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

Article abstract of DOI:10.1016/j.ultsonch.2009.12.019

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.

Full text of DOI:10.1016/j.ultsonch.2009.12.019

Ultrasonics Sonochemistry 17 (2010) 685–689
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Ultrasonics Sonochemistry
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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
a b s t r a c t
Article history:
A mild and efficient method has been developed using ultrasound irradiation for the synthesis of imi-
dazolium and pyridinium salts based on the Zincke reaction. Tertiary nitrogen nucleophiles such as pyr-
idines 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.
Received 15 November 2009
Received in revised form 26 December 2009
Accepted 30 December 2009
Available online 4 January 2010
Ó 2010 Elsevier B.V. All rights reserved.
Keywords:
Zincke reaction
Primary amine
Ultrasound
Pyridinium salts
Imidazolium salts
1. Introduction
nitrogen nucleophiles such as pyridines and imidazoles can be
alkylated with alcohols [6]. Fürstner et al. developed a novel
Over the course of the last decades, pyridinium and imidazo-
lium-based salts have gained much attention for their versatile
properties and variety applications in diverse areas ranging
from synthetic and catalytic chemistry to biotechnology, elec-
trochemistry, 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 ex-
cess of alkyl halides [2]. However, using alkyl halides as alkyl-
ating agents can lead to preparative issues such as time-
consuming and usually requiring a large molar excess of haloal-
kane (10–400%) to achieve good yields [3], these make synthe-
ses 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 ex-
plore another alternative synthetic strategy for the preparation
of imidazolium and pyridinium salts has been the recent re-
search 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
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, how-
ever, 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 un-
der 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).
* Corresponding author. Tel.: +86 350 3048252; fax: +86 350 3031845.
E-mail address: sanhuzhao@yahoo.cn (S. Zhao).
1350-4177/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.ultsonch.2009.12.019

686
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, DMSO-
O2N
d6): d = 148.4, 142.8, 141.2, 134.3, 129.4, 126.4, 125.8. Anal. calcd.
for C11H11ClN2Á0.5H2O: C, 61.25; H, 5.14. Found: C, 61.39; H, 5.47.
N
R
N
NO2
N
O2N
H2N
Cl
Cl
or
80% EtOH
)))))), 35oC
+
R
NH2
+
NO2
or
Cl
2.3.2. 1, 4-Bis (pyridinium) butane chloride (Table 2, entry 8)
1H 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 C14H18Cl2N2Á0.8H2O: C, 56.12; H, 6.06.
Found: C, 56.41; H, 5.82.
O2N
N
R
N
N
Cl
NO2
Scheme 1. Synthesis of imidazolium and pyridinium salts.
2. Experimental
2.3.3. 1,2-Bis (pyridinium) ethane chloride (Table 2, entry 9)
1H 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 C12H14Cl2N2Á1.0H2O: C, 52.38; H, 5.13. Found: C, 52.59;
H, 5.47.
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 inter-
nal 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 chro-
matography (TLC) was carried out using MN Kieselgel G/UV254 (Art.
816320) glass backed plates.
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-methylimid-
azole (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 ultra-
sound at 40 °C for 80 min. The resulting precipitate was col-
lected, 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, DMSO-
d6): 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.
2.2. 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.
2.5. General procedure for the synthesis of imidazolium chloride
A 50 mL round flask was charged with 1-(2,4-dinitrophenyl)-3-
methylimidazolium chloride (2.0 g, 7.0 mmol), primary amine
(7.5 mmol) and 15 mL of 80% ethanol, the reaction flask was lo-
cated in the cleaner bath, where the surface of reactants was
slightly lower than the level of the water. Then the reaction mix-
ture 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-dinitroan-
iline precipitated from the reaction solution was removed by filtra-
tion. The solvent was evaporated under vacuum, and the resulting
2.3. General procedure for the synthesis of pyridinium chloride
A
50 mL round flask was charged with N-(2,4-dinitro-
phenyl)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 reac-
tion progress was monitored by TLC. After the solution was irradi-
ated 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 Â 10 mL). Evaporation of sol-
vent 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 previ-
ously reported [14].
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
320
360
360
360
360
360
360
360
0
25
30
35
40
45
35
35
35
60
60
60
60
60
57
64
71
71
72
80
86
87
56
75
80
100
120
360
720
9c
10c
Reflux
Reflux
0
a
Conditions: With irradiation frequency 40 kHz, N-(2,4-dinitrophenyl)pyridini-
um 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.
2.3.1. 1-(4-Amino-phenyl)-pyridinium chloride (Table 2, entry 5)
1H 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),
b
Refers to work-up yield.
c
Conventional method with magnetic stirring.

S. Zhao et al. / Ultrasonics Sonochemistry 17 (2010) 685–689
687
solid was washed with anhydrous diethyl ether (3 Â 10 mL). Evap-
oration of solvent under reduced pressure gave the desired prod-
uct. 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 [16].
2.5.2. 3-Methyl-1-p-tolylimidazolium chloride(Table 3, entry 2)
1H 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 C11H13ClN2Á0.5H2O: 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)
1H 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 C10H16Cl2N4ÁH2O: C, 42.61; H, 5.74. Found: C, 42.85; H, 5.91.
2.5.1. 1-(4-Methoxy-phenyl)-3-methylimidazolium chloride (Table 3,
entry 1)
1H 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 C11H13ClN2OÁ0.8H2O: C, 55.25;
H, 5.48. Found: C, 55.61; H, 5.46.
2.5.4. 1, 4-Bis (3-methylimidazolium) butane chloride(Table 3, entry 6)
1H 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
O2N
80% EtOH
)))))), 35oC
+ R NH2
NO2
N R
Cl
N
Cl
+
H2N
NO2
1
2
Entry
1
Substrate (1)
Product (2)
M.p. (°C) (Lit) [14]
Time (min)
100
Yield (%)b
67
115.4–116.8 (117)
H C
NH2
N
CH3
Cl
3
Cl
2
3
4
118.9–121.2 (119–122)
121.7–123.2 (122–124)
107.2–108.6 (110)
100
100
100
79
66
62
Cl
NH2
N
Cl
I
NH2
N
Cl
I
NH2
N
Cl
5
6
132.4–134.0
100
100
79
48
H2N
NH2
N
Cl
H C
3
NH2
178.3–179.6 (179–181)
CH
3
NH
N
2
Cl
H C
3
CH
3
7
8
CH3CH2CH2CH2NH2
133.7–134.2 (132–133)
231.3–232.8
80
80
83
N
Cl
H2N–(–CH2–)4–NH2
72.5
Cl
N
Cl
N
CH2
4
9
H2N–(CH2–)2–NH2
245.2–246.7
80
74
Cl
N
Cl
N
CH2
2
10
11
120
120
Trace
Trace
O2N
NH2
N
Cl
NO2
NH2
N
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.

688
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 C12H20Cl2N4Á0.7H2O: 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 ultra-
sonic-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 reac-
tions proceeded smoothly at 35 °C, most of the amines can be con-
verted to their corresponding pyridinium salts with moderate to
high yield. It can easily be seen that reaction yields are indepen-
dent 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-dinitro-
benzene, and the desired 1-(2,4-dinitrophenyl)-3-methylimidazo-
lium chloride was obtained in 92% yield under ultrasound
irradiation after 80 min. Then the similar Zincke reaction of pri-
mary 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 corre-
sponding 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-chloro-
propylene oxide and the products 1-(3-chloro-2-hydroxy-pro-
pyl)-pyridinium chloride can be easily converted to pyridinium io-
nic 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-dinitro-
phenyl)pyridinium chloride was examined. In contrast to tradi-
tional 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, en-
try 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 vis-
cous 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 un-
der 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
O N
2
Cl
80% EtOH
O N
N
N
+ R NH
2
R N
N
+
2
H N
NO
2
2
o
Cl
)))))), 35 C
NO
2
1
2
Entry
1
Substrate (1)
Product (2)
M.p. (°C) (Lit) [16]
Time (min)
120
Yield (%)b
78
96.5–98.4
Cl
N
H CO
NH2
NH2
H3CO
N
3
2
84.5–86.2
120
62
Cl
N
H3C
H3C
N
3
4
CH3NH2
124.3–125.2 (125)
65.2–66.7 (65)
100
90
72
81
N
N
Cl
CH3CH2CH2CH2NH2
Cl
N
N
5
6
H2NCH2CH2CH2CH2NH2
H2NCH2CH2NH2
17.4–118.6
86.3–88.1
90
90
78
84
Cl
N
Cl
N
N
(CH2)2
(CH2)4
N
Cl
N
Cl
N
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
689
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In conclusion, we have developed a simple synthetic method to
prepare imidazolium and pyridinium salts using a laboratory ultra-
sonic cleaning bath. Under the optimized conditions of ultrasonic
irradiation, a series of imidazolium and pyridinium salts were pre-
pared 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.
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Acknowledgements
We gratefully acknowledge financial support from Xinzhou
Teachers University(No. [2008]79).We thank Dr. L.H. Feng for their
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