CO2, COS and CS2 adducts of N-heterocyclic olefins and their application as organocatalysts for carbon dioxide fixation

Abstract

Various CO₂, COS and CS₂ adducts of N-heterocyclic olefins (NHOs) were synthesized and characterised by single crystal X-ray crystallography. The length of the C–O bond in NHO–COS adducts is slightly shorter than that in the corresponding NHO–CO₂ adducts, while the C–S bond length in NHO–COS adducts is significantly longer than the corresponding C–S distance in the NHO–CS₂ adducts, suggesting that the negative charge of the NHO–COS adducts is preferentially delocalized on the sulfur atom. The length of the C_CS2–C_NHO bond is significantly shorter than that of the C_COS–C_NHO or C_CO2–C_NHO bond in the corresponding NHO–COS or NHO–CO₂ adducts, implying the relatively high thermal stability of NHO–CS₂ adducts. The NHO–CO₂, NHO–COS and NHO–CS₂ adducts were found to be efficient in catalyzing the cycloaddition reaction of CO₂ and epoxides to selectively afford the corresponding cyclic carbonates. Among them, NHO–CO₂ adducts were found to be more active, while NHO–CS₂ adducts exhibited the lowest activity for this reaction, probably due to their high stability for difficult release of the highly active NHO.