Radical ions in photochemistry. 16. The photosensitized (electron transfer) carbon–carbon bond-cleavage reaction of radical cations

Abstract

The photosensitized (electron transfer) carbon–carbon bond cleavage of 1,1,2,2-tetraphenylethane (1b), 2-(4-methoxy-phenyl)-1,1-diphenyl-2-niethylpropane (1c), 1,1,2-triphenyl-2-methylpropane (1d), and 2-(4-trifluoromethylphenyl)-1,1-diphenyl-2-methylpropane (1e) has been studied with 1,4-dicyanobenzene (2) serving as the electron-accepting sensitizer. The oxidation potential of 1b–e have been measured by cyclic voltammetry. Estimation of the free-energy associated with the electron transfer between these donors (1b–e) and the first excited singlet state of 2, using the Weller equation, indicates the process is favorable in every case. There is, in every case, a one-to-one correspondence of products derived from reaction of the carbocation fragment and from the carbanion derived from the initially formed radical fragment. The efficiency of the reaction of 1b, which gives a good yield of diphenylmethane (3) and methyl diphenylmethyl ether, has been studied as a function of temperature. The observed activation energy (7.2 kcal mol−1) is equated to the bond dissociation energy of the central bond of the radical cation (1b+•). The ratio of products from the unsymmetric compounds is dependent upon the redox potentials of the fragment radicals and carbocations.