Palladium-catalyzed direct carbonylation of thiophenes and furans under CO/CO2-binary conditions leading to carboxylic acids

Abstract

In this study, novel catalytic carbonylation of thiophenes and furans was successfully achieved by using a catalytic amount of Pd(OAc)₂ (min. 1 mol%). The catalyst showed excellent catalytic performance under CO/CO₂-binary conditions, and the use of p-benzoquinone (p-BQ) as a stoichiometric oxidant for the regeneration of high-valent palladium catalysts active for C–H bond activation of heteroaromatics successfully led to the corresponding carboxylic acids in up to quantitative yields. Conventional palladium-promoted direct carbonylation of heteroaromatics was difficult to conduct as a catalytic reaction due to the thermal decomposition of palladium active species. In sharp contrast, we found for the first time that the thermal decomposition of the catalyst could be suppressed by using a CO/CO₂-binary system; thus, highly efficient conversion of thiophenes and furans to the corresponding carboxylic acids has been attained with only 1 mol% catalyst loading. In the catalytic reactions, CO is the carbonyl source. The control experiments clearly showed that pressurized CO₂ suppresses the thermal decomposition of the active palladium species to inactive palladium black and improves the durability of the catalyst under reaction conditions. The direct and catalytic carbonylation via C–H bond activation has been considered one of the most challenging reactions; therefore, the method established in this study will lead to a novel, practical, and versatile transformation based on direct carbonylation.