Cinchona-derived prolinamide in Brønsted acidic ionic liquids: A novel and recyclable catalytic system for asymmetric aldol reaction

Abstract

Cinchonine-derived prolinamide in Brønsted acidic ionic liquids was utilized for the first time as a novel and recyclable catalytic system for the asymmetric direct aldol reaction of acetone with aromatic aldehydes. The effects of the Brønsted acidity and amount of ionic liquid, co-solvents, catalyst amount, and reaction temperature on the catalytic activity and enantioselectivity were studied in detail. Good to high yields and ee values were achieved for the aldol reaction of various aromatic aldehydes with neat acetone in 1-methylimidazolium tetrafluoroborate ([Hmim]+BF4−) in the presence of 10mol% of catalyst at room temperature. [Hmim]+BF4− was also identified as a Brønsted acid additive for the reaction and beneficial to the enhancement of catalytic performance. Moreover, the enhanced interaction between prolinamide and acidic ionic liquid facilitated the recoverability and reusability of organocatalyst. The catalyst recycling experiments indicated that there was only a slight loss in activity and enantioselectivity after four runs.