Mechanistic exploration of CpRe(CO)3-catalyzed coupling of chloromethyloxirane with CO2: Unexpected potentials of CO ligands

Abstract

Density functional theory (DFT) calculations have been performed to unravel the detailed mechanism of the CpRe(CO)3-catalyzed coupling reaction of CO2 with chloromethyloxirane (R). The mechanisms proposed in previous literature have been examined. However, the computed results seem not to completely rationalize the experimental findings. Alternatively, by performing an exhaustive search on the potential energy surface, we presented a novel CO-assisted mechanism, which provides an effective access to the catalytic coupling. The newly established mechanism involves the CO bond cleavage of R from the less hindered side, release of one CO ligand, CO2 insertion, re-coordination of the dissociated CO ligand, another CO ligand shift, reductive elimination with simultaneous migration of carbonyl substituent, resulting in P and regeneration of CpRe(CO)2. Two CO ligands attached to CpRe(CO)2 play significant roles in lowering the key CO2 insertion and reductive elimination: (i) One CO ligand dissociation prior to CO2 insertion contributes to reduce the electron density on the Re metal, facilitating the CO2 O atom nucleophilic attack. (ii) After performing the migration, another initially inactive CO ligand serves as a carbonyl substituent, which lowers the energy penalty for later reductive elimination through the resultant extra π-interaction and the smaller ring tension energy involved in TS. The theoretical results provide insight into the mechanism of the important coupling reaction and rationalize well the experimental observations.