Electrochemical Birch Reduction: A Molecular-Level Venture into the Solvation Structure at the Electrode Interface

Abstract

Although selective hydrogenation of aromatics through Birch reduction has long been at the heart of synthetic organic chemistry, its various proposed procedures suffer from monumental challenges. The optimized design of salt, solvent, and additives along with a sacrificial anode material has enabled the recent success of Electrochemically driven Birch reduction (E-Birch) reactions, which are procedurally convenient, safe, and sustainable1. The E-Birch reactions are premised upon the concepts applied in Li-ion battery research regarding the surface protection layer formation, also referred as the solid electrolyte interphase (SEI). However, the impacts of these constituents on the surface layer formation in E-Birch reactions is largely unknown. In this study, we have employed a combination of molecular dynamics (MD) simulations and density functional theory (DFT) calculations to provide insights into the liquid structure and the interactions of these constituents, both in bulk of the solution and at the electrode surface. Specifically, an optimal electrolyte design of LiBr salt in tetrahydrofuran (THF) solvent, with 1,3-dimethylurea (DMU) as the proton donor, and tris(pyrrolidino)phosphoramide (TPPA), as surface protector, has been studied as it promoted high yields of 1,4-diene formation from several aromatic-ring containing molecules with a variety of functional groups. We have found that THF, DMU, and TPPA are all present in the Li+ solvation sheath at the bulk of the solution and at the electrode surface. Since the surface layer is formed by the reduction reactions of solution, the reduction voltage was computed for representative solvation structures. It was found that the reduction voltage and reduction process of THF, DMU, and TPPA were altered by the Li+ coordination and are therefore sensitive to the liquid solution structures. In addition, we have demonstrated that the species in the solvation shell can undergo bond breaking events, which lead to protection layer formation at the electrode surface. The role of salt, solvent, additives, and sacrificial anodes in altering the solution structure and reduction reactions was further discussed. References Peters Byron, K.; Rodriguez Kevin, X.; Reisberg Solomon, H.; Beil Sebastian, B.; Hickey David, P.; Kawamata, Y.; Collins, M.; Starr, J.; Chen, L.; Udyavara, S.; Klunder, K.; Gorey Timothy, J.; Anderson Scott, L.; Neurock, M.; Minteer Shelley, D.; Baran Phil, S., Scalable and safe synthetic organic electroreduction inspired by Li-ion battery chemistry. Science 2019, 363 (6429), 838-845.