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

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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.

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