Five-membered cyclic carbonates arise principally from cycloaddition of carbon dioxide to an epoxide, a transformation that inserts CO₂ as the carbonyl and one ring oxygen of the heterocycle. Mechanistically, a Lewis acid activates the epoxide toward ring opening by a nucleophilic cocatalyst (halide or related anion); the resulting alkoxide captures CO₂, and the carbonate anion displaces the leaving group to close the five-membered ring ^[1]. Bimetallic aluminium–salen complexes catalyze this sequence with two metal centers cooperating in epoxide activation and nucleophilic delivery ^[2]. One-component catalysts incorporate both the Lewis-acidic and the nucleophilic functions in a single molecular species, so that no external cocatalyst is required ^[3]. Alternative routes include oxidative carbonylation of diols and rearrangement of acyclic carbonates, but the epoxide–CO₂ coupling is the defining contemporary construction. The product is a cyclic organic carbonate, distinct from polycarbonates (which may form if the intermediate alkoxide polymerizes) and from acyclic dialkyl carbonates.