Unrevealing the Mechanism of Aminoxyl Catalyzed Electrochemical Oxidation of Alcohols By Unpolished Glassy Carbon Electrode

Abstract

2,2,6,6-tetramethyl-1-piperidine N-oxyl (TEMPO, 1) and related aminoxyl derivatives are important catalysts for alcohol oxidation. They are featured in the synthesis of pharmaceuticals and recently found considerable attention for the conversion of hydroxyl bearing biomass-derived feedstocks to value-added products. Aminoxyl compounds undergo facile redox reactions at electrode surfaces, enabling them to mediate electrochemical oxidation of alcohols.The well-known electron transfer reaction of these persistent radicals involves a reversible one electron oxidation to the corresponding oxoammonium species (TEMPO+ , 3), which are relatively strong oxidants. Aminoxyl radicals can also undergo a proton coupled reduction to hydroxylamines (2), which is quasireversible and less known. However, for many of their electrosynthetic reactions, the catalytic cycle involves not only aminoxyl/oxoammonium redox couple, but also formation of hydroxylamines and regeneration of the oxoammonium from the hydroxylamine. This two steps oxidation has been proposed to occur via two distinct pathways: the comproportionation reaction between the hydroxylamine and a second oxoammonium cation to form aminoxyloxyl radical (two) followed by oxidation of aminoxyl radicals to oxoammonium (pathway b), or the two-electron oxidation of the hydroxylamine to the oxoammonium (pathway a). The electron transfer of the hydroxylamine, at the electrode surface, is a quasireversible ill-defined reaction that strongly depends on the electrode material and conditionings, and its detailed understanding is still lacking. Herein, we investigated the electron transfer reaction of TEMPO and its hydroxylamine, is formed during catalytic alcohol oxidation, using the glassy carbon electrodes that treated under different conditions. Well-treated electrode facilitates the electron transfer of hydroxylamine and enables two-electrons oxidation to oxoammonium, whereas on the untreated electrode, two separated one-electron oxidations are observed that enables measuring the rate of generation and consumption of hydroxylamine. Understanding of how each regeneration pathway affects the catalytic behavior made it possible to comprehensively understand all the features of the current profiles obtained under various alcohol oxidation conditions. Figure 1. Possible mechanism for regeneration of oxoammonium during aminoxyl catalyzed alcohol oxidation. Figure 1