Mechanism of chiral recognition in the enantioseparation of 2-aryloxypropionic acids on new brush-type chiral stationary phases

New brush-type chiral stationary phases (CSP I-IV) comprising N-3,5,6-trich loro-2,4-dicyanophenyl-L-alpha -amino acids (1-4) were prepared by binding of chiral selectors 1-4 to gamma -aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N-3,5,6-t richloro-2,4-dicyanophenyl-L-alanyl-(3-trietlioxysilyl)propylamide to unmod ified silica gel. The best separation of racemic 2-aryloxypropionic acids ( TR-1-13) was obtained with CSP I; the -(-)-S enantiomer were regularly elut ed first, as determined by a CD detector. The mechanism of chiral recogniti on implies a synergistic interaction of carboxylic acid analyte with the ch iral selector and achiral free gamma -aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separ ation ability for 2-aryloxypropionic acids, but a similar enantioselective discrimination of esters TR-19-20, in comparison with CSP I. CSP I-IV retai n unaltered separation ability after a few months of continuous work using a large number of various mobile phases. Chirality 13:581-581, 2001. (C) 20 01 Wiley-Liss, Inc.