Mechanistic Insights into Urea‐, Thiourea‐, and Isothiourea‐Based Bifunctional Tetraarylphosphonium Salt Catalysis for Conversion of Carbon Dioxide to Cyclic Carbonates

A novel bifunctional isothiourea derived from a tetraarylphosphonium salt catalyzes the coupling reaction of monosubstituted epoxides with 1 atm of carbon dioxide to afford five‐membered cyclic carbonates in high yields. Thiourea and urea groups are also effective hydrogen bond donors for the carbon dioxide fixation. Comprehensive mechanistic studies on this catalysis using theoretical calculations disclosed key hydrogen bond interactions between catalysts and intermediates, which relied upon orientation of the binding modes.