Efficient catalytic synthesis of 2-imidazolines and bis-imidazolines with silica supported tungstosilicic acid

A rapid and efficient preparation of 2-imidazolines and bis-imidazolines by the reaction of ethylenediamine or 1,2-propanediamine with nitriles in the presence of catalytic amounts of tungstosilicic acid supported on SiO 2 under reflux condition, is reported. The advantages of this procedure are moderate reaction times, good to high yields and the ability to carry out the large scale reactions.


Introduction
In recent years, heteropoly acids (HPAs) especially Keggin type ones and related polyoxometalates (typically represented by the formula H8-x[XM12O40], where X is the heteroatom (e.g., P 5+ or Si 4+ ), x is its oxidation state, and M is the addendum atom (usually W 6+ or Mo 6+ )), have attracted much interest, especially tungstophosphoric acid (H3PW12O40), tungstosilicic acid (H4SiW12O40), and molybdatophosphoric acid (H3PMo12O40). 1They possess very strong acidity and redox properties and have attracted much interest as catalysts in both academic and industrial applications. 2They are highly soluble in polar solvents but insoluble in non-polar ones.They can be used in bulk or supported forms in both homogeneous and heterogeneous systems.Some advantages of HPAs are non-corrosivity, environmentally friendly because of its reusability, economically feasible solid acid catalysts compared to conventional homogeneous acids, high flexibility in modification of the acid strength, ease of handling, nontoxicity and experimental simplicity. 3
From a synthetically point of view, a variety of useful transformations such as oxidation of alcohols, 4 Friedel-Crafts 5 and Mannich reactions, 6 cyanosilylation, 7 ring-opening of epoxides, 8 and dehydration, 9 have been developed using HPAs as catalysts.Increasing of the surface area or even better, increasing of the number of accessible acid sites of the HPAs is important.This can be achieved by dispersing the HPAs on solid supports with a high surface area. 10,11As the sizes of HPA molecules are extremely larger than other typical acidic species, it is assumed that they are much easier to immobilize.Some of the goals of using supported HPAs are the surface area enhancement, higher dispersion of acidic protons, heterogenization and acid strength control.These supported catalysts have been widely studied and found useful in many reactions such as the synthesis of 2,4-dihydropyrimidones, 12 metanethole , 13 α-aminonitriles 14 and esterification. 15,16midazoline derivatives are of great interest and importance because of their pharmaceutical and synthetic material applications.They exhibit significant biological and pharmacological activities including antihypertensive, 17 antihyperglycemic, 18 antidepressive, 19 antihypercholesterolemic 20 and antiinflammatory. 21These compounds are also used as catalysts, 22 synthetic intermediates. 23chiral auxiliaries, 24 chiral catalysts 25 and ligands for asymmetric catalysis. 26There are several methods for the synthesis of 2-imidazolines from carboxylic acids, 27 esters, 28 nitriles, 29 orthoesters, 30 hydroximoylchlorides, 31 hydroxy amides, 32 mono-or disubstituted (chlorodicyanovinyl)benzene, 33 and N-tert-butoxycarbonyl-protected α-amino acids. 34However, some of these methods suffer from disadvantages such as long reaction times, low yields, difficulty in preparation of starting materials and tedious workup.Due to these problems, there is still scope to find new methods for the synthesis of imidazolines and bisimidazolines.
From the point of combinatorial chemistry, Multi-component reactions (MCRs) are of increasing importance in organic and medicinal chemistry for various reasons [35][36][37][38] that as a result of which, the processes are performed without isolation of any intermediates, thus reducing time and saving both energy and raw materials.
In this work, we report an efficient catalytic synthesis of imidazolines and bisimidazolines by use of silica supported tungstosilicic acid.

Results and Discussion
In this paper, we describe an efficient method for the synthesis of 2-imidazolines and bisimidazolines by the reaction of ethylenediamine or 1,2-propanediamine with nitriles in the presence of catalytic amounts of tungstosilicic acid supported on SiO2 under reflux condition (Scheme 1).

Preparation of the supported 12-tungstosilicic acid
The supported tungstosilicic acid catalysts were prepared by the method of incipient wetness.In a typical procedure, a 400 mg portion of acid was dissolved in deionized water (50 mL) and was impregnated drop-wise onto 1600 mg supports under constant agitation.The resulting pastes were dried at 80 °C for 4 h and then calcinated at 170 °C for 4 h. 39After the calcination process; the prepared supported acid was washed three times with water.The amount of loaded acid was measured by acid-base titration of the filtrate with regard to the acid that was used at first (loading was more than 98%).FT-IR spectra can be used as a powerful technique for the investigation of the surface interaction between tungstosilicic acid and inorganic supports.The pure acid compound displays infrared bands at 980 (W=O), 925 (Si-O), 880 (W-Od-W) and 781 cm -1 (W-Ob-W). 40In addition, a broad, intense band centered around 3450 cm −1 (O−H stretching) and a weak absorption at 1640 cm −1 (δH2O bending) indicate the presence of water.
However, small shifts of νW=Od (973 cm -1 ) and νW-Oc-W(790 cm -1 ) vibrations were registered indicating interactions of the support with the most external atoms Od and Oc of the Keggin anion.Such effects decrease with the increasing coverage (Fig. 1).

Effect of tungstosilicic acid loading on SiO2
For the investigation of the effect of different amounts of H4SiW12O40 loading in the synthesis of 2-imidazolines, various weight percents of acid were used.Table 1 shows differences in catalytic activity among catalysts having 10-30 wt.% of H4SiW12O40 on silica.Lowering the loading of the deposited tungstosilicic acid causes the reduction of the catalytic activity.No improvements in the reaction rate and yield were observed by increasing the amount of acid on SiO2 from 20 to 30 wt.%.Since 20 wt.% of H4SiW12O40-SiO2 was the best catalyst loading, it was used to study the effect of various parameters on yields.

Effect of catalyst concentration
The catalyst concentration was varied over a range of 0.02-0.12g (0.1-0.8 mol% of tungstosilicic acid) on the basis of the total volume of the reaction mixture.Table 2 shows the effect of the catalyst concentration on the reaction of benzonitrile and ethylenediamine.The yield of the corresponding imidazoline increased with an increasing catalyst concentration from 0.1 to 0.4 mol%.Further, addition of catalyst had no noticeable effect on the yield.This was due to the fact that beyond a certain concentration, there exist an excess of catalyst sites over what is actually required by the reactant molecules, and hence, the additional catalyst does not increase the rate of the reaction.Therefore, in all further reactions 0.4 mol% was used for 20 wt.% of H4SiW12O40-SiO2.

Synthesis of 2-imidazolines catalyzed by supported tungstosilicic acid
The results from the reaction of ethylenediamine or 1,2-propanediamine and various nitriles in the presence of optimized H4SiW12O40-SiO2 are shown in Table 3.As shown in entries 1-16, the reactions using various aromatic nitriles proceeded well to give the corresponding 2-imidazolines and bis-imidazolines in good to excellent yields. Yields refer to isolated and purified products.
Typically, benzonitrile (1), ethylenediamine and H4SiW12O40-SiO2 were mixed and exposed to heating for 45 min.Cold ethanol was added, the mixture was stirred and the catalyst isolated by filtration.Removal of the solvent and recrystallization of the crude product from cyclohexane gave the corresponding 2-imidazoline in 90% yield.Surprisingly, it was observed that both mono-and bis-imidazolines can be obtained by this catalytic system.As shown in Table 3, mono-imidazolines were produced from dinitriles in 90% yields with shorter reaction times (60-75 min).Increasing the reaction times to 130-135 min produced the bis-imidazolines in 75-87% yields.The preparation of mono-imidazolines from dinitrile compounds is of great interest because the remaining nitrile group can be converted to other functional groups. 44s it has been reported, synthesis of bis-imidazolines from the reaction of dinitriles with ethylenediamine catalyzed by supported tungstophosphoric acid has failed 43 whereas in the presence of H4SiW12O40-SiO2, the corresponding bis-imidazolines were synthesized from dinitriles in the good yields (entries 7, 9; table 2).
One advantage of this method is its large scale applicability so that imidazolines and bisimidazolines were prepared on a 100 mmol scale, and the results were comparable to the small scale experiments.

Effect of unsupported acid in synthesis of imidazolines
As shown in table 4, in the presence of tungstosilicic acid without supporting on SiO2, the synthesis of 2-imidazoline were performed in longer time with reduced yields in comparison to the supported one.For example, in presence of H4SiW12O40-SiO2, the reaction of benzonitrile was completed in 45 min, while by using unsupported catalyst, this reaction was carried out in 8 h with 80% yield.This results show that supporting of H4SiW12O40 on SiO2 increases the catalytic strength of this catalyst.

Effect of SiO2 in the synthesis of imidazolines
For the investigation of the probable effect of SiO2 as catalyst in the synthesis of 2-imidazolines, various weight percents of silicagel were used.In the synthesis of 2-phenylimidazoline from benzonitrile and ethylenediamine in the presence of SiO2, the reaction was carried out in 12 h in 25% yield.While in the presence of H4SiW12O40-SiO2, the reaction of benzonitrile was completed in 45 min.Using an unsupported catalyst, this reaction was carried out in 8 h with 80% yield.These results show that supported and unsupported H4SiW12O40 accelerates the rate of reaction versus the action of SiO2 alone.

Catalyst reusability
The reusability of the catalyst is important for the large scale operation and from an industrial point of view.Therefore, the reusability of the catalysts was examined in the reaction of benzonitrile with ethylenediamine.Since the reaction medium is heterogeneous, the catalysts can be recovered by simple filtration after dilution of the reaction mixture with EtOH.The recovered catalysts were dried at 100 °C and weighed.Afterwards, according to the catalyst amount, the required amount of fresh nitrile and ethylenediamine were added.The results showed that the catalyst can be reused four consecutive times (Table 5).

Comparative results
In order to show the ability of our method with respect to previous reports, some of our results in comparison to some other methods are summarized in table 6.As shown, the yield/time ratios of the present method are better or comparable with the others.

Conclusions
A simple and efficient procedure for the synthesis of 2-imidazolines and bis-imidazolines has been developed.Mild reaction conditions, absence of solvent, moderate reaction times, easy and quick isolation of the products, good to excellent yields, recoverability and reusability of the heterogeneous catalyst and large scale applicability are the main advantages.

Experimental Section
General.Chemicals were purchased from Merck, Fluka and Aldrich chemical companies.All of the products were identified by comparison of their physical and spectral data with those of authentic samples.IR spectra were recorded on a Jasco IR-680 spectrophotometer.MHz) spectrometers.The calcination process was carried out by a (2000W) Carbolite-Elf-ly6B furnace.

Preparation of the supported catalyst
The silica gel supported H4SiW12O40 was prepared by mixing silica gel (1.6 g, Merck grade 40, 0.063-0.2mm) with a solution of acid (0.40 g) in distilled water (20 mL).The resulting mixture was stirred for 30 min.After removal of water in a rotary evaporator, the solid powder was dried at 80 °C for 4 h followed by 4 h calcinations at 170 °C39 .

General procedure for the preparation of 2-imidazolines and bis-imidazolines
A mixture of nitrile (10 mmol), ethylenediamine or 1,2-propanediamine (40 mmol) and 20% H4SiW12O40-SiO2 (0.6 g) was heated under reflux for 45-220 min.After completion of the reaction as indicated by TLC (eluent: EtOAc/MeOH, 4:1), the reaction mixture was cooled to room temperature and then ethanol was added.After stirring, the mixture was filtered and the solvent evaporated.The crude product (1) was recrystallized from cyclohexane, 8 and 10 were recrystallized from methanol and others were recrystallized from n-hexane giving the pure products in 75-95% yields based on the starting nitrile (

Table 1 .
Effect of H4SiW12O40-SiO2 weight ratios in the synthesis of 2-phenylimidazoline from benzonitrile and ethylenediamine under solvent-free conditions a Isolated yields.

Table 2 .
Investigation of catalyst effects in the synthesis of 2-phenylimidazoline from benzonitrile and ethylenediamine under solvent-free conditions a a 20% H4SiW12O40-SiO2 was used as catalyst.b Isolated yields

Table 3 .
Synthesis of imidazolines and bis-imidazolines in the presence of H4SiW12O40-SiO2  ARKAT USA, Inc.

Table 4 .
Synthesis of some imidazolines in the presence of H4SiW12O40  ARKAT USA, Inc.aThe entries refer to that ones in table 3. b Isolated yields.

Table 5 .
Reusability of H4SiW12O40-SiO2 in the reaction of benzonitrile with ethylenediamine b Isolated yields.

Table 6 .
Comparison of some our results with those reported in the litraturea 1H NMR spectra were obtained with a Bruker-Arance AQS 300 MHz or a Bruker 400 Ultrasheilld (400  ARKAT USA, Inc.