Zno as an effective and reusable heterogeneous catalyst for the synthesis of arylaminotetrazoles

Article abstract of DOI:10.1080/00397911.2010.548620

ZnO is an effective heterogeneous catalyst for the reaction between arylcyanamides with sodium azide to synthesize the arylaminotetrazoles in good yields. This method has advantages of good yields, simple methodology, short reaction times, and easy workup

Full text of DOI:10.1080/00397911.2010.548620

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ZnO as an Effective and Reusable
Heterogeneous Catalyst for the Synthesis
of Arylaminotetrazoles
Davood Habibi a & Mahmoud Nasrollahzadeh a
a Department of Organic Chemistry, Bu-Ali Sina University, Hamedan,
Iran
Accepted author version posted online: 12 Jan 2012.Published
online: 26 Mar 2012.
To cite this article: Davood Habibi & Mahmoud Nasrollahzadeh (2012): ZnO as an Effective and
Reusable Heterogeneous Catalyst for the Synthesis of Arylaminotetrazoles, Synthetic Communications:
An International Journal for Rapid Communication of Synthetic Organic Chemistry, 42:14, 2023-2032
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Synthetic Communications1, 42: 2023–2032, 2012
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ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.548620
ZnO AS AN EFFECTIVE AND REUSABLE
HETEROGENEOUS CATALYST FOR THE SYNTHESIS
OF ARYLAMINOTETRAZOLES
Davood Habibi and Mahmoud Nasrollahzadeh
Department of Organic Chemistry, Bu-Ali Sina University, Hamedan, Iran
GRAPHICAL ABSTRACT
Abstract ZnO is an effective heterogeneous catalyst for the reaction between
arylcyanamides with sodium azide to synthesize the arylaminotetrazoles in good yields. This
method has advantages of good yields, simple methodology, short reaction times, and easy
workup. Furthermore, the catalyst can subsequently be reused for several times without any
significant loss of activity.
Keywords 5-Arylamino-1H-tetrazole; 1-aryl-5-amino-1H-tetrazole; arylcyanamide;
heterogeneous catalyst; ZnO
INTRODUCTION
Tetrazoles have been widely used as an isosteric carboxylic acid pharmacophore
in medicinal chemistry,[1] as ligands in coordination chemistry,[2–4] and as plant-
growth regulators, herbicides, fungicides, explosives, and rocket propellants.[5–7]
Hansch has shown that anionic tetrazoles are almost 10 times more lipophilic than
the corresponding carboxylates, which is an important factor in designing a drug
molecule to pass through the cell membranes.[8] Another important application of
tetrazoles is preparation of imidoylazides.[9]
The 5-monosubstitutedamino-1H-tetrazoles were previously synthesized by
thermal isomerization of 1-substituted-5-amino-1H-tetrazoles in boiling ethylene
or melt state (180–200 ꢀC).[10,11] Garbrecht and Herbst have shown that cyanamides
may be converted to aminotetrazoles using hydrazoic acid, which often results, in a
mixture of isomers 5-arylamino-1H-tetrazoles and 1-aryl-5-amino-1H-tetrazoles.[12]
Received October 23, 2010.
Address correspondence to Davood Habibi, Department of Organic Chemistry, Faculty of
Chemistry, Bu Ali Sina University, Abbas Abad Street, Hamedan 65178-38683, Iran. E-mail:
davood.habibi@gmail.com
2023

2024
D. HABIBI AND M. NASROLLAHZADEH
Congreve has reported a two-step synthesis of 1-aryl-5-amino-1H-tetrazoles
from the corresponding 1-aryltetrazoles.[13] The reaction suffers from some draw-
backs such as harsh reaction conditions, low temperatures (ꢁ70 ꢀC), and use of large
excess of sodium azide and organolithium reagents, which are potentially dangerous.
Vorobiov and coworkers published a three-step synthesis of 1-aryl-5-amino-1H-
tetrazoles, which proceeded in poor yields from the corresponding aromatic amines
via isolation of cyanamides intermediate.[14] Unfortunately, this approach is not well
developed because of the insufficient stability of these intermediates. In most cases,
N-arylureas and other by-products were mostly formed, and the cyanamides inter-
mediates were not stable enough to be isolated. Therefore, 1-aryl-5-amino-1H-tetra-
zoles were produced in poor yields.
These syntheses require the use of highly toxic and explosive hydrazoic acid, so
it is desirable to develop a more efficient and convenient method for the regiospecific
synthesis of arylaminotetrazoles. On the other hand, in most cases only the 1-aryl-5-
amino-1H-tetrazoles were obtained. We needed to develop a method without appli-
cation of homogeneous system as well as hydrazoic acid or in situ generation of
hydrazoic acid because of the presence of azide sources as well as the synthesis of
regiospecific products.
In recent years, application and design of environmentally friendly solid cata-
lysts have been increased to reduce the amount of toxic wastes. The use of metal oxide
catalysts have received the considerable attention in organic synthesis because of their
environmental compatibility, ease of handling, nontoxic nature, reusability, and
development of solventless reactions, which have environmental advantages[15–18]
of all the heterogeneous solid catalysts, application of zinc oxide has widely increased
because it is commercially available, inexpensive, moisture stable, reusable, and envir-
onmentally benign, and it has been used in several reactions such as Beckmann rear-
rangements,[15] Friedel–Crafts acylation,[16] and synthesis of cyclic ureas.[18]
Recently, we described a sufficiently simple and convenient synthesis of aryla-
minotetrazoles from the corresponding arylcyanamides using ZnCl2 in water.[19]
ZnBr2 is homogeneous and cannot be separated from the reaction mixture, whereas
zinc oxide is heterogeneous and can easily be recovered and reused.
In continuation of our recent work on the synthesis of heterocycles[20] and
application of heterogeneous reagents for development of the useful synthetic meth-
odologies,[21–26] we herein describe a new, simple, and effective procedure for the
synthesis of arylaminotetrazoles (3a–h) via the reaction of arylcyanamides (1a–h)
and sodium azide (2) in the presence of zinc oxide as a heterogeneous catalyst
(Scheme 1).
Scheme 1. Synthesis of different arylaminotetrazoles.

ZnO AS AN EFFECTIVE AND REUSABLE CATALYST
2025
EXPERIMENTAL
All reagents were purchased from the Merck and Aldrich chemical companies
and used without further purification. Products were characterized by spectroscopic
1
data [infrared (IR), fourier transform (FT)–IR, H NMR, and 13C NMR spectra],
elemental analyses (CHN), and melting points. The NMR spectra were recorded
1
in dinethylsulfoxide (DMSO) and acetone. H NMR spectra were recorded on Bru-
ker Avance DRX 300- and 500-MHz instruments. The chemical shifts (d) are
reported in parts per million (ppm) relative to the tetrametleysilene (TMS) as inter-
nal standard. J values are hertz given in (Hz). 13C NMR spectra were recorded at 125
and 75 Hz. IR (KBr) and FT-IR (KBr) spectra were recorded on Shimadzu 470 and
Perkin-Elmer 781 spectrophotometers, respectively. 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.
Thin-layer chromatography (TLC) was performed on silica gel polygram SIL
G=UV 254 plates.
The arylcyanamides (1a–h) were prepared according to the literature.[27]
Typical Procedure for Preparation of Arylaminotetrazoles 3
ZnO (0.1 g) was added to a mixture of cyanamides (1a–h) (2 mmol), NaN3 (2)
(0.2 g, 3 mmol) in distilled dimethylformamide (6 mL) and stirred at 120 ꢀC for the
appropriate time (Table 1). After completion of the reaction (as monitored by
TLC), the ZnO was centrifuged and the centrifugate was treated with ethyl acetate
(35 mL) and 5 N HCl (20 mL) and stirred vigorously. The resultant organic layer
was separated, and the aqueous layer was again extracted with ethyl acetate
(25 mL). The combined organic layers were washed with water, the solvent was
removed, and the crude solid arylaminotetrazole was recrystallized (aqueous etha-
nol). The physical data (mp, IR, NMR) of known compounds were found to be
identical with those reported in the literature.[19]
1-(4-(5-Amino-1H-tetrazole-1-yl)phenyl)-5-amino-1H-tetrazole (3h,
Table 1, entry 8)
Mp. 264–266 ꢀC; IR (KBr): 3357, 3139, 1652, 1588, 1519, 1426, 1293, 1141,
1
1070, 843 cmꢁ1; H NMR (DMSO-d6, 500 MHz): d 7.03 (2H, s), 7.83 (4H, s); 13C
NMR (DMSO-d6, 125 MHz): d 126.3, 134.6, 155.9; Anal. calcd. for C8H8N10: C,
39.35; H, 3.30; N, 57.35; Found: C, 39.46; H, 3.42; N, 57.23.
RESULTS AND DISCUSSION
Arylcyanamides (1a–h) were treated with sodium azide (2) in dimethylforma-
mide (DMF) in the presence of ZnO to give the desired arylaminotetrazoles (3a–h)
in moderate to good yields (Scheme 1). Reactions were performed at 120 ꢀC and
stirred for the appropriate time. To investigate both the electrical and steric effects,
the arylcyanamides carrying either electron-releasing or electron-withdrawing substi-
tuents studied having different groups in ortho, meta, or para positions. As shown in

2026
D. HABIBI AND M. NASROLLAHZADEH
Table 1. Synthesis of arylaminotetrazoles (3a–h) in the presence of ZnO by reaction of sodium azide (2)
and arylcyanamides (1) at 120 ꢀC
Entry
Substrate (1)
Product
Time (min)
Yielda %
1
120
77
2
115
80, 78b
3
4
110
55
75
73
5
6
55
50
74
77
7
8
75
50
75
75
aYield refers to the pure isolated products.
bYield after the third cycle.
Table 1, among the various cyanamides, those having the electron-releasing groups
on the aromatic rings (entries 4–6) complete at 120 ꢀC after 50–55 min, whereas the
electron-withdrawing species (entries 1–3) require longer reaction times.

ZnO AS AN EFFECTIVE AND REUSABLE CATALYST
2027
In the first set of experiments, the catalytic potential of some catalysts were
investigated for the reaction between 2-chlorophenylcyanamide and sodium azide
at 120 ꢀC (Table 2). We observed that the regiospecificity of the cycloaddition reac-
tion of azide ion with the cyanamides is strongly affected by the type of catalysts
(compare entries 1–6 with 7–11 in Table 2). According to the obtained results,
PPh3, FeCl3-SiO2, SiO2-HClO4, Al2O3-SO3H, LiCl, and glacial HOAc catalysts gave
the mixture of isomers (A þ B), while with ZnO only the sole isomer (A) was pro-
duced. The best result was obtained with 0.1 g of ZnO at 120 ꢀC, which gave
5-(2-chlorophenyl)-amino-1H-tetrazole (3b, entry 2, Table 1) in good yield. This
observation is in contrast with the reports that were presented for the synthesis of
arylaminotetrazoles using hydrazoic acid, which often produced a mixture of isomers
(Scheme 2).[12]
In the research carried out earlier, the nature of the substituent did not have
any effect.[1,10,11,12,28–32] Substituents with different electrical natures such as -methyl
or -nitrophenyl groups will form the same compound, presumably through the for-
mation of a guanyl azide as an intermediate. Surprisingly, ring closure of the substi-
tuted guanyl azide in all of these methods yields 1-alkyl- or 1-aryl-5-aminotetrazole
as a major product (as much as 95% in certain cases), whereas in the our method
tetrazoles (3) are strongly afficted by the type of substituents in arylcyanamides
(1) and the obtained isomers A or B (Table 1). Generally, when the substituent on
the aryl ring of arylcyanamides is electron releasing, formation of 1-aryl-5-amino-
1H-tetrazoles (B) is favored via the guanidine azide intermediate B’ (entries 4–8,
Table 1), and as the electronegativity of substituent is increased, the product is
shifted toward the formation of 5-arylamino-1H-tetrazole (A) via the guanidine
azide intermediate A’ (entries 1–3, Table 1). This observation is similar to the
substituent effect on aryl ring of the mechanism that was presented by Henry and
coworkers for thermal isomerization.[11]
Table 2. Comparison of different amounts of ZnO catalyst with PPh3, FeCl3-SiO2, SiO2-HClO4, Al2O3-
SO3H, glacial acetic acid, and LiCl in the synthesis of 5-(2-chlorophenyl)amino-1H-tetrazole (3b) at 120 ꢀC
Entry
Catalyst
Solvent
DMF
DMF
Time (min)
Yielda %
Product (A or B)
1
2
PPh3
120
120
30
64
75
87
85
65
86
70
74
80
80
81
A þ B
A þ B
A þ B
A þ B
A þ B
A þ B
A
FeCl3-SiO2
SiO2-HClO4
Al2O3-SO3H
LiCl
b
3
—
b
4
—
DMF
30
5
100
24 hd
115
115
115
115
115
6
Glacial HOAc
ZnO (0.05 g)
ZnO (0.07 g)
ZnO (0.10 g)
ZnO (0.10 g)
ZnO (0.14 g)
Glacial HOAcc
DMF
DMF
7
8
A
9
DMF
DMSO
DMF
A
10
11
A
A
aIsolated yield.
bUnder solvent-free and thermal conditions at 80 ꢀC.
cGlacial acetic acid as a solvent and a proton donor source.
dRoom temperature.

2028
D. HABIBI AND M. NASROLLAHZADEH
Scheme 2. Effect of different substituents in the synthesis of arylaminotetrazoles.
1
In all cases, the products were characterized by H NMR, 13C NMR, IR,
FT-IR, elemental analyses (CHN), and melting points. The disappearance of one
strong and sharp absorption band (CN stretching band) and the appearance of a
NH stretching band in the IR spectra provided clear evidence for the formation of
arylaminotetrazoles. 13C NMR spectra displayed signals at d ¼ 154–157.5 ppm,
indicative of C5 in the tetrazole ring (depending on the nature of the substituents
in the amino functionality) (Fig. 1).[19,33–35]
1
On the basis of H NMR spectra, we have considered two possible structures,
A and B. A comparison of 1H NMR spectra revealed that 5-arylamino-1H-tetrazoles
isomers (A) contain two NH bonds [NH of the amine attached to the aryl group
(NHA) and NH of the tetrazole ring (NHT)] and 1-aryl-5-amino-1H-tetrazoles
isomers (B) contain a NH2 bond. The free N–H bond of tetrazoles (NHT) makes
them acidic molecules, and not surprisingly it has been shown that both the aliphatic
and aromatic heterocycles have pKa values that are similar to the corresponding
carboxylic acids, due to the ability of the moiety to stabilize a negative charge
by electron delocalization.[36–38] In general, tetrazolic acids exhibit physical charac-
teristics similar those of to carboxylic acids. Thus, the signal of the NH proton of
1
the tetrazole ring (NHT) shifted downfield (see Fig. 2 and H NMR data of 3a–h).
1
Indeed, H NMR spectra showed signals at d ¼ 9–10 ppm, indicative of NHA in
5-arylamino-1H-tetrazoles isomers (A) (Fig. 2), whereas 1H NMR spectra of
1-aryl-5-amino-1H-tetrazoles isomers (B) showed one peak at d ¼ 6–7 ppm, indica-
tive of the NH2 group (Fig. 3).
Not many organic solvents are suitable for the cycloaddition reactions, which
usually need high temperatures (sometimes as high as 130 ꢀC), and so DMF is a most
commonly used solvent for this purpose.[1,6,39]
ZnO can easily be handled and removed from the reaction mixture by simple
filtration in the workup stages. The recovered catalyst was consecutively reused three

ZnO AS AN EFFECTIVE AND REUSABLE CATALYST
2029
Figure 1. 13C NMR spectrum (125 MHz, DMSO-d6) 1-(1-naphthyl)-5-amino-1H-tetrazole (3g).
times with a minimum variation of the yields of the products (Table 1, entry 2). After
completion of the reaction, the catalyst was filtered, thoroughly washed with
ethanol, dried and used for the subsequent runs.
Figure 2. 1H NMR spectrum (500 MHz, DMSO-d6) 5-(2,5-dicholorophenyl)-amino-1H-tetrazole (3c).

2030
D. HABIBI AND M. NASROLLAHZADEH
Figure 3. 1H NMR spectrum (500 MHz, DMSO-d6) 1-(4-methylphenyl)-5-amino-1H-tetrazole (3d).
CONCLUSION
In conclusion, we have developed a novel and highly efficient method for the
synthesis of arylaminotetrazoles by treatment of cyanamides with sodium azide in
the presence of ZnO as an effective heterogeneous catalyst. The significant advan-
tages of this methodology are good yields, short reaction times, a simple workup
procedure, and easy preparation and handling of the heterogeneous catalyst. This
methodology may find widespread uses in organic synthesis for the preparation of
aminotetrazoles.
ACKNOWLEDGMENT
We gratefully acknowledge the financial support from Bu-Ali Sina University,
Hamedan, Iran.
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