Abstract:

Recognition of chiral molecules is very important for the chemical and pharmaceutical industries. Three kinds of urea containing chiral centers (L-Phe-U, L-Ala-U and L-Val-U) were used as chiral solvating agents (CSAs) to identify chiral organic carboxylic acids with different substituents and their chiral anion recognition properties were examined by NMR spectroscopy. In the presence of 4-dimethylaminopyridine (DMAP), the enantiomeric signals of the benzylic C-H protons of mandelic acid were achieved upon mixing with L-Phe-U, the ΔΔδ values of the C_αH signals were ranged from 2.4 to 16.0 Hz, and the enantiomer excess (ee) value of rac-mandelic acid (rac-MA) can be determined accurately. It was found that the phenyl group contained in chiral solvating agents and the groups connected to the chiral carbon center of the substrates would affect separation of the enantiomeric signals. In addition, DOSY spectra demonstrate diffusion coefficient of the rac-MA enantiomers are different in the chiral solvating agents, and the dynamics of chiral recognition are revealed.

Key words: Nuclear magnetic resonance (NMR), Chiral solvating agents, Chiral carboxylic acid, Enantiomeric recognition, Chemical shift difference, Enantiomeric excess