Diastereoselectivity Controlled by the Hydrogenation Mechanisms during the Electrochemical Reduction of a Carbonyl Group

Stereocontrol is of critical importance in organic synthesis. In this study, we demonstrate how heterogeneous electrochemical hydrogenation enables diastereocontrol simply by tuning electrochemical hydrogenation mechanisms without altering the adsorption conformation of a reactant on the electrode. We use 4-hydroxy-1-tetralone (4-OH-tetralone) as a model reactant, where diastereomers can be produced during the hydrogenation of the carbonyl group. In traditional thermocatalytic hydrogenation, H2 first dissociates on the catalyst surface to form surface-adsorbed hydrogen (H*), and therefore, H* is always added to the organic reactant from the catalyst side via hydrogen atom transfer (HAT). Thus, in order to flip the diastereoselectivity, the adsorbed reactant itself must be physically flipped. In contrast, electrochemical hydrogenation can occur either via HAT, where H is added from the electrode surface, or via proton-coupled electron transfer (PCET), where H is added from the solution side of the adsorbed reactant. Thus, without changing the adsorption conformation of the reactant, opposite diastereomers can be obtained by switching the hydrogenation mechanism (HAT vs PCET). In this work, using a combination of experimental and computational studies, we demonstrate two examples of flipping diastereoselectivity by different electrochemical hydrogenation mechanisms. In the first case, we achieve opposite diastereoselectivities using metals that adopt different hydrogenation mechanisms (HAT vs PCET). In the second case, we flip the diastereoselectivity by varying the applied potential, which switches one hydrogenation mechanism to the other on the same metal electrode. In each case, our results offer an atomic-level understanding of the preferred hydrogenation mechanism that enables the corresponding diastereoselectivity.