9-O-(Phenylcarbamoyl)quinine-bonded Carbon-Clad Zirconia for Chiral Separation of Racemic 2,4-Dinitrophenyl Amino Acids in RPLC

Abstract

Quinine has been widely used for enantiomer separation in HPLC as a chiral ion-pairing agent in the mobile phases and as a CSP ligand. In recent years, chiral stationary phases (CSPs) based on the use of carbamoylated derivatives of quinine and quinidine as selectors were found to be highly stereoselective for the direct resolution of chiral acids using mixtures of aqueous buffers and methanol or acetonitrile as mobile phases. Silica is the most popular choice for support of HPLC stationary phase ligands due to the mechanical strength, wide range of particle size and pore dimensions, pore structure and well-established silane chemistry. However, silica and bonded phase ligands have stability problems. Silica dissolves in mobile phase buffered at or above pH 8 with loss of bonded phase ligand and column packing. Loss of organosilanes from the silica surface via hydrolysis proceeds rapidly at low pH (< 3) and at high temperature (≥ 40 C). These deficiencies of the column packing create problems of poor injection reproducibility, poor peak shape, and high backpressure, thus making method development tasks difficult. Zirconia particles are very robust material; they show no detectable signs of dissolution over the pH range from 1 to 14 and have been used for prolonged periods at temperatures up to 200 C in chromatographic separations. Over the last decade, zirconia has received considerable attention as stationary phase support for HPLC. We have been working to develop efficient and chemically stable CSPs on zirconia substrate. Bare zirconia cannot be covalently modified like silica due to the instability of Zr-C and Zr-OSi bonds in water. Zirconia-based CSPs reported have thus been prepared by coating chiral selectors on zirconia surface by utilizing Lewis acid-base chemistry. Recently we reported chiral separation of N-(2,4-dinitrophenyl) (DNP) amino acids on a 9-O-(phenyloxycarbonyl)quinine bonded carbon-clad zirconia (QNOCZ) in reversedphase liquid chromatography (RPLC). Carbon-clad zirconia is made by passing organic vapors over very hot porous zirconia. Carbon-clad zirconia particles show similar mechanical, thermal and chemical stability to bare zirconia particles but no appreciable Lewis acidity. They do not exhibit peak tailing for amines nor do they adsorb phosphates or carboxylates. Although QNOCZ gave better chiral selectivity for amino acids studied than quinine-coated zirconia and quinine-bonded silica, synthetic yield of the chiral selector, 9-O-(4-aminophenyloxycarbonyl)quinine was very low (14%) and bonding yield of the selector to carbon-clad zirconia was also found to be also very low due to several intermolecular site reactions of the diazonium salt of the selector. In this work we prepared 9-O-(phenylcarbamoyl)quininebonded carbon-clad zirconia (QNCCZ) in the hope of obtaining better synthetic and bonding yields than those for QNOCZ. It was also expected that different functionality in the linkage (carbamoyl vs. oxycarbonyl) would show different retention and selectivity behavior. QNCCZ was used as the chiral stationary phase (CSP) for separation of enantiomers of DNP-amino acids in RPLC. Retention and enantioselectivity for QNCCZ were compared to those for QNOCZ and quinine carbamate-bonded silica (QNS).