An efficient synthesis of 5-substituted 1H-tetrazoles via B(C 6F5)3 catalyzed [3+2] cycloaddition of nitriles and sodium azide

Article abstract of DOI:10.1016/j.tetlet.2014.04.089

A simple and efficient protocol is developed for the synthesis of 5-substituted 1H-tetrazole derivatives from various nitriles and sodium azide (NaN₃) via [3+2] cycloaddition reaction using B(C₆F ₅)₃ as a catalyst. The present synthetic method displayed significant advantages such as low catalyst loading, mild reaction conditions, low toxicity, easy work-up, high yields, and compatibility with other functional groups.

Full text of DOI:10.1016/j.tetlet.2014.04.089

Tetrahedron Letters xxx (2014) xxx–xxx
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An efficient synthesis of 5-substituted 1H-tetrazoles via B(C6F5)3
catalyzed [3+2] cycloaddition of nitriles and sodium azide
⇑
Santosh Kumar Prajapti, Atulya Nagarsenkar, Bathini Nagendra Babu
Department of Medicinal Chemistry, National Institute of Pharmaceutical Education and Research, Balanagar, Hyderabad 500037, India
a r t i c l e i n f o
a b s t r a c t
Article history:
A simple and efficient protocol is developed for the synthesis of 5-substituted 1H-tetrazole derivatives
from various nitriles and sodium azide (NaN3) via [3+2] cycloaddition reaction using B(C6F5)3 as a cata-
lyst. The present synthetic method displayed significant advantages such as low catalyst loading, mild
reaction conditions, low toxicity, easy work-up, high yields, and compatibility with other functional
groups.
Received 25 February 2014
Revised 26 April 2014
Accepted 27 April 2014
Available online xxxx
Ó 2014 Elsevier Ltd. All rights reserved.
Keywords:
5-Substituted 1H-tetrazole
Trispentafluorophenyl borane [B(C6F5)3]
Nontoxic
NaN3
[3+2] Cycloaddition
Tetrazoles are an important class of heterocycles with a wide
range of application in the field of medicinal chemistry.1 The
tetrazoles are representative of active pharmacophores for several
therapeutic active molecules such as antiallergic,2a anti-inflamma-
tory,2b antibiotic,2c antihypertensive,2d antineoplastic,2e,f antiviral,
and receptor modulator activities.2g Angiotensin II receptor block-
ers belonging to the sartan family often contain tetrazole as their
active moiety, for example losartan, and candesartan.3 Tetrazole
derivatives have potential in drug development for HIV and other
immune diseases4 and also play an important role in metabolically
stable surrogate for a carboxylic acid group in biologically active
molecules.5 Tetrazoles are found to be useful in various material
sciences, including photography, and explosive.6 In addition to this,
nanocrystalline ZnO,10c mesoporous ZnS nanospheres,10d Zn
hydroxyapatite,10e Pd(PPh3)4,10f Cu2O,10g and CuFe2O4 nanoparti-
cles.10h Acid catalysts are also employed for the synthesis of tetra-
zoles via cycloaddition of isocyanide to hydrazoic acid.11a Very
recently, AgNO3 has also been described for the same purpose.11b
However, some of the reported methods suffer from drawbacks
such as harsh reaction condition, inferior yields of the desired prod-
uct, formation of side product, stoichiometric amount of catalyst,
water sensitivity, use of organic azide complexes such as tin or
silicon organic azides, and the in situ generated hydrazoic acid,
which is toxic, volatile, and explosive.9g
In view of the demands of organic syntheses, there is still need
to develop new catalytic, environmentally benign and efficient
protocol for the preparation of tetrazoles. In this regard, tris(penta-
fluorophenyl)borane [B(C6F5)3] has been explored as non conven-
tional Lewis acid of comparable strength to BF3, but without the
problem associated with reactive BAF bond.12a B(C6F5)3 gained
prominence because of its thermal stability, less toxicity than other
Lewis acids, air stable, and water tolerant Lewis acid.12b Numerous
research groups are actively involved to explore the potential util-
ity of B(C6F5)3 in modern organic synthesis, such as ring-opening of
epoxides,13a aza-Ferrier glycosylation,13b hydrosilylation of
imines,14 reduction of alcohols with silane,15 and hydrogenation
of imines.16 B(C6F5)3 was utilized in the regio- and stereo-selective
cyclizations of unsaturated alkoxysilanes.17a B(C6F5)3 was also used
as an efficient activator of polymethoxyhydrosiloxane in the reduc-
tion of various functional groups,17b,c and Friedel–Crafts alkylation
of activated arenes and heteroarenes.18
tetrazoles have
a wide range of application in coordination
chemistry as well as in the preparation of imidoylazides.7
This broad utility provoked significant effort toward the
tetrazole synthesis. Various methods have been reported for the
synthesis of 5-substituted 1H-tetrazoles, most of which are based
on [3+2] cycloaddition of azide ion to corresponding organic nitriles.
The reactions were carried out by using numerous catalysts such as
copper triflates,8a CdCl2,8b Fe(OAc)2,8c ZnBr2,8d triethyl amine
hydrochloride,8e Bronsted acid catalyst,8f various Lewis acid
catalysts such as AlCl3,2f,9a BF3-OEt2,9b FeCl3,9c TBAF,9d InCl3,9e I2,9f
(CH3)2SnO9g and by using some heterogeneous catalysts such
as NaHSO4ÁSiO2,9f COY zeolites,10a Zn/Al hydrotalcite,10b
⇑
Corresponding author. Tel.: +91 40 23073740; fax: +91 40 23073751.
E-mail address: bathini@niperhyd.ac.in (B.N. Babu).
http://dx.doi.org/10.1016/j.tetlet.2014.04.089
0040-4039/Ó 2014 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Prajapti, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.089

2
S. K. Prajapti et al. / Tetrahedron Letters xxx (2014) xxx–xxx
N
N N
HN
come was not promising (Table 1, entry 10). We also tried reaction
N
in water and EtOH, but the yields of the desired products were
inferior (Table 1, entries 11 and 12). Thus, 5 mol % of B(C6F5)3
and DMF were selected as the optimized condition for the synthe-
sis of 5-substituted 1H-tetrazoles (Table 1, entry 4). We also com-
pared the catalytic efficiency of B(C6F5)3 with other Lewis acids
(Table 1, entries 13–16). Among the Lewis acid catalysts, B(C6F5)3
was found to be superior in terms of yield and reaction time. We
next examined a variety of structurally diverse organic nitriles to
understand the scope and generality of the B(C6F5)3 promoted
[3+2] cycloaddition reaction to form 5-substituted 1H-tetrazoles
and results are summarized in Table 2.
It was observed that, by using 5 mol % of B(C6F5)3 in solution of
organic nitriles and NaN3 in DMF at 120 °C, the desired 5-substi-
tuted 1H-tetrazoles were obtained in admirable yields (Table 2,
entries 1–18).21 In case of aromatic nitriles, electron donating
and electron withdrawing groups have a significant influence on
the reaction times and product yields. The aromatic nitriles con-
taining electron withdrawing substituents took less time for com-
plete conversion of starting material relatively and the desired
products were obtained in excellent yields (Table 2, entries 2–4),
while bromo substitution at ortho position had given compara-
tively lower yield and longer reaction time due to steric effect
(Table 2, entry 5). Moreover, electron donating groups at para posi-
tion of aromatic ring gave the corresponding tetrazoles in high
yields, although longer reaction times were required (Table 2,
entries 6 and 7). Aryl nitrile containing a free hydroxy group at
para position also gave desired product in high yield after 12 h
(Table 2, entry 8). Similarly, 4-(allyloxy)benzonitrile and ethyl
2-(4-cyanophenoxy)acetate gave corresponding 1H-tetrazoles in
excellent yields after 12 and 14 h, respectively. Other aryl nitriles
such as 3,4-(methylenedioxy) phenyl acetonitrile and phenyl ace-
tonitrile also reacted smoothly to give the corresponding tetrazoles
in 89% and 84%, respectively (Table 2, entries 11 and 12).
B(C6F5)3 (5 mol%)
Na+
DMF, 120 oC, 8 h
94%
N N N
Scheme 1. Synthesis of 5-phenyl 1H-tetrazole.
Recently, B(C6F5)3 catalyzed Sakurai allylation of N-benzyloxy-
carbonylamino p-tolylsulfone is also reported with
allyltrimethylsilane.19
In continuation of our research in the development of novel
synthetic methodology,20 herein we wish to report a remarkable
activity of tris(pentafluorophenyl)borane [B(C6F5)3] to catalyze
[3+2] cycloaddition reaction between organic nitriles and sodium
azide to corresponding 5-substituted 1H-tetrazoles in high yields
and purity. To the best of our knowledge, B(C6F5)3 has not previ-
ously been reported for the preparation of 5-substituted 1H-
tetrazole.
Initially, we screened the reaction conditions for [3+2] cycload-
dition reaction of benzonitrile with sodium azide in the absence
and presence of tris(pentafluorophenyl)borane [B(C6F5)3] (Scheme
1). In the absence of catalyst at 120 °C, no reaction occurred after
10 h (Table 1, entry1). When benzonitrile was treated with sodium
azide using 1 mol % B(C6F5)3 in DMF, the 5-phenyl 1H-tetrazole
was isolated in 33% yield (Table 1, entry 2). The yield was improved
to 72% when the reaction was carried out in the presence of
3 mol % of B(C6F5)3 (Table 1, entry 3). In an attempt to improve
the conversion and yield, the reaction was repeated using
5 mol % of B(C6F5)3 as a catalyst. Pleasingly, this resulted in com-
plete conversion of benzonitrile into 5-phenyl 1H-tetrazole within
8 h in excellent yield (Table 1, entry 4). Further improvement of
yields was not observed on increasing the loading of the catalyst
(Table 1, entry 5). Hence 5 mol % of catalyst was considered as an
optimum catalyst concentration. Furthermore, the reaction was
carried out in different solvents (Table 1, entries 6–8). Among the
various solvents tested, DMF was found to be the best solvent giv-
ing maximum yields of the desired product.
We next examined the reactivity of heterocyclic nitriles. 6-Chlo-
ronicotinonitrile was found to be extremely reactive substrate,
affording the relative tetrazole in 90% yield after 4 h (Table 2, entry
13), while furan-2-carbonitrile gave respective 1-H tetrazole in 89%
yield in 6 h (Table 2, entry 14). Indole 3-acetonitrile and indole
5-carbonitrile produced corresponding tetrazoles in good yields
after 14 and 20 h, respectively (Table 2, entries 15 and 16).
Furthermore, [3+2] cycloaddition reaction was also extended to
1-adamantyl carbonitrile and butyronitrile under standard
We also investigated the reaction by using TMSN3 as azide
source in DMF at 120 °C and the desired product was obtained in
92% yield (Table 1, entry 9). Interestingly, no silylated product
was observed in present reaction conditions. An attempt was also
made to catalyze the reaction in the absence of solvent but out-
Table 1
Optimization of various reaction parameters for the synthesis of 5 substituted 1H-tetrazolea
Entry
Catalyst (mol %)
Solvent
Azide source
Time (h)
Yieldb (%)
1
2
3
4
5
6
7
8
Nil
DMF
DMF
DMF
DMF
DMF
EtOH
EtOH/water (8:2)
THF
DMF
Neat
Water
EtOH
DMF
DMF
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
NaN3
TMSN3
TMSN3
TMSN3
TMSN3
NaN3
NaN3
TMSN3
TMSN3
10
12
12
8
NRc
33
72
94
93
B(C6F5)3 (1)
B(C6F5)3 (3)
B(C6F5)3 (5)
B(C6F5)3 (7)
B(C6F5)3 (5)
B(C6F5)3 (5)
B(C6F5)3 (5)
B(C6F5)3 (5)
B(C6F5)3 (5)
B(C6F5)3 (5)
B(C6F5)3 (5)
BF3-OEt2 (5)
I2
8
48
48
48
10
24
18
48
20
10
24
72
20
20
Trace
92d
25d
15d
40d
80
89
56
60
9
10
11
12
13
149f
158c
169g
Fe(OAc)2
(CH3)SnO
DMF/water (9:1)
Toluene
a
b
c
Reaction and conditions: benzonitrile (1 mmol), NaN3 (1.5 mmol) are used at 120 °C.
Isolated yield.
In the absence of catalyst, no reaction occurred after 10 h.
TMSN3 (1.5 mmol) was used as azide source.
d
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S. K. Prajapti et al. / Tetrahedron Letters xxx (2014) xxx–xxx
3
Table 2
B(C6F5)3 catalyzed synthesis of 5-substituted 1H-tetrazolea
Entry Substrate
Time (h) Product
Yieldb (%) Entry
Substrate
Time (h) Product
Yieldb (%)
90
CN
N
CN
N
HN
HN
N
N
19d
8
94
96
1022
14
N
N
EtOOC
O
EtOOC
O
CN
N
O
O
H
N
HN
CN
O
N
N
N
222
6
118d
12
89
Cl
N
N
N
O
Cl
Cl
Cl
CN
N
H
HN
CN
N
N
39e
7
90
95
82
84
129c,9d
12
4
84
90
89
78
N
F
N
N
F
CN
N
CN
N
HN
HN
N
N
49d
6
138e
N
N
O2N
Cl
N
O2N
N
Cl
O
CN
Br
N
O
N
HN
CN
HN
N
N
511a
15
149d
6
N
N
Br
CN
N
H
HN
N
CN
N
N
610g
14
152f
14
N
MeO
Me
N
N
N
N
H
H
MeO
CN
N
CN
N
N
HN
HN
N
N
N
N
710g
12
88
169d
20
80
N
H
H
N
H
Me
CN
N
CN
N
HN
N
N
810g
12
87
92
178e
14
16
87
76
N
N
HO
HO
CN
N
N
H
N
HN
N
N
N
922
12
1810g
N
O
N
O
a
Reaction and conditions: nitriles (1 mmol), NaN3 (1.5 mmol), B(C6F5)3 (5 mol %), DMF (5 mL), and temperature (120 °C).
Isolated yield.
b
reaction condition, the corresponding 1H-tetrazoles were pro-
duced in 87% and 76% yields, respectively (Table 2, entries 17
and 18). The above results clearly indicate the scope and generality
of catalyst for the synthesis of 5-substituted 1H-tetrzoles using a
wide range of organic nitriles without affecting the presence of
other functional groups and [3+2] cycloaddition proceeds well,
irrespective of the electronic nature and position of the substitu-
ents of aromatic ring. Presumably, the mechanism of reaction insti-
gates through co-ordination of nitriles with B(C6F5)3, resulting in
enhancement of its reactivity with NaN3, which may facilitate
the cycloaddition reaction to yield 5-substituted 1H-tetrazole
products.
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We thank the Department of Pharmaceuticals (Ministry of
Chemicals and Fertilizers) for providing funds and also CSIR-Indian
Institute of Chemical Technology, Hyderabad for providing the
facilities.
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21. Typical experimental procedure: B(C6F5)3 (67.3 mg, 0.13 mmol, 5 mol %) was
added to a stirred solution of 3,4-dichlorobenzaldehyde (172 mg, 1 mmol) and
NaN3 (97.5 mg, 1.5 mmol) in DMF (5 mL) and was heated at 120 °C. After
completion of reaction (as monitored by TLC), the reaction mixture was cooled
to room temperature and was added 5 mL of cold water followed by 10 mL of
2 N HCl and 10 mL of ethyl acetate. The resulting mixture was stirred
vigorously for 15 min. The organic layer was separated and aqueous layer
was again extracted with ethyl acetate (3 Â 15 mL). The combined organic
layer was washed with water and dried over anhydrous sodium sulfate and
was evaporated under reduced pressure. The crude product was purified by
column chromatography (silica gel, EtOAc/hexane 9:1) to obtain pure 5-(3,4-
dichlorophenyl)-1H-tetrazole. The known compounds were characterized and
confirmed by comparison of their spectral data and physical properties with
reported literature.
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1H-tetrazole (entry 2): off white solid, mp 150–152 °C; (KBr) vmax 3481, 2923,
1610, 1425, 1243, 1124, 1096, 886 cmÀ1 1H NMR (300 MHz, DMSO-d6) d 8.25
;
(1H, d, J = 1.51 Hz), 8.03 (1H, dd, J = 1.51 Hz, J = 8.30 Hz), 7.70 (1H, d,
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J = 8.30 Hz); 13C NMR (75 MHz, DMSO-d6) 154.7, 133.3, 132.3, 131.6, 128.5,
126.8, 125.1; HRMS m/z calcd for C7H4Cl2N4 [M+H]+: 214.9891; found:
+
214.9881. 5-(4-(Allyloxy)phenyl)-1H-tetrazole (entry 9): pale yellow solid, mp
181–183 °C; (KBr) vmax 3400, 2921, 1613, 1508, 1500, 1262, 1181, 989,
835 cmÀ1 1H NMR (300 MHz, DMSO-d6) d 7.97 (2H, d, J = 8.87 Hz), 7.17 (2H, d,
;
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J = 8.87 Hz), 6.05 (1H, m), 5.45 (1H, d, J = 17.18 Hz), 5.38 (1H, d, J = 10.57 Hz),
4.66 (2H, d, J = 5.09 Hz); 13C NMR (75 MHz, DMSO-d6) 160.1, 154.8, 133.1,
128.4, 117.7, 116.6, 115.37, 68.2; HRMS m/z calcd for C10H10N4O+ [M+H]+:
203.0933; found: 203.0922. Ethyl 2-(4-(1H-tetrazol-5-yl)phenoxy)acetate
(entry 10): pale brown solid, mp 154–156 °C; (KBr) vmax 3410, 2982, 2920,
1751, 1614, 1500, 1208, 1077, 835 cmÀ1 1H NMR (300 MHz, DMSO-d6) d 7.98
;
15. Blackwell, J. M.; Sonmor, E. R.; Scoccitti, T.; Piers, W. E. Org. Lett. 2000, 2, 3921–
3923.
16. Chen, D.; Klankermayer, J. Chem. Commun. 2008, 2130–2131.
17. (a) Shchepin, R.; Xu, C.; Dussault, P. Org. Lett. 2010, 12, 4772–4775; (b)
Chandrasekhar, S.; Chandrashekar, G.; Vijeender, K.; Reddy, M. S. Tetrahedron
(2H, d, J = 8.87 Hz), 7.17 (2H, d, J = 8.87 Hz), 4.89 (2H, s), 4.21 (2H, q,
J = 7.17 Hz), 1.24 (3H, t, J = 6.98 Hz); 13C NMR (75 MHz, DMSO-d6) 168.2,
159.6, 154.6, 128.4, 116.9, 115.3, 64.6, 60.6, 13.9; HRMS m/z calcd for
C
11H12N4O+3 [M+H]+: 249.0988; found: 249.0982.
Please cite this article in press as: Prajapti, S. K.; et al. Tetrahedron Lett. (2014), http://dx.doi.org/10.1016/j.tetlet.2014.04.089