Computer-aided rational design of Fe(iii)-catalysts for the selective formation of cyclic carbonates from CO2 and internal epoxides

Abstract

The catalytic mechanism of the cyclic carbonate formation reaction between CO₂ and internal epoxides promoted by Fe-salen and the Kleij catalyst was examined in detail to better understand how the catalytic efficiency can be increased. Specifically, we aimed to make the catalyst more chemoselective towards forming cyclic carbonates and preventing the competing side reaction leading to polycarbonates via ring-opening polymerization. A few rational design principles were derived and first tested using computer models based on density functional theory. The most promising candidate that was identified in the computer model was then prepared and found to display significantly enhanced reactivity towards forming the cyclic carbonates, supporting the validity of the mechanistic insights deduced from the computer simulations. We propose that a cyclic carbonate is formed most efficiently via an inner-sphere mechanism where both the CO₂ and epoxide substrates utilize the metal center for the key bond forming events. In contrast, the ring-opening polymerization uses an outer-sphere mechanism, where a carbonate attacks and ring-opens the epoxide bound to the metal without engaging the metal directly. These mechanistic differences are exploited to implement a chemoselective catalyst by enhancing the rate of the cyclic carbonate formation reaction while leaving the polymerization pathway largely unaffected.