Photosensitized (electron transfer) carbon-carbon bond cleavage of radical cations : The diphenylmethyl system

The photosensitized (electron transfer) reaction of methyl 2,2-diphenylethyl ether ( 1), 1,1,2,2-tetraphenylethane ( 5), 2-methyl-1,1,2-triphenylpropane ( 6), and 2-methoxy-2-diphenylmethylnorbornane (11 endo and exo) with 1,4-dicyanobenzene ( 4) in acetonitrile-methanol leads to products indicating cleavage of an intermediate radical cation to give the diphenylmethyl radical and a carbocation. The diphenylmethyl radical is then reduced by the radical anion of the photosensitizer and protonated to yield diphenylmethane. The carbocation fragment reacts with methanol to yield ether and/or acetals. The effect of temperature on the efficiency of cleavage of 5 and 6 has been analyzed. The increase in efficiency observed at higher temperatures reflects an activation energy for the cleavage of the radical cations. In cases where no cleavage is observed, the activation energy for cleavage may be so high that back electron transfer from the radical anion of the pbotosensitizer is the dominant reaction. The C—C bond dissociation energies of the radical cations of 5 and 6 were estimated by analysis of the thermochemical cycle using the bond dissociation energies and the oxidation potentials of the neutral molecules and the oxidation potential of the diphenylmethyl and cumyl radicals. The direction of cleavage of the radical cation is explained in terms of the relative oxidation potentials of the two possible radicals.