Productive Asymmetric Styrene Epoxidation Based on a Next Generation Electroenzymatic Methodology

We have established a novel and scalable methodology for the productive coupling of redox enzymes to reductive electrochemical cofactor regeneration relying on efficient mass transfer of the cofactor to the electron‐delivering cathode. Proof of concept is provided by styrene monooxygenase (StyA) catalyzing the asymmetric ( S )‐epoxidation of styrene with high enantiomeric excess, space‐time yields, and current efficiencies. Highly porous reticulated vitreous carbon electrodes, maximized in volumetric surface area, were employed in a flow‐through mode to rapidly regenerate the consumed FADH 2 cofactor required for StyA activity. A systematic investigation of the parameters determining cofactor mass transfer revealed that low FAD concentrations and high flow rates enabled the continuous synthesis of the product ( S )‐styrene oxide at high rates, while at the same time the accumulation of the side‐products acetophenone and phenylacetaldehyde was minimized. At 10 μ M FAD and a flow rate of 150 mL min −1 , an average space‐time yield of 0.35 g L −1 h −1 could be achieved during 2 h with a final ( S )‐styrene oxide yield of 75.2%. At two‐fold lower aeration rates, the electroenzymatic reaction could be sustained for 12 h, albeit at the expense of lower (59%) overall space‐time yields. Under these conditions, as much as 20.5% of the utilized current could be channeled into ( S )‐styrene oxide formation. In comparison with state‐of‐the‐art electroenzymatic methodologies for the same conversion, ( S )‐styrene oxide synthesis could be improved up to 150‐fold with respect to both reaction time and space‐time yield. These productivities constitute the most efficient reaction reported for asymmetric in vitro epoxidations of styrene.