(240g) Understanding the Molecular Origin of Performance Enhancements Seen Under Rapidly Alternating Polarity (rAP) in Flow Organic Electrosynthesis

Organic electrosynthesis provides a unique opportunity to drive organic reactions using renewable electricity, oftentimes under milder conditions than can be achieved with chemistry driven by heat derived from fossil fuels. An added benefit of using electricity in these reactions is the ability to dynamically change the driving force through time-dependent modulation of the applied potential. In organic electrosynthesis, rapidly alternating the polarity (rAP) applied to an electrode has been shown to enhance product yield and enable new, chemoselective reactivities. However, the molecular origin of the increase in selectivity and yield observed under rAP has yet to be fully investigated.

One hypothesis about the origin of enhancement under rAP is that exposing the electrode to the oppositely charged potential provides time to replenish reagents and eliminate products from the diffusion layer near the electrode surface. Thus, upon renewed application of the reaction potential, the interface is refilled with a high concentration of substrate, enabling high conversion rates while simultaneously avoiding competitive side reactions. Based on this premise, similar performance as observed under rAP should be achievable under an applied DC potential if the supply and removal of species to and from the electrode surface is rationally controlled.

Furthermore, in reactions that required both an oxidation and reduction of intermediates to yield the final product, rAP is hypothesized to eliminate the need to transfer relevant intermediates across the bulk solution. This then enables enhanced performance of a reaction compared to running under DC conditions. However, it can be envisioned that a reactor engineered to have extremely high rates of mass transfer could move intermediates across the bulk solution at sufficient rates to yield the same performance as running under rAP.

To verify these hypotheses, we built electrochemical flow reactors which allow for precisely controlled mass transport rates. We compared conditions of high and low mass transport, with and without rAP, to evaluate the impact of mass transport in several reactions of interest to pharmaceutical electrosynthesis. Our data confirm that high mass transport rates can mimic rAP’s ability to replenish reagents at the interface and cycle intermediates.

Based on our findings, we were then able to employ controlled mass transport to achieve comparable organic electrosynthesis selectivity and yield under DC conditions compared to rAP. Our results provide novel insight into the role of mass transfer in rAP’s ability to mediate interfacial reagent concentrations to enhance reaction performance and chemoselectivity.