Electrochemical CO2 Utilization for the Synthesis of α-Hydroxy Acids

The electrocarboxylation of organic compounds provides a green alternative to the conventional use of cyanides or organometallics. Via a simple electrochemical approach, CO2 can be directly incorporated into aldehydes or ketones to form α-hydroxy acids using electrical energy as a driving force. In this study, propylene carbonate was used as a "green" solvent. Importantly, the use of sacrificial anodes was avoided; instead, cost-efficient and stable graphite anodes were applied. Voltammetric studies on benzaldehyde carboxylation revealed that the presence of CO2 significantly changes the reduction behavior of benzaldehyde at potentials more negative than the potential of the ketyl radical formation. Bulk electrolysis was performed in various solvents and supporting electrolyte systems, leading to more than 55% mandelic acid yield in propylene carbonate. Among the tested solvents (acetonitrile, N,N-dimethylformamide, dimethyl sulfoxide, and propylene carbonate), propylene carbonate resulted in the highest carboxylate yield. Numerous cathode materials were tested. High carboxylate yields were achieved using graphite, glassy carbon, and lead cathodes. The reaction was successfully carried out for various aromatic aldehydes and ketones as substrates, providing yields of up to 63%.