An efficient synthesis of 1-substituted 5-bromo-1 H -tetrazoles

Article abstract of DOI:10.1055/s-0033-1338935

1-Substituted 1H-tetrazole-5-thiols were efficiently converted into the corresponding 1-substituted 5-bromo-1H-tetrazoles by treatment with zinc(II) bromide and 50% hydrogen peroxide or 36% peracetic acid at 70-80 °C. In most cases, the 5-bromotetrazole p

Full text of DOI:10.1055/s-0033-1338935

PAPER
▌2029
paper
An Efficient Synthesis of 1-Substituted 5-Bromo-1H-tetrazoles
1-Substituted 5-Bromo-1H-tetrazoles
Leonid Myznikov,*a Ulyana Dmitrieva,a Tatiana Artamonova,a Jaroslav Roh,b Alexandr Hrabalek,b Yuri Zevatskiia
a
St. Petersburg State University of Technology and Design, ul. Bolshaja Morskaja 18, 191183 Saint Petersburg, Russian Federation
E-mail: Myznikov_lv@mail.ru
b
Charles University in Prague, Faculty of Pharmacy in Hradec Kralove, 50005 Hradec Kralove, Czech Republic
Received: 01.03.2013; Accepted after revision: 16.04.2013
We have developed a simple method for the preparation
Abstract: 1-Substituted 1H-tetrazole-5-thiols were efficiently con-
of 1-substituted 5-bromo-1H-tetrazoles through the bro-
verted into the corresponding 1-substituted 5-bromo-1H-tetrazoles
mination of the corresponding tetrazole-5-thiols with
zinc(II) bromide in the presence of an oxidizing agent
by treatment with zinc(II) bromide and 50% hydrogen peroxide or
36% peracetic acid at 70–80 °C. In most cases, the 5-bromotetrazole
products could be isolated simply by dilution of the reaction mix- such as hydrogen peroxide or peracetic acid (Scheme 1).
ture with water followed by filtration and washing of the precipitat-
ed product. Column chromatography was needed only in the case of
one tetrazole that was additionally brominated on its side chain un-
der the reaction conditions.
Br
SH
R
R
ZnBr2 (2 equiv)
N
N
H2O2 or AcOOH (6 equiv)
N
N
N
N
AcOH, 80 °C, 4 h
N
N
Key words: halogenation, bromine, heterocycles
1–7
8–14
Scheme 1 Synthesis of 1-substituted 5-bromo-1H-tetrazoles from 1-
substituted 1H-tetrazole-5-thiols by treatment with zinc bromide in
the presence of oxidizing agent
Tetrazoles are readily functionalized in the 5-position by
cross-coupling reactions or by nucleophilic substitution of
a halo substituent in the 5-position. 1-Substituted 5-halo-
tetrazoles can undergo Suzuki–Miyaura coupling reac-
tions,1,2 Negishi cross-coupling reactions,3 or palladium-
catalyzed amination reactions.4 The nucleophilic substitu-
tion of a halo substituent has been used in syntheses of
inhibitors of 11β-hydroxysteroid dehydrogenase5 and
syntheses of quinolinomorphinan-4-ol derivatives as d-
opioid receptor agonists.6 It has also been used in the prep-
aration of derivatives of 1-(2-pyrimidinyl)piperazine as
potential antianxiety, antidepressant, and antipsychotic
agents,7 and in the design of potent non-thiourea H3-re-
ceptor histamine antagonists.8
Our initial efforts were focused on studying the bromina-
tion of 1-phenyl-1H-tetrazole-5-thiol (1) in the presence
of various inorganic bromides (Table 1).
Table 1 Bromination of 1-Phenyl-1H-tetrazole-5-thiol (1) in the
Presence of Inorganic Bromides and 50% Hydrogen Peroxide (6
equiv) in Acetic Acid at 80 °C
Entry
Bromide
Amount (equiv) Yield (%)
1
2
3
4
HBr
4
4
2
2
31
42
65
94
KBr
Various routes have been described for the synthesis of 1-
substituted 5-halotetrazoles, including the reaction of 5-
lithiotetrazoles with bromine or iodine;9 the direct bromi-
nation of 5H-tetrazoles with dibromine,10 N-bromosuccin-
imide,1 or iodine in a presence of oxidizing agent;11 or
mercuration and subsequent decomposition of the organo-
mercury intermediate with iodine.12 1,3-Dipolar cycload-
dition reactions of phenylcarbonimidic dichloride with
sodium azide or of phenyl isocyanide with iodine azide
have been used to prepare the corresponding 5-chloro-13
and 5-iodo-1-phenyl-1H-tetrazole, respectively.14 Among
the limitations of these methods are low yields, the need
for toxic reagents, the poor availability of starting materi-
als, and the need to purify the products by column chro-
matography.
MgBr2
ZnBr2
As shown in Table 1, 5-bromo-1-phenyl-1H-tetrazole (8)
was obtained in moderate yields in the presence of hydro-
gen bromide, potassium bromide, or magnesium(II) bro-
mide. In all these cases, large amounts of 1-phenyl-5H-
tetrazole were present in the reaction mixture. The reac-
tion with zinc(II) bromide was the most effective and it
gave bromotetrazole 8 in 94% yield when a mixture of thi-
ol 1, zinc(II) bromide, and 50% hydrogen peroxide in ace-
tic acid was heated at 80 °C for four hours. Under these
conditions, complete conversion of 1 occurred.
Next, we investigated the bromination of 1-phenyl-1H-
tetrazole-5-thiol (1) and 1-(4-tolyl)-1H-tetrazole-5-thiol
(2) in the presence of zinc(II) bromide and various oxidi-
zing agents (Table 2).
As mentioned above, the tetrazole moiety is present in
many drug candidates, and a new and efficient route to 1-
substituted 5-halotetrazoles would therefore be of value.
Hydrogen peroxide and peracetic acid were the best re-
agents, and a higher concentration of hydrogen peroxide
led to an increased yield of the 1-substituted 5-bromo-1H-
SYNTHESIS 2013, 45, 2029–2033
Advanced online publication: 15.05.2013
DOI: 10.1055/s-0033-1338935; Art ID: SS-2013-Z0173-OP
© Georg Thieme Verlag Stuttgart · New York
0
0
3
9
-
7
8
8
1
1
4
3
7
-
2
1
0
X

2030
L. Myznikov et al.
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tion.16 Our original thought was that the first step in the re-
action of 1-substituted 1H-tetrazole-5-thiols with
hydrogen peroxide or peracetic acid results in the forma-
tion of 1-substituted 5H-tetrazoles and that these then re-
act with the zinc(II) bromide and oxidant system to form
the corresponding 1-substituted 5-bromo-1H-tetrazoles.
However, no complete conversion was observed during
bromination of 1-phenyl-5H-tetrazole by the zinc(II) bro-
mide and oxidant system. On the basis of these facts, we
suggest an alternative mechanism for the bromination re-
action (Scheme 2).
Table 2 Bromination of 1-Phenyl-1H-tetrazole-5-thiol (1) or 1-(4-
Tolyl)-1H-tetrazole-5-thiol (2) in the Presence of Zinc(II) Bromide (2
equiv) and Various Oxidizing Agents (6 equiv) in Acetic Acid at
80 °C
Entry
Substrate Oxidant
Time (h) Yield (%)
1
2
3
4
5
6
7
1
1
1
1
1
2
2
H2O2 (30%)
9
4
5
6
6
5
5
81
94
89
77
58
68
74
H2O2 (50%)
AcOOH (36%)
MCPBA
In this mechanism, a dimeric zinc–thiolate complex is
formed in the first step, and this undergoes two successive
oxidations of the sulfur atoms. Finally, the resulting inter-
mediate rearranges to give the 5-bromotetrazole. Because
the corresponding 1-substituted 5H-tetrazole is always
present in the reaction mixture, an excess of zinc bromide
and hydrogen peroxide is needed to complete the conver-
sion into the corresponding 5-bromotetrazole.
urea·H2O2
H2O2 (50%)
AcOOH (36%)
tetrazole. 3-Chloroperoxybenzoic acid gave only a mod-
erate yield of product 8. The lowest yield was obtained
with urea–hydrogen peroxide, as a result of basification of
the reaction mixture. In this case, the product was ob-
tained only after acidification of the basified mixture.
Having identified the optimal conditions for the bromina-
tion reaction, we then began to investigate its scope.
Note that this method for preparing 1-substituted 5-bro-
mo-1H-tetrazoles preparation is regiospecific, and that
our attempts to obtain bromine-substituted 4,6-dimethyl-
pyrimidin-2-thiols and benzo[d]oxazole-2-thiols failed.
In summary, we have described a simple and convenient
method for the direct preparation of 1-substituted 5-bro-
mo-1H-tetrazoles from the corresponding tetrazole-5-thi-
ols. Among the advantages of our system over other
methods for the preparation of 1-substituted 5-bromo-1H-
tetrazoles are its relatively high yields, the ready availabil-
ity of the starting materials, the simple procedures needed
for isolation of the products, and the elimination of the
need to use toxic reagents or harsh reaction conditions.
The results of bromination reactions of various 1-substi-
tuted 1H-tetrazole-5-thiols are summarized in Table 3.
Most of the products were isolated in a pure form by dilu-
tion of the reaction with water and subsequent filtration
(method A). However, the presence of an oxidant and free
bromine in the reaction mixture facilitated bromination of
side chains in one instance. As a result, tetrazole 6 gave
the byproduct 13b together with the desired 5-bromotetra-
zole 13a, and these products were isolated by column
chromatography on silica (method B).
All the reaction mixtures were magnetically stirred and the reac-
The lower yield from tetrazole 7 (entry 7) is the result of tions were monitored by TLC on aluminum-backed SiO2-coated
sheets; compounds were visualized by UV irradiation (254 nm).
Column chromatography was performed on silica gel (60–120
mesh) using EtOAc and hexane. Compounds 2–7 were prepared by
decomposition of the substrate and the product under the
conditions of the reaction; however, even in this case, the
pure product could be isolated without resorting to col-
umn chromatography.
using a known procedure.17 All other chemicals were obtained from
Sigma-Aldrich and used without further purification. IR spectra
were recorded on a Nicolet Impact 400 spectrophotometer or a Ni-
colet 6700 (ATR mode). 1H and 13C NMR spectra were recorded on
a Varian Mercury Vx BB 300 spectrometer; chemical shifts are in-
directly referenced to TMS through the solvent signal. Elemental
analyses were carried out by using an Automatic Microanalyser
EA1110CE (Fisons Instruments SpA, Milan). ESI MS experiments
were performed by using an LCQ Advantage Max spectrometer
During the reaction, thin-layer chromatography always
showed the formation of a significant amount of the cor-
responding 1-substituted 5H-tetrazole. It is known that the
reaction of heterocyclic thiols with hydrogen peroxide can
lead to desulfurization15 and that treatment with a bromide
salt in the presence of an oxidant can be used for bromina-
O
S
O
S
Br
Zn
Br
Br
O
R
O
R
N
R
R
Zn
Br
S
S
N
R
SH
N
N
H2O2
N
N
ZnBr2
+
N
N
N
N
N
N
N
N
N
N
N
N
N
N
HO3SO
Zn
OSO3H
R
Br
R
N
Br
R
Br
H2O2
N
N
N
N
2
+
Zn(HSO4)2
N
N
N
N
N
N
N
Scheme 2 Suggested mechanism for the formation of 1-substituted 5-bromo-1H-tetrazoles
Synthesis 2013, 45, 2029–2033
© Georg Thieme Verlag Stuttgart · New York

PAPER
1-Substituted 5-Bromo-1H-tetrazoles
2031
Table 3 Synthesis of Various 1-Substituted 5-Bromo-1H-tetrazoles in the Presence of 36% Peracetic Acid
Entry
1
Substrate
Isolation method
Product
Yielda (%)
SH
Br
1
Ab
8
89
74
N
N
N
N
N
N
N
N
SH
Br
2
3
2
3
A
A
9
N
N
N
N
N
N
N
N
F
F
Br
SH
10
65
N
N
N
N
N
N
N
N
Cl
Cl
Br
SH
4
5
4
5
A
A
11
12
67
65
N
N
N
N
N
N
N
N
Cl
Cl
SH
Br
N
N
N
N
N
N
N
N
MeO
Br
N
MeO
N
N
N
13a
13b
38
51
Bc
SH
6
6
N
Br
MeO
N
N
N
Br
N
N
N
N
SH
Br
N
N
7
7
A
14
41
N
N
N
N
N
N
a Isolated yield.
b Dilution with H2O and filtration.
c Column chromatography (silica gel).
(Thermo Finnigan, San Jose, CA) equipped with an ESI source.
Mass spectra of the samples were obtained by direct infusion into
the detector of a soln of the sample in 50:50:0.1 MeOH–H2O–
HCO2H. APCI mass spectra was obtained by using an Agilent 500
Ion Trap LC/MS (Agilent Technologies, Santa Clara, CA) with di-
rect infusion into the detector of a soln of the sample in MeOH.
1-Substituted 5-Bromo-1H-tetrazoles; General Procedure
The 1-substituted 1H-tetrazole-5-thiol (8.4 mmol) and ZnBr2 (16.8
mmol) were dissolved in AcOH (12 mL) at 40 °C. 36% AcOOH
(50.4 mmol) or a 50% soln of H2O2 (50.4 mmol) in AcOH (7 mL)
was added dropwise. When the addition was complete, the mixture
was stirred at 80 °C for 5 h and then worked up by method A or B.
© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 2029–2033

2032
L. Myznikov et al.
PAPER
Method A: When the starting material had been consumed (TLC),
the mixture was poured into H2O (75 mL) and the precipitated prod-
uct was collected by filtration, washed with H2O (3 × 15 mL), and
dried.
MS (ESI): m/z (%) = 272.99 (81), 274.98 (100), 276.91 (25) [M +
H]+.
Anal. Calcd for C8H8BrClN4: C, 35.13; H, 2.21; N, 20.48. Found:
C, 34.95; H, 2.40; N, 20.31.
Method B: When the starting material had been consumed (TLC),
the mixture was poured into H2O (75 mL). The precipitated product
was extracted with EtOAc (3 × 10 mL) and the extracts were com-
bined then concentrated under reduced pressure. The resulting
crude products were purified by column chromatography (silica gel,
EtOAc–hexane).
5-Bromo-1-(2-chlorophenyl)-1H-tetrazole (12)
Colorless solid; yield: 1.42 g (65%); mp 123 °C (i-PrOH).
IR (KBr): 3097, 1590, 1491, 1447, 1416, 1399, 1390, 1291, 1262,
1234, 1166, 1140, 1106, 1089, 1036, 1012, 972, 869, 762, 733, 712,
659 cm–1.
1H NMR (300 MHz, acetone-d6): δ = 7.69–7.90 (m, 4 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 136.43, 134.41, 132.05,
5-Bromo-1-phenyl-1H-tetrazole (8)
Colorless solid; yield: 1.67 g (89%); mp 151–152 °C (i-PrOH)
(Lit.18 150–152 °C).
131.63, 130.53, 129.68 (one carbon atom resonance was not detected).
IR (KBr): 3063, 1593, 1499, 1460, 1431, 1423, 1416, 1407, 1387,
1319, 1297, 1265, 1230, 1112, 1073, 1036, 1012, 973, 764, 694,
688 cm–1.
1H NMR (300 MHz, acetone-d6): δ = 7.69–7.77 (m, 5 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 134.7, 134.4, 131.8, 130.7,
126.5.
MS (ESI): m/z (%) = 258.95 (81), 260.96 (100), 262.96 (22) [M +
H]+.
Anal. Calcd for C7H4BrClN4: C, 32.40; H, 1.55; N, 21.59. Found:
C, 32.53; H, 1.72; N, 21.40.
5-Bromo-1-(4-methoxyphenyl)-1H-tetrazole (13a)
Colorless solid; yield: 0.81 g (38%); mp 109–110 °C (EtOH–H2O)
(Lit.1 110–111 °C).
MS (ESI): m/z (%) = 225.02 (100), 227.02 (94) [M + H]+.
Anal. Calcd for C7H5BrN4: C, 37.36; H, 2.24; N, 24.90. Found: C,
37.30; H, 2.32; N, 24.81.
IR (KBr): 3086, 2969, 2841, 1609, 1591, 1464, 1453, 1441, 1413,
1388, 1318, 1309, 1271, 1232, 1188, 1175, 1121, 1106, 1067, 1040,
1027, 1003, 980, 833, 803, 713, 696 cm–1.
1H NMR (300 MHz, acetone-d6): δ = 3.93 (s, 3 H, CH3), 7.19–7.35
(m, 2 H, Ar), 7.63–7.66 (m, 2 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 162.36, 135.05, 128.08,
5-Bromo-1-(4-tolyl)-1H-tetrazole (9)
Colorless solid; yield: 1.48 g (74%); mp 127–128 °C (EtOAc–hex-
ane).
IR (KBr): 3041, 1541, 1514, 1434, 1390, 1268, 1231, 1176, 1110,
1104, 1058, 1037, 1012, 976, 944, 821, 800, 711, 698 cm–1.
126.90, 115.67, 55.14.
1H NMR (300 MHz, acetone-d6): δ = 2.47 (s, 3 H, CH3), 7.48–7.63
(m, 2 H, Ar), 7.58–7.63 (m, 2 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 142.3, 134.7, 131.9, 131.1,
Anal. Calcd for C8H7BrN4O: C, 37.67; H, 2.77; N, 21.97. Found: C,
37.83; H, 2.93; N, 21.84.
MS (ESI): m/z (%) = 255.00 (100), 256.98 (96), [M + H]+.
126.3, 21.2.
5-Bromo-1-(3-bromo-4-methoxyphenyl)-1H-tetrazole (13b)
MS (ESI): m/z (%) = 211.16 (17), 213.15 (18) [M – N2 + H]+, 238.99
Colorless solid; yield: 1.43 g (51%); mp 125 °C (EtOH–H2O).
(100), 240.96 (97) [M + H]+.
IR (KBr): 3116, 3012, 2968, 1599, 1579, 1547, 1504, 1460, 1450,
1417, 1391, 1382, 1295, 1277, 1265, 1223, 1183, 1149, 1106, 1060,
1052, 1036, 1012, 970, 823, 712, 699, 673 cm–1.
Anal. Calcd for C8H7BrN4: C, 40.19; H, 2.95; N, 23.44. Found: C,
40.24; H, 3.12; N, 23.45.
1H NMR (300 MHz, acetone-d6): δ = 4.04 (s, 3 H, CH3), 7.37–7.40
(s, 1 H, Ar), 7.73–7.77 (m, 1 H, Ar), 7.99–8.00 (s, 1 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 158.82, 135.20, 131.35,
5-Bromo-1-(4-fluorophenyl)-1H-tetrazole (10)
Light-pink solid; yield: 1.33 g (65%); mp 114–115 °C (EtOH–
H2O).
127.57, 127.35, 113.47, 112.15, 57.25.
IR (KBr): 3079, 1598, 1510, 1488, 1417, 1396, 1296, 1275, 1235,
1221, 1158, 1108, 1096, 1055, 1030, 1010, 975, 840, 821, 710, 692
cm–1.
1H NMR (300 MHz, acetone-d6): δ = 7.47 (m, 2 H, Ar), 7.81–7.85
MS (APCI): m/z (%) = 224.5 (98), 226.4 (100) [M – N2 – Br + H]+,
335.2 (23) [M + H]+.
Anal. Calcd for C8H6Br2N4O: C, 28.77; H, 1.81; N, 16.78. Found:
C, 28.87; H, 1.97; N, 16.74.
(m, 2 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 164.54 (d, J = 149 Hz), 134.99,
130.62 (d, J = 2 Hz), 129.17 (d, J = 6 Hz), 117.65 (d, J = 14 Hz).
MS (ESI): m/z (%) = 242.99 (100), 244.98 (92) [M + H]+.
1-Benzyl-5-bromo-1H-tetrazole (14)
Beige solid; yield: 0.82 g (41%); mp 48 °C (hexane).
IR (KBr): 3031, 3006, 2965, 1587, 1493, 1460, 1453, 1424, 1384,
1375, 1343, 1325, 1277, 1229, 1204, 1172, 1161, 1107, 1099, 1076,
1050, 984, 915, 824, 708, 691, 681, 580 cm–1.
Anal. Calcd for C7H4BrFN4: C, 34.59; H, 1.66; N, 23.05. Found: C,
34.45; H, 1.85; N, 23.22.
1H NMR (300 MHz, DMSO-d6): δ = 5.71 (s, 2 H, CH2), 7.28–7.41
(m, 5 H, Ph)
13C NMR (75 MHz, DMSO-d6): δ = 134.91, 134.12, 129.44, 129.07,
128.34, 51.68.
5-Bromo-1-(5-chloro-2-methylphenyl)-1H-tetrazole (11)
Colorless solid; yield: 1.54 g (67%); mp 124–125 °C (i-PrOH).
IR (KBr): 3068, 2930, 1602, 1574, 1494, 1474, 1447, 1423, 1408,
1390, 1384, 1284, 1263, 1224, 1204, 1143, 1101, 1029, 993, 970,
881, 825, 720, 709, 691 cm–1.
1H NMR (300 MHz, acetone-d6): δ = 2.09 (s, 3 H, CH3), 7.60–7.70
MS (ESI): m/z (%) = 239.01 (100), 241.01 (79), [M + H]+.
Anal. Calcd for C8H7BrN4: C, 40.19; H, 2.95; N, 23.44. Found: C,
40.01; H, 3.10; N, 23.35.
(m, 3 H, Ar).
13C NMR (75 MHz, acetone-d6): δ = 135.91, 135.48, 134.11,
133.91, 132.84, 132.56, 128.34, 16.77.
Supporting Information for this article is available online at
http://www.thieme-connect.com/ejournals/toc/synthesis.SnoIufproi
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Synthesis 2013, 45, 2029–2033
© Georg Thieme Verlag Stuttgart · New York

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1-Substituted 5-Bromo-1H-tetrazoles
2033
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© Georg Thieme Verlag Stuttgart · New York
Synthesis 2013, 45, 2029–2033