4 Reaction Mechanisms Part (i) Pericyclic Reactions By G. B. GILL Department of Chemistry University of Nottingham Nottingham NG7 2RD 1 Introduction All chemists and particularly those with special interests in pericyclic reactions will take delight in the award of the Nobel Prize for chemistry 1981 to Roald Hoffmann and Kenichi Fukui; the texts of their Stockholm lectures have now been published.' The full papers of several pieces of work previously discussed in Annual Reports have now appeared. Limitations on space however prevent any further discussion this year. Review articles include photochemical cycloadditions uia exciplexes excited complexes and radical ions,2 preparative organic photo~hemistry,~ (2 + 2) cyclorever~ions,~ and some aspects of orbital interactions through bonds.' 2 Cycloadditions and Cycloreversions The Diels-Alder reaction has been modelled by new quantum chemical approxima- tions in terms of a charge flux from space regions in which bonds are broken into those formed rather than in terms of changes in nuclear co-ordinates.The method is 1-2 orders of magnitude faster than the geometrical procedure at a comparable level of sophistication and is suggested to be convenient for the study of medium- sized molecules of ca. 30-40 atoms.6 Volume effects of regioselectivity VAc -VLc and V& -Vk,(where oc mc ot and mt refer to ortho-cis meta-cis ortho-trans and meta-trans adducts respectively) and the volume effects of stereoselectivity V& -VAt and VLc -Vk, have been determined for the Diels-Alder reactions of trans-piperylene with alkyl acrylates CH2=CHC02R (R = Me Pr CMe2Et).The volume effects of regioselectivity became more negative with increasing size of R whereas the volume effects of stereoselectivity varied irregularly; the results were attributed to conformational effects in the ester.' MIND0/3 MO calculations reveal that 1,4-dyotropic" H migrations in H2NC( =NH)C02H H2NCOCONH2 ' R. Hoffmann Angew. Chem. Int. Ed. Engl. 1982,21,711; K. Fukui ibid. p. 801. S. L. Mattes and S. Farid Acc. Chem. Res. 1982 15 80. 'P. Margaretha Top. Curr. Chem. 1982,103 1. E. Schaumann and R. Ketcham Angew. Chem. Int. Ed Engl. 1982 21,225. M. N. Paddon-Row Acc. Chem. Res. 1982,15,245. J. Pantif J. Am. Chem. SOC.,1982 104,7424.'S. K. Shakhova and B. S. El'yanov lzv. Akad. Nauk SSSR Ser. Khim. 1982 1194. * Dyotropic rearrangements are defined by M. T. Reetz Angew. Chem. Znt. Ed. Eng.. 1972 11 129 130. 36 G. B. Gill and CH2=CMeCMe=CH2 proceed by a stepwise mechanism whereas 1,5-and 1,6-migrations in HOCH=C(OH)COCHO and compound (l) respectively pro- ceed by concerted mechanisms.8 0 OH (2) X=H,Y=OH (4) X = H Y = OH (1) (3) X=OH,Y=H (5) X = OH,Y = H x- x@o Y 0 (7) X = D,Y = H (8) X = H,Y = D The simplest enediols (2) and (3) have been generated successfully by flash vacuum pyrolysis at 600 0C/10-5 Torr of the corresponding (4 + 2) adducts with anthracene (4) and (5),respectively. The enediols were characterized specti oscopi- cally.' Results obtained for the (4 + 2) addition of the deuterium-labelled butadiene (6) with maleic anhydride which gave (7) and (8)in 85 :15 ratio leads to a value for AAG* of 1.2 kcal mol-' at 80 "C.Since the endu-transition state is sterically disfavoured in this case the 1.2 kcal mol-' represents the minimum energy advan- tage associated with electronic explanations of the Alder endu rule." Orbital correlation diagrams reveal that the (4 + 2)' reaction is allowed for the case (diene) + (dienophile)' but not for the case (diene)' + (dienophile); thus in the radical cation Diels-Alder reaction diene radical cations function as highly electron- deficient dienophiles. 'la Radical cation polymers with radical cation functions of up to 5% of the cross-linked polystyrene monomer sites have been prepared and are effective in catalysing for example the (4 + 2) dirnerization of cyclohexa-1,3- diene.' Id Danishefsky et al.have published several papers this year on the Lewis acid- catalysed cyclocondensations of (E)-l-methoxy-3-[(trialkylsilyl)oxy]buta-l,3-dienes with aldehyde^'^"^" and with imines,13' affording rapid entry to 5,6-dihydro- B. Ya. Simkin B. V. Golyanskii and V. Minkin Zh. Org. Khim. 1981 17 1793. M.-C. Lasne and J.-L. Ripoll Tetrahedron Lett. 1982 23 1587. L. M. Stephenson D. E. Smith and S. P. Current J. Org. Chem. 1982,47,4170. (a) D. J. Bellville and N. L. Bauld J. Am. Chern. Soc. 1982 104 2665; (b)N. L. Bauld D. J. Bellville S. A. Gardner Y. Migron and G. Cogswell Teruhedron Lett. 1982 23 825.l2 (a) S. Danishefsky J. F. Kerwin and S. Kobayashi J. Am. Chem. SOC. 1982 104 358; (b) S. Danishefsky N. Kato D. Askin and J. F. Kerwin ibid. p. 360; (c) S. Danishefsky and J. F. Kerwin J. Org. Chem. 1982 47 1597; (d) S. Danishefsky S. Kobayashi and J. F. Kerwin ibid. p. 1981; (e) S. Danishefsky E. R. Larson and D. Askin J. Am. Chem. Soc. 1982 104 6457; (f) E. R. Larson and S. Danishefsky ibid. p. 6458. l3 (a) E. R. Larson and S. Danishefsky Tetrahedron Lett. 1982 23 1975; (6) J. F. Kerwin and S. Danishefsky ibid. p. 3739. Reaction Mechanisms -Part (i) Pericyclic Reactions y-pyrones and 5,6-dihydro-y-pyridones, respectively. The ZnC12-catalysed addition of Danishefsky's diene with (R)-glyceraldehyde acetonide in benzene at room temperature afforded the (5S,6R)-heptulose (9) in 72% yield with no detectable contamination by the (5R,6R)-dia~tereoisomer.'~~ The product (9) arises from the reaction of the chiral aldehyde with the diene by &attack in accordance with the classical Cram's rule model.The stereochemical outcome of these cyclocondensa- tions is considerably influenced by the choice of the Lewis acid catalyst. For example predominant trans-addition has been observed using BF,-OEt2 at -78 "C whereas use of ZnC12 at room temperature afforded mainly cis-adducts.12e The ZnC12- catalysed process exhibits all of the characteristics expected from a pericyclic addition mechanism whereas with BF3.0Et2 the evidence indicates that part of the products arise from intermediate siloxonium species.12f*130 Virtually quantitative asymmetric induction (99.3%d.e.) has been achieved in the TiC12(0R)2 catalysed Diels-Alder addition of the acylate ester (10) and its enantiomer [prepared from (R)- (+)-and (S)-(-)-camphor] to ~yclopentadiene.'~ The re-face of the acrylate moiety C carbon atom is sterically blocked by the pendant t-butyl group forcing addition at the si-face of (10) thereby affording adduct (11) of (R)- configuration at the new chiral centre (*). Diels-Alder additions of carbohydrate-derived dienophiles bearing an allylic oxygen function as shown by the generalized structure (12) with the o-quinone methide (13) have been studied to elucidate the stereochemical control exerted by the allylic oxygen func- tion." The preferential formation of adducts of the general type (14) indicates a pronounced selectivity in favour of orbital control over steric control in accordance with previous predictions by Houk ef af.,in that the incoming group should bond to the face opposite to the allylic oxygen function (si-face) so as to minimize secondary orbital antibonding effects.The new chiral centre (*) in (14) was shown to possess the expected (S)configuration. l4 W. Oppolzer C. Chapuis G. M. Dao D. Reichlin and T. Godel Tetrahedron Leu. 1982,23,4781. Is R. W. Franck T. V. John and K. Olejniczak J. Am. Chem. Soc. 1982 104 1106. G. B. Gill Isodicyclopentadiene undergoes a [1,5] H shift at temperatures a170 "Cto form the isomeric highly reactive diene (15) which readily underwent (4 + 2) cycloaddi-tions by attack of the dienophile at the exo-face but giving adducts in accord with Alder's endo rule.16 This reaction has been used as a strategy for the preparation of fused norbornene/norbornadiene adducts.16' The intramolecular Diels-Alder reaction naturally continues to be the focus of considerable interest because of its unique value for the construction of complex molecules of biological importance.Attention has been drawn to the problems associated with such reactions of an (E)-diene;17 these are the ready accessibility of the alternative endo-and exo-transition state topologies and in the case of the introduction of an asymmetric centre at the pentadienylic position of the diene the further complication arising from the four possible modes of addition namely endo-a endo-p exo-a and exo-p.Chiral (2)-dienes on the other hand undergo intramolecular Diels-Alder reaction with enones to afford cis-fused products with complete stereo-and enantio-specificity since the (2)-diene because of its geometry can only attain a single transition-state ge~metry.'~ The utilization of a (2)-diene unit in (16) a relatively unactivated system for the stereoselective construction of the cis-lactone (17) which has previously been transformed into marasmic acid has been reported.'' Comparison of the relative rates of cyclization C0,Me of the (Z,E,Z)-triene with the (Z,Z,Z)-triene (16) at 205 "C indicates the former to be more reactive as expected by a factor of CQ. 42 (AAG*at 205 "C ca.3.5 kcal-mol-'). Comparison with previous work on the stereoselectivity of the (Z,E,Z)-(a) R. Subramanyam P. D. Bartlett G. Y. M. Iglesias W. H. Watson and J. Galloy J. Org. Chem. 1982 47 4491; (6)L. A. Paquette R. V. Williams R. V. C. Carr P. Charumilind and J. F. Blount ibid. p. 4566. " S. G. Pyne M. J. Hensel and P. L. Fuchs J. Am. Chem. SOC.,1982,104 5719; see also S. G. Pyne, D. C. Spellmeyer. S. Chen and P. L. Fuchs ibid.,p. 5728. IX R. K. Boeckman and T. R. Alessi J. Am. Chem. SOC.,1982 104 3216; see also R. K. Boeckman and D. M. Demko J. Org. Chem. 1982 47 1789. Reaction Mechanisms -Part (i)Pericyclic Reactions 39 triene reaction indicates that selection of the transition state is primarily governed by diene geometry that dienophile orientation is independent of dienophile geometry and that the dienophile orientation has no great effect on the activation energy.Of further interest is the finding that [1,5] sigmatropic H shifts apparently do not present a limitation even in highly substituted systems.'* Heating the pentenyl derivative (18) at 150 "C for 16 h in acetonitrile afforded the perhydro- azulene derivative (20) by way of the oxidopyrylium ylide intermediate (19).19 The yield (61%) can be improved by use of a base promoter such as DBN (75%). A claim has been made for the first example in which a monosubstituted benzene ring assumes the role of the diene function in an intramolecular (4 + 2) sycloaddi-tion; thus heading the allenecarboxanilides (21) in xylene at reflux for 4-9 h effects conversion into the spiro-fused lactams (22).20 In one case (R'= R2 = Me R3= SiMe,Ph) an equilibrium process is suggested since the lactam on heating in xylene was partially reconverted into the allene.R2 Much work also continues to be done on 1,3-dipolar cycloadditions and only one or two examples will be considered heie. Thermal or copper-catalysed decomposition of dimethyl diazomalonate [NEN-C(CO~M~)~] in the presence of benzaldehyde afforded mainly the 1,3-dioxolanes (23) and (24) but the oxirane (25) was also formed.21 Although (25) could be converted into (23) and (24) under the reaction conditions this is only a minor pathway and the evidence including that from trapping experiments [e.g. the formation of (26) and (27) in the presence of dimethyl fumarate iqdicates that reaction involves the formation of the carbonyl ylide PhCH= -CC(CO2Me),.Interesting use has been r; made of X=Y-ZH systems as potential 1,3-dip+oles by means of B;onstedy and Fewis acid-catalysis22n (X=Y-ZH $ X=Y(L)-ZH X=Y(L) -Z t* X-Y(L)-Z). Thus for example treatment of the benzylidene imine of methyl- 19 P. G. Sammes and L. J. Street J. Chem. SOC.,Chem. Commun. 1982 1056. *' G. Himbert and L. Henn Angew. Chem. Int. Ed. Engl. 1982,21,620. 21 R. Huisgen and P. de March J. Am. Chem. SOC.,1982,104,4952,4953. 22 (a) R. Grigg and H. Q. N. Gunaratne. J. Chem. Sou Chem. Commun. 1982 384; (b)R. Grigg L. D. Basangoudar D. A. Kennedy J. F. Malone and S. Thianpatangul Tetrahedron Lett. 1982,23,2803. 40 G.B.Gill x (23) X = Ph,Y = H (24) X = H,Y = Ph YX (26) X = H Y = C02Me (27) X = C02Me,Y = H phenylglycinate with a Lewis acid [Z~(OAC)~ > AgOAc > LiOAc > Mg(OAc),] in the presence of methyl propiolate afforded the adduct (28); in the analogous addition catalysed by a Bronsted acid the rate was found to be related to the pKa of the acid.22" The pressure dependence of the rate of the (2 + 2 + 2) cycloaddition of norbor- nadiene to dimethyl acetylenedicarboxylate or tetracyanoethylene has been st~died.'~ The reactions were accelerated by pressure to the same extent as for Diels-Alder reactions and on the basis of the activation volume the establishment of the two new bonds is concluded to occur simultaneously.The AV*:AV ratio (20.85) indicates a product-like transition state. The mechanism of the thermal isomerization of cyclododeca- 1,5,9-triyne to hexaradialene has been investigated by means of the doubly labelled (I3C2)starting material. From chemical degradation of the product and analysis as biacetyl to determine doubly singly or unlabelled material the results indicate that the (2 + 2 + 2) pathway by way of 1,2 :3,4 :5,6-tricyclobutabenzene as intermediate is not followed.24" This conclusion accords with theoretical prediction that thermal alkyne (2 + 2 + 2) cycloadditions are highly unfavourable and instead hexaradialene appears to be formed by a sequence of three Cope rearrangements. Related results have been obtained in the flash and flow pyrolysis of de~a-1,5,9-triyne.~~' In contrast cobalt-mediated intramolecular (2 + 2 + 2) cyclization of a,S,w-diynenes to annulated cyclohexadienes [e.g.(29) + (30)] occurs readily,25" .and Wilkinson's catalyst RhCl(PPh,) is effective at the 0.5-2 mol% level for the intermolecular trimerization of hepta-lY6-diynes with monoacetylenes under mild conditions [e.g. (31) + (32)].25b The mechanism of the photochemical rneta-cycloaddition of olefins to the benzene ring has attracted further study.26 The effects of bulky substituents on the course of the reaction have been rationalized in terms of a mechanism involving an exciplex '' G. Jenner and M. Papadopoulos Tetrahedron Lett. 1982 23,4333. 24 (a)W. V. Dower and K. P. C. Vollhardt J. Am. Chem. Soc.1982,104 6878; (6) W. V. Dower and K. P. C. Vollhardt Angew. Chem. Znt. Ed. Engl. 1982 21 685. '' (a) E. D. Sternberg and K. P. C. Vollhardt J. Org. Chem. 1982 47 3447; (b) R. Grigg R. Scott and P. Stevenson Tetrahedron Lett. 1982 23 2691. 26 (a)D. Bryce-Smith G. A. Fenton and A. Gilbert Tetrahedron Lett. 1982 23 2697; (6) J. Mattay J. Runsink H. Leismann and H.-D. Scharf ibid. p. 4919. Reaction Mechanisms -Part (i ) Pericyclic Reactions intermediate and formation of bonds b and c concertedly but prior to a formula (33) through interaction betweeen S1benzene and Soalkene with meta-cycloaddi-tion being favoured by low vibrational levels of the S1arene.26a The photochemical Y (33) cycloadditions of 1,3-dioxoles to benzene affords predominantly products of exo- configuration in both ortho- and rneta-addition modes.266 The results are rationalized in terms of a mechanism involving an exciplex intermediate with a product-like structure.Wender's group continue to make elegant use of arene-olefin cycloaddi- tions for natural products synthesis; the latest advances are concerned with syntheses of (+)-hir~utene~~" and of (f)-m~dhephene.*~~ Direct excitation at 254 nm of the N-oxides (34)/(35) led to N2/N20metathesis giving (36)/(37) and (38) respectively.28 The analogous azo-compound to (34)/(35) underwent N2 elimination. The reaction between OsO and olefins leading to cis-hydroxylation has been generally assumed to proceed by way of a 67r electron cyclization. Evidence has now been presented in favour of an alternative mechanism involving the formation of an asymmetric (2 + 2) addition intermediate which rearranges and dimerizes in a rate-determining step to give the osmium(v1) ester complex that is subsequently hydrolysed reductively or oxidatively to furnish the cis-diol products.The slow reaction of 1,l-diphenylethylene with Os04 enabled the detection of the asymmetric intermediates (39) and (40) by 'H n.rn.~.'~ Asym-metric (27r + 27r) photopolymerization in chiral crystals has been examined as a " (a)P. A. Wender and J. J. Howbert Tetrahedron Lett. 1982 23,3983; (b)P. A. Wender and G. B. Dreyer J. Am. Chem. Soc. 1982 104 5805. *' H.Prinzbach G. Fischer G. Rihs G. Sedelmeier E. Heilbronner and Yang Z.-z. Tetrahedron Lerr. 1982 23 1251; see also J.M. Mellor R. N. Pathirana and J. H. A. Stibbard ibid. p. 4489; J. M. Mellor and R. N. Pathirana ibid. p. 4493. 29 M. Schroder and E. C. Constable J. Chem. Soc.. Chem. Commun. 1982,734. G. B. Gill X N'N (39) X = Ph,Y = H (40) X = H,Y = Ph means for 'absolute' asymmetric ~ynthesis.~' Thus crystalline chiral phases com- posed of unsymmetrically substituted phenylene-1,4-diacrylateswith the two different double bonds correctly aligned for asymmetric (2 + 2) photodimerization along a translational axis have been engineered. Dimers and oligomers of either chirality with quantitative enantiomeric yield were obtained in several experiments. A new (4 + 4) annulation approach applicable to the synthesis of unsymmetrically substituted and functionalized cyclo-octane derivatives has been devised.31 The strategy involves the (2 + 2) cycloaddition of a vinylketene to a 1,3-diene the 2,3-divinylcyclobutanoneintermediate reacting further by Cope rearrangement to give the eight-membered ring system.Hence reaction of cyclohexa-1,3-diene for example at 120 "C for 4 days in the presence of 2,3-dimethylcyclobut-2-en-l-one (the vinylketene precursor) gave an 81% yield of (41). A high-yield dehydrogena- tion-free synthesis of azulenes is possible through the use of the (6 + 4) cycloaddi- tions of dienamines to fulvenes that possess a potential leaving group at the C-6 position. The 6-p-nitro-benzoyloxy group is particularly well-suited to this pur- The intramolecuIar variant of the (6 + 4) cycloaddition process has now been examined.32b Thermolysis of (42) in o-dichlorobenzene at 180 "C for 7 h gave (43) as the major product of a mixture of olefin isomers arising from [1,5] H shifts following upon the inital (6 + 4) cycloaddition.The analogous intermolecular reaction afforded only endo-(4 + 2) cycloadducts. The alteration in periselectivity may be rationalized in terms of conformational requirements in ring-opened (42) which make the (6 + 4) process favourable in comparison with the spiro-(4 + 2) or ring-(4 + 2) addition modes. It has been suggested that a classification system based on the mode of cycloaddition [i.e. (6 + 2) (4 + 2) or (2 + 2)] of electron- deficient ,2-cycloaddends to tricyclo[5.3.1 .O]undeca-2,4,9-triene (44) could be 30 L.Addadi J. van Mil and M. Lahov J. Am. Chem. SOC. 1982 104 3422; see also J. van Mil L. Addadi E. Gati and M. Lahov ibid. p. 3429; J. van Mil L. Addadi M. Lahov and L. Leiserwitz J. Chem. SOC.,Chem. Commun. 1982,584. 31 R. L. Danheiser S. K. Gee and H. Sard J. Am. Chem. Soc. 1982,104,7670. 32 (a) Y. N. Gupta S. R. Mani and K. N. Houk,Tetrahedron Lett. 1982 23 495; (b)Y. N. Gupta M. J. Doa and K. N. Houk J. Am. Chem. SOC.,1982,104,7336. Reaction Mechanisms -Part (i) Pericyclic Reactions (* +2) (4 +2) valuable in predicting the outcome of new cycloadditions in other These results suggest that 4-phenyl-l,2,4-triazoline-3,5-dione is a better model for singlet oxygen additions than is tetracyanoethylene. Stereospecific endo-(10 + 8) cyclo-addition has been observed between isobenzofuran (8~ component) and 8,8-dimethylisobenz~fulvene.~~ 3 Ene Reactions Intramolecular 'magnesium-ene' reactions of type-13' and t~pe-11~~ have been defined and then employed in a most interesting way for the construction of carbocyclic frameworks.Two type-I processes in sequence were employed in the key steps for the preparation of (f)-A9"2'-~apnellene,35a and the type-I1 reaction for the key step (45)+ (46)in the construction of (f)-khusim~ne.~~' The central carbonyl group of indane-1,2,3-trione exhibits very marked enophilic character; the adduct (47),for example was obtained in high yield from hept-1-yne at moderate temperatures (80-1 10 0C).37 These ene adducts are cleaved essentially quantitatively by periodic acid thus providing a very simple route to allyl- and to 2,3-dienyl-carboxylic acids (i.e.indanetrione is a CO,ene-equivalent).Many potential enophiles are insufficiently reactive to overcome the relatively high energy barriers to the ene additions and hence catalysis of the reactions by Lewis acids continues to be a profitable area for study. Diethylaluminium chloride is an effective promoter for the addition of a-substituted acrylate esters to tri- and 33 L. T. Scott I. Erden W. R. Brunsvold T. H. Schultz K. N. Houk and M. N. Paddon-Row J. Am. Chem. SOC.,1982,104 3659. 34 R. N. Warrener D. A. C. Evans M. N. Paddon-Row and R. A. Russell Ausr. J. Chem. 1982,35,757. " (a) W. Oppolzer and K. Battig Tetrahedron Lett. 1982 23 4669; (6) W.Oppolzer H. F. Strauss and D. P. Simmons ibid. p. 4673. 36 (a)W. Oppolzer R. Pitteloud and H. F. Strauss J. Am. Chem. SOC.,1982,104,6476;(6)W. Oppolzer and R. Pitteloud ibid. p. 6478. 37 G. B. Gill and K. S. Kirollos Tetrahedron Lett. 1982 23 1399. 44 G. B. Gill truns-l,2-di-substitutedalkene~.~~~ The stereochemical courses of the additions occur by way of the least hindered transition state approach geometry and are regio- and stereo-selective in that the ester group adds endo and a hydrogen atom is transferred selectively from the alkyl group syn to the vinylic hydrogen atom. In a synthetic approach to the (*)-pseudomonic acids A and C sequential ene additions of formaldehyde under alkylaluminium chloride 'catalysis' have been used to great effect in constructing the diene (48) from hexa-1,5-diene (Scheme l).38b Elimination of ethane from (48) led to the formation of the Lewis acid (49) Me,AICI (49) Et (i) HCHO (ii) HZO Scheme 1 which in the presence of formaldehyde underwent a quasi intramolecular Diels- Alder reaction giving (after hydrolytic work-up) the dihydropyran (50).Two sequential ene reactions the first intermolecular and the second intramolecular have been used to construct bicyclic alcohols through the Me2A1C1-promoted addition of a,P-unsaturated carbonyl compounds to alkylidene cy~loalkanes.~~" The regio- and stereo-chemical outcome of the BF,-OEt catalysed ene addition of formaldehyde to (E)-and (Z)-l-ethylidene-2-methylcyclopentaneshas been examined and applied to a synthesis of (*)-Prelog-Djerassi la~tone.~~' The stereocontrolled synthesis of (20s)- steroidal side chains from the Lewis acid- catalysed addition of methyl propiolate to the (20E)-steroid has been de~cribed.~~' 38 (a)J.V. Duncia P. T. Lansbury T. Miller and B. B. Snider J. Am. Chem. Soc. 1982 104 1930; (b) B. B. Snider and G. B. Phillips ibid. p. 11 13. 39 (a) B. B. Snider and E. A. Deutsch J. Org. Chem. 1982 47 745; (b) P. M. Wovkulich and M. R. UskokoviC ibid. p. 1600; (c) W. G. Dauben and T. Brookhart ibid. p. 3921. Reaction Mechanisms -Part (i)Pericyclic Reactions 45 The rate of reaction is at least an order of magnitude less rapid than for the (2)-isomer which gives the (20R)-steroid but the reasons for the retardation are not clear at present.Several important papers have appeared this year concerned with the transition- state geometry and mechanisms of ene reactions Lewis acid-catalysed ene reactions and superene reaction^.^^.^' Kinetic isotope-eff ect data for the addition of mesoxalic esters to allylben~ene~~" suggest that a symmetrically structured (2 + 2) charge- transfer complex is formed in a preliminary step followed by a pseudopericyclic transition state similar to that proposed previously41b for superene reactions. In particular the data imply a bent rather than a linear transition state for the H transfer. Further support comes from a study of the activation volumes and pressure dependence of rate constants for the thermal ene reactions of dimethyl mesoxalate with substituted alkene~.~'~ A concerted mechanism in which the transfer of the H atom occurs non-linearly [i.e.(AVf/Ae), ratios are close to unity] in a highly product like transition state is proposed. The SnC14-catalysed addition of diethyl mesoxalate to allylbenzene affords the oxetane (51)rather than the tetrahydrofuran (52).4uc This and previous results can be accomodated by assuming the preliminary formation of a three-membered charge-transfer complex for which there are two possible olefin/electrophile orientations one suitable for the formation of ene adduct (i.e.Markovnikov-type addition with olefins possessing bulky substituents) and the other for formation of oxetane (i.e.anti-Markovnikov-type addition where there is little steric hindrance between the ene and enophile substituents).The Stephenson isotope test using 2-(53) E-(53) and gem-(53) and [2H0]-(53)and 5:. 2 Et12 (51) (52) 2-(53) W = X = CD3 Y = CH3 E-(53) W = Y = CD3 X = CH3 gem-(53) W = CH3,X = Y = CD3 [2H12]-(53)has been applied to the addition of nitrosopentafluorobenzene to obtain the intra- and inter-molecular isotope effect^.^'" The lack of an intermolecular isotope effect compared with the large isotope effects with E-(53)and gern-(53) and the substantial difference in isotope effects for E-(53) compared with 2-(53) are regarded as strong evidence against cleavage of the allylic C-H(D) bond in the rate-determining step. Instead the results are consistent with the rate-deter- mining formation of an intermediate followed by cleavage of the allylic C-H(D) bond in a subsequent faster step with isotopic discrimination when the allylic C-H is cis to C-D [i.e.in E-(53) and gem-(53)].The finding that the reaction rate for the ene addition of 'Ago2 to alkenes is almost entirely determined by changes in 40 (a)H. Kwart and M. W. Brechbiel J. Org. Chem. 1982,47,3353; (b)G.Jenner and M. Papadopoulos ibid. p. 4201; (c)H. Kwart and M. Brechbiel ibid. p. 5409. 41 (a) C. A. Seymour and F. D. Greene J. Org. Chem. 1982 47 5226; (6)H. Miinsterer G. Kresze M. Brechbiel and H. Kwart ibid. p. 2677; (c) J. R. Hurst and G. B. Schuster J. Am. Chem. Soc. 1982,104,6854. 46 G. B. Gill AS*betweeen cis-olefins and non-cis-olefins lends support to a previous proposal that the inital encounter complex of '0 and olefin is not in the required reaction geometry.41" Movement to this geometry is the irreversible entropy-controlled process that brings the reactants to the transition state.The favoured ene transition states for the addition of glyoxylate N-sulphonylimine to prochiral olefins (e.g. cyclohexene and trans-but-2-ene) are those with the ester carbonyl group Highly regio- and stereo-selective SnC1,-catalysed intramolecular ene reaction is observed in the transformation (54) + (55).42bAsym-metric induction levels consistently above 93% were obtained in the additions of olefins to 8-phenylmenthol glyoxylate promoted by SnC14.42c 0 (54) (55) Photobisdecarbonylation of benzonorbornene-2,3-dioneat low temperatures afforded a diastereoisomeric mixture of 1-(indanyl)- lH-indenes which are con- sidered to arise by an [BT + 8~ + 2u] cyclodimerization (56)of intermediate isoindene by an ene-type me~hanism.,~ 4 Sigmatropic Rearrangements Optimized but semi-empirical CND0/2 and MIND0/3 calculations have been performed on the isolated molecule intramolecular [1,3] H shift that converts vinyl alcohol into a~etaldehyde.,~ Surprisingly the barrier to the forbidden suprafacial shift in the plane perpendicular to the molecule is lower than that of the allowed antarafacial shift in the plane of the molecule which is explained by the interaction of the migrating H atom with bonds.Existence of the relatively high barriers indicates that a spontaneous H shift in vinyl alcohol is essentially impossible.The difference between the allowed [1,5]H shift and the forbidden [1,3] and [1,7] H shifts can be traced to the orbital interaction between the migrating proton and the .rr-system of the remaining conjugated anion (the model system employed in the calc~lations).~~ The temperature dependence of the kinetic hydrogen isotope effect for the [1,5] sigmatropic rearrangement (57) (58) has been determined 42 (a) D. M. Tschaen and S. M. Weinreb Tetrahedron Lett. 1982 23 3015; (6) D. L. Lindner J. B. Doherty G. Shoham and R. B. Woodward ibid. p. 5111; (c) J. K. Whitesell A. Bhattacharya D. A. Aguilar and K. Henke J. Chem. SOC.,Chem. Commun.. 1982,989. 43 R. N. Warrener I. G.Pitt and R. A. Russell J. Chem. SOC.,Chem. Cornmun. 1982 1136; see also R. N. Warrener P. A. Harrison and R. A. Russell ibid. p. 1134. 44 M.Zakova and J. Leska Collect. Czech. Chem. Commun. 1982,47 1897. 4s G. Klopman 0.Kikuchi H. Moriishi and K.Suzuki Tetrahedron Lett. 1982 23 3447. Reaction Mechanisms -Part (i) Pericyclic Reactions (57) (E = C0,Me).46 Over a 65 "C temperature range kH/kDwas independent of tem- perature with AH/AD = 5.11 (average value) which can only be equated with an angular geometry for the H transfer; More O'Ferrall calculations provide an estimate of the angle at ca. 145 "C. Accordingly the [1,5] H shift in pentadiene itself [(AE) = 1.4 kcal mol-' and A,/AD = 1.15 from temperature dependence studies] is considered to occur by a concerted linear H-transfer mechanism.Thermal isomerization of (+)-2-deuterio-3,7-dimethyl-7-methoxycycloheptatriene (59) occurs by degenerate [1,5] carbon shifts with inversion by way of the norcaradiene form^.^' This reaction stereochemistry is favoured by least motion but forbidden by orbital symmetry theory and may result from subjacent orbital effects or may involve a diradical intermediate in which rotation at the tertiary radical site occurs much more slowly than closure to form the inversion product. Thermolysis of (0-tolylcarbony1)trimethylsilane at 450 "C-0.05 mm Hg afforded o-trimethylsilyl- rnethylben~aldehyde.~' It appears that the reaction path followed involves acylsilane to siloxycarbene rearrangement insertion of the carbene into the adjacent benzylic C-H bond to give the trimethylsilyloxybenzocyclobutane ring opening to the o-quinone methide and [1,5] sigmatropic shift of the silicon atom from oxygen to carbon.The preparation of 2,6-dicyano- 1,5-dimethylsemibullvalene(60) has been repor- Low-temperature I3C n.m.r. data indicates that (60) possesses an even lower activation barrier (4 kcal mol-') towards degenerate Cope rearrangement than the parent unsubstituted system. The presence of two discrete C=N and two C=C bands in the room-temperature i.r. spectrum however militates against C2-symmetry indicating that (60) hits not achieved homo-aromaticity on the i.r. time scale. The asymmetric induction observed in the thermal (240°C) and PdC12- catalysed Cope rearrangement of (3R,5E)-2,3-dimethyl-3-phenylhepta-1,5-diene 46 H.Kwart M. W. Brechbiel R. M. Acheson and D. C. Ward J. Am. Chem. Soc. 1982,104,4671. 47 J. E. Baldwin and B. M. Broline J. Am. Chem. SOC.,1982 104 2857. 48 C. Shih and J. S. Swenton J. Org. Chem. 1982,47 2668. *9 R. Askani and M. Littmann Tetrahedron Lett. 1982 23 3651; H. Quast J. Christ Y. Gorlach and W. van der Saal ibid. p. 3653; H. Quast Y. Gorlach G. Meichsner K. Peters E.-M. Peters and H. G. von Schnering ibid. p. 4677. 48 G. B. Gill at room temperature occur in the same sense and demonstrates that the PdC1,- catalysed process takes place preferentially (AAG' > 1.9 kcal mol-' at 25 "C) by way of a chair transition Kinetic data and activation parameters are in agreement with a concerted symmetry-allowed unturu-unturu Cope rearrangement for the transformation of (61) into (62) in boiling x~lene.~' Inductively stabilized a-sulphonyl carbanions greatly accelerate the Claisen rear- rangement.Thus treatment of (63) with potassium hydride (1.5 equiv.) in HMPA at 50°C for 3.5 h afforded on work-up the ketone (64).52 The regioselectivity is completely unaffected by the position of the double bond in the vinyl ether portion of the substrate and methyl substitution of the vinyl unit in (63) accelerated the rearrangement. Me Me 5 Electrocyclic Reactions Experimental evidence has been provided that the thermal ring-opening of cis-fused 3-aminocyclobutenes proceeds in a conrotatory fashion to give (initially) the cis,truns-monocyclic system e.g.(65) + (66).53 Irradiation of the diene (-)-(67) at 193 K in pentane with 254 nm light produced the thermally labile chiral triene (+)-(68); irradiation of the triene with 300 nm light at 193 K regenerated (-)-(67) demonstrating that least motion ('accordant') conrotatory processes occurred in the reverse photo reaction^.^^ At 218 K disrotatory cyclization of (68) occurred to give the achiral cis-hexahydronaphthalene (fIl2 = 29 min). g2Me $?2Me S C02Me C0,Me L. E. Overman and E. J. Jacobsen J. Am. Chem. Soc. 1982,104 7225. " R. K. Hall H. J. den Hertog and D. N. Reinhoudt J. Org. Chem. 1982 47,972. " S. E. Denmark and M. A. Harmata J. Am. Chem. SOC.,1982,104,4972. 53 G. W. Visser W. Verboom D. N. Reinhoudt S. Harkema and G. J.Van Hummel J. Am. Chem. SOC. 1982,104,6842. 54 B. Matuszewski A. W. Burgstahler and R. S. Givens 1.Am. Chem. SOC., 1982 104,6874. Reaction Mechanisms -Part (i) Pericyclic Reactions a3 In a series of four short papers Nicolaou et al. have outlined stepwise stereocon- trolled syntheses of endiandric acids A-G including biomimetic studies." The key step in the construction of the common bicyclo[4.2.0]octane moiety present in these systems was the conrotatory cyclization of a 1,8-disubstituted octa- 1,3,5,7-tetraene to a 7,8-disubstituted cyclo-octa-1,3,5-triene which then underwent disrotatory six-electron electrocyclization to give the bicyclic system. Further synthetic elabor- ation included the utilization of intramolecular Diels-Alder reactions to construct the additional carbocyclic rings.The vinylogous fidecene (69) possessed a half-life of ca. 5 min at 150 "Cbeing converted in 55% yield into the pentacyclic hydrocarbon (70).5" The reaction pathway visualized was (in sequence) an 18-electron conrotatory electrocyclization (symmetry-forbidden) sigmatropic [1,9] and [1,5] H shifts and a 14-electron disrotatory electrocyclization. " K. C. Nicolaou N. A. Petasis R. E. Zipkin and J. Uenishi J. Am. Chem. SOC.,1982 104 5555 5557; K. C. Nicolaou R. E. Zipkin and N. A. Petasis ibid. p. 5558 5560. A. Beck L. Knothe D. Hunkler and H. Prinzbach Tetrahedron Lett. 1982,23,2431.