Electrochemical Reductive N-Methylation with CO2 Enabled by a Molecular Catalyst

Abstract

The development of benign methylation reactions utilizing CO₂ as a one-carbon building block would enable a more sustainable chemical industry. Electrochemical CO₂ reduction has been extensively studied, but its application for reductive methylation reactions remains out of the scope of current electrocatalysis. Here, we report the first electrochemical reductive N-methylation reaction with CO₂ and demonstrate its compatibility with amines, hydroxylamines, and hydrazine. Catalyzed by cobalt phthalocyanine molecules supported on carbon nanotubes, the N-methylation reaction proceeds in aqueous media via the chemical condensation of an electrophilic carbon intermediate, proposed to be adsorbed or near-electrode formaldehyde formed from the four-electron reduction of CO₂, with nucleophilic nitrogenous reactants and subsequent reduction. By comparing various amines, we discover that the nucleophilicity of the amine reactant is a descriptor for the C–N coupling efficacy. We extend the scope of the reaction to be compatible with cheap and abundant nitro-compounds by developing a cascade reduction process in which CO₂ and nitro-compounds are reduced concurrently to yield N-methylamines with high monomethylation selectivity via the overall transfer of 12 electrons and 12 protons.