NORMAL PHASE CHIRAL SEPARATION OF HEXAHELICENE ISOMERS USING A CHIRAL SURFACE CONFINED IONIC LIQUID STATIONARY PHASE

Recent advances in the enantioseparation promoted by ionic liquids and their resolution mechanisms

More and more various chemical media are being applied in enantioseparation; among them, ionic liquids (ILs) have attracted the long-term attention in this decade as green designable solvents. This paper provides comprehensive overview for the applications of ILs in chiral extraction, gas chromatography, liquid chromatography, capillary electrophoresis and other techniques for enantioseparation. Additionally, the important resolution mechanisms based on ILs have also been summarized and discussed. This review focuses on the latest development of enantioseparation methods by using ILs in various modes, leading to meaningful and valuable information to related fields and thus promotes further research and application of reported methods.

Chiral Ionic Liquids: Structural Diversity, Properties and Applications in Selected Separation Techniques

Ionic liquids (ILs) are chemical compounds composed of ions with melting points below 100 °C exhibiting a design feature. ILs are commonly used as the so-called green solvents, reagents or highly efficient catalysts in varied chemical processes. The huge application potential of ionic liquids (IL) justifies the growing interest in these compounds. In the last decade, increasing attention has been devoted to the development of new methods in the synthesis of stable chiral ionic liquids (CILs) and their application in various separation techniques. The beginnings of the successful use of CILs to separate enantiomers date back to the 1990 s. Most chiral ILs are based on chiral cations or chiral anions. There is also a limited number of CILs possessing both a chiral cation and a chiral anion. Due to the high molecular diversity of both ions, of which at least one has a chiral center, we have the possibility to design a large variety of optically active structures, thus expanding the range of CIL applications. Research utilizing chiral ionic liquids only recently has become more popular. However, it is the area that still has great potential for future development. This review aimed to describe the diversity of structures, properties and examples of applications of chiral ionic liquids as new chiral solid materials and chiral components of the anisotropic environment, providing chiral recognition of enantiomeric analytes, which is useful in liquid chromatography, countercurrent chromatography and other various CIL-based extraction techniques including aqueous biphasic (ABS) extraction systems, solid-liquid two-phase systems, liquid-liquid extraction systems with hydrophilic CILs, liquid-liquid extraction systems with hydrophobic CILs, solid-phase extraction and induced-precipitation techniques developed in the recent years. The growing demand for pure enantiomers in the pharmaceutical and food industries sparks further development in the field of extraction and separation systems modified with CILs highlighting them as affordable and environmentally friendly both chiral selectors and solvents.

Renewable sources from plants as the starting material for designing new terpene chiral ionic liquids used for the chromatographic separation of acidic enantiomers

Synthesis of cheap and natural resources is an important topic in green chemistry.

Polycyclic aromatic hydrocarbons as model solutes for carbon nanomaterials in ionic liquids

The aim of this work is to understand the details of the interactions of ionic liquids with carbon nanomaterials (graphene and nanotubes) using polyaromatic compounds as model solutes.

Synthesis of novel chiral imidazolium stationary phases and their enantioseparation evaluation by high-performance liquid chromatography

Two novel chiral stationary phases (CSPs) were prepared by bonding chiral imidazoliums on the surface of silica gel. The chiral imidazoles were derivatized from chiral amines, 1-phenylethylamine and 1-(1-naphthyl)ethylamine. The obtained CSPs were characterized by Fourier Transform Infrared (FT-IR) spectroscopy and elemental analysis (EA), demonstrating the bonding densities of CSP 1 and CSP 2 were 0.43 mmol g^-1 and 0.40 mmol g^-1, respectively. These two CSPs could be used to availably separate 8 pharmaceuticals, 7 mandelic acid/its derivatives, 2 1-phenylethylamine derivatives, 1 1,1'-bi-2-naphthol, and 1 camphorsulfonic acid in high-performance liquid chromatography (HPLC). It is found that CSP 1 could effectively enantioseparate most chiral analytes, especially the acidic components, while CSP 2 could enantiorecognize all chiral analytes, although a number of components did not achieve baseline separation. Additionally, the effects of mobile phase composition, mobile phase pH and salt content, chiral selector structures, and analyte structures on the enantiorecognitions of the two CSPs were investigated. It is found that high acetonitrile content in mobile phases was conducive to enantiorecognition. Mobile phase pH and salt content could alter the retention behaviors of different enantiomers of the same chiral compound, resulting in better enantioresolution. Moreover, both chiral selector structures and substituted groups of analytes played a significant role in the separation of chiral solutes.

Recent development of ionic liquid stationary phases for liquid chromatography

Based on their particular physicochemical characteristics, ionic liquids have been widely applied in many fields of analytical chemistry. Many types of ionic liquids were immobilized on a support like silica or monolith as stationary phases for liquid chromatography. Moreover, different approaches were developed to bond covalently ionic liquids onto the supporting materials. The obtained ionic liquid stationary phases show multi-mode mechanism including hydrophobic, hydrophilic, hydrogen bond, anion exchange, π-π, and dipole-dipole interactions. Therefore, they could be used in different chromatographic modes including ion-exchange, RPLC, NPLC and HILIC to separate various classes of compounds. This review mainly summarizes the immobilized patterns and types of ionic liquid stationary phases, their retention mechanisms and applications in the recent five years.