Chiral NMR discrimination of secondary amines using (18-crown-6)-2,3,11,12-tetracarboxylic acid.

[reaction: see text] Enantiomeric discrimination is observed in the (1)H NMR spectra of chiral secondary amines in the presence of (R)-(+)-(18-crown-6)-2,3,11,12-tetracarboxylic acid. Secondary amines are protonated by one of the carboxylic acid groups of the crown ether to produce the corresponding ammonium and carboxylate ions. The secondary ammonium ion likely forms two hydrogen bonds to crown ether oxygen atoms and an ion pair with the carboxylate anion.