Ionic liquids are salts composed of organic cations and inorganic or organic anions that are liquid at or near room temperature. Ionic liquids are usually called room temperature ionic liquids or room temperature molten salts. As a designable substance, ionic liquids have special properties that traditional solvents do not have, such as good thermal stability, strong solubility, moderate viscosity, high conductivity, strong catalytic performance and others. Therefore, ionic liquids have been widely used in the fields of organic chemistry, electrochemistry, biological science, material science and medicine. A chiral ionic liquid is formed by introducing chiral centers into the ionic liquid. Chiral ionic liquids, as an important branch of ionic liquids, have both chiral and ionic liquid functions, which has attracted extensive attention of researchers in recent years. The general structures of chiral ionic liquids are shown in Figure 1.
Figure 1. The general structures of chiral ionic liquids.
With the development of science and technology, chiral ionic liquids with various structures and functions have been synthesized, and they have been widely used in asymmetric synthesis catalysis, spectral recognition analysis and others.
- Asymmetric synthesis catalysis: Asymmetric synthesis, also known as chiral synthesis, stereoselective synthesis, is a chemical reaction in which reactants are introduced into one or more chiral units. High enantioselectivity is required for asymmetric synthesis reactions. In addition, the chiral reagents used in the reaction should be readily available and recyclable. Chiral ionic liquids have both chirality and good solubility, which can meet the above requirements. Therefore, chiral ionic liquids have become a new hotspot in catalytic asymmetric synthesis reactions. At present, chiral ionic liquids have been widely used in asymmetric Michael addition reactions, Diels-Alder reactions, Aldol reactions, Baylis-Hillman addition reactions and other asymmetric synthesis.
Figure 2. An example of chiral ionic liquid used in Asymmetric synthesis.
- Spectral recognition analysis: Chiral recognition ability is an important function of chiral ionic liquids. The realization of chiral recognition is through the chiral enantiomer and the chiral environment generated by hydrogen bonding, hydrophobic interaction, electrostatic interaction and other non-covalent bond forces. Under the action of multi-site recognition, diastereomer complexes are formed, and their physical or chemical differences are used to realize the ability of recognition and separation. This ability has been used in a wide range of spectroscopic analyses, including nuclear magnetic resonance spectroscopy, fluorescence spectroscopy and others.
- Others: In addition to the above fields, chiral ionic liquids have important applications in many other fields including chromatographic separation, synthesis of liquid crystal materials, surface modification of gold nanoparticles, extraction separation and others. For example, chiral ionic liquids play an important role in the field of liquid crystal materials by taking advantage of their wide electrochemical window, structural tunability, good solubility and optical rotation of chiral molecules.
Chiral ionic liquids are divided into ionic liquids containing chiral cations, ionic liquids containing chiral anions and ionic liquids containing both chiral cations and chiral anions. Among them, ionic liquids containing chiral cations are the most common, including chiral imidazole salt chiral ionic liquid, chiral quaternary ammonium salt chiral ionic liquid, chiral thiazole salt chiral ionic liquid, chiral pyridine salts chiral ionic liquids and others.
- Long, Zhang, Lingyun, et al. Functionalized Chiral Ionic Liquid Catalyzed Asymmetric SN1 α-Alkylation of Ketones and Aldehydes[J]. European Journal of Organic Chemistry, 2010.