Covalently anchored 2-amino ethyl-3-propyl imidazolium bromideon SBA-15 as a green, efficient and reusable Brønsted basic ionic liquid nanocatalyst for one-pot solvent-free synthesis of benzopyranopyrimidines under ultrasonic irradiation

Abstract

In the present study, highly ordered mesoporous SBA-15 having Brønsted basic ionic liquid pore channels was synthesized via a surfactant-templated sol–gel methodology and a post modification process. For this, well ordered mesoporous chloro-functionalized SBA-15, SBA-Cl, was first synthesized by the direct incorporation of chloropropyl groups through the co-condensation of TEOS and CPTMS precursors in the presence of Pluronic P123 triblock copolymer as a supramolecular template to direct the organization of polymerizing silica. Subsequently, highly ordered 2-amino ethyl-3-propyl imidazolium bromide functionalized mesoporous SBA-15, SBA-Im-NH₂, with a high surface area was synthesized by a nucleophilic substitution reaction of SBA-Cl with imidazole and then quaternization with 2-bromo ethylamine hydrobromide. The target organic–inorganic nanocomposite was characterized by FT-IR spectroscopy, scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), thermogravimetric analysis (TGA), elemental analysis (CHN) and Brunauer–Emmett–Teller (BET) analysis. The SBA-Im-NH₂ nanocomposite was successfully used as an efficient Brønsted basic ionic liquid nanocomposite for the preparation of benzopyranopyrimidine derivatives by the one-pot pseudo four-components reaction of salicylaldehydes, malononitrile and secondary amines under ultrasonic and solvent-free conditions at room temperature. This method has the advantages of high yields, cleaner reaction, simple methodology, short reaction times, easy work-up, and greener conditions. In addition to the facility of this methodology, the catalyst also enhances product purity and promises economic as well as environmental benefits.

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1. Introduction

The exciting discovery of mesoporous and nanostructured silica-based materials in the early 1990s evoked new windows for the exploration in material research.^1,2 Recently, enormous attention has been directed toward the synthesis of this class of tunable pore size mesoporous materials with different compositions and pore apertures using a variety of templates and inorganic precursors. Therefore, a new type of ordered mesoporous material, i.e. SBA-15, with a honeycomb-like porous structure containing hundreds of empty channels was developed by employing amphiphilic triblock copolymers P123 as a structure-directing agent.^3,4 Owing to its convenient synthesis procedure, stupendous mesoporous structure and surface silanol groups, SBA-15 usually possess exclusive properties such as a large surface area, high pore volume, high thermal and hydrothermal stability, uniform tubular channels with tunable pore diameter, low mass density, non-toxic nature and is easily modifiable.^5,6 All these excellent characteristics and exclusive properties make it attractive in the fields of catalysis and functional materials.^7–9

To develop this strategy, numerous scientists have recently focused their interest on the modification of the inner pore surfaces of SBA-15 with various organic functional groups to render them suitable for specific applications.^10–15

Green chemistry is increasingly seen as a powerful tool that reduces the impact of chemistry on the environment by preventing pollution at its source and using fewer natural resources. To target this objective, ultrasound promoted reactions have been increasingly used in synthetic organic chemistry. In addition of advantages, such as shorter reaction times, milder reaction conditions, higher yields, improved selectivity and clean reactions in comparison to classical methods,^16–18 in this green technique the reaction is carried out at a lower external temperature relative to the usual thermal methods; the possibility of the occurrence of undesired reactions is reduced, and as a result of cleaner reaction work-up is easier.¹⁹

In spite of the significant useful attributes of multicomponent reactions (MCRs) for modern organic chemistry and their suitability for building up large compound libraries, these reactions were of limited interest in the past fifty years. However, in the last decade, with the introduction of high-through put biological screening, the importance of MCRs for drug discovery has been recognized, and considerable efforts from both academic and industrial researchers have been focused, especially on the design and development of multi-component procedures for the generation of libraries of heterocyclic compounds.²⁰ In this context, benzopyranopyrimidines show interesting features, which make them attractive targets for the synthesis via MCRs.

Benzopyranopyrimidines demonstrate anti-inflammatory, anticonvulsant and analgesic activities, importantly in vitro anti-aggregating activities,²¹ as well as pharmacological activities such as antiviral,²² antimicrobial,²³ antifungal,²⁴ antioxidant,²⁵ antileishmanial,²⁶ antitumor,²⁷ hypotensive,²⁸ antiproliferation,²⁹ local anesthetic,³⁰ antiallergenic,³¹ central nervous system (CNS) activities and effects,³² as well as treatment of Alzheimer's disease³³ and Schizophrenia disorder.³⁴ Although to achieve suitable conditions for the synthesis of the benzopyranopyrimidines various Lewis and protic acid catalysts in different solvents have been previously investigated, but due to biological importance of this class of compounds, finding an efficient and facile method is still challenging.

Taking all these facts into account and as a part of our ongoing interest in the synthesis of novel functionalized mesoporous silica and investigation of their applications as nanocatalysts in the one-pot synthesis of biologically relevant heterocyclic compounds,³⁵ herein we have focused our interest on the modification of the inner pore surfaces of mesoporous SBA by the incorporation of Brønsted basic ionic liquid units to render it suitable as an organized mesoporous tunable pore nanocatalyst for green, rapid and efficient multicomponent synthesis of benzopyranopyrimidine derivatives under ultrasonic and solvent-free conditions.

2. Experimental

2.1. General

Tetraethyl orthosilicate (TEOS), 3-chloropropyltrimethoxysilane (CPTMS), Pluronic P123 triblock copolymer (EO20 PO70 EO20, MW = 5800) and 2-bromo ethyl amine hydrobromide were supplied by Aldrich. Other chemical materials were purchased from Fluka and Merck companies and used without further purification. The products were characterized by comparison of their physical data, such as IR, ¹H NMR and ¹³C NMR spectra, with known samples. NMR spectra were recorded in CDCl₃ on a Bruker Advance DPX 400 MHz spectrometer using TMS as the internal standard. The determination of the purity of the products and reaction monitoring were performed by TLC on silica gel Poly Gram SIL G/UV 254 plates. Elemental analysis was performed on a Perkin-Elmer CHN-2400 analyzer. Melting points were determined in open capillaries with a BUCHI 510 melting point apparatus. The FT-IR spectra of the powders were recorded using a BOMEM MB-Series 1998 FT-IR spectrometer. X-ray diffraction (XRD) patterns of the samples were obtained on a Philips X-ray diffraction model PW 1840. Transmission electron microscopy (TEM) images were obtained using a Zeisss-EM10C at 80 kV.

2.2. Synthesis of chloropropyl-grafted SBA-15 (SBA-Cl)

Chloropropyl-grafted SBA-15 (SBA-Cl) was prepared according to the reported method.³⁶ Pluronic 123 (4 g) was dissolved in 125 g of 2.0 M HCl solution at room temperature. Then, TEOS (8.41 g, 40.41 mmol) was added and equilibrated at 40 °C for prehydrolysis. To the mixture, CPTMS (1.3 g, 6.5 mmol) was slowly added and stirred at 40 °C for 20 h and reacted at 95 °C under static conditions for 24 h. The solid product was recovered by filtration and dried at room-temperature overnight. The template was removed from the as-synthesized material by refluxing in 95% ethanol for 48 h (1.5 g of the as-synthesized material per 400 mL of ethanol). Then, the product was filtered, washed several times with water and ethanol, and dried at 50 °C.

2.3. Synthesis of SBA-propyl-3-aminoethyl imidazolium bromide, SBA-IM-NH₂

To a solution of imidazole (0.476 g, 1 mmol) in 25 mL of dry toluene, sodium hydride (0.167 g, 7 mmol) was added and stirred under a nitrogen atmosphere at room temperature for 2 h to obtain sodium imidazole. Then, SBA-Cl (5.00 g) was added and the mixture was refluxed under a nitrogen atmosphere for 24 h. The resulting product was filtered and washed with ethanol (3 × 20 mL) and dried under vacuum at 100 °C for 8 h to obtain 3-(1-imidazole)propyl-SBA (SBA-Im).

To prepare SBA-propyl-3-aminoethyl imidazolium bromide, SBA-IM-NH₂, 0.6 g of SBA-Im was suspended in 25 mL of acetonitrile. 2-Bromo ethyl amine hydrobromide (0.4 g, 2 mmol) was added slowly and the mixture was refluxed at 80 °C for 12 h. The excess of 2-bromo ethyl amine hydrobromide was removed by filtration and washing with ethanol. The resulting solid was washed with NaOH (0.03 g, 50 mmol) for neutralization and dried in an oven at 80 °C for 6 h under vacuum. The reaction sequence and the possible structure of SBA-IM-NH₂ are shown in Scheme 1.

Scheme 1 The synthesis route of SBA-IM-NH₂.

The basic capacity of the nanocomposite was determined by potentiometry and confirmed by acid–base titration. Therefore, SBA-IM-NH₂ (0.1 g) was placed in an aqueous solution of NaCl (1 M, 25 mL, initial pH = 6) and the resulting mixture was stirred for 24 h; the pH of the solution increased to 8.50. The basic capacity of the nanocomposite is 2.5 × 10⁻⁴ mmol OH⁻ per gram of composite. This result was also confirmed by back-titration.

2.4. General procedure for the synthesis of benzopyranopyrimidines

A mixture of 2-hydroxybenzaldehyde (2 mL), malononitrile (1 mL), secondary amine (1 mmol) and 0.005 g of SBA-IM-NH₂ was reacted at room temperature under ultrasonic conditions. After the completion of the reaction, as indicated by TLC, the solid product was dissolved in CH₂Cl₂ and then the catalyst was separated from the reaction mixture by centrifugation. Then, solvent was removed by evaporation and the crude solid product was purified by a recrystallization procedure in ethanol to obtain the corresponding benzopyranopyrimidines as pure products in good yields. To recover the catalyst, SBA-IM-NH₂ was washed with CH₂Cl₂ and dried under reduced pressure.

3. Results and discussion

The route for the preparation of highly ordered mesoporous SBA-15 having Brønsted basic ionic liquid units, SBA-IM-NH₂ nanocomposite, is schematically illustrated in Scheme 1. The SBA-Cl was initially synthesized by the direct incorporation of chloropropyl groups through the co-condensation of TEOS and CPTMS precursors in the presence of Pluronic P123 triblock copolymer as a supra molecular template to direct the organization of polymerizing silica. Subsequently, 2-amino ethyl-3-propyl imidazolium bromide functionalized mesoporous SBA-15, SBA-Im-NH₂, was synthesized by a nucleophilic substitution reaction of SBA-Cl with imidazole and then quaternization with 2-bromo ethylamine hydrobromide.

To characterize the catalyst, and to confirm the immobilization of the active components on the pore surface, FT-IR spectroscopy was utilized. The FT-IR spectra of the SBA-15, SBA-Cl and SBA-IM-NH₂ (Fig. 1) in the range of 400–4000 cm⁻¹ exhibited peaks at 1220, 1078, 804, and 470 cm⁻¹ related to stretching, bending and vibration modes of Si–O–Si. In addition, the FT-IR spectra of the SBA-IM-NH₂ exhibited two new peaks at 1560 and 1645 cm⁻¹, which were assigned to the C=C and C=N bands of the imidazole rings, respectively. The absorbance of the C–N stretching vibration is normally observed around 1000–1200 cm⁻¹, which cannot be resolved due to its overlap with the absorbance of Si–O–Si stretch. The presence of amino groups in the SBA-IM-NH₂ was further corroborated by a broad band at 2800–3400 cm⁻¹ attributed to the O–H vibration of the physically adsorbed water.

Fig. 1 The FT-IR spectra of SBA-15, SBA-Cl and SBA-IM-NH₂.

The grafting of organic units to SBA-15 was also confirmed by CHN analysis. The results of elemental analysis showed the presence of carbon, nitrogen and hydrogen in SBA-Im-NH₂ (Table 1).

Table 1 Elemental compositions of sample analyzed using the CHN technique

Samples	C (wt%)	N (wt%)	H (wt%)
SBA-IM-NH2	13.2	3.2	4.5

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The representative powder X-ray diffraction (XRD) patterns of SBA-IM-NH₂ are shown in Fig. 2. The low-angle XRD pattern of SBA-IM-NH₂ exhibits two weak lines (for 110 and 200) and a single strong peak (100), which confirm the long range order and excellent textural uniformity of the mesoporous material.

Fig. 2 XRD pattern of SBA-IM-NH₂.

The size and morphology of the SBA-IM-NH₂ nanocomposite was characterized by TEM and SEM (Fig. 3). The TEM images of the organocatalyst indicate the mesoporous structure and orderly pore arrangement, with an average diameter of approximate 30 nm. It can be clearly seen that the samples maintain the unique pore structure of the parent support, SBA-15, very well.

Fig. 3 The TEM and SEM images of SBA-IM-NH₂.

Thermogravimetric analysis (TGA) was employed to determine the organic content of the grafted silica and their thermal stability. Fig. 4 shows the thermogravimetric analysis-derivative thermogravimetric analysis (TGA-DTG) for SBA-IM-NH₂. It can be seen that SBA-IM-NH₂ nanocomposite yielded about 9.2% weight loss from ca. 25 to 195 °C, which was attributed to the desorption of physically adsorbed water as well as dehydration of the surface –OH groups. The decline in the temperature range from ca. 280 to 650 °C should be ascribed to the removal of the organic parts on the surface of the nanocomposite during hydrothermal treatment. As shown in Fig. 4, the weight losses of the organic parts are 0.277 g/g of the sample. Therefore, the nanocatalyst is completely stable below 280 °C and can be applied without degradation.

Fig. 4 TGA-DTG analysis of SBA-IM-NH₂.

Fig. 5 shows the characterization of surface area, pore volume, and pore size distribution of SBA-Cl and SBA-IM-NH₂, which were determined through BET and BJH method from the adsorption of nitrogen at 77 K on a Micrometrics Gemini analyser. The hysteresis curves exhibited the characteristics of type IV isotherm. The experimentally determined main structural characteristics are listed in Table 2. The BJH pore size and the surface areas of the samples ranged from 753 to 367.27 m² g⁻¹. After SBA-Cl loading, the surface areas decreased by approximately 48%, indicating that the SBA-Cl pores were occupied by organic molecules. A corresponding decrease in the pore volume of the SBA-IM-NH₂ relative to the SBA-Cl by approximately 25% is also observed, which correlates with the presence of organic molecules inside the SBA-Cl pores. The BJH pore size distribution analysis shows that the material possesses uniform-sized mesopores centered at ca. 97.2 Å for SBA-Cl and centered at 37.6 Å for the SBA-IM-NH₂ samples.

Fig. 5 Pore size distributions of SBA-Cl (a) and SBA-IM-NH₂ (c) and nitrogen adsorption–desorption isotherms of SBA-Cl (b) and SBA-IM-NH₂ (d).

Table 2 Specific surface area (S_BET), diameter pore and total pore volume

Samples	BET surface area (m^2 g^−1)	Diameter (nm)	Pore volume (cm^3 g^−1)
SBA-Cl	753	9.72	1.25
SBA-IM-NH2	367.27	3.76	0.31

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To investigate the catalytic activity of SBA-IM-NH₂ as a heterogeneous catalyst in the synthesis of benzopyranopyrimidines, the reaction of 2-hydroxy benzaldehyde, malononitrile and morpholine in a 2 : 1 : 1 molar ratio was selected as a model system under ultrasonic conditions at room temperature. As shown in Table 3, the best result was obtained when the reaction was carried out in the presence of 5 mg of SBA-IM-NH₂.

Table 3 Optimization of the amount of the SBA-IM-NH₂ nanocomposite

Entry	Catalyst (mg)	Time (min)	Yield (%)
1	—	60	—
2	2	20	70
3	5	8	95
4	10	10	90

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With the optimized conditions in hand, we investigated the use of a wide range of amines 3 and salicylic aldehydes 1 in this pseudo four components condensation using 5 mg of SBA-IM-NH₂ under ultrasonic and solvent-free conditions at room temperature, where the desired products were afforded in high to excellent isolated yields (Table 4). With regard to substituents, both aldehydes with electron-withdrawing and electron-donating groups participated in the reaction, but the former reacted better.

Table 4 The preparation of benzopyranopyrimidines derivatives

Entry	X	Amine	Product	Time (min)	Yield^a (%)	mp (°C) found (reported)
a Isolated yields.
1	H			10	80	178–180 (177–179)^37
2	H			8	90	199–201 (197–199)^38
3	H			10	93	201–203 (—)
4	H			10	85	185–189 (186–188)^39
5	3-Cl			8	80	195–197 (197)^40
6	3-Cl			5	95	246–250 (249–251)^40
7	3-Cl			8	95	242–245 (—)
8	3-Cl			8	89	191–194 (192–195)^41
9	2-OMe			15	65	190–192 (190)^40
10	2-OMe			15	75	225–227 (224–226)^38
11	2-OMe			15	72	220–224 (—)
12	2-OMe			15	70	159–161 (158–160)^39

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Reusability of the SBA-IM-NH₂ catalyst was tested by consecutively recovering and then reusing the catalyst up to four times. The reaction was carried out repeatedly under one constant set of operating conditions (Fig. 6). Therefore, the recyclability of catalyst makes the process economically and potentially viable for commercial applications.

Fig. 6 Catalyst reusability.

4. Conclusion

In the present study, a highly ordered mesoporous SBA-15 having Brønsted basic ionic liquid pore channels was synthesized via a surfactant-templated sol–gel methodology and a post modification process. The catalytic activity of the basic nanocomposite has been successfully applied for the one-pot pseudo four-components reaction of salicylaldehydes, malononitrile and secondary amines under ultrasonic and ambient conditions. This solvent-less catalytic system certainly contributes to better environmental and green technology for the facile preparation of the benzopyranopyrimidine derivatives. This method offers several advantages including high yield, clean reaction media, short reaction time, easy separation and recyclability of the solid catalyst.

Acknowledgements

We are grateful to the Research Council of Shahid Chamran University for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra11006h

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