A Rh complex anchored cathode for cofactor regeneration and asymmetric reduction coupled with immobilized enzymes

Chiral compounds have a huge market demand in fields such as medicine, pesticides, and fine chemicals, among which chiral alcohols are important raw materials for the production of active intermediates. Enzymatic catalysis features high efficiency and high product purity. Asymmetric reduction of prochiral ketones by enzymatic catalysis is an effective method for the synthesis of chiral alcohols and has received increasing attention in recent years. However, expensive cofactor NADH is usually required to participate in electron and proton transfer during the enzymatic catalysis process. Electro-catalytic cofactor regeneration has advantages such as simple operation, low cost, easy process monitoring, and easy product separation. In the electrochemical cofactor regeneration process, electron mediators are needed to overcome problems such as poor selectivity, formation of inactive dimers, and high overpotential. Rhodium complexes ([Cp*Rh(bpy)H₂O], Cp* = pentamethylcyclopentadienyl, bpy = 2,2'-bipyridine, Rh(III)) have attracted increasing attention in recent years due to their high regeneration efficiency, selectivity, and stability. However, rhodium complexes are toxic and can cause enzyme inactivation. Fixing the electron mediator on the electrode can inhibit the toxic effect on the enzyme, simplify the separation of the product, and achieve the recycling and reuse of the electron mediator. Studies have shown that compounds containing -S- or -N parts (commonly used as precursors for electrode surface functionalization) can cause the inactivation of the electron mediator because their interaction with the Rh center prevents the formation of active hydrogenated rhodium complexes. The stepwise grafting strategy can covalently fix -S or -N to reduce the impact on the performance of rhodium complexes. In addition, free enzymes have poor stability, are difficult to recover and reuse, and the precursors of chiral alcohols are usually poorly water-soluble. Designing and selecting appropriate immobilized enzyme carriers can not only improve the stability and reusability of enzymes but also enrich the substrate, thereby enhancing the enzymatic catalytic efficiency.

The team of Yanjun Jiang from Hebei University of Technology has constructed an efficient asymmetric reduction system for the synthesis of chiral alcohols by coupling electrochemical coenzyme regeneration with immobilized enzymes. In the electrocatalytic coenzyme regeneration part, tannic acid (TA) and polyethyleneimine (PEI) are used to form a hydrophilic surface on the carbon felt (CF) electrode, increasing the effective contact area of the electrode and the available functional groups on the electrode surface, thereby further anchoring the electron mediator complex of Rh coordination in the electrode through a stepwise grafting strategy; in the enzyme catalytic part, a hydrophobic COF is selected as the carrier for immobilized enzymes, which can enrich the substrate and increase the local substrate concentration near the enzyme molecules, thereby improving the catalytic efficiency. By combining immobilized enzyme catalysis with electrochemical coenzyme regeneration, an electro-enzyme catalytic system for efficient asymmetric reduction synthesis of chiral alcohols is constructed.

Using the Rh@TA-PEI@CF electrode for electrochemical coenzyme regeneration, the maximum NADH regeneration amount can reach 300 μM, and the yield of (R)-2-Chloro-1-phenylethanol reaches 95.5%, which is 3 times and 2.25 times that of the bare CF electrode and free Rh(III), respectively. The immobilized carrier can protect the enzyme molecules. Compared with COF-OH, the hydrophobic COF-OMe can better enrich the substrate and improve the yield. The yield of (R)-2-Chloro-1-phenylethanol of LfSDR1@COF-OMe is 4.3% higher than that of LfSDR1@COF-OH and 3.1 times that of the free LfSDR1. The relevant parameters of the electro-enzyme synthesis process were optimized, and the maximum TOF of this reaction was 101.1 h^-1. The substrate spectrum was expanded, and the system was proved to have certain universality.

This work was supported by the National Key Research and Development Program of China (2023YFA0914500), the National Natural Science Foundation of China (No. 22378096, 22308083, and 22178083), the Natural Science Foundation of Hebei Province (B2022202014), the Educational Commission of Hebei Province (JZX2023012).

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