Synthesis of Cyclic Carbonates Catalyzed by CaI2–Et3N and Studies on Their Biocatalytic Kinetic Resolution

Abstract

Herein, we report a simple catalytic system consisting of CaI2 in combination with Et3N as a ligand for the efficient cycloaddition of CO2 to a variety of epoxides. Terminal (bio-based) carbonates were obtained under mild conditions (23 °C, 1 bar CO2 pressure) in excellent yields up to 97%. Under elevated temperature (70 °C) and higher CO2 pressure (10 bar), even challenging internal epoxides were converted, leading to the desired carbonates in yields up to 90%. 1H NMR experiments indicated the in situ coordination of the amine to the Lewis acidic calcium center as well as the activation of the epoxide. Furthermore, the enzymatic kinetic resolution of seven cyclic carbonates using readily available pig liver esterase was investigated. Styrene carbonate proved to be the most suitable substrate for the kinetic resolution, leading to (R)-styrene carbonate in 99% ee.