In Situ Kinetic Studies of Rh(II)-Catalyzed Asymmetric Cyclopropanation with Low Catalyst Loadings

Dirhodium tetracarboxylates are versatile catalysts for the reactions of donor/acceptor carbenes. They catalyze a variety of transformations, including enantioselective intermolecular cyclopropanations. This study is focused on understanding the kinetics of the rhodium-catalyzed cyclopropanation, and this information was used to develop conditions for conducting the reactions with very low catalyst loadings. The enantioselective cyclopropanation of styrenes can be conducted with a catalyst loading of 0.001 mol % and still maintain high levels of enantioselectivity (86–99% ee). A triarylcyclopropanecarboxylate (TPCP) catalyst, Rh2(p-Ph-TPCP)4, was the optimum catalyst for maintaining high enantioselectivity with very low catalyst loading. The reaction also benefited from using dimethyl carbonate as the solvent, an environmentally benign and nontoxic material.