Development Of New Chiral Selectors For Liquid Chromatography And Their Performance Relative To Established Chiral Stationary Phases (CSPS)

Abstract

HPLC with chiral stationary phases (CSPs) has proven to be the most widely applicable and versatile technique for enantiomeric separations. This dissertation discusses new research in three important areas of enantiomeric separations: HPLC method developments for target chiral analytes using the existing CSPs, mechanistic investigations by NMR, and development of new HPLC CSPs. Two classes of commercial chiral stationary phases (cyclodextrin-type and macrocyclic glycopeptide-type) were evaluated by separating specific groups of chiral analytes, including β-lactam compounds and ruthenium(II) polypyridyl complexes. The effects of chiral selector structures, mobile phase compositions, and analyte structures have been examined. For Ru(II) complexes, the newly developed Chirobiotic T2 column works more successfully than the other glycopeptide-based stationary phases, while R-naphthylethyl carbamate-functionalized β-cyclodextrin CSP (Cyclobond RN) is the most effective among all tested cyclodextrin-based columns. The exceptional separation capabilities of cyclodextrins (CDs) for Ru complexes prompted us to conduct NMR mechanistic studies. Enantioselective host-guest complexation between Ru complexes and CDs was investigated and correlated to the separation results (HPLC and CE). It was found that aromatic and anionic derivatizing groups on CDs were beneficial for chiral recognition.Also, enantiopure Ru complexes and a new class of cyclic oligosaccharides, cyclofructans (CFs), were introduced as bonded chiral stationary phases. Three different chemistries linking the Ru complex to the silica gel were studied. The CSP synthesized by reductive amination of aldehyde-functionlized silica worked best. Extensive liquid chromatographic studies with UV and CD detectors show Ru complex-bonded CSP provide enantioselectivity toward a wide variety of compounds, in particular, compounds with acidic groups. A new class of chiral selectors, cyclofructans, have been bonded to the silica gel for the first time to develop chiral stationary phases. Native CF6 appeared to be a poor chiral selector due to extensive internal hydrogen bonding, which could possibly block the interactions with chiral analytes. Therefore, new CSPs have been designed by derivatizing CF6, in order to disrupt internal hydrogen bonding. Derivatized-CF6 CSPs appeared to separate a very broad range of chiral analytes. Particularly, aliphatic-functionalized CF6 with a low substitution degree baseline separated all tested primary amines. In addition, this class of CSPs has great potential for preparative separations.