. Can. J. Chem. 69,839 (1991). The effect of mera-and para-methoxy substitution on the reactivity of some radical cations has been determined. The compounds chosen for study were 1-(3-methoxypheny1)-1-phenylethylene (7), 1-(4-methoxypheny1)-1-phenylethylene (S), 3-(3-methoxypheny1)indene (9), 3-(4-methoxypheny1)indene (lo), methyl 2-(3-methoxypheny1)-2-phenylethyl ether ( l l ) , methyl 2-(4-methoxypheny1)-2-phenylethyl ether (l2), cis-and trans-2-methoxy-1-(3-methoxypheny1)indane (13), and cis-and trat1s-2-methoxy-l-(4-methoxyphenyl)indane (14). The radical cations of these compounds were generated by photosensitization (electron transfer) using 1,4-dicyanobenzene (3) as the electron acceptor. The three reactions studied were: (1) The addition of nucleophiles (methanol) to the radical cation of the arylalkenes, a reaction that yields the anti-Markovnikov addition product.(2) The carbon-carbon bond cleavage of radical cations, which yields products derived from the radical and carbocation fragments. (3) The deprotonation of the radical cation, a reaction that can be used to invert the configuration at a saturated carbon centre. The mechanisms of these reactions are discussed and the factors that need to be considered in order to predict reactivity are defined. Molecular orbital calculations (UHFISTO-3G) were carried out on the radical cations of the model compounds 3-and 4-vinylanisole and 3-and 4-methylanisole.Key words: photochemistry, photosensitize (electron transfer), radical cation, radical DONALD R. ARNOLD, XINYAO DU et KERSTIN M. HENSELEIT. Can. J. Chem. 69,839 (1991). On a dttermint l'effet de substituants en me' ta et en para sur la rCactivitC de quelques radicaux cations. Les composCs choisis pour cette ttude sont : les 1-(3-mtthoxyphtny1)-1-phtnylCthyl&ne (7), 1 -(4-mCthoxyphtny1)-1 -phtnyltthylkne (S), 3-(3-mtthoxyphtny1)indkne (9), 3-(4-mtthoxyphtnyl)ind&ne (lo), 1-mtthoxy-2-(3-mtthoxyphtny1)-2-phCnyltthane ( l l ) , I-mtthoxy-2-(4-mCthoxyphtny1-2-phtnyltthane (12), les cis-et trans-2-mtthoxy-I-(3-mtthoxyphtny1)indane (13) et les cis-et trans-2-mtthoxy-1-(4-mtthoxyphtny1)indane (14). On a gCnCrC les cations radicaux de ces composts par photosensibilisation (transfert d'Clectron), en utilisant le 1,4-dicyanobenzkne (3) comme accepteur d'tlectrons. Les trois rtactions ttudites sont: (1) l'addition de nuclCophiles (methanol) aux cations radicaux des arylalcknes, une rtaction qui conduit ?I un produit d'addition anti-Markovnikov; (2) le clivage de la liaison carbone-carbone des cations radicaux qui conduit ?I des produits qui dtrivent des fragments radicalaires et carbocationiques et (3) la dtprotonation des cations radicaux qui peut Ctre utiliste pour inverser la configuration ?I un site carbon6 saturt. On discute des mCcanismes de ces rkactions et on dtfinit les facteurs qu'il faut considtrer afin de pouvoir prtdire la rtactivitC. On a effectut des calculs d'orbitales moltculaires (UHF/ STO-3G) sur les cations radicaux des composts modkles, 3-et 4-vinylanisole et 3-et 4-mtthylanisole.

The effect of electron-withdrawing substituents, mera-orpnr~l-cyano, on the rcactivity of the radical cation of arylalkenes and alkanes has been determined. The radical cations were generated by single electron transfer (set) to an electron-accepting photosensitizer. Three reactions were studied: (i) the addition of nucleophile to the radical cation of arylalkenes, (ii) cleavage of the benzylic carbon-carbon bond of the radical cation of arylalkanes; and (iii) the deprotonation of the benzylic carbon-hydrogen bond of the radical cation of arylalkanes. The radical cations of 4-(1-pheny1ethenyl)benzonitrile (lb), 3-(1-phenylethenyl)benzonitrile (lc), 4-(2-methoxy-I-phenylethy1)benzonitrile (20). 3-(2-methoxy-I-phenylethy1)benzonitrile ( 2~) .cis-and tr.crrls-5-cyano-2-methoxy-1-phenylindane (60-cis and -rratls), and 6-cyano-3-phenylindene (7B) were generated, by single electron transfer to the lowest excited singlet state of 1,4-dicyanobenzene (3), in acetonitrile-methanol. The radical cations of lb, lc, and 70 react with methanol to yield the nrzri-Markovnikov adducts (20,2c, and 60-cis and 60-rmns). The radical cations of 2 0 . 2~. and 60-rrcrrzs cleave at the benzylic carbonsarbon bond to give products derived from the radical and carbocation fragments. The radical cation of 60-cis deprotonates from the benzylic position with subsequent formation of the diastereomer, 60-rrarls. This behaviour can be explained1 predicted on the basis of the proposed mechanisms for these reactions. Molecular orbital calculations (AM 1) support the conclusions.Key words: photosensitized, electron transfer, radical ions, radicals, molecular orbital calculations (AMI).RCsumC : On a dCterminC I'effet de substituants Clectroattracteur, les groupes rrle'ra-et pcrrcr-cyano, sur la rCactivitC de cations radicaux d'arylalcknes et alcanes. On a gCnCrC les cations radicaux par le transfert d'un seul Clectron i un photosensibilisateur accepteur d'klectron. On a CtudiC trois rCactions : (i) l'addition d'un nuclCophile sur le cation radical des arylalcknes; (ii) le clivage de la liaison carbone-carbone benzylique du cation radical des arylalcanes et (iii) la dCprotonation de la liaison carbone-hydrogkne benzylique du cation radical des arylalcanes. On a gCnCrC les cations radicaux des 4-(1-phCnylCthCny1)benzonitrile (10). 3-(1-phCnylCthCny1)-benzonitrile (lc), 4-(2-mCthoxy-1-phCny1Cthyl)benzonitrile (20). 3-(2-mCthoxy-I-phCnylCthy1)benzonitrile (2c), cis-et r,nr1~-5-cyan0-2-mCthoxy-I -phCnylindane (6b-cis et -fmrls) et 6-cyano-3-phCnylindkne (70) par un transfert d'un seul Clectron j. l'Ctat singulet excite le plus bas du 1,4-dicyanobenzkne (3), en solution dans l'acCtonitrile-mCthano1. Les cations radicaux des composCs lb, l c et 70 rkagissent avec le methanol pour donner des adduits arlri-Markovnikov (2b, 2c et 60-cis et 60-rrarls). Les radicaux cations 2 0 . 2~ et 60-rrans se clivent au niveau de la liaison carbone-carbone benzylique pour donner des produits dCrivCs des fragments radicalaires et carbocationiques. Le cation radica...

. Can. J. Chem. 69, 1365 (1991).Carbon~arbon bond cleavage of the radical cations of 1-butene (C4H8t) and 4,4-dimethyl-1-pentene (C7H14t) will generate the allyl and alkyl radical and carbocation fragments. Alternative bonding arrangements between the allyl and methyl moieties in C4H8f and between the allyl and tert-butyl moieties in C7HI4?, possible metastable intermediates or transition states preceding complete separation of the fragments, have been investigated by ab initio molecular orbital calculations. Structures were fully optimized at the UHFl6-31G* or UHFISTO-3G levels, and some of the calculations on C4H8t were expanded with single point MP216-31G*l~16-31G* computations. The C4Hst radical cation, having a structure similar to that of 1-butene, is more stable than the separated fragments: 183 kJ mol-I lower in energy than the sum of the energies of the allyl cation and the methyl radical, and 385 kJ mol-I lower than the sum of the energies of an allyl radical and a methyl cation, at the MP216-31G* level. The corresponding values at the UHFISTO-3G level are 276 and 415 kJ mol-', respectively. There is less bonding interaction between the allyl and tert-butyl moieties in C7HI4t. The summation of the energies of the allyl radical and tert-butyl cation is 123 kJ mol-' lower than the summation of the energies of the allyl cation and tert-butyl radical, and 115 kJ mol-I higher in energy than the bonded radical cation C7H14?, at the UHFISTO-3G level. These calculated values are compared with thermochemical data and with experimental results on the cleavage of these, and related, radical cations.Key words: radical cation, cleavage, ab initio calculations, electron transfer, photochemistry.

. Can. J. Chem. 72,403 (1994).The reactivity of the radical cations of a-and P-pinene (8 and 9), tricyclene (la), and nopol (23) has been studied. The radical ions were generated, in acetonitrile-methanol (3:1), by single electron transfer (set) to the singlet excited state of 1,4-dicyanobenzene (1). Biphenyl (3) was used as a codonor. The cyclobutane rings of the initially formed radical cations of a-and P-pinene (8' ' and 9") cleave to distonic radical cations that react as tertiary alkyl cations and allylic radicals. The results of a b initio molecular orbital calculations (STO-3G) are consistent with the observation that the positive charge is largely associated with the tertiary alkyl moiety while the spin density is largely distributed over the allylic radical. There was no evidence, experimental or theoretical, indicative of a bonding interaction between the cationic and allylic moieties of these distonic radical cations. The cyclobutane ring cleavage is irreversible. The radical cation of tricyclene (18) also gives 1 : 1: 1 (nucleophile:cyclopropy1:aromatic) adducts. The regio-and stereochemistry of the adducts, and the nucleophile selectivity, are all indicative of nucleophile-assisted cleavage of the radical cation (18"). The radical cation of nopol (23") also cleaves to the distonic radical cation. The cyclobutane ring cleavage must be rapid; intramolecular 1,5-endo cyclization of the hydroxyl group cannot compete. DONALD R. ARNOLD et XINYAO Du. Can. J. Chem. 72,403 (1994).On a CtudiC la rCactivitt des cations radicalaires des a-et P-pinbnes (8 et 9), du tricyclbne (18) et du nopol (23). On a gtntrts les ions radicalaires dans un mtlange acttonitrile-mtthanol(3 : 1) par un transfert d'un seul tlectron (set) 2i l'ttat excitt singulet du 1,4-dicyanobenzbne (1). On a utilist le biphtnyle (3) comme codonneur. Les noyaux cyclobutane des cations radicalaires qui se foment initialement B partir des a-et P-pinbnes (8'' et 9") se clivent en cations radicalaires distoniques qui rtagissent comme cations alkyles tertiaires et radicaux allyliques. Les rtsultats de calculs d'orbitales molCculaires a b initio (STO-3G) sont en accord avec l'observation que la charge positive est associte largement avec la portion alkyle tertiaire alors que la densitt de spin est largement distribuke sur le radical allylique. 11 n'y a pas d'indications, soit exptrimentale ou thtorique, pour suggtrer l'existence d'une interaction de liaison entre les portions cationiques et allyliques de ces cations radicalaires distoniques. Le clivage du noyau cyclobutane est irrtversible. Le cation radicalaire du tricyclbne (18) donne aussi des adduits 1 : 1 : 1 (nuclCophile : cyclopropyle : aromatique). La rtgio-et la sttrtochimie des adduits et la stlectivitt du nucltophile indiquent toutes que le clivage du cation radicalaire (18") est assist6 par le nucltophile. Le cation radicalaire du nopol (23") se clive aussi en cation radicalaire distonique. Le clivage du noyau cyclobutane doit Etre rapide car la cyclisation intramoltculaire 1,5-endo du grou...

When a solution of 1,4-dicyanobenzene (1), and 1-methylcyclohexene (3) in acetonitrile-methanol (3:1) is irradiated, the 1:1:1 (alcohol/olefin/aromatic) adducts 4-7 are obtained. Re-