Bifunctional porphyrin catalysts for the synthesis of cyclic carbonates from epoxides and CO ₂: Structural optimization and mechanistic study

We prepared bifunctional Mg ^II porphyrin catalysts 1 for the solvent-free synthesis of cyclic carbonates from epoxides and CO ₂ . The activities of 1d, 1h, and 1i, which have Br ^- , Cl ^- , and I ^- counteranions, respectively, increased in the order 1i < 1h < 1d. Catalysts 1d and 1j-m, which bear four tetraalkylammonium bromide groups with different alkyl chain lengths, showed comparable but slightly different activities. Based on the excellent catalyst 1d, we synthesized Mg ^II porphyrin 1o with eight tetraalkylammonium bromide groups, which showed even higher catalytic activity (turnover number, 138,000; turnover frequency, 19,000 h ^-1 ). The catalytic mechanism was studied by using 1d. The yields were nearly constant at initial CO ₂ pressures in the 1-6 MPa range, suggesting that CO ₂ was not involved in the rate-determining step in this pressure range. No reaction proceeded in supercritical CO ₂ , probably because the epoxide (into which the catalyst dissolved) dissolved in and was diluted by the supercritical CO ₂ . Experiments with ¹⁸ O-labeled CO ₂ and D-labeled epoxide suggested that the catalytic cycle involved initial nucleophilic attack of Br ^- on the less hindered side of the epoxide to generate an oxyanion, which underwent CO ₂ insertion to afford a CO ₂ adduct; subsequent intramolecular ring closure formed the cyclic carbonate and regenerated the catalyst. Density functional theory calculations gave results consistent with the experimental results, revealing that the quaternary ammonium cation underwent conformational changes that stabilized various anionic species generated during the catalytic cycle. The high activity of 1d and 1o was due to the cooperative action of the Mg ^II and Br ^- and a conformational change (induced-fit) of the quaternary ammonium cation.