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. Instead of using expensive palladium-based catalysts and toxic nickel-based catalysts, employing safer transition metals such as cobalt drives the search for developing transformations with cobalt-based electrocatalysts. In this work, first-principles density functional theory (DFT) calculations were carried out to examine the reaction mechanism of carboxylation of secondary benzyl chloride with CO2 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, the active species for each step, and the effect of zinc- and magnesium-based sacrificial anodes. Simulations 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, 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.

Low-coordinate cobalt electrocatalysts for carboxylation of benzyl chloride with CO2

ACS Fall 2021

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