Chiral Ionic Liquids: Applications in Chemistry and Technology

 The current chapter describes the basic concepts, selected physicochemical properties, and general structural supramolecular organization of ionic liquids. The concepts and importance of ion pairs, supramolecular aggregates, and the organization of neat ionic liquids are also addressed in this chapter. These ionic fluids have also been used as chiral inductors, and the basis for this application is also evaluated in this chapter. The main objectives of this opening chapter of the book are to highlight selected examples showcasing the significance of chiral ionic liquids and their applications in chemistry, particularly in promoting chiral transmission.

Carbohydrates are nature's most prevalent bio-organic substance. Because of their benign nature and ubiquitous availability, the most relevant field of research is examining these chemicals for value-added uses. Although carbohydrate-derived chiral ionic liquids have shown promise in asymmetric synthesis, carbohydrate-based chiral auxiliaries, catalysts, and reagents have received little attention. Only CILs derived from isomannide and isosorbide proved useful for a variety of sustainable catalysis and asymmetric reactions. As a result, numerous research groups have recently developed carbohydrate-derived chiral ionic liquids from a naturally available chiral pool and evaluated their application in asymmetric synthesis and sustainable applications. This book chapter will cover the design, synthesis, and applications of chiral carbohydrate ionic liquids. 

Ionic liquids (ILs) are widely useful as catalysts or as a medium for reactions in varied chemical processes since they possess environment-friendly chemical/physical properties. Ionic liquids (ILs) are those chemical entities that consist of a cation and an anion having melting points less than 100 °C. Since the last decade, there has been an increase in the number of chiral ionic liquids (CILs) and their applications. Most chiral ILs have either a chiral cationic or chiral anionic center. There are also some CILs with both chiral cationic and chiral anionic centers. Molecules obtained from nature (biomolecules) are mostly degradable; a number of them are not toxic and are sustainable in nature. So, the development of CILs from biodegradable biomolecules provides an opportunity to further improve their greener aspects. Amino acids are a special kind of biomolecule due to their easy conversion into both anions and cations; the diverse functionalities in their side chains make them chiral and also enhance their properties. In comparison to various other chiral molecules, amino acids are cheaper and plentiful. In the last few years, an array of novel chiral ionic liquids were synthesized from simple, economical, naturally occurring terpenoids. In this chapter, very recent developments about the amino acids and terpenoid-based CILs have been reported and reviewed.

Ionic liquids (ILs) are low-melting compounds composed entirely of ions that exist as liquids at room temperature. Chiral ionic liquids (CILs) are a subclass of ILs that possess chiral characteristics. CILs are gaining immense attention as additives in enantioseparation techniques, such as capillary electrophoresis (CE). Capillary electrophoresis is a powerful analytical technique used for the separation of chiral compounds. CILs can affect the separation process through several mechanisms, including chiral recognition, modification of electrophoretic mobility, acting as a unique solvent system, and providing a chiral stationary phase. The use of CILs in the CE system offers several advantages for enantioseparation, including enhanced separation selectivity, improved resolution, and expanded applicability to a wide range of chiral compounds. However, the selection of an appropriate CIL and optimization of experimental conditions are critical to achieving the desired enantioseparation performance. Taking into account the blossoming research in the field, the present chapter summarizes the advancement in the application of CILs in capillary electrophoretic separations, taking examples from recent literature. 

Ionic liquids (ILs) are exceptional solvents having melting points at or below 100 0C. They are completely made up of ions, often consisting of an organic cation and an inorganic or organic anion. ILs having chiral moiety are referred to as chiral ionic liquids (CILs). Cations, anions, or both can be chiral in CILs. A CIL can have chiral azolinium, imidazolium, ammonium, or pyridinium as its chiral cationic component. Lactic acid, borate, or camphor sulfonate are some examples of the chiral anion. CILs have recently been used in electrophoretic techniques in different forms, such as chiral ligands, background electrolyte (BGE) additives, chiral selectors, and chiral stationary phases for the separation of chiral compounds. As they integrate the benefits of ILs and the features of a chiral moiety, they are thought to be particularly fascinating in chiral investigation. Notably, the use of CILs as chiral selectors offers advantages over other chiral selectors whose employment is typically constrained by a few issues, such as high-temperature instability, high UV absorptivity, complex synthetic methodologies, low solubility, and expensive nature. Therefore, it is crucial to consider how CILs can be used as solvents and chiral selectors. In this chapter, the diverse applications of chiral ionic liquids as stationary phases in electrophoretic separations are discussed in detail. 

Chiral ionic liquids (CILs) are a subcategory of ionic liquids that possess a chiral moiety. The need for chiral separations in several industries, including pharmaceutical, food, and chemical industries, has led to an increasing search for materials capable of performing such separations. CILs have emerged as effective candidates for the separation of enantiomers because of their advantageous properties like low melting point, little vapor pressure, high thermal stability, good electrical conductivity, and low cost. They are being employed in chromatographic methods as chiral ligands, stationary phases, and chiral selectors for the separation of chiral compounds. As compared to other chiral selectors (cyclodextrins, polysaccharides, surfactants, and crown ethers), CILs show better solubility, easy synthesis, and low cost. They represent an intriguing opportunity for use in chromatography because of their wide range of solubility in organic and inorganic solvents, as well as their miscibility with common solvents (methanol and acetonitrile). Considering the flourishing research in the field, the present chapter summarizes the advancement in the application of CILs as chiral ligands, stationary phases, and chiral selectors in liquid and gas chromatographic techniques. Furthermore, the chiral recognition mechanism and prospects for the use of CILs in enantioseparations have been examined.

Chiral recognition and separation methods have received a lot of attention due to the growing need for pure enantiomeric forms of substances. The separation of enantiomers is usually done with the aid of a chiral selector. Numerous chiral selectors, such as crown ethers, polysaccharides, antibiotics, etc., are extensively used in enantiomeric recognition studies; nevertheless, each one of them has limitations of its own. Recently, chiral ionic liquids (CILs), having inherent chirality due to the presence of a chiral cation or anion, have emerged as inexpensive and lucrative chiral selectors for enantiomeric recognition procedures. This article discusses the application of CILs in chiral recognition methods through spectroscopic techniques like UV-visible, NMR, and fluorescence spectroscopy. It also focuses on the mechanism behind chiral recognition.

The functionalized chiral ionic liquids (FCILs) can be considered a sub-class of chiral ionic liquids containing special functional groups for their use in fields like organo-catalysis, separation, purification, etc. The use of FCILs in the field of organocatalysis is particularly important because of the recyclability and high efficiency of these systems. In the current chapter, the applications of functionalized chiral ionic liquids in asymmetric organo-catalysis are summarized with a special emphasis on synthetically important organic reactions like asymmetric Michael reaction, aldol reaction, epoxidation, asymmetric transfer hydrogenation, and Diels-Alder reaction. 

Aqueous two-phase separation (ATPS) is a powerful, greener tool for the partitioning-based extraction and concentration of compounds in a mixture. ATPS is characterized by the separation of the biphasic layers upon the addition of the aqueous salt solution to aqueous ionic liquids. The aqueous biphasic system using chiral ionic liquids (CILs) has been studied for the resolution of the chiral molecules and their enantioseparation. Thus, in this chapter, abrief introduction to the ATPS formation thermodynamics for the polymer-polymer, polymer-salt, and salt-salt type ionic liquid isgiven. Apart from this, the effect of the salt, pH, and temperature on the physicochemical behavior of aqueous biphasic systems is discussed. The chiral ionic liquid-based ATPS is further categorized as tetrabutylammonium, choline, imidazolium, tropine, and quinine-based ionic liquids for enantioseparation. The study includes the effect of salts, cation of IL, anion of IL, temperature, pH, and Cu2+ ions. The chiral resolution of molecules and biomolecules is studied using these ionic liquids for amino acids and proteins.

 Supported ionic liquids (SILs) have been engaged in asymmetric synthesis, providing better recoverability, enantioselectivity, catalytic action, and economical and environmentally benign paths. SILs have minimized the limitations of previous homogeneous and heterogeneous systems and also opened new routes to design chiral heterogeneous systems with improved catalytic efficiency, including stability and recyclability. To carry out asymmetric reactions, both chiral catalysts and ILs have been supported onto a single support material, generally through a physical immobilization approach. In some cases, chiral ionic liquids have been grafted onto supports to obtain chiral SIL. The chirality has also been transferred into the resulting heterogeneous catalyst by covalently grafting chiral catalysts onto catalytic centers of supports by using ionic liquids as linkers. In this chapter, the potential catalytic role of all types of chiral SILs in several asymmetric organic reactions, such as hydrogenation, Mannich, epoxidation, Michael addition, Strecker, Diels-Alder reaction, etc., has been discussed.