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‐trichloro‐2,4‐dicyanophenyl‐L‐α‐amino acids ( 1–4 ) were prepared by binding of chiral selectors 1–4 to γ‐aminopropyl silica gel. To check the role of excess free aminopropyl groups, CSP V was prepared by binding N‐3,5,6‐trichloro‐2,4‐dicyanophenyl‐L‐alanyl‐(3‐triethoxysilyl)propylamide to unmodified silica gel. The best separation of racemic 2‐aryloxypropionic acids ( TR‐1‐13 ) was obtained with CSP I; the ‐(−)‐ S enantiomer were regularly eluted first, as determined by a CD detector. The mechanism of chiral recognition implies a synergistic interaction of carboxylic acid analyte with the chiral selector and achiral free γ‐aminopropyl units on silica. In fact, CSP V, which is lacking an achiral aminopropyl spacer, shows a lower separation ability for 2‐aryloxypropionic acids, but a similar enantioselective discrimination of esters TR‐19‐20, in comparison with CSP I. CSP I–IV retain unaltered separation ability after a few months of continuous work using a large number of various mobile phases. Chirality 13:581–587, 2001. © 2001 Wiley‐Liss, Inc.