Catalytic activity of Cu nanoparticles supported on Fe3O 4-polyethylene glycol nanocomposites for the synthesis of substituted imidazoles

Article abstract of DOI:10.1039/c4nj00645c

In the present study, we carried out chemical synthesis and characterization for a Fe₃O₄-polyethylene glycol-Cu nanocomposite (Fe₃O₄-PEG-Cu). Firstly, poly(ethylene glycol) was functionalized using cyanuric chlo

Full text of DOI:10.1039/c4nj00645c

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Catalytic activity of Cu nanoparticles supported
on Fe3O4–polyethylene glycol nanocomposites
for the synthesis of substituted imidazoles
Cite this: New J. Chem., 2014,
38, 4555
Zohre Zarnegar and Javad Safari*
In the present study, we carried out chemical synthesis and characterization for a Fe3O4–polyethylene
glycol–Cu nanocomposite (Fe3O4–PEG–Cu). Firstly, poly(ethylene glycol) was functionalized using
cyanuric chloride (PEG-Cl4). Then PEG-Cl4 was linked with Fe3O4 nanoparticles via formation of covalent
bonds (Fe3O4–PEG). The Cu nanoparticles were supported by reducing copper ammonia complexes
with hydrazine hydrate on the surface of this nanocomposite. Nanoparticles have been characterized by
FT-IR, XRD, SEM, EDAX, TGA-DTA, AAS and VSM techniques. The catalytic activity of the Fe3O4–PEG–Cu
catalyst was evaluated for the synthesis of highly substituted imidazoles. This new catalyst was found to be
a highly active and green catalyst for the synthesis of 2,4,5-trisubstituted imidazoles and 1,2,4,5-substituted
imidazoles. The synthesized catalysts displayed magnetic properties, which allowed their fast separation
from the reaction medium using a simple magnet.
Received (in Porto Alegre, Brazil)
23rd April 2014,
Accepted 4th July 2014
DOI: 10.1039/c4nj00645c
www.rsc.org/njc
laborious and complex work-up and purification, the use of
hazardous organic solvents and expensive reagents, expensive
1. Introduction
Naturally occurring substituted imidazoles, as well as synthetic moisture-sensitive catalysts, long reaction time and low yields,
derivatives thereof, exhibit wide ranges of biological activities, occurrence of side reactions, and significant amounts of waste
making them attractive compounds for organic chemists. They are materials.1 Consequently, the development of a new catalytic
well-known as inhibitors of P38 MAP kinase, anti-inflammatory system is an important task for organic chemists to overcome
agents, plant growth regulators, fungicides, herbicides, anti- these shortcomings and fulfill the criteria of a mild, efficient,
thrombotic agents, and therapeutic agents. In addition, they and environmentally benign protocol for the synthesis of sub-
are used in photography as photosensitive compounds.1 Some stituted imidazoles.
substituted imidazoles are selective antagonists of the glucagon
Metal nanoparticles have attracted much attention due to their
receptors and inhibitors of the IL-1 biosynthesis.2 They are special optical, electronic and catalytic properties originating from
also the core structural skeleton in many important biological quantum size effects. Recently, metal nanoparticles have been
molecules like histidine, histamine, and biotin, as well as widely used in biomedicine, electronic devices, spectroscopy,
several drug moieties such as trifenagrel, eprosartan, and and especially in catalytic performances.6 Because of their large
losartan.3 Therefore, the development of novel synthetic methods surface area, these nanoparticles show high catalytic activity in
for their synthesis has attracted sustained interest in organic different types of organic reactions. Unfortunately, there are
synthesis.
two major drawbacks that greatly hinder them from being
The traditional methods for the preparation of both 2,4,5- applied in catalytic processes. First, the metal nanoparticles
substituted and 1,2,4,5-substituted imidazoles are based on the easily aggregate to large clusters due to their high surface
multicomponent reaction using 1,2-dicarbonyl compounds, energy, which results in the deactivation of the catalyst Second,
different aldehydes, and a nitrogen source.4 There are a number it is very difficult to separate and recover the metal nano-
of methods in the literature following this strategy with variation particles from the reaction solution owing to their small size.7
of parameters and catalysts.5 However, most of these synthetic Many approaches have been established to solve these two
methods suffer from one or more disadvantages such as issues. It has been verified through much investigation that
the utilization of a catalyst support to load the metal nano-
particles is an effective method in catalytic reactions. Therefore,
many materials have been explored to be candidates for catalyst
Laboratory of Organic Compound Research, Department of Organic Chemistry,
College of Chemistry, University of Kashan, P.O. Box: 87317-51167, Kashan, I.R.,
supports. Compared with other solid supports, the magnetic
supports show obvious superiority, as their superparamagnetic
Iran. E-mail: Safari@kashanu.ac.ir; Fax: +98 (0)361 5912397;
Tel: +98 (0)361 591 2320
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Scheme 1 Synthesis of 1,2,4,5-tetrasubstituted/2,4,5-trisubstituted imidazoles by a multi-component reaction of benzil, aldehydes, NH4OAc, and amine
by the Fe3O4–PEG–Cu catalyst under solvent-free conditions.
nature enables easy and effective separation of catalysts from the ZnO,26 and silica gel,27 have been reported. However, the applica-
reaction solution with the help of an external magnetic field.8
tion of the immobilized catalysts on solid supports frequently
In recent years, more attention has been paid to the magnetic suffers from many disadvantages, such as requiring additives,
core–shell materials with iron oxide cores and functional outer desorption from the solid support surface and also low reactivity
shells of polymers,9 silica,10 zeolites11 and carbon12 for a wide of the catalyst.28
range of applications in chemical separation and catalysis due to
Herein, the magnetite Fe3O4 nanoparticles were prepared by
their unique magnetic properties. However, to be a catalyst support, coprecipitation of Fe3+ and Fe2+ using ammonium hydroxide
the outer shell of polymer shell composites is usually required to (NH4OH) as a precipitating agent. PEG was functionalized using
have both high surface areas and good dispersion characteristics, triazine and PEG containing four chloride functional groups
which are important for maximizing the reactive activity of metal (PEG-Cl4) was obtained. The magnetite nanoparticles coated
nanoparticles.9 If core–shell structure composites use magnetic with PEG-Cl4 were prepared via formation of covalent bonds
catalysts with the Fe3O4 core and the polymer shell, the catalysts between chloride functional groups of PEG and hydroxyl groups of
can be recovered with a magnetic field. Hence, the magnetic nano- Fe3O4. Copper (Cu) nanoparticles were deposited onto the surface
composites are excellent supports for the catalysts, also magnetic of Fe3O4–PEG nanocomposites by reducing copper ammonia
separation from the reaction mixture is simple, economical, and complexes with hydrazine. The catalytic activity of the Fe3O4–
promising for industrial applications.9
PEG–Cu nanocomposites was proved for the synthesis of highly
Various polymers have been adopted as coating materials for substituted imidazoles (Scheme 1). The nanocomposites could
magnetic nanoparticles, such as dextran,13 chitosan,14 poly(ethylene be reused for the catalytic reaction after recycling 6 times with
glycol) (PEG),15 PLGA,16 poly(L-lactide) (PLA),17 poly(glycolide) high efficiency.
(PGA),18 etc.19 Among these, poly(ethylene glycol) (PEG) is known
as a typical non-toxic and biocompatible polymer. The PEG coating
could enhance the compatibility between nanoparticles and aqueous
2. Results and discussion
medium, prevent the particle surface from oxidation, reduce toxicity,
facilitate storage or transport, etc.15
The Cu nanoparticle supported Fe3O4–PEG nanocomposites
Moreover, immobilized copper nanoparticles on various solid were prepared following the procedure shown in Scheme 2.
supports as catalysts, such as Al2O3,20 carbon nanotubes,21 In the first step, the magnetite Fe3O4 nanoparticles were prepared
Fe3O4@SiO2,22 zeolites,23 activated carbon,24 Ti–Ce mixed oxide,25 by coprecipitation of iron(II) and iron(III) ions in alkali medium.
Scheme 2 Synthesis of Cu nanoparticles supported on Fe3O4–PEG nanocomposites.
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Then, PEG-Cl4 was synthesized through reaction between peaks at 2922 cmÀ1 (–CH2 stretching), 1719, 1626 and 1512 cmÀ1
cyanuric chloride and PEG–(OH)2.29 The end cyanuric chloride (CQN stretching) and at 1100 cmÀ1 (C–O stretching). These
segments of PEG-Cl4 were able to react with hydroxyl groups on results provided the evidence that PEG was successfully attached
the surface of the Fe3O4 MNPs via covalent bonds. The Fe3O4– to the surface of Fe3O4 MNPs.
PEG–Cu catalyst was obtained by reducing copper ammonia
Fig. 2(a–c) presents the SEM micrographs of Fe3O4 MNPs,
complexes using hydrazine hydrate on the surface of Fe3O4– Fe3O4–PEG and Cu–Fe3O4–PEG nanocomposites. Uniform
PEG composite nanoparticles. The structure of Fe3O4 MNPs, nanospheres of Fe3O4 with a diameter of B20 nm was observed
Fe3O4–PEG and Fe3O4–PEG–Cu nanocomposites, was evaluated in Fig. 2(a). Fig. 2(b) shows that Fe3O4–PEG nanoparticles still
using Fourier transform infrared spectroscopy (FT-IR), scanning retain the morphological properties of Fe3O4 except for a
electron microscopy (SEM), X-ray powder diffraction (XRD), energy slightly larger particle size and smoother surface, where PEG
dispersive spectrometry (EDS), vibrating sample magnetometry is uniformly coated on the Fe3O4 particles to form polymer
(VSM) and thermogravimetry (TG).
shells. Compared to the Fe3O4 MNPs, the SEM image shown in
The surface chemical structure of magnetic nanoparticles Fig. 2(b) demonstrates that Fe3O4–PEG nanocomposites are
was characterized by FTIR spectroscopy. Fig. 1 depicts the FT-IR nearly spherical with more than 50 nm in size. The SEM and
spectra of the naked Fe3O4 MNPs, PEG-Cl4 and Fe3O4–PEG FE-SEM images of Fe3O4–PEG–Cu (Fig. 2(c) and (d)) confirmed
nanocomposites. The Fe–O stretching vibration near 576 cmÀ1
,
that the nano-Cu combined with the surface of magnetic
O–H stretching vibration near 3430 cmÀ1 and O–H deformed nanocomposites in the form of embedding and their diameter
vibration near 1626 cmÀ1 were observed for both (Fig. 1(a) was about 10 nm.
and 1(c)). The IR spectrum of PEG-Cl4 (Fig. 1(b)) was characterized
The pure and composite Fe3O4 MNPs were characterized,
by the following absorption bands: the –CH2 stretching of PEG-Cl4 respectively, by X-ray powder diffraction using Cu Ka radiation
appearing at 2870 cmÀ1; CQN of cyanuric chloride segments (l = 1.5406 Å). Distances between peaks were compared to
at 1709, 1546 and 1510 cmÀ1; and C–O at 1109 cmÀ1. The the JCDPS card No. 01-1111 of the International Center for
significant features observed in Fig. 1(c) are the appearance of the Diffraction Data to determine crystalline structures. The X-ray
Fig. 1 The comparative FT-IR spectra of (a) Fe3O4MNPs, (b) PEG-Cl4 and (c) Fe3O4–PEG.
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diffraction patterns of bare magnetite, Fe3O4–PEG and Fe3O4–
PEG–Cu samples are shown in Fig. (3a–c), respectively. Fig. 3(a)
shows the spectrum of Fe3O4 MNPs prepared by co-precipitation.
The peaks agree with the standard Fe3O4 (cubic phase) XRD
spectrum. The crystallographic parameters calculated using soft-
ware provided by the Panalytical company (Holland) are: a = b =
c = 8.3941 Å, a = b = g = 90.00001, they are indexed to the cubic
spinel phase of Fe3O4. The average crystallite size D was calcu-
lated using Debye–Scherrer’s equation D = Kl/(bcos y), where K is
the Scherrer constant, l the X-ray wavelength, b the peak width of
half-maximum, and y is the Bragg diffraction angle. The crystal-
lite size thus obtained using this equation was found to be about
12 nm. The same characteristic peaks can also be found in Fig. 3b
and c, which illustrated that the characteristic peaks did not
change but only the peak intensity and width changed after being
coated with PEG and supported Cu nanoparticles, showing that
the crystalline structure of the composite nanoparticles did not
vary.30 No peaks of impurities were detected. In addition, Fig. 3(c)
represents the XRD pattern of Fe3O4–PEG–Cu nanocomposites,
showing that the peaks of both Fe3O4 and Cu appear and some
enhanced peak intensity is caused by overlapping of peaks. The
Cu peaks of Fe3O4–PEG–Cu composite nanoparticles agree well
with the standard Cu (cubic phase) XRD spectrum (JCDPS card
No. 85-1326).
The components of Fe3O4–PEG–Cu were analysed by using an
energy dispersive spectrometer (EDS) (Fig. 4). The EDX spectrum
shows the elemental composition (O, Fe and Cu) of Fe3O4–PEG–Cu.
To investigate the magnetic properties of Fe3O4 MNPs and
Fe3O4–PEG composite nanoparticles, their hysteresis loops were
Fig. 2 The SEM image of (a) Fe3O4 MNPs, (b) Fe3O4–PEG, (c) Fe3O4–PEG–Cu,
(d) FE-SEM Fe3O4–PEG–Cu and (e) SEM recovered Fe3O4–PEG–Cu.
Fig. 3 XRD patterns of (a) Fe3O4 MNPs; (b) Fe3O4–PEG; and (c) Fe3O4–PEG–Cu.
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in the polymer matrix, while the other stage beginning at about
280 1C was due to the decomposition of PEG. The weight loss of
nanocomposites is about 7% at 280–600 1C, corresponding to
the thermal decomposition of PEG chains over Fe3O4 nano-
particles. The organic materials and magnetite of the Fe3O4–
PEG nanocomposite are completely burned to generate gas
products and converted into iron oxides at the elevated tem-
perature, respectively.30 DTA diagrams indicate that Fe3O4–PEG
was decomposed in two steps at 62 and 286 1C which was
followed by an exothermic process. The first region, which
appeared below 62 1C, displayed a mass loss that was attributable
to the loss of adsorbed solvent or trapped water from the
nanocomposites. The exothermic process at higher temperature
(286 1C) could be mainly attributed to the decomposition of the
PEG polymer. Temperatures above this confirm previous results
in TGA. Thus, the catalyst was stable up to 286 1C, confirming
that it could be safely used in organic reactions at temperatures
in the range of 110 1C.
In the next step, we studied the efficacy of the nanocatalyst
in the synthesis of substituted imidazole derivatives. We investi-
gated the synthesis of highly substituted imidazoles in the
presence of Fe3O4–PEG–Cu under the thermal and solvent-free
conditions. Herein, we report a mild and efficient process for the
synthesis of substituted imidazole derivatives by solvent-free
condensation of an aldehyde, benzil, NH4OAc, and a primary
amine using Cu supported magnetic nanocomposites as nano-
catalysts (Scheme 1). A thorough literature survey showed no
reference concerning the use of copper nanoparticles on poly-
meric magnetic supports as catalysts for the synthesis of hetero-
cyclic compounds.
Fig. 4 The EDS spectrum of Fe3O4–PEG–Cu composite nanoparticles.
To optimize the reaction conditions for the synthesis of
1,2,4,5-tetrasubstituted imidazoles, the reaction of benzalde-
hyde, benzil, ammonium acetate, and aniline was used as a
model reaction. Reactions at different solvents, temperatures,
and various amounts of catalyst revealed that the best con-
ditions were solvent-free at 110 1C and 10 mol% of catalyst
(Table 1, entry 6).
Fig. 5 Magnetization curves obtained by VSM at room temperature of
Fe3O4 MNPs and Fe3O4 MNPs coated by PEG.
In order to examine the solvent effect, the reaction was
explored in EtOH, CH3CN, THF, and water. The reaction in
measured by VSM at room temperature. Fig. 5 shows their solvents required relatively longer reaction times, and afforded
magnetization curves. When the magnetic field is cycled between low yields of the product (Table 1, entries 1–4). The reaction
À8000 and 8000, zero coercivity for both samples is obtained, proceeded efficiently under solvent-free conditions, and the
which indicates that the saturation magnetization value (Ms) for 1,2,4,5-tetrasubstituted imidazoles were produced in high
bare MNPS and Fe3O4–PEG nanocomposites is 58.25 emu gÀ1 yields. Also, the effect of the nitrogen source was evaluated in
and 66.6 emu gÀ1, respectively. We would like to point out that carrying out the reactions by replacing NH4OAc with other
the obtained Ms values for both samples are lower than that of ammonium salts (Table 1) and NH4OAc was proved to be the
the bulk material (92 emu gÀ1).31 The reduction of measured most effective nitrogen source. The promising results obtained
saturation magnetization is due to rather small size of the in the case of synthesis of 1,2,4,5-tetrasubstituted imidazoles
Fe3O4 MNPs and the added mass of the monolayer (PEG) which encouraged us to test the developed protocols for the synthesis
was nonmagnetic on them.30
of 2,4,5-trisubstituted imidazoles.
Thermal gravimetric analysis (TGA) and differential thermal
After optimizing the reaction conditions, in order to explore
analysis (DTA) of the Fe3O4–PEG composite nanoparticles were the scope and generality of this protocol, various aldehydes and
also performed at the range of 25 to 600 1C, with a temperature amines were used as substrates for the synthesis of 1,2,4,5-
ramp rate of 10 1C minÀ1 under a nitrogen atmosphere (Fig. 6). tetrasubstituted imidazoles and 2,4,5-trisubstituted imidazoles.
As shown in Fig. 6, the first weight loss stage (below 150 1C) can The results are shown in Tables 2 and 3. These results show
be ascribed to the evaporation of water and solvent molecules that the reactions are equally facile with both electron-donating
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Fig. 6 TGA curves of (a) Fe3O4, (b) Fe3O4–PEG and (c) DTA curve of Fe3O4–PEG composite nanoparticles.
and electron-withdrawing substituents present on the aromatic were characterized by spectroscopic and elemental analysis data.
aldehydes and primary amines, resulting in high yields of the The known products were characterized by comparing their
corresponding imidazoles. Structures of the new compounds physical properties with those reported in the literature.
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Table 1 Condensation of benzil, benzaldehyde, benzylamine and ammonium
acetate using different catalysts, solvents and ammonium saltsa
Table 3 Solventless synthesis of 2,4,5-trisubstituted imidazoles catalyzed
by Fe3O4–PEG–Cu
Time Yield
Product (min) (%)
Entry Ar
mprep/mplit. (1C)
Catalyst
loading Nitrogen
(mol%) source
1
2
3
4
5
6
7
8
H
p-OMe
m-OMe, p-OMe 6c
p-Cl
m-OMe
2-Naphthyl
m-OMe, m-OMe 6g
m-OH, p-OMe
m-OH
m-NO2
m-Br
6a
6b
30
35
30
25
25
30
20
30
25
20
20
30
30
45
98
94
92
95
98
97
98
92
96
97
98
94
96
88
(271–273)/(270–272)36
(230–231)/(228–231)36
(214–216)/213–216 37
(260–262)/(260–261)37
(258–260)/(259–262)36
(274–275)/(273–276)29
(255–257)/(254–256)37
(215–217)/(214–216)37
(259–260)/(259)37
Temp. Time Yieldb
Entry Catalyst
Solvent (1C)
(min) (%)
6d
6e
6f
1
2
3
4
5
6
Fe3O4–PEG–Cu 10
NH4OAc EtOH
NH4OAc CH3CN 80
80
120 50
120 40
120 32
120 20
55 96
55 96
70 80
70 90
55 94
120 45
90 60
80 55
55 70
120 40
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
NH4OAc THF
NH4OAc H2O
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
NH4OAc Neat
80
100
120
110
100
110
110
110
110
110
110
110
110
110
110
110
110
6h
6i
6j
6k
6l
6m
6n
9
10
11
12
13
14
267–268/265–267 37
(302–303)/(301–303)37
230–232/230–23337
258–260/259–260 37
256–258/255–257 37
7
8
Fe3O4–PEG–Cu
5
p-Me
p-OH
p-(Me)2N
9
Fe3O4–PEG–Cu 15
10
11
12
13
14
15
16
17
18
19
Fe3O4
Nano-Fe3O4
Cu
Nano-Cu
Fe3O4–PEG
10
10
10
10
10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
Fe3O4–PEG–Cu 10
NH4Cl
Neat
300
300
300
—
—
—
rearrangement through the imino intermediate [B] yielded the
desired product.38
NH4ClO4 Neat
(NH4)2SO4 Neat
(NH4)2CO3 Neat
NH4SCN Neat
One of the purposes for designing this magnetic nanocatalyst
is to enable recycling of the catalyst for use in subsequent
reactions. The reusability of the nanocatalyst was tested upon
the reaction of benzil (1.0 mmol), benzaldehyde (1.0 mmol),
NH4OAc (2.5 mmol), and n-Pr amine (1.0 mmol), as the representa-
tive reactants, and in the presence of a recycled catalyst (10 mol%)
300 38
300 45
a
Reaction conditions: benzil (1.0 mmol), benzaldehyde (1.0 mmol),
aniline (1.0 mmol), NH4OAc (4.0 mmol), catalyst and solvent (10 mL), or
b
neat. Isolated yield.
The mechanism of formation of highly substituted imid- in order to study the recyclability of this heterogeneous catalyst
azoles mediated by the Fe3O4–PEG–Cu catalyst is given in under identical conditions. After each run, the catalyst was
Scheme 3. The reaction proceeds via the diamine intermediate [A], recovered from the reaction mixture by magnetic separation,
which is formed by the activation of an aldehyde carbonyl group washed with ethyl acetate, and subsequently dried at 100 1C and
by the Cu on the surface of nanocomposites. Condensation of then reused. After six consecutive reuses, the catalyst exhibited
diamine with 1,2-diketone followed by dehydration, and then almost identical catalytic activity (Fig. 7).
Table 2 Solventless synthesis of 1,2,4,5-tetrasubstituted imidazoles catalyzed by Fe3O4–PEG–Cu
Entry
Ar
R
Product
Time (min)
Yield (%)
mprep/mplit. (1C)
1
2
3
4
5
6
7
8
H
C6H5
C6H5
C6H5
C6H5
C6H5
C6H5
4-EtC6H4
4-EtC6H4
4-EtC6H4
n-Pr–NH2
n-Pr–NH2
n-Pr–NH2
n-Pr–NH2
n-Pr–NH2
5a
5b
5c
5d
5e
5f
5g
5h
5i
5j
5k
5l
5m
5n
55
55
45
55
55
50
50
50
40
40
40
35
35
35
96
94
98
92
95
96
97
95
98
95
92
98
98
96
(215–217)/(216–218)32
(187–188)/(185–188)32
(255–257)/(255)33
(172–174)/(171–173)34
(175–178)/(175–178)34
(150–152)/(149–151)32
(142–144)
4-Me
3-NO2
4-OMe
3,4-(OMe)2
4-Cl
H
4-Me
3-NO2
H
4-Me
4-NO2
3-NO2
4-Cl
(159–161)
9
(131–132)
10
11
12
13
14
(88–89)/(87–89)35
(80–81)/(78–83)35
161–163
141–142
(85–87)/(85–87)35
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Scheme 3 Proposed mechanism for the formation of 1,2,4,5-tetrasubstituted/2,4,5-trisubstituted imidazoles from benzil, aldehydes, NH4OAc, and
amine in the presence of Fe3O4–PEG–Cu.
Table 5 Comparison of catalytic efficiency of Fe3O4–PEG–Cu with
several catalysts in the literature for the synthesis of imidazole 6a
Time
(min)
Yield
(%)
Entry
Catalyst
Ref.
1
2
3
Fe3O4–PEG–Cu
Zr(acac)2
Zinc(II) [tetra(4-methylphenyl)]
porphyrin
30
150
70
98
90
94
This work
50
51
4
5
6
7
8
9
10
11
12
[Hbim]BF4
Montmorillonite K10
Zeolite
Polymer–ZnCl2
SSA
CAN
25
90
60
240
240
360
160
120
540
93
70
80
96
73
75
92
97
90
52
53
54
55
56
57
58
59
60
Fig. 7 Condensation of benzil, benzaldehyde, ammonium acetate and
n-Pr amine using recycled catalysts.
Yb(OTf)3
Nano-SnCl4ÁSiO2
L-Proline
Table 4 Comparison of catalytic efficiency of Fe3O4–PEG–Cu with
several catalysts in the literature for the synthesis of imidazole 5a
Fe3O4–PEG–Cu is an efficient and environmentally benign nano-
catalyst which could be used in the synthesis of a series of highly
substituted imidazoles.
Entry
Catalyst
Time (min)
Yield (%)
Ref.
1
2
3
4
5
6
7
8
Fe3O4–PEG–Cu
K5CoW12O40Á3H2O
PEG-400
55
160
360
720–900
498
120
60
190
120
96
95
86
92
82
92
95
75
This work
39
40
41
42
43
44
45
46
47
48
49
DABCO
3. Conclusions
InCl3Á3H2O
BF3/SiO2
In this study, we have described a simple and novel method-
ology for the synthesis of tri and tetrasubstituted imidazoles by
using Fe3O4–PEG–Cu as a highly effective and heterogeneous
catalyst. The advantages of this catalyst are green, inexpensive
and easy to prepare, easiness of separation, and recycled
several times. Moreover the mild reaction conditions, opera-
tional simplicity, giving the desired products in high yields,
SBPPSA
TiCl4ÁSiO2
9
SiO2/NaHSO4
[(CH2)4SO3HMIM]
Glycerol as solvent
NaH2PO4
92
85–95
96
10
11
12
160–190
180
35
90
A comparative study of the reported methods in the literature easy work-up, and purification of compounds by a nonchromato-
to synthesize 5a and 6a is compiled in Tables 4 and 5 to show the graphic method (crystallization only) are the key advantages of
efficiency of this catalyst. It is clear from the two tables that this protocol. We believe that this procedure is convenient,
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economic, and a user-friendly process for the synthesis of sub- 4.4. Preparation of the magnetic Fe3O4–PEG–Cu
stituted imidazoles of biological and medicinal importance.
nanocomposites
Firstly, a certain concentration of copper acetate (0.02 g) aqueous
solution was prepared. 0.20 g of Fe3O4–PEG was dispersed in the
blue solution for 15 min under ultrasonication in a water bath.
28 wt% ammonia aqueous solution was added dropwise up to
pH 13, and then hydrazine hydrate was added until a brown
aqueous solution appeared. After stewing for 10 min, the
product was separated using an external magnet and washed
using ethanol 3 times, dried at 50 1C for 12 h. In order to
evaluate the content of Cu supported on Fe3O4–PEG, the catalyst
was analyzed using atomic absorption spectroscopy (AAS) analyzers.
The results revealed that the content of Cu doped on Fe3O4–PEG
was 14% (w/w).
4. Experimental
4.1. Chemicals and apparatus
Chemical reagents in high purity were purchased from Merck
and Aldrich and were used without further purification. Melting
points were determined in open capillaries using an Electrothermal
Mk3 apparatus and are uncorrected. 1H NMR and 13C NMR spectra
were recorded using a Bruker DRX-400 spectrometer at 400 and
100 MHz respectively. FT-IR spectra were obtained with potassium
bromide pellets in the range of 400–4000 cmÀ1 using a Perkin–
Elmer 550 spectrometer. The elemental analyses (C, H, N) were
obtained using a Carlo ERBA Model EA 1108 analyzer carried out
on a Perkin–Elmer 240c analyzer. Nanostructures were charac-
terized using a Holland Philips Xpert X-ray powder diffraction
(XRD) diffractometer (CuK, radiation, l = 0.154056 nm), at a
scanning speed of 21 per min from 101 to 1001 (2y). Scanning
electron microscopy (SEM) was performed on a FEI Quanta
200 SEM operated at a 20 kV accelerating voltage. The samples
for SEM were prepared by spreading a small drop containing
nanoparticles onto a silicon wafer and drying almost com-
pletely in air at room temperature for 2 h, and then were
transferred onto SEM conductive tapes. The transferred sample
was coated with a thin layer of gold before measurement. The
purity of the compounds synthesized was monitored by TLC,
visualizing using ultraviolet light. The magnetic properties were
characterized using a vibrating sample magnetometer (VSM,
Lakeshore7407) at room temperature. Thermogravimetric analysis
(TGA) was performed on a PL-STA 1500 thermal analyzer at a
heating rate of 10 1C minÀ1 under N2 flow (40 cm3 minÀ1).
4.5. General synthesis for the preparation of
1,2,4,5-tetrasubstituted imidazoles
Fe3O4–PEG–Cu (10 mol%) as an efficient magnetic nanocatalyst
was added to a mixture of benzil (1 mmol), benzaldehyde
(1 mmol), NH4OAc (4 mmol), primary aliphatic and aromatic
amines (1 mmol). The resulting mixture was heated at 110 1C in
an oil bath under neat conditions for appropriate time till the
completion of the reaction. After the TLC (eluent: petroleum
ether : ethyl acetate: 7 : 3) indicates the disappearance of starting
materials, the reaction was cooled to room temperature. The
resultant solid was dissolved in acetone and the magnetic
catalyst was separated using an external magnet. The mixture
was concentrated on a rotary evaporator under reduced pres-
sure and the solid product obtained was washed with water and
recrystallized from acetone–water 9 : 1 (v/v) to offer the pure
1,2,4,5-tetrasubstituted imidazole derivatives.
4.5.1. Spectral data for new derivatives of
1,2,4,5-tetrasubstituted imidazoles
1-(4-Ethylphenyl)-2,4,5-triphenyl-1H-imidazole (C29H24N2, 5i).
Cream crystals; petroleum ether : ethyl acetate: 7 : 3 (v/v) = 0.79;
UV-Vis (CHCl3) lmax: 356 nm; IR (KBr) n: 3052 (C–H aromatic),
2964 (C–H aliphatic), 1601 (CQC), 1503 (CQN) cmÀ1; 1H NMR
(400 MHz, DMSO-d6): dH 1.11 (t, J = 7.6 Hz, 3H), 2.54 (q, J = 7.6 Hz,
2H), 7.13 (d, J = 7.9 Hz, 1H, H–Ar), 7.23 (d, J = 8.4 Hz, 2H, H–Ar),
7.29 (m, 4H, H–Ar), 7.36–7.38 (m, 3H, H–Ar), 7.46–7.48 (m, 3H,
H–Ar), 7.53–7.54 (m, 4H, H–Ar), 8.06–8.08 (t, J = 7.9 Hz, 2H,
H–Ar), ppm; 13C NMR (100 MHz, DMSO-d6): dC 15.9, 28.4, 121.1,
122.1, 123.6, 126.1, 126.7, 127.7, 128.3, 128.8, 130.4, 132.4,
139.1, 142.9, 154.3 ppm; anal. calcd for C29H24N2: C 86.97,
H 6.04, N 6.99%; found: C 86.94, H 7.05, N 6.01%.
4.2. Preparation of PEG-Cl4
PEG (Mn = 1000) was functionalized using cyanuric chloride
according to a reported procedure in the literature.29 In short,
PEG was dissolved in a solution of sodium hydroxide in water,
and it was then added to a solution of cyanuric chloride in
dichloromethane at 0 1C. The mixture was stirred for 1 h at
room temperature and it was refluxed for 6 h. It was then filtrated,
and the product was precipitated in diethyl ether as a white solid
compound in 70% yield.
4.3. Preparation of the magnetic Fe3O4–PEG nanocomposites
Fe3O4 MNPs were prepared using chemical coprecipitation
described in the literature.10 To synthesize Fe3O4–PEG com-
1-(4-Ethylphenyl)-2-(4-methylphenyl)-4,5-diphenyl-1H-imidazole
posite nanoparticles, 0.2 g (0.15 mmol) of functionalized PEG (C30H26N2, 5j). White powder; petroleum ether : ethyl acetate:
(PEG-Cl4) was dissolved in 20 mL of dried THF and added to a 7 : 3 (v/v) = 0.85; UV-Vis (CHCl3) lmax: 344 nm; IR (KBr) n:
reaction flask equipped with a magnet, argon, and vacuum 3029 (C–H aromatic), 2966 (C–H aliphatic), 1602 (CQC), 1510
inlets. Then, 0.1 g of Fe3O4 MNPs was added to this solution at (CQN) cmÀ1 1H NMR (400 MHz, DMSO-d6) dH 1.11 (t, J =
;
room temperature. The mixture was stirred at room temperature 7.6 Hz, 3H), 2.24 (s, 3H), 2.49–2.56 (q, J = 7.6 Hz, 2H), 7.06–7.08
for 1 h and then refluxed for 12 h. Finally, with the help of a (d, J = 7.9 Hz, 2H, H–Ar), 7.12 (m, 4H, H–Ar), 7.22–7.24 (m, 5H,
magnet, the PEG-coated Fe3O4 nanoparticles were harvested and H–Ar), 7.27 (m, 4H, H–Ar), 7.46–7.7 (t, J = 7.9 Hz, 3H, H–Ar),
repeatedly washed with THF. The Fe3O4–PEG nanocomposites ppm 13C NMR (100 MHz, DMSO-d6): dC 15.5, 21.2, 28.0, 126.8,
were dried at 50 1C under vacuum.
128.2, 128.6, 128.8. 128.9, 129.2, 131.0, 131.6, 134.8, 135.0,
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137.2, 138.2, 144.6, 146.7 ppm; M.S. (70 eV) m/z (%): 414 (M+, 98), Acknowledgements
179 (56), 165 (98), 89 (49), 79 (60); anal. calcd for C30H26N2: C 86.92,
H 6.32, N 6.76%; found: C 86.90, H 6.35, N 6.80%.
This study is part of Zohre Zarnegar PhD thesis entitled: ‘‘The
preparation of magnetic iron oxide nanostructuers and their
application in host–guest systems and some of the organic
reactions’’ which has been conducted in the University of
Kashan. We gratefully acknowledge the financial support from
the Research Council of the University of Kashan for supporting
this work by Grant NO. 363022/1.
1-(4-Ethylphenyl)-2-(3-nitrophenyl)-4,5-diphenyl-1H-imidazole
(C29H23N3O2, 5k). Yellow powder; petroleum ether : ethyl acetate:
7 : 3 (v/v) = 0.75; UV-Vis (CHCl3) lmax: 356 nm; IR (KBr) n:
3062 (C–H aromatic), 2961 (C–H aliphatic), 1604 (CQC), 1528
(CQN) cmÀ1 1H NMR (400 MHz, DMSO-d6): dH 1.12 (t, J =
;
7.6 Hz, 3H), 2.56 (q, J = 7.6 Hz, 2H), 3.81 (d, J = 7.9 Hz, 2H,
H–Ar), 7.25–7.27 (m, 4H, H–Ar), 7.30 (t, J = 7.0 Hz, 3H, H–Ar),
7.49–7.50 (t, J = 7.0 Hz, 3H, H–Ar), 7.64 (d, J = 7.9 Hz, 2H, H–Ar),
7.8 (d, J = 7.9 Hz, 1H, H–Ar), 8.05 (s, 1H), 8.11 (d, J = 8 Hz, 1H,
H–Ar) ppm; 13C NMR (100 MHz, DMSO-d6): dC 15.6, 28.2, 122.5,
123.2, 126.5, 126.8, 126.9, 127.1, 128.1, 128.6, 128.7. 128.8,
128.9, 129.0, 129.1, 129.2, 129.4, 130.3, 130.4, 130.5, 131.5, 132.2,
132.7, 134.2, 134.3, 134.5, 135.0, 137.2, 137.9, 144.1, 145.5,
148.0 ppm; anal. calcd for C29H23N3O2: C 78.18, H 5.208,
N 9.43, O 7.18%; found: C 79.25, H 5.04, N 9.41, O 6.30%.
References
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N 10.96, O 8.34%; found: C 75.12, H 5.56, N 10.83, O 8.47%.
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C 75.18, H 5.52, N 10.96, O 8.34%; found: C 75.18, H 5.51,
N 10.75, O 8.42%.
4.6. General synthesis for the preparation of
2,4,5-trirasubstituted imidazoles
In a 50 mL round bottom flask a mixture of benzil (1 mmol),
benzaldehyde (1 mmol), ammonium acetate (4 mmol), and
Cu–Fe3O4–PEG (10 mol%) was added at 110 1C in an oil bath
under solvent-free conditions. The pure 2,4,5-trisubstituted
imidazole derivatives were obtained by a similar method used
for obtaining 1,2,4,5-tetrasubstituted imidazoles.
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