Aminoborate-Catalyzed Reductive Counterreactions for Oxidative Electrosynthetic Transformations.

Abstract

Electrooxidative transformations frequently rely on proton reduction as the terminal electron sink. However, this cathodic counterreaction can be slow in organic solvents and can operate at reducing potentials that are incompatible with catalysts and reagents needed for oxidative reactions. We report aminoborate adducts as redox mediators for proton reduction that operate at mild reducing potentials. This reliable cathodic couple ultimately enables successful oxidative organic transformations, including chlorodeborylation, developed herein, and Cu-catalyzed Chan-Lam coupling, reported previously by our group. Pyridinium borate adducts formed during electrooxidative chlorination of aryl trifluoroborates serve as easily reduced complexes (-1.1 V vs Fc/Fc+) to catalyze proton reduction. Reactions that promote the formation of borate adducts result in high yields, operate at low cell potentials, suppress aryl trifluoroborate decomposition, and mitigate electrode passivation. These studies illustrate the utility of Lewis acid-base complexes in cathodic counterreactions and underscore the importance of developing both anodic and cathodic reactions in electrosynthesis.