Electroenzymatic NADH Recycling for Selective in-Flow Biocatalytic Reductions

Abstract

Demand for greater environmental and economic sustainability in organic syntheses is driven by the chemical industry’s desire to reduce energy consumption, eliminate undesirable side products and develop more efficient reactions. There is growing interest in electrosynthesis for organic redox reactions, utilizing electrodes as a very clean electron source or sink,[1] but it is currently underrepresented in organic synthesis due to a range of limitations, particularly with selectivity and difficulties in implementation. Currently 10-20% of industrial transformations are catalytic hydrogenations, often using noble-metals.[2] These catalysts frequently require high pressures of hydrogen and can suffer from poor chemo- and enantio-selectivity. This study demonstrates bioelectrocatalysis for selective C=X bond reductions, offering an alternative to traditional hydrogenations. Biocatalysts such as oxidoreductase enzymes offer exquisite selectivity under mild reaction conditions, but have proved difficult to exploit in electrosynthesis. These enzymes often require stoichiometric hydride transfer from the expensive biological NAD(P)H cofactor, which means that their use is only viable when they are coupled with an efficient cofactor recycling method. Glucose or isopropanol in super-stoichiometric quantities are often used to recycle the reduced cofactor, NADH, but this approach suffers from poor atom economy. Electrochemically-driven regeneration of the NADH cofactor is problematic at unfunctionalized electrodes due to formation of non-active forms of the reduced cofactor. Here, we present an electrochemically-driven NADH regeneration system with NAD+-reductase modified carbon electrodes, offering perfect selectivity for the active NADH cofactor. We show that we can immobilize this enzyme onto carbon interfaces and that it is electrochemically active at much more modest potentials than using unmodified electrodes.[3] We demonstrate this recycling system as a modular approach to biocatalytic reductions,[4,5] coupling NADH-dependent reductases onto the same carbon interface for efficient C=X reductions (Figure 1). Application of this bioelectrocatalytic interface to an electrochemical flow cell increases catalytic activity further and we observe that this interface is stable over many days of continuous use.