Tetrabutylphosphonium amino acid ionic liquids as efficient catalysts for solvent-free Knoevenagel condensation reactions

Five tetrabutylphosphonium amino acid ionic liquids ([P-4444][AA]) were prepared, characterized, and used as catalysts for solvent-free Knoevenagel condensation reactions. The tetrabutylphosphonium prolinate ([P-4444][Pro]) showed excellent catalytic activity and selectivity in Knoevenagel condensation reactions of active methylene compounds with various aromatic aldehydes, and all the yields of corresponding products were more than 85% under mild conditions. Furthermore, a plausible reaction mechanism for the excellent performance of [P-4444][Pro] has been proposed, and [P-4444][Pro] could be used repetitively at least six times without obvious decrease in activity and quantity.