Ionic liquid: A green solvent for organic transformations I

Ionic liquids are a class of unconventional solvent having unique chemical and physical properties. The entire molecular framework of ionic liquids is made up of ions. Molten sodium chloride, for example, is an ionic liquid but a solution of sodium chloride in water is an ionic solution. The term molten salts, which were previously used to describe such materials, conjure an image of hot, viscous and highly corrosive medium. The term ionic liquid, in contrast, refers to a material that is fluid at temperature as low as -96°C, colourless, low viscosity and is easily handled. In patent and academic literature, the term "ionic liquids" now refers to liquids composed entirely of ions that are fluid around or below 100°C. Ionic liquids are not new. Some of them have been known for many years. For instance, ethylammonium nitrite, [EtNH₃][NO₃], which has a melting point of 12°C, was first known in 1914 (1). It was proposed that these ionic liquids provide a useful extension to the range of solvents that are available for synthetic chemistry. However, it was only until recently that the research into ionic liquids intensifies (2). This is mainly due to their favourable properties, such as non-flammability, no measurable vapour pressure, low toxicity, reusability, low cost and high thermal stability. In addition to the polar properties of the ionic liquids, they are non-coordinating, which can avoid any undesired solvent binding in pretransition states. Ionic liquids have also been referred to as "designer solvent". This is because the polarity and hydrophilicity can be controlled by careful selection of cations or anions. Moreover, the ionic liquids can be designed and tuned to optimize yield, selectivity, substrate solubility, product separation, and even enantioselectivity. As a result, ionic liquids are considered to be promising non-conventional solvents for organic reactions. Note that ionic liquids are not intrinsically "green" (some are extremely toxic), but they can be designed to be environmentally friendly, with many potential benefits for sustainable chemistry (3). Because research into ionic liquids is at an early stage, many of their properties remain to be uncovered. Nevertheless, ionic liquids have been found to be potentially viable solvents for organic synthesis with promising results in many organic reactions, such as hydrogenation (4), hydroformylation (5), Friedel-Crafts acylation (6), Diels-Alder reaction (7), asymmetric allylation reactions (8), asymmetric epoxidation of alkenes (9), and asymmetric ring-opening of epoxides (10). Thus, ionic liquids can be a useful green alternative to replace organic solvents used in organic transformations. Due to their unique properties, we envisage that ionic liquids will provide interesting perspectives of how green chemistry can be integrated into organic chemistry. In this article, we will provide an up-to-date discussion with regards to the investigations we have done on using ionic liquids in reactions involving the formation of C-C bonds, namely, three-component Mannich-type reaction, direct aldol reaction and Mukaiyama aldol reaction. In the next chapter, we will further discuss our investigation on using ionic liquids in allylation reaction and the synthesis of bis(indolyl)methanes and polyhydroquinoline derivatives.