Sulfuric Acid Immobilized on Silica Gel As Highly Efficient and Heterogeneous Catalyst for the One-Pot Synthesis of 2,4,5-Triaryl-1H-Imidazoles
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International Journal of Organic Chemistry, 2012, 2, 93-99 doi:10.4236/ijoc.2012.21015 Published Online March 2012 (http://www.SciRP.org/journal/ijoc) Sulfuric Acid Immobilized on Silica Gel as Highly Efficient and Heterogeneous Catalyst for the One-Pot Synthesis of 2,4,5-Triaryl-1H-imidazoles Behrooz Maleki1*, Hossein Keshvari Shirvan1, Fereshteh Taimazi1, Elahe Akbarzadeh2 1Department of Chemistry, Sabzevar Tarbiat Moallem University, Sabzevar, Iran 2Department of Chemistry, Tarbiat Moallem University, Tehran, Iran Email: *[email protected] Received January 4, 2012; revised February 7, 2012; accepted February 15, 2012 ABSTRACT Application of sulfuric acid immobilized on silica gel as an efficient and benign catalyst has been explored in the syn- thesis of 2,4,5-Triaryl-1H-imidazoles via condensation reaction of benzil or benzoin, aldehyde and ammonium acetate. The key advantages of this process are high yields, cost effectiveness of catalyst, easy work-up, purification of products by non-chromatographic method and the reusability of the H2SO4·SiO2 catalyst. Keywords: 2,4,5-Trisubstituted Imidazoles; Solvent-Free Synthesis; Multicomponent Reaction; Silica-Supported Sulfuric Acid; Heterogeneous Catalyst 1. Introduction 1,3-di-butyl imidazolium salts) [11], ceric (IV) ammo- nium nitrate (CAN) [12], oxalic acid [13], Eu(OTf) [14], Multicomponent reactions (MCRs) have drawn great in- 3 [Hmim]HSO [15], ZrCl [16], Yb(OTf) [17], NiCl ·6H O terest enjoying an outstanding status in modern organic 4 4 3 2 2 [18], sodium bisulfate [19], iodine [20], nanocrystalline synthesis and medicinal chemistry because they are one- magnesium Oxide [21], oxalic acid [22], silica sulfuric pot processes bringing together three or more components acid [23], acetic acid [24], L-proline [25], PEG-400 [26], and show high atom economy and high selectivity [1,2]. Cu(TFA) [27], tetrabutylammoniumbromide (TBAB) [28], MCRs have great contribution in convergent synthesis 2 (NH4)6Mo7O24·4H2O [29], InCl3·6H2O [30], Zr(acac)4 of complex and important organic molecules from simple [31], anhydrous FePO4 [32] and uranyl nitrate hexahy- and readily available starting materials, and have emerged drate [UO2(NO3)2·6H2O] supported on acidic alumina as powerful tools for drug discovery [3,4]. The imidazole [33]. nucleus is a fertile source of biologically important mole- Many of these methods, however, suffer from longer cules. Compounds containing imidazole moiety have reaction times, unsatisfactory yields, difficult workup, many pharmacological properties and play important and excessive use of reagents and catalyst. It is therefore roles in biochemical processes. They are well known as important to find more convenient methods for the pre- inhibitors of P38MAP kinase, fungicides, herbicides, anti- paration of these compounds. Therefore, the develop- inflammatory agents, antithrombotic agents, plant growth ment of a new mild method to overcome these disadvan- regulators and therapeutic agents. In addition, they are tages still remains a challenge for organic chemists. One used in photography as photosensitive compounds. Some of the aims we have in mind is to introduce a new cata- substituted triarylimidazoles are selective antagonists of lyst for synthesis of 2,4,5-trisubstituted imidazoles with the glucagons receptor and inhibitors of IL-1 biosynthe- cost effectiveness and mild condition in high yields. sis [5]. Radziszewski and Jaap proposed the first synthe- sis of the imidazole core in 1882, starting from 1,2-di- 2. Result and Discussion carbonyl compounds, aldehydes and ammonia to obtain Several methods are used in the synthesis of these trisub- 2,4,5-triphenylimidazole [6,7]. There are several methods stituted imidazoles and their derivatives. In addition, the for the synthesis of 2,4,5-triarylimidazoles using synthesis of these heterocycles has been usually carried H PO ·12MoO ·24H O, KH PO [8], catalyst-free under 3 4 3 2 2 4 out in polar organic solvents such as ethanol, methanol, microwave irradiation [9,10], ionic liquid (1-n-butyl and acetic acid, DMF and DMSO leading to complex isola- *Corresponding author. tion and recovery procedures. These processes also gene- Copyright © 2012 SciRes. IJOC 94 B. MALEKI ET AL. rate waste containing catalyst and solvent, which have to lyst for synthesis of trisubstituted imidazoles by using be recovered, treated and disposed of. The toxicity and different aromatic aldehydes with a wide range of ortho-, volatile nature of many organic solvents, particularly meta- and para-substitutions under solvent-free classical chlorinated hydrocarbons that are widely used in huge heating conditions to establish the catalytic importance of amounts for organic reactions have posed a serious threat H2SO4 immobilized on SiO2 for this reaction. to the environment [34]. Thus, design of solvent-free ca- Generally, the synthetic procedure involves stirring the talytic reaction has received tremendous attention in re- mixture of aldehyde (1 mmol), benzil (1 mmol), ammo- cent times in the area of green synthesis [35]. nium acetate (5 mmol) and H2SO4 immobilized on SiO2 Solid acids and especially those based on micelle- (0.01 gr) for 45 - 60 min at 110˚C. The corresponding template silica’s and other mesoporous high surface area results are given in Table 2. We found that the reaction support materials are beginning to play a significant role proceeded very efficiently either electron-releasing or in the greening of fine and specialty chemicals manufac- electron-withdrawing substituents on aryl ring of alde- turing processes. A wide range of important organic re- hyde. actions can be efficiently catalyzed by these materials, Also, due to direct use of benzoin rather than benzil in which can be designed to provide different types of acid- the synthesis of imidazoles a significant improvement in ity as well as high degrees of reaction selectivity. The the synthesis toward the greener chemistry is represented. solid acids generally have high turnover numbers and can We have repeated the reaction with benzoin instead of be easily separated from the organic components [36,37]. benzil and the desired product has been efficiently pro- In recent years the H2SO4 immobilized on SiO2 was used duced. As indicated in Table 2, when we used benzoin as a catalyst for synthesis of organic compounds [38-42]. instead of benzil, the reaction time increased and also the In this work, we report the solvent-free synthesis of yield of the reaction decreased partially. 2,4,5-trisubstituted imidazoles using H2SO4 immobilized Possible mechanism for the sulfuric acid immobilized on SiO2 as a catalyst under classical heating (Scheme 1). on silica gel catalysed synthesis of trisubstituted imida- Efficiency of this reaction is mainly affected by the zoles has been proposed in Scheme 2. amount of catalyst, temperature and reaction time. For In summary, this paper describes a convenient and ef- getting the best conditions, initially we started the con- ficient process for the Solvent-free synthesis of trisubsti- densation of benzil (1 mmol), 4-chloro benzaldehyde (1 tuted imidazoles through the three-components coupling mmol) and ammonium acetate (5 mmol) in the presence of benzil or benzoin, aldehydes and ammonium acetate of H2SO4 immobilized on SiO2 (0.005 gr) as a catalyst at using H2SO4 immobilized on SiO2 as a catalyst. Reaction 100˚C for 1 h, which led to low yield (40%) of 2,4, profile is very clean and no side products are formed. All 5-trisubstituted imidazole (Table 1, entry 1). To enhance the synthesized imidazoles have been characterized on the yield of the desired product the temperature of the the basis of elemental and spectral studies. We believe reaction was increased to 120˚C (entry 2, 3). With in- that this procedure is convenient, economic, and a user- creasing the temperature, the productivity of the reaction friendly process for the synthesis of trisubstituted imida- increased but was not very high. Then, it was thought zoles of biological and medicinal importance. worthwhile to carry out the reaction in the presence of Also, we investigated the reusability and recycling ef- higher amount of the catalyst (entry 4, 5). As indicated in fect of H2SO4·SiO2 catalyst in these reactions. At the end Table 1, Maximum yield was obtained (94%) when the of each reaction, the catalyst was filtered, washed with reaction was loaded with 0.01 gr of the catalyst at the diethyl ether, dried at 120˚C for 3 h, and reused in a sub- 110˚C. A further increasing of catalyst loading does not sequent reaction cycle. The recycled catalyst was em- affect the yield (entry 6). ployed consecutively for three reactions and no signifi- After optimizing the conditions, we applied this cata- cant loss in its efficiency was observed (Table 2. 2a, 2d, Scheme 1. Sulfuric acid immobilized on silica gel catalysed synthesis of 2,4,5-trisubstituted imidazole. Copyright © 2012 SciRes. IJOC B. MALEKI ET AL. 95 H+ H H O O H NH2 H2SO4.SiO2 NH3 [NH OAc] HO Ph 4 Ph Ph 1a-l NH3 + HOAc H2SO4.SiO2 H NH2 H2N NH2 +H NH3 O H -H2O Ph Ph H Ph O Ph O Ph O Ph OH H H +H +H H H O O Ph N Ph N Ph Ph Ph N Ph H2N O O H +H +H H H -2H2O -2H2O Ph Ph N N H air oxidation H H N Ph N Ph Ph Ph H [1,5]H Ph N Ph N Ph H 2a-l Scheme 2. Probable mechanism for the formation of triarylimidazoles using benzil or benzoin, ammonium acetate, aromatic aldehydes using sulfuric acid immobilized on silica gel as catalyst. Table 1. Optimization one-pot synthesis of trisubstituted imidazoles under classical heating conditionsa. Entry H2SO4·SiO2 (gr) T(˚C) Time(min) Yield 1 0.005 100 60 40 2 0.005 110 60 63 3 0.005 120 60 63 4 0.008 110 60 70 5 0.01 110 60 94 6 0.015 110 60 90 aBenzil (1 mmol), 4-chloro benzaldehyde (1 mmol) and ammonium acetate (5 mmol).