Homoselective synthesis of 5-substituted 1H-tetrazoles and one-pot synthesis of 2,4,5-trisubstuted imidazole compounds using BNPs@SiO2-TPPTSA as a stable and new reusable nanocatalyst

Article abstract of DOI:10.1002/aoc.6144

Considering the importance of tetrazole and imidazole derivatives in pharmacy, industry, and explosives, BNPs@SiO₂-TPPTSA was easily prepared and used as an effective, stable, and renewable nanocatalyst for the homoselective synthesis of different 5-substituted 1H-tetrazoles and atom economic synthesis of 2,4,5-trisubstituted-1H-imidazoles in solventless conditions. BNPs@SiO₂-TPPTSA was characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy dispersive X-ray analysis (EDX), thermal gravimetric-differential thermal analysis (TGA-DTA), mapping, pH analysis, and Fourier transform infrared (FT-IR) techniques. Furthermore, the catalyst recycled for at least sequential five loads without a remarkable drop-in catalytic activity.

Full text of DOI:10.1002/aoc.6144

Received: 11 September 2020
DOI: 10.1002/aoc.6144
Revised: 11 December 2020
Accepted: 19 December 2020
F U L L P A P E R
Homoselective synthesis of 5-substituted 1H-tetrazoles and
one-pot synthesis of 2,4,5-trisubstuted imidazole compounds
using BNPs@SiO2-TPPTSA as a stable and new reusable
nanocatalyst
Minoo Khodamorady1
| Nazanin Ghobadi2,3
| Kiumars Bahrami1,4
1Department of Organic Chemistry,
Faculty of Chemistry, Razi University,
Kermanshah, Iran
Considering the importance of tetrazole and imidazole derivatives in
pharmacy, industry, and explosives, BNPs@SiO2-TPPTSA was easily prepared
and used as an effective, stable, and renewable nanocatalyst for the
homoselective synthesis of different 5-substituted 1H-tetrazoles and atom eco-
nomic synthesis of 2,4,5-trisubstituted-1H-imidazoles in solventless conditions.
BNPs@SiO2-TPPTSA was characterized by transmission electron microscopy
(TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), energy
dispersive X-ray analysis (EDX), thermal gravimetric-differential thermal
analysis (TGA-DTA), mapping, pH analysis, and Fourier transform infrared
(FT-IR) techniques. Furthermore, the catalyst recycled for at least sequential
five loads without a remarkable drop-in catalytic activity.
2Department of Chemistry, School of
Science, Alzahra University, Tehran, Iran
3Department of Chemistry and
Biochemistry, Ohio University, Athens,
OH, USA
4Nanoscience and Nanotechnology
Research Center (NNRC), Razi University,
Kermanshah, Iran
Correspondence
Kiumars Bahrami, Department of Organic
Chemistry, Faculty of Chemistry, Razi
University, Kermanshah 67144-14971,
Iran.
K E Y W O R D S
Email: k.bahrami@razi.ac.ir;
kbahrami2@hotmail.com
2,4,5-trisubstituted imidazoles, BNPs@SiO2-TPPTSA, heterogeneous nanocatalyst,
multicomponent reactions, tetrazole
Funding information
Razi University
1 | INTRODUCTION
and have recyclability as heterogeneous catalysts.[2]
Nanoparticles are the bridge between heterogeneous and
In recent decades, nanocatalysis is one of the most pro-
voking subfields of catalysis. Its main aim is to speed up
some useful chemical reactions by varying the size, mor-
phology, or surface chemical compositions of the
nanomaterials with elite and tunable chemical activity,
specificity, and selectivity. Today, chemistry synthesis is
moving towards greener techniques of synthesis.
Nanocatalyst systems play an important role in the green
exchange.[1]
homogenous catalysts.[2a,3]
Boehmite is an oxyhydroxide (γ-AlOOH) with the
orthorhombic structure with the surface covered by
hydroxyl groups and has a good surface-to-volume
ratio.[4] Thermal, chemical, and mechanical stability,
good specific surface area, and easy to carry and low-cost
required materials, synthesizing in mild conditions and
using water as solvent, are reasons for introducing
boehmite as an ideal support.[5]
Nanostructured materials are attractive candidates as
heterogeneous catalysts, which not only have the merits
of homogeneous catalysts such as selectivity and activity
but also are readily removed from the reaction mixture
Sol–gel, sediment, solvothermal, and hydrothermal
are diverse methods to produce γ-AlOOH. In the mean-
time, the most popular method to produce different
forms of boehmite is the hydrothermal method. In
Appl Organomet Chem. 2021;35:e6144.
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© 2021 John Wiley & Sons, Ltd.
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https://doi.org/10.1002/aoc.6144

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KHODAMORADY ET AL.
many protocols for the construction of tetrazole deriva-
tives have been reported, but the development of useful
techniques is still needed because of some drawbacks of
previous methods like the use of poisonous metals, usage
of organic solvents in particular (DMF), strong Lewis
acids, rigid reaction conditions, high-cost reagents, low
yields, and use of hydrazoic acid, which is poisonous and
dangerous.[26] Highly substituted imidazoles are impor-
tant molecules in nature and industries. This class of het-
erocyclic compounds has prevalent uses in the
pharmaceutical industries (Figure 2) and agriculture and
is known as light-sensitive substances in photography,
inhibitors, plant growth regularizes, and therapeutic
agents.[27]
FIGURE 1 Some drugs based on tetrazoles
addition, some factors like temperature, reaction time,
and solution pH strongly affect the size, shape, and mor-
phology of boehmite's nanoparticles.[6]
Because the 2,4,5-trisubstituted-imidazoles and the
5-substituted tetrazoles have particular importance in
the pharmaceutical, textiles, and explosives industries,
we were interested in the synthesis of these
compounds.[7]
One of the most important reactions in organic chem-
istry
is
borrowing
hydrogen
reactions
and
dehydrogenative cyclizations.[28] This reaction involves
dehydrogenation and hydrogenation processes and is
used to form carbon–nitrogen and carbon–carbon
bonds.[29] This method is an effective and valuable
method for the synthesis of amines, ketones, benzimid-
azoles, and aromatic ring compounds.[30]
A significant class of heterocyclic substances is
tetrazoles. They consist of a five-membered ring con-
taining four nitrogen atoms and one carbon atom. The
most important reason for these compounds to be com-
mon in pharmaceutical and medicinal chemistry is their
ability to regard as a biological equivalent for the
carboxylic acid group due to the equivalent strength
acidic of the free N–H group.[7] Tetrazole derivatives
display numerous biological properties, including
antimicrobial,[8] anti-inflammatory,[9] anticancer,[10]
antibacterial,[11] anticonvulsant,[12] and as a component
of the information recording systems[13] (Figure 1).
Keller in 1932 first synthesized tetrazoles from the
reaction of hydrazoic acid (HN3) and cyanides.[14]
1,3-Dipolar cycloaddition is an effective route for the
preparation of tetrazoles.[15] Up to now, varied catalysts
like Lewis acids, AgNO3,[16] Fe(OAc)2,[17] In(OTf)3,[18]
BF3-OEt2,[19] Yb(OTf)3,[20] Cu2O,[21] modified montmoril-
lonite clays,[22] MNPs@NHC(O)CH2CH2PPh2,[23] and
natrolite zeolite[24] have also been documented to the
preparation of 5-substituted 1H-tetrazoles.[25] So far,
The first imidazole was prepared by Heinrich Debus
in 1858. So far, different protocols have been documented
in the scientific papers for the formation of
2,4,5-trisubstituted imidazoles, and in most of these
methods, imidazoles are obtained from the reaction of
1 and 2 diketones with aldehydes and ammonium salts.
For the synthesis of 2,4,5-trisubstituted imidazoles,
several protocols have been reported using catalysts such
as nano Fe3O4@SiO2–OSO3H,[31] L-proline,[32] montmo-
rillonite K10, and nanosulfated zirconia,[33] zeolites
and [EMIM]OAc,[34] graphene oxide–chitosan,[35] silica
gel/NaHSO4,[36] MCM-41,[37] Fe3O4@SiO2–OSO3H,[38]
propylpiperazine N-sulfamic acid,[39] NiCl2.6H2O with
acidic alumina,[40] [HeMIM]BF4,[41] and CoFe2O4
nanoparticles.[42]
In the last decade, multicomponent reactions (MCRs)
have gained remarkable attention in chemistry due to sig-
nificant features including great synthetic efficiency, the
simplicity of operation, less waste production, reduce
cost and time, atom economic, easy separation, and
purification.
FIGURE 2 Some drugs with imidazole structure

KHODAMORADY ET AL.
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strength shows that BNPs@SiO2-TPPTSA in saturated
NaCl aqueous solution has a low pH value of 1.92.
2.1 | Catalyst characterization
Transmission electron microscopy (TEM), field emission
scanning electron microscopy (FESEM), X-ray diffraction
(XRD), energy dispersive X-ray analysis (EDX), mapping,
thermal
gravimetric-differential
thermal
analysis
(TGA-DTA), pH analysis, and the Fourier transform
infrared (FT-IR) techniques were used to characterize
and confirm the synthesis of the BNPs@SiO2-TPPTSA.
The morphology and size of the catalyst have been
affirmed using TEM and SEM. As explained below, all of
these techniques confirmed the modification of the
boehmite surface.
TEM image exhibited the nearly orthorhombic
structure for BNPs@SiO2-TPPTSA nanoparticles, and the
average particle size is estimated to be 40–60 nm
(Figure 3a). FESEM image of the final nanocatalyst
demonstrated that the shape of the particles is spheral,
and they are well dispersed (Figure 3b). Based on
Figure 3b, particle size was estimated at 65–110 nm. The
FESEM of the BNPs is presented in Figure 3c, and the
particles are distributed uniformly, and the average
particle size for BNPs is 80–100 nm.
SCHEME 1 Synthesis of 5-substituted 1H-tetrazoles and
2,4,5-trisubstituted imidazoles with BNPs@SiO2-TPPTSA
Due to the special properties of boehmite mentioned
above, and the ease of synthesis and modification of its
surface, and the use of cheap raw materials for boehmite
synthesis compared with other supports, we decided to
use boehmite nanoparticles (BNPs) as a solid substrate.
In light of this, we introduce triphenylphosphine
trisulphonic acid (TPPTSA) anchored on BNPs as highly
reusable organometallic catalyst (denoted as BNPs@SiO2-
TPPTSA) for the homoselective synthesis of 5-substituted
1H-tetrazoles using sodium azide, malononitrile, and
substituted aldehydes using a [2 + 3] dipolar cycloaddi-
tion reaction in solvent-free conditions (Scheme 1)
and highly atom economic one-pot preparation of
2,4,5-trisubstituted imidazoles using benzoin or benzil,
different aromatic aldehydes, and ammonium acetate in
solvent-free conditions at 100ꢀC (Scheme 1).
Grafting of the PPh3 and SO3H groups on the surface
of boehmite is corroborated by FT-IR spectra (Figure 4).
In Figure 4a, the absorption peaks at 494.10, 650.27, and
766.18 cm−1 are related to the Al–O, and Si–O stretching
frequency peak is appeared at 927.66 cm−1 [43]
Also
.
2 | RESULT AND DISCUSSION
strong signal at 1095.80–1300 cm−1 corresponds to the
hydrogen bond (OH …. OH) between the boehmite
plates, and OH stretching frequency appeared in 3411.24
and 3473.93 cm−1. FT-IR spectra of BNPs@TEOS and
The BNPs@SiO2-TPPTSA was prepared by the route
depicted in Scheme 2. The measurement of acidity
SCHEME 2 Schematic preparation of
BNPs@SiO2-TPPTSA

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KHODAMORADY ET AL.
FIGURE 3 (a) Transmission electron
microscopy (TEM) image, (b) field emission
scanning electron microscopy (FESEM) of
the BNPs@SiO2-TPPTSA, and (c) FESEM of
the boehmite nanoparticles (BNPs)
BNPs@SiO2-TPPTSA (Figure 4d). Of course, the OH
stretching
frequency
may
be
appearing
in
3100–3500 cm−1. A broad vibration peak at 1000–1300
cm−1 is related to the Al–O stretching and bending
frequencies of hydrogen bands of boehmite plates (OH …
OH). This peak overlaps with the stretching vibrations of
p-Ar at 1090 to 1130 cm−1 and S-O at 1030 to 1275 cm−1
(Figure 4).
EDX analysis was used to determine the essential
elements in the structure. EDX patterns of all synthetic
steps are shown in Figure 5. EDX patterns of BNPs
(Figure 5a), BNPs@TEOS (Figure 5b), EDX pattern of
BNPs@SiO2(CH2)3PPh3+Cl− (Figure 5c), and eventually
EDX pattern of BNPs@SiO2-TPPTSA are illustrated in
Figure 5d. Based on Figure 5d, the presence of Si (35.7%),
O (20.1%), S (12.1%), Al (15.5%), P (3.9%), Cl (3.5%), and
C (9.2%) in the structure of the BNPs@SiO2-TPPTSA is
confirmed. Also, the surface of the nanocatalyst has been
coated with SO3H groups (Figure 5d).
Another technique used to identify the catalyst is
mapping, which shows the distribution of elements, and
as we can see from the mapping images, elements P,
C, O, Si, S, and Al are well homogeneously dispersed
throughout the selected area of the SEM image
(Figure 6).
FIGURE 4 Fourier transform infrared (FT-IR) spectra of
(a) boehmite nanoparticles (BNPs), (b) BNPs@TEOS,
(c) BNPs@SiO2(CH2)3PPh3+Cl−, and (d) BNPs@SiO2-TPPTSA
BNPs@SiO2(CH2)3PPh3 are depicted in Figure 4b,c. As a
result, due to the grafting of linker and ligand on the
boehmite surface, all frequencies have shifted to the
original BNPs, and the PPh3 stretching frequency appears
in 1630.25 cm−1 (Figure 4c). After sulfonating of the
BNPs@SiO2(CH2)3PPh3, C C stretching adsorption of
aromatic rings appeared at 1631.25 cm−1 and shifting the
absorption frequency of PPh3 to a lower frequency due to
the attachment of SO3H to the surface of the benzene
ring of PPh3. Also, broad absorption at 3100–3500 cm−1 is
a good reason for the presence of the SO3H group on the
The XRD spectra of (a) BNPs, (b) BNPs@SiO2,
(c) BNPs@SiO2(CH2)3PPh3+Cl−, and (d) BNPs@SiO2-
TPPTSA are depicted in Figure 7. In Figure 7b,c,
diffraction peaks that appeared at 20.50ꢀ, 21.55ꢀ, 24.00ꢀ,

KHODAMORADY ET AL.
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FIGURE 5 Field emission scanning
electron microscopy (FESEM) images
and energy dispersive X-ray analysis
(EDX) patterns of (a) boehmite
nanoparticles (BNPs), (b) BNPs@SiO2,
(c) BNPs@SiO2(CH2)3PPh3+Cl−, and
(d) BNPs@SiO2-TPPTSA
and 29.80ꢀ are related to the amorphous silica groups
on the catalyst surface, which are not seen in the XRD
pattern of boehmite. Besides, in all XRD curves shown
in Figure 7, the peaks that appeared in 2Ѳ: 30.11ꢀ
(120), 32.20ꢀ (110), 41.15ꢀ (031), 45.30ꢀ (060), 50.35ꢀ
(051), 56.1ꢀ (151), 59.1ꢀ (112), and 63.45ꢀ (132),
are associated with the boehmite crystalline
structure.[1a,5a,44]
Bohemite nanoparticles have an orthorhombic struc-
ture, but as silica increases, the spectrum widens, and the
structure shifts from crystalline to amorphous. It is note-
worthy that in all XRD spectra, the boehmite structure
has been preserved, and most of its main peaks
have appeared. It should be noted that the structure of
nanobohemite is nearly orthorombic based on difraction
peaks appearing in all spectra. After step-by-step modifi-
cation of the boehmite surface, only the intensity of the
peaks decreased slightly, and a slight displacement was
observed at the peaks (Figure 7).
Physical information such as absorption, desorption,
and phase transfer and chemical information like the
thermal decomposition, chemisorption, and solid–gas
reactions are obtained from TGA. As the TGA survey
indicated, physically adsorbed water and organic solvent
mass loss (21%) can occur at temperatures below 200ꢀC.
A strong endothermic peak at 92.8ꢀC in the DTA curve
(Figure 8b) was attributed to solvents and volatile compo-
nents removal in the sample.

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KHODAMORADY ET AL.
FIGURE 6 Mapping of BNPs@SiO2-TPPTSA
spectrum also shows a small exothermic peak at 610ꢀC.
These results were affirmed that ligands and linkers
have been successfully connected to the surface of
boehmite (Figure 8a). According to the DTA diagram,
the process of separating of ligand, organic, and inor-
ganic materials from the boehmite surface is endother-
mic (Figure 8b).
2.2 | Catalytic studies
To discover the best reaction conditions, some important
factors like the amount of catalyst, solvent, and tempera-
ture were surveyed (Table 1). As a model reaction, benz-
aldehyde (1 mmol) and sodium azide (1.5 mmol) reacted
with malononitrile (1 mmol) using different amount of
BNPs@SiO2-TPPTSA under conditions shown in Table 1
(Entries 1–4). The optimal reaction conditions were
obtained when 0.05 g of the catalyst was used (96%)
(Table 1, Entry 3). It can be seen, when 0.07 g of
BNPs@SiO2-TPPTSA was used, no remarkable enhance-
ment in yield was seen (98%), but the time of the reaction
was reduced (45 min) (Table 1, Entry 4). Next, we
checked model reaction in solvents such as ethanol, ace-
tonitrile, H2O, DMF, and n-octane under optimized
FIGURE 7 X-ray diffraction (XRD) patterns of (a) boehmite
nanoparticles (BNPs), (b) BNPs@SiO2,
(c) BNPs@SiO2(CH2)3PPh3+Cl−, and (d) BNPs@SiO2-TPPTSA
The weight loss of about 23.47% from 200 to 600ꢀC is
attributed to the elimination of TPPTSA, CPTMS, and
TEOS from the catalyst surface. Three weak exothermic
peaks at 200ꢀC, 380ꢀC, and 550ꢀC were observed in the
DTA curve that can be related to the degradation, oxida-
tion, and combustion of organic parts (TPPTSA, CPTMS,
and TEOS). Ultimate weight loss that occurs at
600–800ꢀC (5.92%) is related to the boehmite crystalline
structure change. The phase change process in the DTA

KHODAMORADY ET AL.
7 of 17
the synthesis of tetrazole derivatives (Table 1, Entries
12–14). Based on the obtained results of SEM images, the
morphology of BNPs@CPTMS, BNPs@SiO2-TPP, and
final catalyst is similar. As the catalyst is synthesized step
by step, the spherical structure becomes more regular.
Besides, based on experimental tests and SEM images
corresponding to different stages of catalyst synthesis,
morphology has no effect on catalyst activity, and
only Bronsted acid groups located on the surface of
tri-phenylphosphine ligand are effective on tetrazol
synthesis.
To illustrate the generality of this approach, a series
of different aryl aldehydes with both electron-rich and
electron-poor groups on the different ortho-, meta-, and
para- situations were applied to preparation tetrazoles
under the optimal reaction conditions, and the findings
are demonstrated in Table 2. As a result, aldehydes con-
taining various electron-withdrawing groups (EWGs)
and electron-donating produced desired tetrazoles with
excellent yields. It should be noted that the electronic
effects and the nature of the groups do not have a
remarkable influence on the reaction yields, but in over-
all, electron-poor aromatic aldehydes react quickly
than those with electron-donating substituents. The
synergistic effect of the BNPs@SiO2-TPPTSA resulted in
high selectivity. Furan-2-carbaldehyde and pyridine-
2-carbaldehyde as heteroaromatic aldehydes worked well
in this procedure to generate the 5-substituted 1H-
tetrazoles in decent yields and showed high selectivity
for this reaction (Table 2, Entries 10 and 11). Under sim-
ilar reaction conditions, this procedure failed to produce
the desired product with hexanal as an aliphatic alde-
hyde and produced mixtures of products that are not
identified (Table 2, Entry 13). In addition, all the reac-
tions were finished during 30–60 min at 70ꢀC, and the
products were generated with excellent yields with high
purity. Interestingly, during the reaction, just one of the
two cyano groups of the malononitrile molecule reacted
with NaN3 for the synthesis of tetrazole, and another
remains intact.
FIGURE 8 (a) Thermal gravimetric analysis (TGA) diagram of
the catalyst and (b) differential thermal analysis (DTA) diagram of
the catalyst
reaction conditions (Table 1, Entries 5–9). It is notewor-
thy that polar solvents produce the product in 80–92%
yields. While the product was not detected when
n-octane (a nonpolar solvent) was used (Table 1, Entry
9). However, solvent-free conditions were selected
because it is especially adapted to organic synthesis under
green chemistry conditions. Besides, increasing the
temperature to 100ꢀC does not have a significant effect
on the yield and only reduces the time (Table 1, Entry 11).
Also, a decrease in yield and enhancement in reaction
time was obtained by reducing the temperature to 25ꢀC
(Table 1, Entry 11).
Also, after obtaining the best conditions, the model
reaction in the presence of BNPs, BNPs@CPTMS, and
BNPs@SiO2-TTP nanoparticles was investigated, and
after 60 min, the resulting product was negligible, which
demonstrates that only BNPs@SiO2-TPPTSA can catalyze
2.3 | Homoselective preparation of
5-substituted-1H-tetrazoles
Homoselectivity is the property of choosing a site from
several similar sites. This key term is used in many
specific reactions, as there are two or more identical
functional groups in a substance, only one of which selec-
tively reacts and produces a specific product. The com-
pound may have two or more hydroxyl groups, or it may
have several double bonds in the structure of its com-
pound at the same time, only one of which may react

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KHODAMORADY ET AL.
TABLE 1 Optimization of the
reaction conditions[a]
Entry
Catalyst (g)
0.02
Solvent
T (ꢀC)
70
Time (min)
Yieldb (%)
1
-
-
-
-
60
60
60
45
60
60
60
60
60
55
100
60
60
60
65
85
2
0.04
70
3
0.05
70
96
4
0.07
70
97
5
0.05
Ethanol
CH3CN
H2O
70
80
6
0.05
70
80
7
0.05
70
90
8
0.05
DMF
70
92
9
0.05
n-Octane
-
70
-
10
11
12
13
14
0.05
100
25
96
0.05
70
0.05c
0.05d
0.05e
-
-
-
70
Trace
Trace
Trace
70
70
aReaction conditions: benzadehyde (1 mmol), NaN3 (1.5 mmol), malononitrile (1 mmol), catalyst
BNPs@SiO2-TPPTSA, 70ꢀC.
bIsolated yield.
cCatalyst: BNPs.
dCatalyst: BNPs@CPTMS.
eCatalyst: BNPs@SiO2-TPP.
TABLE 2 BNPs@SiO2-TPPTSA catalyzed multicomponent synthesis of 5-substituted 1H-tetrazoles[a]
Entry
Aldehyde
Yield (%)b, time: min
M.p. (ꢀC)[Ref]
TONc
TOFd (h−1
)
1
96 (60)
164–166[47]
19.20
19.20
2
3
96 (40)
98 (30)
165–167[48]
142–144[49]
19.20
19.60
29.09
39.20
(Continues)

KHODAMORADY ET AL.
9 of 17
TABLE 2 (Continued)
Entry
Aldehyde
Yield (%)b, time: min
M.p. (ꢀC)[Ref]
TONc
TOFd (h−1
)
4
98 (45)
165–167[47]
19.60
26.13
5
98 (40)
161–163[48]
19.60
29.69
6
7
8
9
96 (55)
98 (55)
97 (50)
95 (50)
152–154[47]
167–170[48]
157–159[48]
169–171[49]
19.20
19.60
19.40
19.00
20.96
21.39
23.28
22.80
10
11
98 (45)
95 (55)
253–254[49]
185–187[49]
19.60
19.00
26.13
20.74
12
- (60)
Mixture of products
-
aReaction conditions: aldehyde (1 mmol), NaN3 (1.5 mmol), malononitrile (1 mmol), catalyst BNPs@SiO2-TPPTSA: 0.05 g), solvent-free conditions 70ꢀC.
bYields: isolated products.
cTurnover number (TON) = (mmol of product/mmol of catalyst).
dTurnover frequency (TOF) = TON/time (h).

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KHODAMORADY ET AL.
SCHEME 3 Investigation of the
homoselectivity of this method under best
reaction conditions
selectively. Here, in the tetrazole synthesis reaction, the
malononitrile has the same two cyano groups, and
selectively, one of them enters the reaction, and the other
remains unchanged.[5c,45]
Then, this intermediate tolerates [2 + 3] cycloaddition
reaction with NaN3 to construct the desirable tetrazole
(Scheme 4).
For scrutiny of homoselectivity of this study, the syn-
thesis of (E)-3-phenyl-2-(1H-tetrazole-5-yl) acrylonitrile
in the presence of BNPs@SiO2-TPPTSA was chosen
under optimal reaction conditions (Scheme 3). As
Scheme 3 illustrated, the 5,50-(2-phenylethene-1,1-diyl)bis
(1H-tetrazole) was not generated, and just (E)-3-phenyl-
2-(1H-tetrazole-5-yl) acrylonitrile was gained in 96%
yield. It is worth noting that even by doubling the quanti-
ties of raw materials, only one of the cyano groups of
malononitrile reacts. According to these findings, the
present protocol is homoselective for the formation of
tetrazoles.
2.4 | Reusability of the catalyst
To investigate the reusability of the BNPs@SiO2-TPPTSA,
we checked the reaction of benzaldehyde (1 mmol),
sodium azide (1.5 mmol), and malononitrile (1 mmol) in
the presence of BNPs@SiO2-TPPTSA as a model reaction.
After completion of the reaction, which is screened by
thin layer chromatography (TLC), the nanocatalyst was
separated from the reaction media by centrifuge, washed
twice with EtOH, and dried prior to subjection to the
next reaction. It should be noted that the average
tetrazole yield after five consecutive times is 94%, which
confirms the recyclability of this introduced catalyst
(Figure 9).
The possible mechanism for the synthesis of
5-substituted 1H-tetrazoles in the presence of
BNPs@SiO2-TPPTSA is summarized in Scheme 4. At
first,
a complex between aromatic aldehyde and
BNPs@SiO2-TPPTSA was formed. Then, by removing a
molecule of water, arylidenemalononitrile is produced
through a condensation reaction with malononitrile.
2.5 | Leaching test
To check the leaching test, the model reaction between
benzaldehyde (1 mmol), sodium azide (1.5 mmol), and
malononitrile (1 mmol) in the presence of BNPs@SiO2-
TPPTSA was studied under optimized reaction. At the
half-reaction time (19 min), the yield was 54%, and the
catalyst was isolated by simple filtration. Then, the
SCHEME 4 Proposed mechanism for the formation of
FIGURE 9 Reusability of the catalyst for the synthesis of
(E)-3-phenyl-2-(1H-tetrazol-5-yl) acrylonitrile
tetrazoles

KHODAMORADY ET AL.
11 of 17
filtrate was placed in another vessel, and the reaction was
continued without catalyst for long time. After 3 h, the
amount of product did not change significantly (56%)
(as monitored by TLC), which indicates a slight leaching
of the acidic active groups from the nanobohmite surface
to the reaction medium. Based on this result, the
BNPs@SiO2-TPPTSA catalyzes the tetrazole synthesis
reaction. In Figure 10, the FT-IR spectrum and EDX
analysis of nanocatalyst after liching test are shown.
Also, the pH test was taken from the catalyst after the
fifth run, which showed a slight increase in the pH of the
catalyst surface (pH = 2.04) after the fifth load compared
with the pH of the fresh catalyst (pH = 1.92). However,
after several consecutive uses of the catalyst, there is a
possibility of blocking the active sites of the catalyst
surface and leaching of acidic groups attached to the
bohemite surface due to washing the catalyst.
To further evaluate the performance of this catalytic
system, we concentrated on the formation of
2,4,5-trisubstituted imidazoles. To find the best reaction
conditions, we checked some parameters like amount of
catalyst, polar and nonpolar solvents, and different tem-
peratures. At first, we used different amounts of
BNPs@SiO2-TPPTSA in the reaction between benzalde-
hyde, benzoin, and ammonium acetate as a model reac-
tion. Based on the findings, the best result was obtained
in the presence of 0.05 g of the nanocatalyst (Table 3,
Entry 6) at 100ꢀC. The higher amount of the catalyst did
not improve the amount of yield (Table 3, Entry 7). Next,
we studied the effect of polar and nonpolar solvents on
the reaction times and yields (Table 3, Entries 8–12). It is
noteworthy that the best results are achieved when the
model reaction was performed under solvent-free condi-
tions. No product was produced in the presence of BNPs
(Table 3, Entry 13), BNPs@CPTMS (Table 3, Entry 14),
and BNPs@SiO2-TTP nanoparticles (Table 3, Entry 15)
confirming that the only requirement to promote the
reaction is our prepared catalyst. According to the find-
ing, with decreasing temperature to 75ꢀC, 50ꢀC, and
25ꢀC, the yield of product decreases (Table 3, Entries
1–3). The model reaction was also examined at tempera-
tures >100 (140ꢀC), and both the reaction time and the
amount of product did not change (Table 3, Entry 16).
In order to study the generality of this procedure, a
variety of aromatic aldehydes was used to construct
2,4,5-trisubstituted imidazoles under the optimized reac-
tion conditions. The obtained results are presented in
Table 4. As shown, aldehydes including EWGs and alde-
hydes containing electron-donating groups (EDGs) gen-
erated the desired imidazoles in excellent yields with
high purity. As can be seen from Table 4, aldehydes with
EDGs like OH, OCH3, and CH3 substitutes (Table 4,
Entries 3–5) are reacted faster than aldehydes with EWGs
such as NO2 and Cl (Table 4, Entries 8 and 11). To survey
the reactivity of benzil in comparison with benzoin, the
synthesis of titled compounds in the presence of benzoin
was repeated, and the results are summarized in Table 4.
As shown, benzoin reactions are completed in longer
reaction times compared with the benzil reactions.
The plausible mechanism for the synthesis of
2,4,5-trisubstituted imidazoles from benzyl and benzoein
is depicted in Scheme 5.
The reusability of the catalyst is significant in indus-
try, particularly for large-scale processes. In this regard,
the recyclability of this nanocatalyst was studied using
the reaction of benzaldehyde, benzil, and NH4OAc in the
presence of BNPs@SiO2-TPPTSA under optimal condi-
tions (Figure 11). The catalyst was retrieved by filtration,
washed with ethanol, dried, and then reused at least for
the next five runs with a small drop in its catalytic effi-
ciency. The observed catalyst deactivation after consecu-
tive cycles was due to fouling of the acid sites and
removal from the catalyst surface after each wash cycle.
Table 5 was provided to show the efficiency of this
method in comparison with some reported methods in
the synthesis of tetrazoles and 2,4,5-triphenyl-1H- imid-
azoles. As shown, this catalyst is superior to some previ-
ously reported procedures in terms of yields and reaction
times. Several other methodologies for the synthesis of
FIGURE 10 (a) Fourier transform infrared (FT-IR) spectrum
and (b) energy dispersive X-ray analysis (EDX) analysis of
nanocatalyst after leaching test

12 of 17
KHODAMORADY ET AL.
TABLE 3 Optimization of the
reaction conditions for the preparation
of 2,4,5-triphenyl-1H-imidazole[a]
Entry
1
Catalyst (g)
0.05
Solvent
Temp. (ꢀC)
25
Yield (%)b, time (min)
50 (40)
70 (40)
85 (40)
50 (90)
75 (60)
97 (40)
97 (38)
40 (40)
70 (40)
80 (40)
90 (40)
20 (40)
- (40)
-
-
-
-
-
-
-
2
0.05
50
3
0.05
75
4
0.02
100
100
100
100
100
100
100
100
100
100
100
100
140
5
0.035
0.05
6
7
0.07
8
0.05
H2O
9
0.05
CH3CN
10
11
12
13
14
15
16
0.05
EtOH
0.05
DMF
0.05
n-Octane
0.05c
0.05d
0.05e
0.05
-
-
-
-
- (40)
-(40)
97(38)
aReaction conditions: benzil (1 mmol), benzaldehyde (1 mmol), NH4OAc (2 mmol), catalyst, solvent
(5 mL) or solvent-free conditions.
bIsolated yields.
cCatalyst: BNPs.
dCatalyst: BNPs@CPTMS.
eCatalyst: BNPs@SiO2-TTP.
TABLE 4 One-pot synthesis of 2,4,5-trisubstituted imidazoles by BNPs@SiO2-TPPTSA[a]
Time (min)
Yield%b
TOFd (h−1
)
Entry
Ar
Benzil
40
Benzoin
46
Benzil
97
Benzoin
96
M.p. (ꢀC)[Ref]
270–271[50]
202–204[51]
266–268[41]
230–231[32]
231–232[32]
255–257[50]
265–267[41]
260–262[50]
190–192[51]
172–173[35]
170–172[34]
314–316[51]
TONc
16.16
15.33
16.00
15.83
16.00
15.83
15.83
15.16
15.00
15.50
15.00
15.33
Benzil
24.48
20.44
27.58
27.29
21.33
23.98
21.10
18.26
15
Benzoin
21.26
18.46
24.24
22.61
19.27
17.02
19.54
16.30
13.51
13.47
14.15
13.33
1
C6H5
2
2-HOC6H4
4-HOC6H4
4-MeOC6H4
4-MeC6H4
45
50
92
93
3
35
40
96
93
4
35
42
95
94
5
45
50
96
96
6
4-N (Me)2C6H4
3-MeOC6H4
4-ClC6H4
40
56
95
94
7
45
49
95
95
8
50
54
91
93
9
2-ClC6H4
60
67
90
91
10
11
12
2,4-DiCl2C6H3
4-O2NC6H4
3-O2NC6H4
65
69
93
90
14.35
15
60
64
90
91
65
69
92
93
14.19
aReaction conditions: aromatic aldehyde (1 mmol), benzoin or benzil (1 mmol), ammonium acetate (2 mmol), BNPs@SiO2-TPPTSA (0.05 g), solvent-
free, 100ꢀC.
bIsolated yields.
cTON = (mmol of product/mmol of catalyst).
dTOF = TON/time (h).

KHODAMORADY ET AL.
13 of 17
FIGURE 11 The recyclability of the catalyst for the synthesis
of 2,4,5-triphenyl-1H-imidazole
3 | EXPERIMENTAL SECTION
All of the known products are determined by comparison
of their physical properties and spectral data with those
of documented examples. Progress of the reactions was
followed by TLC. Melting points were obtained using a
Stuart Scientific SMP2 apparatus. FT-IR spectra were
defined by
a Perkin-Elmer 683 instrument. The
substances were procured from Merck Company and
used without any purification.
3.1 | General procedure for the synthesis
of tetrazoles in the presence of BNPs@SiO2-
TPPTSA
BNPs@SiO2-TPPTSA (0.05 g, 0.06 mmol, SO3H groups)
was added to a mixture of aldehyde (1 mmol), sodium
azide (1.5 mmol), and malononitrile (1 mmol). Then
the reaction mixture was stirred at 70ꢀC in solvent-less
conditions for the desired time (Table 2). When the raw
materials were consumed, as was checked by TLC
(n-hexane/EtOAc, 70:30), the catalyst was removed
from the reaction medium with centrifuge and washed
with ethanol (2 × 10). Next, a solution of aqueous HCl
(2 N, 10 mL) was added to the organic layer under
vigorous stirring. The mixture of the product was
extracted with ethyl acetate (3 × 10) and washed by
distilled water. Then, anhydrous Na2SO4 was added to
dehydrate the organic phase, and the organic layer was
evaporated under reduced pressure to obtain the desired
tetrazole in great purity and high yields. Most of the
compounds were obtained with high purity, but in
some cases, the desired product was recrystallized from
EtOH to afford the pure product.
SCHEME 5 A plausible mechanism for the preparation of
2,4,5-trisubstituted imidazoles from benzil and benzoin,
respectively
tetrazoles with different tetrazole products were also
compared, and the results are shown in Table 5.
The FESEM image of the reused catalyst after the
fifth load and FT-IR spectra of the fresh catalyst and
reused catalyst after the fifth run are shown in Figure 12.
As you can see, there is no particular change in the
structure and morphology of the catalyst.

14 of 17
KHODAMORADY ET AL.
TABLE 5 Comparison of the efficiency of this method with previously reported methods for the synthesis of tetrazoles and
2,4,5-triphenyl-1H- imidazoles
Entry
Product
Conditions
Time (h or min)
3.5–5.5 h
15 min
Yield (%)[Ref]
77–87[48]
89–94[48]
85–95[52]
86–98[53]
95–98
1
2
3
4
5
SMA (silica molybdic acid), H2O, 50ꢀC
SMA (silica molybdic acid), H2O, microwave
Nano-NiO, DMF, 70ꢀC
7 h
Fe3O4-AMPD-Cu, PEG, 120ꢀC
8–80 min
30–60 min
This work
6
7
Ba0.5Sr0.5Fe12O19@PU-SO3H, DES, 40ꢀC
60 min
84–95[54]
8
9
Cellulose/Fe2O3/Ag, EtOH, 40ꢀC
Fe3O4@SiO2-NH-GA, EtOH, Reflux
60 min
85–93[55]
81–90[56]
15–20 min
10
11
12
13
14
Polymer-supported zinc chloride, EtOH, reflux
Silica sulfuric acid (0.5 g), H2O, reflux
Potassium aluminum sulfate, EtOH, 70ꢀC
H2SO4.SiO2, Solvent-free 110ꢀC
4 h
4 h
2.5 h
60
96[57]
73[58]
93[59]
90[60]
90[39]
Silica bonded propyl piperazin-N-sulfamic acid, solvent-
free,140ꢀC
60
15
16
CoFe2O4, EtOH, u. s, 40ꢀC
20
40
95[42]
97
This work
3.2 | General procedure for the synthesis
of 2,4,5-trisubstituted imidazoles with
BNPs@SiO2-TPPTSA
of their physical data with those of known
compounds.
In a round bottom flask, a mixture of BNPs@SiO2-
TPPTSA (0.05 g), benzil or benzoin (1.0 mmol), alde-
hyde (1.0 mmol), and NH4OAc (2.0 mmol) was heated
at 100ꢀC in solvent-free conditions as long as the reac-
tion continues (Table 4). The reaction progress was
screened by TLC (n-hexane/EtOAc, 70:30). After com-
pletion of the reaction, the reaction mixture was fil-
tered to remove the heterogeneous catalyst. After that,
the anhydrous MgSO4 was added to the organic layer,
and the organic solvent was evaporated under reduced
pressure to gain crude product. Recrystallization in
ethanol was used to further purify the products. All of
the desired products were characterized by comparison
3.3 | Preparation of BNPs@SiO2-TPPTSA
First, BNPs (1 g) were dispersed in dry toluene 40 mL,
and 3-chloropropyltrimethoxysilane (CPTMS) (3 mL)
and 4 g triphenylphosphine (PPh3) were added to the
reaction media. The mixtures were refluxed for 48 with
severe stirring at 110ꢀC. Then, the milky powder was
filtered, washed twice with EtOH, and dried in
vacuum at 80ꢀC.[46] Then, for the sulfonation of
BNPs@SiO2(CH2)3PPh3+Cl−, a mixture of 10 mL of
(98 wt.%) H2SO4/oleum was added and the mixture was
stirred for 48
sulfonated BNPs were washed with hot deionized
h
at 70 ꢀC. Finally, the obtained

KHODAMORADY ET AL.
15 of 17
5-substituted 1H-tetrazoles and 2,4,5-trisubstituted-
imidazoles. Chemical and mechanical stability, use of
cheap raw materials, good acidic strength, excellent
yields (up to 90%), shorter reaction times, easy
separation, and recyclability are the benefits of this
catalyst. We believe this protocol provides directions
for the design and production of new heterogeneous
nanocatalysts and has also opened up new ways for the
synthesis of precious organic substances.
DATA AVAILABILITY STATEMENT
The data that support the findings of this study are
available in the supplementary material of this article.
ACKNOWLEDGEMENT
The authors gratefully appreciated the financial
support of this work by the research council of Razi
University.
ORCID
Minoo Khodamorady https://orcid.org/0000-0003-0255-
0527
Nazanin Ghobadi https://orcid.org/0000-0001-9704-
1092
Kiumars Bahrami https://orcid.org/0000-0001-9229-
0890
FIGURE 12 (a) Field emission scanning electron microscopy
(FESEM) image of the catalyst after fifth run and (b) Fourier
transform infrared (FT-IR) of the fresh and after Run 5 for the
BNPs@SiO2-TPPTSA
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How to cite this article: Khodamorady M,
Ghobadi N, Bahrami K. Homoselective synthesis of
5-substituted 1H-tetrazoles and one-pot synthesis
of 2,4,5-trisubstuted imidazole compounds using
BNPs@SiO2-TPPTSA as a stable and new reusable
nanocatalyst. Appl Organomet Chem. 2021;35:
e6144. https://doi.org/10.1002/aoc.6144
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