Hydrogen atom transfer oxidation of primary and secondary alcoholates into aldehydes and ketones by aromatic halides in liquid ammonia. A new electrochemically induceable reaction

Abstract

It is possible to induce the oxidation of alcoholates into the corresponding carbonyl compounds by electrochemical reduction of aromatic halides in liquid ammonia, Le., to electrochemically trigger the reaction ArX + >CH-OArH + >C=O + X-. H-Atom transfer from the alcoholate to the aryl radical formed upon reduction of the aryl halide appears as the key step of the oxidation process. The ketyl anion radical thus formed can be oxidized into the parent carbonyl compound, remain electrochemically stable, or be reduced into the dianion depending upon the location of the two corresponding standard potentials toward the reduction potential of the aryl halide. Electricity consumption thus tends toward 0, 1, and 2 F/mol for the three cases, respectively. The reactions competing with H-atom transfer, thus lowering the efficiency of the electrochemical inducement of the oxidation process, are electron transfers to the aryl radical which occur at the electrode surface and/or in the solution. These will play the role of termination steps for the corresponding chain systems involving homogeneous initiation of the reaction. The kinetic analysis of the competition between H-atom transfer and homogeneous or heterogeneous electron transfer allows a detailed investigation of the reaction mechanism by electrochemical techniques such as cyclic voltammetry. This also leads to the determination of the rate constant of H-atom transfer of the alcoholate-aryl radical couple. Electrochemical inducement of a chemical reactionza is based upon the reactivity of the system being larger at a higher or a lower oxidation level than at the original one. A few examples of such processes have been described in organic or coordination chemistry? This reaction that has been the most extensively studied in this respect is SRNl aromatic nucleophilic s~bs t i t u t ion ,~ the electrochemical inducement”Sk of which is based on the following reaction sequence: ArX + l e 9 ArX-. ArX-. Ar. + XAr. + NuArNu-.