The Solvent as H-Atom Donor in Organic Electrochemical Reactions. Reduction of Aromatic Halides

Article abstract of DOI:10.1021/ja00532a023

The first step following the initial formation of the anionic radical in the electrochemical reduction of aromatic halides in the cleavage of the C-X bond leading to the neutral Ar^. radical.The latter species undergoes three concurrent reactions: H-atom abstraction from the solvent and electron transfer at the electrode and/or from the initial anion radical.The quantitative analysis of this threefold competition allows one to predict the effect of the intrinsic (rate constants) and operational (concentration, stirring rate, cell geometry) parameters.It is the basis on which the results of deuterium incorporation by deuterated water or solvent can be used as a tool to investigate the reaction mechanism and to determine the characteristic rate constants.This is illustrated by the study of the chloro, bromo, and iodo derivatives of three aromatic residues (9-anthryl, 1-naphthyl, and 4-cyanophenyl) in Me2SO and acetonitrile.It is shown that the proposed reduction mechanism is indeed followed in each case and the degree of competition between the three concurrent steps can be evaluated.Using the values obtained independently (by cyclic voltammetry or redox catalysis) for the cleavage rate of the anion radicals of the chloro compounds, the corresponding rates for the bromo and iodo derivatives are determined as well as the rates of H-atom transfer to the three aromatic radicals and the magnitude of the corresponding deuterium isotope effects.