Chiral NMR discrimination of amines: Analysis of secondary, tertiary, and prochiral amines using (18-crown-6)-2,3,11,12-tetracarboxylic acid

Enantiomeric discrimination is observed in the H-1 and C-13 NMR spectra of secondary and tertiary amines in the presence of (-)-(18-crown-6)-2,3,11,12-tetracarboxylic acid (1). Nonequivalence of the resonances of prochiral nuclei in primary and secondary amines is also observed when they associate with 1. The amines are added in their neutral form and are protonated by the carboxylic acid groups of 1 to produce the corresponding ammonium and carboxylate ions. Secondary amines associate with 1 through two hydrogen bonds and an ion pair interaction. Tertiary amines can only form one hydrogen bond to accompany the ion pairing. Chiral discrimination in the H-1 and C-13 NMR spectra of a series of aryl-containing secondary amines is of sufficient magnitude to determine enantiometic purities. The discrimination in the spectra of tertiary amines with 1 is smaller, but C-13 NMR spectra provided enough distinction for the determination of enantiomeric purity.