Mechanistic insights into the rhodium-catalyzed aryl C–H carboxylation

The recently reported Rh(II)-catalyzed direct C–H bond activation and lactonization of 2-arylphenols uncovers an attractive strategy to prepare coumarin derivatives with novel chemoselectivity. Motivated by the mechanistic ambiguity (on the origin of the chemoselectivity and the details for lactonization etc.), we conducted a detailed mechanistic study for the rhodium-catalyzed lactonization of 2-arylphenols with density functional theory (DFT) calculations. The results suggest that the reaction occurs via the coordination exchange, C–H bond activation, carboxylation, protonation and lactonization steps. The rate-determining step is the carboxylation, in which CO2 favorably inserts into the Rh–C bond (instead of the more nucleophilic Rh–O bond). The protonation step after carboxylation is critical, which makes the subsequent CO2-assisted lactonization feasible. Interestingly, the corresponding pKa value of the base can reasonably predict the reaction energy barrier of the C–H bond activation step. The calculations will provide insights and suggestions for developing and advancing the subsequent C–H bond activation carboxylation reaction.