. Can. J. Chem. 65, 2734Chem. 65, (1987.The photosensitized (electron transfer) reactions of 3-phenyl-2.3-dihydrobenzofuran @a), 5-methyl-3-phenyl-2,3-dihydrobenzofuran (8b), cis and trans-2-methoxy-1-phenylindane (9, cis and trans), 3,3-diphenyltetrahydrofuran (lo), and 22-diphenyl-1-methoxycyclopentane (11) have been studied using 1,4-dicyanobenzene as an electron-accepting photosensitizer and acetonitrile-methanol (3: 1) as solvent. These reaction conditions cause carbon-carbon bond cleavage of analogous acyclic P,P-diphenylethyl ethers to give products derived from the diphenylmethyl radical and the a-oxycarbocation intermediates. The purpose of this study was to determine if this reaction could be applied to five-membered cyclic derivatives to give 1,5-radical cations.The primary products from 8 a and 8b were the dehydrogenated, aromatized 3-phenylbenzofurans 140 and 14b. These products react further; continued irradiation gave the methanol adducts, cis and tmns-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a and 15b, cis and trans). The only observed reaction of the indanes (9, cis and trans) was cis-trans isomerization. Deuterium was incorporated at the bis-benzylic position of 8 and 9 when the irradiation was carried out in acetonitrilemethanol-0-d. These results are consistent with reversible deprotonation from the radical cations. 'There was no evidence for carbon-carbon bond cleavage with either 8 or 9. The relative rate, deprotonation faster than carbon-carbon bond cleavage, is explained in terms of the conformation of the bond that cleaves in relation to the singly occupied molecular orbital (SOMO) of the radical cation. Oxidation potential measurements support the conclusion that the SOMO of 8 and 9 is largely associated with the fused phenyl ring and is therefore orthogonal to the benzylic carbon-carbon bond. Irradiation of cis or trans-2-methoxy-3-phenyl-2,3-dihydrobenzofuran (15a, cis or trans), under these conditions, leads to cis-trans isomerization. The mechanism in this case involves the reversible loss of methanol. There is evidence that the addition of methanol to 14 involves the sensitizer radical anion -14 radical cation pair.In contrast with the fused bicyclic systems, the monocyclic tetrahydrofuran 10 and the methoxycyclopentane 11 both cleave under these conditions; the products are the expected acetals 22 and 29 formed from the intermediate 1,5-radical cations. In 10 and 11 the SOMO, which is largely associated with the diphenylmethyl moiety, can overlap with the adjacent carbon-carbon bond and cleavage occurs as in analogous acyclic systems. Both 10 and 11 are relatively stable to irradiation under conditions that are identical except with acetonitrile as solvent (without methanol). We found no evidence for cyclization of the intermediates (1,hadical cation or 1,5-diradical) into the terminal phenyl ring. DONALD R. ARNOLD, BRIAN J. FAHIE, LAURIE J. LAMONT, JACEK WIERZCHOWSKI et KENT M. YOUNG. Can. J. Chem. 65, 2734Chem. 65, (1987.Utilisant le dicyano-1,4 benzkne comme photosen...

The scope of the photosensitized (electron transfer) carbon-carbon bond cleavage involving radical cations has been defined for 2-phenylethyl ethers and acetals. The thresholds for reactivity of the monophenylethyl and gem-diphenylethyl derivatives are compared. While the radical cation of methyl 2,2-diphenylethyl ether (7) cleaves to give ultimately diphenylmethane (2) and dimethoxymethane (8), the radical cation of methyl 2-phenylethyl ether (9) was stable under these conditions. In contrast to the lack of reactivity of the radical cation of 9, the radical cations of methyl 2-phenyl-2-propyl ether ( l l ) , methyl 2-phenylcyclopentyl ether (13), and 2-phenylmethyl-l,3-dioxolane (16) cleave. Cleavage in the monophenylethyl series is limited to formation of a carbocation at least as stable as the secondary a-oxyalkyl or di-a-oxyalkyl. The basis for predicting this type of reactivity of radical cations is defined. The rate of carbon-carbon bond cleavage is increased by increasing the oxidation potential of the molecule, by decreasing the carbon-carbon bond strength, and (or) by decreasing the oxidation potential of that fragment that will become the carbocation. The results obtained from the reactions of 2-diphenylmethyl-l,3-dioxolane (14) and 2-phenylmethyl-1,3-dioxolane (16) cast doubt on the published oxidation potential for the 1,3-dioxolan-2-yl radical.Key words: photochemistry, radical cation, electron transfer, bond cleavage, radical. (8), le cation radical du mCthoxy-1-2-phtnylCthane (9) est stable dans ces conditions. En opposition au fait que le cation radical du composC 9 n'est pas reactif, les cations radicaux du 2-mCthoxy-2-phCnylpropane ( l l ) , du I-mtthoxy-2-phtnylcyclopentane (13) et du 2-phCnylmtthy1-1,3-dioxolane (16) se clivent. Le clivage dans la sCrie des composCs monophtnylCthylts est limit6 2 la formation d'un cation au moins aussi stable que les a-oxyalkyles ou di-a-oxyalkyles secondaires. On a dCfini les bases permettant de pridire ce type de reactivitt des cations radicaux. Les vitesses de clivage de la liaison carbone-carbone augmentent lorsqu'on augmente le potentiel d'oxydation de la molCcule, lorsqu'on diminue la force de la liaison carbone-carbone et (ou) lorsqu'on diminue le potentiel d'oxydation du fragment qui deviendra le carbocation. Les rtsultats obtenus lors des rtactions des 2,2-diphCnylmtthyl-l,3-dioxolane (14) et 2-phtnylmtthyl-l,3-dioxolane (16) permettent de mettre en doute les rCsultats qui ont Ct C publits relativement au potentiel d'oxydation du radical 1,3-dioxolan-2-yle.

. Can. J. Chem. 69, 225 (1991). The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-l,4-dioxaspiro[4,5]decane (8), methyl cis-and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-l,4-dioxaspiro[4.5]decane (lo), generated by photosensitized (electron transfer) irradiation, has been studied. Solutions of the ethers and acetals in acetonitrile-methanol (3:1), with 1,4-dicyanobenzene (2) serving as the electron acceptor, were irradiated with a medium-pressure mercury vapour lamp through Pyrex. The diphenyl derivatives 7 and 8 were reactive; 7 gave 6,6-diphenylhexanal dimethyl acetal(11) and 8 gave 2-methoxy-2-(5,5-diphenylpenty1)-1,3-dioxolane (12). These are the products expected from the intermediate 1,6-radical cation, formed upon carbon-carbon bond cleavage of the cyclic radical cation.The monophenyl derivatives 9cis and trans and 10 were stable under these irradiation conditions. The mechanism for the carbon-carbon bond cleavage and for the cis-trans isomerization is discussed. An explanation, based upon conformation, is offered for the lack of reactivity of 9 and 10. Molecular mechanics (MM2) calculations were used to determine the preferred conformation of 9cis and trans, and 10.Key words: photosensitization, electron transfer, radical cation, carbon-carbon bond cleavage, conformation. Chem. 69, 225 (1991). On a CtudiC la rCactivitC des cations radicalaires du mkthoxy-2,2,-diphCnylcyclohexane (7), du 6,6-diphCnyl-l,4-dioxaspiro [4,5] Introduction Photosensitized (electron transfer) carbon-carbon bond cleavage of five-membered cyclic compounds generates 1,5-radical cations (2). When the irradiation is carried out in the presence of a nucleophile, often the solvent, further reaction of these intermediates proceeds by nucleophilic attack on the cationic site, and reduction of the radical site by electron transfer from the radical anion of the ~hotosensitizer. followed by protonation of the resulting anion 12). A typical example is shown in reaction [I]: irradiation of an acetonitrile-methanol (3: 1) solution of methyl 2,2-diphenylcyclopentyl ether (1) and 1,4-dicyanobenzene (2) (an electron-accepting photosensitizer)

Irradiation of an acetonitrile solution of para-dicyanobenzene (I), 3,3-diphenyltetrahydrofuran (2), and magnesium perchlorate leads to the formation of 4,4-diphenyl-1,3-dioxane (7). Similarly, irradiation of an acetonitrile solution of 1, methyl 2,2-diphenylcyclopentyl ether (5), and magnesium perchlorate leads to the formation of 6,6-diphenyl-2-methoxytetrahydropyran (8). The mechanism proposed for these reactions involves formation of the radical cations 2f and Sf with the first excited singlet state of 1 acting as the photosensitizer (electron transfer). The radical cations 2f and 5: then cleave to give 1,5 radical cations. Reaction of the cationic site with water, followed by futher oxidation of the radical site by the perchlorate anion, gives the diphenylalkyl carbocation that can cyclize via a six-membered intermediate to the observed products 7 and 8. While the addition of other perchlorate salts (lithium and tetra-n-butylammonium) also lead to the formation of 7 and 8, lithium trifluoroacetate is ineffective. The proposal that water is involved as the nucleophile is supported by incorporation of 1 7 0 in the products 7 and 8 when 170-enriched water was added to the reaction mixture.Key words: photosensitization, electron transfer, radical cations, bond cleavage, 2,2-diphenylethyl ether.

In order to provide a firm basis for predicting the reactivity of diphenylethyl and phenylethyl ethers and acetals, the photolysis of compounds such as (I), (V), (VI), and (X) is studied.