Carbon-carbon bond cleavage in radical anions of strained diphenylethane derivatives

Fragmentation reactions of radical anions (mesolytic cleavages) of 1,1,2,2-tetraalkyl-1-(4'-nitrophenyl)-2-phenylethanes with varying degrees of steric strain have been studied in polar solvents. The rates and activation parameters obtained were compared to the corresponding values obtained for homolytic bond scissions in the neutrals. The free energies of activations for both types of reactions strongly depend on the strain present in the molecule. With the exception of the most crowded members of the series, nearly all of the strain is released in the transition state of homolytic reactions, and ca. 77% of it is relieved in the transition states of mesolysis. The mesolytic fragmentations have activation energies that are on average ca. 12.5 kcal/mol lower than the homolytic processes. The observed redox activation is shown to have thermodynamic origins.