Advantages of polar organic solvent chromatography for enantioseparation of chiral liquid crystals

Abstract

Three sets of fluorinated chiral liquid crystals were used to explore the polar organic solvent chromatography mode for their enantioseparation. The materials include a set of newly synthesized compounds with chiral center derived from 2-hexanol and two sets of compounds with chiral center derived from 2-/3-octanol. Baseline enantioseparation of all materials was achieved using binary mobile phases without additives. For some of the compounds exceedingly high values of enantioresolution (> 20) and enantioselectivity (> 4) were found. The chromatographic behavior of the sample set was studied on three different polysaccharide-based chiral columns – Chiralpak IA-U, IG-U and IB-U. Comparison of results from Chiralpak IA-U and IB-U shows the effect of amylose vs. cellulose polysaccharide backbone while comparison of Chiralpak IA-U and IG-U reveals the effect of 3,5-dimethylphenylcarbamate vs. 3‑chloro-5-methylphenylcarbamate substituent. The mobile phases tested included whole range of acetonitrile/methanol mixtures to demonstrate that acetonitrile-rich and alcohol-rich mobile phases offer different enantiorecognition mechanisms and can provide complementarity to some extent. The effect of temperature on enantioseparation was investigated on Chiralpak IA-U by constructing van't Hoff plots for selected liquid crystals in pure acetonitrile and pure methanol as mobile phases.