Advances in Organic Synthesis: Volume 10

Atta-ur-Rahman

• Publisher: Bentham Science Publishers
• Year: 2018
• ISBN:
  9781681087436
• Topic: Physical Sciences
• Subtopic: Organic Chemistry

Advances in Organic Synthesis is a book series devoted to the latest advances in synthetic approaches towards challenging structures. The series presents comprehensive reviews written by eminent authorities on different synthetic approaches to selected ta

Acidic and Basic Functionalized Ionic Liquid Systems for Advanced Synthesis of Five and Six Membered Nitrogenates Heterocycles

Pinky Gogoi¹, Susmita Saikia¹, Arup Kumar Dutta², Ruli Borah^{*, 1}

¹ Department of Chemical Sciences, Tezpur University, Napaam-784028, Tezpur, Assam, India

² Department of Chemistry, Pandit Deen Dayal Upadhyaya Adarsha Mahavidalaya, Behali, Jinjia-784184, Assam, India

Abstract

This book chapter describes a brief introduction on acidic and basic functional group tethered ionic liquid systems with various ion-pairs and their utilization as recyclable catalyst/medium in designing sustainable advanced synthetic methods of selected five and six membered nitrogenated heterocycles via one-pot approach.

Keywords: Acidic ionic liquids, Basic ionic liquids, Multicomponent, N-Heterocycles, One-pot reaction, Reusable catalysts, Sustainable method.

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^* Corresponding author Ruli Borah: Department of Chemical Sciences, Tezpur University, Napaam-784028, Tezpur, Assam, India; Tel: [phone]; Fax: [phone]; E-mail: [email protected]

INTRODUCTION ON IONIC LIQUID SYSTEMS

The development of target oriented functionalized ionic liquids (FILs) with unique physicochemical properties has received major recognition in different research fields with diversified applications in catalysis [1], such as reaction medium for organic reactions [2], biocatalysis [3], biomass treatment and processing [4], electrochemistry [5], analytical chemistry [6, 7], coordination chemistry [8], polymer chemistry [9], fluorine chemistry [10], nanotechnology [11], material science [12] and other miscellaneous uses [13].

The various types of ionic liquids (ILs) are represented in Fig. (1) which can be extended to cover new families and generations of ionic liquids with target oriented properties.

This chapter covers a brief description of target-oriented acidic/basic character ionic liquids of different ion pairs and also their modification with support material as recyclable catalyst/medium for advanced synthetic routes of some five and six membered nitrogen heterocycles. The first review on room temperature ionic liquids was published by Welton in 1999 and from then onwards ionic liquids have grown as a major research area in the last 18 years [1, 14, 15]. Ionic liquids include a wide variety of molten organic salts comprising organic cations and inorganic or organic anions having variable physicochemical properties based on the nature of constituent ion pairs. The inherent properties of ionic liquids can be considered as a function of ionic interaction of the corresponding ion-pairs and coexistence of distinct structural environment of the component ions [16]. These ionic interactions can be varied by tethering appropriate functional groups or atoms either in the cation or anion of the ionic liquid.

Fig. (1))
Representation of IL generations.

They show low vapor pressures, wide range of thermal stability and high boiling points owing to their strong secondary ionic interactions. The term “designer liquids” is also used to express their environment-friendly behavior in chemical reactions as medium or catalyst and as electrolyte in electrochemistry with specific physical and chemical properties. They are known to be ideal replacement of volatile organic solvents in reaction due to their advantageous non-volatile, nonflammable, recyclable and outstanding solvating properties. Furthermore, the study of molecular mechanism of reaction in ionic liquid environment suggested faster rate of reaction as compared to conventional process [17^-19].

Ionic liquids that contain aromatic heterocyclic cations tend to have lower melting points than those containing aliphatic ammonium salts. More electronegative nature of the anion decreases the melting point of ionic liquid. Structural modification of the cations or anions of ionic liquids can result in unique phase transition from liquid to solid state that dramatically influences the strategies of IL catalyzed reactions either in homogeneous or heterogeneous conditions. Davis et al. in 2004 [20] introduced the term “task specific ionic liquid” in a brief review presented on target-oriented ionic liquids for organic synthesis. He also utilized thiazolium based organic ionic liquids as dual functionalized solvent / catalyst system for benzoin condensation [21].

These target-oriented Functionalized Ionic Liquids (FILs) are the latest generation of ionic liquids where a functional group is covalently attached with either the cation or anion (or both) of the organic salt as per requirement of a particular use. The incorporation of this functionality should imbue the ionic liquid with a capacity to behave not only as reaction medium but also as a reagent or catalyst in some reactions or processes. The functionalization of cation or anion of ionic liquids allows amazing tunability of ionic-liquid properties, including acidity, basicity, melting point...