Enantioselective Liquid–Liquid Extraction of Underivatized Amino Acids with Simple Chiral Aminophenyl‐Aldehyde

The derivative of aminophenyl‐aldehyde with an asymmetric carbon and an uryl group, ( S )‐ 2 , was synthesized. The combination of ( S )‐ 2 and aliquat‐336 in CDCl 3 extracted underivatized amino acids in water layer by imine formation with enantioselectivities of 12/1 for Phe, 13/1 for Val, and 12/1 for Leu, which are comparable with those of previously reported binaphthol‐based extractor ( S )‐ 1 . The enantioselectivities of ( S )‐ 2 is remarkable considering the low molecular weight compared to ( S )‐ 1 . Density functional theory computations and experimental data demonstrate that imine bond is strengthened by resonance‐assisted hydrogen bond with the nearby NH group.