The use of CO as a C1 building block will be of essential importance in the future. In this context the synthesis of cyclic carbonates from epoxides and CO gained great attention recently. These products are valuable compounds in a variety of chemical fields. The development of new catalysts and catalytic systems for this atom-economic, scalable, and industrially relevant reaction is a highly active research field. Over the past 17 years great advances have been made in this area of research. This chapter covers the survey of the important known classes of homogeneous catalysts for the addition of CO to epoxides. Besides pioneering work, recent developments and procedures that allow this transformation under mild reaction conditions (reaction temperatures of ≤100 °C and/or CO pressures of 0.1 MPa) are especially emphasized.

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.

Numerous bifunctional organocatalysts were synthesized and tested for the atom-efficient addition of carbon dioxide and epoxides to produce cyclic carbonates. These catalysts are based on phosphonium salts containing an alcohol moiety in the side chain for substrate activation through hydrogen bonding. In the model reaction, converting 1,2-butylene oxide with CO2 , 19 catalysts were tested to determine structure-activity relationships. In total, 28 epoxides were converted with CO2 to give the respective cyclic carbonates in yields of up to 99%. Even at 45 °C, the most active catalyst was able to produce cyclic carbonates selectively in high yields. The carbonates were generally obtained as analytically pure products after simple filtration over silica gel. This single-component catalyst system works under neat and mild reaction conditions and tolerates several useful moieties.

Calcium punched beyond its weight: An in situ formed Ca2+–crown ether complex showed unprecedented efficiency in cyclic carbonate synthesis.