Alkoxide-functionalized imidazolium betaines for CO2 activation and catalytic transformation

Abstract

Alkoxide-functionalized imidazolium betaines (AFIBs), including an alkoxide anion and an imidazolium cation, were synthesized by treating potassium tert-butoxide with 1-(2-hydroxyethyl)-2,3-disubstituted imidazolium bromide. The novel betaines were able to quickly capture CO₂, affording carboxylate zwitterions (AFIB-CO₂ adducts). In the presence of adventitious water, the transformation of the AFIB-CO₂ adducts into the corresponding bicarbonate salts was observed by ¹H and ¹³C NMR spectroscopy. The structures of the AFIB bicarbonate salts were solved using single crystal X-ray crystallography. Furthermore, the dithiocarboxylate zwitterions (AFIB-CS₂ adducts), which are more stable to moisture in comparison with their CO₂ adducts, were prepared by reacting CS₂ with the corresponding betaines. X-Ray single crystal analysis revealed the bent geometry of the binding CS₂ in the dithiocarboxylate zwitterions with a S–C–S angle of 126.6–126.9°, which indirectly confirms the structures of the AFIB-CO₂ adducts in hand. These AFIB-CO₂ adducts were found to function as organocatalysts for the coupling reaction of propargylic alcohols with CO₂ for selectively producing valuable cyclic carbonates under mild and solvent-free reaction conditions.