(650f) Electrochemical Carboxylation of Benzyl Chloride with CO2 Using Low-Coordinate Cobalt Electrocatalysts

The synthesis of phenylacetic acids has high utility as they serve as motifs for pharmaceuticals and biologically active compounds. Drugs such as Ibuprofen and Naproxen, which are phenylacetic acid derivatives, are already used commercially in common pain-relievers. Electrochemistry provides new routes for carboxylation of benzyl halides with CO₂ to the corresponding phenylacetic acids in a more sustainable fashion, where reactions can be carried out under milder conditions removing the use of unstable and toxic reagents. Employing safer transition metals such as cobalt drives the search for developing transformations with cobalt-based electrocatalysts instead of using expensive palladium-based catalysts and toxic nickel-based catalysts. First-principles density functional theory (DFT) calculations were carried out in this work to examine the reaction mechanism that controls the carboxylation of secondary benzyl chloride with CO₂ to the corresponding phenylacetic acid using the Cobalt-pyrox electrocatalyst. The low-coordinate Cobalt-pyrox catalyst is selective towards carboxylation than Cobalt-salen, which predominantly gives homocoupling products, and Cobalt-bipyridine catalysts, which gives a mixture of homocoupling and other hydrogenated and dehydrogenated products. We examine the plausible elementary reaction steps involved, the active species for each step, and the effect of zinc- and magnesium-based sacrificial anodes. Our results support the experimental observations that suggest that the direct carboxylation of the benzyl-Cobalt intermediate with CO₂ is favored over carboxylation by an organo-zinc or organo-magnesium reagent. Salts derived from the sacrificial anodes are essential to enable this transformation (Figure 1), which is also observed experimentally, where the yields are low in the absence of any sacrificial anode. Simulations are combined with electroanalytical results to answer other critical mechanistic questions concerning the reactive transformations, including the role of each cobalt species in the mechanism and the nature of oxidative addition and carboxylation steps, and a mechanism is proposed based on our findings.