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.
