Inclusion complexes of organic salts with β-cyclodextrin as organocatalysts for CO2 cycloaddition with epoxides†
The inclusion complexes between β-cyclodextrin (β-CD) and the organic bases 1,8-diazabicyclo-[5.4.0]undec-7-ene (DBU)-based phenolates have been prepared by a simple method and also characterized by FT-IR, 1H NMR, 19F NMR, TGA etc. Among these inclusion complexes, DBU-based 2,3,4,5,6-pentafluorophenolate as a guest compound bound by β-CD ([DBUH][PFPhO]/β-CD) has been employed as an easily-separable organocatalyst for the cycloaddition of CO2 into cyclic carbonate and exhibited the best catalytic performance. High conversion of epoxides and excellent selectivity to carbonates could be achieved at 130 °C and under the 3.0 MPa CO2 without additional organic solvents or additives. Additionally, the organocatalyst [DBUH][PFPhO]/β-CD exhibited the better recycability in consecutive catalytic recycles, as compared with that of corresponding DBU-based phenolates. The β-CD played a crucial role in immobilizing catalytically active species and thus improving the recyclability of the present organocatalysts. The detailed characterization indicated that phenolate anions had been bound inside the cavity of β-CD, while the [DBUH]+ cation was located outside of β-CD. More interestingly, it was observed that phenolate anions could dissociate from the β-CD cavity under the reaction temperature, but the inclusion compound could form on cooling the reaction mixture after reaction, which was extremely attractive for separation and recycling of the supramolecular organocatalysts. Finally, on the basis of the characterization above, a reaction mechanism for the present organocatalysts has been proposed.