An efficient multicomponent synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles catalyzed by a magnetic nanoparticle supported Lewis acidic deep eutectic solvent

Article abstract of DOI:10.1039/c9ra08074k

A mild and highly efficient reaction for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles catalyzed by a magnetically supported Lewis acidic deep eutectic solvent on magnetic nanoparticles (LADES?MNP) has been developed via one-pot multicomponent processes under solvent-free sonication. These reactions have good to excellent yields, mild conditions, and work-up simplicity. This method represents a new method for the preparation of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles. More importantly, LADES?MNP can be easily recovered by magnetic separation and reused five times without significant loss of catalytic activity.

Full text of DOI:10.1039/c9ra08074k

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An efficient multicomponent synthesis of 2,4,5-
trisubstituted and 1,2,4,5-tetrasubstituted
Cite this: RSC Adv., 2019, 9, 38148
imidazoles catalyzed by a magnetic nanoparticle
supported Lewis acidic deep eutectic solvent†
Thanh Thi Nguyen, Ngoc-Phuong Thi Le, The Thai Nguyen
*
and Phuong Hoang Tran
A mild and highly efficient reaction for the synthesis of 2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted
imidazoles catalyzed by a magnetically supported Lewis acidic deep eutectic solvent on magnetic
nanoparticles (LADES@MNP) has been developed via one-pot multicomponent processes under solvent-
free sonication. These reactions have good to excellent yields, mild conditions, and work-up simplicity.
Received 5th October 2019
Accepted 16th November 2019
This method represents
a new method for the preparation of 2,4,5-trisubstituted and 1,2,4,5-
DOI: 10.1039/c9ra08074k
tetrasubstituted imidazoles. More importantly, LADES@MNP can be easily recovered by magnetic
rsc.li/rsc-advances
separation and reused five times without significant loss of catalytic activity.
receiving much attention due to the advantages of their high
surface areas, good stability, catalyst loading capacity, easy
Introduction
synthesis and functionalization, good dispersion, facile sepa-
ration by an external permanent magnet.15–17 More recently,
owing to the unique properties, magnetic nanoparticle sup-
ported DES as a highly efficient and easily recoverable hetero-
geneous catalyst in organic synthesis.18–20
In past decade, deep eutectic solvents (DESs) have attracted
much attention in both reaction media and catalysts due to
their unique properties such as wide liquid range, biodegrad-
ability, excellent thermal stability, and negligible vapor pres-
sure.1,2 Among them, Lewis acidic deep eutectic solvents
(LADESs) have been intensively studied as efficient media for
organic syntheses.3,4 The stable and efficient LADESs can afford
the desired products in high yield and selectivity against the
conventional metal halide catalysts.5–11 However, some draw-
backs such as the use of a large amount of LADESs, difficulty in
catalyst recovery and product separation, which can cause
environmental pollution and a high-cost to the procedure, limit
their application in industrial processes. The development of
heterogeneous DES catalysts in organic reactions has become
a signicant area of research due to the advantages of these
materials over traditional homogeneous DESs such as easy
product separation, efficient recovery and reusability, and work-
up simplicity.12 However, the disadvantage of heterogeneous
catalysts is their lower catalytic activity and selectivity due to the
poor dispersion of heterogeneous catalysts.13 Moreover, the
tedious ltration and inevitable loss of heterogeneous catalysts
in the recovery can cause the high-cost issue when applied to
the industrial processes.14 Recently, magnetic nanoparticle
supported Lewis acidic ionic liquid catalysts have been
Imidazoles are important bioactive heterocyclic compounds
which have been proved to possess antitumor, antibacterial,
fungicides, herbicides, plant growth regulators, antiviral activi-
ties.21–24 Various protocols for preparation of imidazoles via
multicomponent reaction have been tested in literature, which
employs a three-component reaction of 1,2-diketone or ketomo-
noxime, aldehyde, and ammonium acetate, or four-component
reaction of 1,2-diketone, aldehyde, primary amines, and ammo-
nium acetate.25–28 Many of the synthetic approaches for the prep-
aration of imidazoles suffer from one or more drawbacks such as
drastic reaction conditions, low yields, by-products, prolonged
reaction time, volatile organic solvent, tedious work-up processes,
expensive and unrecyclable catalysts.29–32 Thus, the development of
efficient and environmentally benign catalysts is still highly
desirable. Herein, we disclose a one-pot multicomponent synthesis
of imidazole catalyzed by magnetic nanoparticle supported Lewis
acidic deep eutectic solvent (LADES@MNP) under solvent-free
sonication. Interestingly, no additives and no toxic reagents were
included in the current method.
Department of Organic Chemistry, Faculty of Chemistry, University of Science,
Vietnam National University, Ho Chi Minh City 721337, Vietnam. E-mail:
thphuong@hcmus.edu.vn; Tel: +84 903706762
Results and discussion
Firstly, magnetic nanoparticle supported Lewis acidic ionic
liquid was synthesized following the previous procedure, which
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c9ra08074k
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removed the coating process with TEOS. The procedure was
shown in Scheme 1. Magnetite nanoparticles (Fe3O4) were easily
prepared through a co-precipitation method. Subsequently,
these nanoparticles were coated using TEOS, functionalized
using (3-chloropropyl)triethoxy-silane in toluene under sonica-
tion, and further reacted with the [Urea]4[ZnCl2] to yield the
magnetically retrievable Lewis acidic deep eutectic solvent
LADES@MNP.
Fig. 1 illustrates the FT-IR spectra of LADES@MNP. The peak
at around 580, 1095, 1645, and 3419 cmÀ1 were attributed to the
absorption of Fe–O–Fe, Si–O–Si, and O–H vibrations, respec-
tively (Fig. 1b). The absorption peak at 2935 cmÀ1 was associ-
ated with stretching vibration of C–H groups in
(CH2)3Cl@SiO2@Fe3O4 (Fig. 1c). The peak around 1613, 1465,
1243, 3330, and 3449 cmÀ1 were caused by vibration of C]O, C–
N, N–H bond in amide groups, which conrms successful
immobilization. The characteristic peaks of LADES@MNP were
at the same wavenumbers with the precursors, with a small shi
due to the interaction between DES and the support. Besides,
the Raman peak at 485 cmÀ1 was assigned to the Zn–Cl
stretching vibrations (Fig. S1, ESI†). These results showed that
magnetite nanoparticles were successfully coated with the SiO2
and then the [Urea]4[ZnCl2].
Fig.
1
FT-IR spectrum of Fe3O4 (a), SiO2@Fe3O4 (b), (CH2)3-
Cl@SiO2@Fe3O4 (c), [Urea]4[ZnCl2] (d), and LADES@MNP (e).
Thermogravimetric analysis (TGA) of LADES@MNP dis-
LADES@MNP catalyst, the SEM image was recorded (Fig. 3a).
The image showed slight aggregation presumably due to the
interaction of the layer of DES surrounding with each other and
the SEM image showed the nano-sized structure of the
LADES@MNP. Also, the TEM image conrmed more detailed
structural information about the nano-sized structure. TEM
image showed a dark Fe3O4 surrounded by a grey shell of silica,
and the average size of the obtained particles are 15–25 nm
(Fig. 3b).
The energy-dispersive X-ray spectroscopy (EDX) of
LADES@MNP catalyst showed the presence of iron (Fe), silicon
(Si), carbon (C), nitrogen (N), oxygen (O), and chlorine (Cl)
components in the structure (Fig. 3a). The EDX revealed that the
mass percent of Zn was 26.69%. Also, the content of elemental
ꢀ
played thermal stability up to 250 C with a total loss of 40%
ꢀ
mass of organic species from the nanoparticles at 250À650 C
(Fig. S2, ESI†). The magnetic properties of the LADES@MNP
were recorded by vibrating sample magnetometry (VSM). Fig. S3
(ESI†) showed the magnetic hysteresis loops of Fe3O4 and
LADES@MNP. The saturation magnetization value of
LADES@MNP (30.8 emu gÀ1) was less than that of Fe3O4 (69.2
emu gÀ1) due to the silica coating and the organic graing on
the surface of Fe3O4. However, it was sufficient for the recovery
of the dispersed LADES@MNP from the reaction solution by
merely using an external magnet, indicating the prepared
catalyst could be separated without ltration or centrifugation.
The XRD pattern of the coated nanoparticles functionalized
DES was presented in Fig. 2. The diffraction peak positions and
the relative intensities of these signals in the XRD pattern of
LADES@MNP are consistent with the standard Fe3O4 sample
(COD 9006920), which conrm that the coating process does
not induce any phase change of Fe3O4. To determine the
morphologies and nanometer-sized particles of the
Fig. 2 X-ray diffraction pattern of LADES@MNP and Fe3O4. The
bottom row of tick marks shows standard magnetite pattern (COD
9006920).
Scheme 1 The preparation of LADES@MNP catalyst.
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Table 2 Effect of solventsa
Entry
Solvent
Time (min)
Isolated yield (%)
1
2
3
4
5
6
7
8
Tetrahydrofuran
1,4-Dioxane
Toluene
1,4-Dioxane
Chloroform
Dichloromethane
Acetonitrile
Dimethylsulfoxide
Dimethylformamide
Methanol
Water
Methanol : water (1 : 1)
Solvent-free
120
120
120
120
120
120
120
120
120
120
120
120
120
20
50
60
50
30
0
40
55
80
55
15
15
83
Fig. 3 SEM-EDX (a) and TEM (b) images of LADES@MNP.
9
10
11
12
13
Zn of the immobilized catalyst was found to be 277600 ppm
(corresponding to Zn2+ loading of 4.10 mmol gÀ1) based on
inductively coupled plasma mass spectrometry (ICP-MS)
analysis.
a
Condition reaction: benzil (0.5 mmol), benzaldehyde (0.5 mmol),
ammonium acetate (1.0 mmol), and LADES@MNP (7.5% mmol) in
the presence of solvent (1.0 mL) under ultrasound irradiation.
The preparation of trisubstituted imidazoles was chosen as
the model reaction for the catalytic testing activity of
LADES@MNP. Benzaldehyde (0.5 mmol), benzil (0.5 mmol),
and ammonium acetate (1 mmol) were sonicated under
a variety of experimental conditions. The results reported in
Tables 1–3 pointing out that optimal conditions were obtained
using catalytic amounts of LADES@MNP (7.5 mol%) under
solvent-free sonication for 150 min under solvent-free condition
afforded 2,4,5-triphenyl-1H-imidazole as the desired product in
83% isolated yield. The use of ZnCl2, as well as other Lewis acids
in optimized conditions, afforded comparably lower yields than
LADES@MNP. These results were exciting and vindicated the
feasibility of our approach.
catalyzed by the LADES@MNP was amenable to several
substrates of aromatic aldehydes with either electron-donating
or electron-withdrawing substituents under mild conditions
(Table 4, entries 1–13). The present method was highly useful
for the synthesis of trisubstituted imidazoles in high yields (64–
93%) with the absence of byproducts. Interestingly, hetero-
aromatic aldehyde such as 2-indole-carbaldehyde also afforded
the corresponding imidazoles in good yield (Table 4, entry 14).
Thus, the LADES@MNP acts as a highly efficient catalyst for the
preparation of polysubstituted imidazoles.
To widen the scope of the current method, the preparation of
tetrasubstituted imidazoles was screened through the multi-
component reactions of benzil, aromatic aldehydes, aniline,
and ammonium acetate under the optimized conditions of
trisubstituted imidazoles. Initially, the reaction was carried out
by using stoichiometric amount of ammonium acetate (0.5
mmol) but the desired product was obtained in low yield (62%).
If the amount of ammonium acetate was increased to 1.0 mmol,
the yield was 73% and the yields were slightly improved with
1.5 mmol and 2.0 mmol (74% and 76%, respectively). Thus, the
preparation of tetrasubstituted imidazoles was tested by using
aldehydes (0.5 mmol), benzil (0.5 mmol), aniline (0.5 mmol),
and ammonium acetate (1 mmol). The results were summarized
Inspired by these results, in order to study the scope of this
procedure, the reactions of a wide variety of aldehydes with
benzil and ammonium acetate were tested under the optimal
conditions (Table 4). Gratifyingly, the multicomponent reaction
Table
1 Screening catalysts for the synthesis of trisubstituted
imidazolesa
Entry
Catalyst
Isolated yield (%) Table 3 The amount of catalysta
1
2
3
4
5
6
7
8
Fe3O4
Cu(CH3COO)2
CrCl3
ZnCl2
AlCl3$6H2O
FeSO4
35
30
50
40
60
60
25
83
Entry
LADES@MNP (mol%)
Isolated yield (%)
1
2
3
4
5
6
0
2.5
5
7.5
10
15
20
75
60
83
83
83
FeCl3$6H2O
LADES@MNP
a
a
Condition reaction: benzil (0.5 mmol), benzaldehyde (0.5 mmol),
Condition reactions: benzil (0.5 mmol), benzaldehyde (0.5 mmol)
ammonium acetate (1.0 mmol), and catalyst (7.5% mmol) under ammonium acetate (1.0 mmol) and LADES@MNP under solvent-free
solvent-free sonication for 2 h.
sonication for 2 h.
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Table 4 One-pot synthesis of trisubstituted imidazoles in presence of LADES@MNP as a catalyst under solvent-free sonicationa
Entry
R
Product
Time (min)
Isolated yield (%)
Melt point (ꢀC)
1
2
3
4
5
6
7
8
H
2-OH
3-OH
4-OH
2-F
4-F
4- NO2
2-Cl
4-Cl
4-OMe
4-N(CH3)2
4-C(CH3)3
4-Me
1a
1b
1c
1d
1e
1f
1g
1h
1i
1k
1l
1m
1n
1o
120
120
180
120
120
120
120
120
120
120
240
120
120
180
83
93
80
91
76
64
82
88
85
84
88
94
89
93
276–277
212–213
263–264
264–265
217–218
253–254
202–203
212–214
267–268
258–259
258–259
285–286
231–232
306–307
9
10
11
12
13
14
3-Indolecarbaldehyde
a
Condition reaction: benzil (0.5 mmol), benzaldehyde (0.5 mmol), ammonium acetate (1.0 mmol) and catalyst (7.5% mmol) under solvent-free
sonication.
in Table 5. As can be seen, electron-withdrawing substituents FT-IR spectra of the fresh and used LADES@MNP for ve times
and electro-donating groups on the aromatic ring of aldehydes were observed with no considerable change in functionality
could proceed smoothly under solvent-free sonication. (Fig. 5).
Regardless of the steric effects of the 2-OH, 2-NO2, 2-COOH
groups, these substrates afforded the tetrasubstituted imidaz-
oles products in high yields. Remarkably, the reactions were
clean, the products were isolated easily, and no side products
were detected.
Table
5 One-pot synthesis of tetrasubstituted imidazoles in the
presence of LADES@MNP under solvent-free sonicationa
The plausible mechanism is disclosed in Scheme 2. The
mechanism are entirely consistent with previous litera-
ture.27,32,33 The LADES@MNP has Lewis acidic on Zn atoms on
the surface which can participate in the bonding with the
oxygen atoms on the carbonyl groups to accept nucleophiles.
Moreover, we believe that a partial contribution of the nano-
particles can facilitate a high active catalysis in our method
because nano-support features have a good dispersion in the
reaction mixture, just like a “quasi-homogeneous” catalyst.
The turnover numbers (TONs) should be useful to evaluate
the catalytic activity of solid acid. The synthesis of trisubstituted
and tetrasubstituted imidazoles using benzaldehyde as
substrate under optimized condition results in an attained TON
of 11.1 and 9.8, respectively. Besides, the recycling of the cata-
lyst is highly desirability for a greener process. The recovered
LADES@MNP was also tested for the synthesis of trisubstituted
and tetrasubstituted imidazoles (Fig. 4). LADES@MNP was
easily recovered by an external magnet and washed with acetone
and ethanol, dried under vacuum over 2 h at 80 ꢀC, and reused
in the next run. Interestingly, the catalyst was recovered in
almost quantitative yield from each run. In the recyclable test,
the catalytic activity of LADES@MNP could be preserved aer
ve consecutive runs, with a slight decrease in reaction yield.
Entry R1
Product Time (min) Isolated yield (%) Melt point (ꢀC)
1
H
2a
2b
2c
2d
2e
2f
2g
2h
2k
150
180
120
120
120
120
120
180
180
120
120
240
120
120
73 (62)b
78
80
82
70
90
84
74
71
90
93
88
75
213–217
250–254
202–205
285–289
213–216
194–198
194–197
184–188
174–177
207–209
294–298
194–197
187–190
200–202
2
3
4
5
6
7
2-OH
3-OH
4-OH
2-NO2
4-NO2
4-F
8
4-Cl
9
4-OMe
10
11
12
13
14
4-N(CH3)2 2l
2-COOH 2m
4-C(CH3)3 2o
4-Me
2-OH-5-
Me
2p
2q
93
a
Condition reaction: benzil (0.5 mmol), benzaldehyde (0.5 mmol),
aniline (0.5 mmol), ammonium acetate (1.0 mmol) and catalyst (7.5%
b
mmol) under solvent-free sonication. Ammonium acetate (0.5 mmol)
was used.
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Fig. 4 Catalyst recycling studies.
with silica by reuxing mixture Fe3O4, 3.0 mL ammonia solu-
tion and, 3.0 mL of tetraethylorthosilicate (TEOS) in 100 mL
ethanol for 8 h. Then, Fe3O4@SiO2 and (3-chloropropyl)trie-
thoxysilane (20.0 mmol, 4.820 g) in 100 mL toluene were
reuxed for 12 h. The prepared Fe3O4@SiO2@(CH2)3Cl was
separated, washed with ethanol and dried at 80 ꢀC for 6 h.
Besides, a mixture of urea (20.0 mmol, 1.200 g) and zinc chlo-
ride (5.0 mmol, 0.680 g) was stirred at 100 ꢀC until a clear
colorless liquid [Urea]4[ZnCl2] was obtained. Finally, Fe3O4@-
SiO2@(CH2)3Cl (3.000 g) and [Urea]4[ZnCl2] (20 mmol, 1.880 g)
were added round-bottomed ask and stirred vigorously at
100 ꢀC for 18 h. Aer completion of the reaction, the
LADES@MNP was collected by an external magnet, washed with
Scheme 2 Proposed mechanism for the synthesis of trisubstituted
and tetrasubstituted imidazoles in the presence of LADES@MNP.
ꢀ
ethanol (3Â 20 mL) and dried under vacuum at 60 C for 6 h.
There is a capability that the active species could migrate
from the LADES@MNP to the liquid phase, and the leaching
acted as a homogeneous catalyst. To explore the catalyst
leaching, the reaction of benzaldehyde, benzil, aniline, and
ammonium acetate catalyzed by LADES@MNP was examined
under optimized conditions. When the reaction time reached
60 min, acetone (10 mL) was added, and the LADES@MNP was
easily separated by an external magnet. The reaction mixture
was divided into two parts. The desired product of part 1 was
obtained in a 50% yield. Part 2 was removed the acetone and
heated under the above conditions for another 90 min to give
the product in 53% yield (less than 73% yield in general
procedure). Consequently, the leaching test exhibited no
contribution from homogeneous leached species of the catalyst.
Synthesis of polysubstituted imidazoles
Benzil (0.5 mmol, 105 mg), benzaldehyde (0.5 mmol, 53 mg),
and ammonium acetate (1.0 mmol, 77 mg) were added into
Experimental
Synthesis of LADES@MNP catalyst
The catalyst was prepared via a ve-step procedure in Scheme 1.
The mixture of FeCl2 and FeCl3 was dissolved in 200 mL water
under ultrasonic irradiation. The potassium hydroxide solution
(1.0 M) was added slowly until pH 12.0. Aer 120 minutes, Fe3O4
was washed by water and ethanol to pH 7.5, separated by
ꢀ
a magnet, and dried at 80 C for 12 h. Next, Fe3O4 was coated Fig. 5 FT-IR spectra of the fresh catalyst and the reused catalyst.
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a vessel with LADES@MNP (7.5% mmol, 11 mg) for the
synthesis of 2,4,5-triphenyl-1H-imidazole. Benzil (0.5 mmol, 105
mg), benzaldehyde (0.5 mmol, 53 mg), aniline (0.5 mmol, 47
mg) and ammonium acetate (1.0 mmol, 77 mg) were added into
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Conclusions
In conclusion, we have developed a novel magnetic nano-
particle supported Lewis acidic deep eutectic solvent and used
as a catalyst in the one-pot multicomponent reactions to afford
2,4,5-trisubstituted and 1,2,4,5-tetrasubstituted imidazoles in
good to excellent yields for the rst time. Prominent features of
the current method include high yield, milder and cleaner
reaction, and work-up simplicity, magnetic separability, and
reusability of the catalyst making it an exciting alternative to the
existing methodologies.
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Conflicts of interest
There are no conicts to declare.
Acknowledgements
This research is funded by Vietnam National Foundation for
Science and Technology Development (NAFOSTED) under grant
number 104.01-2019.26.
26 M. B. Gawande, V. D. Bonifacio, R. Luque, P. S. Branco and
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