Reductive and oxidative dissociative electron transfers: transition between the concerted and stepwise mechanistic pathways
The dissociative electron transfer reactions of a series of α-epoxyketones and tetra-n-butylammonium acetate have been examined by electrochemical and computational techniques.
Results for both the direct electrochemical (linear sweep voltammetry and convolution voltammetry) and indirect electrochemical (homogeneous redox catalysis) reductions of the epoxyketones are presented. In cases where the ring-closed radical anion generated by reduction of the epoxyketones is resonance stabilized (aromatic epoxyketones) the mechanism proceeds in a stepwise fashion, where the electron transfer and bond breaking reactions occur in sequential, discrete steps. On the other hand, where there is no additional resonance stabilization afforded to the ring-closed epoxide radical anion (aliphatic epoxyketones) the reaction proceeds in a concerted fashion, where electron transfer and ring cleavage occur simultaneously. The presence (or absence) of resonance stabilization in the ring-opened distonic radical anion plays little role in the kinetics of these dissociative electron transfers. Computations with the Density Functional Theory (B3-LYP and BHandH-LYP) on α-epoxyketones are also presented, and are in good agreement with the electrochemical results.
The oxidative dissociative electron transfers of the acetate anion in "dry" and "wet" (0.5 M H2O) acetonitrile were also characterized with direct and indirect electrochemical experiments, again utilizing linear sweep voltammetry, convolution voltammetry, and homogeneous redox catalysis. There is a significant change in the observed oxidation potential of the anion upon addition of water, as well as an apparent decrease in the intrinsic barrier to the electron transfer. The possible transition from a concerted to stepwise mechanism for the dissociative electron transfer of acetate upon addition of water is examined - the electrochemical data is compared to theoretical models for both the concerted and stepwise processes. It is determined that the indirect electrochemical experiments do not proceed through an outer sphere electron transfer. Additionally, it is shown that the difference between the direct oxidation of acetate in anhydrous and wet acetonitrile is unlikely to be the result of transition from a purely concerted mechanism to a purely stepwise mechanism based on thermodynamic considerations.