1,n-Radical ions. Photosensitized (electron transfer) carbon–carbon bond cleavage. Formation of 1,6-radical cations

Abstract

The reactivity of the radical cations of methyl 2,2-diphenylcyclohexyl ether (7), 6,6-diphenyl-1,4-dioxaspiro[4.5]decane (8), methyl cis- and trans-2-phenylcyclohexyl ether (9cis and trans), and 6-phenyl-1,4-dioxaspiro[4.5]decane (10), 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-diphenylpentyl)-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.