1, n -Radical ions. Photosensitized (electron transfer) and electrochemical oxidation of 1,1,2,2-tetraphenylcyclopropane

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. Can. J . Chem. 70, 272 ( 1992). Irradiation of an acetonitrile solution of cis 1-methyl-2-phenylcyclopentane ( l b cis); 1,4-dicyanobenzene (2), an electronaccepting photosensitizer; and 2,4,6-collidine (3), a nonnucleophilic base, leads to configurational isomerization of the cyclopentane; the photostationary state lies >99% in favour of the trans isomer. The mechanism proposed for this reaction involves formation of the radical cation of l b cis by photoinduced electron transfer to the singlet excited state of 2, deprotonation of the radical cation assisted by the base 3, reduction of the resulting benzylic radical by the radical anion 2;, and reprotonation of the benzylic anion to give both the cis and the tratls isomers of lb. The photostationary state is controlled by the relative rates of deprotonation of the radical cations of l b cis and trans; these rates are dependent upon the extent of overlap of the SOMO of the radical cation, which is largely associated with the phenyl ring, and the benzylic carbon-hydrogen bond. Molecular mechanics calculations (MM3 and MMP2) are used to calculate the preferred conformations of the isomers. The required orbital overlap is 3 1 % effective with the global minimum conformation of the cis isomer and essentially ineffective for the low-lying conformations of the trcrns isomer. This proposed mechanism is supported by Stern-Volmer quenching studies, which indicate that both isomers quench the singlet excited state of 2 at the diffusion-controlled rate, and by deuterium incorporation studies. When irradiation of the cis isomer is carried out in acetonitrile-methanol-0-d as solvent, isomerization is accompanied by deuterium exchange at the benzylic position; the trat~s isomer is stable under these conditions.Key words: photosensitized electron transfer, radical cation, deprotonation, configurational isomerization, conformation, molecular mechanics (MM3). conduit a une isomerisation configurationnelle du cyclopentane; 1'Ctat photostationnaire favorise I'isorqere trans i plus de 99%. Le mecanisme propose pour cette reaction implique la formation du cation radicalaire du produit l b cis par le biais d'un transfert d'electron photo-induit vers 1'Ctat excite singulet du produit 2, une deprotonation du cation radicalaire assistee par la base 3 , la reduction, par I'anion radicalaire 2;; du radical benzylique qui en resulte et un reprotonation de l'anion benzylique qui fournit les isomeres cis et trans du produit l b . L'etat photostationnaire est contr61C par les vitesses relatives de deprotonation des cations radicalaires des produits l b cis et trans; ces vitesses dependent du degre de recouvrement de la liaison carbone-hydrogene benzylique et des orbitales molCculaires SO du cation radicalaire qui sont trks associkes au noyau phknyle. On a fait appel a des calculs de mkcanique molCculaire (MM3 et MMP2) pour calculer les conformations privilCgiCs des isomkres. Le recouvrement orbitalaire requis est efficace a 3 1% dans la conformation globale minimale de I'isomkre cis et est ...

Section: Electrochetnical Tneasuretnetztsmentioning

confidence: 99%

The importance of conformation in the reactivity of radical cations. Changing configuration at saturated carbon centres

Perrott¹,

Arnold²

1992

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“…The apparatus has been described previously (30). The working electrode was a platinum sphere (1 mm), the counter electrode was a platinum wire, and a saturated calomel electrode was used as the reference.…”

Section: General Infortnationmentioning

confidence: 99%

The effect of meta- and para-methoxy substitution on the reactivity of the radical cations of arylalkenes and alkanes. Radical ions in photochemistry. Part 26

Arnold¹,

Du²,

Henseleit³

1991

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. 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.

“…Dichloromethane (Fisher) was distilled from P2O5 and stored over 4A molecular sieves. The electrochemical cell and apparatus have been described previously (2). Typically, concentrations of 0.001 M (1) were used in a total cell volume of 10 mL.…”

Section: Methodsmentioning

confidence: 99%

“…The photosensitized (electron transfer) reactions of l a l a involve the same intermediate (2~'). In this case, the ratio of 3 to 4 depends on the choice of sensitizer for the reaction (2).…”

Section: Introductionmentioning

confidence: 99%

“…Because of the stabilizing effect of the tetraphenyl substitution, we had hoped to obtain some direct evidence for the involvement of this species during the electrochemical oxidation of l a . Radical cations are generally more stable in dichloromethane than in acetonitrile (3), but even in this solvent, at sweep rates as high as 20 V s-', no evidence for a single one-electron transfer was obtained (2).…”

Section: Introductionmentioning

confidence: 99%

See 1 more Smart Citation

The effect of substituents on the electrochemical oxidation potentials of 1,1,2,2-tetraphenylcyclopropane and 1,1,3,3-tetraphenylpropenyl radical

Wayner¹,

Arnold²

1986

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. Can. J. Chem. 64, 100 (1986).The effects of multiple substitution by methoxy and cyano in the 4-position(s) of 1,1,2,2-tetraphenylcyclopropane (I) and 1,1,3,3-tetraphenylpropenyl radical (2') on the oxidation potentials have been measured. The results indicate that the oxidation process for the cyclopropanes (la-m) is irreversible. Nevertheless, the substituent effect on the potential is essentially additive and correlates reasonably well with Xuf. A slow electron transfer process (an overall conversion of 1 to 2+ ), with a transition state resembling the ring-opened radical cation, is consistent with these observations. The oxidation of the radical 2' is quasi-reversible in dichloromethane. In this case also, the substituent effect on the oxidation potential is additive and correlates with Xu'. Any deviation from planarity of 2' is not sufficient to prevent substituents from exerting normal (additive) behaviour.