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 bo nds. The observed enthalpies of activation are significantly lower tha n those measured for the homolysis of the corresponding neutral substr ates. The entropies of activation are small or negative, despite produ ction of fragments and partial freeing of rotational degrees of freedo m upon fragmentation. These activation parameters are solvent dependen t, generally decreasing in solvents with higher polarity in a self-con pensatory manner, leading to the free energies of activation (and rate constants) that remain constant in different solvents. The free energ ies of activation depend on strain present in the structures, with ca. 63% of the strain relatived in the transition state. The fragmentatio n reactions of 2(.+) have activation energies that are on average 23 k cal/mol lower than those for homolysis of 2. The observed activation o f single bonds for scission has thermodynamics origins. The fragmentat ion reactions of radical cations are also compared to the fragmentatio n reactions of the analogous radical anions.