Mesolytic scission of C-C bonds in radical cations of amino derivatives: Steric and solvent effects

The radical cations of 1,2-dialkyl-1-(4'-(dimethylamino)phenyl (2(.+)) have been observed to undergo unimolecular cleavage of central C-C bonds. The observed enthalpies of activation are significantly lower than those measured for the homolysis of the corresponding neutral substrates. The entropies of activation are small or negative, despite production of fragments and partial freeing of rotational degrees of freedom upon fragmentation. These activation parameters are solvent dependent, generally decreasing in solvents with higher polarity in a self-conpensatory manner, leading to the free energies of activation (and rate constants) that remain constant in different solvents. The free energies of activation depend on strain present in the structures, with ca. 63% of the strain relatived in the transition state. The fragmentation reactions of 2(.+) have activation energies that are on average 23 kcal/mol lower than those for homolysis of 2. The observed activation of single bonds for scission has thermodynamics origins. The fragmentation reactions of radical cations are also compared to the fragmentation reactions of the analogous radical anions.