Thermodynamic control in ion radical cleavages through out-of-cage diffusion of products. Dynamics of C-C fragmentation in cation radicals of tert-butylated NADH analogues and other ion radicals

According to the nature of the alkyl group, cation radicals of NADH analogues alkylated para to the nitrogen atom (AHR), generated by direct or indirect electrochemical means, may undergo C-C fragmentation or deprotonation. The former reaction is dominant with the tert-butyl substituent and the latter with methyl and phenyl substituents. It is shown that the cleavage reaction produces AH(+) and the tert-butyl radical which is then rapidly oxidized to form the tert-butyl cation. Changing the A group allows a variation of the C-C fragmentation rate constant (determined by cyclic voltammetry or redox catalysis) by ca. six orders of magnitude for a change of ca. 0.4 eV in the standard free energy of the reaction. The logarithm of the rate constant varies linearly with the standard free energy of the reaction with a slope of 1/(60 meV) showing that fragmentation is kinetically controlled by the diffusion of the two fragments out of the solvent cage rather than by activation. The kinetic data thus allow an easy determination of the thermodynamics of the fragmentation. Analysis of previous rate data concerning an extended series of bibenzylic cation and anion radicals shows that they follow the same behavior.