Synthesis and characterization of eight arylpentazoles

Article abstract of DOI:10.1002/jhet.988

p-Nitrophenyl-, p-methoxyphenyl-, p-hydroxyphenyl-, p-t-butylphenyl-, p-HOSO₂-phenyl-, ¹⁵N-p-N,N-dimethylaminophenyl-, ¹⁵N₂-p-N,N-dimethylaminophenyl-, and dicyanoimidazopentazole were obtained via different synthetic routes. Cesium, barium, potassium, and sodium salts of the arylpentazoles bearing acidic hydrogens were prepared. NMR spectra (¹H, ¹³C) are reported for the arylpentazoles, their corresponding arylazides, and their salts.

Full text of DOI:10.1002/jhet.988

Month 2013
Synthesis and Characterization of Eight Arylpentazoles
*
Stefan Ek, Stanley Rehn, Larisa Yudina Wahlström, and Henric Östmark
Department of Energetic Materials, Swedish Defence Research Agency, FOI, Grindsjön Research
Center, Stockholm SE‐172 90, Sweden
*
E-mail: stefan.ek@foi.se
Received February 24, 2011
DOI 10.1002/jhet.988
Published online 00 Month 2013 in Wiley Online Library (wileyonlinelibrary.com).
p‐Nitrophenyl‐, p‐methoxyphenyl‐, p‐hydroxyphenyl‐, p‐t‐butylphenyl‐, p‐HOSO2‐phenyl‐, 15N‐p‐N,N‐
dimethylaminophenyl‐, 15N2‐p‐N,N‐dimethylaminophenyl‐, and dicyanoimidazopentazole were obtained
via different synthetic routes. Cesium, barium, potassium, and sodium salts of the arylpentazoles bearing
acidic hydrogens were prepared. NMR spectra (1H, 13C) are reported for the arylpentazoles, their corre-
sponding arylazides, and their salts.
J. Heterocyclic Chem., 50, 00 (2013).
INTRODUCTION
available. The scope of this article is to describe the synthetic
procedure of some arylpentazoles together with the isolation
of solid arylpentazoles.
In recent years, there has been an increased interest in
polynitrogen compounds, mainly due to their use of them
as candidates for high‐energy density materials (HEDM),
whose crucial characteristic from an energetic point of
view is the ratio between the energy released in a fragmen-
tation reaction and the specific weight. In such a reaction,
only dinitrogen forms. This renders nitrogen clusters prime
candidates as environmentally benign HEDMs. Some
years ago, the novel v‐shaped N+5 ion was synthesized
[1], and since then, several N+5 salts have been reported
[2,3]. This breakthrough in polynitrogen chemistry
prompted much effort in isolating N−5 from arylpentazoles.
Calculations have shown that the cyclo‐N−5 is stable toward
fragmentation to N−3 and N2 [4,5]. Beside the interest as an
HEDM, pentazole is the last constituent of the azole series
still remaining to be synthesized. Hitherto, the only way of
constructing the N−5 ring moiety is in the form of arylpenta-
zoles. Arylpentazoles [6–8] were first synthesized and
studied by Huisgen and Ugi in the late 1950s and early
1960s [9–13].
Some years ago, the pentazolate ion was detected experi-
mentally by two groups independently [14,15]. Later on, But-
ler et al. [16] claimed the synthesis of the novel pentazole ring
by oxidative cleavage of p‐methoxyphenylpentazole. How-
ever, Schroer et al. [17] indicated that the synthesis and the
conclusions were defective. Butler et al. [18] later published
a thorough reinvestigation of the reaction of the dearylation
of a multitude of heterocycles. However, only indirect proofs
for the presence of pentazole or the pentazolate ion were
obtained. These first two contradictory papers and the latter
with indirect proofs show that more research is required in
the area of pentazoles. To enable such research, arylpentazoles
must be synthesized. Despite the need of good procedures to
prepare different arylpentazoles, no general collection
of synthetic procedures to yield solid arylpentazoles is
RESULTS AND DISCUSSION
The only available route to arylpentazoles is a two‐step
reaction starting from an aryl amine 1, which is diazotized
to give a diazocompound 2, followed by addition of an
azide ion yielding both the arylpentazole 4 and the aryla-
zide 5 (Scheme 1). The only available route to arylpenta-
zoles is a two‐step reaction starting from an aryl amine 1,
which is diazotized to give a diazocompound 2, followed
by addition of an azide ion yielding both the arylpentazole
4 and the arylazide 5 (Scheme 1). According to Butler et al.
[19], the mechanism of the pentazole formation does not
proceed via a concerted [3+4] cycloaddition but through
a stepwise formation via the pentazenes 3, as shown in
Scheme 1. If the azide adds in a cis fashion, in relation to
the aryl ring, ring‐closure to the pentazole cannot occur.
The pentazene must first isomerize into a trans‐configuration
around the N‐N double bond originating from the diazo-
nium salt. The trans,s‐trans‐pentazene must also isomerize
into the trans,s‐cis‐pentazene to allow cyclization, which
could occur via rotation around the N—N single bond.
Scheme 1 shows all possible configurations. The relative en-
ergies and the theoretical distribution between the different
structures are currently being quantum mechanically
calculated. Despite Butler’s conclusions [19], a concerted
cycloaddition will be evaluated in these calculations. The
possibility of cis‐trans isomerizations is uncertain and
depends among other things on the conjugation in the
pentazene system. However, this has little bearing on the
results of the syntheses presented in this publication. N2
can be expelled from all pentazenes to provide the arylazide.
The prevalent problem encountered in the synthesis of
© 2013 HeteroCorporation

S. Ek, S. Rehn, L. Y. Wahlström, and H. Östmark
Vol 50
Table 1
Reaction conditions and yields of arylpentazole syntheses.
R
Diazotization conditions
NaN3 mixture
Yield (%)
a
b
c
d
e
OH
NaNO2/HCl (conc)
n.a.
Isoamyl nitrite/MeOH/HCl (conc)
NaNO2/HCl (conc)
H2O
n.a.
58
56a
10
8
OCs
OMe
t‐Bub
NO2
MeOH‐H2O/n‐heptane
MeOH‐H2O
Isoamyl nitrite/MeOH/HCl (conc)
H2O, Zn(NO3)2
aYield from 4a.
bThe diazonium salt was added to an azide solution.
arylpentazoles is the formation of the arylazide, which can
be cumbersome to remove from the pentazole.
explained by pentazene formation. If the temperature is
very low, the trans,s‐trans‐pentazene cannot isomerize
into the trans,s‐cis‐pentazene, which is required for the
ring‐closure to the arylpentazole, comparing Scheme 1.
Thus, raising the reaction temperature when the azide ion
is introduced into the reaction mixture should favor the
formation of trans,s‐cis‐pentazene. However, a higher
temperature will increase the thermal decomposition of
the arylpentazole and arylpentazenes into the arylazide.
These contradicting trends ought to be valid for all arylpen-
tazole syntheses. The reaction temperature must thus be
carefully chosen to obtain a maximum yield of the desired
product.
Different approaches to the synthesis of p‐hydroxyphe-
nylpentazole, 4a, were evaluated. The method described
by Benin et al. [20] turned out to be troublesome. The p‐
hydroxypentazole 4a appeared to be very soluble in the
reaction solvent, which led to low yields. In our hands, this
procedure also gave irreproducible results. On the contrary,
the procedure described by Vij et al. [14] worked well and
was used with minor modifications (i.e., diazotization in
an acidic aqueous solution at 0°C followed by addition of
the azide).
1
This yielded a greenish solid. H‐NMR in CD3CN and
CD3OD at −10°C of this product showed that it was pure p‐
hydroxyphenylpentazole 4a at 58% yield. Heating of the
NMR sample to ambient temperature resulted in expulsion of
nitrogen gas and formation of p‐hydroxyphenylazide 5a. Com-
parison of the UV‐spectra of the p‐hydroxypehenylpentazole
4a and the p‐hydroxyphenylazide 5a revealed some differ-
ences, where the pentazole 4a had a symmetrical peak with
maximum at 275 nm. The UV max of azide 5a was shifted
to a lower wavelength at 225 nm together with a small shoul-
der on the red side of the main peak.
Benin et al. [20] discussed temperature dependence in
the ratio between the formed arylazide and arylpentazole.
The ratio of arylpentazole 4a to arylazide 5a changed from
1:2.0 at −30°C to 1:1.2 at 0°C. This behavior could be
The cesium salt of p‐hydroxyphenylpentazole 4b was
prepared by treatment of the p‐hydroxyphenylentazole 4a
with a cold methanolic solution of cesium hydroxide (see
Scheme 2). A mixture of the p‐hydroxyphenylpentazole 4a
and the p‐hydroxyphenylazide 5a could easily be separated
in this manner, as the cesium p‐hydroxyphenylazide 5b
was removed by washing with cold acetone. The proton
NMR showed no OH signal and all other signals were
shifted compared to the p‐hydroxyphenylpentazole 4a.
It was also of interest to prepare arylpentazoles with elec-
tron‐rich aryl moieties, since electrondonating groups on
the aryl ring increase the stability of the arylpentazoles
[21]. When the procedure described for the synthesis of p‐
hydroxyphenylpentazole 4a in aqueous solution was applied
Scheme 1. Synthesis of arylpentazoles via arylpentazenes.
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Synthesis and Characterization of Eight Arylpentazoles
Scheme 2. Synthesis of arylpentazoles. Conditions and yields are given in Table 1.
to p‐anisidine 1c, a 1:0.95 mixture of p‐methoxyphenylpenta-
zole 4c and p‐methoxyphenylazide 5c was collected. The ratio
improved to 1:0.11, when the conditions described by
Butler et al. [18,22] (i.e., diazotization of p‐anisidine with
iso‐amylnitrite in methanol) were used. To the resulting
p‐methoxyphenyldiazonium chloride 2c, n‐heptane was added
to create a two‐phase system, which subsequently was treated
with sodium azide in a solution of methanol and water at –
40°C to yield the corresponding arylpentazole 4c in 10% yield.
Although p‐methoxyphenylpentazole 4c has been prepared
earlier, little analytical data were available [22]. Our in-
vestigations showed that this compound had its UV absorption
maximum at 270 nm in acetonitrile. Also the UV‐spectrum of
p‐methoxyphenylazide 5c exhibit a shoulder (280 and 300 nm)
on the red side of the main peak. The total maximum was at
256 nm. The Raman spectrum of the p‐methoxyphenylpentazole
4c and the p‐methoxyphenylazide 5c yielded spectrum of sim-
ilar kind. The main difference was a big peak at 1374 cm−1 for
the pentazole 4c but in the same region the azide 5c had a big
peak at 1303 cm−1.
p‐t‐Butylphenylpentazole 4d was obtained from p‐t‐
butylphenylamine 1d by diazotization with sodium nitrite,
followed by treatment with sodium azide. Only small
amounts of the pentazole 4d was collected and the azide
5d was the major product. The stability of p‐t‐butylphe-
nylpentazole 4d was significantly lower than the ones of
p‐methoxy‐ or p‐hydroxyphenylpentazoles. The reprodu-
bility of the yield of p‐t‐butylphenylpentazole 4d was
low, due to the high solubility of the product.
yielded a solid product which according to 1HNMR
contained 7% of the desired p‐nitrophenylpentazole 4e,
although the main product was still the p‐nitrophenylazide
5e. p‐Nitrophenylpentazole 4e was extremely unstable.
When the NMR sample was allowed to reach room temper-
ature, the peaks corresponding to the pentazole 4e disap-
peared quickly and the only signals present belonged to
the azide 5e.
The first attempt to synthesize the salts of p‐pentazolephe-
nylsulfonic acid 8 (M = H) only yielded the zwitter‐ionic di-
azonium salt of sulfanilic acid 7, which is remarkably stable
and did not react with the azide. One sample was dissolved
and heated in DMSO‐d6 to approximately 45–50°C.
1H‐NMR at 25°C of this sample showed a mixture of the
internal salt, the corresponding azide and a small amount of
sulfanilic acid, although the internal diazonium salt was
still the major component.
To obtain the p‐pentazolephenylsulfonic acid 8, the
reaction conditions had to be modified (Scheme 3). The
zwitter‐ionic diazonium salt of sulfanilic acid 7 was iso-
lated and purified. It was then subjected to a solution of so-
dium azide in anhydrous methanol. This protocol produced
a solution of the desired sodium salt of p‐pentazolephenyl-
sulfonic acid 8 together with the corresponding azide 9.
NMR samples, taken from reaction mixture and analyzed
in CD3OD at −20°C, showed no signals that correspond
to the diazonium salt 7. There were two sets of signals, be-
longing to Na+(OSO2PhN5) and Na+(OSO2PhN3) in a 1:4
ratio. Pentazole 8 and azide 9 were not isolated. Treatment
of the cold methanolic solution with Ba(OH)2 resulted in a
As electron‐donating substituents stabilize the pentazole
ring, the contrary is true to electron‐withdrawing substitu-
ents [21]. An attempt was made to synthesize p‐nitrophe-
nylpentazole 4e by the same procedure, as described
above for p‐methoxyphenylpentazole 4c. Only a very pure,
light yellowish powder of p‐nitrophenylazide 5e, without
any trace of corresponding arylpentazole, was obtained.
The next attempt was made with the same reaction condi-
tions, but the addition of NaN3 to the mixture was followed
by addition of Zn(NO3)2 to stabilize the product. This
1
precipitate, which was filtered off and analyzed. H‐NMR
(in CD3OD at −20°C) showed two sets of signals that cor-
responded to Ba2+(OSO2PhN5)2 10 and Ba2+(OSO2PhN3)2
11 in a 1:1.9 ratio. When KOH was added to the methano-
lic solution of pentazole 8 and azide 9, the corresponding
potassium salts 12 and 13 were observed with same ratio
as in the case with barium. The choice of counter ion did
not influence the NMR spectra. Different solvents led
to some shifts of NMR signals. Biesemeier et al. [23]
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S. Ek, S. Rehn, L. Y. Wahlström, and H. Östmark
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Scheme 3. Synthesis of the salts of p‐pentazolephenylsulfonic acid.
13C‐NMR in cold solutions and at ambient temperature. None
of the compounds 14, 15, and 16 in Scheme 4 had any signals
in 1H‐NMR, as there are no protons in the products.
synthesized the sodium and potassium salts of 8 in a
similar manner, but did not discuss the high stability/low
reactivity of 7.
Arylpentazoles with electron‐withdrawing substituents,
such as p‐nitrophenylpenbtazole 4e, are very unstable. None-
theless, arylpentazoles with such properties and, furthermore,
pentazoles soluble in water were prepared. The synthesis
was performed, starting from the commercial available 2‐
amino‐4,5‐imidazoledicarbonitrile 12 (Scheme 4). Anhy-
drous diazotization [24] yielded the isolable, diazonium
trifluoroacetate 13. Addition of sodium azide yielded so-
dium 2‐pentazolylimidazole‐4,5‐dicarbonitril 14 together
with the sodium 2‐azidoimidazole‐4,5‐dicarbonitrile 15. The
corresponding cesium 2‐pentazolylimidazole‐4,5‐dicarboni-
trile 16 was obtained by treating the solution with CsOH.
The identification of the arylpentazole and azide was possible
by analysis of the changes in the UV absorption pattern and
For the synthesis of the labeled compounds, diazotization
was performed with Na15NO2 to yield the labeled diazonium
salt 19, which in turned was used to synthesize either the
mono‐labeled pentazole 20 or the di‐labeled pentazole 21
(Scheme 5). The mono‐labeled pentazole 19 contained 28%
1
of the azide, according to H‐NMR. The di‐labeled p‐N,N‐
dimethylaminophenylpentazole was synthesized by the use
of labeled sodium azide in the cyclization step. The second
labeled nitrogen was incorporated either in the third position
(21b) or in the fifth position (21a) of the pentazole ring.
Scheme 5. Synthesis of mono‐ and di‐labeled p-N,N‐dimethylaminophenyl-
pentazole. Formation of the corresponding arylazides is omitted for clarity.
Scheme 4. Synthesis of salts of 3,4‐dicyanoimidazopentazole and 3,4‐
dicyano‐2‐azidoimidazole.
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Synthesis and Characterization of Eight Arylpentazoles
in vacuo at −20°C. The remaining brown solid was collected
CONCLUSIONS
and washed on a filter with cooling (−30°C) with cold, dry
acetone (3 × 10 mL, −40°C). The acetone solution contained
CsOPhN3 (2.24 g, 100%). The creamy residue was nearly pure
We have prepared, isolated, and characterized eight aryl-
pentazoles in their solid state. The choice of synthetic proce-
dure depended on the desired product. The factors varied to
obtain higher yields were the solvent, diazotization procedure,
and reaction temperature during the cyclization into the de-
sired arylpentazole. Furthermore, the preparation of 15N
mono‐ and di‐labeled p‐N,N‐dimethylaminophenylpentazole
was accomplished.
1
CsOPhN5 (1.41 g, 56%). H‐NMR (CD3OD, −10°C): 7.77 (2H,
d, J = 7.1), 6.73 (2H, d, J = 7.1); 13C‐NMR (CD3OD, −10°C):
153.40 (s), 142.64 (s), 123.30 (d), 120.60 (d). ); ms: m/z: 161.9
[OPhN5]− (100), 133.9 [M‐N2]− (35, 106.0 [M‐2N2]− (40).
Cesium salt of p‐hydroxyphenylazide (5b). 1H‐NMR
(CD3OD, −10°C): 6.70 (2H, d, J = 6.7), 6.61 (2H, d, J = 6.7);
13C‐NMR (CD3OD, +25°C): 165.04 (s), 126.62 (s) 120.60 (d),
120.42 (d).
p‐Methoxyphenylpentazole (4c). This compound was prepared
according to Butler et al. [18]. 1H‐NMR (CD3CN): 8.03 (2H, d, J
= 6.9), 7.17 (2H, d, J = 6.9), 3.87 (3H, s); 13C‐NMR (CD3CN):
162.46 (s), 133.61 (s), 123.60 (d), 115.64 (d), 55.80 (q).
p‐Methoxyphenylazide (5c). Tm 34–37°C; 1H‐NMR
(CD3CN, +25°C): 6.97 (2H, d, J = 6.8), 6.91 (2H, d, J = 6.8),
3.72 (3H, s); 13C‐NMR (CD3CN, +25°C): 158.18 (s), 133.19
(s), 121.04 (d), 116.20 (d).
p‐t‐Butylphenylpentazole (4d). p‐t‐Butylphenylamine
(3.73 g, 25 mmol) was suspended in water (18 mL), cooled to
0°C and treated with conc. HCl (5.18 mL, 62.5 mmol).
Sodium nitrite (1.90 g, 27.5 mmol) was added over 15 min.
The reaction mixture was kept for 40 min at −3 to −5°C
until everything had dissolved. A solution of sodium azide
(1.79 g, 27.5 mmol) in methanol (22 mL) and water (4 mL)
was cooled to −45°C and the prepared diazonium salt (vide
supra) was added carefully over 15 min. During the
addition, the temperature of reaction mixture was kept
between −40 and −50°C. After completion of the addition,
the reaction was stirred for another 10 min and then filtered
EXPERIMENTAL
General. The prepared pentazoles were stored under liquid
N2 to prevent any thermal decomposition. The samples were at
all times handled at low temperatures. The samples were for
instance weighed into precooled bottles and NMR and UV/vis
samples were prepared by dissolution of the substances in
cooled solvents. NMR and UV/vis experiments of arylpentazole
samples were run at –20°C, unless otherwise stated. The 1H‐
NMR and 13C‐NMR spectra were recorded at 400 and 100 MHz,
respectively, on a Bruker Avance 400 spectrometer. UV/vis
spectra were recorded on a PerkinElmer Lambda 40. IR Raman
spectra were recorded on a Bruker IFS 50/FRA106. Melting
points were determined on a Mettler DSC. The low thermal
stability of the arylpentazoles prevented any elemental analysis.
p‐Hydroxyphenylpentazole (4a). This compound was
prepared according to the published procedure [14] with minor
modifications. p‐Hydroxyphenylamine hydrochloride (2.62 g, 18
mmol) was dissolved in water (45 mL), cooled in an ice‐salt bath
to −3°C, slightly acidified with conc. HCl (3.07 mL, 36 mmol).
Sodium nitrite (1.35 g, 19.5 mmol) was added to the solution over
7–8 min, and the solution was stirred for another 7–8 min. An ice‐
cold aqueous (7 mL) solution of NaN3 (1.22 g, 18.75 mmol) was
added to the produced p‐hydroxyphenyldiazonium chloride. The
formed suspension was stirred at 0–2°C for another 10–12 min
before it was quickly filtered, washed with ice‐cold water (2 × 5
mL), and dried in vacuum at –10 to 0°C to produce a green–grey
solid (1.70 g, 58%).
through
a precooled (−30°C) filter. The obtained white
precipitate was not washed to avoid loss of material. Drying
on the filter yielded 0.92 g of a crude product as a 1:1.3
mixture of p‐t‐butylphenylpentazole 0.43 g (8%) and p‐t‐
butylphenylazide (0.49 g, 11%). One extraction of the
mother liquor with ether yielded additionally 2.12 g (48%)
p‐t‐butylphenylazide as a pale yellow oil.
p‐t‐Bu‐phenylpentazole (4d). 1H‐NMR (CD2Cl2): 8.08
1
(2H, d, J = 8.2), 7.67 (2H, d, J = 8.2), 1.38 (9H, s); H‐NMR
p‐Hydroxyphenylpentazole (4a). Tdec 53–58°C, 1H‐NMR
(CD3OD): 7.99 (2H, d, J = 8.0), 7.03 (2H, d, J = 8.0); H‐NMR
(D2O): 7.69 (2H, d, J = 8.9), 6.77 (2H, d, J = 8.9); H‐NMR
(CD3CN); 8.05 (2H, d, J = 8.2), 7.72 (2H, d, J = 8.2), 1.34
(9H, s); 13C‐NMR (CD2Cl2): 120.55 (d), 118.55 (d), 30.99 (q);
13C‐NMR (CD3CN): 169.82 (s), 139.80 (s), 128.00 (d), 121.39
(d), 35.80 (s), 30.92 (q).
1
1
(CD3CN) 9.11 (1H, s, OH), 7.96 (2H, d, J = 8.1); 7.08 (2H, d,
J = 8.1); 13C‐NMR (CD3CN) 160.77 (q), 123.86 (d), 119.97
(q), 117.25 (d).
p‐t‐Bu‐phenylazide (5d). 1H‐NMR (CD2Cl2, +25°C):
7.41 (2H, d), 6.99 (2H, d), 1.33 (9H, s); 1H‐NMR (CD3CN,
+25°C): 7.40 (2H, d, J = 8.0), 6.98 (2H, d, J = 8.0), 1.25
(9H, s); 13C‐NMR (CD2Cl2, +25°C): 148.75 (s), 137.64 (s),
127.29 (d), 119.08 (d), 34.68 (s), 31.62 (q); 13C‐NMR
(CD3CN, +25°C): 149.15 (s), 138.14 (s), 127.75 (d), 119.59
(d), 35.08 (s), 31.56 (q).
p‐Hydroxyphenylazide (5a). 1H‐NMR (CD3OD, +25°C)
6.88 (2H, d, J = 6.7), 6.80 (2H, d, J = 6.7). 1H‐NMR (D2O,
1
+25°C) 6.66 (2H, d, J = 8.8), 6.55 (2H, d, J = 8.8); H‐NMR
(DMSO‐d6, +25°C) 9.50 (1H, s, OH), 6.93 (2H, d, J = 8.0),
6.80 (2H, d, J = 8.0); 13C‐NMR (CD3OD, +25°C) 146.45 (s),
122.87 (s), 111.47 (d), 108.07 (d).
p‐Nitrophenylpentazole (4e). The diazotization was
performed as described above for p‐anisidine, vide supra, starting
from 15 mmol p‐nitroaniline. The addition of NaN3 (1 equiv.)
to the mixture was followed by addition of 3 equiv. of Zn(NO3)
Cesium salt of p‐hydroxyphenylpentazole (4b). This
compound was prepared according to the published procedure
[14] with minor modifications. A crude 1:1 mixture of p‐
hydroxyphenylpentazole (1.364 g, 8.4 mmol) and the
corresponding azide (1.129g, 8.4 mmol) (i.e., 2.493 g total) was
added to dry methanol (15 mL, −45°C). To this suspension,
CsOH·H2O (20 mmol) in dry methanol (15 mL −45°C), was
added slowly via syringe. The temperature was raised to −20°C
over 2 h. The obtained brown solution was concentrated
in a minimum amount of water. After 30 min of stirring, a
2
solid was filtered off, washed with methanol:water 1:1
mixture (2 × 5 mL), and dried (work up at –32°C) gave
1.82 g of a solid product. According to 1H‐NMR, the
product was a 1:0.07 mixture of p‐nitrophenylazide and
p‐nitrophenylpentazole.
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S. Ek, S. Rehn, L. Y. Wahlström, and H. Östmark
Vol 50
p‐Nitrophenylpentazole (4e). 1H‐NMR (CD3CN): 8.51
(2H, d, J = 7.8), 8.39 (2H, d, J = 7.8).
to produce a light suspension. The reaction mixture was
additionally cooled to −70°C before the addition of cold (−70°C)
dry ether (30 mL). The suspension was filtered on a filter with a
cooling jacket (−28°C). The solid diazonium trifluoroacetate 13
was washed with cold dry ether (5 mL), slightly dried, and
immediately transferred into a three‐neck flask with cold dry
methanol (20 mL). WARNING: This must be done with utmost
care, as the diazonium salt was very sensitive to mechanical
stimuli and released much energy, when investigated by DSC. Its
melting point was 148°C.
p‐Nitrophenylazide (5e). 1H‐NMR: (CD3CN, +25°C) 8.22
(2H, d, J = 7.2), 7.22 (2H, d, J = 7.2); 13C‐NMR (CD3CN,
+25°C: 148.14 (s), 142.15 (s), 126.50 (d), 120.71 (d).
p‐Nitrophenyldiazonium salt (2e). 1H‐NMR (CD3CN,
+25°C): 8.28 (2H, d, J = 8.8), 7.65 (2H, d, J = 8.8).
p‐Pentazolphenylsulfonates (8, 10, 12). To a suspension of
sulfanilic acid (2.60 g, 15.0 mmol) in water (15 mL), aqueous
solution of Na2CO3 (8 mL H2O, 0.88 g NaHCO3, 8.3 mmol)
was added dropwise. After 10 min, when everything had
dissolved, the solution was cooled down to 15°C and NaNO2
(1.14 g, 16.5 mmol) was added over 10 min. After 10 min of
stirring at the same temperature, the reaction mixture was
poured onto crushed ice (17.0 g), containing conc. hydrochloric
acid (3.6 mL, 43.5 mmol). The product precipitated and formed
a suspension. After 15 min of stirring the resulting cold (0°C),
thick, white suspension of p‐sulfobenzenediazonium betaine 7
was quickly filtered, washed with ice‐cold water (2 × 5 mL),
and dried on a cold filter by letting air pass through it for 10
min. The betaine 7 was then suspended in precooled (−20°C)
dry methanol (35 mL). The suspension was cooled to −50°C
before dropwise addition of a saturated solution of NaN3 (975
mg, 15 mmol) in dry methanol (30 mL, −50°C) over 30 min.
Stirring continued for 4.5 h at maintained temperature, before a
light solution formed. A suspension of Ba(OH)2·H2O (789 mg,
2.5 mmol) in methanol (20 mL, −50°C) was added to the
solution over 5 min. After 2.5 hours of stirring at −50°C, the
formed suspension was filtered through a filter with cooling
jacket (−30°C) and washed with cold (−50°C) dry methanol (2
× 5 mL) and dried on the same filter at the same temperature
(−30°C) to obtain 1.24 g product as a white solid (30%
pentazole and 59% azide with respect to the Ba(OH)2). The use
of KOH (0.35 equiv.) provided identical results with almost
exactly the same ratio of K+(−OSO2PhN5) to K+(−OSO2PhN3).
p‐Sulfobenzenediazonium betaine (7). 1H‐NMR (CD3CN)
8.65 (2H, d, J = 8.9), 8.10 (2H, d, J = 8.9); 1H‐NMR
(DMSO‐d6) 8.66 (2H, d, J = 8.9), 8.10 (2H, d, J = 8.9);
13C‐NMR (DMSO‐d6) 144,10 (s), 142.30 (s), 133.32 (d), 127.95
The suspension was cooled to −70°C. Then a suspension
(maintained below −40°C) of NaN3 (1.43 g, 22 mmol) in metha-
nol (10 mL) was added slowly, and the reaction mixture was
stirred for 1.5 h. Temperature was increased to −45°C. Samples
were taken from the reaction mixture for UV and NMR analysis
at low temperatures. The solution was reduced to half its volume
in vacuo at low temperature to produce the sodium salt as a solid.
Sodium 3,4‐dicyanoimidazopentazolate (14). 13C‐NMR
(CD3OD): 152.02 (s), 118.43 (s), 115.06 (s).
Cesium salt of 3,4‐dicyano‐2‐azidoimidazole (15). 13C‐NMR
(CD3OD, +25°C): 152.50 (s), 118.82 (s), 115.49 (s).
The remaining methanolic solution, containing about 10 mmol,
was treated at −40°C with 4 mL of a methanolic solution of
CsOH·H2O. Forty minutes of stirring produced a suspension,
which was filtered, washed with cold methanol (1 mL), and dried
under vacuum at −30°C to produce the cesium salt of 3,4‐
dicyanoimidazopentazole.
Cesium 3,4‐dicyanoimidazopentazolate (16). 13C‐NMR
(CD3OD): 152.24 (s), 118.65 (s), 115.25 (s).
Cesium salt of 3,4‐dicyano‐2‐azidoimidazole (17). 13C‐NMR
(CD3OD, +25°C): 152.52 (s), 118.85 (s), 115.56 (s).
15N‐p‐N,N‐Dimethylaminophenylpentazole (20). The
diazotization was performed as described below for 21, vide infra.
A cold (0°C) solution of p‐N,N‐dimethylaminophenyldiazonium
dihydrochloride (15 mmol) in water (8 mL) was added in small
portions to a cold (−40°C) solution of sodium azide (1.04 g, 16
mmol) in water (3 mL) and methanol (14 mL) over 12 min. This
produced a thick green–grey suspension, which was stirred for
another 20 min at the same (−40°C) temperature, before it was
filtered off on a filter with a cooling jacket (−30°C). The mother
liquor was stirred for 15 min at −30°C, before being filtered again
in the same manner. The combined precipitates were washed with a
cold (−40 to −50°C, 2 × 7 mL) 1:1 mixture of water and methanol
and then two times with acetone (3 mL): water (4 mL) mixture, and
dried to get 15N‐p‐N,N‐dimethylaminophenylpentazole as a grey
powder that contained about 15% p‐N,N‐dimethylaminophenylazide.
The methanol from the mother liquor was evaporated and the
remaining p‐N,N‐dimethylaminophenylazide was extracted with
ethylacetate. The crude product was purified by sublimation at
100°C to produce bright yellow needles.
1
(d). H‐NMR (D2O) 8.73 (2H, d, J = 8.9), 8.31 (2H, d, J = 8.9).
1H‐NMR (CD3OD) 8.69 (2H, d, J = 8.0), 8.29 (2H, d, J = 8.0).
Sodium (8), barium (10), and potassium (12) p‐
pentazolephenylsulfonates. 1H‐NMR (CD3OD) 8.35 (2H, d,
J = 8.8), 8.15 (2H, d, J = 8.8); 13C‐NMR (CD3OD) 148.8 (s),
136.2 (s), 129.2 (d), 122.4 (d).
Sodium (9), barium (11) and potassium (13) salts of p‐
azidophenylsulfonate. 1H‐NMR (D2O) 7.78 (2H, d, J = 7.5),
7.19 (2H, J = 7.5); 13C‐NMR (D2O) 143.48 (s), 138.90 (s),
127.55 (d), 119.61 (d); 1H‐NMR (CD3CN) 7.61 (2H, d), 7.06
(2H, d); 13C‐NMR (CD3CN) 145.44 (s), 139.24 (s), 127.35 (d),
118.27 (d); H‐NMR (CD3OD) 7.84 (2H, d, J = 7.8), 7.17 (2H,
15N2‐p‐N,N‐Dimethylaminophenylpentazole (21). To
a
1
J = 7.8).
solution of p‐N,N‐dimethylphenylenediamine dihydrochloride
(1248 mg, 5.97 mmol) in water (8 mL), precooled to 0°C. HCl
(0.10 mL, 1.19 mmol) and solid Na15NO2 (460 mg, 6.57 mmol)
were added over 15 min, keeping the temperature of between
−2 and 0°C. After the addition was completed, the reaction
mixture was stirred at the same temperature for 30 min. During
the last 3 min of the reaction, air was bubbled through the
solution to remove any free nitrogen oxides. A mixture of
MeOH (8 mL) and n‐heptane (1 mL) was cooled to −35°C,
and the reaction mixture was transferred by syringe under N2
flow to the solvent mixture, keeping the temperature of the
p‐Hydroxyphenylsulfonic acid. 1H‐NMR (D2O) 7.65 (2H, d,
1
J = 7.9), 6.93 (2H, d, J = 7.9); H‐NMR (CD3OD) 7.66 (2H, d,
J = 8.6), 6.80 (2H, d, J = 8.6).
Sodium (14) and cesium salts (16) of 3,4‐dicyanoimidazopentazole.
2‐Amino‐4,5‐imidazoledicarbonitrile 12 (2.74 g, 20.0 mmol) was
dissolved in dry CH2Cl2 (15 mL) and CH3CN (8 mL). The
solution was cooled to 0°C and TFA (3.21 mL, 42 mmol) was
added dropwise. The solution was further cooled to −30°C before
the dropwise addition of iso‐amylnitrite (3.22 mL, 24 mmol). The
clear, yellow solution was stirred for 1 h at the same temperature
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

January 2013
Synthesis and Characterization of Eight Arylpentazoles
solution stable. A saturated solution of Na15NNN (434 mg, 6.57
mmol) in aqueous MeOH (8 mL, 50%) was added under intensive
stirring in such a rate the temperature remained between −30 and
−40°C. After 1 h of stirring, the reaction mixture was filtered (p4),
and the filter cake washed with cold MeOH (3 mL), acetone
(2 mL), and n‐heptane (1 mL). Then a small amount of crude product
(40 mg, mixture of 15N2‐p‐N,N‐dimethylaminophenylpentazole
[2] Vij, A.; Wilson, W. W.; Vij, V.; Tham, F. S.; Sheehy, J. A.;
Christe, K. O. J Am Chem Soc 2001, 123, 6308.
[3] Wilson, W. W.; Vij, A.; Vij, V.; Bernhardt, E.; Christe, K. O.
Chem Eur J 2003, 9, 2840.
[4] Fau, S.; Wilson, K. J.; Bartlett, R. J. J Phys Chem A 2004, 108,
236.
[5] Fau, S.; Wilson, K. J.; Bartlett, R. J. J Phys Chem A 2002, 106,
4639.
[6] Ugi, I. Advances in heterocyclic chemistry 1964, 3, 373.
[7] Ugi, I. Comprehensive Heterocyclic Chemistry; Pergamon
Press: Oxford, 1984; Vol.5.
[8] Butler, R. N. Comprehensive Heterocyclic Chemistry II, A.;
Pergamon Press: Oxford, 1996; Vol.4.
[9] Huisgen, R.; Ugi, I. Chem Ber 1957, 90, 2914.
[10] Ugi, I.; Huisgen, R. Chem Ber 1958, 91, 531.
[11] Ugi, I.; Perlinger, H.; Behringer, L. Chem Ber 1958, 91,
2324.
[12] Ugi, I.; Perlinger, H.; Behringer, L. Chem Ber 1959, 92,
1864.
[13] Ugi, I. Tetrahedron 1963, 19, 1801.
[14] Vij, A.; Pavlovich, J. G.; Wilson, W. W.; Vij, V.; Christe, K.
O. Angew Chem Int Ed 2002, 41, 3051.
[15] Östmark, H.; Wallin, S.; Brinck, T.; Carlqvist, P.; Claridge, R.;
Hedlund, E.; Yudina, L. Chem Phys Lett 2003, 379, 539.
[16] Butler, R. N.; Stephens, J. C.; Burke, L. A. Chem Commun
2003,1016.
[17] Schroer, T.; Haiges, R.; Schneider, S.; Christe, K. O. Chem
Commun 2005,1607.
[18] Butler, R. N.; Hanniffy, J. M.; Stephens, J. C.; Burke, L. A. J
Org Chem 2008, 73, 1354.
[19] Butler, R. N.; Fox, A.; Collier, S.; Burke, L. A. J Chem Soc
Perkin Trans2,1998,2243.
1
and p‐N,N‐dimethylaminophenylazide 1:0.5 according to H‐NMR)
was obtained. The cold filtrate was stirred for another 15 min
at −30°C and then filtered again. The obtained filter cake was
washed and dried as described above to get the product (65 mg,
mixture 1:0.18 15N2‐p‐N,N‐dimethylaminophenylpentazole and p‐N,
N‐dimethylaminophenylazide) as a grey solid. Then the filtrate
was left stirring for 2.5 h at −20 to −25°C. Additional product
precipitated and was filtered off. The same washing and drying
procedure as above yielded another 120 mg of a grey solid as
a 1:0.15 mixture of 15N2‐p‐N,N‐dimethylaminophenylpentazole and
p‐N,N‐dimethylaminophenylazide.
15N2‐p‐N,N‐Dimethylaminophenylpentazole. 1H‐NMR
(CD2Cl2) 7.96 (2H, d, J = 9.1 Hz), 6.83 (2H, d, J = 9.1 Hz), 3.10
(6H, s, Me); 13C‐NMR (CD2Cl2) 151.98 (s), 141.18 (s), 122.06
(d), 111.77 (d), 40.47 (q).
p‐N,N‐Dimethylaminophenylazide. 1H‐NMR (CD2Cl2) 6.92
(2H, d, J = 9.0 Hz), 6.77 (2H, d, J = 9.0 Hz), 2.91 (6H, s, Me);
1H‐NMR (CD3CN) 6.94 (2H, d, J = 9.0 Hz), 6.76 (2H, d,
J = 9.0 Hz), 2.96 (6H, s, Me); 13C‐NMR (CD3CN) 149.76 (s),
128.87 (s), 120.71 (d), 114.83 (d), 41.04 (q); 13C‐NMR
(CD2Cl2) 149.00 (s), 128.54 (s), 120.20 (d), 114.19 (d), 41.13 (q).
[20] Benin, V.; Kaszynski, P.; Radziszewski, J. G. J Org Chem
2002, 67, 1354.
[21] Carlqvist, P.; Östmark, H.; Brinck, T. J Phys Chem A 2004,
108, 7463.
Acknowledgments. The financial support from DSTA, Singapore,
and the Swedish Armed Forces is gratefully acknowledged.
[22] Butler, R. N.; Stephens, J. C.; Hanniffy, J. M. Tetrahedron Lett
2004, 45, 1977.
[23] Biesemaier, F.; Harms, K.; Müller, U. Z Anorg Allg Chem
2004, 630, 787.
REFERENCES AND NOTES
[1] Christie, K. O.; Wilson, W. W.; Sheehy, J. A.; Boatz, J. A.
Angew Chem Int Ed 1999, 38, 2004.
[24] Colas, C.; Goeldner, M. Eur J Org Chem 1999,1357.
Journal of Heterocyclic Chemistry
DOI 10.1002/jhet

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