Mechanistic Understanding of Dinuclear Cobalt(III) Complex Mediated Highly Enantioselective Copolymerization of meso-Epoxides with CO2

The desymmetrization copolymerization of meso-epoxides with CO2 using chiral catalysts or reagents is regarded as a valuable strategy for the synthesis of optically active polycarbonates with main-chain chirality. The present study demonstrates that the biphenol-linked dinuclear Co(III) complexes show unprecedented activity, enantioselectivity, and broad substrate scope for coupling CO2 with various meso-epoxides to afford the corresponding isotactic polycarbonates. This investigation also focuses on the mechanistic understanding on the origin of enantioselectivity and highly catalytic activity in the enantiopure dinuclear Co(III) complex mediated copolymerization process, using cyclohexene oxide as a model monomer of meso-epoxides. The kinetic study by in situ infrared spectroscopy revealed a first-order dependence on the catalyst concentration in the systems of biphenol-linked dinuclear Co(III) complex alone or in the presence of an ionic cocatalyst. An intramolecular bimetallic cooperation mechanism was proposed to be predominantly responsible for the copolymerization process, wherein alternating chain growth and dissociation take turns between two Co(III) ions from the inside cleft of the catalyst by the nucleophilic attack of the growing carboxylate species at one metal center toward the activated epoxide at the other. Density functional theory calculations suggested that in the two diastereoisomers of the biphenol-linked dinuclear Co(III) complexes the matched configuration was (S,S,S,S,S)- rather than (S,S,R,S,S)-conformer for CO2/meso-epoxide copolymerization to give the corresponding polycarbonate with S,S-configuration. The addition of an ionic cocatalyst with bulky cation significantly improves both the catalytic activity and enantioselectivity, while the presence of a coordination Lewis base caused dramatically a change in the chiral induction orientation.