Photosensitized (electron transfer) carbon–carbon bond cleavage of radical cations: the 2-phenylethyl ether and acetal systems

Abstract

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 (11), methyl 2-phenylcyclopentyl ether (13), and 2-phenylmethyl-1,3-dioxolane (16) cleave. Cleavage in the monophenylethyl series is limited to formation of a carbocation at least as stable as the secondary α-oxyalkyl or di-α-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-1,3-dioxolane (14) and 2-phenylmethyl-1,3-dioxolane (16) cast doubt on the published oxidation potential for the 1,3-dioxolan-2-yl radical. Keywords: photochemistry, radical cation, electron transfer, bond cleavage, radical.