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, oils, and terpenes with CO₂, and is able to facilitate the reaction under mild conditions. Various internal epoxides were converted at only 45 °C, 0.5 MPa CO₂ pressure, a catalyst loading of 5 mol %, and a reaction time of 24 h with isolated yields up to 98% of the respective carbonate. The challenging terpene-based carbonates were isolated in yields up to 81%, although harsher reaction conditions were necessary.