Asymmetric Electroreduction of Ketone and Aldehyde Derivatives to the Corresponding Alcohols Using Alcohol Dehydrogenase as an Electrocatalyst

Abstract

Asymmetric electroreduction of ketone and aldehyde derivatives was examined for two electrochemical reduction systems using alcohol dehydrogenase (ADH) as an electrocatalyst. The reaction system A is concerned with reduction of substrates catalyzed by ADH coupled with regeneration of cofactors by another enzyme with assistance of methyl viologen as an electron mediator, and the reaction system B is concerned with the use of ADH as the sole enzyme which catalyzes both reduction of substrates and regeneration of cofactors. In the latter case, a redox couple of phenethyl alcohol/acetophenone is used as an electron mediator to induce the reaction. The electrolysis using the system A allowed asymmetric reduction of acetophenone, propiophenone, phenoxy-2-propanone, pyruvic acid, and 2-phenylpropionaldehyde to the corresponding optically active alcohols with the enantiomer excesses (ee) close to 100% and the current efficiencies larger than 92%, and the turnover number of the cofactor higher than 50 was obtained for electrochemical reduction of phenoxy-2-propanone for 30 h. The reaction system B gave 100% ee for reduction of propiophenone, phenoxy-2-propanone, and pyruvic acid. However, the amount of products obtained was very small for reduction of benzoylformic acid, and a low enantiomer excess was obtained for reduction of phenylpropionaldehyde. Discussion is made focusing on what substrates are suitable for asymmetric reduction induced by the reaction system B.