ar ticle

Advanced Science, Engineering and Medicine Vol. 6, pp. 1–7, 2014 (www.aspbs.com/asem)

Copyright © 2014 by American Scientific Publishers All rights reserved. Printed in the United States of America

Single-Walled Carbon Nanotube-Ammonium Ionic Liquid a New Catalyst for Synthesis of 3,4-Dihydropyrimidinones Pankaj Attri1 , Rohit Bhatia2, ∗ , Bharti Arora3 , Jitender Gaur4, 5 , Ruchita Pal6 , Arun Lal2 , Varun Chopra7 , and Ankit Attri5 1

Plasma Bioscience Research Center, Department of Electrical and Biological Physics, Kwangwoon University, Seoul, Korea 2 Department of Chemistry, University of Delhi, Delhi 110007, India 3 Department of Applied Sciences and Humanities, ITM University, Sector-23(A) Gurgaon 122017, Haryana, India 4 CSIR-National Physical Laboratory, Dr. K. S. Krishnan Marg, New Delhi 110012, India 5 J&S Research and Innovations, New Delhi 110092, India 6 Advanced Instrumentation Research Facility (AIRF), Jawaharlal Nehru University (JNU), New Delhi 110067, India 7 C2-C3 Plant, Oil and Natural Gas Corporation, Baroda, India

KEYWORDS: Single-Walled Carbon Nanotubes, 3,4-Dihydropyrimidinones, Ionic Liquid, Bucky Gel.

1. INTRODUCTION Carbon nanotubes (CNTs) and ionic liquids (ILs) represent interesting class materials due to their unique properties and wide range of applications.1–8 ILs completely consist of weakly coordinating ions i.e., organic cation and inorganic/organic anion possessing desirable properties, and are liquids at or close to room temperature.5 6 ILs are emerging as more promising solvents in the various fields such as organic synthesis, protein folding, materials science, electrochemistry and separation technology.9–15 On the other hand, the discovery of CNTs has enhanced intensive research, that has resulted in producing extraordinary scientific and technological advances in the development of nanostructured materials.1 2 It is very difficult to disperse CNTs homogeneously in both organic and aqueous solvents due to their strong van der Waals attraction ∗

Author to whom correspondence should be addressed. Email: [email protected] Received: 20 August 2013 Accepted: 13 October 2013

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among tubes, which limits their application in many fields.16–19 To solve this problem, the use of ILs to disperse CNTs can prove to be very interesting.16 17 Recently, there has been significant curiosity in understanding the properties and interactions of ILs with CNTs.16 17 20–27 The interactions between CNTs and ILs provide with a benefit to obtain the dispersed CNTs. And, these dispersed CNTs are nowadays in high demand and are commonly known as bucky gels or soft materials.25 26 These bucky gels are found to be of great use in chemical, physical and biological applications.25 Fukushima et al.16 were first to report that imidazolium based ILs, such as 1-butyl-3-methylimidazolium tetrafluoroborate can form gel called “Bucky gels” by grinding them with single-walled carbon nanotubes (SWCNTs). Later, Wang et al.27 studied the dispersion mechanism of SWCNTs in an imidazolium-based ILs. Kocharova et al.28 reported that carbon nanotubes can be effectively dispersed in aqueous solutions by 1-dodecyl-3methylimidazolium bromide and 1-(12-mercaptododecyl)3-methylimidazolium bromide ILs. Further, Aida and Lee

2164-6627/2014/6/001/007

doi:10.1166/asem.2014.1507

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A new catalytic method has been developed for the synthesis of 3,4-dihydropyrimidinones via one-pot three component Biginelli condensation, which provides higher product yields. This catalyst is a combination of single-walled carbon nanotubes (SWCNT) with triethylammonium hydrogen phosphate (TEAP) ionic liquid (IL), commonly referred to as bucky gel. To gain insight into the interactions involved between IL and SWCNT, we utilized Raman spectroscopy for our analysis. The interactions between SWCNT with TEAP were clearly shown by the increasing intensity ratios and spectral shift in wavelength of the Raman D and G bands of SWCNT. The morphological studies of the resulting composite materials of TEAP and SWCNT (bucky gel) were carried out using scanning electron microscopy (SEM). The key advantage of using bucky gel as a catalyst is that higher product yield is obtained at a reduced reaction time for Biginelli reactions.

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Single-Walled Carbon Nanotube-Ammonium Ionic Liquid a New Catalyst for Synthesis of 3,4-Dihydropyrimidinones

reported that imidazolium ion-based ILs, when being grounded with SWCNTs, get transformed into bucky gels.29 Although Zhou et al.30 observed that CNTs could be dispersed stably in water with the aid of small amounts of the 1-aminoethyl-3-methylimidazolium bromide and 1(2-aminoethyl)-pyridinium bromide ILs. Crescenzo et al.31 explained the stability of 1-hexadecyl-3-vinyl-imidazolium, a water-soluble long-chain IL having surfactant properties, forming a stable homogeneous aqueous dispersion with SWCNTs. Further, Liu et al.24 showed the effect of alkyl chain length on the dispersion ability of multiwalled carbon nanotubes (MWCNTs). Additional, Fukuda et al.32 fabricated a Braille sheet with a CNT-based actuator containing SWCNT and ILs. In another study, Lu et al.33 found MWCNTs to be effectively dispersed in aqueous solutions containing IL-based phosphonium surfactants {e.g., alkyl-triphenyl phosphonium bromide (CnTPB, n = 12 14)}. The MWCNT dispersibility increased with increasing length of the hydrocarbon chain in CnTPB. Recently, Mohammadi and Foroutan studied the structural characteristics and the dispersion of the aggregated non-bundled and bundled CNTs in an IL (1-n-propyl-4amino-1,2,4-triazolium bromide).20 These results revealed that, ILs due to their properties (e.g., low flammability, low or zero volatility, high thermal stability and ionic conductivity) can be considered as better CNT dispersants than the conventional organic solvents. In addition, many researchers used bucky gel for a wide variety of electrochemical applications such as biosensors, capacitors and actuators.20 26 In electrochemical biosensors, these composite materials (bucky gel) can also be used as immobilizing matrices to entrap proteins and enzymes to provide a favorable microenvironment for redox proteins and enzymes to retain their bioactivity, and to perform direct electrochemistry and electrocatalysis.26 However, the use of CNT-IL in organic synthesis is limited. A report from Chen et al.21 revealed the aerobic oxidation of 1-phenylethanol is catalyzed by carbon nanotube supported palladium catalyst and the reaction was improved by further addition of IL. To boost the applicability of bucky gel in organic reaction, we explored the Biginelli reaction. 3,4-Dihydropyrimidinones, known as Biginelli compounds are significant heterocyclic units that possess diverse therapeutic and pharmacological properties, including anti-viral, anti-tumor, anti-bacterial and anti-inflammatory activities.34 Furthermore, these compounds have emerged as calcium channel blockers, anti-hypertensive agents and -1a-adrenergic antagonists. Also, several alkaloids containing dihydropyrimidine nucleus isolated from marine sources have been found to possess interesting biological activities.35 36 Owing to the wide range of pharmacological and biological activities, the synthesis of these compounds has become an important challenge in recent years.37–55 Unfortunately, Biginelli reaction are suffering from many limitations, such as the use of expensive reagents, harsh conditions, long reaction times, high 2

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catalyst loading, low selectivity, presence of side reactions, and tedious work-up procedures for their separation, recycling, or disposal problems and effluent pollution. All these limitations enforced us to explore a new, more efficient catalyst with limited drawbacks.37–48 In this research article, we have tried to explore the new catalyst system that has characteristic properties, such as good thermal and mechanical stabilities of supported reagents, is easy to handle, of low toxicity, non-corrosive, easy to separate from the reaction mixture through filtration, and feasible for reuse. In light of these considerations, we have explored the combination of triethylammonium dihydrogen phosphate (TEAP) IL and SWCNT as a catalyst system for organic reaction commonly known as bucky gel. We also investigated the interaction between SWCNT and TEAP using confocal Raman spectroscopy. We are further motivated to carry out morphological studies of the resulting composite materials of TEAP and SWCNT using scanning electron microscopy (SEM). In addition, the synthesis of 3,4-dihydropyrimidinones reaction products were carried out using this bucky gel as a catalyst system.

2. MATERIAL AND METHOD SWCNT was obtained from Sigma–Aldrich (USA). All the reagents used were of analytical grade. Melting points were determined using a Thomas Hoover melting point apparatus. 1 H (400 MHz) and 13 C (75 MHz) nuclear magnetic resonance (NMR) spectra were recorded on a Jeol 400 NMR spectrometer in CDCl3 (with tetramethylsilane (TMS) for 1 H and chloroform-d for 13 C as internal references). The Raman spectra were measured at room temperature using a confocal Raman microscope (WITec, Alpha 300 R) with a 514 nm He–Ne laser. And, the reactions were monitored by thin layer chromatography (TLC) using aluminum sheets with silica gel 60 F254 . CNTs could be easily dispersed in the TEAP based room temperature IL by mechanically milling, forming a thermally stable bucky gel as follows.16 26 27 CNTs (3 mg) were suspended in a TEAP (0.01 mg), and the mixture was crushed with an agate mortar for 1 hr, and gradually the suspension turned into a uniform black paste. Further, the resulting paste was centrifuged for 2 hr, followed by the removal of excess IL, to isolate a gel phase. 2.1. General Procedure for the Preparation of 3,4-Dihydropyrimidinones In a typical experimental procedure, a mixture of four components was added in a round bottom flask containing aldehyde (1 mmol), urea (1.6 mmol), -dicarbonyl compound (1 mmol) and bucky gel ( 0.5 mmol) which were then stirred thoroughly for 45 min at 60 0 C. And, the progress of the reaction was monitored using TLC. After completion of the reaction, addition of dichloromethane (5 ml × 3) was accompanied by the washing of the resulting organic phase extract with saturated sol. of Adv. Sci. Eng. Med., 6, 1–7, 2014

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Fig. 1.


Raman spectra of (a) TEAP IL and (b) SWCNT (magenta) and SWCNT-TEAP (Cyan).

NaHCO3 , water, and then dried over Na2 SO4 . The solvent was then removed and the residue was purified by recrystallization or silica gel chromatography. The reaction products were analyzed using 1 H and 13 C NMR spectroscopy.

Fig. 2.

3. RESULTS AND DISCUSSION Our present investigation has revealed a new catalyst system (bucky gel) for the synthesis of 3,4dihydropyrimidinones products. It is well-known that SWCNT can be easily dispersed in the TEAP IL by

SEM image of (a) pure SWCNT, (b) TEAP IL and (c) SWCNT-TEAP composite.

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2.2. Synthesis of Triethylammonium Dihydrogen Phospahte (TEAP) The synthesis of ionic liquids was carried out in a 250 mL round-bottomed flask, which was immersed in a water-bath and fitted with a reflux condenser. Phosphoric acid (1 mol) was dropped into the triethyl amine (1 mol) at 70 C for 1 hr. And, the reaction mixture was heated at 80 C with

stirring for 2 hrs to ensure the reaction proceeds to completion. The reaction mixture was then dried at 80 C until the weight of the residue remained constant. The sample analyzed by Karl Fisher titration revealed low levels of water (below 70 ppm). 1 H NMR (DMSO-d6 ): (ppm) 1.18 (t, 9H), 3.06 (m, 6H), 6.37 (s, 1H).


O O R

O H

Y

O OR'

Me

H2N

Bucky gel NH2

Attri et al.

R

R'O

NH

60 ºC, 45 min Me

N H

Y

R = C6H5, NO2-C6H4, Cl-C6H4, Cl2-C6H4, CH3O-C6H4, C5H11 R' = C2H5, CH3

Y = S,O

Fig. 3. Synthesis of 3, 4-Dihydropyrimidinones from aldehyde (1 mmol), urea (1.6 mmol), -dicarbonyl compound (1 mmol) catalyzed by bucky gel (1 mmol) for 45 min at 60 C. Table I. The Biginelli of compound 1 of Table III with ethylacetoacetate, urea, aldehyde and bucky gel.

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Entry 1. 2. 4. 6. 7. 8. 9. 10. 11.

Catalyst No catalyst SWCNT SWCNT TEAP TEAP TEAP SWCNT-TEAP SWCNT-TEAP SWCNT-TEAP

Temperature Time for complete ( C) conversion 25 25 60 25 60 90 25 60 60

24 hr 24 hr 24 hr 24 hr 45 min 45 min 24 hr 45 min 1 hr

Yield (%) No product No product 30 16 54 70 40 96 96

mechanical milling, forming a thermally stable gel.16 26 27 Different kinds of bucky gel or soft materials have been reported in literature, with varying ILs or CNTs, which have been found to be of great use.17–33 However, the utility of bucky gel as a catalyst for organic reactions is still not documented well. We have utilized the confocal Raman spectroscopy to ascertain the interactions between TEAP and SWCNT during the formation of bucky gel.

In order to explicate the interactions involved, confocal Raman spectroscopy gives an important advantage of serving the vibrational energy of the target solutes as a reporter, which then eliminates the need for invasive monitoring aids such as molecular probes as is the case of confocal scanning laser microscopy.56 However, the Raman signal typically diminishes strongly with the increasing penetration of the bucky gel, owing to the Raman scattering as well as attenuation of the excitation laser power. Thus, the laser can be directed inside the bucky gel without damaging it, while maintaining a high selectivity and sensitivity. Hence Raman spectroscopy proves to be a powerful tool for the structural characterization of CNTs. As per the reported literature Du et al.57 an upshift of 22 cm−1 in the tangential G-band in the Raman spectra of SWCNTs is a characteristic feature. Similarily, Zhao et al.58 reported a slight upshift of around 2 cm−1 in the G-band in the Raman spectra of MWCNTs. These shifts clearly lead to affirmation the cation- or – interactions result in the charge transfer from IL to CNTs. The Raman spectra of TEAP IL is as displayed in Figure 1(a) and SWCNT (magenta) and SWCNT-TEAP

Table II. Comparison of catalytic ability for methyl 6-methyl-2-oxo-4-phenyl-1,2,3,4-tetrahydropyrimidine-5-carboxylate in various catalysis. Entry 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12 13. 14. 15. 16. 17. 18. 19. 21.

4

Catalyst

Time (h)

Condition

Yield (%)

References

10 mol% [Al (H2 O)6 ](BF4 3 TFA LiBr Bentonite/PS-SO3 H nanocomposite Silica sulfuric acid H5 PW10 V2 O40 /Pip-SBA-15 H3 PMo12 O40 H3 PW12 O40 TiO2 MAI · Fe2 Cl7 CMImHSO4 CuCl2 · 2H2 O in an aqueous surfactant system. PEG-400 glyNO3 FeCl3 · 6H2 O [MSEI][Cl] [BMIM][FeCl4 ] [BMIM][BF4 ] [HMIM][HSO4 ] SWCNT-TEAP

20 015 3 030 6 033 5 009 020 1 015 6 045 010 5 30 120 30 90 075

Reflux, CH3 CN 95 C, THF, MW Reflux, CH3 CN 120 C, solvent free Reflux, solvent free 120 C, solvent free AcOH/reflux MW/solvent free 70 C/solvent free 80 C/solvent free 80 C/solvent free 60 C/H2 O 100 C/solvent free MW, ethanol Reflux/EtOH 80 C, solvent free 90 C, solvent free 100 C, solvent free 90 C, solvent free 60 C, solvent free

85 84 92 89 90 90 65 95 96 93 93 90 94 84 86 93 90 99 96 96

[37] [38] [39] [40] [41] [42] [43] [44] [45] [46] [47] [48] [49] [50] [51] [52] [53] [54] [55]

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Table III. SWCNT-TEAP catalysis the synthesis of 3,4dihydropyrimidinones under solvent-free conditions at room temperature.

R

Yield Y (%)a

Mp ( C)

Entry

R

1

CHO

–C2 H5

O

96

2

CHO

–C2 H5

O

93

–C2 H5

O

92

214–215

213–215

–C2 H5

O

89

193–194

192–19331

–C2 H5

O

93

223–224

222–22332

Found

Reported

202–203

201–203

206.5–207 207–208.5

NO2

3

CHO

Cl

4

CHO

Cl

5

CHO

Cl

Cl

6

CHO

–CH3

O

88

208–210

209–21030

7

CHO

–CH3

O

96

235–238

236–23730

–CH3

O

94

193–194

192–194

–CH3

O

97

205–206

204–20730

NO2

8

CHO

OCH3

9

CHO

Cl

5

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composite (cyan) (Fig. 1(b)). The D band shifts from 1349 to 1343 cm−1 and the G band shifts from 1583 to 1575 cm−1 in the Raman spectra of SWCNT (magenta) and SWCNT-TEAP composite (cyan) (Fig. 1(b)). The Raman spectrum shows a typical D and G-peaks in case of SWCNT-TEAP composite which is credited due to the presence of sp3 defects and vibration of sp2 carbon atoms in SWCNT sidewall, respectively. In addition, the position of a G-peak SWCNT-TEAP composite down shifts by 8 cm−1 as compared to SWCNT, suggesting a noncovalent functionalization of TEAP on the graphitic structure of SWCNTs. Meanwhile, the driving force may come from the electrostatic attraction between the SWCNT and TEAP. The possible interactions between SWCNT and TEAP may include the cation- and/or – interactions, being evidenced by the shift in wavelength of the Raman spectrum. These findings are well supported by the work of other research groups.22 25–27 To quantify the morphologies of the resulting SWCNTTEAP composites, we used SEM for further analysis. On comparing the SEM images of pure SWCNT, TEAP and SWCNT-TEAP composite, morphology was found to be distinct from each other (Fig. 2). Figure 2(c) clearly illustrates that SWCNT is buried inside the TEAP. The above discussed results apparently suggest there is a possible change in morphology of SWCNT after interaction with TEAP. And, the SEM images strongly suggest that SWCNT-TEAP gel composites produce more flat film morphology with a smooth surface. This further recommends that TEAP uniformly covers the outer surface of the SWCNT in SWCNT-TEAP composites, and the CNTs bundles are weekly interlocked with one another, which allow the formation of gels. We find these results to be in good agreement with the reported results.22 26 To investigate the applicability of bucky gel in organic reactions, we have further studied Biginelli reaction. A variety of aldehydes, -dicarbonyl compounds and urea/ thiourea were made to react to yield corresponding 3,4dihydropyrimidinones under appropriate reaction conditions (as depicted in Fig. 3). Specifically, to study the effect of bucky gel as a catalyst, we have used benzaldehyde, ethylacetoacetate and urea yielding 5-ethoxycarbonyl4-phenyl-6-methyl-3,4-dihydropyrimidin-2 (1H )-one (2a). The standardized results are well represented in Table I. It was observed that TEAP can catalyze the reaction in 45 min, at 60 C with 54% yield whereas the addition of SWCNT, resulted in an increase in yield up to 96%. Hence, the above results evidence the importance of bucky gel in organic reaction. In addition, Table II clearly reveals that SWCNT-TEAP catalyst system offers good results compared to other catalysts at mild reaction conditions. The emphasis here is on the idea that SWCNT have limited catalytic activity but with the addition of IL, we can increase the efficiency of SWCNT as a catalyst. Using standardization results, we have synthesized the 3,4-dihydropyrimidinones in excellent yields, from various


Table III. Continued.

Entry

R

10

CHO

11

OCH3

Mp ( C)

R

Y

Yield (%)a

–C2 H5

S

94

192–193

191–193

–C2 H5

S

91

151–152

150–15234

–C2 H5

S

94

152–153

151–15334

Found

Reported

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to be further explored. In the present study, bucky gel has shown its potential as catalyst for higher yields with both urea and thiourea, and seems to serve as an alternative reagent/catalyst. Being a cheap, cost effective, ecofriendly reagent, it is therefore employed as a catalyst for the Biginelli reaction. Further, we have also investigated the reusability and recycling of catalyst for Biginelli reaction. Figure 4, reveals that SWCNT-TEAP catalyst has the recyclability ability for Biginelli reaction. The decrease in yield is more for Biginelli reaction after six cycles.

CHO

12

CHO

OCH3

13

CH3 CH2 CH2 CHO

–C2 H5

O

89

152–153

151–15334

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Notes: a Yields determined by GC analysis.

aromatic aldehydes (containing both electron donating and withdrawing groups), using SWCNT-TEAP catalyst system under mild reaction conditions, as demonstrated in Table III. It is well documented that both -ketoesters (ethyl and methylacetoacetate) and 1,3-dicarbonyl compound (acetylacetone) react under the above mentioned reaction conditions with great ease. Furthermore, the use of thiourea in place of urea yielded corresponding 3,4-dihydropyrimidinones-2(1H )-thiones in comparable amount, as shown in Table III, entries (10–12). To evaluate the effect of SWCNT-TEAP catalyst system, a mixture of benzaldehyde, urea and ethylacetoacetate in a molar ratio (1:1.6:1) was stirred at 60 C for 24 hr in the absence of a catalyst. It was surprising that the reaction did not proceed, indicating SWCNT-TEAP to be an essential catalyst for this reaction. The exact role played by bucky gel in this reaction remains imprecise and needs

Fig. 4.

6

Recycling of catalyst for the synthesis of Table III entry 1.

4. CONCLUSIONS In conclusion, the interaction between SWCNT and TEAP results in a new catalyst system. This catalyst system is well documented as an accomplished catalyst for the synthesis of Bignilli reaction under solvent-free conditions. The use of this catalyst provides several advantages; (i) SWCNT-TEAP is a cost effective and environmentally benign reagent, (ii) green synthesis (avoiding hazardous and toxic organic solvents for work up), (iii) applicability to a wide range of substituted aldehydes and (iv) mild temperature reaction condition. Although, the SWCNT-TEAP act as an excellent catalyst for the reaction medium and provides the better product yield, simple reaction conditions, shorter reaction times, easy work up and recyclability making it a green, easy and superior method for the synthesis. Acknowledgments: We gratefully acknowledge the SRC program of the National Research Foundation of Korea (NRF) Grant funded by the Korean Government (MEST) (No. 20100029418) and in part by Kwangwoon University 2014.

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