Calcium-Based Catalytic System for the Synthesis of Bio-Derived Cyclic Carbonates under Mild Conditions

Recently, bio-derived cyclic carbonates have gained significant importance: e.g., as building blocks in non-isocyanate polyurethanes (NIPUs). Herein we report the development of a calcium-based catalyst system for the synthesis of challenging internal and trisubstituted cyclic carbonates from bio-derived epoxides and CO2 under mild reaction conditions. Several crown ethers were tested as ligands in combination with various cocatalysts for the possible activation of CO2. The most active system consists of a dicyclohexyl-functionalized 18-crown-6 ether and triphenylphosphane in addition to calcium iodide. The in situ complexation of Ca2+ by the crown ether was detected by 1H NMR spectroscopy. Interestingly, the addition of triphenylphosphane as a cocatalyst leads to a significant increase in activity, which is similar to or even higher than that of organic superbases such as DBU and TBD. The catalytic system was employed in the conversion of 16 different bio-derived epoxides, including fatty acid esters, oi...