Electrocarboxylation of Benzylic Phosphates and Phosphinates with Carbon Dioxide

Abstract

Carbon dioxide (CO₂) serves as a non-toxic, abundant, cheap, and renewable C1 feedstock in synthetic chemistry. The synthesis of high value-added fine chemicals, such as organic carboxylic acids, using CO₂, is always a focal point of research. Due to the thermodynamic stability and kinetic inertness of carbon dioxide, traditional carboxylation reactions utilizing CO₂ often require harsh reaction conditions. However, organic electrochemical synthesis, which employs electrons as clean reagents to drive the reaction and avoids additional chemical oxidants or reductants, has emerged as a safer, more economical, highly selective, sustainable, and environmentally friendly method for preparing fine chemicals. Electrocarboxylation, which leverages organic electrochemical synthesis to catalytically transform CO₂, provides a milder and more efficient route for CO₂ utilization. Among these approaches, electrocarboxylation of organic halides or pseudohalides containing C―X bonds with CO₂ has been extensively investigated as a means to access value-added carboxylic acids. Phosphates, known for their good leaving group properties, find extensive applications in organic synthesis. Under reductive conditions, the radical anion generated by benzyl phosphate easily dissociates into a benzyl radical and a phosphate anion. Hence, it can serve as an attractive substrate for participating in electrocarboxylation reactions. In this study, we report the highly efficient electrocarboxylation of benzylic phosphate and phosphinate derivatives using CO₂ as the carboxyl source. The constant current reaction took place in an undivided cell, employing glassy carbon as the cathode, and magnesium as the sacrificial anode, in a mixed solvent of DMF and THF. Additionally, this mild electrolysis can be carried out under nonsacrificial anode conditions, using cheap carbon felt electrode as both the nonsacrificial anode and cathode and N,N-diisopropylethylamine as an external reductant, therefore provided operationally simple and highly efficient synthetic method toward aryl acetic acids in moderate to good yield. The broad substrate scope, simple operation, facile scalability, and highly efficient transformation of phosphates into high value-added aryl acetic acids under mild conditions demonstrate the potential applicability of this reaction. To gain insight into the possible reaction mechanism, several control experiments were conducted. Isotope-labeling [¹³]CO₂ experiment, cyclic voltammetry experiments, radical trapping reactions, and deuterium-labeling experiment indicated that cathodically generated benzylic radical and benzylic anion were key intermediates. Moreover, the single electron reduction of CO₂ to ${\text{CO}}_2^{\cdot -}$ might also occur during the reaction.

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