AlCl3 as an effective lewis acid for the synthesis of arylaminotetrazoles

Article abstract of DOI:10.1080/00397911.2010.497728

An efficient method for preparation of the arylaminotetrazoles derivatives is reported using aluminium chloride as an effective Lewis acid. Generally, 5-arylamino-1- H-tetrazole isomer can be obtained from arylcyanamides carrying electron-withdrawing subs

Full text of DOI:10.1080/00397911.2010.497728

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AlCl3 as an Effective Lewis Acid for the
Synthesis of Arylaminotetrazoles
Davood Habibi a , Mahmoud Nasrollahzadeh a & Yadollah Bayat b
a Department of Organic Chemistry, Bu-Ali Sina University, Hamedan,
Iran
b Department of Chemistry, Malek-Ashtar University of Technology,
Tehran, Iran
Published online: 20 May 2011.
To cite this article: Davood Habibi , Mahmoud Nasrollahzadeh & Yadollah Bayat (2011) AlCl3 as
an Effective Lewis Acid for the Synthesis of Arylaminotetrazoles, Synthetic Communications: An
International Journal for Rapid Communication of Synthetic Organic Chemistry, 41:14, 2135-2145,
DOI: 10.1080/00397911.2010.497728
To link to this article: http://dx.doi.org/10.1080/00397911.2010.497728
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Synthetic Communications1, 41: 2135–2145, 2011
Copyright # Taylor & Francis Group, LLC
ISSN: 0039-7911 print=1532-2432 online
DOI: 10.1080/00397911.2010.497728
AlCl3 AS AN EFFECTIVE LEWIS ACID FOR THE
SYNTHESIS OF ARYLAMINOTETRAZOLES
Davood Habibi,1 Mahmoud Nasrollahzadeh,1 and
Yadollah Bayat2
1Department of Organic Chemistry, Bu-Ali Sina University, Hamedan, Iran
2Department of Chemistry, Malek-Ashtar University of Technology,
Tehran, Iran
GRAPHICAL ABSTRACT
Abstract An efficient method for preparation of the arylaminotetrazoles derivatives is
reported using aluminium chloride as an effective Lewis acid. Generally, 5-arylamino-1-
H-tetrazole isomer can be obtained from arylcyanamides carrying electron-withdrawing
substituents on the aryl ring. As the electropositivity of the substituent is increased, the
product is shifted toward the formation of 1-aryl-5-amino-1H-tetrazole isomer.
Keywords AlCl3; 5-arylamino-1H-tetrazole; 1-aryl-5-amino-1H-tetrazole; arylcyanamide;
Lewis acid
INTRODUCTION
Despite the scarcity of tetrazoles in natural systems, the chemistry of this
heterocycle has gained increasing attention since the early 1980s.[1] Tetrazoles are
currently used as an isosteric replacement for carboxylic acid in drug design.[2] In
the design of drug molecules, one advantage of tetrazolic acid over carboxylic acid
is that they are resistant to many biological metabolic degradation pathways.[3–5]
Tetrazoles are also as famous as ligands in coordination chemistry,[6–8] as explosives
and rocket propellants, and also in agriculture as plant growth regulators, herbi-
cides, and fungicides stabilizers.[9–11] Another important application of tetrazoles
is the preparation of imidoylazides.[12]
Received May 4, 2010.
Address correspondence to Professor Davood Habibi, Department of Organic Chemistry, Faculty
of Chemistry, Bu Ali Sina University, Abbas Abad Street, Hamedan 6517838683, Iran. E-mail: davood.
habibi@gmail.com
2135

2136
D. HABIBI, M. NASROLLAHZADEH, AND Y. BAYAT
The earliest published methods for preparation of the aminotetrazole
derivatives were reactions including the following: (1) addition of NaNO2 to
aminoguanidine,[13,14] (2) addition of NaN3 to carbodiimides[15] or cyanamides,[16,17]
(3) substitution of a leaving group in the tetrazoles 5-position with amines,[18,19] and
(4) nucleophilic substitution by N3ꢀ of (a) the sulfite anion in aminoiminomethane-
sulfonic acids,[20] and (b) sulfur from thioureas in the presence of mercury.[21,22]
Previously, the 5-monosubstituted amino-1H-tetrazoles were synthesized by thermal
isomerization of 1-substituted-5-amino-1H-tetrazoles in boiling ethylene or molten
state (180–200 ꢁC).[23] Herbst and Garbrecht have shown that cyanamides may be
converted to aminotetrazoles using hydrazoic acid, which often results in a mixture
of isomers.[24]
Congreve has reported a two-step synthesis of 1-aryl-5-amino-1H-tetrazoles
from the corresponding 1-aryltetrazoles via cyanamide intermediates.[25] The
reaction suffers from some drawbacks such as harsh reaction conditions, low
temperatures (ꢀ70 ꢁC), use of large excess of sodium azide, and organolithium
reagents that are potentially dangerous. Vorobiov and coworkers published a
three-step synthesis of 1-aryl-5-amino-1H-tetrazoles in poor yields from the
corresponding aromatic amines via isolation of cyanamides intermediate.[26]
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 intermediate were not stable enough to
be isolated. Therefore, the 1-aryl-5-amino-1H-tetrazoles were produced in poor
yields.
These methods suffer from one or more disadvantages, such as poor yield,
long reaction times, harsh reaction conditions; difficulty of obtaining and=or
starting materials preparation, difficulty of workup due to the application of
homogeneous catalyst, use of expensive and toxic reagents, and the in
situ–generated hydrazoic acid, which is highly toxic and explosive. On the other
hand, in most cases only the 1-aryl-5-amino-1H-tetrazoles were obtained. Because
of the safety considerations, we required a method that did not use hydrazoic acid
or apply an azide source because of the in situ production of hydrazoic acid.
Thus, a convenient and efficient method was required for preparation of the
arylaminotetrazoles.
In continuation of our recent work on the synthesis of heterocycles and
application of heterogeneous reagents,[27–32] we herein report the synthesis of
arylaminotetrazoles (3a–h) from a wide variety of organic cyanamides (1a–h) using
AlCl3 as an effective Lewis acid at 120 ꢁC in dimethylformamide (DMF) (Scheme 1).
Scheme 1.

AlCl3 IN ARYLAMINOTETRAZOLE SYNTHESIS
2137
EXPERIMENTAL
All reagents were purchased from the Merck and Aldrich chemical companies
and used without further purification. Products were characterized by spectroscopy
1
data [infrared (IR), Fourier transform (FT)–IR, H NMR, and 13C NMR spectra],
elemental analysis (CHN), and melting points. The NMR spectra were recorded in
dimethylsulfoxide (DMSO) and acetone. 1H NMR spectra were recorded on a
Bruker Avance DRX 300- and 500-MHz instruments. The chemical shifts (d) are
reported in parts per million (ppm) relative to the tetramethylsilane (TMS) as inter-
nal standard. J values are given in hertz. 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.
Preparation of the Arylcyanamides
The arylcyanamides (1a–h) were prepared according to the literature.[33]
Typical Procedure for Preparation of Arylaminotetrazoles 3
Aluminium choloride (0.09 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 AlCl3 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
ethanol). The spectral data of eight representative arylaminotetrazoles are given:
Spectroscopic Data
5-(4-Nitrophenyl)amino-1H-tetrazole (3a, Table 1, entry 1). IR (KBr):
3550, 3235, 3100, 1635, 1572, 1488, 1337, 1290, 1257, 1111, 1052, 839, 745 cmꢀ1
;
1H NMR (DMSO-d6, 500 MHz): d 7.76 (2H, d, J ¼ 7.3 Hz), 8.21 (2H, d, J ¼ 7.3 Hz),
10.96 (1H, s, br); 13C NMR (DMSO-d6, 125 MHz): d 116.8, 124.9, 140.4, 147.2,
155.2. Anal. calcd. for C7H6N6O2: C, 40.78; H, 2.93; N, 40.77. Found: C, 40.90; H,
3.03; N, 40.64.
5-(4-Bromophenyl)amino-1H-tetrazole (3b, Table 1, entry 2). IR (KBr):
1
3285, 3205, 1617, 1574, 1534, 1485, 1240, 1054, 1023, 828, 772, 720 cmꢀ1; H NMR
(DMSO-d6, 300 MHz): d 7.45–7.53 (4H, m), 9.98 (1H, s, br), 15.57 (1H, s, br);
13C NMR (DMSO-d6, 75 MHz): d 112.8, 119.2, 132.2, 140.3, 156.1. Anal. calcd. for
C7H6N5Br: C, 35.02; H, 2.52; N, 29.17; Found: C, 35.11; H, 2.64; N, 29.24.

2138
D. HABIBI, M. NASROLLAHZADEH, AND Y. BAYAT
Table 1. Synthesis of arylaminotetrazoles (3) via secondary arylcyanamides (1) catalyzed by AlCl3
3
(A or B) (min)
Time Yielda
Entry
1
R
Product
%
Mp (ꢁC) [lit. Mp[ref]
218–220
[—]
4-NO2-C6H4
3a (A)
3b (A)
120
100
74
249–250
[—]
2
3
4-Br-C6H4
78
77
272–274
[—]
2,5-(Cl)2-C6H3
3c (A)
95
215–217
[211–212[13]
4
5
C6H5
3d (A)
3e (B)
75
65
76
75
]
178–179
[175.5–177[23]
4-Me-C6H4
]
191–192
[191–192[13,23]
6
2-Me-C6H4
3f (B)
65
82
]
199–201
[199–201[23]
7
8
2,4-(Me)2-C6H3
1,4-C6H4
3g (B)
3h (B)
65
55
76
80
]
220–221
[—]
aYield refers to pure isolated products.
5-(2,5-Dichlorophenyl)amino-1H-tetrazole (3c, Table 1, entry 3). IR
(KBr): 3245, 3184, 3156, 3115, 1620, 1592, 1569, 1544, 1467, 1411, 1090, 1054,
1
1038, 836, 799, 772, 664, 563, 538 cmꢀ1; H NMR (DMSO-d6, 500 MHz): d 7.06
(1H, d, J ¼ 8.5 Hz), 7.48 (1H, d, J ¼ 8.6 Hz), 8.19 (1H, s), 9.38 (1H, s), 14.72 (1H,
s, br); 13C NMR (DMSO-d6, 125 MHz): d 118.7, 120.3, 122.5, 130.8, 132.3, 137.8,
154.4. Anal. calcd. for C7H5N5Cl2: C, 36.52; H, 2.17; N, 30.43. Found: C, 36.66;
H, 2.22; N, 30.56.
5-Phenylamino-1H-tetrazole (3d, Table 1, entry 4). IR (KBr): 3325, 3220,
1620, 1580, 1533, 1498, 1243, 1053, 785, 742, 735, 688, 660 cmꢀ1 1H NMR
;
(DMSO-d6, 500 MHz): d 6.93 (1H, t, J ¼ 7.3 Hz), 7.29 (2H, t, J ¼ 7.9 Hz), 7.49
(2H, d, J ¼ 8.2 Hz), 9.74 (1H, s), 15.33 (1H, s, br); 13C NMR (DMSO-d6,
125 MHz): d 155.8, 140.3, 128.9, 121.0, 116.7.
1-(4-Methylphenyl)-5-amino-1H-tetrazole (3e, Table 1, entry 5). FT-IR
(KBr): 3306, 3141, 1655, 1594, 1572, 1519, 1467, 1320, 1306, 1142, 1090, 1017,
1
839, 818, 618, 545, 483 cmꢀ1; H NMR (DMSO-d6, 500 MHz): d 2.38 (3H, s), 6.80

AlCl3 IN ARYLAMINOTETRAZOLE SYNTHESIS
2139
(2H, s), 7.39 (2H, d, J ¼ 7.8 Hz), 7.43 (2H, d, J ¼ 8.3 Hz); 13C NMR (DMSO-d6,
125 MHz): d 21.6, 124.8, 131.1, 131.8, 139.8, 155.8.
1-(2-Methylphenyl)-5-amino-1H-tetrazole (3f, Table 1, entry 6). FT-IR
(KBr): 3323, 3158, 1655, 1593, 1575, 1503, 1473, 1313, 1126, 1091, 1026, 772, 757,
1
715, 673, 564 cmꢀ1; H NMR (DMSO-d6, 300 MHz): d 2.06 (3H, s), 6.74 (2H, s),
7.36–7.51 (4H, s); 13C NMR (DMSO-d6, 75 MHz): d 17.4, 127.7, 127.9, 130.9,
131.8, 132.5, 135.8, 156.1.
1-(2,4-Dimethylphenyl)-5-amino-1H-tetrazole (3g, Table 1, entry 7). IR
(KBr): 3310, 3150, 1651, 1573, 1506, 1458, 1377, 1312, 1134, 1088, 1027, 868, 823,
614, 562 cmꢀ1 1H NMR (DMSO-d6, 500 MHz): d 7.28 (1H, s), 7.19 (1H, d,
;
J ¼ 8.3 Hz), 7.22 (1H, d, J ¼ 8.1 Hz), 6.63 (2H, s) 1.99 (3H, s), 2.36 (3H, s); 13C NMR
(DMSO-d6, 125 MHz): d 16.7, 20.6, 127.1, 127.6, 129.4, 131.7, 134.9, 140.0, 155.6.
1-(4-(5-Amino-1H-tetrazole-1-yl)phenyl)-5-amino-1H-tetrazole (3h,
Table 1, entry 8). IR (KBr): 3357, 3139, 1652, 1588, 1519, 1426, 1293, 1141,
1
1070, 843 cmꢀ1; 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
The general synthetic method is depicted in Scheme 1. Arylaminotetrazoles
(3a–h) were obtained from the reaction of cyanamides (1a–h) with sodium azide
(2) in the presence of AlCl3 at 120 ꢁC for the appropriate time in good yields, as
summarized in Table 1.
To show the advantages of AlCl3, its reaction was compared with PPh3,
Fe(HSO4)3, SiO2-HClO4, Al2O3-SO3H, LiCl, and glacial HOAc in the synthesis of
5-(4-bromophenyl)amino-1H-tetrazole (3b). As shown in Table 2, AlCl3 is an effective
Lewis acid since the products are regiospecific, whereas with PPh3, Fe(HSO4)3, SiO2-
HClO4, Al2O3-SO3H, LiCl, and glacial HOAc, a mixture of isomers will be produced.
In addition, to optimize the yields, different amounts of AlCl3 were used for the
synthesis of 3b from the reaction of 1b with 2, and the best result was obtained with
0.09 g of AlCl3.
Not many organic solvents are stable at high temperatures, which are necessary
for the cycloaddition reactions. For this reason, DMF is the most commonly used
solvent for this purpose.[34,35]
The following four important results can be extracted from the data collected
in the tables:
1. As shown in Table 1, among the various cyanamides, those having electron-
releasing groups on the aromatic rings (entries 5–7) complete at 120 ꢁC after
55–65 min, whereas the electron-withdrawing species (entries 1–3) require longer
reaction times.
2. The processes were completely regiospecific. This observation is in contrast with
reports that hydrazoic acid was used for the synthesis of aminotetrazoles, in
which the mixture of isomers is often obtained.[24]

2140
D. HABIBI, M. NASROLLAHZADEH, AND Y. BAYAT
Table 2. Comparison of different amounts of AlCl3 catalyst with PPh3, LiCl, SiO2-HClO4, Al2O3-SO3H,
Fe(HSO4)3, and glacial HOAc in the synthesis of 3b at 120 ꢁC
Entry
Catalyst
Solvent
Time (min)
Yielda
%
Product (A or B)
1
2
PPh3
LiCl
DMF
DMF
120
100
25
75
70
85
86
90
87
74
76
77
78
79
A þ B
A þ B
A þ B
A þ B
A þ B
A þ B
A
b
3
SiO2-HClO4
Al2O3-SO3H
Fe(HSO4)3
—
—
b
4
30
b
5
—
30
6
Glacial HOAc
AlCl3 (0.05 g)
AlCl3 (0.07 g)
AlCl3 (0.09 g)
AlCl3 (0.09 g)
AlCl3 (0.12 g)
Glacial HOAcc
DMF
DMF
30 hd
100
100
100
100
100
7
8
A
9
DMSO
DMF
DMF
A
10
11
A
A
aIsolated yield.
bUnder solvent-free and thermal conditions at 80 ꢁC.
cGlacial acetic acid as both solvent and proton donor source.
dRoom temperature.
3. Tetrazoles (3) are strongly under the effect of the type of substituents in arylcya-
namides (1) and the obtained isomers A or B (Table 1). Generally, when the
substituent on the aryl ring of 1 is electron-releasing, formation of 1-aryl-5-
amino-1H-tetrazoles (B) is favored via the guanidine azide intermediate B’ (entries
6–8, 11, and 12, Table 1), and as the electronegativity of substituent is increased,
the product is shifted toward the formation of 5-arylamino-1H-tetrazoles (A) via
the guanidine azide intermediate A’ (entries 1–3, Table 1). In the research carried
out before, the nature of the substituent did not have any effect.[36–42] In other
words, in the synthesis of aminotetrazoles from cyanamides, only the 1-aryl-5-
Scheme 2.

AlCl3 IN ARYLAMINOTETRAZOLE SYNTHESIS
2141
Figure 1. 13C NMR spectrum (125 MHz, DMSO-d6) 5-(2,5-dicholorophenyl)-amino-1H-tetrazole (3c).
amino-1H-tetrazole (B) or the mixture of isomers (A þ B) were obtained
(Scheme 2).[24]
4. Because of the presence of two CN groups, 1h (entry 8, Table 1) interestingly
afforded the double-addition product.
After removal of the solvent and purification, the products were characterized
by melting points, elemental analysis (CHN), infrared (IR), 1H NMR, and 13C
NMR. Elimination of one strong and sharp absorption band (CN stretching band)
Figure 2. 1H NMR spectrum (300 MHz, DMSO-d6) 5-(4-bromophenyl)amino-1H-tetrazole (3b).

2142
D. HABIBI, M. NASROLLAHZADEH, AND Y. BAYAT
Figure 3. 1H NMR spectrum (100 MHz, acetone-d6) 5-(4-bromophenyl)amino-1H-tetrazole (3b).
and appearance of the two absorption bands in the range of 3139–3550 cmꢀ1 (NH
stretching bands) in the IR spectrum confirmed formation of the arylaminotetra-
zoles. 13C NMR spectra showed signals for tetrazole ring carbons of arylaminotetra-
zoles in the range of 154–157 ppm (Fig. 1).[43–46]
Compound 5-(4-bromophenyl)amino-1H-tetrazole (3b) shows a singlet for
aryl ring protons (Fig. 2). It is probably due to the electronegativity of aminotetra-
zole fragment, which is roughly equivalent to one bromine atom.[47] However, also
as expected, small chemical shift differences are observed for these compounds in
the two applied solvents. Chemical shifts observed in DMSO are not equivalent to
1
those observed in acetone. This general medium effect in H NMR causes a large
separation for aryl ring proton shifts in acetone rather than in DMSO for com-
pound 5-(4-bromophenyl)amino-1H-tetrazole (3b) as shown in Figs. 2 and 3. These
chemical shift differences are a reflection of the solvent dielectric constants and are
perhaps consistent with the ability of the solvents to participate in the hydrogen
bonding.[48]
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 AlCl3 as an effective Lewis acid. The significant advantages
of this methodology are good yields, short reaction times, a simple workup
procedure, and easy preparation and handling of the Lewis acid catalyst. This
methodology may find widespread uses in organic synthesis for preparation of the
aminotetrazoles.
ACKNOWLEDGMENT
The authors are thankful to the Bu-Ali Sina University for the partial support
of this work.

AlCl3 IN ARYLAMINOTETRAZOLE SYNTHESIS
2143
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