) with fumaronitrile addition syn to the fluorine is observed. This was attributed to a hyperconjugative interaction between the developing u*bonds and the electron-rich trans-periplanar 1/8 and 3/ 10 bonds. Whilst it is not claimed that this hyperconjuga- tion argument constitutes a universal theory of facial selectivity other examples that can be treated in this way are cited. In a systematic study of Diels-Alder reactions of the cyclopentadienes (10; X = various S- 0-,and N-containing groups) facial selectivity was attributed to a mixture of effects." Particularly noteworthy is the change from syn selectivity in the cases X = OH and OMe to anti selectivity for the cases X = SMe SOMe and S02Me (reaction with maleic anhydride).The system X = SH gives an almost equal synlanti mixture. Me X 10& F Me Me Ab initio MO calculations have been reported for various thermal 2u + 2u,13 2a + 27r,14 and 27r + 27r" cycloaddition reactions and it is disturbing to note that ab initio/3-31G calculations find no true transition state for 7r2s + 7r2a cycloaddition of ketene to ethylene. The authors suggest that concerted pathways of this type do not exi~t.'~ Conversely thermochemical arguments suggest that some thern,al 2u + 2u (retro 7r2s + ~2s) reactions follow a concerted symmetry-forbidden pathway16 12 L. A. Paquette and M. Gugelchuk J.Org. Chem. 1988 53 1835; cf R. Gleiter and L. A. Paquette Acc. Chem. Res. 1983 16 328. 13 P. B. Shevlin and M. L. McKee J. Am. Chem. Soc. 1988 110 1666. 14 S. Yamabe T. Minato and S. Inagaki J. Chem. Soc. Chem. Commun. 1988 532. IS F. Bernardi A. Bottoni M. Olivucci M. A. Robb H. B. Schlegel and G. Tonachini J. Am. Chem. SOC. 1988 110 5993. 16 W. v. E. Doering W. R. Roth R. Breukmann L. Frigge H.-W. Lennartz W.-D. Fessner and H. Prinzbach Chem. Ber. 1988 121 1. R. J. Bushby rather than involving a 1,4-biradical intermediate. Studies of the effect of geometry on the 1,4-biradicals (1 1) generated photochemically in rigid crystalline media” show that biradicals in geometry (12) give mostly cleavage product and biradicals in geometry (13) cyclization product.This also neatly explains the tendency of cyclohexanone to give trans fused 27~ + 27r photoadducts without needing to invoke an initial cis/trans isomerization of the cyclohexanone double bond. Only biradicals whose geometries are analogous to (13) will give adducts. Those whose geometry is analogous to (12) will revert to starting materials. H H Studies of the singlet state 27r + 27r photodimerization of p-MeOC,H,CH=CHMe show that this too proceeds through 1,4-biradical intermedi- ates rather than a concerted 7~2s+ 7r2s pathway. The E precursor gives a product (14) in which the stereochemistries of both olefins are preserved but the 2 precursor gives products (15) and (16) in which one stereochemistry is inverted.18 Intramolcular ketene and keteniminium cycloadditions have been reviewed” and there have been several interesting studies of intramolecular Diels- Alder and 1,3- l7 S.Ariel S. V. Evans M. Garcia-Garibay B. R. Harkness N. Ornkaram J. R. Scheffer and J. Trotter J. Am. Chem. Soc. 1988 110 5591. ’* F. D. Lewis and M. Kojirna J. Am. Chem. Soc. 1988 110 8660. B. B. Snider Chem. Rev. 1988,88 793. Reaction Mechanisms -Part (i) Pericyclic Reactions dipolar cycloaddition reaction^.^'-^^ In the case of the furan derivatives (17) reac- tivity is shown to relate to the effect of substituents R on the population of relevant rotomeric states.20 In the intramolecular cycloaddition reaction of the yne-dienes (18; X or Y = -0-or -CH,-) steric interactions between the centres marked with an asterisk are a controlling factor.Hence reaction for the systems Y = -0-is considerably faster than for those where Y = -CH2-.” Molecular mechanics calculations for the intramolecular 1,3-dipolar additions (19) show a correlation between product ratios and product stabilities suggesting a late transition state.22 * CH,-X-CH,-Y-CH2-C=C-C02Me hO+C02Me RRO Me As usual there have been many reports of ways of speeding up 27r + 47r cycloaddi-tion reactions. These include the use of pres~ure,~~,~~ water formamide or ethylene glycol as solvent,26927 catalysis with aluminium trichloride28 or shift reagents,29 and the use of ultras~und.~~ In the case of reaction (20) (Pht = phthalide) it was shown NPht 2o M.E. Jung and J. Gervay Tetrahedron Lett. 1988 29 2429. 21 K. J. Shea L. D. Burke and W. P. England J. Am. Chem. SOC.,1988 110 860. 22 A. Hassner K. S. Murthy A. Padwa W. H. Bullock and P. D. Stull J. Org. Chem. 1988 53 5063. 23 E. C. Angell F. Fringuelli F. Pizzo A. Taticchi and E. Wenkert J. Org. Chem. 1988 53 1424; S. Lamothe A. Ndibwami and P. Deslongchamps Tetrahedron Lett. 1988 29 1639 1641; G. R. Krow Y.B. Lee R. Raghavachan P. V. Alston and A. D. Baker Tetrahedron Lett. 1988,29,3187; A. Marinier and P. Deslongchamps Tetrahedron Lett. 1988 29 6215; L. F. Tietze T. Brumby S. Brand and M. Bratz Chem. Ber. 1988 121 499. 24 A. Sera M. Ohara T. Kubo K. Itoh H. Yamada Y. Mikata C. Kaneto and N. Katagiri J. Org. Chem. 1988 53 5460; R. M. Ortuno A.Guingnant and J. d’Angelo Tetrahedron Lett. 1988 29 6989; D. L. Boger and K. D. Robage J. Org. Chem. 1988 53 3373. 25 L. F. Tietze T. Hubsch E. Voss M. Buback and W. Trost J. Am. Chem. SOC.,1988 110 4065. 26 S. Colonna A. Manfredi and R. Annunziata Tetrahedron Lett. 1988 29 3347. 27 A. G. Griebeck Tetrahedron Lett. 1988,29 3477; T. Dunams W. Hoekstra M. Pentaleri and D. Liotta ibid. p. 3745; R. Breslow and T. Guo J. Am. Chem. SOC.,1988 110 5613. 28 E. Wenkert P. D. R. Moeller and S. R. Piettre J. Am. Chem. SOC.,1988 110 7188. 29 R. P. Gandhi M. P. S. Ishar and A. Wali J. Chem. SOC.,Chem Commwn. 1988 1074. 30 D. R. Borthakur and J. S. Sandhu J. Chem. Soc. Chem. Comun. 1988 1444. R. J. Bushby that the diastereoselectivity is also significantly pressure dependent,25 whereas for reaction (21) in water a degree of asymmetric induction (38% ee) is obtained by adding bovine serum albumin.26 There have also been reports that 27r + 47r cyclo-addition reactions can be accelerated by adsorption of the reactants on chromato- graphic supports.31 4-31G Ab initio MO calculations on the reaction between ethylene and butadiene suggest that it is synchronous but that this energy surface is not far separated from that for a non-synchronous mechanism.32 Experimental evidence for the synchronous nature of the Diels-Alder reaction has been adduced from an LFER study of the reaction between cyclopentadiene and the sulphones (22).33 Secondary isotope effects for 2a + 2a + 27r cycloadditions of quadricyclane and 27r + 271-+ 27r cyclo-additions of norbornadiene suggest that these reactions are non-synchronous but concerted,34 and calculations on the reaction of nitrile oxides with acetylene also suggest non-synchronous bond formation.35 Ar’ -SO,-CH=CH -SO,-ArZ (22) It has recently been suggested that endo selectivity in the Diels-Alder reaction is the result of steric rather than secondary orbital factors.Indeed steric effects seem satisfactorily to account for endo selectivity in the 271-+ 47r addition of thioaldehydes to ~yclopentadiene.~~ However a detailed study of Diels- Alder reactions of trans-penta-1,3-diene has come down in favour of the ‘more traditional’ secondary orbital interaction interpretation of the stere~chemistry.~’ Among interesting routes to reactive 47r system^^^-^' are the generation of the azomethine ylides (23; R = H alkyl or aryl) by decarboxylative addition of aromatic 3’ V.V. Veselovsky A. S. Gybin A. V. Lozanova A. M. Moiseenkov W. A. Smit and R. Caple Tetrahedron Lett. 1988 29 175; M. Koreeda D. J. Ricca and J. I. Luenge J. Org. Chem. 1988 53 5586. 32 F. Bernardi A. Bottoni M. J. Field M. F. Guest I. H. Hillier M. A. Robb and A. Venturini J. Am. Chem. Soc. 1988 110 3050. 33 R. A Hancock and B. F. Wood 1.Chem. SOC.,Chem. Commun. 1988 351. 34 L. A. Paquette M. A. Kesselmayer and H. Kiinzer J. Org. Chem. 1988 53 5183. 35 R. Sustmann and W. Sicking Tetrahedron 1988 44,379. 36 E. Vedejs J. S. Stults and R. G. Wilde J. Am. Chem. SOC.,1988 110 5452. 37 0.F. Giiner R. M. Ottenbrite D. D. Shillady and P. V. Alston J. Org. Chem. 1988 53 5348. 38 R. Grigg S. Surendrakurnar S. Thianpatanagul and D. Vipond J. Chem. SOC.,Perkin Trans. 1 1988 2693; R. Grigg J. Idle P. McMeekin S. Surendrakumar and D. Vipond ibid. p. 2073. 39 A. Padwa and B. H. Norman Tetrahedron Lett. 1988 29 2417. 40 C. W. G. Fishwick A. D. Jones M. B. Mitchell and C. Szhntay Tetrahedron Lett. 1988 29 5325; Y. Terao A. Aono and K. Achiwa Heterocycles 1988 27 981. 41 A. Padwa and P. E. Yeske J. Am. Chem. SOC.,1988 110 1617. Reaction Mechanisms -Part (i) Pericyclic Reactions 45 R I A=+ N+\-N+ CR'R" R2CH '*-+ CH,=CH-C( OEt) aldehydes to amino the generation of the nitrones (24) by the reaction of vinyl sulphones and ~ximes,~~ various desilyation routes to 1,3-dip0les,4~ and the formation of the ally1 anions (25) by the addition of X-(CN- NO2- or PhS02-) to CH2=C=CHS02Ph?1 The reactive 27r system S2can be obtained by thermolysis of the disulphide (26),42 and further evidence has been advanced of remarkably high reactivity of cation (27) in 27r + 47r cycloaddition reactions.43 2 Sigmatropic Reactions The 1,3-sigmatropic rearrangement leading from compound (28) to compound (29) proceeds as shown with predominant inversion at the migrating carbon although some ST product is also obtained.The stereochemistry of this reaction was assessed directly from small amounts of (29) isolated from the reaction mixture and also on the major product ethylene which is formed by a retro-Diels- Alder reaction.The stereochemistry was explained in terms of competing concerted and biradical pathways.44 An equivalent biradical pathway for the 1,3-sigmatropic rearrangement of bicyclo[3.1 .O]hex-2-ene (30) would give the symmetrical intermediate (3 1). Previous studies have failed to detect such a symmetrical intermediate but clear evidence has been provided for an equivalent vinyl-TMM intermediate (32) in the rearrangement of derivatives of the 6-methylene compound (33)."' The 'forbidden' 1,3-sigmatropic rearrangement of compound (34) can be greatly accelerated by forming the radical cation with a small amount of Ar3Nt SbF6-.46 42 W. Ando H. Sonobe and T. Akasaka Tetrahedron Lett. 1987 28 6652. 43 P. G. Gassman and S.P. Chavan J. Org. Chem. 1988 53 2393. 44 J. E. Baldwin and K. D. Belfield J. Am. Chem. SOC.,1988 110 296; F.-G. Klarner R. Drews and D. Hasselmann ibid.,p. 247. 45 S. Pikulin and J. A. Berson J. Am. Chem. Soc. 1988 110 8500. 46 J. P. Dinnocenzo and D. A. Conlon J. Am. Chem. SOC.,1988 110 2324. R. J. Bushby An interesting example of regioselectivity in a 1,Shydrogen shift is shown by compound (35; Ar = 3,4,5-trimethoxyphenyl),where the hydrogen indicated is the one to migrate. The conformation required for this particular 1,5-shift is easy to attain but that required for a 1,5-shift of the neighbouring hydrogen results in a severe steric interaction between the Ar and C02Me grouping^.^' The endo conforma-tion shown (36) is that required for the observed formation of cis-hexa-1,4-diene by a homodienyl 1,5-shift.Ab initio MO calculations at the 3-21G level confirm that this should be the case.48 Discrepancies between the calculated and experimental kinetic isotope effects for the simplest 1,Shydrogen shift (37; L = H or D) have been explained by the assumption that some tunnelling OCCU~S,~~ and experimental evidence for such tunnelling has been claimed in a study of the 1,7-hydrogen shift (38; L = H or D) for which kH/k = 7.0 at 60"C.50 The claim of a remarkably high kinetic isotope effect (kH/kD -45 at 80 "C) for the 1,7-hydrogen shift in the conversion of previtamin D into vitamin D3 has now been withdrawn" and new measurements for a 19,19,19-trideuteriated derivative give the more acceptable value of k,/kD -6.1 at 80 0C.52 Several reinvestigations of the vitamin D system have been reported this A measurement of the volume of activation for the conversion of previtamin D3 into vitamin D, A V# = -5.14 cm3 mol-' at 20 0C,54 is taken as evidence for a concerted 1,7-hydrogen shift and n.m.r.studies have challenged previous claims that previtamin D3 exists entirely in the S-cis-S-cis conformation (39) providing evidence for significant population of the S-trans-S-cis conformation (40).55 47 D. W. Jones and A. M. Thompson J. Chem. SOC.,Chem. Comrnun. 1988 1095. 48 R. J. Loncharich and K. N. Houk J. Am. Chem. Soc. 1988 110 2089. 49 M. J. S. Dewar E. F. Healy and J. M. Ruiz J. Am. Chem. SOC.,1988 110 2666. 50 J. E. Baldwin and V.P. Reddy J. Am. Chem. SOC.,1988 110 8223. 51 Y. Mazur and M. Sheves in ref. 52. 52 W. H. Okamura C. A. Hoeger K. J. Miller and W. Reischl J. Am. Chem. SOC.,1988 110 973. 53 W. G. Dauben P. E. Share and R. R. Ollmann J. Am. Chem. Soc. 1988 110 2548. 54 W. G. Dauben B. A. Kowalczyk and D. J. H. Funhoff Tetrahedron Lett. 1988 29 3021. 55 W. G. Dauben and D. J. H. Funhoff J. Org. Chem. 1988 53 5376. Reaction Mechanisms -Part (i) Pericyclic Reactions R R 14C and l80kinetic isotope effects have been measured for the aromatic Claisen rearrangement (41). Together with previous measurements of *H kinetic isotope effects it is claimed that these show a concerted non-synchronous mechanism in which the C,-0 bond is 50-60% broken and the C,-Co bond 10-20% formed in the transition state.56 A rather similar picture with a concerted non-synchronous mechanism and appreciable C-0 bond breaking was arrived at from ab initio MO calculations for the corresponding aliphatic Claisen rea~angement.~’ Calculations on the Cope rearrangement (including CI) suggest that it is concerted and syn- chrono~s.’~ Despite these indications that 3,3-sigmatropic rearrangements are con- certed the suggestion that such rearrangements involve 1,4-biradical intermediates still attracts attention providing cause for lively debate and innovative experimenta- tion.It has been argued that the stereospecificity of the 3,3-sigmatropic rearrange- ment (42) is only consistent with a concerted mechanism or the 1,4-biradical (43) if involved being exceedingly short lived.59 Further evidence against a 1,4-biradical intermediate in the Cope rearrangement has come from a comparison of the normal reaction and that of the radical cation.60 By way of contrast to the normal reaction these radical cation reactions do proceed through a 1,4-radical cation which can be trapped.This is shown by the isolation of peroxide (44) from the catalysed rearrange- ment of compound (45).61 The 1,4-biradical mechanism for 3,3-sigmatropic re- 04“. / 56 L. Kupczyk-Subotkowska W. H. Saunders and H. J. Shine J. Am. Chem. SOC.,1988 110 7153. 57 R. L. Vance N. G. Rondan K. N. Houk F. Jenson W. T. Borden A. Komornicki and E. Wimmer J. Am. Chem. SOC.,1988 110 2315. 58 K. Morokuma W.T. Borden and D. Hrovat J. Am. Chem. SOC.,1988 110 4474. 59 K. A. Owens and J. A. Berson J. Am. Chem. SOC.,1988 110 627. 60 Q.-X. Guo X.-Z. Qin J. T. Wang and F. Williams J. Am. Chem. SOC.,1988 110 1974. 61 T. Miyashi A. Konno and Y. Takahashi J. Am. Chem. SOC.,1988 110 3676. R. J. Bushby arrangements represents one end of the mechanistic spectrum. The other end of this spectrum is a mechanism in which bond breaking precedes bond formation and a pair of allyl radicals is involved. Possible evidence for such a mechanism has been provided from studies of the 1,5-diene (46; E = C0,Me). When this is heated in the presence of oxygen the peroxides (48) and (49) are isolated which can be formulated as arising from the di-ally1 radical (47).62 Undeterred by exceptions to his facial selectivity rule for cycloaddition reac- tion~,~-"Hehre has suggested that a similar treatment can explain asymmetric induction in 3,3-sigmatropic rearrangement^.^^ In Ireland-Claisen rearrangements the oxyallyl portion is treated as being nucleophilic and the allyl portion as elec- trophilic.Mapping electrostatic potentials on the surface of each portion and matching the more electron-rich surface of the ester enolate to the more electron-poor surface of the allylic olefin leads to the chair-like transition state (50). Hence the fact that compound (52) is the predominant diastereoisomer obtained by rearrange- ment of the diene (51) (THP = tetrahydropyranyl) is interpreted by Hehre in terms of the transition state (53) in which (provided the C-H bond eclipses the C=C bond) the electron-poor face of the allylic portion is uppermost.NUCLEOPHILICITY ELECTROPHILICITY (50) 62 R. Iyengar R. Pina K. Grohmann and L. Todaro J. Am. Chem. Soc. 1988 110 2643. 63 S. D. Kahn and W. J. Hehre J. Org. Chem. 1988 53 301. Reaction Mechanisms -Part (i) Pericyclic Reactions O(THP) (53) The 3,3-sigmatropic rearrangement of chorismate (54; R = H) to prephenate is normally catalysed by the enzyme chorismate mutase. It has been shown that this enzyme will also accept the methylated analogue (54; R = Me) and the stereochemistry of the product confirms previous indications that the enzyme- catalysed rearrangement proceeds through a chair-like transition state.64 It has also been shown that the stereospecific 3,3-sigmatropic rearrangement of chorismate (54; R = H) can be catalysed by an antibody elicited against a chorismate mutase inhibit~r.~’ Labelling studies have now confirmed that the thermal rearrangement of the bicyclo[4.1 .O]-system (55) proceeds by the 3,5-sigmatropic shift mechanism shown and not a 1,3-sigmatropic mechanism.66 3 Electrocyclic Reactions The electrocyclic ring opening of cyclobutene has been used as a test case in a comparison of a wide range of MO methods.A6 initio MO calculations using large basis sets give results that compare well with experimental data but in this test semi-empirical and STO-3G a6 initio methods perform rather p~orly.~’ In this reaction C3 or C4 alkoxy substituents tend to rotate outwards and a dramatic demonstration of this rotational preference is provided by the t-butyl-substituted systems (56; R = Me and Me,Si) where the products obtained were the olefins (57)? 64 D.Lesuisse and G. A. Berchtold J. Org. Chem. 1988 53 4992. 65 D. Hilvert and K. D. Nared J. Am. Chem Soc. 1988 110 5593. 66 P. J. Battye D. W. Jones and H. P. Tucker J. Chem. Soc. Chem. Commun. 1988,495. 67 D. C. Spellmeyer and K. N. Houk J. Am. Chem Soc. 1988 110 34-12. 68 K.N. Houk D. C. Spellmeyer C. W. Jefford C. G. Rimbault Y. Wang and R.D. Miller J. Org. Chem. 1988 53 2125. R. J. Bushby 40R I But Bu' Perhaps the most interesting finding of the past year however has been that in reactions of this sort there is a different kinetic secondary isotope effect for the hydrogens rotating inwards and those rotating outwards.For the conversion of (58) into (59) kH/kD = 1.04 * 0.03 (140 "C),while for (58) -+ (60) kH/kD = 1.15 * 0.03; for the conversion of (61) into (63) kH/kD = 1.05 0.03 while for (62) -+ (63) kH/kD = 0.88 f 0.06. These kinetic secondary isotope effects can be fitted to calcu- lated values from which it is seen that the differences arise from various causes. In the four-electron electrocyclic reaction the major factor is differences in the p-character of the C-H bonds but in the six-electron electrocyclic reaction the difference is one of steric comp~ession.~~ 4 Miscellaneous Pericyclic Reactions Ab initio MO calculations on the thermal elimination of carbon monoxide from 9-ketonorbomadiene (64) suggest a linear suprafacial process which is concerted and synchrono~s.~~ When this molecule is generated in a vibrationally excited state it tends to eliminate carbon monoxide spontaneously.This occurs when the pyrazo- line (65) is photolysed in an argon matrix at 10 K. However when the pyrazoline (65) is photolysed in a frozen toluene matrix at 195 K much less decarbonylation occurs and quite a high yield of the ketone is isolated. The explanation given to %JN CN I1 N=N CN 69 J. E. Baldwin V. P. Reddy B. A. Hess and L. J. Schaad J. Am. Chem. SOC 1988 110 8554 8555. D. M. Birney K. B. Wiberg and J. A. Berson J. Am. Chem. Soc. 1988 110 6631. Reaction Mechanisms -Part (i) Pericyclic Reactions this seeming paradox is that a frozen toluene matrix has vibrational modes which can couple with those of the ketone (64) effectively removing excess vibrational energy but equivalent vibrational modes are not present in an argon matrix.” A study of the dehydrogenation of cyclohexa-l,4-diene derivatives with tetracyanoethylene has concluded that the first step is normally an ene reaction giving (from the parent system) compound (66),which then probably breaks down by a heterolytic route.Some reaction may however occur through a stepwise electron-proton transfer me~hanism.’~ 71 B. F. LeBlanc and R. S. Sheridan J. Am. Chem. SOC.,1988 110 7250. 72 B. M. Jacobson P. Soteropoulos and S. Bahadori J. Org. Chern 1988 53 3247.