Efficient synthesis of 5-substituted tetrazoles catalysed by palladium–S-methylisothiourea complex supported on boehmite nanoparticles

Article abstract of DOI:10.1002/aoc.3602

An efficient and general method is reported for the synthesis of 5-substituted 1H-tetrazole derivatives in the presence of S-methylisothiourea complex of palladium immobilized on boehmite nanoparticles (Pd-SMTU@boehmite) as an efficient and recyclable nan

Full text of DOI:10.1002/aoc.3602

Received: 15 June 2016
DOI 10.1002/aoc.3602
Revised: 28 July 2016
Accepted: 7 August 2016
F U L L PA P E R
Efficient synthesis of 5‐substituted tetrazoles catalysed by
palladium–S‐methylisothiourea complex supported on
boehmite nanoparticles
*
Parisa Moradi | Arash Ghorbani‐Choghamarani
Department of Chemistry, Faculty of Science, Ilam
An efficient and general method is reported for the synthesis of 5‐substituted 1H‐
University, PO Box 69315516, Ilam, Iran
tetrazole derivatives in the presence of S‐methylisothiourea complex of palladium
immobilized on boehmite nanoparticles (Pd‐SMTU@boehmite) as an efficient
and recyclable nanocatalyst. Boehmite nanoparticles were not sensitive to air or
moisture and were prepared without inert atmosphere in water at room temperature.
Correspondence
Arash Ghorbani‐Choghamarani, Department of
Chemistry, Ilam University, PO Box 69315516,
Ilam, Iran.
Email: arashghch58@yahoo.com;
a.ghorbani@mail.ilam.ac.ir
Then a novel type of phosphine‐free palladium complex was immobilized on these
nanoparticles. This catalyst was characterized using Fourier transform infrared,
thermogravimetric, Brunauer–Emmett–Teller, transmission and scanning electron
microscopic, energy‐dispersive X‐ray spectroscopic, X‐ray diffraction and induc-
tively coupled plasma optical emission spectroscopic techniques. The catalyst was
reused several times without palladium leaching or change in its structure.
KEYWORDS
5‐substituted 1H‐tetrazole, boehmite nanoparticles, organometallic catalyst,
palladium, S‐methylisothiourea
1
|
INTRODUCTION
herein a moisture‐ and air‐stable S‐methylisothiourea
complex of palladium immobilized on boehmite
nanoparticles (Pd‐SMTU@boehmite) is reported as new
organometallic catalyst for the synthesis of tetrazole
derivatives.
In the last few decades, supporting of homogeneous
catalysts on various heterogeneous solids has been studied
as a means to realize recyclable catalysts.[1] Many supports
such as TiO2 nanoparticles,[2] MCM‐41,[3] SBA‐15,[4] iron
oxide,[5] carbon nanotubes,[6] heteropolyacids,[7] graphene
oxide,[8] ionic liquids,[9] molecular sieves[10] and various
polymers[11] have been used as heterogeneous catalysts or
as supports for homogeneous catalysts. However, most of
them are expensive, require inert atmosphere or high tem-
perature for calcination and involve a lot of time and tedious
conditions to prepare. In contrast, the synthesis of boehmite
nanoparticles is not moisture or air sensitive, and also can
be synthesized in water at room temperature using available
materials.[12] Nanoboehmite has several attractive features
such as stability, non‐toxicity, high dispersion of the active
phases, high specific surface area, ease of surface modifica-
tion, readily and easily available and favourable biocompat-
ibility.[13,14] However, boehmite nanoparticles have rarely
been employed as a heterogeneous support.[15] Therefore
Tetrazoles are important heterocyclic compounds, having
several applications in organic synthesis, material science,
coordination chemistry, in organometallic chemistry as effec-
tive stabilizers of metallopeptide structures, stable surrogates
for carboxylic acids and medicinal chemistry.[16–18] For
example, valsartan and losartan are two typical examples of
the extensive application of these compounds in drugs.[19]
Also, recently tetrazole derivatives were used for binding aryl
thiotetrazolylacetanilides with HIV‐1 reverse transcriptase.[20]
Tetrazoles and their derivatives have been reported as anti‐
viral, anti‐bacterial, anti‐inflammatory and herbicidal agents,
potential anti‐HIV drug candidates and anti‐proliferative,
analgesic and anti‐tumour agents.[21–23] Tetrazoles can be
used as isosteric replacements for carboxylic acids in drug
design[24] and have found use in information recording
systems.[25]
Appl Organometal Chem 2016; 1–6
wileyonlinelibrary.com/journal/aoc
Copyright © 2016 John Wiley & Sons, Ltd.
1

2
MORADI AND GHORBANI‐CHOGHAMARANI
Conventional synthesis of 5‐substituted 1H‐tetrazoles is
via [3 + 2] cycloaddition of azides to nitriles. Based on this
method, several procedures have been reported. Unfortu-
nately, most of them suffer from some disadvantages such
as: the use of organic solvents, harsh reaction conditions,
water sensitivity, long reaction time and difficulty in
separation and recovery of the catalysts.[26–28] Thus, the
synthesis of these heterocycles has become an area of great
interest.
SCHEME 1 Synthesis of 5‐substituted 1H‐tetrazole derivatives in the pres-
ence of Pd‐SMTU@boehmite as catalyst.
using TLC), the reaction mixture was cooled to room temper-
ature. The catalyst was removed by simple filtration and HCl
(4 N, 10 ml) was added to the filtered solution and the corre-
sponding tetrazole extracted with ethyl acetate (2 × 10 ml).
The resulting organic layer was washed with distilled water,
dried over anhydrous sodium sulfate and concentrated to
afford the crude solid product.
2
| EXPERIMENTAL
2.1 | Preparation of catalyst
The catalyst was prepared via the following procedure. A
solution of NaOH (6.490 g) in 50 ml of distilled water
was added dropwise to a solution of Al(NO3)3⋅9H2O
(20 g) in 30 ml of distilled water under vigorous stirring.
The resulting milky mixture was subjected to mixing in an
ultrasonic bath for 3 h at 25°C. The resulting nanoboehmite
was filtered and washed with distilled water and kept in an
oven at 220°C for 4 h. The obtained boehmite nanoparticles
(1.5 g) were dispersed in 50 ml of toluene by sonication for
30 min, and then 2.5 ml of (3‐chloropropyl)triethoxysilane
was added. The reaction mixture was stirred at 40°C for
8 h. Then, the prepared nanoparticles (nPr‐Cl‐boehmite)
were filtered, washed with ethanol and dried at room
temperature. The obtained Cl‐boehmite (1 g) was dispersed
in 50 ml of ethanol for 20 min, and then S‐methylisothiourea
hemisulfate salt (2.5 mmol) and potassium carbonate
(2.5 mmol) were added to the reaction mixture and stirred for
18 h at 80°C. The resulting nanoparticles (SMTU@boehmite)
were filtered, washed with ethanol and dried at room
temperature. Thermogravimetric analysis (TGA) was used
to determine the percentage of organic functional groups
chemisorbed onto the boehmite nanoparticles. The
obtained SMTU@boehmite (0.5 g) was dispersed in
25 ml of ethanol by sonication for 20 min, and then pal-
ladium acetate (0.25 g) was added to the reaction mixture.
The reaction mixture was stirred at 80°C for 20 h. Then,
NaBH4 (0.3 mmol) was added to the reaction mixture and
stirred for 2 h. The final product (Pd‐SMTU@boehmite)
was filtered, washed with ethanol and dried at room tem-
perature. The amount of palladium was determined using
energy‐dispersive X‐ray spectroscopy (EDS) and induc-
tively coupled plasma optical emission spectroscopy
(ICP‐OES).
3
| RESULTS AND DISCUSSION
3.1 | Catalyst preparation
Herein report the preparation and characterization of mois-
ture‐ and air‐stable Pd‐SMTU@boehmite. Initially, the
boehmite nanoparticles were prepared, and subsequently
were modified with (3‐chloropropyl)triethoxysilane. Then,
in order to prepare SMTU@boehmite, S‐methylisothiourea
was grafted on the surface of nPr‐Cl@boehmite and finally
Pd nanoparticles were immobilized on SMTU@boehmite
by a concise route (Scheme 2).
3.2 | Catalyst characterization
Pd‐SMTU@boehmite was characterized using Fourier trans-
form infrared (FT‐IR) spectroscopy, TGA, Brunauer–
Emmett–Teller (BET) measurements, transmission electron
microscopy (TEM), scanning electron microscopy (SEM),
EDS, X‐ray diffraction (XRD) and ICP‐OES.
The XRD patterns of nanoboehmite and Pd‐
SMTU@boehmite are presented in Figure 1. The boehmite
phase is identified by the peaks at 2θ values of 14.40°,
28.41°, 38.55°, 46.45°, 49.55°, 51.94°, 56.02°, 59.35°,
2.2 | General procedure for synthesis of 5‐substituted
1H‐tetrazoles
A mixture of nitrile (1 mmol) and sodium azide (1.4 mmol) in
the presence of 0.020 g (0.03 mmol) of Pd‐SMTU@boehmite
was stirred at 120°C in poly(ethylene glycol) (PEG)
(Scheme 1). After completion of the reaction (monitored
SCHEME 2 Synthesis of Pd‐SMTU@boehmite.

MORADI AND GHORBANI‐CHOGHAMARANI
3
FIGURE
1
XRD patterns of boehmite nanoparticles (black), Pd‐
SMTU@boehmite (green) and recovered Pd‐SMTU@boehmite (red).
65.04°, 65.56°, 68.09° and 72.38° in the XRD patterns.[12,13]
Also, the recovered catalyst was investigated using XRD
(Figure 1, red curve). This indicates that the catalyst can be
recycled without any change in its structure.
The sizes of boehmite nanoparticles and catalyst were
confirmed using SEM and TEM (Figure 2a–c) techniques.
TEM and SEM images reveal most of the particles are pre-
pared at the nanoscale with an average diameter of about
5–15 nm.
In order to show the presence of Pd metal in
Pd‐SMTU@boehmite, EDS (Figure 2d) and ICP‐OES
analyses were conducted. The EDS spectrum of Pd‐
SMTU@boehmite shows the presence of Al, O, Si, C, N, S
and Pd species in this catalyst. In order to determine the exact
amount of Pd, ICP‐OES was applied. According to the ICP‐
OES results, the exact amount of Pd in this catalyst is
1.64 mmol g−1
.
The nitrogen adsorption–desorption isotherms were
obtained at 120°C (Figure 3I), from which the pore volume
and average pore diameter of Pd‐SMTU@boehmite
(0.19 cm3 g−1 and 1.64 nm) are lower than those of
nanoboehmite (0.22 cm3 g−1 and 1.21 nm). Also, when the
Pd complex is grafted on boehmite nanoparticles, the BET
specific surface area decreases from 122.8 to 82.90 m2 g−1
.
The decrease in average pore diameter, surface area and pore
volume of the modified boehmite nanoparticles is related to
the grafting of organic layers and Pd complex on the surface
of nanoboehmite.
Successful
functionalization
of
the
boehmite
nanoparticles can be confirmed from FT‐IR spectra
(Figure 3II). In Figure 3(IIa), two bands at 3086 and
3308 cm−1 are indicative of the OH surface[13] and several
peaks at 480, 605 and 735 cm−1 can be attributed to Al─O
bonds.[15] Also, the vibrations of hydrogen bonds appear at
FIGURE
2
SEM images of (a) boehmite nanoparticles and (b) Pd‐
SMTU@boehmite. (c) TEM image of Pd‐SMTU@boehmite. (d) EDS spec-
trum of Pd‐SMTU@boehmite.
lower frequency (1631 cm−1) in the spectrum of the catalyst
(Figure 3IId), which indicates the formation of Pd complex
on surface of boehmite nanoparticles.[29]
Also the FT‐IR spectrum of recovered Pd‐SMTU@
boehmite (Fig. 3IIe) indicates that this catalyst is stable
after recovery and can be reused without any change in
its structure.
1164 and 1069 cm−1 [13]
In Figure 3(IIb), the anchored
.
(3‐chloropropyl)triethoxysilane is confirmed by C─H and
O─Si stretching vibrations that appear at 2955 and 1073 cm
−1, respectively.[3,5] In Figure 3(IIc), the existence of the
grafted S‐methylisothiourea is characterized by C═N vibra-
tions that appear at 1638 cm−1, this band being shifted to

4
MORADI AND GHORBANI‐CHOGHAMARANI
FIGURE 4 TGA curves of (a) boehmite, (b) SMTU@boehmite and (c) Pd‐
SMTU@boehmite.
a small weight loss below 200°C due to desorption of physi-
cally adsorbed solvents and surface hydroxyl groups.[5] In the
TGA curve of the boehmite nanoparticles, a weight loss of
about of 16% from 25 to 200°C is observed. Meanwhile,
weight losses of about 29 and 40% from 200 to 800°C are
observed for SMTU@boehmite and Pd‐SMTU@boehmite,
respectively. These results indicate that organic layers and
Pd complex have been grafted on the surface of boehmite
nanoparticles.
3.3 | Catalytic study
We examined the application of Pd‐SMTU@boehmite as an
efficient, stable, reusable and commercially available catalyst
for the synthesis of tetrazole derivatives. In order to optimize
the reaction conditions, the reaction of benzonitrile and
sodium azide (NaN3) was selected as a model reaction. The
effects of various parameters such as temperature, solvent
TABLE 1 Optimization of reaction conditions for synthesis of 5‐substituted
1H‐tetrazole derivatives
Catalyst
(mg)
NaN3 Temperature
Solvent (mmol) (°C)
Time
(min)
Yield
(%)a
Entry
1
2
15
15
15
15
10
17
20
20
20
20
20
20
PEG
PEG
PEG
PEG
PEG
PEG
PEG
DMSO
DMF
EtOH
PEG
PEG
1.2
1.3
1.4
1.5
1.4
1.4
1.4
1.4
1.4
1.4
1.4
1.4
120
240
240
240
180
180
180
150
150
150
150
150
150
38
45
120
120
3
69
4
120
73
5
120
62
6
120
77
FIGURE
3
(I) Nitrogen adsorption–desorption isotherms of boehmite
7
120
95
nanoparticles and Pd‐SMTU@boehmite. (II) FT‐IR spectra of (a) boehmite,
(b) nPr‐Cl@boehmite, (c) SMTU@boehmite, (d) Pd‐SMTU@boehmite and
(e) recovered Pd‐SMTU@boehmite.
8
120
77
9
120
83
10
11
12
Reflux
100
Trace
64
TGA was used to determine the amount of functional
groups chemisorbed onto the surface of boehmite
nanoparticles (Fig. 4). The TGA curves of the samples show
120
39
aIsolated yield.

MORADI AND GHORBANI‐CHOGHAMARANI
5
TABLE 2 Synthesis of 5‐substituted 1H‐tetrazole derivatives in presence of
Pd‐SMTU@boehmite
After optimization of the reaction condition, the catalytic
activity of Pd‐SMTU@boehmite in the reaction of various
benzonitriles including different functional groups was exam-
ined under optimized conditions (Table 2). As variety of
benzonitriles bearing electron‐donating and electron‐with-
drawing substituents were successfully employed and the cor-
responding 5‐substituted 1H‐tetrazole derivatives are
obtained in good to excellent yields.
Time
(h)
Yield
(%)a
M.p.
(°C)
Entry
Nitrile
Benzonitrile
Ref.
[17]
[17]
[31]
[18]
[31]
[30]
[31]
[16]
[31]
[16]
1
2
2.5
20
48
8
95
95
90
89
91
89
89
87
92
94
213–214
219–221
177–179
265–268
128–130
181–182
221–224
251–254
149–150
229–232
4‐Nitrobenzonitrile
4‐Acetylbenzonitrile
4‐Bromobenzonitrile
3‐Chlorobenzonitrile
2‐Chlorobenzonitrile
2‐Hydroxybenzonitrile
Terephthalonitrile
3.4 | Reusability of catalyst
3
4
The recyclability of the Pd‐SMTU@boehmite catalyst was
examined in the synthesis of 2‐(1H‐tetrazol‐5‐yl)phenol
(Fig. 5). After the completion of the reaction, the catalyst
was recovered by filtration and washed with ethyl acetate.
Then, the reaction vessel was charged with fresh substrates
and subjected to the next run. As shown, the Pd‐
SMTU@boehmite could be reused over 10 runs without any
significant loss of its activity or palladium leaching, which
clearly demonstrates the practical recyclability of this catalyst.
Metal leaching of the catalyst was studied using hot
filtration test and ICP analysis. Based on the results from
ICP‐OES analysis, the amount of palladium in fresh catalyst
and recovered catalyst after five runs is 1.64 and
1.62 mmol g−1, respectively, which indicates that Pd leaching
from the catalyst is very low. In order to examine the leaching
of Pd in the reaction mixture and heterogeneity of this cata-
lyst, we performed hot filtration in the reaction of 2‐
chlorobenzonitrile and NaN3. In this study we found the yield
of product in half the time of the reaction was 52%. Then the
reaction was repeated and in half the time of the reaction, the
catalyst was separated and the filtrate allowed to react further.
The yield of the reaction in this stage was 53%, confirming
that leaching of Pd does not occur.
5
4.5
4
6
7
0.5
1.75
8
8
9
3‐Nitrobenzonitrile
4‐Methoxybenzonitrile
10
20
aIsolated yield.
FIGURE
5
Recyclability of Pd‐SMTU@boehmite in the synthesis of
2‐(1H‐tetrazol‐5‐yl)phenol.
3.5 | Comparison of catalyst
(PEG‐400, dimethylsulfoxide (DMSO), dimethylformamide
(DMF) and ethanol), amount of catalyst and amount of
NaN3 were considered in the model reaction (Table 1). As
evident from Table 1, 0.02 g of Pd‐SMTU@boehmite in
PEG‐400 at 120°C using 1.4 mmol of NaN3 are found to be
ideal reaction conditions for the synthesis of 5‐substituted
1H‐tetrazole derivatives.
In order to show the efficiency of described catalytic
system, the results obtained for the model compounds in
this project were compared with previously reported pro-
cedures. Comparison of the results shows a better catalytic
activity of Pd‐SMTU@boehmite in the synthesis of
5‐substituted 1H‐tetrazoles (Table 3). Also, a comparison
TABLE 3 Comparison of results for Pd‐SMTU@boehmite with other catalysts for synthesis of 5‐phenyl‐1H‐tetrazole
Entry
Catalyst
Condition
DMF, 120°C
Time (h)
Yield (%)a
TOF (h−1
)
Ref.
[16]
[19]
[30]
[31]
[32]
[33]
[34]
1
2
3
4
5
6
7
8
Fe3O4@SiO2/Salen Cu(II)
(NH4)Ce(NO3)6
7
6
90
97
83
98
79
93
95
95
—
1.61
1.66
0.32
—
DMF, 110°C
AgNO3
DMF, 120°C
5
Cu(OAc)2
DMF, 120°C
12
12
4
FeCl3–SiO2
DMF, 120°C
CAN‐supported HY‐zeolite
Ammonium chloride and ammonium fluoride
Pd‐SMTU@boehmite
DMF, 110°C
0.77
0.10
12.66
DPOL/H2O (6/4), 160°C
PEG, 120°C
48
2.5
This work
aIsolated yield.

6
MORADI AND GHORBANI‐CHOGHAMARANI
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excellent results for Pd‐SMTU@boehmite in the synthesis
of 5‐substituted 1H‐tetrazoles. As evident from Table 3,
Pd‐SMTU@boehmite shows a high TOF number in
comparison with other catalysts. This result indicates that
Pd‐SMTU@boehmite is more effective and more efficient
compared to other catalysts. This new catalyst is compara-
ble in terms of price, non‐toxicity, catalyst recycling,
stability and ease of separation.
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4
| CONCLUSIONS
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In summary, a novel type of recoverable nanocatalyst was
prepared via grafting of palladium on boehmite
nanoparticles. This catalyst showed excellent catalytic activ-
ity, high reusability and stability to air and moisture for the
synthesis of 5‐substituted 1H‐tetrazole derivatives in PEG‐
400. The advantages of this protocol are the use of eco‐
friendly, commercially available, chemically stable materials,
operational simplicity and good to high yields, and, more
importantly, the catalyst can be synthesized from inexpensive
and commercially available starting materials. The catalyst
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How to cite this article: Moradi, P., and Ghorbani‐
Choghamarani, A. (2016), Efficient synthesis of
5‐substituted tetrazoles catalysed by palladium–S‐
methylisothiourea complex supported on boehmite
nanoparticles, Appl. Organometal. Chem., doi:
10.1002/aoc.3602
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