A DFT Study on the Mechanism of the Coupling Reaction between Chloromethyloxirane and Carbon Dioxide Catalyzed by Re(CO)5Br

The detailed mechanism of the experimentally observed formation of five-membered cyclic carbonates in the coupling reaction between chloromethyloxirane and CO2 catalyzed by the Re(CO)(5)Br complex is revealed by means of density functional theory (DFT) calculations. All possible pathways are examined, and their corresponding energetics are demonstrated. Our calculations indicate that the real active catalyst is the unsaturated complex Re(CO)(4)Br rather than free radical species (Re(CO)(5) or Br radicals). The preferred mechanism (path I) for the catalytic production of cyclic carbonates can be divided into three main stages involving epoxide oxidative addition, carbon dioxide insertion, and reductive elimination of cyclic carbonate, none of which contains significantly large barriers. Our results provide support for Jiang's proposal that the reaction proceeds through the reactive oxametallacyclobutane 2b. Furthermore, we have found CO dissociation from 2b is an essential step, which facilitates CO2 coordination and insertion leading to metallacarbonate 6. The whole CO2 insertion is predicted to be an endergonic process. From the unstable metallacyclic species 6, the cyclic carbonate reductive elimination occurs via a three-center transition-state structure. Reductive elimination is a very facile step as compared with epoxide oxidative addition and carbon dioxide insertion. As for the ring-opening of epoxide, the activation energy barrier from 1d to 2b is 36.2 kcal/mol in supercritical CO2, which is slightly higher, by 2.2 kcal/mol, than that of the CO2 multistep insertion (2b -> 6). Thus, each of them can be the rate-determining step with variation in the reaction conditions (temperature and pressure). The high-energy barriers along the alternative reaction pathway indicate path II is not competitive. Our present theoretical study provides a clear profile for the cycloaddition of carbon dioxide with chloromethyloxirane catalyzed by Re(CO)(5)Br.