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 CO_(2) 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 CO_(2)-assisted lactonization feasible.Interestingly,the corresponding pK_(a) 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.