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Preparation, characterization and application of novel ionic liquid as an efficient and reusable catalyst for the solvent-free synthesis of hexahydroquinolines
Journal of Molecular Liquids 209 (2015) 224–232
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Journal of Molecular Liquids journal homepage: www.elsevier.com/locate/molliq
Preparation, characterization and application of novel ionic liquid as an efficient and reusable catalyst for the solvent-free synthesis of hexahydroquinolines Mahdi Ghorbani ⁎, Soheila Noura, Mohsen Oftadeh, Mahdi Narimani Chemistry Department, Payame Noor University, 19395-4697 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 30 May 2015 Accepted 3 June 2015 Available online xxxx Keywords: Brønsted acidic ionic liquid 1-Butyl-3-sulfonic acid imidazolium chloride [Bsim]Cl Solvent-free Catalyst
a b s t r a c t A novel Brønsted acidic ionic liquid 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, was synthesized. Its structure was investigated using FT-IR, 1H NMR, 13C NMR, mass, UV, TGA and DTA spectra. This ionic liquid, with one acidic functional group, is utilized as a highly efficient and homogeneous catalyst for the promotion of hexahydroquinolines via one-pot multi-component condensation of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free conditions. This new method consistently has the advantages of excellent yields and short reaction times. Further, the catalyst could be reused and recovered at least four times without appreciable loss of activity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids, IL, (based imidazolium or other organic cations) have received considerable interest as eco-friendly solvents, catalysts and reagents in green synthesis because of their unique properties, such as low volatility, nonflammability, high thermal stability, negligible vapor pressure and ability to dissolve a wide range of materials [1–4]. Recently ionic liquids have been successfully employed as dual reagents (solvents + catalysts) for a variety of the reactions, but their use as catalyst under solvent-free conditions needs to be given more attention [5]. Among them, Brønsted acidic ionic liquids have been designed in order to replace solid acids and traditional mineral liquid acids like sulfuric acid and hydrochloric acid in chemical procedures [6]. A one-pot process is a promising plan of the novel organic synthesis in which a sequence of reactions without performing isolating intermediatesis. Thus proceeding with studies on the synthesis of the compounds by the one-pot multi-component reactions (MCRs) have been of ongoing interest, since MCRs preferably are facile, fast, and efficient with a minimal workup [7–9]. IL-MCR causes the rapid synthesis of highly functionalized heterocyclic molecules with high potential applications in
⁎ Corresponding author. E-mail address: [email protected] (M. Ghorbani).
http://dx.doi.org/10.1016/j.molliq.2015.06.011 0167-7322/© 2015 Elsevier B.V. All rights reserved.
medicinal chemistry [10]. In 1882, Arthur Hantzsch reported first synthesis of symmetrically substituted 1,4-dihydropyridines by the one-pot, four component condensation of two molecules of ethyl acetoacetate, aromatic aldehyde and ammonia [11]. Hantzsch 1,4dihydropyridines (1,4-DHPs) form a class of heterocyclic compounds which represent interesting pharmacological and biological properties [12]. To realize the importance of polyhydroquinoline derivatives in the synthesis of various drug sources, many characteristic methods were reported. They include conventional heating [13,14], L-proline [15], various catalysts such as ammonium nitrate (CAN) [16], silica perchloric acid (HClO4–SiO2) [17], trimethylsilyl chloride [18], nickel nanoparticle [19], FeF3 [20], K7[PW11CoO40] [21], p-TSA [22], solar heat [23], hafnium (IV) [24], SBA-Pr-SO3H [25], Bakers' yeast [26], and
Scheme 1. The synthesis of 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl.
M. Ghorbani et al. / Journal of Molecular Liquids 209 (2015) 224–232
Scheme 2. The one-pot multi-component preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl.
Fig. 1. FT-IR spectra of 1-butyl imidazole (top), 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, (middle) and recycle [Bsim]Cl, after four runs.
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iron (III) trifluoroacetate [27]. The above mentioned subjects encouraged us to synthesize a new ionic liquid with Brønsted acidic property, named as 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, for the first time (Scheme 1), and its full characterization was investigated by using FT-IR, 1H NMR, 13C NMR, UV as well as mass spectra, thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). Our observations showed that this reagent can be used as an efficient catalyst in the Hantzsch reaction. The advantages of the method are easy, high-yield products and short reaction times (Scheme 2). 2. Experimental 2.1. Materials All chemicals were purchased from Merck or Fluka chemical companies. All yields refer to the isolated products. Products were characterized by their physical constants and compared with authentic samples. The purity of substrates and the reaction monitoring were accompanied by TLC using silica gel SIL G/UV 254 plates. 2.2. Instrumentation Melting points were measured using a Galen Kamp melting point apparatus and are uncorrected. 1H NMR spectra were recorded by Bruker Avance 400.22 using DMSO-d6 as solvent and TMS as internal standard. FT-IR spectra were recorded with PerkinElmer Spectrum Version 10.00.00. Mass Spectra were recorded on a Shimadzu Gas Chromatograph Mass Spectrometer GCMS-QP5050A/QP5000 apparatus. TG and DTA data were obtained by a PerkinElmer pyris diamond TG-DTA. UV–vis spectra were recorded by a Shimadzu UV-mini-1240 V spectrophotometer with 1 cm quartz cells (0.5 ml). 2.3. Procedure for the preparation of ionic liquid [Bsim]Cl To a round-bottomed flask (100 mL) containing 1-butyl imidazole (1.2418 g, 10 mmol) in dry CH2Cl2 (50 mL), was added to chlorosulfonic acid (1.1768 g, 10.1 mmol) dropwise over a period of 20 min at room temperature. Then the reaction mixture was stirred for 2 h, let it stand for 5 min, and the CH2Cl2 was dried under vacuum to give [Bsim]Cl a viscous pale yellow oil in 96% yield, 2.31 g. 2.4. General procedure for the synthesis of hexahydroquinolines under solvent-free conditions The mixture of the aldehydes (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol), ammonium acetate (15 mmol) and [Bsim]Cl containing 10 mol% of acidic ionic liquid catalyst was stirred at 60 °C for specific time. After completion of the reaction (monitored by TLC), it was cooled to room temperature. Then, 3 mL of water was added to the mixture. The ionic liquid was dissolved in water and filtered for separation of the crude product. The separated product was washed twice with water (2 × 3 mL). For recycling the catalysts, after washing the solid products with water completely, the water containing the ionic liquid (IL is soluble in water) was evaporated under reduced pressure and the ionic liquid was recovered and reused. The solid product was purified by recrystallization procedure in ethanol. 3. Results and discussion 3.1. Characterization of the catalyst Fig. 2. 1H NMR and 13C NMR of 1-butyl-3-sulfonic acid imidazolium chloride [Bsim]Cl.
3.1.1. FT-IR studies FT-IR (KBr, cm−1) υmax: 764, 853, 1016, 1173, 1463, 1547, 1579, 1651, 2550–3500 cm−1. The corresponding FT-IR spectral data of [Bsim]Cl are presented in Fig. 1. The strong absorptions at 1173.76 cm−1 in [Bsim]Cl
ionic liquid assign to the stretching and bending for S–O bond of sulfonic acid, which is absent in 1-butyl imidazole. The N–S stretching vibration also appears at 1016.64 cm−1. These special IR peaks indicate that the
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Fig. 3. The TG/DTG diagrams of [Bsim]Cl.
sulfonic group is successfully introduced in the 1-butyl imidazole molecule. On the other hand, the presence of the sulfonic acid group on the nitrogen of 1-butyl imidazole in the [Bsim]Cl ionic liquid increases the number of vibrational modes and brought completely different FT-IR spectra. In addition, C_N and C_C vibrations are observed at 1579.29 and 1547.64 cm−1, respectively. The broad and strong bands at 2600–3500 cm−1 can be arising from the stretching of the hydroxyl group in the ionic liquid catalyst. 3.1.2. 1H NMR and 13C NMR studies 1 H and 13C NMR spectra of [Bsim]Cl ionic liquid have been presented in Fig. 2. The important peak of 1H NMR spectra of [Bsim]Cl is related to the acidic hydrogen (SO3H) which observed in 14.33 ppm. The peak (14.33 ppm) is really related to the hydrogen of SO3H in the compound, not hydrogen of unreacted starting material ClSO3H (ClSO3H:1H NMR (DMSO-d6, 300 MHz, ppm): δ = 13.45 (s, 1H)) [28]. The difference between the peaks of the acidic hydrogens in [Bsim]Cl and ClSO3H [28]
Fig. 4. Molecular self-assembly of [Bsim]Cl via hydrogen bonding.
confirmed the peak observed in 14.33 ppm of the 1H NMR spectra of [Bsim]Cl is correctly related to the SO3H group of this compound. The structure of [Bsim]Cl confident this reagent can act as an efficient catalyst in the reactions need acidic reagents to speed up. Our investigations clarified that it can act as a promotion of the synthesis of hexahydroquinolines (Scheme 2). Spectral data of [Bsim]Cl viscous pale yellow oil; 1HNMR (400.22 MHz, DMSO-d6): δ (ppm) 0.889 (t, J = 7.2, 3H, CH3), 1.193– 1.285 (m, 2H, CH2), 1.743–1.817 (m, 2H, CH2), 4.218 (t, J = 7.2, 2H, CH2), 7.698 (s, 1H), 7.818 (s, 1H), 9.166 (s, 1H), 14.335 (s, 1H); 13C NMR (100.64 MHz, DMSO-d6): (ppm) 13.75, 19.27, 31.94, 48.67, 120.40, 122.50, 135.69. 3.1.3. MS studies The important peaks of MS spectrum of the ionic liquid relate to 241 (M + + 1), 240 (M + ), 183 (M + − But), 205 (M + − Cl), 124 (M+ − ClSO3H), 67 (M+ − ButClSO3).
Fig. 5. The structure of [Bsim]Cl.
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273
273
a
0.25
b
1.4 1.2
0.2 1 0.15
Abs.
0.8
Abs. 0.6
0.1
0.4 0.05 0.2 0
0 250
300
250
350
350
300
nm
nm
Fig. 6. UV spectra of a) 1-butylimidazole and b) [BSim]Cl at room temperature in DMSO (the concentration of these compounds in DMSO was 0.005 mol L−1).
3.1.4. Study on thermal gravimetric analysis (TGA) of [Bsim]Cl The corresponding diagrams have been shown in Fig. 3. As it can be seen in Fig. 3, TGA and differential thermal analysis (DTA) of the catalysts showed weight loss in one step; [Bsim]Cl was decomposed above 348.1 °C in one step. It confirms that the catalyst is one-system, not binary systems. In binary systems, the thermogravimetry (TG) diagrams show weight losses in two or more steps. Moreover, the TGA and DTA diagrams of [Bsim]Cl are different. Therefore, the molecular decomposition of the ionic liquid occurs above 348.1 °C. Molecular self-assembly including intramolecular and intermolecular, is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. Most often the term molecular self-assembly refers to intermolecular self-assembly, and assembly of molecules is directed through non-covalent interactions (such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π–π interactions and electrostatic) as well as electromagnetic interactions [29]. The specific structure of [Bsim]Cl as a functional Brønsted acidic ionic liquid with hydrogen-bond donor, enables to produce a molecular self-assembly through hydrogen bonds (Fig. 4). On the basis of the structure of [Bsim]Cl, it can act as an efficient catalyst in reactions in which need the use of acidic catalysts to accelerate the rate of reaction. These functional groups enable to produce a molecular self-assembly through intermolecular hydrogen bonds (Fig. 5) and as a result, go up its thermal stability.
carried out with no catalyst at 60 °C, the product yield was only 19%. Using [Bsim]Cl as homogenous catalyst the yield increases to 92% in 8 min at 60 °C. However, [Bsim]Cl catalyzed the reaction very well due to its strong acidity (–SO3H group). When 1-butyl imidazole was used as catalyst, no significant amount of product was produced. After sulfonation of 1-butyl imidazole for the synthesis of acidic ionic liquid, the product yield was considerably increased with containing of [Bsim]Cl (–SO3H group). This results show this reaction needs an acidic catalyst for completion and 1-butyl imidazole cannot catalyze this reaction. Further investigation shows that 10 mol% of catalyst at 60 °C is an optimum condition and the product yield was excellent. To investigate the generality of the present protocol, various aldehydes were used and converted to hexahydroquinolines under the optimum conditions (Table 2). Considering the reaction mechanism (Scheme 3), it can be proposed that in the first step of the reaction, dimedone is converted to its enol form by using [Bsim]Cl and readily undergoes Knoevenagel condensation with benzaldehyde to generate 2-benzylidenedimedone (1). On the other hand, the activated β-ketoester (by the catalyst) with liberation of ammonia from NH4OAc gives enamine (2). Afterwards, the 2benzylidenedimedone (1) and enamine (2) react together by Michael addition to afford intermediate (3). The intermediate (3) is converted to intermediate (4) by tautomerization, and the intermediate (4) affords intermediate (5) by intramolecular nucleophilic attack of the NH2 group
3.1.5. UV spectra UV spectra were another evidence to confirm that [BSim]Cl was really synthesized. For this purpose, UV–vis spectra of 1-butyl imidazole and [BSim]Cl were recorded. At first, some solutions of the mentioned compounds in DMSO with the same concentration were prepared. The concentration of these compounds in DMSO for UV study was 0.005 mol L− 1. The maximum absorptions of 1-butyl imidazole and [BSim]Cl appeared at 273 nm, 0.2286 and 1.2678, respectively. The difference between maximum absorptions of 1-butyl imidazole and [BSim]Cl confirmed the production of the catalyst (Fig. 6).
Table 1 Optimization conditions for the one-pot synthesis of ethyl 4-(4-methoxyphenyl)-2,7,7trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3 carboxylate in the presence of [Bsim] Cl under solvent-free conditions.
3.1.6. Catalyst activity The activity of catalyst was tested in the synthesis of hexahydroquinolines by multi-component reaction during the reaction of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free conditions for the first time (Scheme 2). The control experiment and the optimization of the reaction condition were examined by choosing the reaction of 4-methoxybenzaldehyde (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol) as model reaction. As shown in Table 1, if the reaction is
Entry
Catalyst (mol%)
Temp (°C)
Time (min)
Yielda (%)
1 2 3 4 5 6 7 8 9 10 10
– 3 5 7 10 12 10 10 10 10 10
60 60 60 60 60 60 25 40 50 60 70
90 49 26 15 8 4 60 42 19 8 5
19 79 84 89 92 93 63 78 84 92 92
a Yields refer to the pure isolated products based on the reaction of 4-methoxybenzaldehyde (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol).
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Table 2 Synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl. Entry
Aldehydes
Product
Time (min)
Yielda (%)
M.P./M.P. [ref] (°C)
1
6
91
201–203/203–205 [30]
2
5
90
232–234/233–235 [31]
3
5
90
205–207/207–209 [30]
4
4
93
242–244/242–244 [31]
5
5
91
171–173/172–174 [30]
6
8
92
258–259/257–259 [30]
(continued on next page)
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Table 2 (continued) Entry
Aldehydes
Product
Time (min)
Yielda (%)
M.P./M.P. [ref] (°C)
7
6
94
261–263/262–264 [30]
8
7
93
228–229/228–230 [30]
9
6
90
261–263/262–263 [26]
10
10
89
184–186/184–186 [30]
to the activated carbonyl group. The mechanism is confirmed by the literatures [17,32,33]. 3.1.7. Catalyst recycling An important aspect of catalysts in large scale production is recyclability and reusability of applied catalysts. In order to examine the recyclability of [Bsim]Cl, the reaction of 4-methoxybenzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate was chosen (Fig. 7). For recycling experiment, the reaction was carried out in the same condition. At the end of the reaction, it was cooled to room temperature. Then, 3 mL of water was added to the mixture. The ionic liquid was dissolved in water, and filtered for separation of the crude product. The separated product was washed twice with water (2 × 3 mL). For catalyst recycling, after washing the solid products with water completely, the water containing the
ionic liquid (IL is soluble in water) was evaporated under reduced pressure and the ionic liquid was recovered and reused. The solid product was purified by recrystallization procedure in ethanol. All of the desired product(s) were characterized by comparison of their physical and spectral data (melting points, IR, 1H NMR) with those of known compounds [26,30,31,33].
3.1.8. Comparison of the synthesized catalyst with literatures In order to show the accessibility of the present work, we summarized the described results of acidic IL besides the other reported acidic catalysis especially acidic ionic liquids in Table 3. The results show after sulfonation, imidazolium ring with a longer chain derivative (–butyl) will have a smooth decrease in acidic activity rather some of acidic ionic liquids, however our IL is superior and the most efficient catalyst with respect
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Scheme 3. Plausible mechanism for the catalytic synthesis of ethyl 4-(phenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl.
to the reaction time, low amount of the catalyst, low temperature, and exhibits broad applicability in terms of the obtained yields. 4. Conclusions In this research, Brønsted acidic ionic liquid [Bsim]Cl was introduced as a novel, highly efficient, general and homogeneous catalyst for the one-pot multi-component reaction between various aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate leading to hexahydroquinolines. The synthesized catalyst showed an excellent
activity in the synthesis of various hexahydroquinoline in a green way. Also, the catalyst was recycled four times without the significant loss of activity. In the green chemistry point of view, the present protocol could find industrial application.
Acknowledgments The authors gratefully acknowledge partial support of this work by the University of Payame Noor (Grant No. 48910).
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Fig. 7. The investigation of the reusability of the ionic liquid under solvent-free conditions using a model reaction of 4-methoxybenzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate in the presence of reused [Bsim]Cl as catalyst.
Table 3 Comparison of the results of 1-butyl-3-sulfonic acid imidazolium chloride [Bsim] Cl with other catalysts reported in the literature in the synthesis of ethyl 4-(4-methoxyphenyl)2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate. Entry
Catalyst
Conditions
Time (min)
Yield (%) [ref]
1 2 3 4 5 6 7 8 9 10 11
[Bsim]Cl TiO2NPS Hf(NPf2)4 PEG-400 FePO4 TFE FeF3 [DMBSI]HSO4 [HMIM]BF4 [Dsim]HSO4 [pyridine-SO3H]Cl
Solvent-free, 60 °C Ethanol, under reflux In C10H18, 60 °C Solvent-free, 75 °C Ethanol, under reflux 70 °C Ethanol, 75–80 °C Solvent-free, 60 °C Solvent-free, 90 °C Solvent-free, 50 °C Solvent-free, 50 °C
8 30 240 180 30 180 60 12 10 25 12
92 (the present work) 93 [30] 89 [24] 93 [35] 84 [36] 98 [34] 95 [31] 80 [37] 95 [38] 96 [39] 91 [40]
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Preparation, characterization and application of novel ionic liquid as an efficient and reusable catalyst for the solvent-free synthesis of hexahydroquinolines Mahdi Ghorbani ⁎, Soheila Noura, Mohsen Oftadeh, Mahdi Narimani Chemistry Department, Payame Noor University, 19395-4697 Tehran, Iran
a r t i c l e
i n f o
Article history: Received 4 May 2015 Received in revised form 30 May 2015 Accepted 3 June 2015 Available online xxxx Keywords: Brønsted acidic ionic liquid 1-Butyl-3-sulfonic acid imidazolium chloride [Bsim]Cl Solvent-free Catalyst
a b s t r a c t A novel Brønsted acidic ionic liquid 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, was synthesized. Its structure was investigated using FT-IR, 1H NMR, 13C NMR, mass, UV, TGA and DTA spectra. This ionic liquid, with one acidic functional group, is utilized as a highly efficient and homogeneous catalyst for the promotion of hexahydroquinolines via one-pot multi-component condensation of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free conditions. This new method consistently has the advantages of excellent yields and short reaction times. Further, the catalyst could be reused and recovered at least four times without appreciable loss of activity. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Ionic liquids, IL, (based imidazolium or other organic cations) have received considerable interest as eco-friendly solvents, catalysts and reagents in green synthesis because of their unique properties, such as low volatility, nonflammability, high thermal stability, negligible vapor pressure and ability to dissolve a wide range of materials [1–4]. Recently ionic liquids have been successfully employed as dual reagents (solvents + catalysts) for a variety of the reactions, but their use as catalyst under solvent-free conditions needs to be given more attention [5]. Among them, Brønsted acidic ionic liquids have been designed in order to replace solid acids and traditional mineral liquid acids like sulfuric acid and hydrochloric acid in chemical procedures [6]. A one-pot process is a promising plan of the novel organic synthesis in which a sequence of reactions without performing isolating intermediatesis. Thus proceeding with studies on the synthesis of the compounds by the one-pot multi-component reactions (MCRs) have been of ongoing interest, since MCRs preferably are facile, fast, and efficient with a minimal workup [7–9]. IL-MCR causes the rapid synthesis of highly functionalized heterocyclic molecules with high potential applications in
⁎ Corresponding author. E-mail address: [email protected] (M. Ghorbani).
http://dx.doi.org/10.1016/j.molliq.2015.06.011 0167-7322/© 2015 Elsevier B.V. All rights reserved.
medicinal chemistry [10]. In 1882, Arthur Hantzsch reported first synthesis of symmetrically substituted 1,4-dihydropyridines by the one-pot, four component condensation of two molecules of ethyl acetoacetate, aromatic aldehyde and ammonia [11]. Hantzsch 1,4dihydropyridines (1,4-DHPs) form a class of heterocyclic compounds which represent interesting pharmacological and biological properties [12]. To realize the importance of polyhydroquinoline derivatives in the synthesis of various drug sources, many characteristic methods were reported. They include conventional heating [13,14], L-proline [15], various catalysts such as ammonium nitrate (CAN) [16], silica perchloric acid (HClO4–SiO2) [17], trimethylsilyl chloride [18], nickel nanoparticle [19], FeF3 [20], K7[PW11CoO40] [21], p-TSA [22], solar heat [23], hafnium (IV) [24], SBA-Pr-SO3H [25], Bakers' yeast [26], and
Scheme 1. The synthesis of 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl.
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Scheme 2. The one-pot multi-component preparation of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl.
Fig. 1. FT-IR spectra of 1-butyl imidazole (top), 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, (middle) and recycle [Bsim]Cl, after four runs.
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iron (III) trifluoroacetate [27]. The above mentioned subjects encouraged us to synthesize a new ionic liquid with Brønsted acidic property, named as 1-butyl-3-sulfonic acid imidazolium chloride, [Bsim]Cl, for the first time (Scheme 1), and its full characterization was investigated by using FT-IR, 1H NMR, 13C NMR, UV as well as mass spectra, thermal gravimetric analysis (TGA) and differential thermal analysis (DTA). Our observations showed that this reagent can be used as an efficient catalyst in the Hantzsch reaction. The advantages of the method are easy, high-yield products and short reaction times (Scheme 2). 2. Experimental 2.1. Materials All chemicals were purchased from Merck or Fluka chemical companies. All yields refer to the isolated products. Products were characterized by their physical constants and compared with authentic samples. The purity of substrates and the reaction monitoring were accompanied by TLC using silica gel SIL G/UV 254 plates. 2.2. Instrumentation Melting points were measured using a Galen Kamp melting point apparatus and are uncorrected. 1H NMR spectra were recorded by Bruker Avance 400.22 using DMSO-d6 as solvent and TMS as internal standard. FT-IR spectra were recorded with PerkinElmer Spectrum Version 10.00.00. Mass Spectra were recorded on a Shimadzu Gas Chromatograph Mass Spectrometer GCMS-QP5050A/QP5000 apparatus. TG and DTA data were obtained by a PerkinElmer pyris diamond TG-DTA. UV–vis spectra were recorded by a Shimadzu UV-mini-1240 V spectrophotometer with 1 cm quartz cells (0.5 ml). 2.3. Procedure for the preparation of ionic liquid [Bsim]Cl To a round-bottomed flask (100 mL) containing 1-butyl imidazole (1.2418 g, 10 mmol) in dry CH2Cl2 (50 mL), was added to chlorosulfonic acid (1.1768 g, 10.1 mmol) dropwise over a period of 20 min at room temperature. Then the reaction mixture was stirred for 2 h, let it stand for 5 min, and the CH2Cl2 was dried under vacuum to give [Bsim]Cl a viscous pale yellow oil in 96% yield, 2.31 g. 2.4. General procedure for the synthesis of hexahydroquinolines under solvent-free conditions The mixture of the aldehydes (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol), ammonium acetate (15 mmol) and [Bsim]Cl containing 10 mol% of acidic ionic liquid catalyst was stirred at 60 °C for specific time. After completion of the reaction (monitored by TLC), it was cooled to room temperature. Then, 3 mL of water was added to the mixture. The ionic liquid was dissolved in water and filtered for separation of the crude product. The separated product was washed twice with water (2 × 3 mL). For recycling the catalysts, after washing the solid products with water completely, the water containing the ionic liquid (IL is soluble in water) was evaporated under reduced pressure and the ionic liquid was recovered and reused. The solid product was purified by recrystallization procedure in ethanol. 3. Results and discussion 3.1. Characterization of the catalyst Fig. 2. 1H NMR and 13C NMR of 1-butyl-3-sulfonic acid imidazolium chloride [Bsim]Cl.
3.1.1. FT-IR studies FT-IR (KBr, cm−1) υmax: 764, 853, 1016, 1173, 1463, 1547, 1579, 1651, 2550–3500 cm−1. The corresponding FT-IR spectral data of [Bsim]Cl are presented in Fig. 1. The strong absorptions at 1173.76 cm−1 in [Bsim]Cl
ionic liquid assign to the stretching and bending for S–O bond of sulfonic acid, which is absent in 1-butyl imidazole. The N–S stretching vibration also appears at 1016.64 cm−1. These special IR peaks indicate that the
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Fig. 3. The TG/DTG diagrams of [Bsim]Cl.
sulfonic group is successfully introduced in the 1-butyl imidazole molecule. On the other hand, the presence of the sulfonic acid group on the nitrogen of 1-butyl imidazole in the [Bsim]Cl ionic liquid increases the number of vibrational modes and brought completely different FT-IR spectra. In addition, C_N and C_C vibrations are observed at 1579.29 and 1547.64 cm−1, respectively. The broad and strong bands at 2600–3500 cm−1 can be arising from the stretching of the hydroxyl group in the ionic liquid catalyst. 3.1.2. 1H NMR and 13C NMR studies 1 H and 13C NMR spectra of [Bsim]Cl ionic liquid have been presented in Fig. 2. The important peak of 1H NMR spectra of [Bsim]Cl is related to the acidic hydrogen (SO3H) which observed in 14.33 ppm. The peak (14.33 ppm) is really related to the hydrogen of SO3H in the compound, not hydrogen of unreacted starting material ClSO3H (ClSO3H:1H NMR (DMSO-d6, 300 MHz, ppm): δ = 13.45 (s, 1H)) [28]. The difference between the peaks of the acidic hydrogens in [Bsim]Cl and ClSO3H [28]
Fig. 4. Molecular self-assembly of [Bsim]Cl via hydrogen bonding.
confirmed the peak observed in 14.33 ppm of the 1H NMR spectra of [Bsim]Cl is correctly related to the SO3H group of this compound. The structure of [Bsim]Cl confident this reagent can act as an efficient catalyst in the reactions need acidic reagents to speed up. Our investigations clarified that it can act as a promotion of the synthesis of hexahydroquinolines (Scheme 2). Spectral data of [Bsim]Cl viscous pale yellow oil; 1HNMR (400.22 MHz, DMSO-d6): δ (ppm) 0.889 (t, J = 7.2, 3H, CH3), 1.193– 1.285 (m, 2H, CH2), 1.743–1.817 (m, 2H, CH2), 4.218 (t, J = 7.2, 2H, CH2), 7.698 (s, 1H), 7.818 (s, 1H), 9.166 (s, 1H), 14.335 (s, 1H); 13C NMR (100.64 MHz, DMSO-d6): (ppm) 13.75, 19.27, 31.94, 48.67, 120.40, 122.50, 135.69. 3.1.3. MS studies The important peaks of MS spectrum of the ionic liquid relate to 241 (M + + 1), 240 (M + ), 183 (M + − But), 205 (M + − Cl), 124 (M+ − ClSO3H), 67 (M+ − ButClSO3).
Fig. 5. The structure of [Bsim]Cl.
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273
273
a
0.25
b
1.4 1.2
0.2 1 0.15
Abs.
0.8
Abs. 0.6
0.1
0.4 0.05 0.2 0
0 250
300
250
350
350
300
nm
nm
Fig. 6. UV spectra of a) 1-butylimidazole and b) [BSim]Cl at room temperature in DMSO (the concentration of these compounds in DMSO was 0.005 mol L−1).
3.1.4. Study on thermal gravimetric analysis (TGA) of [Bsim]Cl The corresponding diagrams have been shown in Fig. 3. As it can be seen in Fig. 3, TGA and differential thermal analysis (DTA) of the catalysts showed weight loss in one step; [Bsim]Cl was decomposed above 348.1 °C in one step. It confirms that the catalyst is one-system, not binary systems. In binary systems, the thermogravimetry (TG) diagrams show weight losses in two or more steps. Moreover, the TGA and DTA diagrams of [Bsim]Cl are different. Therefore, the molecular decomposition of the ionic liquid occurs above 348.1 °C. Molecular self-assembly including intramolecular and intermolecular, is the process by which molecules adopt a defined arrangement without guidance or management from an outside source. Most often the term molecular self-assembly refers to intermolecular self-assembly, and assembly of molecules is directed through non-covalent interactions (such as hydrogen bonding, metal coordination, hydrophobic forces, van der Waals forces, π–π interactions and electrostatic) as well as electromagnetic interactions [29]. The specific structure of [Bsim]Cl as a functional Brønsted acidic ionic liquid with hydrogen-bond donor, enables to produce a molecular self-assembly through hydrogen bonds (Fig. 4). On the basis of the structure of [Bsim]Cl, it can act as an efficient catalyst in reactions in which need the use of acidic catalysts to accelerate the rate of reaction. These functional groups enable to produce a molecular self-assembly through intermolecular hydrogen bonds (Fig. 5) and as a result, go up its thermal stability.
carried out with no catalyst at 60 °C, the product yield was only 19%. Using [Bsim]Cl as homogenous catalyst the yield increases to 92% in 8 min at 60 °C. However, [Bsim]Cl catalyzed the reaction very well due to its strong acidity (–SO3H group). When 1-butyl imidazole was used as catalyst, no significant amount of product was produced. After sulfonation of 1-butyl imidazole for the synthesis of acidic ionic liquid, the product yield was considerably increased with containing of [Bsim]Cl (–SO3H group). This results show this reaction needs an acidic catalyst for completion and 1-butyl imidazole cannot catalyze this reaction. Further investigation shows that 10 mol% of catalyst at 60 °C is an optimum condition and the product yield was excellent. To investigate the generality of the present protocol, various aldehydes were used and converted to hexahydroquinolines under the optimum conditions (Table 2). Considering the reaction mechanism (Scheme 3), it can be proposed that in the first step of the reaction, dimedone is converted to its enol form by using [Bsim]Cl and readily undergoes Knoevenagel condensation with benzaldehyde to generate 2-benzylidenedimedone (1). On the other hand, the activated β-ketoester (by the catalyst) with liberation of ammonia from NH4OAc gives enamine (2). Afterwards, the 2benzylidenedimedone (1) and enamine (2) react together by Michael addition to afford intermediate (3). The intermediate (3) is converted to intermediate (4) by tautomerization, and the intermediate (4) affords intermediate (5) by intramolecular nucleophilic attack of the NH2 group
3.1.5. UV spectra UV spectra were another evidence to confirm that [BSim]Cl was really synthesized. For this purpose, UV–vis spectra of 1-butyl imidazole and [BSim]Cl were recorded. At first, some solutions of the mentioned compounds in DMSO with the same concentration were prepared. The concentration of these compounds in DMSO for UV study was 0.005 mol L− 1. The maximum absorptions of 1-butyl imidazole and [BSim]Cl appeared at 273 nm, 0.2286 and 1.2678, respectively. The difference between maximum absorptions of 1-butyl imidazole and [BSim]Cl confirmed the production of the catalyst (Fig. 6).
Table 1 Optimization conditions for the one-pot synthesis of ethyl 4-(4-methoxyphenyl)-2,7,7trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3 carboxylate in the presence of [Bsim] Cl under solvent-free conditions.
3.1.6. Catalyst activity The activity of catalyst was tested in the synthesis of hexahydroquinolines by multi-component reaction during the reaction of aromatic aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate under solvent-free conditions for the first time (Scheme 2). The control experiment and the optimization of the reaction condition were examined by choosing the reaction of 4-methoxybenzaldehyde (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol) as model reaction. As shown in Table 1, if the reaction is
Entry
Catalyst (mol%)
Temp (°C)
Time (min)
Yielda (%)
1 2 3 4 5 6 7 8 9 10 10
– 3 5 7 10 12 10 10 10 10 10
60 60 60 60 60 60 25 40 50 60 70
90 49 26 15 8 4 60 42 19 8 5
19 79 84 89 92 93 63 78 84 92 92
a Yields refer to the pure isolated products based on the reaction of 4-methoxybenzaldehyde (10 mmol), dimedone (10 mmol), ethyl acetoacetate (10 mmol) and ammonium acetate (15 mmol).
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Table 2 Synthesis of ethyl 4-(aryl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl. Entry
Aldehydes
Product
Time (min)
Yielda (%)
M.P./M.P. [ref] (°C)
1
6
91
201–203/203–205 [30]
2
5
90
232–234/233–235 [31]
3
5
90
205–207/207–209 [30]
4
4
93
242–244/242–244 [31]
5
5
91
171–173/172–174 [30]
6
8
92
258–259/257–259 [30]
(continued on next page)
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Table 2 (continued) Entry
Aldehydes
Product
Time (min)
Yielda (%)
M.P./M.P. [ref] (°C)
7
6
94
261–263/262–264 [30]
8
7
93
228–229/228–230 [30]
9
6
90
261–263/262–263 [26]
10
10
89
184–186/184–186 [30]
to the activated carbonyl group. The mechanism is confirmed by the literatures [17,32,33]. 3.1.7. Catalyst recycling An important aspect of catalysts in large scale production is recyclability and reusability of applied catalysts. In order to examine the recyclability of [Bsim]Cl, the reaction of 4-methoxybenzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate was chosen (Fig. 7). For recycling experiment, the reaction was carried out in the same condition. At the end of the reaction, it was cooled to room temperature. Then, 3 mL of water was added to the mixture. The ionic liquid was dissolved in water, and filtered for separation of the crude product. The separated product was washed twice with water (2 × 3 mL). For catalyst recycling, after washing the solid products with water completely, the water containing the
ionic liquid (IL is soluble in water) was evaporated under reduced pressure and the ionic liquid was recovered and reused. The solid product was purified by recrystallization procedure in ethanol. All of the desired product(s) were characterized by comparison of their physical and spectral data (melting points, IR, 1H NMR) with those of known compounds [26,30,31,33].
3.1.8. Comparison of the synthesized catalyst with literatures In order to show the accessibility of the present work, we summarized the described results of acidic IL besides the other reported acidic catalysis especially acidic ionic liquids in Table 3. The results show after sulfonation, imidazolium ring with a longer chain derivative (–butyl) will have a smooth decrease in acidic activity rather some of acidic ionic liquids, however our IL is superior and the most efficient catalyst with respect
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Scheme 3. Plausible mechanism for the catalytic synthesis of ethyl 4-(phenyl)-2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate derivatives catalyzed by [Bsim]Cl.
to the reaction time, low amount of the catalyst, low temperature, and exhibits broad applicability in terms of the obtained yields. 4. Conclusions In this research, Brønsted acidic ionic liquid [Bsim]Cl was introduced as a novel, highly efficient, general and homogeneous catalyst for the one-pot multi-component reaction between various aldehydes, dimedone, ethyl acetoacetate, and ammonium acetate leading to hexahydroquinolines. The synthesized catalyst showed an excellent
activity in the synthesis of various hexahydroquinoline in a green way. Also, the catalyst was recycled four times without the significant loss of activity. In the green chemistry point of view, the present protocol could find industrial application.
Acknowledgments The authors gratefully acknowledge partial support of this work by the University of Payame Noor (Grant No. 48910).
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Fig. 7. The investigation of the reusability of the ionic liquid under solvent-free conditions using a model reaction of 4-methoxybenzaldehyde, dimedone, ethyl acetoacetate, and ammonium acetate in the presence of reused [Bsim]Cl as catalyst.
Table 3 Comparison of the results of 1-butyl-3-sulfonic acid imidazolium chloride [Bsim] Cl with other catalysts reported in the literature in the synthesis of ethyl 4-(4-methoxyphenyl)2,7,7-trimethyl-5-oxo-1,4,5,6,7,8-hexahydroquinoline-3-carboxylate. Entry
Catalyst
Conditions
Time (min)
Yield (%) [ref]
1 2 3 4 5 6 7 8 9 10 11
[Bsim]Cl TiO2NPS Hf(NPf2)4 PEG-400 FePO4 TFE FeF3 [DMBSI]HSO4 [HMIM]BF4 [Dsim]HSO4 [pyridine-SO3H]Cl
Solvent-free, 60 °C Ethanol, under reflux In C10H18, 60 °C Solvent-free, 75 °C Ethanol, under reflux 70 °C Ethanol, 75–80 °C Solvent-free, 60 °C Solvent-free, 90 °C Solvent-free, 50 °C Solvent-free, 50 °C
8 30 240 180 30 180 60 12 10 25 12
92 (the present work) 93 [30] 89 [24] 93 [35] 84 [36] 98 [34] 95 [31] 80 [37] 95 [38] 96 [39] 91 [40]
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