Efficient bifunctional urea–phosphonium halide catalysts for the cycloaddition of CO2 to epoxides

The cycloaddition of CO 2 to epoxides (CCE) reaction is one of the few chemical processes viable commercially in the valorization of CO 2 . A bifunctional urea–phosphonium halide with both electrophilic and nucleophilic centers was proposed as an effective catalyst for CCE reactions here. The urea–phosphonium halides with different urea substituents and halide anions were simply synthesized and systematically evaluated in CCE reactions. It was found that the H-bond donicity of the urea N–Hs, the nucleophilicity of the halide anion, and the affinity between the urea and halide anions jointly affected the catalytic activity of the urea–phosphonium halide in CCE reactions. The H-bonding strength between the N–Hs and nucleophilic anions was disclosed via NMR titrations. The catalyst featuring a 3,5-bis(trifluoromethyl)phenyl-substituted urea structure and an iodide anion was selected as the optimal structure, which promoted the cycloaddition of styrene oxide with excellent conversion (92 %) and selectivity (99 %) in 6 h under mild conditions (1 mol% catalyst loading, 100 °C, 0.1 MPa CO 2 ). Furthermore, the bifunctional catalyst performed exceptional recyclability, exhibiting no discernible decline in catalytic activity over five consecutive reaction cycles. The structure−performance relationship of the urea–phosphonium halides and a wide range of substrates applicability provided valuable insights for the optimization of the existing catalysts or the further design of new effective organocatalysts.