Transition-State Polarization in Cleavage of C-C Bonds in Radical Anions

The substituent effect on the rate of C-C bond cleavage in radical anions of l-(4-nitrophenyl)-2-(substituted-phenyl)-l,l,2,2-tetraethylethanes has been explored. The data provide evidence for two distinctive modes of bond scission. One mode is characterized by a significant negative charge transfer across the scissile bond in the transition state. Such polarization of the transition state is in contradiction to the prediction based on the fragments’ stability. The second mode, dominant in cases where the charge shift leads to negative charge accumulation on an already electron-rich fragment, involves a σ* radical anion. Both modes point to a general kinetic preference for a cleavage of radical anions that allows for charge delocalization across the scissile bond.