Bifunctional catalysts based on m-phenylene-bridged porphyrin dimer and trimer platforms: Synthesis of cyclic carbonates from carbon dioxide and epoxides

Highly active bifunctional diporphyrin and triporphyrin catalysts were synthesized through Stille coupling reactions. As compared with a porphyrin monomer, both exhibited improved catalytic activities for the reaction of CO₂ with epoxides to form cyclic carbonates, because of the multiple catalytic sites which cooperatively activate the epoxide. Catalytic activities were carefully investigated by controlling temperature, reaction time, and catalyst loading, and very high turnover number and turnover frequency were obtained: 220000 and 46000 h^-1, respectively, for the magnesium catalyst, and 310000 and 40000 h^-1, respectively, for the zinc catalyst. Results obtained with a zinc/free-base hybrid diporphyrin catalyst demonstrated that the Br^- ions on the adjacent porphyrin moiety also function as nucleophiles.