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Origin of Selectivity in Alternating Current-Enabled Partial Reduction of (Hetero)Arenes: A Case Study of Two Consecutive Irreversible Electrochemical Steps
Herein, we investigate the origin of selectivity in the alternating current (AC)-enabled partial reduction of (hetero)arenes to cyclic alkenes. Reduction of (hetero)arenes can be considered as a reaction involving two consecutive irreversible electrochemical steps: the first generates the desired cyclic alkene, while the second leads to its undesired overreduction. Conventional constant current or voltage (DC) electrolysis results in poor selectivity toward the partial reduction products, originating from overreduction and base-induced decomposition of the desired product. Fast-scan cyclic voltammetry shows that the rate constant for the first reduction (k1) exceeds that of the second one (k2). Finite element simulations based on this experimental finding semiquantitatively capture the frequency-dependent selectivity observed in AC electrolysis experiments (i.e., increasing the AC frequency enhances selectivity). The results further reveal that AC electrolysis mitigates the low selectivity by only collecting the products at the initial stage of the reduction reaction, which is mostly under a kinetically controlled regime. We then extend the finite element model and introduce ΔEFOW, the foot-of-the-wave potential difference between cyclic voltammograms of substrate and partial reduction product, as an accessible proxy for k2/k1. A ΔEFOW > 80 mV predicts synthetically useful selectivity (>30%) toward the partial reduction product below 100 Hz.
