Ag–TiO2 nanocomposite-catalyzed one-pot synthesis of 1,2,4,5-tetrasubstituted imidazoles: a green and benign approach

Article abstract of DOI:10.1007/s13738-021-02192-1

Ag–TiO₂ nanocomposite as an efficient catalyst was synthesized and characterized by AFM analysis. This nanocomposite acts as a heterogeneous and recyclable catalyst for the room temperature synthesis of 1,2,4,5-tetrasubstituted imidazoles from aldehydes, benzil, ammonium acetate and several anilines or aliphatic amines via one-pot four-component condensation reaction in H₂O as a green solvent. Ag–TiO₂ nanocomposite was simply recovered by filtration and was reused up to three times.

Full text of DOI:10.1007/s13738-021-02192-1

Journal of the Iranian Chemical Society
https://doi.org/10.1007/s13738-021-02192-1
ORIGINAL PAPER
Ag–TiO2 nanocomposite‑catalyzed one‑pot synthesis
of 1,2,4,5‑tetrasubstituted imidazoles: a green and benign approach
Soghra Alipour1 · Seyed Mohammad Vahdat1 · Fereshteh Chekin1
Received: 14 May 2020 / Accepted: 21 January 2021
© Iranian Chemical Society 2021
Abstract
Ag–TiO2 nanocomposite as an efcient catalyst was synthesized and characterized by AFM analysis. This nanocomposite
acts as a heterogeneous and recyclable catalyst for the room temperature synthesis of 1,2,4,5-tetrasubstituted imidazoles from
aldehydes, benzil, ammonium acetate and several anilines or aliphatic amines via one-pot four-component condensation reac-
tion in H2O as a green solvent. Ag–TiO2 nanocomposite was simply recovered by fltration and was reused up to three times.
Keywords Ag–TiO2 nanocomposite · Four-component condensation reaction · 1,2,4,5-tetrasubstituted imidazoles
Introduction
efective biological activities such as antifungal, anti-infam-
matory, antibiotic, anti-allergenic and antitumor [25–27].
Numerous approaches and catalysts have been described to
produce 1,2,4,5-tetrasubstituted imidazoles such as nano-
sized Cu/Zn-modifed MCM-41 [28], isocyanurate-based
periodic mesoporous organosilica [29], sodium lauryl sulfate
[30], {[2,6-DMPyH]C(NO2)3} [31], Fe3O4-polyethylene gly-
col-Cu nanocomposite [32], NiFe2O4 nanoparticle supported
phosphomolybdic acid [33], Wells–Dawson heteropolyacid
supported on silica [34], copper(I) complex of 1,3‐dimeth-
ylbarbituric acid-modifed SBA‐15 [35], peptide nanofber-
templated zinc oxide nanostructures [36] and nanoaluminum
nitride [37].
Heterogeneously catalyzed approaches have useful rule in
generalization of the procedure, easy separation, shrinking
of waste production and recycling of the catalysts [1–4]. It
is important for the green chemistry to build up efcient
and eco-friendly mild catalytic methods which have become
one of the chief themes of present synthetic chemistry [5,
6]. One of the most substantial feld in terms of its efcacy
is nano-catalysis [7]. These catalysts are identifed as efec-
tive techniques for increasing recyclable catalysts because of
their efortlessness of separation [8–11]. The nanoparticles
improve the revealed surface area of the active component of
the catalyst, thus improving the contact between substrates
and catalyst [12–16].
In continuation of our previous investigation on the
application of nano-Ag–TiO2 catalyst and other nano-metal
oxide catalysts in organic synthesis and multi-component
reactions [38–45], we herein report an efcient approach
to give 1,2,4,5-tetrasubstituted imidazoles with the aid of a
recyclable heterogeneous Ag–TiO2 nanocomposite catalyst
starting from aldehydes, benzil, ammonium acetate and sev-
eral anilines or aliphatic amines in H2O as a green solvent at
room temperature (Scheme 1).
Multi-component reactions (MCRs) are important
‘medicinal toolkit’ reactions approving fast assembly of
three or further components in a one-pot sequence to aford
a target product [17–20]. This infuential synthetic tool gives
an informal and suitable access to archives of organic prod-
ucts with various substitution frameworks [21, 22].
Among the numerous N-containing skeletons, imidazole
derivatives are main building blocks for the preparation of
biologically active molecules and pharmacologically active
compounds [23, 24]. Imidazole rings have a varied range of
Experimental
General methods
* Seyed Mohammad Vahdat
vahdat_mohammad@yahoo.com
Ag–TiO2 nanocomposite was purchased from commercial
centers and then characterized by atomic force microscope
1
Department of Chemistry, Ayatollah Amoli Branch, Islamic
Azad University, Amol, Iran
Vol.:(0123456789)
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Journal of the Iranian Chemical Society
Scheme 1 Synthesis of
1,2,4,5-tetrasubstituted
imidazoles by using Ag–TiO2
nanocomposite catalyst
NH4OAc
4
O
2
O
O
N
H
+
Ag-TiO2 nanocomposite
N
R'
(1 mol%)
R
R
NH2
R'
3
H2O (5 mL), r.t.
5
1
(AFM) and transmission electron microscopy (TEM) tech-
niques [38–40]. All reagents were purchased from Sigma-
Aldrich or Merck and were applied without purifcation.
Solvents were dried, distilled and stored over molecular
sieves. All the reactions and the purity of the products
were observed by TLC by using TLC aluminum sheets
silica gel F254. All melting points measured on a Thermo
Scientifc apparatus and also are uncorrected. The infra-
red spectra were recorded in KBr disks on a FT-IR Bruker
(WQF-510) instrument. The 1H NMR and 13C NMR spec-
tra were recorded on a Bruker DRX‐400 MHz instrument,
using TMS as an internal standard in DMSO-d6. Chemical
shifts are measured in ppm. The surface morphologies were
described by AFM.
Results and discussion
Characterization of Ag–TiO2 nanocomposite
AFM analysis was also performed to confrm the structure
morphology and topography of Ag–TiO2 nanocomposite
(Figure S1) and transmission electron micrograph showing
the size of nanocomposite (Figure S2) [38–40].
Synthesis of 1,2,4,5‑tetrasubstituted imidazoles
by using Ag–TiO2 nanocomposite
Firstly, we studied the condensation of 4-chlorobenzalde-
hyde, benzil, ammonium acetate and 4-chloroaniline as a
model of reaction in the presence of Ag–TiO2 nanocom-
posite in numerous conditions. As revealed in Table 1, we
used several solvents containing H2O, C2H5OH, CH3CN,
CH2Cl2, ethyl acetate, benzene and toluene (Table 1,
entries 1–7) at room temperature. According to reaction
time and yield of desired product, the suitable result was
attained by using 1 mol% catalytic amount of Ag–TiO2
nanocomposite in H2O as a solvent with respect to pos-
sessing a green nature and clean workup process (Table 1,
entry 1). Also, various amounts of Ag–TiO2 nanocompos-
ite (Table 1, entries 1 and 8–11) were used and the results
showed that 1 mol% of catalyst is appropriate (Table 1,
entry1).
General procedure for the synthesis
of 1,2,4,5‑tetrasubstituted imidazoles
To a solution of aldehydes (1 mmol), benzil (1 mmol; 0.210
g), ammonium acetate (1 mmol; 0.077 g) and several ani-
lines or aliphatic amines (1 mmol) in H2O (5 mL) at room
temperature with continuous stirring, Ag–TiO2 nanocom-
posite (1 mol%) was added and reacted for appropriate time
(Table 2). After completion of the reaction monitored by
TLC (n-hexane/ethyl acetate; 4:1), the precipitate was col-
lected by fltration. The fltrate was washed with ethanol and
dried to obtain desired pure products.
Moreover, in the absence of any catalyst, the reaction
did not progress and no desired product was attained even
when the reaction was heated under refux conditions in
H2O for 120 min, and the substrates were recovered together
(Table 1, entry 12), therefore suggesting the need of catalyst.
The results showed that on increasing the temperature from
room temperature to refux condition the desired product
was achieved in similar isolated yield. Additionally, the reac-
tion was carried out in the presence of several quantities of
starting materials. The best result was attained with a 1:1:1:1
molar ratio for 4-chlorobenzaldehyde, benzil, ammonium
acetate and 4-chloroaniline, respectively.
Selected spectral data analysis for desired product
1,2‑Bis(4‑chlorophenyl)‑4,5‑diphenyl‑1H‑imidazole (Table 2,
entry 25): White solid, M.p.: 187–189 °C; Yield: 85%;
FT-IR (KBr)(υmax, cm−1): 3050, 1486, 1089, 835; 1H NMR
(400 MHz, DMSO-d6): δ = 7.18–7.20 (d, 2H, J = 8.0 Hz,
Ar–H), 7.23–7.27 (dd, 4H, J=8.0 Hz and J=4.0 Hz, Ar–H),
7.30–7.34 (dd, 4H, J=8.0 Hz and J=4.0 Hz, Ar–H), 7.40–
7.43( dd, 6H, J=6.0 Hz and J=4.0 Hz, Ar–H), 7.48–7.50
(d, 2H, J = 8.0 Hz, Ar–H); 13C NMR (100 MHz, DMSO-
d6): 126.8, 127.1, 128.7, 128.9, 129.1, 129.2, 129.3, 129.5,
129.8, 130.5, 131.0, 131.6, 131.9, 133.8, 133.9, 134.5,
135.8, 137.5, 145.4.
Based on the above optimized conditions, we have taken
several aldehydes having diferent electron-withdrawing
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Journal of the Iranian Chemical Society
Table 1 Optimization of reaction condition for the synthesis of 1,2,4,5-tetrasubstituted imidazolesa
Entry
Solvent
Catalyst loading (mol%)
Time (min)
Yield (%)b
1
2
3
4
5
6
7
8
H2O
1
1
1
1
1
1
1
0.5
2
10
10
10
20
35
120
60
25
10
10
10
120
85
85
78
75
70
45
49
75
85
85
80
–
C2H5OH
CH3CN
CH2Cl2
Ethyl acetate
Benzene
Toluene
H2O
H2O
H2O
H2O
H2O
9
10
11
12
5
10
–
aReaction condition: 4-chlorobenzaldehyde (1 mmol; 0140 g), benzil (1 mmol; 0.210 g), ammonium acetate (1 mmol; 0.077 g), 4-chloroaniline
(1 mmol; 0.127 g), r.t.; b Isolated yield
and releasing groups in the ortho, meta and para posi-
tions and hetero-aromatic aldehydes for the synthesis of
1,2,4,5-tetrasubstituted imidazoles (Table 2). As summa-
rized in Table 2, the substituents of the aldehydes shown
displayed that the reaction time for the aromatic aldehydes
bearing electron-withdrawing substituents is rather faster
and with slightly higher yield than those for electron-
donating substituents. Furthermore, aniline derivatives
and aliphatic amines aforded appropriate yields of the
desired products.
ammonium acetate 4 and then nucleophilic attack of ammonia
to intermediate 7 to aford aminal intermediate 8. In the next
step, aminal intermediate 8 is condensed with activated benzyl
1′ to provide intermediate 9 which rearranges via elimination
of water to the 1,2,4,5-trisubstituted imidazoles target mol-
ecule 5.
To show the signifcant properties of our study, we have
compared our result with the identifed data from the other
works for the synthesis of 1,2,4,5-tetraphenyl-1H-imidazole
(Table 2, entry 7). For comparison, this target molecule was
chosen, as revealed in Table 3. No use of hazardous solvent,
short reaction time and high yields of products make this
method as a valuable approach for the synthesis of library of
1,2,4,5-trisubstituted imidazoles.
In another investigation, reusability of Ag–TiO2 nano-
composite was studied in model reaction. Subsequent to the
completion of the reaction, hot ethanol was added to the
reaction mixture and the resulted solid (recycled catalyst)
was fltered. The recycled catalyst was washed three times
with hot ethanol (3×5 mL) and dried at 80 °C and reused
at least six times without any loss of its catalytic activity
(Fig. 1). The structure of recycled catalyst was investigated
by AFM analysis, and the results showed that the catalyst is
stable (Figure S3).
So as to assess the efectiveness of Ag–TiO2 nanocom-
posite catalyst on the synthesis of 1,2,4,5-trisubstituted imi-
dazoles, turn over frequency (TOF) value was calculated
via the equation: TOF=yield of product (%)/[reaction time
(min)×amount of catalyst (mol%)], TOF value is min−1.
Also, turn over number (TON) was calculated via the equa-
tion: TON=[TOF×reaction time (min)], and TON is a useless
value (Table 4) [63–66].
A suggested mechanism for the synthesis of 1,2,4,5-trisub-
stituted imidazoles is shown in Scheme 2 [31]. The Ag–TiO2
nanocomposite catalyst activates the carbonyl functional
group of aldehyde 2 for the nucleophilic addition of anilines
or aliphatic amines 3 on the intermediate 2′ to give intermedi-
ates 6. The synthesis of intermediate 7 occurs via elimina-
tion of water. Ammonia is formed in situ by decomposition of
1 3

Journal of the Iranian Chemical Society
Table 2 Synthesis of 1,2,4,5-tetrasubstituted imidazoles in the presence of Ag-TiO2nanocomposite.a,b
46
46
46
49
49
[51]
[50]
[46]
[46]
[46]
[52]
[48]
[52]
[46]
[46]
[46]
[46]
[46]
[46]
[47]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[46]
[51]
[46]
aReaction condition: aldehyde (1 mmol), benzil (1 mmol; 0.210 g), ammonium acetate (1 mmol; 0.077 g), several anilines or aliphatic amines
(1 mmol), Ag–TiO2 nanocomposite (1 mol%); H2O (5 mL), room temperature; b Isolated yield
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Journal of the Iranian Chemical Society
Conclusion
In summary, we have revealed an efficient and benign
approach for one-pot, four-component synthesis of
1,2,4,5-trisubstituted imidazoles utilizing Ag–TiO2 nano-
composite catalyst in H2O as a green solvent at room tem-
perature. The combination of easy experimental processes
and efortlessness of recovery and reuse of catalyst as well as
the reaction medium is expected to provide to the progress of
a green approach for the synthesis of target molecules. High
isolated yields, improved reaction rates, eco-friendly and no
requirement of column chromatography for separation are
also the signifcant advantages of this procedure.
Fig. 1 Recyclability studies of Ag–TiO2 nanocomposite
Scheme 2 Proposed mecha-
nism for the synthesis of
Ag-TiO2
O
Ag-TiO2
1,2,4,5-trisubstituted imidazoles
Ag-TiO2
by using Ag–TiO2 nanocom-
H
H
O
posite
O
H
NH
H
2
6
R
R'
R
R
H2N
2'
R'
3
Ag-TiO2
NH4OAc
Ag-TiO2
H
4
H
N
R'
O
CH3CO2H
NH3
N
R'
1'
7
N
H
H
O
R
H
8
Ag-TiO2
H
N
R
N
N
R'
N
R'
R
Ag-TiO2
Ag-TiO2
5
9
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Journal of the Iranian Chemical Society
Table 3 Comparison of this procedure with some other procedures for the synthesis of 1,2,4,5-tetraphenyl-1H-imidazole
Entry
Reaction condition
Time (min)
Yield (%)a
Reported reference
1
2
3
4
5
6
7
8
Ag–TiO2 nanocomposite (1 mol%), H2O, r.t
TFA (20 mol%), solvent free, MW (150 W)
n-Bu4NBr (10 mol%), t-BuOH, 80 °C
Pivalic acid (1 equiv), DMSO/H2O (1:1), 140 °C, air
DBSA (20 mol%), H2O, refux
CuFe2O4 (0.05 g), solvent free, 90 °C
SLS (0.02 g), H2O, 90 °C
Zeolite H-ZSM 22 (0.08 g), C2H5OH, 140 °C
Nano-crystalline MgAl2O4 (0.05 g), C2H5OH, 60 °C
Fe2O3@TiO2-EG-CuII (2.5 mol%), solvent free, 100 °C
Fe3O4@SiO2@TiO2 (10 mol%), solvent free, 110 °C
Nano-TiO2@SiO2 (0.04 g), CH3OH, r.t
20
4
480
24–48 h
240
80
81
88
87
88
81
77
90
81
91
98
82
78
This work
[53]
[54]
[55]
[56]
[57]
[33]
[58]
[59]
60
15–30
12–25
25
15
180
9
10
11
12
[60]
[61]
[62]
aIsolated yield
Table 4 TON and TOF of products 1 to 35
1; TOF=3.12 min−1
2; TOF=3.20 min−1
3; TOF=4.05 min−1
4; TOF=3.16 min−1
5; TOF=4.00 min−1
TON=78
TON=80
TON=81
TON=79
TON=80
6; TOF=4.05 min−1
7; TOF=3.20 min−1
8; TOF=3.28 min−1
9; TOF=4.05 min−1
10; TOF=5.53 min−1
TON=81
TON=80
TON=82
TON=81
TON=83
11; TOF=5.60 min−1
12; TOF=4.15 min−1
13; TOF=5.53 min−1
14; TOF=5.73 min−1
15; TOF=4.05 min−1
TON=84
TON=83
TON=83
TON=86
TON=81
16; TOF=5.60 min−1
17; TOF=4.05 min−1
18; TOF=4.15 min−1
19; TOF=5.60 min−1
20; TOF=5.60 min−1
TON=84
TON=81
TON=83
TON=84
TON=84
21; TOF=8.60 min−1
22; TOF=8.10 min−1
23; TOF=5.47 min−1
24; TOF=8.30 min−1
25; TOF=8.50 min−1
TON=86
TON=81
TON=82
TON=83
TON=85
26; TOF=3.24 min−1
27; TOF=4.15 min−1
28; TOF=4.20 min−1
29; TOF=4.15 min−1
30; TOF=4.20 min−1
TON=81
TON=83
TON=84
TON=83
TON=84
31; TOF=4.10 min−1
32 TOF=5.60 min−1
33 TOF=5.73 min−1
34; TOF=4.15 min−1
35; TOF=5.67 min−1
TON=82
TON=84
TON=86
TON=83
TON=85
Acknowledgements The authors are thankful for the facilities provided
to carry out research in chemistry research laboratory at Ayatollah
Amoli Branch, Islamic Azad University.
Supporting Information
The supporting information includes AFM and TEM analy-
sis and spectral images of FT-IR, 1H NMR and 13C NMR of
selected product.
Compliance with ethical standards
Conflict of interest The authors declare no confict of interest.
Supplementary Information The online version contains supplemen-
tary material available at https://doi.org/10.1007/s13738-021-02192-1.
1 3

Journal of the Iranian Chemical Society
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