Cycloaddition of CO2 and epoxides catalyzed by imidazolium bromides under mild conditions: influence of the cation on catalyst activity

Abstract

The synthesis and characterization of a library of imidazolium-based compounds of the type [R¹R²R³im]Br (R¹ = H, CH₃, benzyl (Bz), 1-(2,3,4,5,6-pentafluoro)benzyl (Bz^F5); R² = H, CH₃, C₂H₅; R³ = n-butyl, n-octyl) are reported. All compounds were characterized by means of ¹H and ¹³C NMR spectroscopy, elemental analysis and single crystal X-ray diffraction in the case of 1-Bz^F5-3-n-octylimidazolium bromide and examined as catalysts for the cycloaddition of carbon dioxide and propylene oxide (PO) to yield propylene carbonate (PC). The catalyst screening shows the influence of different substituents (steric and ion pairing effects) on the catalytic activity. It is shown that the most active catalyst, 1-Bz^F5-3-n-octylimidazolium bromide (10), gives very good conversions and carbonate yields at 70 °C and below 5 bar CO₂ pressure, which is the lowest temperature for the synthesis of PC from PO and CO₂ using metal-free catalysts. Catalyst 10 can be recycled and reused at least 10 times without loss of yield and activity.