Elsevier

Catalysis Communications

Volume 9, Issue 13, 20 July 2008, Pages 2264-2268
Catalysis Communications

Solvent-free synthesis of benzoic esters and benzyl esters in novel Brønsted acidic ionic liquids under microwave irradiation

https://doi.org/10.1016/j.catcom.2008.05.015Get rights and content

Abstract

Fischer esterifications of benzoic acid with a variety of alcohols and a variety of acids with benzyl alcohol could be promoted by four novel Brønsted acidic ionic liquids a–d that bear a benzyl on the imidazolium or pyridinium cation. The reactions were carried out smoothly without the need of added catalyst with good to excellent isolated yields (77–98.5%) under solvent-free microwave irradiation conditions in the presence of 50 mol% ionic liquids. These ionic liquids could be recovered easily and recycled three times without any significant loss in their catalytic activity.

Introduction

Aromatic esters are very important chemicals having widely applications in a variety of areas in chemical industry such as perfumes, flavours, pharmaceuticals, plasticisers, solvents and intermediates [1], there are many routes [2], [3] available for their preparation, the simpler and most widely employed method is the Fisher esterification. However, the conventional Fisher esterification suffers from the problems associated with the generation of side reactions (such as oxidation, etherification), corrosion of the equipments, tedious purification procedures, long reaction time and discharge large volumes of acidic wastes. In addition to these problems, this industrially important process also suffers from using volatile organic solvents. Facing the ecological and economic increasing challenge, there is an urgent need to develop an eco-friendly, simple and efficient method for the synthesis of aromatic esters. Recently, the functional ionic liquids have crossed the barrier of solvent and entered successfully into the area of catalyst [4], [5], [6], [7], [8], [9], [10], [11], [12]. For example, Brønsted acidic ionic liquids [BAILs], which can take dual role in organic reactions as solvents and catalysts. Many typical acid-catalyzed organic reactions have been carried out successfully in them [13], [14], [15], [16], [17], [18], among which esterifications are hot topic [19], [20], [21], [22], [23], [24], [25]. Compare with conventional homogeneous and heterogeneous Brønsted acid catalysts, these published reports of esterifications using BAILs demonstrated the advantages of reactions carried out under solvent-free condition, the esters can be separated by easy decantation and BAILs can be recycled.

The application of microwave irradiation in synthetic chemistry is a fast-growing research area [26], this has opened up the possibility of realizing fast synthesis of organic compounds. From the perspective of microwave chemistry, the strong polar nature of ionic liquids makes them ideal reaction medium in solvent-free microwave irradiation.

To combine the advantages of microwave irradiation and ionic liquids, recently, many interesting studies [27], [28], [29], [30], [31] have been published. However in these reports the ionic liquids acted only as reaction medium and require additional catalyst to promote the reaction proceed. Bazureau et al. [21] firstly reported the Fischer esterification in BAILs under microwave irradiation and satisfactory results were obtained, but this report has not been entirely satisfactory with drawbacks such as the BAILs need modified with 5% concentrated sulfuric, using an excess alcohol and higher amount of BAILs (3 equiv.). In continuation of our studies on green Fischer esterification [32], herein we wish to report the first study on application of four inexpensive and easily accessible BAILs ad for esterifications of benzoic acid with aliphatic alcohols and aliphatic acids, benzoic acid with benzyl alcohol under solvent-free microwave irradiation conditions without any added catalyst (Scheme 1). This synthetic strategy combines the merits of both ionic liquids and microwave irradiation realized the green synthesis of benzoic esters and benzyl esters.

Section snippets

Experimental

All reagents and solvents were pure analytical grade materials purchased from commercial sources and were used without further purification if not stated otherwise.

Melting points were measured using Gallen Kamp melting point apparatus. IR spectra were recorded with a Perkin-Elmer FT-IR 240-c spectrophotometer using KBr optics. 1HNMR (400 MHz) spectra were recorded on NOVO-400H spectrometer in D2O with DSS as internal standard. The compositions of the esters were analyzed by GC (HP-6890) using an

Synthesis of four novel BAILs ad

Four novel BAILs a-d was prepared readily from the commercially available starting materials via two-step process in excellent yields (Scheme 2). N-methyllimidazole and pyridine was independently alkylated by using benzyl chloride at the first step, and the resulting quaternary halides 12 was finally transformed into the target compounds by using concentrated sulphuric acid and phosphoric acid as anion exchange agent, respectively. The resulting water-soluble BAILs ad was isolated as somewhat

Conclusions

In summary, we successfully developed a simple, fast, efficient, and eco-friendly Fisher esterification for the synthesis of widespread aromatic esters using four novel readily synthesized BAILs ad as catalysts and reaction medium under solvent-free microwave irradiation conditions. Several noteworthy features of this approach as follows: (1) reactions could be completed in shorter time; (2) the esters produced can be separated by easy decantation with high yields and purity; (3) BAILs could

Acknowledgments

We wish to thank the financial support from the project “Light of Western” of Chinese Academic Science.

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