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Brønsted acidic ionic liquid catalyzed an efficient and eco-friendly protocol for the synthesis of 2,4,5-trisubstituted-1H-imidazoles under solvent-free conditions
⁎Corresponding author. Tel.: +91 0870 2459445; fax: +91 0870 2459547. rajitha_nitw@yahoo.com (Rajitha Bavantula) rajitabhargavi@yahoo.com (Rajitha Bavantula)
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Received: ,
Accepted: ,
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.
Peer review under responsibility of King Saud University.
Abstract
A simple, highly efficient and eco-friendly protocol for the synthesis of 2,4,5-trisubstituted-1H-imidazoles via one-pot three component condensation of benzil/benzoin, aldehydes and ammonium acetate under solvent-free conditions has been achieved utilizing Brønsted acidic ionic liquid, (4-sulfobutyl)tris(4-sulfophenyl) phosphonium hydrogen sulfate as catalyst. The remarkable features of this methodology are excellent yields in shorter reaction times, cleaner reaction profile, and environmentally benign nature, use of non-toxic, readily synthesizable, inexpensive and recyclable catalyst.
Keywords
Eco-friendly
Imidazole
Ionic liquid
Solvent-free
Three component reaction
1 Introduction
Heterocyclic compounds containing imidazole moiety have many pharmacological properties and play an important role in biochemical processes (Breslow, 1995). Highly substituted imidazoles are key intermediates in the synthesis of various therapeutic agents and act as a subunit in drugs such as Olmesartan, Losartan, Eprosartan (angiotensin II receptor antagonist), Metronidazole (antibiotic), Trifenagrel (platelet aggregation inhibitor), Dacarbazine (antineoplastic), Cimetidine (H2-receptor antagonist) (Fig. 1), Methimazole (antithyroid), Pilocarpine (muscarinic receptor agonist), Etomidate (intravenous anesthetic) and also act as plant growth regulators (Freedman and Loscalzo, 2009), fluorescence labeling agents (Kuroda et al., 2000), biological imaging (Sun et al., 2009) and chromophores for non-linear optic systems (Staehelin et al., 1992). These are also found to possess antibacterial, anti-inflammatory (Lombardino and Wiseman, 1974), antihypertensive (Antolini et al., 1999), antithrombotic, anti-viral (Horton et al., 2003), anti-allergic, analgesic, fungicidal (Pozherskii et al., 1997) and herbicidal properties. On the other hand ionic liquid catalyzed reactions have gained considerable attention because of their interesting properties like high thermal stability, non volatility, eco-friendly and reusability (Martyn and Kenneth, 2000) leading to proceed the reaction effectively with high yields in shorter reaction times.
Potent highly substituted imidazole derivatives.
In view of the diverse pharmacological properties of these compounds, many methods have been developed using various catalytic systems such as InF3 (Reddy and Jeong, 2012), InCl3.3H2O (Saikat et al., 2008), BF3.SiO2 (Sadeghi et al., 2008), Zr(acac)4 (Khosropour, 2008), I2 (Mazaahir et al., 2007), TBAB (Chary et al., 2008), CAN (Rajanarendar et al., 2011), DABCO (Murthy et al., 2010), Yb(OTf)3 (Wang et al., 2006), l-Proline (Subhasis et al., 2009), Zirconium(IV)-modified silica gel (Sharma and Sharma, 2011), p-TSA (Mohammad et al., 2007), Wells–Dawson heteropolyacid (Ali et al., 2012), MCM-41-SO3H (Mahdavinia et al., 2012), p-Dodecylbenzenesulfonic acid (Biswanath et al., 2013), Cellulose sulfuric acid (Shelke et al., 2010), Silica-bonded S-sulfonic acid (Niknam et al., 2010), Boric acid (Shelke et al., 2009) and Ammonium metavanadate (Niralwad et al., 2011). Ionic liquid catalyzed reactions were also reported using [EMIM]OAc (Zang et al., 2010), [Et3NH][HSO4] (Deng et al., 2013), [HeMIM]BF4 (Xia and Lu, 2007), [(CH2)4SO3HMIM][HSO4] (Majid et al., 2010), N-Methyl-2-pyrrolidonium hydrogen sulfate (Shaterian and Ranjbar, 2011) and Triphenyl(propyl-3-sulphonyl)phosphonium toluenesulfonate (Shaterian et al., 2011). However, many of these reported methods suffer from one or several drawbacks such as low yields, prolonged reaction times, use of hazardous, expensive, moisture-sensitive, large quantity of reagents, harsh reaction conditions, special apparatus, tedious workup procedure and difficulty in recovery and reusability of the catalysts. Therefore, still there is a need to develop an efficient, eco-friendly and versatile method for the synthesis of imidazole derivatives.
In continuation of our effort to develop Brønsted acidic ionic liquid catalyzed synthetic methodologies (Janardhan and Rajitha, 2012; Janardhan et al., 2012), we report herein, a simple, highly efficient and eco-friendly method for the synthesis of 2,4,5-trisubstituted-1H-imidazoles under solvent-free conditions (Scheme 1) in excellent yields utilizing inexpensive Brønsted acidic ionic liquid, (4-sulfobutyl)tris(4-sulfophenyl)phosphonium hydrogen sulfate [(4-SB)T(4-SPh)PHSO4] as catalyst.
Synthesis of 2,4,5-trisubstituted-1H-imidazoles catalyzed by (4-SB)T(4-SPh)PHSO4.
2 Experimental
All the reagents were purchased from Aldrich/Merck and used without further purification. Melting points were determined in open capillaries using Stuart SMP30 apparatus and are uncorrected. The progress of the reactions as well as purity of compounds was monitored by thin layer chromatography with F254 silica-gel precoated sheets (Merck, Darmstadt, Germany) using hexane/ethyl acetate 8/2 as eluent; UV light was used for detection. Products were characterized by comparing with authentic samples and by spectroscopy data (IR, 1H NMR and 13C NMR). IR spectra were recorded on Perkin–Elmer 100S spectrophotometer using KBr pellet, values are expressed in cm−1. NMR spectra were recorded on Bruker 400 MHz spectrometer using appropriate solvent and TMS as internal standard, chemical shifts are expressed in ppm. Elemental analyses were performed on a Carlo Erba model EA1108 and the values are ±0.4% of the theoretical values. Mass spectra were recorded on a Jeol JMSD-300 spectrometer.
2.1 General procedure for the synthesis of 2,4,5-trisubstituted-1H-imidazoles (4a-t)
To a mixture of benzil/benzoin (1 mmol), aldehyde (1 mmol) and ammonium acetate (3 mmol); (4-SB)T(4-SPh)PHSO4 (0.11 g, 15 mol%) was added and stirred at 120 °C for an appropriate time as indicated in Table 2. After completion of the reaction shown by TLC, 5 mL of water was added and stirred at room temperature for additional 5 min, the solid separated out was filtered washed with water, dried and recrystallized from ethanol to afford the analytically pure product. Aqueous layer containing catalyst was recovered under reduced pressure, washed with acetone, dried and reused for subsequent reactions.
2.2 Spectral data of selected compounds
2.2.1 2-(4-Methoxyphenyl)-4,5-diphenyl-1H-imidazole (4e)
A white solid; IR (KBr) υmax (cm−1): 3449 (NH), 1610 (C⚌N), 1576 (C⚌C), 1285 (C–O–C); 1H NMR (400 MHz, CDCl3): δ 3.83 (s, 3H), 6.98 (d, J = 7.6 Hz, 2H), 7.18-7.68 (m, 10H), 7.82 (d, J = 7.6 Hz, 2H), 9.14 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 160.1, 146.0, 128.5, 127.7, 126.6, 122.7, 114.2, 110.0, 55.3; MS (ESI) m/z: 327 (M + 1); Anal. Calcd. for C22H18N2O: C, 80.96; H, 5.56; N, 8.58; Found: C, 80.71; H, 5.83; N, 8.36.
2.2.2 4-Chloro-2-(4,5-diphenyl-1H-imidazol-2-yl)phenol (4j)
A white solid; IR (KBr) υmax (cm−1): 3429 (NH), 3171 (OH), 1636 (C⚌N), 1601 (C⚌C), 822 (C–Cl); 1H NMR (400 MHz, CDCl3): δ 6.98 (s, 1H), 7.14 (d, J = 8.0 Hz, 1H), 7.24–7.65 (m, 11H), 9.55 (s, 1H), 12.47 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 156.2, 146.4, 134.6, 132.8, 130.0, 128.5, 128.4, 128.1, 125.7, 120.3, 115.2, 112.8; MS (ESI) m/z: 347 (M + 1); Anal. Calcd. for C21H15ClN2O: C, 72.73; H, 4.36; N, 8.08; Found: C, 72.58; H, 4.54; N, 8.22.
2.2.3 4,5-Diphenyl-2-styryl-1H-imidazole (4n)
A white solid; IR (KBr) υmax (cm−1): 3415 (NH), 1645 (C⚌N), 1600 (C⚌C); 1H NMR (400 MHz, DMSO-d6): δ 7.08 (d, J = 16.4 Hz, 2H), 7.28-7.68 (m, 15H), 12.58 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ 146.1, 138.3, 134.2, 130.0, 128.5, 128.4, 128.1, 128.0, 127.8, 125.7, 123.6, 110.9; MS (ESI) m/z: 323 (M + 1); Anal. Calcd. for C23H18N2: C, 85.68; H, 5.63; N, 8.69; Found: C, 85.49; H, 5.76; N, 8.48.
2.2.4 N-(2-(4-(4,5-Diphenyl-1H-imidazol-2yl)phenoxy)ethyl)-N-methylpyridin-2amine (4p)
A white solid; IR (KBr) υmax (cm−1): 3429 (NH), 1620 (C⚌N), 1595 (C⚌C), 1250 (C–O–C); 1H NMR (400 MHz, CDCl3): δ 3.15 (s, 3H), 3.99 (t, J = 5.6 Hz, 2H), 4.21 (t, J = 5.6 Hz, 2H), 6.52-6.57 (m, 2H), 6.94 (d, J = 8.8 Hz, 2H), 7.32-7.63 (m, 11H), 7.79 (d, J = 8.8 Hz, 2H), 8.15-8.17 (m, 1H), 9.40 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 159.3, 158.2, 147.8, 146.1, 137.3, 128.5, 127.7, 126.7, 122.7, 114.7, 111.8, 105.7, 66.3, 49.4, 37.7; MS (ESI) m/z: 447 (M + 1); Anal. Calcd. for C29H26N4O: C, 78.00; H, 5.87; N, 12.55; Found: C, 78.14; H, 5.97; N, 12.26.
2.2.5 3-(4,5-Diphenyl-1H-imidazol-2-yl)-1H-indole (4q)
A white solid; IR (KBr) υmax (cm−1): 3412 (NH), 1621 (C⚌N), 1598 (C⚌C); 1H NMR (400 MHz, DMSO-d6): δ 7.12-7.99 (m, 14H), 8.47 (d, J = 7.6 Hz, 1H), 11.37 (s, 1H), 12.28 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ 143.7, 136.2, 135.9, 135.7, 131.6, 128.6, 128.1, 127.3, 126.8, 126.1, 125.7, 125.0, 123.7, 121.8, 121.4, 119.6, 111.5, 106.8; MS (ESI) m/z: 336 (M + 1); Anal. Calcd. for C23H17N3: C, 82.36; H, 5.11; N, 12.53; Found: C, 82.18; H, 5.31; N, 12.40.
2.2.6 2-Methyl-3-(4,5-diphenyl-1H-imidazol-2-yl)-1H-indole (4r)
A white solid; IR (KBr) υmax (cm−1): 3415 (NH), 1624 (C⚌N), 1588 (C⚌C); 1H NMR (400 MHz, DMSO-d6): δ 2.97 (s, 3H), 7.14-8.03 (m, 13H), 8.48 (d, J = 8.0 Hz, 1H), 11.35 (s, 1H), 12.27 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ 143.7, 138.1, 136.3, 136.0, 135.5, 131.4, 128.6, 128.1, 126.9, 125.8, 125.5, 124.9, 123.7, 121.7, 121.3, 119.6, 111.2, 104.5, 22.4; MS (ESI) m/z: 350 (M + 1); Anal. Calcd. for C24H19N3: C, 82.49; H, 5.48; N, 12.03; Found: C, 82.30; H, 5.54; N, 12.12.
2.2.7 2-Chloro-3-(4,5-diphenyl-1H-imidazol-2-yl)quinoline (4s)
A white solid; IR (KBr) υmax (cm−1): 3435 (NH), 1602 (C⚌N), 1581 (C⚌C), 746 (C–Cl); 1H NMR (400 MHz, CDCl3): δ 7.29-7.79 (m, 12H), 7.95-8.05 (m, 2H), 9.27 (s, 1H), 10.39 (s, 1H); 13C NMR (100 MHz, CDCl3): δ 146.8, 145.1, 141.1, 139.1, 138.7, 134.2, 131.1, 130.4, 129.0, 128.4, 128.2, 128.1, 127.8, 127.7, 127.3, 127.2, 122.5; MS (ESI) m/z: 382 (M + 1); Anal. Calcd. for C24H16ClN3: C, 75.49; H, 4.22; N, 11.00; Found: C, 75.32; H, 4.36; N, 11.24.
2.2.8 1,3-Diphenyl-4-(4,5-diphenyl-1H-imidazol-2-yl)-1H-pyrazole (4t)
A white solid; IR (KBr) υmax (cm−1): 3428 (NH), 1621 (C⚌N), 1595 (C⚌C); 1H NMR (400 MHz, DMSO-d6): δ 7.20-7.60 (m, 16H), 7.95 (d, J = 8.0 Hz, 2H), 8.09 (d, J = 7.2 Hz, 2H), 8.99 (s, 1H), 12.52 (s, 1H); 13C NMR (100 MHz, DMSO-d6): δ 149.9, 139.4, 139.1, 136.3, 135.2, 132.5, 131.1, 129.7, 129.4, 128.6, 128.2, 128.0, 127.9, 127.6, 127.2, 126.9, 126.7, 126.4, 118.3, 112.8; MS (ESI) m/z: 439 (M + 1); Anal. Calcd. for C30H22N4: C, 82.17; H, 5.06; N, 12.78; Found: C, 82.05; H, 5.23; N, 12.97.
3 Results and discussions
2,4,5-Trisubstituted-1H-imidazoles (4a-t) were synthesized by the condensation of benzil or benzoin with various aldehydes and ammonium acetate under neat conditions at 120 °C using Brønsted acidic ionic liquid, [(4-SB)T(4-SPh)PHSO4] as catalyst (Scheme 1). This ionic liquid has been prepared according to the literature procedure (Hamid et al., 2011) from the readily available starting materials as shown in Scheme 2.
Synthesis of (4-sulfobutyl)tris(4-sulfophenyl)phosphonium hydrogen sulfate.
To optimize the reaction conditions in terms of temperature and amount of catalyst, a modal reaction was performed by the condensation of benzil, 4-chloro benzaldehyde and ammonium acetate under solvent-free conditions at different temperatures with variant amounts (5, 10, 15, 20 mol%) of catalyst, and the observations are as follows: At room temperature with 5 mol% of catalyst formation of the product (4b) was not observed even after 150 min, as the temperature increased to 120 °C the yield has enormously increased to 68%, further increment of temperature to 150 °C has not shown any affect on product (4b) yield and reaction time. To improve the yield of the product (4b) the above reaction was tested with 10, 15 and 20 mol% at 120 °C and observed the maximum yields (98%) in shorter reaction times (10 min) with 15 mol% of catalyst. Same reaction was also carried out with benzoin and observed 96% of product (4b) yield in 15 min (Table 1). At these optimistic conditions (15 mol% of catalyst at 120 °C under solvent-free conditions) we performed a reaction using different aldehydes and the results were postulated in Table 2.
| Entrya | Product | With benzil | With benzoin | Melting points (°C) | |||
|---|---|---|---|---|---|---|---|
| Time (min) | Yieldb (%) | Time (min) | Yieldc (%) | Found | Lit. (Ref.) | ||
| 1 |
 (4a) |
15 | 94 | 25 | 91 | 270–272 | 274–276 (Ali et al., 2012) |
| 2 |
 (4b) |
15 | 94 | 20 | 91 | 194–196 | 190–192 (Majid et al., 2010) |
| 3 |
 (4c) |
10 | 98 | 25 | 96 | 260–262 | 256–260 (Ali et al., 2012) |
| 4 |
 (4d) |
10 | 92 | 20 | 90 | 230–231 | 228–229 (Ali et al., 2012) |
| 5 |
 (4e) |
10 | 94 | 25 | 93 | 228 | 227–229 (Ali et al., 2012) |
| 6 |
 (4f) |
15 | 93 | 25 | 91 | 216–218 | 217–220 (Shaterian and Ranjbar, 2011) |
| 7 |
 (4g) |
20 | 91 | 30 | 90 | 205–206 | 204–207 (Shaterian and Ranjbar, 2011) |
| 8 |
 (4h) |
15 | 92 | 25 | 90 | 234–236 | 233 (Shaterian and Ranjbar, 2011) |
| 9 |
 (4i) |
10 | 91 | 20 | 89 | 198–200 | 199 (Shaterian and Ranjbar, 2011) |
| 10 |
 (4j) |
15 | 90 | 20 | 89 | >330 | – |
| 11 |
 (4k) |
20 | 92 | 25 | 90 | 232–234 | 230–231 (Subhasis et al., 2009) |
| 12 |
 (4l) |
15 | 94 | 20 | 92 | 318–320 | 312–314 (Ali et al., 2012) |
| 13 |
 (4m) |
15 | 92 | 20 | 89 | 256–257 | 256–258 (Shaterian and Ranjbar, 2011) |
| 14 |
 (4n) |
10 | 92 | 20 | 90 | 70–72 | – |
| 15 |
 (4o) |
15 | 90 | 30 | 87 | 200 | 200–201 (Shaterian and Ranjbar, 2011) |
| 16 |
 (4p) |
10 | 98 | 20 | 93 | 190 | – |
| 17 |
 (4q) |
10 | 97 | 20 | 92 | 308–310 | 302–305 (Niknam et al., 2010) |
| 18 |
 (4r) |
15 | 95 | 25 | 91 | 281–283 | – |
| 19 |
 (4s) |
10 | 96 | 25 | 90 | 219–221 | – |
| 20 |
 (4t) |
10 | 98 | 25 | 92 | 260 | – |
The efficiency of [(4-SB)T(4-SPh)PHSO4] has been compared with those of reported acid catalysts in the synthesis of 2-(4-chlorophenyl)-4,5-diphenyl-1H-imidazole (4b) and the results have shown that, the (4-SB)T(4-SPh)PHSO4 has proved to be efficient in terms of product yields and reaction times (Table 3). After completion of the reaction shown by TLC, the catalyst was recovered under reduced pressure, washed with acetone, dried and reused for subsequent reactions for additional six cycles, and observed a slight decrease in its activity in terms of product yield (Table 4). All the synthesized compounds were confirmed by their analytical and spectroscopic data (IR, 1H NMR and 13C NMR) and the known compounds were compared with the authentic samples.
| Entry | Catalyst (amount in grams) | Reaction conditions | With benzil | With benzoin | References | ||
|---|---|---|---|---|---|---|---|
| Time (min) | Yielda (%) | Time (min) | Yieldb (%) | ||||
| 1 | H3BO3 (0.003 g) | Ultrasonication, EtOH:H2O (1:1) | 30 | 98 | 55 | 96 | Shelke et al. (2009) |
| 2 | CSAc (0.1 g) | MWI/180 W, solvent- free | 1.5 | 98 | 2 | 95 | Shelke et al. (2010) |
| 3 | MCM-41-SO3H (0.04 g) | Solvent-free, 100 °C | 10 | 92 | – | – | Mahdavinia et al. (2012) |
| 4 | SBSSAd (0.002 g) | Solvent-free, 130 oC | 30 | 90 | – | – | Niknam et al. (2010) |
| 5 | [Et3NH][HSO4] (0.0199 g) | Solvent-free, 130 °C | 20 | 93 | – | – | Deng et al. (2013) |
| 6 | [SABMIM]HSO4 e (0.016 g) | Solvent-free, 120 °C | 5 | 88 | – | – | Majid et al. (2010) |
| 7 | DBSAf (0.065 g) | H2O , Reflux | 240 | 75 | – | – | Biswanath et al. (2013) |
| 8 | TP(P-3-S)PTSg (0.09 g) | Solvent-free, 100 °C | 60 | 95 | 90 | 97 | Shaterian et al. (2011) |
| 9 | N-M-2-PHSO4h (0.08 g) | Solvent-free, 100 °C | 90 | 97 | 120 | 95 | Shaterian and Ranjbar (2011) |
| 10 | p-TSA (0.038 g) | Solvent-free, 145 °C | 120 | 80 | 180 | 80 | Mohammad et al. (2007) |
| 11 | WD/SiO2 i (0.3 g) | Solvent-free, 140 °C | 120 | 90 | – | – | Ali et al. (2012) |
| 12 | (4-SB)T(4-SPh)PHSO4 j (0.11 g) | Solvent-free, 120 °C | 10 | 98 | 15 | 96 | Present work |
Note: For comparison mole percentages has converted into grams.
A plausible mechanism for the formation of 2,4,5-trisubstituted-1H-imidazoles catalyzed by (4-SB)T(4-SPh)PHSO4 is shown in Scheme 3. Here the carbonyl oxygen of benzil/benzoin was activated with the acid part of the catalyst and reacted with the intermediate substituted methanediamine [A], undergoes dehydration followed by the rearrangement and elimination of proton gives 2,4,5-trisubstituted-1H-imidazoles.
Plausible mechanism for the synthesis of imidazoles catalyzed by (4-SB)T(4-SPh)PHSO4.
4 Conclusion
In conclusion, we have developed a simple, highly efficient and environmentally benign protocol for the synthesis of 2,4,5-trisubstituted-1H-imidazoles by one-pot three component condensation of benzil/benzoin, aldehydes, and ammonium acetate under solvent-free conditions utilizing inexpensive and eco-friendly Brønsted acidic ionic liquid, (4-sulfobutyl)tris(4-sulfophenyl) phosphonium hydrogen sulfate as catalyst. This methodology has advantages of higher yields in shorter reaction times, reusability of the catalyst over seven runs without much losing its activity, simple experimental and work-up procedures. We believe that, this method will be more useful than the existing literature methods for the synthesis of 2,4,5-triaryl-1H-imidazoles.
Acknowledgements
The author J.B. thanks The Ministry of Human Resource Development, R.G. and R.V. thank UGC for the research fellowships.
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