A novel imidazolium-based acidic ionic liquid as an efficient and reusable catalyst for the synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazoles

Article abstract of DOI:10.1007/s11164-013-1522-4

A novel acidic ionic liquid based on imidazoliumcation is designed, synthesized, and successfully used as a catalyst for the one-pot synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazole derivatives. The remarkable feature of this new catalyst is its ethyleneoxy bridge which participates in dissolving organic compounds in ionic liquids. The application of this acidic ionic liquid is studied in a new one-pot method for the synthesis of imidazole derivatives under solvent-free conditions. The advantages of this method are the reusability of the catalyst, high conversion, short reaction time, and simple experimental procedure.

Full text of DOI:10.1007/s11164-013-1522-4

Res Chem Intermed
DOI 10.1007/s11164-013-1522-4
A novel imidazolium-based acidic ionic liquid
as an efficient and reusable catalyst for the synthesis
of 2-aryl-1H-phenanthro[9,10-d]imidazoles
•
Hossein Eshghi Mohammad Rahimizadeh
•
•
Maede Hasanpour Mehdi Bakavoli
Received: 28 September 2013 / Accepted: 18 December 2013
Ó Springer Science+Business Media Dordrecht 2014
Abstract A novel acidic ionic liquid based on imidazoliumcation is designed,
synthesized, and successfully used as a catalyst for the one-pot synthesis of 2-aryl-
1H-phenanthro[9,10-d]imidazole derivatives. The remarkable feature of this new
catalyst is its ethyleneoxy bridge which participates in dissolving organic com-
pounds in ionic liquids. The application of this acidic ionic liquid is studied in a new
one-pot method for the synthesis of imidazole derivatives under solvent-free con-
ditions. The advantages of this method are the reusability of the catalyst, high
conversion, short reaction time, and simple experimental procedure.
Keywords Acidic ionic liquid ꢀ Imidazolium cation ꢀ
Phenanthro[9,10-d]imidazole ꢀ Reusable catalyst
Introduction
Applications of solvent-free reactions in organic synthesis [1–3], particularly those
based on imidazolium cations, have gained attention in recent years [4, 5]. Some of
the unique physical and chemical properties of imidazolium ionic liquids are thermal
stability, negligible vapor pressure, recyclability, ability to dissolve a wide range of
organic and inorganic compounds, fast reaction rate, selectivity, and tendency to
immobilize starting materials and catalyst [6]. For these reasons, they have been used
in different areas of organic synthesis, as both catalyst and solvent [7, 8].
Imidazole and its derivatives are also receiving growing attention for their
pharmacological properties, such as herbicidal, fungicidal, analgesic, anti-inflam-
matory, and antithromobotic [9]. Several methods have reported on the synthesis of
H. Eshghi (&) ꢀ M. Rahimizadeh ꢀ M. Hasanpour ꢀ M. Bakavoli
Department of Chemistry, School of Sciences, Ferdowsi University of Mashhad,
91775-1436 Mashhad, Iran
e-mail: heshghi@um.ac.ir
123

H. Eshghi et al.
2,4,5-triarylimidazoles in the presence of a catalyst such as copper(II) acetate [10],
ZrCl4 [11], AcOH [12, 13], Yb(OTf)3 [14], NbCl3, [14], LaCl3 [14], FeCl3 [14],
AlCl3 [14], NiCl2 [15], and [HBim]BF4 [16]. However, some of these methods suf-
fer from one or more disadvantages such as prolonged reaction time, low yield of
products, and volatile and hazardous organic solvents. Therefore, the search for a
clean procedure using a green and eco-friendly catalyst with high catalytic activity
and short reaction time for the production of 2,4,5-triarylimidazoles has gained
considerable attention.
In recent years, dicationic ionic liquids (ILs) have become interesting research
targets due to their high thermal stability, broad liquid range, and biological
activities such as antiviral, antifungal, and anticancer. In particular, imidazolium-
based dicationic ILs showing high thermal stability were used as effective reaction
media for high temperature organic reactions [7, 17–19]. Some studies have
reported imidazolium-based dicationic ILs that bridged via alkyl and aryl and
alkoxy chains as a linker between imidazolium rings [20–23]. Here, we report for
the first time a novel acidic ionic liquid (AIL) which contains an ethyleneoxy bridge
and a sulfonic acid group, that participates in dissolving organic compounds in ILs
with its ethyleneoxy bridge and SO3H functionalizing as a Brønsted AIL catalyst.
The first ability of this novel catalyst is shown by the rapid synthesis of 2-aryl-
1H-phenanthro[9,10-d]imidazole derivatives from 9,10 phenanthroquinone with
various aromatic aldehydes and ammonium acetate in the presence of a catalytic
amount of this AIL under solvent-free conditions. We believe that this novel AIL
can be applied as a catalyst in many different organic transformations.
Experimental
Materials and instrumentation
All reagents were purchased from Merck and used without further purification. The
melting points of the products were determined with an Electrothermal Type 9100
melting point apparatus. The FT-IR spectra were recorded on an Avatar 370 FT-IR
Therma Nicolet spectrometer. The mass spectra were recorded on a 5973 Network
1
Mass Selective Detector. The H and 13C NMR spectra were recorded on a Bruker
AC 100 and Bruker DRX-400 Avance spectrometers at 400 and 100.65 MHz,
respectively, using DMSO-d6 or CDCl3 as the deuterated solvents. Chemical shifts
are reported in ppm downfield from TMS as internal standard; coupling constants
J are given in Hz.
Synthesis of 1,2-bis(2-(1H-imidazol-1-yl)ethoxy)ethane (1):
A mixture of imidazole (32 mmol, 2.176 g) and potassium hydroxide (32 mmol,
1.792 g) in DMSO (25 ml) was stirred at 70 °C for 1 h. After this time, 1,2-bis(2-
chloroethoxy)ethane (16 mmol, 2.48 ml) was added to the reaction mixture and
stirred for 24 h. The resulting mixture was poured into 100 mL of water and
extracted with methylene chloride (5 9 20 ml). The combined organic layer was
123

A novel imidazolium based acidic ionic liquid
washed with water (3 9 20 ml), dried with anhydrous Na2SO4, and concentrated to
give 1,2-bis(2-(1H-imidazol-1-yl)ethoxy)ethane (1) as a pale yellow viscous oil,
3.10 g, 77 % yield.1H NMR (CDCl3, 400 MHz) d:3.45 (s, 4H), 3.59 (t, 4H,
J = 4.8 Hz), 4.01 (t, 4H, J = 4.8 Hz), 6.90 (t, 2H, J = 1.2 Hz), 6.94 (t, 2H,
J = 1.2 Hz), 7.45 (s, 2H). 13C NMR (CDCl3, 400 MHz) d: 46.98, 70.38, 70.49,
119.39, 128.93, 137.48. IR (KBr, cm-1): 1,642, 1,525, 1,425. m/z, calcd. for
C12H18N4O2 [M]?: 250.30, found: 250.0.
Synthesis of 1,10-((ethane-1,2-diylbis(oxy))bis(ethane-2,1-diyl))bis(3-sulfo-1H-
imidazol-3-ium) chloride (2):
A 100-ml round-bottomed flask was charged with 1,2-bis(2-(1H-imidazol-1-
yl)ethoxy)ethane (1) (12.4 mmol, 3.1 g) in dry CH2Cl2 (50 ml), and then
chlorosulfonic acid (24.8 mmol, 1.64 ml) was added dropwise over a period of
15 min at room temperature. Afterward, the reaction mixture was stirred for 45 min,
stand for 5 min, and the CH2Cl2 was decanted. The residue was washed with dry
CH2Cl2 (3 9 20 mL) and dried under vacuum to give 1,10-((ethane-1,2-diyl-
bis(oxy))bis(ethane-2,1-diyl))bis(3-sulfo-1H-imidazol-3-ium) chloride (2) as a vis-
1
cous colorless oil. H NMR (DMSO-d6, 400 MHz) d:3.46 (s, 4H), 3.69 (t, 4H,
J = 4.8 Hz), 4.33 (t, 4H, J = 4.8 Hz), 7.60 (t, 2H, J = 1.6 Hz), 7.66 (t, 2H,
J = 1.6 Hz), 8.98 (t, 2H, J = 1.6 Hz), 14.32 (bs, 2H). 13C NMR (DMSO-d6,
400 MHz) d: 48.80, 68.60, 69.75, 120.13, 122.71, 135.94. IR (KBr, cm-1): 3425
(broad, SO3H), 1,642, 1,580, 1,549. m/z, calcd. for C12H20Cl2N4O8S2 [M]?: 482.01,
found: 481.9.
General procedure for preparation of 2-aryl-1H-phenanthro[9,10-d]imidazole
derivatives
A 10-ml round-bottom flask was charged with the mixture of phenanthraquinon
(1 mmol), aromatic aldehyde (1 mmol), ammonium acetate (6 mmol), and IL
(0.05 mmol). The mixture was stirred at 120 °C for 5–10 min. After completion of
the reaction (monitored by thin-layer chromatography, TLC), the mixture was
cooled to room temperature and extracted thoroughly with acetone (3 9 10 ml).
The combined extracts were filtered and the solvent was removed under reduced
pressure to afford the crude product, which was purified by recrystallization from
ethyl acetate. The catalyst which does not dissolve in acetone, remained in the
residue.
Selected spectroscopic data
2-p-Tolyl-1H-phenanthro[9,10-d]imidazole (5)
IR (KBr, cm-1): 3,407, 1,616, 1,525, 1,425. 1H NMR (CDCl3, 100 MHz) d: 2.40 (s,
3H), 7.40 (d, 2H, J = 8 Hz), 7.60–7.80 (m, 4H,), 8.30 (d, 2H, J = 8 Hz), 8.60 (d,
2H, J = 7.6 Hz), 8.75(d, 2H, J = 8 Hz).
123

H. Eshghi et al.
N,N-Diethyl-4-(1H-phenanthro[9,10-d]imidazol-2-yl)aniline (6)
IR (KBr, cm-1): 3,432, 3,068, 1,609, 1,525, 1,425. 1H NMR (DMSO, 100 MHz) d:
1.2 (t, 6H, J = 5.4 Hz), 3.45 (m, 4H), 6.81 (d, 2H, J = 8 Hz), 7.50-7.72 (m, 4H),
8.09 (d, 2H, J = 8 Hz), 8.53(d, 2H, J = 7.4), 8.83 (d, 2H, J = 7.6 Hz).
2-(4-Isopropylphenyl)-1H-phenanthro[9,10-d]imidazole (8)
mp 178 °C. IR (KBr, cm-1): 3,434, 1,615, 1,540, 1,425. 1H NMR (CDCl3,
400 MHz) d:1.35 (d, 6H, J = 6.8 Hz), 3.05 (m, 1H), 7.45 (d, 4H, J = 8.4 Hz), 7.75
(m, 8H), 8.34 (d, 4H, J = 8.4 Hz), 8.4 (d, 2H, J = 7.6 Hz), 8.68 (d, 2H, J = 8 Hz),
8.79 (t, 4H, J = 8.4 Hz). 13C NMR (CDCl3, 400 MHz) d: 23.8, 34.2, 120.8, 121.1,
123.0, 123.4, 123.78, 125.2, 126.0, 126.3, 127.0, 127.2, 127.3, 127.4, 128.9, 129.2,
152.2. m/z, calcd. for C24H20N2 [M]?: 336.16, found: 336.0.
2-(4-Hydroxyphenyl)-2H-phenanthro[9,10-d]imidazole (11)
IR (KBr, cm-1): 3,410, 3,357, 1,611, 1,552. 1H NMR (DMSO, 100 MHz) d: 6.95 (d,
2H, J = 8 Hz), 7.50–7.75 (m, 4H), 8.12 (d, 2H, J = 8 Hz), 8.52(d, 2H,
J = 7.4 Hz), 8.82 (d, 2H, J = 7.6 Hz), 13.20 (s, OH).
Results and discussion
Preparation and characterization of dicationic acidic ionic liquid
We designed and synthesized a novel AIL based on imidazolium cations with a
triethylenoxy spacer. In this protocol, we first synthesized 1,2-bis(2-(1H-imidazol-1-
yl)ethoxy)ethane (1) through the reaction between imidazole and 1,2-bis(2-
chloroethoxy)ethane in the presence of potassium hydroxide in DMSO. Then, for
acidic functionalization, we used chlorosulfonic acid in dry dichloromethane and an
AIL (2) was obtained in 95 % yield as a viscous oil (Scheme 1).
One of the key properties of Brønsted AILs is their Brønsted acidity which has
a strong correlation to the catalytic activity of this compound in the many organic
synthesis reactions [24]. SO3H-functionalized ILs have been employed as Brønsted
AIL catalysts for the synthesis of several acid-catalyzed organic reactions. In
recent years, there have been reports about Brønsted AILs based on the sulfonic
acid group which have good activity [25, 26]. The purpose of the preparation of
this novel AIL is to increase the solubility of the organic compounds in the AIL
through triethyleneoxy spacer that increase its flexibility and hydrophobisity. In
addition, longer oligo (ethylene glycol) chains may improve the hydrophobisity of
the overall AIL [29, 30]. In the synthesis of this novel AIL, the sulfonic acid
group is covalently tethered to the AIL imidazolium cation and this part of the
AIL can increase its hydrophilisity. So, features of both hydrophilisity and
hydrophobisity are responsible for increasing the dissolving of organic compounds
in this AIL. In addition, strong acidity by determination of pKa and high catalyst
123

A novel imidazolium based acidic ionic liquid
Scheme 1 Synthesis of acidic ionic liquid
activity for the synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazole were observed
for this AIL. This catalyst can be described as a sulfuric acid derivative with the
difference that the AIL participates in dissolving organic compounds by its
ethyleneoxy bridge.
pH measurement of dicationic AIL
The pKa value for the mentioned AIL was determined using a 0.1 mol L-1 solution
of IL and was titrated with 0.1 mol L-1 of NaOH. The pH of the solution was
measured using a calibrated glass electrode pH meter at 25 °C. As shown in Fig 1,
the pKa value of the AIL is 2.41. This novel AIL can be used as a catalyst in many
different organic transformations.
The application of this AIL has been studied in a new one-pot method for the
synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazole from 9,10-phenanthraquinone
with various aromatic aldehydes and ammonium acetate in the presence of a
catalytic amount of the AIL under solvent-free conditions (Scheme 2). The
corresponding desired products were isolated in excellent yields and the results are
summarized in Table 1. Our results show that these reactions are rapid and
completed in a few minutes. However, a logic substitution effect was not observed
by changing the R group in the aldehyde.
Thermal stability of dicationic AIL
The thermal stability of the dicathionic AIL was measured by a thermogravimetric
analyzer (TGA) and differential thermal analysis (DTA) under argon atmosphere
with a heating rate of 10 °C/min. The TGA and DTA curves obtained for
synthesized AIL are shown in Fig 2. Using TG–DTA, one can tell the melting or
decomposition points of the material. Thermal decomposition temperature (Td)
values for the mentioned AIL were obtained at 348 °C. It was observed that the
thermal stability of the synthesized dicationic AIL in this study was significantly
higher than that of many traditional imidazolium-based monocationic analogs [18,
19]. In the DTA and TGA curves, a weight loss (8.84 %) was observed at 131 °C.
With the loss of one equivalent HCl from AIL, a 7.5 % weight loss was calculated
with the remaining 1.34 % pertaining to water removal. So both HCl and water were
removed at 131 °C.
123

H. Eshghi et al.
8
7
6
5
4
3
2
1
0
0
5
10
15
20
25
30
35
40
NaOH (ml)
Fig. 1 pH meter titration curve of the novel dicationic AIL
Scheme 2 Synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazole derivatives
Molecular geometry of AIL
The optimized geometry of the synthesized dicationic AIL is shown in Fig. 2 and
the minimum energy geometries of the AIL were determined by performing
ab initio geometry optimizations at the RHF/6-31G level [31]. All calculations were
carried out using the Gaussian 09 series programs and the results are shown in
Table 2. In the optimized molecular structure of the AIL demonstrated in Fig 3,
there is a free HCl molecule which correlates with the acidity strength of sulfonic
acid and hydrochloric acid. On the other hand, the distances of hydrogen from the
nitrogen of the imidazolium ring, for two active sites, are H44–N35 = 2.57712 and
H46–N36 = 3.17257, which shows that the distance for one side of the free HCl
molecule is longer than the other side. This optimized structure reveals that the most
stable form of AIL is anti-direction of the HCl molecule and chloride and also that
the two imidazolium rings are opposite to each other.
Edgar et al. [32] isolated 9,l0-phenanthraquinone di-imine intermediates in the
reactions of phenanthraquinone, aldehydes, and ammonium acetate in acetic acid
solution and suggested a mechanism of reaction for imidazole formation under these
conditions. In their mechanism, the initial reaction is the formation of the 9,l0-
phenanthraquinone di-imine. The latter then undergoes an aminal condensation with
the aldehyde to form trisubstitutedimidazoles. In a possible mechanism, in the initial
step an aminal intermediate is formed which then reacts with phenantraquinone to
give the desired product (Scheme 3) [33].
123

A novel imidazolium based acidic ionic liquid
Table 1 Results of 2-phenyl-1H-phenanthro[9,10-d]imidazole derivatives synthesis using a novel acidic
IL (2)
Entry Aldehyde
Product
Time Yield Mp
(min) (%)
(°C)
Reported Ref.
1
5
7
5
4
5
7
7
93
88
91
87
94
95
95
312 314
[27]
2
3
4
5
6
7
339 341
[27]
254 254–255 [27]
276 275–276 [28]
290 290–292 [12]
257 259–260 [27]
320 319–320 [13]
123

H. Eshghi et al.
Time Yield Mp Reported Ref.
Table 1 continued
Entry Aldehyde
Product
(min) (%)
(°C)
8
5
5
7
5
92
92
85
82
178
–
–
9
272 271–272 [27]
10
11
a
233 235
[27]
350 [360
[27]
All compounds were characterized on the basis of 1H NMR and IR spectral data, which were consistent with those
reported in the literature
Because of high catalyst activity, the initial step, the formation of imine with
aromatic aldehyde, is very farapidst and aromatic aldehydes with electron-donating
and electron-withdrawing groups both gave the desired products in excellent yields
within very short times, and the electronic effect and the steric effects of substitutes
did not show significant effects on the yields and reaction times. For example, ortho-
substituted aromatic aldehydes such as 2-chlorobenzaldehyde (Table 1, entry 10)
gave 85 % yields in comparison to 87 % yields of para-substituted aromatic
aldehydes (Table 1, entry 4).
Many of the reported synthetic methods for the synthesis of trisubstituted
imidazoles suffer from one or more disadvantages such as harsh reaction conditions,
poor yields, prolonged time periods, and use of hazardous organic solvents in the
reaction (Table 3).
The synthesis of these heterocycles has usually been carried out in polar solvents
such as ethanol, methanol, acetic acid, and dimethylsulfoxide (DMSO), which give
complex isolations [16]. Therefore, the discovery of clean procedures and the use of
a green and eco-friendly catalyst with high catalytic activity and short reaction time
for the preparation of 2,4,5-triarylimidazoles has gained considerable attention.
123

A novel imidazolium based acidic ionic liquid
Fig. 2 Thermal analysis (TG–DTA) of synthesized dicationic RTILs
Table 2 The geometry parameters of AIL calculated at the RHF/6-31G basis set
Parameter
˚
Bond distance (A)
(N)2C–H
(N)2C–H–Cl
SO3–H
C3–H7 = 1.06790
H7–Cl48 = 3.07264
O43–H44 = 1.02740
C30–H33 = 1.06900
H33–Cl47 = 3.89662
O45–H46 = 1.93601
H46–Cl47 = 1.28266
H46–N36 = 3.17257
H–Cl
H
H
44–Cl48 = 1.84593
44–N35 = 2.57712
H–N
E (a.u.)
-2996.59632770
Dipole moment (D)
12.5536
Recycling of dicationic AIL
One of the advantages of this AIL is its ability to be recycled from the reaction
medium. We were able to readily separate the catalyst from the residue by washing
with ethyl acetate. The catalyst was recovered by evaporation of the water of the
aqueous layer at 50–60 °C in a vacuum oven which gave 95 % yields and could be
reused for the same experiment for four more times (Table 4). The 1H NMR
spectrum of the recycled AIL catalyst compared with the freshly prepared catalyst
and at significant purity was observed.
123

H. Eshghi et al.
Fig. 3 Optimized molecular structure of 2 AIL using RHF/6-31G
Scheme 3 The suggested mechanism of synthesis of 2-aryl-1H-phenanthro[9,10-d]imidazole derivatives
Table 3 Reported synthetic methods for the synthesis of trisubstituted imidazole (1)
Entry Catalyst
Solvent
Temperature( °C) Time
Yield (%) Ref.
1
2
3
4
5
NH3SO3
Ethanol
Reflux
Reflux
5 h
93
41
70
85
[1]
CH3COOH
Acetic acid
1 h
[2]
Cu(OAc)2/NaOH Alcohol/NH3(aq) Heating
2 h
[27]
–
DMSO
–
95
0.5 h
[3]
AIL
120
5 min 93
This work
Table 4 Recycling of AIL in the reaction of phenanthraquinone with N,N-Diethylbenzaldehyde and
ammonium acetate
Reusabilitya
AIL
95
1b
2c
3d
4e
Yield (%)f
92
90
91
89
a
Reusability of the recovered catalyst.
b–e
Reusability of the recovered catalyst in new runs from run 2(b) to run 5(e)
Isolated yields
f
123

A novel imidazolium based acidic ionic liquid
Conclusion
In conclusion, we have designed and synthesized a novel AIL based on imidazolium
cations and have successfully used it as a catalyst for the one-pot synthesis of 2-aryl-
1H-phenanthro[9,10-d]imidazole derivatives. The remarkable feature of this new
catalyst is its ethyleneoxy bridge which participates in dissolving organic
compounds. The application of this AIL is studied in a new one-pot method for
the synthesis of imidazole derivatives under solvent-free conditions. The advantages
offered by this protocol include reusability of the catalyst, high conversion, short
reaction time, and simple experimental procedure.
Acknowledgment We are grateful to Ferdowsi University of Mashhad Research Council for their
financial support of this work (GN: 3/28349).
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