A Magnetically Recoverable and Reusable Catalyst for Synthesis of 5-Substituted 1H-Tetrazoles

Full text of DOI:10.1080/00304948.2018.1525673

Organic Preparations and Procedures International
The New Journal for Organic Synthesis
ISSN: 0030-4948 (Print) 1945-5453 (Online) Journal homepage: http://www.tandfonline.com/loi/uopp20
A Magnetically Recoverable and Reusable Catalyst
for Synthesis of 5-Substituted 1H-Tetrazoles
Ali Elhampour & Firouzeh Nemati
To cite this article: Ali Elhampour & Firouzeh Nemati (2018) A Magnetically Recoverable and
Reusable Catalyst for Synthesis of 5-Substituted 1H-Tetrazoles, Organic Preparations and
Procedures International, 50:5, 493-501, DOI: 10.1080/00304948.2018.1525673
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Organic Preparations and Procedures International, 50:493–501, 2018
2018 Taylor & Francis Group, LLC
#
ISSN: 0030-4948 print / 1945-5453 online
DOI: 10.1080/00304948.2018.1525673
A Magnetically Recoverable and Reusable Catalyst
for Synthesis of 5-Substituted 1H-Tetrazoles
Ali Elhampour and Firouzeh Nemati
Department of Chemistry, Semnan University, 35131-19111, Semnan, Iran
The nitrile group is valuable in organic synthesis and can be transformed to a variety of
functional groups, including thiazoles, oxazolidones, triazoles, and tetrazoles.
Moreover, aromatic nitriles constitute a key component of numerous natural products
1
and pharmaceutically important compounds such as dyes and herbicides. Among the
different heterocycles generated from benzonitriles, tetrazoles are an increasingly popu-
lar functionality with a wide range of applications in the field of medicinal chemistry as
2
metabolically stable surrogates for the related carboxylic acids. Thus, one way to
potentiate anti-inflammatory activity is to mimic a carboxylic acid with a tetrazole moi-
3
ety. Indomethacin and intrazole are interesting examples of anti-arthritis drugs whose
carboxylic acid groups may be replaced by tetrazole with no loss of activity. In add-
ition, as the tetrazoles generally offer a more favorable pharmacokinetic profile, they
have been widely incorporated into angiotensin II antagonist structures, including losar-
4–6
tan and valsartan (Figure 1).
Aryl nitriles are traditionally synthesized via the Rosenmund-von Braun reaction
from the corresponding halides or the Sandmeyer reaction from anilines. On the other
hand, numerous methods have been developed to insert nitrile groups into mole-
7
–10
cules.
Among these methods, substantial research efforts have been developed for
11,12
the direct transition metal catalyzed transformation of aryl halides to aryl nitriles.
Aryl halides are now more attractive as starting materials in lead optimization proc-
esses since they can act as precursors for so many reactions. Recently, transition metal-
catalyzed cyanation of aryl halides was developed as a useful alternative for the prepar-
ation of aryl nitriles. The cyanating agents used were MCN (M ¼ Cu, K, Na, Zn),
1
3
TMSCN, and K Fe[(CN) ] ꢀ 3H O.
4
6
2
On the other hand, the most adaptable method for the preparation of 5-substituted
14
1
H-tetrazoles is via [2 þ 3] cycloaddition between azides and nitriles. This procedure
has advantages but was troubled with such drawbacks as strong Lewis acidity, use of
costly or toxic metal organic azides, hydrazoic acid or highly moisture-sensitive reac-
tion conditions. Thus a catalytic method for preparation of tetrazole derivatives is of
importance. The most attractive feature of heterogeneous catalysis is the production and
convenient separation of large quantities of products with the use of a small amount of
catalyst. Recently, several heterogeneous catalytic systems were reported for the synthe-
1
5–22
ses of 5-substituted tetrazoles.
Received November 18, 2017; in final form July 31, 2018.
Address correspondence to Firouzeh Nemati, Department of Chemistry, Semnan University,
5131-19111, Semnan, Iran. E-mail: fnemati@semnan.ac.ir.
3
493

4
94
Nemati and Elhampour
Figure 1. Bioactive tetrazole moieties.
In the past decade, magnetic nanoparticles (MNPs) have become a useful group of
heterogeneous catalysts because they have high surface area, unique magnetic proper-
2
3,24
ties and high catalytic activity.
Importantly, the easy separation of magnetic nano-
particles is simple, economical, and an attractive alternative to filtration or
centrifugation because it prevents loss of catalyst and enhances reusability, making the
25
catalyst cost-effective and promising for industrial applications. However, MNPs suf-
26
fer from drawbacks, including the tendency to aggregation in the reaction mixture. To
solve this problem, MNPs are coated by using suitable stabilizing materials such as sil-
2
7–30
ica, titanium dioxide, polymers or zeolites.
Titanium dioxide (TiO ) nanoparticles have enjoyed widespread application.
31
2
However, one of the major disadvantages of TiO nanoparticles is the inconvenience of
2
recycling these catalysts. Filtration or centrifugation may lead to catalyst loss and
energy consumption. A strategy developed to solve this issue is coating the magnetic
nanoparticle core with a TiO insulation layer to avoid an unfavorable heterojunction.
2
In terms of convenience, TiO -based catalysts are cheap, easy to prepare, and are insol-
2
uble in all organic solvents. Among various TiO -based heterogeneous catalysts, Fe O
3 4
2
cores within shells of TiO have the merits of low cost, ease of preparation and can be
2
32–35
recycled.
They are easy to recover by simply applying a magnetic field.
Recently, we have used a type of magnetic nano-Fe O as an easily separable cata-
3
4
3
5–40
lyst in different organic reactions.
In continuation of our interest, we have studied
the application of the magnetic Fe O core and TiO shell with microcrystals of Cu O
3
4
2
2
for the regioselective synthesis of 1,4-disubstituated 1,2,3-triazoles, via the click reac-
3
2
tion and A and KA coupling reaction. We have previously described our rationale for
3
5,36
using TiO and Cu O together.
We now report a new process for the cyanation of
2
2
aryl iodides to the corresponding aryl nitriles and synthesis of 5-substituted 1-H-tetra-
zoles using nano-Fe O @TiO /Cu O core-shell magnetic composites as a safe, environ-
3
4
2
2
mentally benign, and inexpensive catalyst (Scheme 1).
The reaction was carried out in two steps in one pot, including: 1) preparation of
aryl nitriles and 2) synthesis of 5-substituted 1H-tetrazoles. In an effort to develop bet-
ter reaction conditions in the first step, loading of catalyst and different solvents were
screened for the synthesis of benzonitrile from the reaction of phenyl iodide and
K [Fe(CN) ] ꢀ 3H O as a template reaction (Step 1, Scheme 1). The results of the opti-
4
6
2
mization of the reaction conditions are summarized in Table 1. Solvents, temperatures
and reaction times were studied.
The data suggested that an excellent yield was obtained in DMF as the solvent at
ꢁ
1
20 C by applying 20 mg of nano magnetic Fe O @TiO /Cu O core-shell composite
3
4
2
2
and a 1:0.22 molar ratio of aryl iodide: K [Fe(CN) ] ꢀ 3H O (Table 1, entry 3). Notably,
4
6
2

5-Substituted 1H-Tetrazoles
495
Scheme 1.
Table 1
Effects of Amount of Catalyst, Duration of Reaction and Solvent on the Yield of
Benzonitrile Prepared from K [Fe(CN) ] ꢀ 3H O and Aryl Iodidea
4
6
2
Entry
Fe O @TiO /Cu O (g)
Condition/ Temp
Time (h)
Yieldb (%)
3
4
2
2
1
2
3
4
5
6
7
8
9
0.020
0.020
0.020
0.020
0.030
0.010
0.020
0.020
0.020
0.020
–
DMF / r.to
6
3
3
3
3
3
5
3
3
3
5
6
3
3
Trace
60
90
90
90
71
11
75
47
DMF / 100 C
o
DMF / 120 C
o
DMF/ 130 C
o
DMF / 120 C
o
DMF / 120 C
H O / Reflux
2
o
DMSO / 120 C
o
Toluene / 120 C
o
10
11
11
12
13
Solvent free/120 C
42
o
DMF / 120 C
n.r.
Trace
40
0.020
DMF / r.to
Fe O @TiO (0.02 g)
DMF / 120 C
3
4
2
o
Fe O /Cu O(0.02 g)
DMF / 120 C
76
3
4
2
a
Reaction condition: Iodobenzene (1 equiv.),
4 6 2
K [Fe(CN) ] ꢀ 3H O
(0.22 equiv.),
Solvent (2 mL).
b
Isolated yield.
the reaction proceeds smoothly to completion and the corresponding products were
obtained with high purity. We then investigated the reaction between the formed benzo-
nitrile with sodium azide to provide the 5-substituted 1H-tetrazole in one pot. The best
results were obtained by using a 1:1.5 ratio of benzonitrile and sodium azide after 5h.
Using our optimized conditions, we next explored the diversity and limitations of
substrates as well as the efficiency of our catalysts for the one-pot reaction to prepare
5
-substituted 1H-tetrazoles. The results are summarized in Table 2. The aryl iodides
included aromatic and heteroaromatic compounds. Yields were uniformly good and not
particulary sensitive to iodide structure.
Notably, o-iodo naphthalene and m-iodo naphthalene also reacted smoothly and
gave the corresponding products in good yield (Table 2, entries 8 and 9). It is interest-
ing to note that heteroaromatic iodides such as o-iodo pyridine and p-iodo pyridine are
compatible in the tetrazole synthesis (Table 2, entries 12 and 13). This study reveals

4
96
Nemati and Elhampour
Table 2
Synthesis of 5-Substituted 1H-Tetrazoles from Aryl Iodide with K [Fe(CN) ].3H O
4
6
2
a
and Sodium Azide Catalyzed by Nano Magnetic Fe O @TiO /Cu O (Scheme 1)
3
4
2
2
a
Reaction conditions: a mixture of Aryl iodide (1equiv.) (or its 2-Iodo pyridine and 4-Iodo pyri-
dine), K [Fe(CN) ] ꢀ 3H O (0.22 equiv.), Sodium azide (1.5 equiv.) and Nano magnetic
4
6
2
o
Fe
3
O
4
2 2
@TiO /Cu O (20 mg) was heated in 2 mL of DMF at 120 C.
b
Isolated yield.

5-Substituted 1H-Tetrazoles
497
that the nano magnetic Fe O @TiO /Cu O core-shell composite is an efficient catalyst
3
4
2
2
for both aromatic and heterocyclic tetrazole synthesis from aryl iodide derivatives.
The reusability of the catalyst is one of the most important benefits. In a typical
experiment, after the reaction was completed, the nano magnetic catalyst was isolated
from the reaction mixture by an external magnet in the work-up stage, washed with
acetone, distilled water and ethanol, respectively, and then dried in a desiccator and
subjected to the next run. The Fe O @TiO /Cu O magnetic composite was reused for
3
4
2
2
five cycles with steady activity. Moreover, there was little decrease in the yield of the
corresponding product indicating high potency of the catalyst (Run 1, 90%, Run 2,
8
9%, Run 3, 87%, Run 4, 86%, Run 5, 85%).
In summary, a very useful procedure for the preparation of 5-substituted 1H-tetra-
zoles was developed using magnetically separable nanoparticle Fe O @TiO /Cu O
3
4
2
2
core-shell composite as a heterogeneous catalyst. The important benefits of this method-
ology are high yields, an easy procedure and simple recovery and reusability of
the catalyst.
Experimental Section
All reagents were purchased from Merck and Aldrich and used without further purifica-
tion. The progress of the reactions was monitored by TLC on commercial aluminum-
backed plates of silica gel 60 F254, visualized using ultraviolet light. Melting points
were determined using an Electrothermal 9100 and were uncorrected. FT-IR spectra
were obtained with potassium bromide pellets in the range of 400–4000 cm-1 using a
Shimadzu 8400s spectrometer.
1
NMR spectra were obtained on a Bruker Avance 400 spectrometer ( H NMR at
1
3
4
00 and 250 Hz, C NMR at 100 and 62.9 Hz) in CDCl or DMSO using TMS as an
3
internal standard. Chemical shifts (d) are given in ppm and coupling constants (J) in
Hz. Nano magnetic-Fe O @TiO /Cu O core-shell composite was prepared and charac-
3
4
2
2
3
5
terized according to our previous report. The model reaction was also carried out in
the presence of Fe O @TiO and Fe O Cu O as catalyst, and lower yield of the corre-
3
4
2
3
4/
2
sponding product was obtained (Table 1, entries 12 and 13).
Preparation of nano-Fe O
3
4
37
The Fe O nanoparticles were prepared in accordance with our earlier report.
3
4
Preparation of nano-Fe O @TiO
2
3
4
42
Nano-Fe O @TiO was synthesized according to a previously reported method,
3
4
2
subjected to modification. The above prepared nano-magnetic Fe O particles were dis-
3
4
persed in a mixture of absolute ethanol and acetonitrile (250 mL/90 mL). This mixture
was sonicated for 15 min, and then 1.5 mL of NH OH solution (25 wt.%) was added.
4
After continuous mechanical stirring for 30 min, 3 mL (8 mmol) of tetrabutyl titanate
(
TBOT), dissolved in 20 mL of absolute ethanol was added drop-wise to the above
ꢁ
suspension, under continuous mechanical stirring at 30 C. The reaction mixture was
further stirred for 1.5 h to obtain Fe O @TiO core/shell nanoparticles. Magnetic separation
3
4
2

4
98
Nemati and Elhampour
was used for collection of nanoparticles. After that, they were washed three times with
absolute ethanol.
Preparation of nano-Fe O @TiO /Cu O Core-Shell Magnetic Composite
3
4
2
2
Four types of composites with different molar ratio of Fe O @TiO :Cu O were synthe-
3
4
2
2
sized. Here we describe the most favorable ratio. Nano-Fe O @TiO (0.036-0.324 g)
3
4
2
was dispersed in 80 mL of deionized water. 5 mL of (0.1 mol/L) CuCl2 solution
was added to the aqueous Fe O @TiO and sonicated for 15 min. Then, 1.8 mL of
3
4
2
(1.0 mol/L) NaOH solution was added dropwise under ultrasonic radiation. The result-
ing solution turned light blue instantly, indicating the formation of Cu(OH) .
2
.
Eventually, 12 mL of NH OH HCl (0.1 mol/L) was immediately injected over 5 sec.
2
into the solution. The solution was kept in the water bath for 1 h and centrifuged for
3
min. Then the solution was decanted as much as possible. After the liquid was deca-
nted, the precipitate was washed with 6 mL of a 1:1 volume ratio of water and ethanol
three times. The nano-Fe O @TiO /Cu O was collected as a brown solid which could
3
4
2
2
be stored in a tight vessel for several months without any changes in color and activity.
36
We found that the most appropriate ratio of nano-Fe O @TiO :Cu O was 80:20%.
3
4
2
2
Therefore, the optimal weight ratio of Fe O @TiO :Cu O 8:2 was selected for this
3
4
2
2
methodology.
General Procedure for Synthesis of 5-Phenyl-1H-tetrazole (entry 1)
.
Nano-Fe O @TiO /Cu O (20 mg) and K [Fe(CN) ] 3H O (0.09 g, 0.22 mmol), was
3
4
2
2
4
6
2
added to a solution of aryl iodide (1 mmol) in distilled DMF (2 mL) and the reaction
ꢁ
mixture was stirred under heating at 120 C for the appropriate time to obtain the nitrile
compound. To the nitrile compound generated in situ was added sodium azide (Safety
Note: Sodium azide is highly toxic. All workers should be thoroughly familiar with its
safe use before attempting experiments and proper personal protective gear must be
used. 0.1g, 1.5 mmol) and the reaction was continued until the complete conversion of
the nitrile to the tetrazole during 5 hours. After the completion of the reaction (as moni-
tored by TLC, 75:25 ethyl acetate: n-hexane), the catalyst was easily separated out from
the reaction mixture by using an external magnet, washed with acetone, dried in the
oven and re-used for a consecutive run under the same reaction conditions. After the
separation of the catalyst the crude material was then taken up in ethyl acetate and
washed with HCl (5 N) and the layers separated. The combined organic layers were
then washed with water and concentrated to obtain the crude product. The crude prod-
uct was purified by short column chromatography to obtain the pure products in high
yield (see Tables 1 and 2). All the products are known compounds and the spectral data
and melting points were identical to those reported in the literature.
Spectral Data of Selected Products
ꢁ
1
5
(
-Phenyl-1H-tetrazole (Table 2, entry 1): White powder, m.p. 213-215 C; H-NMR
13
400 MHz, DMSO): d 7.92 (s, 2H), 7.68 (s, 3H), C-NMR (100 MHz, DMSO): d
1
55.0, 134.6, 130.3, 128.6, 126.6.

5-Substituted 1H-Tetrazoles
499
ꢁ
5
-(4-Nitrophenyl)-1H-tetrazole (Table 2, entry 2): Yellow solid, m.p. 218-219 C;
1
1
H-NMR (400 MHz, DMSO): d 8.39 (d, 2H, J 8.8, Ar-H), 8.30 (d, 2H, J 8.4, Ar-H),
3
C-NMR (100 MHz, DMSO): d 149.51, 131.08, 129.13, 127.66.
ꢁ
5
-(4-Methylphenyl)-1H-tetrazole (Table 2, entry 3): White solid, m.p. 249-251 C.
1
H NMR (250 MHz, DMSO): d 7.90 (d, 2H, J 7.5 Hz, ArH), 7.37 (d, 2H, J 7.6 Hz,
ArH), 2.35 (s, 3H, CH3).
ꢁ
5
-(3-Chlorophenyl)-1H-tetrazole (Table 2, entry 5): White solid, m.p. 138-140 C;
1
1
H NMR (250 MHz, DMSO): d 7.99, (s, 1H), 7.96 (d, 1H, J 7.6, Ar-H), 7.55 (m, 2H).
3
C NMR (62.9 MHz, DMSO): d 154.6, 133.9, 131.1, 130.7, 126.4, 126.2, 125.4.
5
-(4-Chlorophenyl)-1H-tetrazole (Table 2, entry 6): White powder, m.p. 251-
ꢁ
1
2
53 C; H-NMR (400 MHz, DMSO): d 8.09 (d, 2H, J 8.8, Ar-H); 7.61 (d, 2H, J 8.4,
Ar-H).
-(4-Hydroxyphenyl)-1H-tetrazole (Table 2, entry 10): White powder, m.p. 235 C;
H-NMR (400 MHz, DMSO): d 10.11 (s broad, OH); 7.58 (d, 2H, J 8.4, Ar-H); 6.91
ꢁ
5
1
1
3
(d, 2H, J 8.4, Ar-H);
C-NMR (100 MHz, DMSO): d 159.8; 153.2; 128.8;
1
17.4; 116.1.
ꢁ
4
-(1H-Tetrazol-5-yl)-benzonitrile (Table 2, entry 11): White solid, m.p. 257-259 C.
1
H NMR (250 MHz, DMSO): d 8.19 (d, 2H, J 8.6, Ar-H), 8.06 (d, 2H, J 7.1, Ar-H).
13
C NMR (62.9 MHz, DMSO): d 162.2, 155.2, 133.17, 128.7, 127.6, 118.1, 113.3.
ꢁ
1
2
-(1H-Tetrazol-5-yl)pyridine (Table 2, entry 12): White solid, m.p. 210-213 C; H-
NMR (400 MHz, DMSO): d 8.63 (s, 1H); 8.20 (d, 1H, J 8.4 Ar-H); 8.07 (s, 1H); 7.75
s, 1H).
-(1H-Tetrazol-5-yl)pyridine (Table 2, entry 13): White solid, m.p. 256-258 C; H
(
ꢁ
1
4
13
NMR (250 MHz, DMSO): d 8.77 (d, 2H, J 6.5, Ar-H), 8.10 (d, 2H, J 6.0, Ar-H),
NMR (62.9 MHz, DMSO): d 165.7, 149.9, 133.8, 121.3, 120.9.
C
Acknowledgments
The authors gratefully acknowledge The Research Council of Semnan University
Science and Technology Park and the Vice-chancellor for Education and Postgraduates
for financial support of this work.
ORCID
Firouzeh Nemati
http://orcid.org/0000-0002-9446-8936
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