Concerted and stepwise dissociative electron transfers. Oxidability of the leaving group and strength of the breaking bond as mechanism and reactivity governing factors illustrated by the electrochemical reduction of alpha-substituted acetophenones

The cyclic voltammetric investigation of a series of a-substituted acetophenones allowed the identification of the concerted and stepwise character of the dissociative electron transfer reaction, and, in the stepwise cases, the determination of the cleavage rate constants and the standard potentials for the formation of the anion radical. Analysis of the data, using thermodynamical parameters derived from experiment and from literature points to three mechanism governing factors, the oxidability of the leaving group, the bond dissociation energy of the bond being broken, and the LUMO energy. The first of these factors appears to be largely predominant in many cases in the control of the concerted vs stepwise dichotomy. The fluoro substituent provides a reverse example where the bond strength overcomes the unfavorable effect of the leaving group oxidability. It is also an exception, in terms of anion radical cleavage reactivity, where the strength of the C-F bond significantly contributes to slow down the cleavage as opposed to the other substituents where solvent reorganization appears as largely predominant. In the concerted cases, the estimated lifetime of the anion radical is clearly larger than the time of a vibration. The concerted character of the reaction thus results from an energetic advantage rather than from the ''nonexistence'' of the anion radical intermediate.