The cycloaddition of CO2 to epoxides (CCE) reactions produced valuable cyclic carbonates useful in electrolytes of lithium-ion batteries, as organic solvents, and in polymeric materials. However, halide-containing catalysts were predominant in the reactions, while the halide was notorious for its corrosion to steel process equipment and the residue halide was harmful. To obviate the dependence of halide as the cocatalyst in most CCE reactions, halide-free catalyst was highly desirable. Herein, a series of halide-free organocatalysts of stoichiometric base-acid binary adducts of super strong nitrogen base and natural α-hydroxy acid was designed. The adduct was virtually biobased ionic liquid composed of a protonated base cation and an α-hydroxy carboxylate anion. Ionic liquids from four super strong bases (two amidines and two guanidines) and four α-hydroxy acids were evaluated as the halide-free organocatalysts in the CCE reactions. From amidine 1,8-diazabicyclo [5.4.0] undec-7-ene (DBU) and mandelic acid (HMAc) generated an optimal ionic pair catalyst [DBUH][MAc] that showed good catalytic activity in the cycloaddition of CO2 to styrene oxide by 92% yield and 99% selectivity by 2.5 mol% catalyst loading, under conditions of 120 °C, 1.0 MPa, and 12 h. A reasonable bifunctional activation mechanism was proposed in which the protonated base DBUH coordinated to the epoxide by H-bonding, and the carboxylate interacted with CO2 facilitating the formation of acyl carbonate intermediate that attacked the epoxide. The mechanism was validated by 1H NMR titrations, by the intermediate capture, and by controlled experiments by cation and/or anion switches.

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