Origin of Selectivity in Alternating Current-Enabled Partial Reduction of (Hetero)Arenes: A Case Study of Two Consecutive Irreversible Electrochemical Steps

Alternating current (AC) electrolysis is emerging as a powerful electrosynthesis strategy for achieving unique reactivities by introducing a temporal dimension through the periodic modulation of voltage or current. Recent advances have shown that AC electrolysis can overcome key limitations of DC electrolysis across a range of organic reactions (e.g., Baran et al, Science 2023, 380, 81, and Lei et al., Science 2024, 385, 216). Despite these successes in AC-enabled reaction discovery, the mechanistic understanding of these reactivities remains limited due to the inherent complexity of AC-driven processes. In this work, we uncover the origin of AC-enabled selectivity for reactions involving two consecutive irreversible electrochemical steps using partial reduction of (hetero)arenes to cyclic alkenes as our model reaction. Using fast-scan voltammetry, we found that the reduction of thiophene is primarily mass transfer-controlled, whereas the reduction of the cyclic alkene product is limited by slow electron-transfer kinetics, at the time scale relevant to AC electrolysis. By increasing AC frequency, the products in the initial stage of the cathodic pulse are sampled, where the product selectivity reflects the fast reaction kinetics for the first reduction, enhancing selectivity toward the partially reduced product. This mechanism is semi-quantitatively validated through finite element simulations. More importantly, our findings can be generalized to guide the design of AC-enabled selective transformations involving two consecutive irreversible electrochemical steps. A practical rule of thumb emerges: when the foot-of-the-wave potential difference between cyclic voltammograms of substrate and partial reduction product, |∆EFOW|, > 80 mV, AC electrolysis can achieve synthetically useful selectivity (>30%) toward the intermediate product within a practical frequency range (≤100 Hz).