Metformin-Based Bio-Inspired Organocatalysts for CO₂ Fixation of Terminal/Internal Epoxides

In this study, we developed two classes of metformin-based organic catalysts that efficiently promote the formation of high-value cyclic carbonates and oxazolidinone derivatives from CO₂ and epoxides. Three bifunctional metforminium halide salts-chloride, bromide, and iodide (Met-I)-were prepared and evaluated in CO₂ cycloaddition reactions. Met-I exhibited the highest activity, affording 14 mono-substituted cyclic carbonates under mild conditions. It also enabled the synthesis of di- and three-substituted cyclic carbonates. A second catalyst library comprising 12 halide-free metformin-derived organic salts containing pyridinolate and phenolate anions was developed. Among these, metforminium 2-pyridin-1-olate delivered the highest performance, providing cyclic carbonates in high yields under metal-free and solvent-free reaction conditions. Furthermore, the synthesis of 5-hydroxymethyl oxazolidinone derivatives was achieved using metforminium 2-pyridin-1-olate. The simple preparation, low cost, commercial availability, nontoxicity, easy purification, and low environmental footprint of these catalysts highlight metformin-based systems as practical and sustainable platforms for efficient CO₂ utilization.