Multifunctional Phosphonium-Based Deep Eutectic Ionic Liquids: Insights into Simultaneous Activation of CO2 and Epoxide and Their Subsequent Cycloaddition

Novel phosphonium-based deep eutectic ionic liquids were well designed and facilely synthesized; their structures and physical properties were studied in detail. As single-component and sustainable media they were successfully used for efficient cycloaddition of CO2 to epoxides under mild conditions. The effects of ionic liquid structures, reaction parameters, and substrate scope on the cycloaddition reaction were examined thoroughly. Among the designed ionic liquids, the tetra-n-butylphosphonium bromide (TBPB) combined with 3-aminophenol (3-AP) in a molar ratio of 1:2 was proved to be the most active catalyst, which could afford good to excellent cyclic carbonate yields with satisfactory selectivities under the optimum conditions (80 °C, 1 MPa, 1 h). Moreover, the TBPB/3-AP was easily recovered and showed excellent reusability. The TBPB/3-AP medium realized the simultaneous activation of CO2 and epoxide; finally, the cooperative activation and reaction pathways were proposed.