Current PerspectivesSynthesis and characterization of ionic liquid immobilized on magnetic nanoparticles: A recyclable heterogeneous organocatalyst for the acetylation of alcohols
Graphical abstract
Introduction
The development of heterogeneous catalysts for organic synthesis has become a major area of research, because of potential advantages of these materials over homogeneous systems (such as: simplified recovery, reusability and the potential for incorporation in continuous reactors and microreactors), they could be as environmentally benign chemical procedures for academia and industry [1], [2].
In recent years, ionic liquids (ILs) based heterogeneous catalysis has attracted a considerable attention under the heading of “green chemistry” as they permit mutual advantages of both homogeneous as well as heterogeneous catalysts in organic synthesis [3], [4], [5], [6], [7].
They are non-volatile, non-flammable, thermally stable, and have excellent solvation properties [8], [9], because of their unique physicochemical properties, including negligible vapor pressure, wide liquid range, higher ionic conductivity and excellent solubility [10], [11], [12].
Although ILs possess some advantages but their practical applications have been restricted by some difficulties in its recovery which lead to economical and environmental problems. On the other hand, their high viscosity not only limits their mass transfer during catalytic reactions but also makes their handling difficult. Moreover, the use of relatively large amounts of ILs is costly and may cause toxicological concerns. These problems can be overcome by immobilization of ILs on solid supports to obtain heterogeneous catalysts [13], [14], [15], [16].
In this regard, the use of supported magnetic nanoparticles (MNPs) materials has been efficiently employed as heterogeneous catalysts. The main advantage of a catalytic system based on magnetic nanoparticles is that the nanoparticles can be efficiently isolated from the reaction mixture through a simple magnetic separation process after reaction completion [17]. In the other hand, nanoparticles have emerged as efficient alternative support materials for the immobilization homogeneous catalyst [18], [19]. These nanoparticles, because of their high surface area and unique magnetic properties, have a broad range of potential uses in disciplines including physics, biomedicine, biotechnology, material science and catalysis support applications [20], [21], [22]. Additionally, the MNP-supported catalysts also show high dispersion and reactivity with a high degree of chemical stability. By utilizing these advantages of magnetic nanoparticles over other supporting materials, various catalysts and ligands have been immobilized on these particles [23].
Protection of hydroxyl groups is very important for their organic and analytical chemistry applications [24], [25]. Among the protecting groups for alcohols, the acetate esters are the most important. Also acetylation of hydroxyl groups is one of the most important and frequently used transformations in organic synthesis, especially in the synthesis of natural compounds, biologically active compounds, and polyfunctional molecules such as nucleosides, carbohydrates, flavanones, naphthoquinones, pesticides, and steroids [26], [27], [28], [29].
Acetic anhydride is a cheap and commercially available reagent, which it has frequently used for the acetylation of hydroxyl groups, giving ammonia as only by-product. Various catalytic systems have been developed for this transformation [30], [31], [32], [33]. Although these methods provide good reaction conditions and short reaction times, only a few procedures have been reported for the selective acetylation of alcohols in the presence of other functional groups such as phenols and amines. There is therefore still an urgent need for the development of new catalysts capable of affecting the chemoselective acetylation of hydroxyl groups in the presence of other reactive groups.
In this regards, as part of our research program to develop selective, non-corrosiveness, low cost and ease of isolation [34], [35], [36], [37], [38], [39], we report here the preparation of 3-((3-(trisilyloxy)propyl)propionamide)-1-methylimidazolium chloride ionic liquid supported on magnetic nanoparticle (TPPA–IL–Fe3O4) as a novel organocatalyst for the protection of the hydroxyl group in solvent-free media at room temperature.
Section snippets
Materials
Chemicals were purchased from Fisher and Merck chemical companies. The reagents and solvents were obtained from Sigma-Aldrich, Fluka or Merck and used without further purification. TPPA–IL–Fe3O4 were characterized using a Holland Philips X'pert X-ray powder diffraction (XRD) diffractometer (Co Ka, radiation=0.154056 nm), at a scanning speed of 2°/min from 10° to 100°. The particle size and morphology were investigated using scanning electron microscopy (SEM) by FESEM-TESCAN MIRA3 on an
Results and discussion
The whole preparation route to obtain TPPA–IL–Fe3O4 includes three steps as illustrated in Scheme 1. At first, the magnetite prepared by coprecipitation from Fe2+ and Fe3+ using NH3 solution. Then, the 3-aminopropyltriethoxysilane was added to the ethanolic solution of Fe3O4 nanoparticles to achieve amine-coated Fe3O4 nanoparticles through coordination bond with hydroxyl group. In the second step, the reaction of amino groups with acryloyl chloride led to acryloxyl groups-functionalized
Conclusions
In conclusion, in this project we have synthesized a novel supported ionic liquid on magnetic nanoparticles. This ionic liquid was fully characterized. Also we have developed an efficient procedure for the chemoselective acetylation of alcohols with acetic anhydride. Attractive features of this method are the simple experimental procedure, good yields and green content of the procedure. This catalytic system showed an excellent reactivity combined with efficient catalyst recyclability without
Acknowledgments
This work was supported by the research facilities of Ilam University, Ilam, Iran (Grant no. 32/972).
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