The electrochemistry of TEMPO-mediated oxidation of alcohols in ionic liquid

The detailed electrochemistry of TEMPO-mediated oxidation of alcohols to carbonyl compounds in ionic liquid medium is presented. It is shown that TEMPO diffusion currents are suppressed in IL medium, relative to acetonitrile solvent, due to the high viscosity of the IL medium; however, the presence of substrate and base reagent significantly reduces IL viscosity, improving performances significantly. Cyclic voltametry shows that the kinetics of the reaction between electrooxidized TEMPO and the alcohol are similar in both media. It was also observed that in the case where the oxidation product is non-enolizable, electrolyses (diaphragm cell) with catalytic quantities of TEMPO, and in the presence of base (2,6-lutidine), results in close to 100% Faradaic and 100% chemical efficiencies. In contrast, prolonged electrolysis of alcohols yielding enolizable products results in catalyst deactivation due to the irreversible reaction between the active form of the catalyst and the product, which ultimately leads to low product yields and catalyst loss. Under these circumstances, reasonable Faradaic and chemical selectivities can only be obtained with partial electrolyses (to ≤1F/mol).