Improving Carbonate-Promoted C–H Carboxylation Using Mesoporous Carbon Supports

Abstract

C–H carboxylation is an attractive way to utilize CO2 for chemical production provided that it does not consume resource-intensive reagents. Alkali carbonates dispersed into the pores of mesoporous supports display strongly basic reactivity under CO2, allowing them to be used as base promoters for C–H carboxylation of (hetero)arenes in the absence of other reagents or catalysts. Mesoporous oxides are convenient support materials, but only a relatively small fraction of the dispersed carbonate (ca. 10–20%) is converted to carboxylate products when metal oxide supports are used. Here, we compare mesoporous oxide and carbon supports and investigate the dependence of carbonate reactivity on the pore structure. We show that using mesoporous carbon supports can increase the carbonate conversion by 2–4× when compared to oxide supports. This improved carbonate reactivity is maintained across a variety of mesoporous carbons with different pore structures (ordered vs disordered) and pore diameters, indicating that the dispersed carbonate is intrinsically more reactive on the surface of a carbon material compared to an oxide surface. Reaction of the carboxylate products with dimethyl carbonate yields isolable methyl esters as the final product and regenerates the dispersed carbonate. We show that mesoporous carbon supports are robust to at least five cycles of successive C–H carboxylation and methylation. Understanding how the support structure affects dispersed carbonate reactivity is valuable for advancing C–H carboxylation toward practical application and utilizing these materials in other CO2 transformations.