An efficient and robust heterogeneous mesoporous montmorillonite clay catalyst for the Biginelli type reactions

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2

Original Research Paper

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An efficient and robust heterogeneous mesoporous montmorillonite clay catalyst for the Biginelli type reactions

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Ankana Phukan, Subrat Jyoti Borah, Priyanku Bordoloi, Kiran Sharma, Bibek Jyoti Borah, Podma Pollov Sarmah, Dipak Kumar Dutta ⇑ Advanced Materials Group, Materials Sciences and Technology Division, North-East Institute of Science & Technology (CSIR), Jorhat 785 006, Assam, India

a r t i c l e

i n f o

Article history: Received 22 November 2016 Received in revised form 23 March 2017 Accepted 31 March 2017 Available online xxxx Keywords: Montmorillonite Acid-activated montmorillonite 3,4-Dihydropyrimi-din-2(1H)-ones Nanopores

a b s t r a c t Three component coupling reactions of aldehyde, b-ketoester (or b-diketone) and urea (or substituted urea) were carried out by using acid activated montmorillonite clay (AT-Mont.) catalyst having surface area about 400 m2/g for the facile synthesis of 3,4-dihydropyrimidin-2(1H)-ones via the Biginelli reactions under mild reaction conditions. The activation of montmorillonite clay was carried out with HCl under controlled conditions for generating nanopores (size 2–7 nm) into the matrix. This porous catalyst has promising feature for Biginelli reaction such as easy removal of the catalyst, short reaction time, high yield with 100% selectivity and easy workup procedure. Powder XRD, SEM-EDX, N2 adsorption, pyridine adsorbed FT-IR and TPD analysis were carried out to characterize the solid porous AT-Mont. The catalysts can be recycled and reused several times without significant loss of their catalytic activity. Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

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41 42

1. Introduction

43

Pyrimidinones or dihydropyrimidinones (DHPMs) are well known for their wide range of bioactivities and applications in the field of drug research have stimulated the invention of a wide range of synthetic methods for their preparation and chemical transformations [1–4]. DHPM as an N-contained heterocycle attracted much attention due to their pharmaceutical and therapeutic activities such as anticancer, anti-inflammatory, antibacterial, antifungal, anthelmintic activities [5–8]. 4-Aryl-1,4dihydropyridines (DHPs) of the nifedipine type were first introduced into clinical medicine in 1975 and are still the most potent group of calcium channel modulators available for the treatment of cardiovascular diseases [1–4]. The clinically important antiretroviral agents like AZT, DDC and DDI possess the pyrimidinescaffold [1–4]. Some marine natural products such as the alkaloid Batzlladine B are found to be potent HIV gp-120-CD4 inhibitors which contain the dihydropyrimidine-5-carboxylate as a sole unit [5,6]. The original method for the synthesis of DHPMs reported by Biginelli in 1893, involves the HCl-catalyzed condensation reaction of ethyl acetoacetate, aldehydes, and urea or thiourea in ethanol [9]. One major drawback of this reaction, however, is the low to

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⇑ Corresponding author. Fax: +91 376 2370 011. E-mail addresses: [email protected], [email protected] (D.K. Dutta).

moderate yield that is often encountered when substituted aromatic (20–60%) or aliphatic aldehydes (10%) is used [10]. Hence, the Biginelli reaction for the synthesis of DHPMs became an attractive topic, and the investigation on this reaction mainly focused on the exploration of catalysts [11–13]. The use of BF3 as catalyst resulted in a modest yield of DHPMs by one pot reaction of bketoester, aryl aldehyde, and urea [14]. Transition metal salts such as Fe(NO3)3.9H2O, HCl-FeCl3, SnCl2, ZnCl2,CuCl2,TiCl4, and InCl3 were also used as homogeneous catalysts for this reaction [15– 22]. Some non-acidic inorganic salts such as CaF2, CeCl3
7H2O, iodotrimethylsilane/NaI and some basic compounds such as t(CH3)3COK, Ph3P, and L-proline were found to be catalysts for the Biginelli condensation [15–22]. The problems traditionally associated with the homogeneous catalysts include high temperature, prolonged reaction time and strong Lewis acidity of the catalyst. These problems have been solved to some extent by use of heterogeneous acid catalysts like Al-MCM-41 or FeCl3 embedded in Al-MCM-41, EPZ10 and heteropolyacids such as H3PW12O40, H3PMo12O40, organosilane sulfonated grapheme oxide, zirconia sulfuric acid, nano ZrO2 sulfuric acid, and metaliophthalocyanines to catalyze this reaction [23–29]. However, in spite of their potential utility, many of the existing heterogeneous catalytic processes involve the use of expensive reagents, strong acidic conditions, tedious work-up, multi step preparation of catalyst and environmental disposal problems. Therefore, there is still need to develop a more efficient, less expensive, high yielding environmentally

http://dx.doi.org/10.1016/j.apt.2017.03.030 0921-8831/Ó 2017 Published by Elsevier B.V. on behalf of The Society of Powder Technology Japan. All rights reserved.

Please cite this article in press as: A. Phukan et al., An efficient and robust heterogeneous mesoporous montmorillonite clay catalyst for the Biginelli type reactions, Advanced Powder Technology (2017), http://dx.doi.org/10.1016/j.apt.2017.03.030

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benign catalyst involving mild reaction conditions such as less toxic solvent used, simple work up procedure etc. for Biginelli synthesis. The use of heterogeneous catalyst system in organic reaction also complies with the requirement of ‘‘green” chemistry as it offers easy separation, recovery of the catalyst from the reaction products and its recyclability. Recently considerable effort has been devoted to the application of environmentally benign, cheap, easily available and robust materials for catalysis. In this context, there has also been increasing interest in employing montmorillonite clay for the development of ecofriendly and sustainable synthetic methods [30–33]. Herein, we report a procedure for development of acid clay catalyst by using environmentally benign, cheap montmorillonite clay which is collected from the natural deposition in the western part of India. The virgin montmorillonite clay was purified, homoionised with NaCl and activated with mineral acid (HCl) under controlled conditions to generate a matrix having high surface area and contain micro- and mesopores with diameter less than 10 nm on the surface. This acid activated clay (AT-Mont.) catalyses the one pot three component coupling of aldehyde, b-ketoester (or b-diketone) and urea (or substituted urea) to give the products DHPMs (88–98% yield) with 100% selectivity under mild reaction condition. The AT-Mont. maintains their catalytic efficiency for several cycles and serves as potential reusable catalyst.

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2. Experimental

114

2.1. Materials and methods

115

Montmorillonite clay (procured from Gujarat Mines Bentonite, India) was purified to collect the <2 lm fraction (rich in montmorillonite) by standard sedimentation method before use [34]. The purified clay was converted in to homoionic Na-exchanged form (Na-Mont.) by stirring in 2 M NaCl solution for about 72 h, which was washed and finally dialyzed against distilled water until conductivity of the dialyzate approached to that of distilled water [35,36]. The Na-montmorillonite (Na-Mont.) was then treated with 4 M HCl in order to obtain amorphous solid product AT-Mont. FTIR studies of Na-Mont. and AT-Mont. were conducted by using Perkin-Elmer system 2000 FTIR spectrometer. Surface area, pore volume and average pore diameter were measured by using Autosorb-1 (Quantachrome, USA). Surface areas were determined by adsorption of nitrogen gas at 77 K and applying BrunauerEmmett-Teller (BET) isotherm. Prior to adsorption, samples were degassed at 200 °C for about 1.5 h. Pore size distributions were derived from desorption isotherms at P/Po value of >0.35 and using Barrett-Joyner-Halenda (BJH) method. Powder XRD spectra were recorded on a Rigaku, Ultima IV X-ray diffractometer in the range of 2b range 2–80° using a Cu Ka source. Scanning Electron Microscopy (SEM) images and Energy Dispersive X-ray spectroscopy (EDX) patterns were obtained from the samples by using Carl Zeiss SIGMA FE-SEM operated at 5 and 20 KV and Oxford X-Max 20 EDS detector. Prior to examination, samples were coated with gold. 29Si and 27Al MAS-NMR spectra of selected samples were recorded in a DSX 300 NMR spectrometer. Acidity of Na-Mont. and AT-Mont. were determined by NH3-TPD measurement using Chem BET Pulsar (Quantachrome, USA). The samples were prepared by outgassing at 140 °C under He gas flow. NH3 gas was passed through the samples for 30 min for complete saturation and then free absorbed NH3 was removed by passing He for another 30 min. NH3-TPD analysis was achieved by heating the samples at 20 °C min1 under He gas flow rate at 80 ml min1. The analysis was carried out in the temperature range 100–600 °C. The acidity measurement of Na-Mont. and AT-Mont. were also carried out by pyridine-adsorbed FT-IR analysis. The samples were first finely

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ground and allowed to adsorb pyridine in a closed chamber for 30 min and followed by removal of excess pyridine absorbed on the sample by keeping in a hot air oven at 120 °C for 1 h. Samples were analyzed in a FT-IR spectrophotometer.

151

2.2. Catalyst preparation

155

Na-Mont. (10 g) was dispersed in 200 ml 4 M hydrochloric acid and refluxed for 2 h. After cooling, the supernatant liquid was discarded and the activated montmorillonite was repeatedly redispersed in deionised water until no Cl ions could be detected by the AgNO3 test. The montmorillonite was recovered, dried in air oven at 50 ± 5 °C over night to obtain the solid product. The activated montmorillonite was designated as AT-Mont.

156

2.3. General procedure for one-pot synthesis of DHPMs

163

Aldehyde (2 mmol), b-ketoester (or b-diketone) (2 mmol), urea (3 mmol), 20 mg catalyst (AT-Mont.) and 5 ml ethanol were taken in a 25 ml round bottom flask and reaction mixture was refluxed at 78 °C for stipulated time period. The progress of the reactions was monitored by TLC. After completion of the reaction, the reaction mixture was filtered under hot condition to separate the catalyst. The filtrate was then evaporated in a rotavapour to obtain a solid residue. The solid residue was washed with water to remove the excess urea and then filtered. It was finally recrystallized from ethanol to obtain the pure product. The isolated pure product was characterized by 1H and 13C NMR, FT-IR data and melting point respectively.

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3. Results and discussion

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3.1. Characterization of catalyst

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The oxide compositions of the clay before and after acid activation were determined by wet chemical and flame photometric methods and are presented in Table S1 (supporting information). The Na-Mont. exhibited an intense basal reflection at 7.06° 2h corresponding to a basal spacing of 12.5 Å (Fig. 1). This reflection intensity becomes decreased with acid activation. A low intense broad reflection in the range 20–30° 2h confirmed that the materials turned to amorphorous silica [37]. Upon acid activation for 2 h, the surface area and the pore volume of AT-Mont. increases to

178

Counts

2

AT-Mont.

Na-Mont.

0

10

20

30

40

50

two theta degree Fig. 1. Powder XRD pattern of AT-Mont. and Na-Mont.

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about 400 m2/g from 141 m2/g and 0.6037 cm3/g from 0.216 cm3/g of Na-Mont. respectively (Table 1). Acid activation leads to modification of the layered structure of parent montmorillonite by leaching out Al3+ ions from their octahedral sites with the destruction of layered structure and simultaneous creation of uniform pores having diameter in the nano range with high surface area (Fig. 2).

The N2 adsorption-desorption isotherms of the Na-Mont. and AT-Mont. were shown in Fig. 3A which clearly indicates the increase of surface area. The adsorption-desorption isotherm (Fig. 3A) shows type-IV with a H3 hysteresis loop at P/Po  0.5– 0.9, which is a characteristic of a mesoporous solid. On acid treatment, the increase of intensity of the BJH distribution curve of ATMont. (Fig. 3B) compared to the Na-Mont. indicates the generation

Table 1 Surface properties of AT-Mont. and Na-Mont. Sample

Surface area (m2/g)

Pore diameter (nm)

Pore volume (cm3/g)

Acidity (mmol/g)

AT-Mont. Na-montmorillonite

400 141

6.33 6.11

0.604 0.216

0.56 0.07

Si

Si

Si

Si

Al

Al

Al

Al

Si

Si

Si

Si

Si

Si

Si

Si

Al

Si

Conc. HCl

Si

Si

Si

Si

Si

Al

Al

Al

Al

Al

Si

Si

Si

Si

Si

Si

H+

H+

Si

Si

Pores Generate

Al

Si

H+

An+

Si

H+

Si

Si

Al

Al

Si

Si

Al3+ Si

Silicate tetrahedral layer

Si

Al

n+ A

Al

+

2+

Na , Mg , Ca

2+

etc.

Alumina octahedral layer

Fig. 2. HCl treatment on montmorillonite clay.

175

1.50

AT-Mont.

150

(A)

125

Na-Mont.

100 75 50 25

Desorption Dv (log d) ( cc g-1)

188

Volume (cc/g)

187

AT-Mont.

1.25

(B)

1.00 Na-Mont

0.75 0.50 0.25 0.00

0 0.0

0.2

0.4

0.6

0.8

Relative Pressure, P/P0

1.0

0

2

4

6

8

10

Diameter (nm)

Fig. 3. (A) N2 adsorption/desorption isotherm and (B) pore size distribution curve of AT-Mont. and Na-Mont.

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(b)

(b)

(a)

(a)

Chemical shift (ppm) (A)

206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233 234 235 236 237 238 239 240

29

Si and (B)

27

Al MAS-NMR spectra of (a) Na-montmorillonite and (b) AT-Mont.

of pore on acid activation which is also substantiated by the increase of total pore volume of AT-Mont from 0.216 to 0.604 cm3/g. The pore size distribution plot in Fig. 3B shows that the average pore sizes of AT-Mont are in the range of 2–10 nm suggesting that pore sizes of the catalyst lie in the mesoporous region. The FTIR spectra of Na-Mont. and AT-Mont. reflect the structural degradation of the clay components and formation of an amorphous silica phase during acid activation. The Na-Mont. exhibited an intense absorption IR band at 1034 cm1 for SiAO stretching vibrations of tetrahedral sheet. The shape and position of this band changed during acid activation. The band shifts from 1034 to 1083 cm1, indicating the change in bonding environment in tetrahedral layer and formation of an amorphous silica phase [ESI Fig. 1]. The appearance of a pronounced band near 800 cm1 also indicated amorphous silica [37]. Na-Mont. showed absorption bands at 3633 cm1 due to stretching vibrations of OH groups of Al-OH. Other bands at 917, 875 and 792 cm1 were related to Al-Al-OH, Al-Fe-OH and AlMg-OH vibrations [37]. The gradual disappearance of these bands during acid-activation indicated the removal of Al, Fe and Mg ions from the clay mineral structure. The extraction of octahedral or tetrahedral Al3+ ions from the clay during acid activation was confirmed by 27Al and 29Si MAS-NMR spectroscopy. The 29Si MAS-NMR spectra of untreated montmorillonite contained one sharp peak at d = 93 ppm (Q3(0Al) due to SiO4 tetrahedra surrounded by three other silicate units in the tetrahedral sheet and one Al (or Mg) atom through oxygen bridges of montmorillonite. With the acid treatment, the intensity of this peak, i.e. d = 93 ppm (Q3(0Al) of montmorillonite, gradually disappeared with simultaneous appearance of the new peaks at about d = 111 ppm [Q4(0Al) (SiAOASi bond)] and d = 102 ppm [(SiO3)SiAOH bond] showing the decomposition of montmorillonite [Fig. 4A]. The 27Al MAS-NMR spectra of untreated montmorillonite showed an intense signal at d = 3.92 ppm for octahedrally coordinated alumina whereas a weak signal at d = 67 ppm for

tetrahedrally coordinated alumina in the framework of the clay. During acid activation, the intensity of the peak due to octahedral Al decreased considerably along with shifting the peak position from 3.92 ppm to about 1.6 ppm due to leaching of octahedral Al, but the peak value at d = 67 ppm due to tetrahedral Al remained almost similar to untreated montmorillonite indicating residual intactness of the tetrahedral Al structure (Fig. 4B) [38–40]. The total acidity of Na-Mont. and AT-Mont. were measured from NH3-TPD analysis. The total acidity of AT-Mont increases to 0.56 mmol/gm from 0.07 mmol/gm of Na-Mont. on acid activation. The NH3-TPD curve of AT-Mont (Fig. 5) showed three desorption peak centered at 195, 265 and 390 °C. The peaks at 195 and 265 °C are assigned to Bronsted acid sites and the peak at 390 °C is assigned to Lewis acid sites [41,42]. The acid activated montmo-

14

265 195

12 10

Signal (mV)

Fig. 4. (A)

Chemical shift (ppm) (B)

390

8 6 4 2 0 50

100

150

200

250

300

350

400

450

500

Temperature ( OC) Fig. 5. NH3-TPD profile for AT-Mont.

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rillonite possesses Bronsted acid sites both on the surface and interlayer cation exchange sites. So decomposition of NH3 from the surface Bronsted acid sites takes place relatively at low temperature compared to that of interlayer sites and consequently shows two peaks in the NH3-TPD curve [43,44]. The intensity of the peaks corresponding to Bronsted acid sites are much higher compared to Lewis acid site indicating higher Bronsted acidity of the catalyst. This may be correlated to the leaching of Al3+ ions during acid activation which increases Bronsted acidity [40]. The types of acid sites on the catalyst AT-Mont. were also determined by pyridine adsorption FTIR study. AT-Mont. shows (Fig. 6) two characteristic IR absorption bands at 1547 and 1427 cm1 for Bronsted and Lewis acid sites respectively due to the CAC stretching vibration of pyri-

AT-Mont. 1427

Transmittance (%)

255

1547 1481 3425

3623

Na-Mont.

3452 3623

4000

3500

2000

1500

1000

Wave number ( cm-1 )

500

5

dinium ion attached to the Bronsted acid and CAC stretching vibration of coordinatively bonded pyridine complex to the Lewis acid sites indicating the generation of both Bronstead and Lewis acidity upon 2 h acid activation. Another band at 1481 cm1 present on AT-Mont. is due to CAC stretching of pyridine bonded to total acid sites [45,46]. The negligible appearance of these three bands in NaMont. indicates the low acidity of Na-Mont. The relative intensity of the peak at 1547 cm1 due to AT-Mont. indicates the presence of Bronstead acid sites in high concentration as observed in the NH3-TPD analysis. It is worth to mention here that the catalyst AT-Mont was prepared from environmentally benign montmorillonite clay by simple acid activation. The catalyst exhibits both micro- and mesopores and possesses high Bronsted and Lewis acidity. On the other hand, MCM-41 is synthetically prepared ordered mesoporous material of hexagonal symmetry having no Bronsted acidity on its surface. To enhance the surface acidity, direct metal ion like Al is incorporated to the structure of MCM41. The total acidity of Al-MCM-41 as determined by NH3-TPD and pyridine FTIR analysis [47] is about 0.125 mmol/g while in case of the present catalyst AT-Mont. the total acidity value is found to be much higher i.e. 0.56 mmol/g (Table 1). From the stability point of view, MCM-41 has limited lifetimes and collapses its mesoporous structure after few runs; while the structure of the acid activated montmorillonite clay is highly stable and lasts for very long during in catalytic use. Furthermore, the bands (Fig. 6 ) observed near 3623 cm1 in both Na-Mont. and AT-Mont. are due to silanol (Si-OH) groups, while broad bands near 3425 and 3452 cm1 are due to Al2OH groups formed in the interlayer region. The relative intensity of bands for interlayer Al2OH decreases while for silanol groups (Si-OH) increases on acid activation. This may be correlated to the leaching of Al3+ ions during acid activation with increase in time [48]. The SEM images of parent Na-Mont. and AT-Mont. (Fig. 7(A) and (C)) indicate the rough surface on the both. The SEM image confirms the generation of pores in the nano range on the clay matrix during acid activation (Fig. 7(C)) while Na-Mont. contains both Si and Al in predominant amounts (Fig. 7

Fig. 6. Pyridine adsorbed FTIR spectra of AT-Mont. and Na-Mont.

Fig. 7. (A) SEM image of Na-Mont., (B) EDX analysis of Na-Mont. at the surface, (C) SEM image of AT-Mont., (D) EDX analysis of AT-Mont. at the pores.

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O H2N

O

CHO

O

NH2

AT-Mont. Ethanol,

O

O

780C,

2h

O

NH N H

O

Scheme 1. Schematic representation of the reaction between Urea, Ethylacetoacetate and Benzaldehyde to yield 3,4-dihydropyrimidin-2(1H)-one.

Table 2 Reaction conditions: Benzaldehyde (2 mmol), ethylacetoacetate (2 mmol), urea (3 mmol), catalyst (20 mg) and ethanol (5 ml), time-2 h. Entry

Catalyst

Solvent

Temp. (°C)

Yield

1 2 3

Without catalyst Na-Mont. AT.-Mont

Ethanol Ethanol Ethanol

78 78 78

– Trace 98%

306

(B)). The EDX spot analysis at the pore indicates that only Si is present (Fig. 7(D)) and Al was leached out during acid activation.

307

3.2. Catalytic activity

308

The synthesized acid clay catalyst was investigated as heterogeneous catalyst in three-component coupling reaction of aldehyde, urea and b-ketoester (b-diketone). To optimize the reaction conditions that are required to afford excellent yields of DHPMs, a model reaction (Scheme 1) is carried out at different temperatures and

305

309 310 311 312

different time intervals. A maximum of 98% conversion was observed after 2 hr refluxing of the reaction mixtures in ethanol (Table 2). Therefore, ethanol clearly stands out as the solvent of our choice by considering its high yield, selectivity, green solvent nature and environmental acceptability. To test the requirement of a catalyst, a control reaction was carried out by stirring the reagents without any catalyst or with the Na-Mont. as catalyst. No desirable products could be detected in both cases (Table 2). Using the optimized reaction conditions and catalyst we explored the versatility and limitation of various substrates. The presence of electron donating or withdrawing groups on the aldehyde had no effect on the yields of the reaction. The aliphatic aldehydes in general showed slightly lower yield than aromatic ones. Another variation in the reaction was done by using substituted urea such as 1,3-dimethyl urea and acetylacetone (b-diketone) instead of ethylacetoacetate for the synthesis of different DHPMs. All the reactions showed very good to excellent yields (88–98%) with 100% selectivity (Table 3 and ESI Table 2). The commercially available catalyst, montmorillonite K 10, with the lowest surface area of 230 m2/g and the lowest acidity of

Table 3 Three-component coupling of aldehyde, b-ketoester (or b-diketone) and urea for the synthesis of DHPMs.a Entry

Urea

b-ketoester (or b-diketone)

O

O

S1

H2N

O

Aldehyde

CHO O

O

NH2

Yieldb (%)

Product

O

NH N H

O

O

S2

H2N

O

CHO

O

O H2N

O

CHO

O

O H2N

O

CHO

O

H2N

NH2

O

O

CH 3

98

O NH

CH 3 O

N H

O

CHO

O

97

NH N H

3

O

S5

3

O

O

NH2

O

OCH

O

OCH

S4

NH N H

O

NH2

97a 96b 96c 95d

O O

NO 2 S3

O

NO2

O

NH2

98a 98b 97c 96d

95

OH

O

OH

O

NH N H

O

a Reaction conditions: Aldehyde (2 mmol), ethylacetoacetate or acetylacetone (2 mmol), urea or 1,3-dimethyl urea (3 mmol), catalyst (20 mg) and ethanol (5 ml); refluxing temp. 78 °C, time 2 h. b Yields are isolated products based on the aldehyde after recrystallization with ethanol a-1st run, b-2nd run, c-3rd run, d-4th run.

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Catalyst

Solvent

Temp. (°C)

Time (h)

Conversion (%)

1 2 3 4 5 6

Zn(NTf2)2a

Ionic liquid [bmim][PF6] Solvent free Ethanol/reflux H2O Solvent free Ethanol

60 100 80 100 60 78

3 4 8 5 3 2

92 93 93 80 91 98

References

380

354

0.20 mmol/g, shows the lowest conversion of 54.71% only. The separation of the catalyst from the reaction mixture was done by filtration and the products were purified by recrystallization with ethanol and no chromatographic separation was required. A comparison of different reported catalysts with the present one (Table 4) indicates that the latter catalyst is better in activity. The recyclability of our catalyst was investigated in the synthesis of DHPMs (Table 3, Entry S1, S2 and 10, 13 from ESI Table 2). The recovered catalyst was washed with acetone, dried in a desiccator and reused directly with fresh reaction mixture upto the 4th run and showed only a slight decrease in activity (Table 3). Thus, it appears that the catalyst AT-Mont. is very much effective for the synthesis of different DHPMs. The proposed mechanism for the synthesis of DHPMs catalyzed by AT-Mont. is presented in ESI: Scheme 1. The aldehyde and urea react first in the presence of AT-Mont. to form an imine which eliminates water molecule from it to form an iminium ion. This is a very favourable step catalysed by the Bronsted acid catalyst. Thereafter the iminium ion reacts with the enolate form of ethylacetoacetate to give an intermediate product (Ureido) which finally releases a molecule of water to yield the desired product DHPMs.

355

4. Conclusion

356

368

Facile synthesis of DHPMs has been carried out by using ATMont. catalyst through three-component coupling reaction of Aldehyde, b-ketoester (or b-diketone) and Urea in ethanol as solvent. The specific surface area and the pore diameter as well as the pore size distribution of montmorillonite were tuned by controlled acid activation. Further, the catalysts were reused for new batch of reactions (upto 4th run) without significant loss of their activity under the same reaction conditions. High catalytic activity up to about 98% conversion with nearly 100% selectivity were observed. The operational simplicity and robustness of the catalyst make it attractive not only for the large scale synthesis of this class of biologically active molecules, but also for the exploration in the synthesis of important drug intermediates.

369

Acknowledgement

370

376

The authors are grateful to Dr. D. Ramaiah, Director, CSIRNorth-East Institute of Science and Technology, Jorhat, Assam, India, for his kind permission to publish the work. The authors thank Dr. P. Sengupta, Head, Materials Sciences and Technology Division, CSIR-NEIST, Jorhat, for his encouragement. Thanks are also given to CSIR, New Delhi for a financial support (Network projects: CSC-0125, 0135 and MLP-6000/1).

377

Appendix A. Supplementary material

378

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.apt.2017.03.030.

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Nano TiO2-NH2 Ce(LS)3b CSAc Nano-Fe2O3-SO3H AT-Mont

[49] [50] [51] [52] [53] (present work)

Zinc di[bis(trifluoromethylsulfonyl)imide]. p-Dodecylbenzenesulfonic acid. Cellulose sulfuric acid.

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