CARBON-CARBON BOND-CLEAVAGE IN RADICAL-ANIONS OF STRAINED DIPHENYLETHANE DERIVATIVES

Fragmentation reactions of radical anions (mesolytic cleavages) of ,2, 2-tetraalkyl-1-(4'-nitrophenyl)-2-phenylethanes with varying degrees o f steric strain have been studied in polar solvents. The rates and act ivation parameters obtained were compared to the corresponding values obtained for homolytic bond scissions in the neutrals. The free energi es of activations for both types of reactions strongly depend on the s train present in the molecule. With the exception of the most crowded members of the series, nearly all of the strain is released in the tra nsition 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 hav e thermodynamic origins.