Heterogeneous and homogeneous redox catalysed reduction of some sulfilimines

Abstract

The homogeneous and electrode two-electron reduction of some sulfonylsfulfilimines was investigated in order to obtain more quantitative kinetic and thermodynamic information.The two processes were analysed on the basis of homogeneous redox catalysis and voltammetric results, respectively. Both reactions were shown, in most cases, to be controlled by the first endergonic electron uptake. The kinetic data for the process were interpreted on the basis of simple theories for electron-transfer reactions, allowing intrinsic thermodynamic and activation parameters of the charge-transfer steps to be estimated. The results were found to be more consistent with the hypothesis of a simple outer-sphere electron transfer followed by a fast first-order reaction rather than with a concerted bond breaking-charge transfer step. No relevant differences were observed owing to the replacement of ethyl with phenyl groups in the sulfilimine moiety, the substitution effect being apparent only from a thermodynamic point of view. On the contrary, the introduction of a CN group in the sulfonyl moiety increased sensibly the stability of the intermediate radical anion. This caused the electrode process kinetics to be partially dependent on the rdaical anion decay, allowing the quantitative evaluation of its lifetime.