Efficient uncatalyzed conversion of primary and secondary thioamides into 1-substituted, 5-substituted, 1, 5-disubstituted and annulated tetrazoles

Article abstract of DOI:10.1080/10426507.2011.586385

Unprecedented high-yield simple and mild conversion of primary aliphatic and aromatic thioamides into 5-substituted tetrazoles on treatment with a combination of tetrachlorosilane and sodium azide in refluxing acetonitrile has been achieved. Secondary acyclic, cyclic, and heterocyclic thioamides could also be transformed in high yields into 1-substituted, 1,5-disubstituted, or annulated tetrazoles under the same reaction condition. Copyright Taylor & Francis Group, LLC.

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Phosphorus, Sulfur, and Silicon and the
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Efficient Uncatalyzed Conversion of
Primary and Secondary Thioamides
into 1-Substituted, 5-Substituted, 1, 5-
Disubstituted and Annulated Tetrazoles
Abdel-Aziz S. El-Ahl a b , Fatty A. Amer b & Akram H. Elbeheery b
a Department of Chemistry, Makah University College, Umm Al-Qura
University, Makah, Kingdom of Saudi Arabia
b Department of Chemistry, Faculty of Science, Mansoura University,
Mansoura-, 35516, Egypt
Published online: 31 Oct 2011.
To cite this article: Abdel-Aziz S. El-Ahl , Fatty A. Amer & Akram H. Elbeheery (2011): Efficient
Uncatalyzed Conversion of Primary and Secondary Thioamides into 1-Substituted, 5-Substituted, 1,
5-Disubstituted and Annulated Tetrazoles, Phosphorus, Sulfur, and Silicon and the Related Elements,
186:11, 2226-2235
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Phosphorus, Sulfur, and Silicon, 186:2226–2235, 2011
Copyright ꢀC 2011 Umm Al-Qura University
ISSN: 1042-6507 print / 1563-5325 online
DOI: 10.1080/10426507.2011.586385
EFFICIENT UNCATALYZED CONVERSION OF PRIMARY
AND SECONDARY THIOAMIDES INTO 1-SUBSTITUTED,
5-SUBSTITUTED, 1, 5-DISUBSTITUTED AND ANNULATED
TETRAZOLES
Abdel-Aziz S. El-Ahl,1,2 Fatty A. Amer,2
and Akram H. Elbeheery2
1Department of Chemistry, Makah University College, Umm Al-Qura University,
Makah, Kingdom of Saudi Arabia
2Department of Chemistry, Faculty of Science, Mansoura University, Mansoura-
35516, Egypt
GRAPHICAL ABSTRACT
Abstract Unprecedented high-yield simple and mild conversion of primary aliphatic and
aromatic thioamides into 5-substituted tetrazoles on treatment with a combination of tetra-
chlorosilane and sodium azide in refluxing acetonitrile has been achieved. Secondary acyclic,
cyclic, and heterocyclic thioamides could also be transformed in high yields into 1-substituted,
1,5-disubstituted, or annulated tetrazoles under the same reaction condition.
Keywords Tetrachlorosilane; sodium azide; thioamides; tetrazoles
INTRODUCTION
Tetrazoles are a class of nitrogen-rich heterocycles with a wide range of applications.1
They have found use in various material science applications, including photography,2 spe-
cial explosives for rocket propellants,3 information recording systems,4 and agricultural
composition.5 In addition, extensive work has been carried out in the field of medici-
nal chemistry where tetrazoles are frequently used as metabolically stable surrogates for
carboxylic acids.6–8
Received 16 March 2011; accepted 1 May 2011.
Address correspondence to Abdel-Aziz S. El-Ahl, Department of Chemistry, Makah University College,
Umm Al-Qura University, P.O. Box 2064, Makah, Kingdom of Saudi Arabia. E-mail: elahl2002@yahoo.com
2226

UNCATALYZED CONVERSION OF PRIMARY AND SECONDARY THIOAMIDES
2227
Tetrazoles serve as precursors for the synthesis of several interesting nitrogen het-
erocycles.9,10 These are also employed as catalysts in asymmetric synthesis. Recently,
proline-derived tetrazoles have been reported as a powerful enantioselective catalysts used
in conjugate addition reactions, asymmetric aldol, Mannich reaction, and multi-component
reactions.11
Therefore, many synthetic efforts have been made for the development of efficient
routes to prepare tetrazoles. The most convenient routes to synthesize tetrazoles involved
the addition of hydrazoic acid or inorganic azide salts into nitriles,12 ketones,13 oximes,14
amides,15 imidoyl chlorides,16 and nitrlium triflates.17 The use of Bronsted18 or Lewis
acids,19 heterogeneous20 or nanocrystalline catalysts,21 or stoichiometric amounts of Zn II
salts22,23 have been reported as suitable additives in above-mentioned methodologies. As
an alternative to inorganic azide salts, trimethylsilyl,24 trialkyl tin,25 and organoaluminum
azides26 have been introduced as comparatively safe azide sources (sometimes prepared in
situ) that have the added benefit of being soluble in organic solvents. Unfortunately, with
very few exceptions, several of these protocols included the use of toxic metals, expensive
reagents, drastic reaction conditions, and/or the possible presence of dangerous hydrazoic
acid or other explosive sublimates.
In this paper, we report a simple, safe, and cheap one-pot methodology for the
uncatalyzed direct conversion of readily available primary and secondary thioamides into
tetrazoles.
RESULTS AND DISCUSSION
During the last decade, we have being involved in developing a new methodology
for the construction of tetrazole systems via reaction of SiCl4/NaN3 reagent system with
ketones, α, ß-unsaturated ketones, and primary and secondary amides.27–29
To our knowledge the conversion of primary thioamides into 5-substituted tetrazoles
has not been reported in the literature and only few reports on the conversion of secondary
thioamides into tetrazoles have been reported using aluminum azide30 or trimethylsilyl
azide.31
Sulfur has lower electronegativity than oxygen. This means that thioamides should
be less polar than amides, with more equable charge density, and less reactivity. In fact,
thioamides are more polar (and especially more polarizable) compared with respective
amides and participate in more reactions.32
Therefore, we envisioned that the investigation of the uncatalyzed reaction of the
reagent system of tetrachlorosilane and sodium azide with wide varieties of thioamides as
a new route for the synthesis of tetrazoles would be a new addition to the existing methods.
Thus, we have investigated the reaction of thiobenzamide 1a with different stoichio-
metric amounts of tetrachlorosilane and sodium azide in dry acetonitrile under different
conditions. It has been found that the reaction of 1a with two equivalents of tetrachlorosilane
and six equivalents of sodium azide in acetonitrile under reflux for 1.5 h to afford 5-phenyl-
1H-tetrazole 2a in 88% yield. This reaction is considered as the first direct conversion of
primary thioamides into 5-substituted tetrazoles. The potency of this transformation came
from the comparison with previously reported studies, which emphasized that 1H-tetrazoles
could not be made directly from primary amides. They must be prepared using a cleavable
N-substituent, e.g., ß-cyanoethyl group.33
We have found that dry acetonitrile is the best solvent of choice, which is unreactive
toward this reagent system, and the possibility of liberation of hydrazoic acid is very low.

2228
A.-A. S. EL-AHL ET AL.
The risk of evolution of hydrazoic acid by pouring the reaction mixture into 5% ice-cooled
sodium carbonate solution during work-up is minimal. By this method a variety of ring-
substituted thiobezamides were converted into the corresponding tetrazoles (Scheme 1).
The results of this reaction are summarized in Table 1.
As demonstrated in Table 1, the reaction tolerates both electron positive or elec-
tron negative groups on the aromatic ring. This is exemplified by the conversion of 4-
methoxythiobenzamide 1e and 4-nitrothiobenzamide 1f into the corresponding tetrazoles
2e,f in 90% and 80% yield, respectively. The reaction worked well with ortho-substituted
thiobenzamides. In other methods, ortho-substituted substrates tend to be recalcitrant.34 Of
particular interest is ortho-chlorothiobenzamide 1c, which produced 5-(2-chlorophenyl)-
1H-tetrazole 2c in 90% yield. The importance of this tetrazole is that it can be further
elaborated into more complex structures using the fast possibilities of organometallic
chemistry.
Primary thioamides with a bulkier C-aryl group afforded the corresponding tetra-
zoles in high yields. This is demonstrated by the conversion of 4-phenylthiobenzamide
1g, which has a bulky biphenyl nucleus, to the corresponding tetrazole 2g in 85% yield
(1.5-h reflux in acetonitrile). Several highly potent AT1 selective antagonists contain a
biphenyltetrazole moiety appended to a five-membered or a six-membered heterocycle.
The biphenyl-tetrazole moiety is considered to be an essential acidic functional group for
antagonism.35
While the conversion of aliphatic nitriles into tetrazoles has been reported to require
very high temperature using a sealed glass pressure reactor,36 aliphatic thioamides such as
phenoxythioacetamide 1h was readily converted to the corresponding tetrazole 2h in 85%
yield (2-h reflux in acetonitrile).
We have also investigated the reaction of secondary thioamides with SiCl4/NaN3
reagent system. The reaction of N-aryl thioformamides 1i–r with SiCl4/NaN3 in acetonitrile
under reflux conditions afforded 1-aryl-1H-tetrazoles 2i–r in 80–90% yields (cf. Table 1).
The reaction tolerates various substituents at the aryl group such as chloro-, methoxy-, and
methyl- or nitro- groups. The reaction rate was retarded as the N-substituent gets bulkier.
Similarly, N-aryl thioacetamides 1s–z furnished the corresponding 1,5-disubstituted
tetrazoles 2s–z in yields ranging from 86–92% under the same reaction conditions (cf.
Table 1).
For steric reasons N-(1-naphthyl)thioacetamide 1y and N-benzyl-2-phenyl-
thioacetamide 1z required relatively longer reaction time to be completely converted into
the corresponding 2y and 2z tetrazoles (6–7-h reflux in acetonitrile).
In order to investigate the potentiality and generality of this reaction in the synthesis of
different tetrazoles, we have investigated the reaction of SiCl4/NaN3 with both aliphatic and
aromatic heterocyclic thioamides (Scheme 2). Thus, the reaction of SiCl4/NaN3 reagent
system with azepan-2-thione 3 in refluxing acetonitrile afforded 6,7,8,9-tetrahydro-5H-
tetrazolo-[1,5-a]azepine 4 in 88% yield.
Also, the reaction of 4-methylquinoline-2(1H)-thione 5 with SiCl4/NaN3 under the
same conditions furnished 5-methyl tetrazolo[1,5-a]quinoline 6 in 86% yield.
The structures of the previously reported tetrazoles 2a–f, 2h–p, and 2r–x and the
four hitherto unknown tetrazoles 2g, 2q, 2y, and 2z have been confirmed by analytical
and spectral methods and comparison of their spectroscopic data with those reported in
the literature. The 1H-NMR of 5-methyl-1-(1-naphthyl)-1H-tetrazole 2y displayed a three-
proton singlet at δ 2.43 ppm assigned for the 5-CH3 group, three two-proton doublets each at
δ 8.11, 8.0, 7.17 ppm and a four-proton multiplet at δ 7.67–7.47 ppm assigned for aromatic

2229

2230
A.-A. S. EL-AHL ET AL.
Scheme 1 Reaction of primary and secondary aromatic thioamides with SiCl4/NaN3 reagent system.
Scheme 2 Reaction of aliphatic and aromatic heterocyclic thioamides with SiCl4/NaN3 reagent system.

UNCATALYZED CONVERSION OF PRIMARY AND SECONDARY THIOAMIDES
2231
protons. The mass spectra of compounds 2a, 2n, 2t, and 6 were found to show a regular
fragmentation giving molecular ion peaks at m/z of 146, 160, 174, and 184, respectively,
along with fragment results from elimination of nitrogen or hydrazoic acid molecules. In
the Infrared (IR) spectra, the bands due to –N N- and C N group, which are present in all
studied compounds, were observed at about 1595 and 1516 cm−1, respectively. The bands
at about 1185 and 1086 cm−1 were characteristic for the CN4 (tetrazole ring).
The mechanism of transformation of thioamides into tetrazoles may be explained
by the in situ formation of an azidochlorosilane species, represented as SiCln(N3)4-n, from
tetrachlorosilane and sodium azide in 1:3 ratio, respectively.27b In this example the azidosi-
lane reagent forms N,S-bis silyl imidate A,39 which in turn reacts with an equivalent of
sodium azide to give the imidoyl azide B via a nucleophilic addition elimination mecha-
nism. The intermediate imidoyl azide B tautomerizes to N-silyl tetrazole C (presumably
as the N-1 regioisomer), which is hydrolyzed in the workup to provide 5-substituted tetra-
zoles (Scheme 3). A similar mechanism has been proposed for the reaction of amides
with tetrachlorosilane and sodium azide.29 Also, the reported formation of tetrazoles via
desulfurization of thiourea derivatives in presence of sodium azide gives support for the
proposed reaction mechanism.40 It is interesting to note that this reagent apparently does
not react with the acetonitrile solvent.
Scheme 3 Mechanism of reaction of thioamides with SiCl4/NaN3 reagent system.
CONCLUSION
In conclusion, a very simple and convenient chemoselective method has been devel-
oped for the synthesis of different substituted and annulated tetrazoles from primary and
secondary cyclic and acyclic thioamides. The isolation and purification steps were par-
ticularly straightforward in comparison to other methods. This method has advantages in
terms of yields, short reaction times, ease of operation, absence of any catalyst, and making
a useful and important addition to the present methodologies. This methodology might
also find widespread use in organic synthesis for the preparation of different substituted
tetrazoles. In addition, we demonstrated that primary thioamides are much more reactive
toward SiCl4/NaN3 reagent system than nitriles exemplified by the solvent.

2232
A.-A. S. EL-AHL ET AL.
Table 2 Spectroscopic data of compounds 2a–z, 4 and 6
Compound
no.
IR cm−1
(KBr, selected peaks)); MS (m/z)
1H-NMR, δ ppm
2a
3060, 2600, 1608, 1563, 1163, 1057; MS
(m/z,%) 146 (M+, 22), 118 (M+ −N2, 100),
104 (M+ −N3, 11.9), 103 (M+ −HN3, 35.5)
3090, 2550, 1606, 1580, 1164, 1095, 744
δ (CD3OD): 8.03–7.99 (m, 2H),
7.59–7.56 (m, 3H)
2b
2c
2d
2e
2f
δ (CD3OD): 8.03–7.99 (d, J = 7.8 Hz,
2H), 7.61–7.57 (d, J = 7.8 Hz, 2H)
δ (CD3OD): 7.83–7.72 (m, 1H),
7.62–7.44 (m, 3H)
3085, 2700, 1600, 1560, 1160, 1070, 744
3000, 2847, 2615, 1613, 1571, 1161, 1052
3090, 2925, 2720, 1613, 1590, 1178, 1055
δ (CDCl3): 7.73 (d, J = 7.6, 2H),
7.01–7.06 (d, J = 7.6, 2H), 2.18 (s, 3H)
δ (CDCl3): 7.97 (d, J = 8.2 Hz, 2H), 7.14
(d, J = 8.2, 2H), 3.83 (s, 3H)
3208, 3100 1610, 1565, 1161, 1549,1109,
1063
3092, 2847, 2800, 1660, 1607, 1159, 1092
δ (CD3OD): 8.43–8.38 (d, J = 7.9 Hz,
2H), 8.31–8.27 (d, J = 7.9, 2H)
δ (CDCl3):7.94 (d, J = 7.9 Hz, 2H),
7.61–7.46 (m, 5H), 7.30 (d, J = 7.9 Hz,
2H)
2g
2h
3050, 2750, 1597, 1570, 1108, 1077
δ (CD3OD): 7.37–7.26 (m, 2H),
7.03–6.91 (m, 3H), 5.47 (s, 2H)
δ (CDCl3): 9.0 (s, 1H), 7.75–7.52 (m, 5H)
δ (CDCl3): 9.01 (s, 1H), 7.72–7.65 (m,
2H) 7.59–7.52 (m, 2H)
2i
2j
3120, 1596, 1501, 1090, 1047
3123, 3040, 1503, 1091, 1037
2k
2l
3125, 3090, 1592, 1183, 1086, 776
δ (CDCl3): 9.12 (s, 1H), 7.75–7.42 (m,
4H)
3127, 3095, 2930 1607, 1517, 1093, 1046
3130, 3018, 2971 (CH3) 1612, 1595, 1090
δ (CDCl3): 8.95 (s, 1H), 7.71 (d, J = 8,
2H), 7.5 (d, J = 8 Hz, 2H), 3.88 (s, 3H)
δ (CDCl3): 8.95 (s, 1H), 7.5–7.44 (m,
2H), 6.98–6.95 (m, 2H), 3.85 (s, 3H)
δ (CDCl3): 8.95 (s, 1H), 7.58–7.54 (m
2H), 7.39–7.7.35 (m, 2H), 2.44 (s, 3H)
2m
2n
3131, 3086, 2920 (CH3), 1594, 1514, 1090,
1043; MS (m/z,%) 160 (M+, 3.4), 132 (M+
−N2, 60.6), 131 (M+ −HN2, 100), 91
(PhCH2−, 61.3), 77 (C6H5−, 30.9), 65
(C5H5−, 33.8)
2o
3107, 3066, 2970, 2929, 1610, 1588, 1171,
1094
δ (CDCl3): 8.96 (s, 1H), 7.58 (s, 1H), 7.44
(m, 1H), 7.35 (m, 1H), 7.24 (m, 1H),
2.35 (s, 3H)
2p
2q
2r
2s
2t
3123, 3093, 1594, 1522, 1462, 1350, 1110,
1084
3088, 1595, 1516, 1172, 1086
δ (CDCl3): 9.13 (s, 1H), 8.53–8.46 (m
2H), 8.02–7.95 (m, 2H)
δ (CDCl3): 8.94 (s, 1H), 8.1–8.05 (m, 1H),
8.01–7.96 (m, 1H), 7.64–7.40 (m, 5H)
δ (CDCl3): 8.51 (s, 1H), 7.43–7.37 (m,
3H), 7.36–7.25 (m, 2H), 5.59 (s, 2H)
δ (CDCl3): 7.62–7.55 (m, 3H), 7.49–7.43
(m, 5H), 2.61 (s, 3H)
3109 3038, 2963 1478, 1456, 1101, 1075
3061, 2952 1592, 1500, 1113, 1083
3038, 2987, 2954 (CH3) 1514, 1460, 1109,
1087; MS (m/z,%) 174 (M+, 5.3), 146 (M+
−N2, 84.0), 145 (M+ −HN2, 100), 105
(52.3), 91 (PhCH2−, 73.8), 77(C6H5−, 32),
65(43.3)
δ (CDCl3): 7.30 (m, 4H), 2.57 (s, 3H),
2.44 (s, 3H)
2u
3061, 2920 1593, 1520, 1098, 1076
δ (CDCl3): 7.45–7.25 (m, 4H), 2.59 (s,
3H), 2.45 (s, 3H)
(Continued on next page)

UNCATALYZED CONVERSION OF PRIMARY AND SECONDARY THIOAMIDES
2233
Table 2 Spectroscopic data of compounds 2a–z, 4 and 6 (Continued)
Compound
no.
IR cm−1
(KBr, selected peaks)); MS (m/z)
1H-NMR, δ ppm
2v
3069, 2987, 2940 1606, 1513, 1103, 1020
δ (CDCl3): 7.54 (d, J = 8.0 Hz, 2H), 7.12
(d, J = 8.0, 2H), 3.85 (s, 3H), 2.52 (s,
3H)
2w
2x
2y
3071, 2938 (CH3) 1495, 1463, 1092, 1041, 850
δ (CDCl3): 7.65 (d, J = 8.0 Hz 2H), 7.47
(d, J = 8.0, 2H), 2.59 (s, 3H)
δ (CDCl3): 7.72–7.68 (m, 2H), 7.37–7.33
(m, 2H), 2. 59 (s, 3H)
δ (CDCl3): 8.13–8.09 (d, 1H, J = 8 Hz),
8.02–7.98 (d, 1H, J = 8 Hz), 7.67–7.47
(m, 4H), 7.17 (d, J = 8 Hz, 1H), 2.43
(s, 3H)
3055, 2945 (CH3), 1490, 1463, 1111, 1072,
820
3059, 2982 1595, 1517, 1113, 1083, 820
2z
4
3064, 2962 1618, 1555, 1153, 1091, 820
2955 (CH2) 1590, 1490, 1117, 1089
δ(CDCl3): 7.3–7.26 (m, 5H),7.04 (m,5H),
5.30 (s, 2H), 4.14 (s, 2H)
δ (CDCl3): 4.46 (t, 2H, J = 7.5 Hz),
3.10–3.04 (t, 2H, J = 7.5), 2.0–1.68 (m,
6H)
6
3079, 2920 1616, 1560, 1148, 1086; MS
(m/z,%) 184 (M+, 18.4), 156 (M+ −N2,
42.5), 129 (100), 102 (50), 77 (C6H5−,
20.5), 51 (38.4)
δ (CDCl3): 8.88 (d, 1H, J = 8.2 Hz), 8.10
(d, 1H, J = 8.2 Hz, 7.9–7.85 (m, 2H),
7.65 (s, 1H), 2.85 (s, 3H)
EXPERIMENTAL
Infrared spectra were recorded on Thermo Nicolet nexus 470 FT-IR and Mattson
5000 FT-IR spectrophotometer with only selected absorptions being recorded. Absorption
maxima were recorded in per centimeter. Nuclear magnetic resonance (NMR) spectra were
run at 1H-NMR Varian Mercury (200 MHz) FT-NMR spectrometer. Spectra were measured
using CDCl3 and CD3OD as solvents with chemical shifts quoted in parts per million (δ
ppm) using TMS as internal standard. The Mass Spectra (MS) were recorded on GC-MS
QP-1000 EX Schmiadzu (Japan) mass spectrometer. Elemental analyses were obtained on
a Heraeus CHN-OS Rapid analyzer in the elemental-analysis unit at the Faculty of Science,
Cairo University, Egypt.
Melting points (uncorrected) were determined in an open capillary with a Griffin
melting point apparatus. All thioamides were synthesized using Lawesson’s reagent.
Tetrachlorosilane was used as obtained from commercial sources. The solvents were
distilled and dried before use. Acetonitrile was dried by refluxing over phosphorus pen-
taoxide.
Reaction of SiCl4/NaN3 Reagent System with Primary and Secondary
Thioamides (General Procedure)
Tetrachlorosilane (2.4 ml, 20 mmol) was added into a mixture of thioamide (10 mmol)
and sodium azide (3.9 g, 60 mmol) in anhydrous acetonitrile (15 ml). The reaction mixture
was stirred at room temperature for 10 min, then refluxed at 82◦C until thioamide was
consumed (TLC analyses). It was then poured into 5% cooled sodium carbonate solution
(50 ml) and extracted with chloroform (2 × 50 ml). The organic extracts were dried

2234
A.-A. S. EL-AHL ET AL.
over anhydrous magnesium sulfate, filtered, and concentrated to give the corresponding
substituted tetrazoles (cf. Tables 1 and 2).
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