Halide-free ion pair organocatalyst from biobased α-hydroxy acid for cycloaddition of CO₂ to epoxide

The cycloaddition of CO₂ to epoxide (CCE) reactions produce valuable cyclic carbonates useful in the electrolytes of lithium-ion batteries, as organic solvents, and in polymeric materials. However, halide-containing catalysts are predominantly used in these reactions, despite halides being notoriously corrosive to steel processing equipment and residual halides also having harmful effects. To eliminate the reliance on halides as cocatalyst in most CCE reactions, halide-free catalysts are highly desirable. Herein, a series of halide-free organocatalysts composed of stoichiometric base-acid binary adducts of super strong nitrogen bases and natural α-hydroxy acids was designed. The adducts were virtually biobased ionic liquids 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 halide-free organocatalysts in the CCE reactions. Combining amidine 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) and mandelic acid (HMAc) generated an optimal ion pair catalyst [DBUH][MAc], which showed good catalytic activity in the cycloaddition of CO₂ to styrene oxide with a 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 the 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.