InCl3-catalyzed [2+3] cycloaddition reaction: A rapid synthesis of 5-substituted 1H-tetrazole under microwave irradiation

Article abstract of DOI:10.1155/2012/615891

A series of 5-substituted 1H-tetrazole were efficiently prepared by InCl3 catalyzed (10 mol %) from structurally divert organic nitriles with sodium azide under the influence of microwave irradiation. The present protocol was successfully applied to the aliphatic, aryl, benzylic and heterocyclic nitriles and corresponding 5-substituted 1H-tetrazole were obtained in good to excellent yield (70-96%). This method gives remarkable advantages such as short reaction time, simple work-up procedure and economical beneficial.

Full text of DOI:10.1155/2012/615891

ISSN: 0973-4945; CODEN ECJHAO
E-Journal of Chemistry
http://www.ejchem.net
2012, 9(3), 1145-1152
InCl3-Catalyzed [2+3] Cycloaddition Reaction: A
Rapid Synthesis of 5-Substituted 1H-tetrazole under
Microwave Irradiation
VIJAY S. PATILA, KAMALAKAR P. NANDREA, AMULRAO U. BORSEB,
SIDHANATH V. BHOSALEA*
aDepartment of Organic Chemistry, North Maharashtra University, Jalgaon, INDIA.
bSchool of Chemical Sciences, North Maharashtra University, Jalgaon, INDIA.
sidhanath2003@yahoo.co.in
Received 22 November 2011; Accepted 18 January 2012
Abstract: A series of 5-substituted 1H-tetrazole were efficiently prepared by
InCl3 catalyzed (10 mol %) from structurally divert organic nitriles with
sodium azide under the influence of microwave irradiation. The present
protocol was successfully applied to the aliphatic, aryl, benzylic and
heterocyclic nitriles and corresponding 5-substituted 1H-tetrazole were
obtained in good to excellent yield (70-96%). This method gives remarkable
advantages such as short reaction time, simple work-up procedure and
economical beneficial.
Key words: InCl3, Nitriles, [2+3] cycloaddition, 5-Substituted 1H-tetrazole, Microwave irradiation.
Introduction
The growing interest in heterocyclic compounds and their methods for synthesis are of
paramount importance to medicinal, chemical and agricultural research because they make
novel materials with unique properties. In recent years, attention towards synthesis of
tetrazole and its analogues has been constantly increased because of a number of highly
efficient drugs based on tetrazole.1 Tetrazole compounds have broad range of
pharmaceutical applications2 as antiallergic, anti-inflammatory, antipemilic, antimicrobial
and act as stimulants or sedatives on the central nervous system. They also used as trigger
explosives, components of mixed propellants and gas-generating compositions.3 In addition,
these compounds have a significant role as oxidizers and plant growth regulators.4
Furthermore, tetrazole compounds are significant precursors for synthesis of various
nitrogen containing heterocycles and many useful transformation.5 Particularly, 5-substituted
1H-tetrazole were conveniently prepared by direct [2+3] cycloaddition between a nitrile and
an azide.6 Many synthetic methods were reported for this transformation, requires amine
salts7, strong lewis acid8, expensive and toxic metals9 by in situ generated hydrazoic acid
which is highly toxic and explosive. Recently, Sharpless and co-workers reported an
innovative and safe procedure for the synthesis of tetrazoles using stoichiometric amounts of

1146 SIDHANATH V. BHOSALE
Zn (II) salts in water.10 However, many of the established approaches for this [2+3]
cycloaddition reaction are still severely limited in their use by the lack of generality, the
harsh reaction conditions such as high temperatures and long reaction times, or the multistep
procedures required.
Encouraged by the intense research activity in the field of tetrazole and in pursuit of our
continuing interest in developing greener methodologies by using microwave irradiations
and metal salts11a, we envisioned the synthesis of 5-substituted 1H-tetrazole by milder lewis
acid under the microwave influence. Microwave assisted organic reaction11b currently
gaining popularity due to many advantages such as great selectivity, enhanced reaction rates,
economical beneficial, short reactions time, operational simplicity with minimal stockpiling
of chemicals and avoiding transportation of hazardous substances. These increasing
demands offer many opportunities for microwave chemistry in the development of
environmentally benign methods for the preparation of intermediated, speciality chemicals
and pharmaceuticals.12 Recently, indium (III) chloride was emerged as a powerful lewis acid
catalyst imparting high region- and chemoselectivity in various transformations.13 For
instance, Indium(III) chloride are effectively used in promoting the Diels-Alder reaction,14
cycloaddition reactions,15 rearrangement of epoxides,16 trans-esterification processes,17 and
synthesis of amino phosphates and quinolines.18 We wish to report here a remarkable
catalytic activity of InCl3 for the [2+3] cycloaddition reaction between organic nitriles and
sodium azide under microwave irradiation in DMF (Scheme 1).
Experimental
Melting points were measured by open capillary method and are uncorrected. IR spectra
were recorded on a Shimadzu FT-IR -8900 spectrometer. Mass spectra were recorded on
LC/MS and EI mode and mass spectrometer operating at an ionization potential of 70 eV.
1H and 13C NMR spectra were recorded on a Varian mercury plus-400 spectrometer using
TMS as internal standard. All compounds were characterized by melting point, IR, Mass and
1H NMR spectroscopy and identified by comparison in the literature.
General Procedure for Synthesis of 5-substituted 1H- tetrazoles:
In an oven-dried flask sodium azide (0.012 mmol) and InCl3 (10 mol %) were added to a
solution of nitrile (0.01 mmol) in dry DMF (1.0 ml). Then, the flask was placed in
microwave and irradiated at 70- 90 power (2450 W) for 6- 12 min. (see in Table 2). When
TLC analysis showed complete conversion, the reaction mixture was cooled to room
temperature and poured in to ice cold water after which it was acidified with 3 N HCl. The
resulted crude product was collected by filtration and recrystalized by ethyl acetate and
hexane mixture to give desired product in good to excellent yield (70-96 %) under
microwave irradiation. Particularly, in some cases for benzylic tetrazoles product does not
precipitated after acidification and required extractive work-up by ethyl acetate.
Physical and Spectral Data for the Selected Compounds:-
5-Phenyltetrazole (2a)10:
White solid with 95 % yield; mp = 214-216 oC (lit. 215-216 oC); IR (KBr): νmax 3055 (m, C-
H), (2980-2460 (brs, N-H), 1608 (s, C=C), 1564 (s, tetrazole), 727-688 (s, mono substituted
1
benzene) cm-1; H NMR (400 MHz, DMSO-d6): δ 7.58-7.62 (m, 3H), 8.02- 8.08 (m, 2H);
LC/MS: m/z (%) = 147 (100) [M+H] +, 104 (14) [MH-HN3] +.
5-(4-Bromophenyl) tetrazole (2b)5:
White solid with 81 % yield; mp =266-268 oC (lit. 268-269 oC); IR (KBr): νmax 3088 (m, C-
H), (2999-2422 (brs, N-H), 1604 (s, C=C), 1483-1431 (s, tetrazole), 829 (s, p-disubstituted

InCl3-Catalyzed [2+3] Cycloaddition Reaction 1147
benzene) cm-1; 1H NMR (400 MHz, DMSO-d6): δ 7.8 (d, J = 8.4 Hz, 2H), 7.9 (d, J= 8.4 Hz,
2H); LC/MS: m/z (%) = 225 (100) [M+H] +, 182 (17) [MH-HN3] +.
5-(4-Chlorophenyl) tetrazole (2c)4:
White solid with 94 % yield; mp =252-254 oC (lit. 252-253 oC); IR (KBr): νmax 3068 (m, C-
H), (2997-2426 (brs, N-H), 1610 (s, C=C), 1489-1436 (s, tetrazole), 831 (s, p-disubstituted
benzene) cm-1; 1H NMR (400 MHz, DMSO-d6): δ 7.7 (d, J = 8.8 Hz, 2H), 8.05 (d, J=8.8 Hz,
2H); MS(EI): m/z = 179 [M-H]- , 137 [M-HN3]-.
5-(3-Chlorophenyl) tetrazole (2d)5:
White solid with 79 % yield; mp =110- 115 oC; (KBr): νmax 3068 (m, C-H), (2968-2430 (brs,
N-H), 1558 (s, C=C), 1473 (s, tetrazole), 891 (s, m-disubstituted benzene) cm-1; 1H NMR
(400 MHz, CDCl3): δ 7.45-7.52 (m, 2H), 8.0 (d, J = 7.6 Hz, 1H), 8.12 (s, 1H); IR MS(EI):
m/z = 179 [M-H]-.
5-(4-Fluorophenyl) tetrazole (2f):
o
White solid with 78 % yield; mp = 210 C; IR (KBr): νmax 3074 (m, C-H), (2920-2416 (brs,
N-H), 1610 (s, C=C), 1500 (s, tetrazole), 842 (s, p-disubstituted benzene) cm-1; H NMR
1
(400 MHz, DMSO-d6): δ 7.47 (t, J = 8.8 Hz, 2H), 8.07-8.11 (m, 2H); LC/MS: m/z (%) = 165
(100) [M+H] +,122 (17) [MH-HN3]+; HRMS calc. for C7H6FN4 (M++ H) 165.0576, found
165.0568.
5-(4-tolyl) tetrazole (2g) 10
:
White solid with 80 % yield; mp = 248-250 oC (lit. 248-249 oC); IR (KBr): νmax 3062 (m, C-
H), 2983-2400 (brs, N-H), 2972 (s, CH3) 1590 (s, C=C), 1492(s, tetrazole), 823 (s, p-
1
disubstituted benzene) cm-1; H NMR (400 MHz, DMSO-d6): δ 2.38 (s, 3H), 7.41 (d, J=8.4
Hz, 2H), 7.96 (d, J=8.4 Hz 2H); MS (EI): m/z = 159 [M-H] - , 130 [M-HN2]-.
5-(3-tolyl) tetrazole (2h)6:
o
White solid with 85 % yield; mp= 150-152 C; IR (KBr): νmax 3068 (m, C-H), 2983-2426
(brs, N-H), 2910 (s, CH3) 1560 (s, C=C), 1485 (s, tetrazole), 798 (s, p-disubstituted benzene)
cm-1; 1H NMR (400 MHz, DMSO-d6): δ 2.4 (s, 3H), 7.39-7.51 (m, 2H ), 7.84 (d, J = 8.0 Hz,
1H), 7.87 (s, 1H); LC/MS: m/z (%) = 161 (100) [M+H] +, 118 (18) [MH-HN3] +.
5-(2-nitro,4-tolyl) tetrazole (2i):
Yellow solid with 70 % yield; mp= 180- 182 oC; IR (KBr): νmax 3065 (m, C-H), 2980-2432
(brs, N-H), 2955 (s, CH3), 1603 (s, C=C), 1464 (s, tetrazole), 869 (s, substituted benzene)
cm-1; 1H NMR (400 MHz, DMSO-d6): δ 2.45 (s, 3H), 7.74-7.80 (m, 2H), 8.1 (s, 1H); 13CMR
(400 MHz, CDCl3+DMSO-d6): 19.76, 94.06, 115.96, 130.06, 132.4, 133.64, 141.2, 146.8;
+
LC/MS: m/z (%) = 206 (100) [M+H] , 163 (9) [MH-HN3] +; HRMS calc. for C8H8N5O2
(M++ H) 206.0678, found 206.0672.
5-(4-Methoxyphenyl) tetrazole (2j)10:
White solid with 76 % yield; mp= 230-231 oC (lit. 231-232 oC); IR (KBr): νmax 3078 (m, C-
H), (2983-2480 (brs, N-H), 1612 (s, C=C), 1500 (s, tetrazole), 1267 (s, aromatic C-O), 1182
1
(s, methoxy C-O), 835 (s, p-disubstituted benzene) cm-1; H NMR (400 MHz, DMSO-d6): δ
3.85 (s, 3H), 7.15 (d, J = 8.8 Hz, 2H), 7.98 (d, J = 8.8 Hz, 2H); MS (EI): m/z = 177 [M+H]+,
134 [MH-HN3]+.
5-Benzyltetrazole (2k)20:
White solid with 80 % yield; mp= 120-122 oC (lit. 121-122 oC); IR (KBr): νmax 3078 (m, C-
H), 2962-2460 (brs, N-H), 1608 (s, C=C), 1542 (s, tetrazole), 698 (s, mono substituted

1148 SIDHANATH V. BHOSALE
benzene) cm-1; 1H NMR (400 MHz, DMSO-d6): δ 4.28 (s, 2H), 7.25-7.27 (m, 3H), 7.31-7.35
(m, 2H); MS (EI): m/z = 160 [M+H]+ , 91 [M-CHN4]+.
5-((4-methoxyphenyl)methyl) tetrazole (2l):
White solid with 71 % yield; IR (KBr): νmax 3105 (m, C-H), 2966-2484 (brs, N-H), 1610 (s,
1
C=C), 1512 (s, tetrazole),1247 (s, CH2), 848 (s, p-disubstituted benzene) cm-1; H NMR
(400 MHz, DMSO-d6) δ: 7.19 (d, J = 8.4 Hz, 2H), 6.88 (d, J = 8.8 Hz, 2H), 4.21(s, 2H), 3.75
(s, 3H); LC/MS: m/z =191 (100)[M+H]+ , 121 (27) [M-CHN4]+ ; HRMS calc. for C9H11N4O
(M++ H) 191.0933, found 191.0930.
5-Benzhydryltetrazole (2m)7:
o
o
White solid with 96 % yield; mp=164-165 C (lit. 164-165 C); IR (KBr): νmax 3088 (m, C-
H), 3003-2455 (brs, N-H), 1599 (s, C=C), 1496 (s, tetrazole), 1246 (s, CH2), 744 (s,
1
substituted benzene) cm-1; H NMR (400 MHz, CDCl3) δ: 5.86 (s, 1H), 7.19-7.21 (m, 4H),
7.30-7.36 (m, 6H); LC/MS: m/z (%) = 237 (100)[M+H]+, 167 (29) [M-CHN4]+.
3-((2H-tetrazol-5-yl)methyl)-6-methyl-2-p-tolylimidazo[1,2-a]pyridine (2n):
Faint greenish solid with 76 % yield; mp=248-250 oC; IR (Nujol): νmax 2920 (m, C-H), 3181
(brs, N-H), 1612 (s, C=C), 1463 (s, tetrazole), 1377 (s, CH2), 721 (s, substituted benzene)
1
cm-1; H NMR (300 MHz, CDCl3and CD3OD) δ: 2.39 (s, 6H), 4.63 (s, 2H), 7.25-7.28 (d,
2H, J = 8.3Hz) 7.42-7.45 (d, 2H, J = 9Hz) 7.52-7.57 (m,3H) 8.21 (s, 1H); LC/MS: m/z (%) =
305.15 (100)[M+H]+ , 277.10 (32). MS (ESI): m/z = 305.15 (100)[M+H]+.
Ethyl 2-(2H-tetrazol-5-yl)acetate (2o):
o
White solid with 84 % yield; mp=130-132 C; IR (KBr): νmax 3472 (brs, N-H), 1747(s,
1
C=O), 1568 (s, tetrazole) cm-1; H NMR (300 MHz, DMSO-d6) δ: 1.20 (t, 3H J = 7.0Hz),
4.14 (q, 2H J = 7.0Hz), 4.17 (s, 2H); LC/MS: m/z (%) = 157 (100)[M+H]+ .
Result and Discussion
Initially, the reaction of benzonitrile and sodium azide was selected as a model reaction to
optimize the reaction conditions under microwave irradiation and the obtained results are
listed in Table 1. When the reaction was carried out in without solvent with different catalyst
(20 mol %) (Table 1, entries 1-5), it was found that InCl3 was more effective than other
catalysts and a moderate yield (61%) could be obtained (Table 1, entry 5). For improving the
yield of this cycloaddition reaction, we verify the reaction with different solvents (Table 1,
entries 6-10) and remarkably DMF (Table 1, entry 10) found to be most suitable solvent for
this transformation. With these results in hand we investigated catalytic amount required for
this transformation and found that 10 mol% of InCl3 (Table 1, entry 13) catalyst gives
excellent yield (95 %). The reaction was completed only in few minutes, a reaction time
considerably shorter than those previously reported19 by using ZnCl2/H2O under microwave
irradiation (6 hrs. with 78 % yield). While, in absence of catalyst reaction does not proceeds
forward (Table 1, entry 11) and excess amount of the catalyst did not improve the yield to a
greater extent.
N
N
N
CN
Conditions
M.W.
N
H
NaN3
Scheme 1. Synthesis of 5-phenyltetrazole under microwave irradiation.

InCl3-Catalyzed [2+3] Cycloaddition Reaction 1149
Table 1. Screening of reaction parameters for the formation 5-phenyltetrazolea
Entry
1
Catalyst (mol %)
MoO3-SiO2 (20 mol %)
Alum (20 mol %)
ZnCl2 (20 mol %)
ZnO (20 mol %)
InCl3 (20 mol %)
InCl3 (20 mol %)
InCl3 (20 mol %)
InCl3 (20 mol %)
InCl3 (20 mol %)
InCl3 (20 mol %)
InCl3 (0 mol %)
Solvent (1 ml)
----
Time (min.)
Yieldb (%)
NA
< 50
< 20
NA
61
15
10
15
15
10
10
10
10
10
5
2
----
3
----
4
----
5
----
6
DMSO
Xylene
Isopropanol
PEG-400
DMF
82
7
79
8
54
9
65
10
11
12
13
95
DMF
30
10
5
NA
78
InCl3 (5 mol %)
DMF
InCl3 (10 mol %)
DMF
95
a Reaction condition: Benzonitrile (0.01 mmol), NaN3 (0.012 mmol), Solvent (1 ml).
b Isolated product recrystallized by EtOAc: Hexane.
After an optimization study 10 mol % of InCl3 furnished good results, we achieved a higher
degree of reproducibility when we used the DMF as a solvent in our studies. The objective
of the present research was to examine with a wider range of organic nitriles that contained
aliphatic, heterocyclic, benzylic and aromatic with electron-withdrawing and electron-
releasing groups to develop a reproducible protocol. Our results are summarized in Table 2,
it is evident that there is a highly influential steric factor, para-substituted-benzonitrile
afforded higher conversion rates with short reaction times as compared with the ortho
analogue. For example, 4-chlorolbenzonitrile converted to the corresponding tetrazole in
excellent yield (94 %) in only 8.0 min. (Table 2, entries 3), while the ortho-derivative
rendered only 71% conversion during 11.5 min. (Table 2, entries 5). This pattern was
consistently observed with all other substituent’s (Table 2, entries 7, 8, 9). The present
results also indicate that benzylic nitriles were successfully converted to their tetrazoles
(Table 2, entries 11-13) but required extractive work up because of their solubility in water.
It is interesting to note that both, activating and
N
N
InCl3(10 %), DMF
M.W., 6-12 min.
CN
N
NaN3
R
N
H
2a
R
1a
Scheme 2. Synthesis of 5-substituted 1H- tetrazole.

1150 SIDHANATH V. BHOSALE
Table 2. Synthesis of 5-substituted 1H- tetrazole catalysed by InCl3 (10 mol %) under M.W.
irradiationa.
Time
(min.)
Product
Code
Entry
1a
Nitrile
Product
Yieldb(%)
95
H
CN
N
N
N
8.0
10.0
8.0
2a
2b
2c
N
H
CN
N
N
N
1b
1c
81
94
Br
Cl
Br
N
H
N
CN
CN
N
N
Cl
Cl
N
Cl
H
N
1d
1e
6.0
2d
2e
79
71
N
N
N
Cl
CN
H
N
N
11.5
N
Cl
CN
N
H
N
N
N
1f
10.0
10.0
2f
78
80
F
F
N
H
CN
N
N
1g
2g
H3C
N
H3C
H3C
N
H3C
H
CN
N
1h
1i
8.5
N
N
2h
2i
85
70
N
H
N
CN
N
N
H3C
12.0
N
H3C
NO2
CN
NO2
H
N
N
N
1j
10.0
7.0
2j
76
80
MeO
MeO
N
H
N
CN
1k
2k
N
N
N
H
N
CN
1l
11.5
6.0
2l
71
95
N
N
N
MeO
MeO
H
N
N
1m
2m
CN
N
N

InCl3-Catalyzed [2+3] Cycloaddition Reaction 1151
N
CH3
N
N
CH3
H3C
N
1n
1o
10.0
6.0
2n
2o
76
84
H3C
N
NC
HN
N
N
N
HN
O
O
N
CN
EtO
N
EtO
a Reaction condition: nitrile (0.01 mmol), NaN3 (0.012 mmol), InCl3 (10 %), DMF (1 ml).
b Isolated pure product .
deactivating groups furnished surprisingly good conversions. A modified reaction procedure
made the easy isolation and purification of compounds. As indicated in the experimental
section, the yields of the compounds made under this investigation under microwave-
assisted conditions are consistently higher than those reported under conventional and
microwave conditions. No explosion will occurred in the development of this work. The
necessary adequate precautions were taken against the eventuality. In all cases products are
isolated and characterized by IR, M.P., 1H NMR and Mass spectroscopy.
Conclusion
In conclusion, we have developed a highly reproducible protocol for the preparation of 5-
substituted 1H-tetrazole from variety of organic nitriles. The use of InCl3 (10 mol %) under
microwave irradiation allows the easy preparation of 5-substituted 1H-tetrazole in good to
excellent yield. This method has the advantages of higher yields, milder reaction conditions,
shorter reaction time and convenient procedure.
Acknowledgments
The authors are thankful to the DAE-BRNS (2009/37/39 /BRNS/2268), Mumbai India for
financial assistance and Principal, Dr. Sudhir Meshram Vice-chancellor, North Maharashtra
University, Jalgaon for providing necessary facilities.
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1152 SIDHANATH V. BHOSALE
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