A magnetic nanoparticle-supported sulfuric acid as a highly efficient and reusable catalyst for rapid synthesis of amidoalkyl naphthols
Introduction
Sulfuric acid is an essential and efficient catalyst for the production of industrial important chemicals. Over 15 million tons of sulfuric acid are annually consumed as “an unrecyclable catalyst” which requires costly and inefficient separation of the catalyst from homogeneous reaction mixtures [1]. The use of recyclable strong solid acids in organic reactions are often considered to follow the principles of green chemistry, that is these catalyzed processes consume a minimum of energy and reagents or auxiliaries and minimize waste [2].
Magnetic nanoparticles (MNPs) have recently appeared as an excellent type of catalyst support because of their good stability, easy synthesis and functionalization, high surface area and facile separation by magnetic forces, as well as low toxicity and price [3]. Also, surface functionalization of magnetic particles is an elegant way to bridge the gap between heterogeneous and homogeneous catalysis. These attractive features have made MNPs a promising alternative to porous/mesoporous catalyst supports. Thus, the development of silica-coated magnetite nanoparticles as attractive candidates in the search for supporting of catalysts is currently a subject of increasing interest in chemical process [4], [5]. Sulfuric acid functionalized silica-coated magnetite nanoparticles as recyclable strong solid acid catalyst open up new avenue to introduce an amazing and efficient system for facilitating catalyst recovery in different organic reactions. This catalyst allows the combination of well known procedures for catalyst heterogenization with techniques for magnetic separation [6], [7].
The developing of one-pot multicomponent reactions (MCRs) and improving the known MCRs have attracted considerable attention in recent years [8], [9]. One of these MCRs is the preparation of amidoalkyl naphthol derivatives. 1-Amidoalkyl-2-naphthol derivatives are important class of compounds as they can be easily converted to biologically active compounds, 1-aminoalkyl-2-naphthols, by amide hydrolysis reaction [10], [11]. The synthesis of 1-amidoalkyl-2-naphthols can be carried out by multicomponent condensation of aldehydes, 2-naphthol and amide in the presence of various catalysts such as SnCl4·5H2O [12], nano-sulfated zirconia [13], zirconium(IV) oxide chloride [14], dodecylphosphonic acid [15], ZrO(OTf)2 [16], and Bi(NO3)3·5H2O [17]. However, some of these methods are not environmentally friendly and suffer from one or more disadvantages, such as prolonged reaction times, low yields, toxicity and recovery and reusability of the catalyst. Therefore, the development of clean processes and utilizing eco-friendly and green catalysts which can be simply recycled at the end of reactions have been under permanent attention. The demand for environmentally benign procedure with heterogeneous and reusable catalyst, promoted us to develop a safe alternate method for the preparation of amidoalkyl naphthols [18].
The aim of this protocol is to highlight the synergistic effects of the combined use of multicomponent reactions under solvent-free conditions and application of recyclable strong solid acid catalyst with inherent magnetic property for the development of new eco-compatible strategy for amidoalkyl naphthols synthesis. Therefore, we now wish to explore a straight forward convergent synthesis of 1-amidoalkyl-2-naphthols via the one-pot three-component reaction of β-naphthol, aldehydes derivatives, amides or urea under solvent free conditions using MNPs–SO3H as an proficient, mild, harmless to the environment, non-toxic and magnetic powerful solid acid catalyst with good stability (Scheme 1). To the best of our knowledge, the use of magnetically solid acid catalyst for the synthesis of amidoalkyl naphthols previously has not been reported.
Section snippets
Experimental
Chemical reagents in high purity were purchased from the Merck and Sigma Chemical Company. All materials were of commercial reagent grade. 1H NMR and 13C NMR spectra were recorded with a Bruker DRX-400 spectrometer at 400 and 100 MHz respectively. FT-IR spectra were obtained with potassium bromide pellets in the range 400–4000 cm−1 with a Perkin-Elmer 550 spectrometer. The element analyses (C, H, N) were obtained from a Carlo ERBA Model EA 1108 analyzer carried out on Perkin-Elmer 240c analyzer.
Results and discussion
The magnetite nanoparticles were prepared by coprecipitation of iron(II) and iron(III) ions in basic solution. To improve the chemical stability of Fe3O4 nanoparticles, their surface engineering was successfully performed by the suitable deposition of silica onto nanoparticles surface by the ammonia-catalyzed hydrolysis of tetraethylorthosilicate (TEOS). Then, the Fe3O4@SiO2 served as support for the immobilization of SO3H groups by simple mixing of core–shell nanocomposite and chlorosulfonic
Conclusion
Sulfuric acid functionalized Fe3O4 nanoparticles is successfully achieved by a multiple synthetic procedure which is confirmed with XRD, FT-IR, TGA, VSM, and SEM. The most interesting features of the present work include durability as well as efficient catalytic activity for one-pot synthesis of amidoalkyl naphthols by one-pot three-component coupling of β-naphthol, aromatic and aliphatic aldehydes and urea or amides under solvent free at 100 °C, MNPs-SO3H as an inexpensive and non-hazardous
Acknowledgment
We gratefully acknowledge the financial support from the Research Council of the University of Kashan for supporting this work by Grant No. 256722/IV.
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