Reduction by Oxidation: Selective Hydrodehalogenation of Aryl Halides by Mediated Oxalate Oxidation

Abstract

Electro-organic reduction reactions are canonically carried out at a cathode at which a significant negative potential is applied. Specifically, at carbon electrodes, aryl bromides and chlorides undergo heterogeneous reduction in organic solvents at potentials more negative than −2 V vs E^0′ for the Fc/Fc⁺ couple (Fc = ferrocene). To decrease the overpotential for reduction reactions, homogeneous or heterogeneous electrocatalysis strategies are often employed. Here, we present an electrochemical method to reduce aryl bromides and chlorides that is initiated by an oxidation reaction at very mild potentials (∼0 V vs Fc/Fc⁺). Specifically, electrochemical oxidation of an outer-sphere redox mediator, 1,1-dimethylferrocene, in dry N,N-dimethylformamide (DMF) containing oxalate (C₂O₄^2–), results in the homogeneous one-electron oxidation of C₂O₄^2–. The resulting C₂O₄^•– decomposes in ∼1 μs to release the carbon dioxide radical anion (CO₂^•–), a potent reductant that is oxidized to CO₂ at –2.68 V vs Fc/Fc⁺. In this way, an oxidation reaction at low electrode potentials enables homogeneous reduction of aryl bromides and chlorides, which are otherwise directly reduced at very negative potentials. Using this method, selective hydrodehalogenations of electron-deficient aryl bromides and chlorides are carried out at a reticulated vitreous carbon anode with up to quantitative conversion yields. Cyclic voltammetry and finite difference simulations are used to characterize the hydrodehalogenation of 4-bromobenzonitrile via C₂O₄^2– oxidation. Additionally, we show that the efficiency of hydrodehalogenation is tuned by deliberate additions of water to DMF solutions, leading to a substantial improvement in overall conversion yields without interference from water or proton reduction.