An efficient ultrasonic-assisted synthesis of imidazolium and pyridinium salts based on the Zincke reaction

Ultrasonics Sonochemistry 17 (2010) 685–689

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An efficient ultrasonic-assisted synthesis of imidazolium and pyridinium salts based on the Zincke reaction Sanhu Zhao *, Xiaoming Xu, Lu Zheng, Hai Liu Department of Chemistry, Xinzhou Teachers University, Xinzhou 034000, China

a r t i c l e

i n f o

Article history: Received 15 November 2009 Received in revised form 26 December 2009 Accepted 30 December 2009 Available online 4 January 2010 Keywords: Zincke reaction Primary amine Ultrasound Pyridinium salts Imidazolium salts

a b s t r a c t A mild and efficient method has been developed using ultrasound irradiation for the synthesis of imidazolium and pyridinium salts based on the Zincke reaction. Tertiary nitrogen nucleophiles such as pyridines and imidazoles can be alkylated with primary amine by simply using their ammonium form Zincke salts. In almost all cases, a clear yield increase results and a dramatic reduction of the reaction time accompanied by an improved quality of the products occurs. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Over the course of the last decades, pyridinium and imidazolium-based salts have gained much attention for their versatile properties and variety applications in diverse areas ranging from synthetic and catalytic chemistry to biotechnology, electrochemistry, and material science [1]. One of the most practical and widely used routes for the synthesis of these compounds is direct alkylation of pyridine or 1-methylimidazole with an excess of alkyl halides [2]. However, using alkyl halides as alkylating agents can lead to preparative issues such as timeconsuming and usually requiring a large molar excess of haloalkane (10–400%) to achieve good yields [3], these make syntheses both dirty and expensive, and most importantly, a number of halides such as sec-alkyl halides, tert-alkyl halides and aryl halides work difficult or even impossible [4]. So to find or explore another alternative synthetic strategy for the preparation of imidazolium and pyridinium salts has been the recent research focus. In earlier reports, dimethyl sulfate, diethyl sulfate, dimethyl carbonate and propylene oxide were successively used as an alternative alkylating agents [5], a series of imidazolium and pyridinium salts were prepared. Most recently, Bischoff and co-workers demonstrated the synthesis of imidazolium and pyridinium salts based on the Mitsunobu reaction, tertiary

* Corresponding author. Tel.: +86 350 3048252; fax: +86 350 3031845. E-mail address: [email protected] (S. Zhao). 1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ultsonch.2009.12.019

nitrogen nucleophiles such as pyridines and imidazoles can be alkylated with alcohols [6]. Fürstner et al. developed a novel route to synthesis unsymmetrical imidazolium salts bearing two different aryl groups on the N-atoms, which using various substituted anilines as well as secondary and tertiary amines as the reaction partners [4]. To our knowledge, The Zincke reaction is a versatile method in which a pyridine is transformed into a pyridinium salt by reaction with 2,4-dinitrochlorobenzene and a primary amine [7]. Synthesis of N-arylpyridinium salts from the Zincke salt was for the first time reported by Marvell and Ise [8]. Recently, pyridinium salts containing reactive hydroxyl, amine and/or pyridyl groups have also been synthesized [9]. In spite of their potential utility, however, some of draw-backs such as lower yield and relatively long reaction time commonly associated with this useful reaction [9,10]. Ultrasound irradiation has been considered as a clean and useful protocol in organic synthesis in the last three decades [11], compared with traditional methods, the procedure is more convenient. A large number of organic reactions can be carried out in higher yield, shorter reaction time or milder conditions under ultrasonic irradiation [12]. Due to the absence of reported ultrasonic-assisted Zincke reaction and our general interest in the development of clean chemical processes, herein, we wish to report an improved procedure for the Zincke reaction, under ultrasound irradiation, not only a variety of pyridinium salts but also many imidazolium salts were prepared with mild reaction conditions, short reaction time and moderate to high yields (Scheme 1).

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S. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 685–689

6.79 (d, J = 8.8 Hz, 2H), 5.94 (s, 2H). 13C NMR (300 MHz, DMSOd6): d = 148.4, 142.8, 141.2, 134.3, 129.4, 126.4, 125.8. Anal. calcd. for C11H11ClN2
0.5H2O: C, 61.25; H, 5.14. Found: C, 61.39; H, 5.47.

O2N N Cl

+ R NH2

or Cl O2N

N R Cl

NO2

N

N

80% EtOH

+ H2N

or

)))))), 35oC R N

O2N NO2

N Cl

NO2

Scheme 1. Synthesis of imidazolium and pyridinium salts.

2.3.2. 1, 4-Bis (pyridinium) butane chloride (Table 2, entry 8) 1 H NMR (300 MHz, D2O): d = 8.70 (d, J = 5.5 Hz, 4H), 8.48 (t, J = 7.8 Hz, 2H), 7.90 (t, J = 6.9 Hz, 4H), 4.74 (t, J = 7.2 Hz, 4H), 2.22 (m, 4H). 13C NMR (300 MHz, D2O): d = 145.5, 144.1, 128.4, 62.2, 34.6. Anal. calcd. for C14H18Cl2N2
0.8H2O: C, 56.12; H, 6.06. Found: C, 56.41; H, 5.82.

2. Experimental 2.1. Apparatus and analysis Melting points were measured on WRS-1B digital Melting point meter and are uncorrected. 1H NMR and 13C NMR spectra were measured on a DRX300 NMR Spectrometer using TMS as an internal standard in D2O, DCCl3 or DMSO-d6. The elemental analyses were performed in the Institute of Chemistry, Chinese Academy of Sciences. Sonication was performed in Kunshan KQ-400KDE ultrasonic cleaner (with a frequency 40 kHz and a nominal power 400 W), and the reaction temperature was controlled by exchange of the water in ultrasonic cleaning bath. Analytical thin layer chromatography (TLC) was carried out using MN Kieselgel G/UV254 (Art. 816320) glass backed plates. 2.2. N-(2,4-Dinitrophenyl)pyridinium chloride [13] To a solution of finely powdered 1-chloro-2,4-dinitrobenzene (10.12 g, 50 mmol) in acetone (15 mL) was added pyridine (4.5 mL, 55 mmol), the reaction vessel equipped with a drying tube was then placed in a laboratory ultrasonic cleaning bath, and the reaction mixture was irradiated by 40 kHz ultrasound at 40 °C for 1.5 h. The resulting precipitate was collected, washed with acetone, and recrystallized from MeOH/AcOEt/hexane to give a product (12.39 g, 88%) as a slightly yellow crystal, m.p. 192.4–193.1 °C (lit. 197–200 °C). 1H NMR (300 MHz, DMSO-d6): d = 9.40 (d, J = 6.9 Hz, 2H), 9.31 (s, 1H), 9.02 (t, J = 7.9 Hz, 1H), 8.97 (d, J = 8.7 Hz, 1H), 8.47 (d, J = 7.9 Hz, 2H), 8.38 (d, J = 8.7 Hz, 1H). 13C NMR (300 MHz, DMSO-d6): d = 121.4, 128.0, 130.2, 131.9, 138.7, 143.2, 146.2, 148.8, 149.0. 2.3. General procedure for the synthesis of pyridinium chloride A 50 mL round flask was charged with N-(2,4-dinitrophenyl)pyridinium chloride (1.41 g, 5.0 mmol), primary amine (5.5 mmol) and 15 mL 80% ethanol, the reaction flask was located in the cleaner bath, where the surface of reactants was slightly lower than the level of the water. Then the reaction mixture was irradiated by 40 kHz ultrasound at 35 °C under nitrogen. The reaction progress was monitored by TLC. After the solution was irradiated for the period as indicated in Table 2, 2,4-dinitroaniline precipitated from the reaction solution was removed by filtration. The solvent was evaporated under vacuum, and the resulting solid was washed with petroleum ether (3  10 mL). Evaporation of solvent under reduced pressure gave the desired product. For the new products, their structures were determined by 1H and 13C NMR spectroscopy and elemental analysis. For the known compounds, their structures were determined by 1H and 13C NMR spectroscopy and the spectral data of the products were identical to those previously reported [14]. 2.3.1. 1-(4-Amino-phenyl)-pyridinium chloride (Table 2, entry 5) 1 H NMR (300 MHz, DMSO-d6): d = 9.03 (d, J = 5.6 Hz, 2H), 8.78 ((t, J = 7.4 Hz, 1H), 8.21 (t, J = 6.7 Hz, 2H), 7.71 (d, J = 7.4 Hz, 2H),

2.3.3. 1,2-Bis (pyridinium) ethane chloride (Table 2, entry 9) 1 H NMR (300 MHz, DMSO-d6): d = 8.72 (d, J = 5.5 Hz, 4H), 8.63 (t, J = 7.8 Hz, 2H), 8.10 (t, J = 6.9 Hz, 4H), 4.98 (s, 4H) 13C NMR (300 MHz, DMSO-d6): d = 146.3, 139.8, 128.7, 62.9, 35.2. Anal. calcd. for C12H14Cl2N2
1.0H2O: C, 52.38; H, 5.13. Found: C, 52.59; H, 5.47. 2.4. N-(2,4-Dinitrophenyl)-3-methylimidazolium chloride [15] To a solution of finely powdered 1-chloro-2,4-dinitrobenzene (10.12 g, 50 mmol) in acetone (15 mL) was added 1-methylimidazole (4.06 mL, 51 mmol), the reaction vessel equipped with a drying tube was then placed in a laboratory ultrasonic cleaning bath, and the reaction mixture was irradiated by 40 kHz ultrasound at 40 °C for 80 min. The resulting precipitate was collected, washed with acetone, and recrystallized from MeOH/ AcOEt/hexane to give a product (13.09 g, 92%) as a white crystal. Mp 245.4–246.9 °C (lit. 244–247 °C). 1H NMR (300 MHz, DMSOd6): d = 9.20 (d, J = 2.51 Hz, 1H), 8.32–8.90 (m, 3H), 8.14 (d J = 2.05 Hz, 1H), 7.97 (d, J = 2.07 Hz, 1H), 4.16 (s, 3H). 13C NMR (300 MHz, DMSO-d6): d = 149.6, 144.6, 132.8, 131.9, 129.6, 124.6, 124.5, 122.1, 36.3. 2.5. General procedure for the synthesis of imidazolium chloride A 50 mL round flask was charged with 1-(2,4-dinitrophenyl)-3methylimidazolium chloride (2.0 g, 7.0 mmol), primary amine (7.5 mmol) and 15 mL of 80% ethanol, the reaction flask was located in the cleaner bath, where the surface of reactants was slightly lower than the level of the water. Then the reaction mixture was irradiated by 40 kHz ultrasound at 35 °C under nitrogen. The reaction progress was monitored by TLC. After the solution was irradiated for the period as indicated in Table 3, 2,4-dinitroaniline precipitated from the reaction solution was removed by filtration. The solvent was evaporated under vacuum, and the resulting

Table 1 The effect of the reaction conditions on the synthesis of 1-(4-methoxy-phenyl)pyridinium chloridea. Entry

Power (W)

Temperature (°C)

Irradiation time (min)

Yield (%)b

1 2 3 4 5 6 7 8 9c 10c

320 360 360 360 360 360 360 360 0 0

25 30 35 40 45 35 35 35 Reflux Reflux

60 60 60 60 60 80 100 120 360 720

57 64 71 71 72 80 86 87 56 75

a Conditions: With irradiation frequency 40 kHz, N-(2,4-dinitrophenyl)pyridinium chloride (1.41 g, 5. 0 mmol) and 4-methoxy-phenylamine (0.68 g, 5. 5 mmol) were dissolved in 15 mL of 80% ethanol under N2. b Refers to work-up yield. c Conventional method with magnetic stirring.

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solid was washed with anhydrous diethyl ether (3  10 mL). Evaporation of solvent under reduced pressure gave the desired product. For the new products, their structures were determined by 1 H and 13C NMR spectroscopy and elemental analysis. For the known compounds, their structures were determined by 1H and 13 C NMR spectroscopy and the spectral data of the products were identical to those previously reported [16].

2.5.1. 1-(4-Methoxy-phenyl)-3-methylimidazolium chloride (Table 3, entry 1) 1 H NMR (300 MHz, DMSO-d6): d = 10.08 (s, 1H), 8.48 (d, J = 2.06 Hz, 1H), 8.19 (d, J = 2.04 Hz, 1H), 7.30 (d, J = 7.87 Hz, 2H), 7.07 (d, J = 8.37 Hz, 2H), 3.80 (s, 3H), 3.31 (s, 3H). 13C NMR (300 MHz, DMSO-d6): d = 149.6, 144.6, 132.8, 131.9, 129.6, 124.6, 124.5, 122.1, 36.3. Anal. calcd. for C11H13ClN2O
0.8H2O: C, 55.25; H, 5.48. Found: C, 55.61; H, 5.46.

2.5.2. 3-Methyl-1-p-tolylimidazolium chloride(Table 3, entry 2) 1 H NMR (300 MHz, DMSO-d6): d = 10.12 (s, 1H), 7.81 (d, J = 2.27 Hz, 1H), 7.61 (d, J = 2.15 Hz, 1H), 7.32 (d, J = 8.21 Hz, 2H), 7.26 (d, J = 7.69 Hz, 2H), 3.37 (s, 3H), 2.36 (s, 3H). 13C NMR (300 MHz, DMSO-d6): d = 138.1, 137.8, 128.2, 126.2, 123.9, 122.8, 119.6, 35.9, 20.9. Anal. calcd. for C11H13ClN2
0.5H2O: C, 60.69; H, 6.02. Found: C, 60.93; H, 5.98. 2.5.3. 1, 2-Bis (3-methylimidazolium) ethane chloride(Table 3, entry 5) 1 H NMR (300 MHz, D2O): d = 8.70 (s, 2H), 7.42 (d, J = 1.43 Hz, 2H), 7.35 (d, J = 1.73 Hz, 2H), 4.65 (s, 2H), 3.81 (s, 6H). 13C NMR (300 MHz, D2O): d = 138.3, 124.4, 123.8, 57.5, 35.8. Anal. calcd. for C10H16Cl2N4
H2O: C, 42.61; H, 5.74. Found: C, 42.85; H, 5.91. 2.5.4. 1, 4-Bis (3-methylimidazolium) butane chloride(Table 3, entry 6) 1 H NMR (300 MHz, D2O): d = 8.68 (s, 2H), 7.43 (d, J = 2.07 Hz, 2H), 7.37 (d, J = 2.43 Hz, 2H), 4.57 (t, 4H), 3.94 (s, 6H), 1.78 (m,

Table 2 Synthesis of pyridinium salts using Zincke reaction under ultrasound irradiation.a

O2N N Cl

O2N NO2 + R NH2

80% EtOH )))))), 35o C

1

Entry

Substrate (1)

1

NH2

N Cl

CH3

N Cl

Cl

N Cl

I

2

Cl

NH2

3

I

NH2

4

NH2

6

NH2

CH3 NH2

7

M.p. (°C) (Lit) [14]

Time (min)

Yield (%)b

115.4–116.8 (117)

100

67

118.9–121.2 (119–122)

100

79

121.7–123.2 (122–124)

100

66

107.2–108.6 (110)

100

62

132.4–134.0

100

79

178.3–179.6 (179–181)

100

48

133.7–134.2 (132–133)

80

83

231.3–232.8

80

72.5

245.2–246.7

80

74

N Cl

5

H 2N

NO2

2

Product (2)

H3C

N R + H N 2 Cl

N Cl H3C N Cl

H3C CH3CH2CH2CH2NH2

8

H2N–(–CH2–)4–NH2

9

H2N–(CH2–)2–NH2

NH2

CH3

N Cl Cl Cl N CH2 N 4

Cl Cl N CH2 N 2

10

O2N

NH2

N Cl

120

Trace

120

Trace

NO2

11

N

NH2 Cl

a All reactions were performed with N-(2,4-dinitrophenyl)pyridinium chloride (5 mmol, 1.41 g), primary amine (5.5 mmol) in 80% ethanol (15 mL) and the ultrasonic power 360 W, irradiation frequency 40 kHz. b Refers to work-up yield.

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S. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 685–689

4H). 13C NMR (300 MHz, D2O): d = 138.2, 123.4, 123.1, 56.8, 35.6, 34.7. Anal. calcd. for C12H20Cl2N4
0.7H2O: C, 47.43; H, 6.64. Found: C, 47.71; H, 6.29.7.

As therein revealed, using conventional magnetic stirring, the resulting product was obtained with 75% yield after 720 min (Table 1, entry 10), however, under ultrasonic irradiation, with the power 360 W, irradiation frequency 40 kHz and the reaction temperature 35 °C, the excellent products yield (86%) was obtained after only 100 min (Table 1, entry 7). To establish the generality of the ultrasonic-assisted Zincke reaction, a series of primary amine including aromatic amines, aliphatic amines and aliphatic diamines were employed in this reaction, the results are listed in Table 2. As is evident from Table 2, the ultrasound assisted Zincke reactions proceeded smoothly at 35 °C, most of the amines can be converted to their corresponding pyridinium salts with moderate to high yield. It can easily be seen that reaction yields are independent of the group attached to the amine, with good conversion (74–83%) for alkyl amines in short reaction time (80 min), with moderate to high conversion (48–79%) for aromatic amine in 100 min, It was noted that the reactions of 4-nitro-phenylamine (Table 2, entry 10) and 1-naphthylamine (Table 2, entry 11) gave almost no pyridinium salts after 120 min, a possible reason for this is that the weak nucleophilic ability of amino group reduced by the electron-withdrawing group attached to the aromatic amine and the steric hindrance of the 1-naphthylamine prevent nucleophilic addition to the pyridinium ring of Zincke salt. To broaden the scope of this method, 1-methylimidazole was employed as a starting material to react with 1-chloro-2,4-dinitrobenzene, and the desired 1-(2,4-dinitrophenyl)-3-methylimidazolium chloride was obtained in 92% yield under ultrasound irradiation after 80 min. Then the similar Zincke reaction of primary amine with 1-(2,4-dinitrophenyl)-3-methylimidazolium chloride were conducted, the results are listed in Table 3. As shown in Table 3, with 40 kHz, 360 W ultrasound irradiation, some primary amine via similar Zincke reaction can give corresponding imidazolium salts with 62–84% yield in 90–120 min. Compared with aromatic amine, the aliphatic amine showed more reactivity under this modified conditions.

3. Results and discussion Ultrasound-accelerated chemical reactions are well-known and proceed via the formation and adiabatic collapse of the transient cavitation bubbles [17]. In previous studies, we found that the ultrasound can accelerate the reaction of pyridine with 3-chloropropylene oxide and the products 1-(3-chloro-2-hydroxy-propyl)-pyridinium chloride can be easily converted to pyridinium ionic liquids with different anion [18]. As a continuation of our research interest in synthesis of pyridinium or imidazolium salts, a laboratory ultrasonic cleaning bath was used for Zincke reaction. Initially, the effect of ultrasound on the synthesis of N-(2,4-dinitrophenyl)pyridinium chloride was examined. In contrast to traditional method (reaction time 3 h and the yield 67%), the use of ultrasound irradiation leads to a faster reaction (1.5 h) and a higher yield (88%). With the Zincke salt in hand, the first ultrasonic-assisted Zincke reaction of 4-methoxy-phenylamine with N-(2,4-dinitrophenyl) pyridinium chloride in 80% ethanol was investigated (Table 1, entry 1). During sonication, the formation of the pyridium salt could be visibly monitored as the reaction contents turned from a clear solution to opaque solution, then the opaque solution became viscous and finally the complete separation of the solid–liquid phase occurred. The precipitated 2,4-dinitroaniline was removed from the reaction solution by filtration, the filtrate was evaporated under vacuum, and the desired product was obtained in 57% yield. Since we were not satisfied with this result, factors that affect the reaction such as temperature, ultrasonic power and reaction time were carefully investigated and the results were summarized in Table 1.

Table 3 Synthesis of imidazolium salts using similar Zincke reaction under ultrasound irradiation.a

O2N

Cl O2N

N NO2

Entry

Substrate (1)

1 Product (2)

H3CO

NH2

Cl

H3CO

2

3 4

5

NH2

CH3NH2 CH3CH2CH2CH2NH2

N

H2NCH2CH2CH2CH2NH2

Cl

H2NCH2CH2NH2

N

NO2

M.p. (°C) (Lit) [16]

Time (min)

Yield (%)b

96.5–98.4

120

78

84.5–86.2

120

62

124.3–125.2 (125)

100

72

65.2–66.7 (65)

90

81

17.4–118.6

90

78

86.3–88.1

90

84

N

N N

N

N Cl

N (CH2)2 N Cl

+ H2N

Cl

Cl N

N 6

Cl

H3C N

N Cl 2

1

H3C

80% EtOH R N )))))), 35oC

N + R NH2

N

Cl

N (CH2)4 N

N

a All reactions were performed with 1-(2,4-dinitrophenyl)-3-methylimidazolium chloride (2.0 g, 7. 0 mmol), primary amine (7. 5 mmol) in 80% ethanol (15 mL), and the ultrasonic power 360 W, irradiation frequency 40 kHz. b Refers to work-up yield.

S. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 685–689

4. Conclusions In conclusion, we have developed a simple synthetic method to prepare imidazolium and pyridinium salts using a laboratory ultrasonic cleaning bath. Under the optimized conditions of ultrasonic irradiation, a series of imidazolium and pyridinium salts were prepared and characterized by 1H and 13C NMR spectral and elemental analysis. This method display dramatically reduced reaction time compared to conventional methods, and also afford the desired products in moderate to high yields and purity. Acknowledgements We gratefully acknowledge financial support from Xinzhou Teachers University (No. [2008] 79).We thank Dr. L.H. Feng for their support in the measurements of NMR. References [1] (a) T. Welton, Chem. Rev. 99 (1999) 2071; (b) P. Wasserscheid, W. Keim, Angew. Chem. Int. Ed. 39 (2000) 3772; (c) J. Dupont, R.F. de Souza, P.A.Z. Suarez, Chem. Rev. 102 (2002) 3667; (d) D. Zhao, M. Wu, Y. Kou, E. Min, Catal. Today 74 (2002) 157; (e) I. Kawasaki, K. Tsunoda, T. Tsuji, T. Yamaguchi, H. Shibuta, N. Uchida, M. Yamashita, S. Ohta, Chem. Commun. (2005) 2134; (f) N. Jain, A. Kumar, S. Chauhan, S.M.S. Chauhan, Tetrahedron 61 (2005) 1015; (g) X. Mi, S. Luo, H. Xu, L. Zhang, J. Cheng, Tetrahedron 62 (2006) 2537; (h) S.J. Loeb, J. Tiburcio, S.J. Vella, J.A. Wisner, Org. Biomol. Chem. 4 (2006) 667. [2] (a) J.S. Wilkes, J.A. Levisky, R.A. Wilson, C.L. Hussey, Inorg. Chem. 21 (1982) 1263; (b) J.D. Holbrey, W.M. Reichert, R.P. Swatloski, G.A. Broker, W.R. Pitner, K.R. Seddon, R.D. Rogers, Green Chem. 4 (2002) 407. [3] M. Deetlefs, K.R. Seddon, Green Chem. 5 (2003) 181. [4] A. Fürstner, M. Alcarazo, V. César, C.W. Lehmann, Chem. Commun. (2006) 2176.

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