Green synthesis of the 1-substituted 1H-1,2,3,4-tetrazoles by application of the Natrolite zeolite as a new and reusable heterogeneous catalyst

Article abstract of DOI:10.1039/c1gc15245a

An efficient method for the preparation of 1-substituted 1H-1,2,3,4-tetrazoles is reported using Natrolite zeolite as a natural and heterogeneous catalyst under solvent-free conditions. This method has the advantages of high yields, simple methodology and easy work-up. The catalyst can be recovered by simple filtration and reused in good yields.

Full text of DOI:10.1039/c1gc15245a

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PAPER
Green synthesis of the 1-substituted 1H-1,2,3,4-tetrazoles by application of
the Natrolite zeolite as a new and reusable heterogeneous catalyst†
a
a
b
Davood Habibi,* Mahmoud Nasrollahzadeh and Taghi A. Kamali
Received 6th March 2011, Accepted 4th October 2011
DOI: 10.1039/c1gc15245a
An efficient method for the preparation of 1-substituted 1H-1,2,3,4-tetrazoles is reported using
Natrolite zeolite as a natural and heterogeneous catalyst under solvent-free conditions. This
method has the advantages of high yields, simple methodology and easy work-up. The catalyst can
be recovered by simple filtration and reused in good yields.
Introduction
A substitute for hydrazoic acid is a mixture of sodium azide
2d
and ammonium chloride in DMF as a solvent. Combining
azides and acids may yield gaseous HN , which is toxic and
Tetrazoles have received considerable attention because of their
wide range of applications in pharmaceuticals, information
recording systems, photography and explosives and rocket
3
flammable. Also, the reaction mixture in DMF must be heated
◦
to 150 C for several hours to several days. An additional
1,2
propellants. They are important ligands for many useful
transformations and also precursors for a variety of nitrogen-
disadvantage of DMF is its solubility in both organic solvents
and water, thus removing DMF from tetrazole is difficult. From
the standpoint of ‘green chemistry’, significant efforts have been
made to find an alternative to organic solvents, such as solvent-
free conditions, which are industrially important due to reduced
3,4
containing heterocycles.
Among tetrazoles, 1-substituted tetrazoles have received much
attention because of their wide utility.
of convenient methods for the preparation of 1-substituted
tetrazoles strongly restricts their potential application in medical
practice. Although many 5-substituted tetrazoles are known,
1b,1e
However, the lack
11
pollution, low cost, and simplicity in processing and handling.
The first principle of green chemistry states that it is better
to prevent waste production than to treat waste or clean it up
5,6
only very few 1-substituted tetrazoles have been described.
11g
after it has been created. In our methods, tetrazoles were
The acid-catalyzed cycloaddition between isocyanides and
hydrazoic acid or trimethyl azide has long been one of the main
routes for the synthesis of 1-substituted tetrazoles, which often
include a number of steps and are insufficiently effective. One
of the most promising methods of synthesis for 1-substituted
tetrazoles involves the reaction of amines with ethyl orthofor-
mate or orthocarboxylic acid ester and sodium azide in the
prepared by application of stoichiometric amounts of sodium
azide (2.0 mmol) and amine (2.0 mmol) under solvent-free
conditions. So, after completion of the reaction, there is no
sodium azide residue in the reaction mixture, which can form
a heavy-metal azide or potentially explosive hydrazoic acid and
its toxic vapours. Sodium azide is not explosive except when
7,8
◦
heated near its decomposition temperature (300 C) or reacted
presence of acetic acid, trifluoroacetic acid or PCl
5
, ionic liquid
with metals. Compared to the reported methods, our method is
convenient, fast, safe and is easy to work-up.
9
and DMSO and ytterbium triflate in highly polar solvents.
Earlier reported methods for the synthesis of tetrazoles suffer
from drawbacks, such as the use of expensive and toxic reagents,
application of high boiling solvents, low yields, long reaction
times, harsh reaction conditions, difficulty in obtaining and/or
preparing the starting materials, tedious work-up, and the
presence of hydrazoic acid, which is highly toxic, explosive and
In recent years there has been a tremendous interests in
various chemical transformations performed under the hetero-
12
geneous catalysis. Moreover, application of the inorganic solid
acids, especially zeolites, has received considerable attention
due to their unique physical and chemical properties, such as
13a
shape, selectivity, acidic and basic nature and thermal stability.
10
volatile.
Natrolite zeolite, Na
zeolites with the framework constructed from chains of corner-
sharing Al and Si oxygen tetrahedra.
2
[Al
2
Si
3
O
10]·2H
2
O, is one of the fibrous
1
3b
a
Department of Organic Chemistry, Faculty of Chemistry, Bu-Ali Sina
University, Hamedan, 6517838683, Iran.
In continuation of our work on the synthesis of tetrazoles
E-mail: davood.habibi@gmail.com; Fax: +98 811 8257407; Tel: +98
5,14
and the application of heterogeneous reagents,
we hereby
8
11 8282807
report a new protocol for the preparation of 1-substituted
tetrazoles from a wide variety of primary amines with sodium
azide and triethyl orthoformate under solvent-free conditions
b
Gas Division, Research Institute of Petroleum Industry, Tehran, Iran
†
1
Electronic supplementary information (ESI) available. See DOI:
0.1039/c1gc15245a
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Table 1 Formation of 1-substituted tetrazoles 2a–o via primary amines
1
a–o
a
Entry Substrate
1
Product
Yield (%) Ref.
b
94, 91
7b,9c,16,17
Scheme 1
14a,15a,b
2
89
9c
using Natrolite zeolite
Scheme 1).
as a reusable and natural catalyst
(
3
4
5
6
83
82
93
80
9c,16,17
7b,16
Results and Discussion
The reaction of aniline and triethyl orthoformate with sodium
azide in the presence of Natrolite zeolite were carried out in
various temperatures and catalyst amounts under solvent-free
conditions to optimize the reaction conditions (Fig. 1). In the
9c,16
◦
absence of catalyst at 120 C, no reaction was carried out, even
9c,17
after 6 h. The optimum amount of Natrolite zeolite was found
to be 0.02 g in the presence of amine (2.0 mmol), sodium azide
(
2.0 mmol) and orthoformate (2.4 mmol) in 4 h. Increasing
7
8
82
81
This work
This work
the amount of Natrolite zeolite to more than 0.02 g showed
no substantial improvement in the yield, while decreasing the
catalyst reduced the yield to 0.015, 0.01 and 0.007 g.
9
85
83
This work
This work
1
0
1
1
1
2
85
86
16
This work
Fig.
1 Optimization of the amount of Natrolite zeolite and
temperature.
1
3
85
This work
A variety of amines possessing both electron-releasing and
electron-withdrawing groups were employed (Table 1). Accord-
ing to the table, the nature of the substituent on the benzene ring
did not affect the reaction time and the yields were excellent. Due
1
1
4
5
86
81
7b
to the presence of the two NH groups, 1l interestingly afforded
2
9c,16
the double-addition product (Table 1, entry 12).
The suggested Natrolite zeolite catalyzed transformation
mechanism is shown in Scheme 2, in which the Lewis acidity
of the catalyst probably has an important role in the promotion
of the cyclization process. The breakdown of triethyl orthofor-
mate in the presence of the catalyst facilitates the elimination
of ethanol, as well as the formation of the imidoylazide
a
b
Yields are after work-up. Yield after the fifth cycle.
5,17,18,19
intermediate,
which will produce the final 1-substituted
A number of synthetic methods have been reported in recent
1
H-1,2,3,4-tetrazole upon cyclization.
The second principle of green chemistry states that the
years for the preparation of 1-substituted tetrazoles (Scheme
3, equations 1–5). From the view point of atom economy, the
present methodology (Scheme 3, equation 6) compared with
those reported in the literature (Scheme 3 and Table 2).
Disadvantages of equation (1): Application of large excess
amounts of hydrazoic acid (highly toxic, explosive and volatile),
low yield, long reaction times (24 h), harsh reaction conditions
synthetic systems should be focused to improve the atom
economy or atom efficiency by the employment of equimolar
amounts of starting materials. Atom economy (AE) is a useful
concept for estimating the environmental impacts of starting
11g
8
20
materials in a reaction to reach a desired product.
3
500 | Green Chem., 2011, 13, 3499–3504
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Table 2 The Atomic Economy comparison of the present methodology
with other methods for the synthesis of 1-substituted tetrazoles
a
Entry
1
Equation
1
Desired product
Yield (%)
16
AE (%)
100
2
4
5
6
1
67
85
86
94
34
58.39
47.71
47.71
47.71
100
2
Scheme 2
4
5
6
1
89
89
89
88
49.99
49.99
49.99
100
3
4
5
6
4
5
6
1
91
88
83
35
49.99
49.99
49.99
100
2
5
6
1
92
85
82
32
62.91
52.37
52.37
100
4
5
6
1
88
88
93
41
52.99
52.99
52.99
100
Scheme 3
4
6
6
82
80
82
52.99
52.99
58.42
and use of sulfuric acid, which is toxic, extremely corrosive, and
cannot be easily recovered and recycled.
7
8
7b
Disadvantages of equation (2): In situ formation of hydra-
zoic acid (highly toxic, explosive and volatile), use of hydrochlo-
ric acid (toxic, corrosive and homogeneous reagent that cannot
easily be recovered and reused) and application of organic
solvents. Trimethylsilyl azide is a highly flammable substance
and incompatible with the presence of moisture, strong oxidizing
agents and strong acids. It is toxic by inhalation, in contact
with eye, skin and if swallowed or when it is exposed to water.
Also, isocyanides are toxic and very reactive with any materials
containing active hydrogens, such as water, alcohol, amines,
bases and acids. Therefore, working with these materials is very
hard. In addition, a by-product of this reaction is trimethylsilane
6
6
81
85
57.21
52.09
9
10
6
83
57.31
1
1
1
2
6
6
85
86
54.02
66.66
(
Me
3
SiH), which is a toxic and flammable substance and may
cause flash fire and target organ damage.
9e
Disadvantages of equation (3): In situ formation of hydrazoic
acid (highly toxic, explosive and volatile), application of large
excess amounts of glacial acetic acid, which is homogeneous and
cannot easily be recovered and reused.
1
3
6
85
49.99
9c
Disadvantage of equation (4): Application of a volatile
organic solvent (CH OC OH).
3
2
H
4
9d
Disadvantage of equation (5): Application of a high boiling
point organic solvent (DMSO) leading to complicated isolation
and recovery procedures.
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Table 2 (Contd.)
spectral data with authentic samples. The NMR spectra were
recorded in DMSO and acetone. H NMR spectra were recorded
on a Bruker Avance DRX 90 and 300 MHz instruments. The
1
a
Entry
Equation
2
Desired product
Yield (%)
AE (%)
chemical shifts (d) are reported in ppm relative to the TMS as an
1
4
5
77
64.71
1
3
internal standard and J values are given in Hz. C NMR spectra
were recorded at 24.5 and 75 Hz. FT-IR (KBr) spectra were
recorded on a Perkin-Elmer 781 spectrophotometer. Melting
points were taken in open capillary tubes with a BUCHI 510
melting point apparatus and were uncorrected. The elemental
analysis was performed using Heraeus CHN-O-Rapid analyzer.
TLC was performed on silica gel polygram SIL G/UV 254
plates. Natural Iranian Natrolite zeolite was obtained from
the Hormak area (city of Zahedan, province of Sistan &
6
6
86
81
54.41
47.88
1
a
Yields are after work-up.
Advantages of equation (6): None of the above mentioned
14a
Baluchestan, Iran).
disadvantages are observed. Also, the application of Natrolite
zeolite as a natural, benign, inexpensive and non-corrosive
heterogeneous catalyst represents an interesting synthetic route
with relatively high atom economy. The only by-product of this
reaction is ethanol, which is a green solvent.
All known compounds were characterized by comparing their
physical and spectral data with those reported in the literature.
The structures of the 1-substituted tetrazoles were in agreement
with their IR and NMR spectra. In the IR spectra of the 1-
Catalyst characterization
The natural Natrolite zeolite was characterized by using powder
XRD, SEM and FT-IR spectroscopy. Also, X-ray powder
diffraction was applied for its phase purity in which the pattern
15a
is completely matched with that of the Natrolite zeolite. The
actual phases for this catalyst were silicon oxide cristabolite-SiO
2
(
cubic) and aluminum silicate zeolite. The composition of the
substituted tetrazoles, the NH
strong absorptions band was detected (C N stretching band,
2
peaks were disappeared and one
15a
natural zeolite is Na
of the material is 1.77 corresponding to a Natrolite zeolite.
2
[Al
2
Si
3
O10]·2H
2
O. The SiO
2
/Al
2
O
3
ratio
15c
-
1
~
1640–1690 cm ). The 1-substituted tetrazoles are generally
In the relevant FT-IR spectra (Electronic Supporting
acidic substances and the relevant proton signal will be shifted
-1
Information†), peaks 950–1200 cm are probably related to
1
to downfield (see Fig. 1 and H NMR data), so the peak at d =
the Si–O–Si and Si–O–Al vibrations, peaks 3619, 3440 and
7
.80–8.30 ppm can be attributed to the proton of the tetrazole
-1
1
8
640 cm to the presence of zeolitic water and peaks 500–
ring. The carbon of the tetrazole ring can be seen at about d =
-1
14a,15a
-1
00 cm to the pseudo-lattice vibrations.
the familiar bending HOH will appear at 1640 cm , while
Above 1500 cm ,
1
3
5,6,9,14a,14e,16
1
41–157.2 ppm in the C NMR.
-1
their asymmetric and symmetric stretching appear at 3440, and
Catalyst reuse and stability
-1
3619 cm , respectively.
The SEM pictures for this new type of zeolite are given
The catalyst recycling is an important step as it reduces the
cost of the process. The reusability of the Natrolite zeolite
catalyst was checked in the reaction of different anilines with
sodium azide and orthoformate under the reaction conditions
in Fig. 2. The surface of the natural zeolite specimens is
highly heterogeneous due to the coexistence of different zeolite
phases together with other crystalline and amorphous materials.
Crystals of various shapes and sizes, together with amorphous
masses, incorporate into the friable grains. Their size is in the
range 1–50 mm.
(
Table 1, entry 1). In a typical experiment, after completion of
the reaction, Natrolite zeolite was isolated by simple filtration,
washed with water (10 mL) and ethanol (5 mL) three times, dried
and successively used 5 times without any significant loss of
activity (yields: ~91–94%). The XRD patterns before and after
the reaction revealed that the zeolite retained its crystallinity
throughout.
The chemical composition of silica and alumina was estab-
lished by a wavelength dispersive XRF spectrophotometer (Elec-
tronic Supporting Information). Based on the XRF technique,
the mineral content of the zeolitic material from quarry face was
15b
Natrolite 99.63% (w/w).
Conclusion
CAUTION
In conclusion, we have developed an efficient procedure for the
preparation of 1-substituted tetrazoles using Natrolite zeolite
as a natural and reusable heterogeneous catalyst. This method
has the advantages of high yields, simple methodology and
easy work-up in which the chromatographic separation is not
necessary to get the pure compounds.
Although sodium azide is a valuable substance in organic
synthesis, it is nonetheless a toxic, reactive and energetic material
and appropriate safety precautions should be taken (for more
information see the Electronic Supporting Information†).
Typical procedure for preparation of 1-substituted
1H-1,2,3,4-tetrazoles
Experimental
Natrolite zeolite (0.02 g) was added to a mixture of amine
All reagents were purchased from the Merck and Aldrich
chemical companies and used without further purification.
Products were characterized by comparison of their physical and
(2.0 mmol), NaN (2.0 mmol), triethyl orthoformate (2.4 mmol)
and stirred at 120 C for 4 h. After completion (as monitored by
TLC), the reaction mixture was diluted with cold water (5 mL)
3
◦
3
502 | Green Chem., 2011, 13, 3499–3504
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1
13
The elemental analysis (CHN), IR, H NMR and C NMR
data of the unknown 1-substituted tetrazoles (Table 1, entries
1
7
–10 and 12–14) and H NMR data of the known compounds
(Table 1, entries 1–6, 11 and 15) are given below:
1
-Phenyl-1H-1,2,3,4-tetrazole (2a, Table 1, entry 1). M.p.
◦
9c
◦
1
6
7
5–66 C (lit. 64–65 C); H NMR (90 MHz, CDCl
.39–6.99 (m, 4H), 8.21 (s, 1H).
3
): d =
1
-(3-Methylphenyl)-1H-1,2,3,4-tetrazole (2b, Table 1, entry 2).
◦
9c
◦
1
M.p. 53–55 C (lit. 53–54 C); H NMR (90 MHz, CDCl
.30 (s, 1H), 6.82–7.19 (m, 4H), 8.19 (s, 1H).
3
): d =
2
1
-(4-Methylphenyl)-1H-1,2,3,4-tetrazole (2c, Table 1, entry 3).
◦
9c
◦
1
M.p. 94–95 C (lit. 93–94 C); H NMR (90 MHz, CDCl
.30 (s, 1H), 6.92–7.27 (m, 4H), 8.16 (s, 1H).
3
): d =
2
1
-(4-Chlorophenyl)-1H-1,2,3,4-tetrazole (2e, Table 1, entry 5).
◦
9c
◦
1
M.p. 156–158 C (lit. 157–158 C); H NMR (90 MHz, CDCl
3
):
d = 6.96 (d, J = 6.8 Hz, 2H), 7.28 (d, J = 6.7 Hz, 2H), 8.09 (s,
1
H).
1
-(2-Chlorophenyl)-1H-1,2,3,4-tetrazole (2f, Table 1, entry 6).
1
H NMR (90 MHz, CDCl ): d = 7.10–7.44 (m, 4H), 8.07 (s, 1H).
3
1
-(4-Bromophenyl)-1H-1,2,3,4-tetrazole (2g, Table 1, entry 7).
◦
M.p. 183–185 C; FT-IR (Nujol) v 3527, 3151, 3045, 2855, 1660,
1
589, 1575, 1483, 1402, 1312, 1291, 1224, 1205, 1173, 1101, 1073,
-
1
1
1
003, 983, 827 cm ; H NMR (90 MHz, CDCl ): d = 6.90 (d,
13
3
J = 8.7 Hz, 2H), 7.40 (d, J = 7.7 Hz, 2H), 8.07 (s, 1H); C NMR
22.5 MHz, DMSO-d ): d = 152.4, 135.6, 128.2, 124.8, 117.6;
Anal. calcd for C Br: C, 37.36; H, 2.24; N, 24.90. Found:
C, 37.42; H, 2.26; N, 24.79.
(
6
7
H
5
N
4
1
-(3-Trifluoromethylphenyl)-1H-1,2,3,4-tetrazole (2h, Table 1,
◦
entry 8). M.p. 125–127 C; FT-IR (KBr): v 3275, 3220, 3140,
005, 2981, 2940, 2840, 1620, 1585 cm ; H NMR (90 MHz,
-
1
1
3
1
3
DMSO-d
6
): d = 7.60–7.20 (m, 4H), 8.19 (s, 1H); C NMR (22.5
): d = 148.8, 145.1, 132.5, 131.0, 129.8, 121.9,
20.1, 115.7; Anal. calcd for C : C, 44.87; H, 2.35; N,
6.16. Found: C, 44.80; H, 2.46; N, 26.33.
MHz, DMSO-d
6
1
2
8
H
5
F
3
N
4
1
-(2,4-Dimethylphenyl)-1H-1,2,3,4-tetrazole (2i, Table 1, en-
◦
try 9). M.p. 133–135 C; FT-IR (Nujol): v 2917, 1655, 1606,
-
1
1
1
496, 1301, 1204, 1119, 811 cm ; H NMR (90 MHz, DMSO-
): d = 6.94–7.87 (d, J = 5.9 Hz, 2H), 7.80 (s, 1H), 8.69 (s, 1H);
3
d
6
1
C NMR (22.5 MHz, DMSO-d
26.7, 122.8, 118.8, 20.2, 17.8; Anal. calcd for C
H, 4.79; N, 26.65. Found: C, 68.61; H, 4.70; N, 26.60.
6
): d = 147.7, 131.3, 130.7, 128.7,
1
9
H
10
N
4
: C, 68.56;
1
-(2,5-Dichlorophenyl)-1H-1,2,3,4-tetrazole (2j, Table 1, en-
Fig. 2 Scanning electron microscopy analysis (SEM) of the Natrolite
zeolite.
◦
try 10). M.p. 99–105 C; FT-IR (Nujol): v 3387, 2854, 1649,
575, 1523, 1466, 1416, 1377, 1295, 1094, 1045, 802 cm ; H
-
1
1
1
NMR (90 MHz, DMSO-d ): d = 6.89–7.05 (m, 1H), 7.24–7.54
6
1
3
(
m, 1H), 7.62 (s, 1H), 8.03 (s, 1H); C NMR (22.5 MHz, DMSO-
and extracted with ethyl acetate (3 ¥ 10 mL). The catalyst
was separated by filtration and the combined organic layers
d
for C
1
6
): d = 157.1, 149.3, 132.1, 130.3, 124.6, 123.5, 120.3; Anal. calcd
Cl : C, 39.09; H, 1.87; N, 26.06. Found: C, 39.13; H,
.85; N, 26.13.
7
H
4
2
N
4
were washed with brine and dried over anhydrous Na
2
SO .
4
After concentration, a crystallizaton step was performed using
EtOAc–hexane to afford the pure product. The physical data
1-(4-Acethylphenyl)-1H-1,2,3,4-tetrazole (2k, Table 1, en-
1
(
mp, IR, NMR) of known compounds were found to be identical
try 11). H NMR (90 MHz, CDCl ): d = 2.59 (s, 3H), 7.09–8.00
(m, 4H), 8.29 (s, 1H).
3
7b,9c,16
with those reported in the literature.
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1
,1¢-(1,2-Phenylene)bis(1H-1,2,3,4-tetrazole) (2l, Table 1, en-
K. Genguly and R. W. Versace, Eur. Patent EP 274867, 1988; V. M.
Girijavallabhan, P. A. Pinto, A. K. Genguly and R. W. Versace, Chem.
Abstr., 1989, 110, 23890; (d) H. Akimoto, K. Ootsu and F. Itoh, Eur.
Patent EP 530537, 1993; H. Akimoto, K. Ootsu and F. Itoh, Chem.
Abstr., 1993, 119, 226417; (e) A. G. Taveras, A. K. Mallams and A.
Afonso, Int. Patent WO 9811093, 1998; A. G. Taveras, A. K. Mallams
and A. Afonso, Chem. Abstr., 1998, 128, 230253.
◦
try 12). M.p. 167–169 C; FT-IR (Nujol): v 3108, 3003, 2940,
618, 1587, 1476, 1457, 1408, 1364, 1299, 1271, 1245, 1201, 1133,
002, 956, 886, 767, 744, 634, 626 cm ; H NMR (90 MHz,
1
1
-
1
1
1
3
DMSO-d
6
): d = 7.12– 7.69 (m, 4H), 8.30 (s, 2H); C NMR (22.5
): d = 141.9, 138.1, 121.7, 115.3; Anal. calcd for
: C, 44.85; H, 2.82; N, 52.32. Found: C, 44.65; H, 2.71;
N, 53.25.
MHz, DMSO-d
6
5
M. Nasrollahzadeh, Y. Bayat, D. Habibi and S. Moshaee, Tetrahedron
Lett., 2009, 50, 4435.
C
8
H
6
N
8
6 T. Jin, F. Kitahara, S. Kamijo and Y. Yamamoto, Tetrahedron Lett.,
2
008, 49, 2824.
1
-(2-Methylphenyl)-1H-1,2,3,4-tetrazole (2m, Table 1, en-
7
(a) D. M. Zimmerman and R. A. Olofson, Tetrahedron Lett., 1969,
10, 5081; (b) T. Jin, S. Kamijo and Y. Yamamoto, Tetrahedron Lett.,
◦
try 13). M.p. 150–153 C; FT-IR (Nujol): v 3010, 2869, 1666,
592, 1569, 1488, 1467, 375, 1304, 1221, 1186, 1112, 1049, 997,
34, 834, 782, 752, 722 cm ; H NMR (90 MHz, DMSO-d ):
d = 3.38 (s, 3H), 6.88–7.17 (m, 4H), 7.86 (s, 1H); C NMR (22.5
MHz, DMSO-d ): d = 147.8, 144.6, 130.0, 128.8, 126.5, 122.6,
18.9, 17.8; Anal. calcd for C : C, 59.99; H, 5.03; N, 34.98.
Found: C, 60.05; H, 4.95; N, 34.92.
2
004, 45, 9435.
1
9
8
9
F. G. Fallon and R. M. Herbst, J. Org. Chem., 1957, 22, 933.
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-
1
1
6
1
3
(
2
c) W. Su, Z. Hong, W. Shan and X. Zhang, Eur. J. Org. Chem., 2006,
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1
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H
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N
4
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4
1, 999.
1
-(4-Nitrophenyl)-1H-1,2,3,4-tetrazole (2n, Table 1, entry 14).
H NMR (90 MHz, acetone-d ): d = 7.57 (d, J = 8.7 Hz, 2H),
.23 (d, J = 8.8 Hz, 2H), 8.52 (s, 1H).
1
1
0 (a) K. Koguro, T. Oga, S. Mitsui and R. Orita, Synthesis, 1998, 6, 910;
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1
6
8
1 (a) K. Tanaka, Solvent-Free Organic Synthesis, Wiley-VCH: Wein-
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1
-(2-pyridine)-1H-1,2,3,4-tetrazole (2o, Table 1, entry 15).
◦
9c
◦
1
M.p. 128–130 C (lit. 129–130 C); H NMR (90 MHz, CDCl ):
d = 6.86–8.52 (m, 4H), 9.29 (s, 1H).
3
5
8, 1235; (e) K. Tanaka and F. Toda, Chem. Rev., 2000, 100, 1025;
(
(
f) J. H. Clark and D. J. Macquarrie, Chem. Soc. Rev., 1996, 25, 303;
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Acknowledgements
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1
´
Lett., 2007, 48, 5181; (b) V. Polshettiwar and A. Moln a´ r, Tetrahedron,
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We gratefully acknowledge the financial support from the Bu-Ali
Sina University, Hamedan, 6517838683 Iran.
2
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relationships in heterogeneous catalysis, Elsevier Publishers: Amster-
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504 | Green Chem., 2011, 13, 3499–3504
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