Ionic Liquids: Applications in Heterocyclic Synthesis

Abstract

Ionic liquids (ILs) have become omnipresent in the recent chemical literature; for they can be used as highly customizable solvents for almost any synthetic purpose [Wasserscheid & Welton, 2008]. Especially in the industry, their application goes beyond their use as solvents. The highly diverse properties of these materials make possible a surprising number of applications. In organic reactions, although ionic liquids were initially introduced as alternative green reaction media because of their unique chemical and physical properties of nonvolatility, nonflammability, thermal stability, and controlled miscibility, today they have marched far beyond this boundary, showing their significant role in controlling reactions as solvent or catalysts [Wasserscheid & Welton, 2008]. It is well-known that the microenvironment generated by a solvent can change the outcome of a reaction, in terms of both equilibria and rates [Pârvulescu & Hardacre, 2007]. Since ionic liquids have the potential to provide reaction media that are quite unlike any other available at room temperature, it is possible that they will dramatically affect reactions carried out in them. Undeniably, there have been many claims of great improvements in reaction yields and rates when using ionic liquids [Chiappe & Pieraccini, 2005]. Over the past decade, some authors have manifested interest in providing facts to clarify the question: “how do ionic liquids act in organic reactions?” They have found answers for particular reactions, in that ionic liquids play specific roles depending on the reaction [Martins et al., 2008]. This chapter presents some questions and the best results to afford answers about the role of ILs in the most important reactions involved in heterocyclic synthesis: cyclocondensation and 1,3dipolar cycloaddition reactions. Heterocycles form by far the largest of the classical divisions of organic chemistry. Moreover, they are of immense importance not only both biologically and industrially but to the functioning of any developed human society as well. Their participation in a wide range of areas cannot be underestimated. The majority of pharmaceutical products that mimic natural products with biological activity are heterocycles. Most of the significant advances against disease have been made by designing and testing new structures, which are often heteroaromatic derivatives. In fact, in the Comprehensive Medicinal Chemistry (CMC) database, more than 67% of the compounds listed contain heterocyclic rings [Xu & Stevenson, 2000]. Other important practical applications of heterocycles can also be cited, for instance, additives and modifiers in a wide variety of industries including cosmetics,