Synthesis of branched polynuclear 1,3,4-oxadiazoles

Article abstract of DOI:10.1007/s10593-008-0163-9

The reaction of 5-substituted mono-and polytetrazoles with heterocyclic acid chlorides and trifluoroacetic acid anhydride gave branched polynuclear 1,3,4-oxadiazole systems.

Full text of DOI:10.1007/s10593-008-0163-9

Chemistry of Heterocyclic Compounds, Vol. 44, No. 9, 2008
SYNTHESIS OF BRANCHED
POLYNUCLEAR 1,3,4-OXADIAZOLES
L. I. Vereshchagin, O. N. Verkhozina, A. G. Proidakov,
A. I. Smirnov, and V. N. Kizhnyaev
The reaction of 5-substituted mono- and polytetrazoles with heterocyclic acid chlorides and
trifluoroacetic acid anhydride gave branched polynuclear 1,3,4-oxadiazole systems.
Keywords: trifluoroacetic acid anhydride, polynuclear 1,3,4-oxadiazoles, polynuclear tetrazoles,
heterocyclic acid anhydrides, thermolysis of tetrazoles.
In searches for novel photosensitive [1] and biologically active materials [2-4] a broad series of 1,3,4-oxa-
diazoles with different substituents on the carbon atoms have been reported. The usual basis for their synthesis is
mainly the dehydration of the corresponding acid hydrazides [5]. However, this reaction is done in relatively
severe conditions and does not permit preparation of polynuclear systems with different proportions of
oxadiazole rings and other heterocycles as substituents on the oxadiazoles. To a certain extent this restriction is
Ph
R
Ph
N
N
N
N
N
+
N
N
N
N
N
N
COOCl
R
N
O
N
N
H
3a-e
4a-e
1
N
N
COCl
R
O
N
N
N
N
3a-c
+
N
N
2
Ph
Ph
5a-c
3 - 5
3, 4
R=Me ,
a
c
b
R=
R=Ph
N
N
N
Ph
Ph
N
R=
e
d
R=
N
N
CH2
N
N
N
CH2
__________________________________________________________________________________________
Research Institute of Oil and Coal Chemistry and Synthesis, Irkutsk State University, Irkutsk 664003;
Russia, e-mail: kiznyaev@chem.isu.ru. Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 9, pp. 1432-
1437, September, 2008. Original article submitted July 20, 2007.
1158
0009-3122/08/4409-1158©2008 Springer Science+Business Media, Inc.

removed via thermolysis of tetrazoles in the presence of mono- and dicarboxylic acid chlorides [6, 7]. In this
report we discuss the possible synthesis of mono- and polynuclear branched 1,3,4-oxadiazoles including those
having 1,2,3-triazole and tetrazole components. With this in view the (5-phenyltetrazol-2-yl)acetic (1) and
(2-phenyl-1,2,3-triazol-4-yl)carboxylic acid chlorides (2) were used in the reaction with 5-substituted tetrazoles.
The reaction of these acid chlorides with tetrazoles 3a-e occurs quite rapidly at 70-85ºC to give the
corresponding tetrazole and triazole-substituted oxadiazoles 4, 5 in 60-70% yield.
The use of the heterocyclic acid chlorides makes possible the preparation of polynuclear systems with a
different combination of triazole, tetrazole, and oxadiazole rings.
N
N
N
N
or 2
PhCOCl, 1
N
R
X
N
X
O
H
n
n
6a,b
7a–c
, n = 2; b X =
C(COOEt)2
;
CNO2, n = 3
6, 7 a X =
;
CNO2, n = 3
7 a,b R = Ph, c R = phenyltriazol-5-yl, X =
The successful use of acid chlorides 1 and 2 in the reaction with the monotetrazoles 3a-e led us to expect
them to be no less active in reaction with the polytetrazoles 6a-c. However, not all of the polynuclear tetrazoles
and the acid chlorides 1, 2 and also benzoyl chloride form the corresponding branched oxadiazoles. We were
only able to separate and identify the products of reaction of compounds 6a and 6b with benzoyl chloride (7a
and 7b respectively) and also compound 6b with acid chloride 2 (compound 7c) in 35-46% yields.
The thermolysis reaction of tetrazoles 6a-c with acid chlorides 1 and 2 and benzoyl chloride at 90-100°C
led to formation of tarry products which markedly hindered or made impossible the separation of pure
compounds. Hence the diethyl bis[2-(tetrazol-5-yl)ethyl]malonate 6a and acid chlorides 1 and 2 gave total
tarring. The latter and the low yields of targeted products is evidently connected with their low stability in rather
rigid conditions of both the polytetrazoles 6a-c and the oxadiazoles 7a-c formed. For this reason it was not
possible to carry out a reaction of different acid chlorides with the polynuclear tetrazole 6c (where X is a
cyclohexane residue with unsubstituted atoms C-2 and C-6, n = 4).
On the other hand it was found that the polynuclear tetrazoles 6a-c react with trifluoroacetic acid
anhydride even at 10-15°C in low boiling solvents (methylene chloride, ether, benzene) without signs of tarring
to give the corresponding trifluoromethyl-substituted branched polyoxadiazoles 8a-c in high yields.
N
N
N
N
(F3CCO)2O
N
CF3
X
N
X
O
H
n
n
6a–c
8a–c
, n = 2; b X =
C(COOEt)2
;
CNO2, n = 3
6, 8 a X =
O
; n = 4
c X =
The composition and structure of the synthesized compounds 4-8 were confirmed by the results of
elemental analysis and 13C NMR spectroscopic data.
1159

The 13C NMR spectra of compounds 4, 5 show signals for the carbon atoms of the oxadiazole, tetrazole,
and triazole rings in the regions 154-170, 150-160, and 130-145 ppm respectively. The chemical shifts of the
carbon atoms signals for the phenyl substituents on different heterocycles are typical. Hence in the case of a
benzene ring at atom N-2 of a triazole ring they are found at 117-131 ppm. More specifically, the Co (double
intensity) chemical shift occurs at 117-118 ppm and Ci at 130 ppm. The carbon atom signals for the benzene
ring on atom C-5 of the tetrazole ring occur at 126-131 ppm and on atom C-2 of the oxadiazole ring at 122-132
ppm. In the latter case the high field shift of Ci (122 ppm) and low field shift of Cp (132 ppm) are typical. The
observed chemical shift features allow an unambiguous assignment of the signals in the 13C NMR spectra and to
establish or confirm the structures of the compounds obtained. The 13C NMR spectra of compounds 8a-c show
characteristic signals in the regions 160-173 and 155-159 ppm respectively for the atoms C-2 and C-5 of the
1,3,4-oxadiazole ring and also within the range 116-120 ppm assigned to the trifluoromethyl group. Additional
confirmation of the positioning of the CF3 group at position 5 of the oxadiazole ring is the equel splitting of the
heterocycle C-5 atom signal as a quartet with the intensity ratio of the lines 1:3:3:1. The 13C–19F spin-spin
splittings are 267-272 and 42-44 Hz respectively. Unfortunately a marked lowering of solubility was noted for
these compounds with an increasing number of azole rings in the polynuclear oxadiazole molecules. The
majority of the 1,3,4-oxadiazoles prepared have certain photoactivity as displayed by luminosity upon UV
irradiation.
EXPERIMENTAL
1H and 13C NMR spectra were recorded on a Varian VXR-500S spectrometer (500 and 126 MHz
respectively) using acetone-d6, acetone, DMSO, or HMPA. In the case of acetone-d6 the internal standard was
HMDS but for the other solvents acetone-d6 was added for "capture". Elemental analysis was performed on a
FLASH EA 1112 Series CHN-analyzer. The tetrazoles 3c-e were prepared by method [8] and the branched
polynuclear tetrazoles 6a-c as previously described [9, 10].
5-Phenyltetrazol-2-ylacetic Acid Chloride (1). A mixture of 5-phenyltetrazol-2-ylacetic acid (1.25 g,
6.1 mmol) and PCl5 (1.5 g, 7.4 mmol) in benzene (10 ml) was heated to 60ºC and held until hydrogen chloride
evolution had ceased and the starting acid had dissolved. After distillation of solvent and liquid products in
vacuo on a water pump the dry residue was recrystallized from hexane to give colorless needle-like crystals of
the acid chloride 1 (1.2 g, 88%) with mp 101°C (mp 103-104ºC [11]).
2-Phenyl-1,2,3-triazol-4-ylcarboxylic Acid Chloride (2). PCl5 (0.7 g, 3.2 mmol) was added in small
portions to 2-phenyl-1,2,3-triazol-4-ylcarboxylic acid (0.5 g, 2.6 mmol) in benzene (5 ml). The reaction product
was held at room temperature until evolution of hydrogen chloride had ceased and the acid had dissolved
completely. After distillation of solvent and liquid products the dry residue was recrystallized from hexane to
give the acid chloride 2 (0.51 g, 94%) with mp 107-108°C.
2-Methyl-5-(5-phenyltetrazol-2-ylmethyl)-1,3,4-oxadiazole (4a). A mixture of the acid chloride 1
(1.15 g, 5.1 mmol) and 5-methyltetrazole 3a (0.36 g, 4.3 mmol) in toluene (5 ml) was held at 85°C for 3 h until
gas formation had ceased and a homogeneous mass had been formed. The reaction product was cooled and
neutralized over 30 min with saturated Na2CO3 solution. The organic layer was separated and dried over
MgSO4. After removal of solvent the dry residue was recrystallized from ethanol to give the product 4a (0.7 g,
67%) with mp 112-113°C. 13C NMR spectrum (DMSO), δ, ppm: 8.22 (CH3); 44.6 (CH2); 124-128.8 (m, C6H5);
157.7 (C tetrazole); 162.6 (C-2 oxadiazole); 163.4 (C-5 oxadiazole). Found, %: C 54.21; H 3.85; N 34.22.
C11H10N6O. Calculated, %: C 54.55; H 4.13; N 34.71.
Compounds 4b,d, 5a-c and 7a-c were prepared similarly.
1160

5-Phenyl-2-(5-phenyltetrazol-2-ylmethyl)-1,3,4-oxadiazole (4b) is prepared from the acid chloride 1
(0.54 g, 2.5 mmol) and 5-phenyltetrazole 3b (0.24 g, 1.6 mmol) in toluene (5 ml). Yield 0.35 g (71%);
1
mp 158-160°C (a mixture of ethanol and DMF). H NMR spectrum (acetone-d6), δ, ppm: 6.40 (2H, s, CH2);
7.52-7.59 (5H, m, HPh at tetrazole); 8.02-8.13 (5H, m, HPh at oxadiazole). 13C NMR spectrum (HMPA), δ, ppm:
46.9 (CH2); 122.83 (Ci Ph at oxadiazole); 125.94 (Co Ph at tetrazole); 126.06 (Cm Ph at tetrazole); 126.63 (Cp Ph
at tetrazole); 128 87 (Co Ph at oxadiazole); 129.3 (Cm Ph at oxadiazole); 130.35 (Ci Ph at tetrazole); 132.02 (Cp
Ph at oxadiazole); 160.2 (C tetrazole); 164.5 (C-2 oxadiazole); 164.8 (C-5 oxadiazole). Found, %: C 63.31;
H 3.82; N 27.24. C16H12N6O. Calculated, %: C 63.15; H 3.95; N 27.63.
2-(2-Phenyl-1,2,3-triazol-4-yl)-5-(5-phenytetrazol-2-ylmethyl)-1,3,4-oxadiazole (4c) is prepared
from the acid chloride 1 (1 g, 4.5 mmol) and 5-(2-phenyl-1,2,3-triazol-4-yl)tetrazole 3c (0.64 g, 3 mmol) in
toluene (7 ml). Yield 0.8 g (72%); mp 142-143ºC (benzene). 13C NMR spectrum (DMSO), δ, ppm: 46.3 (CH2);
118.6 (Co Ph at triazole); 125.7 (Cp Ph at triazole); 126.03 (Cm Ph at triazole); 130.5 (Ci Ph at triazole); 128.65
(Cp Ph at tetrazole); 128.9 (Cm Ph at tetrazole); 129.5 (Co Ph at tetrazole); 133.8 (Ci Ph at tetrazole); 135.9 (C-4
triazole); 137.9 (C-5 triazole); 158.2 (C tetrazole); 159.9 (C-2 oxadiazole); 164.4 (C-5 oxadiazole). Found, %:
C 58.95; H 3.67; N 33.85. C18H13N9O. Calculated, %: C 58.55; H 3.50; N 33.96.
2-(Benzotriazol-1-ylmethyl)-5-(5-phenyltetrazol-2-ylmethyl)-1,3,4-oxadiazole (4d) is prepared from
the acid chloride 1 (1 g, 4.5 mmol) and (N-methyltetrazol-5-yl)benzotriazole 3d (0.9 g, 4.5 mmol) in toluene
(7 ml). Yield 0.35 g (22%); mp 245-247°C (decomp., ethanol). 13C NMR spectrum (HMPA), δ, ppm: 46.9 (CH2
at tetrazole); 51.5 (CH2 at benzotriazole); 109.6-132 (Ph at benzotriazole and at Ph tetrazole, assignments
ambiguous); 142.6 (C-4 tetrazole); 143.9 (C-5 triazole); 160.9 (C tetrazole); 162.3 (C-2 oxadiazole); 162.6 (C-5
oxadiazole). Found, %: C 56.32; H 3.25; N 34.72. C17H13N9O. Calculated, %: C 56.82; H 3.62; N 35.1.
2-(4-Phenyl-1,2,3-triazol-1-ylmethyl)-5-(5-phenyltetrazol-2-ylmethyl)-1,3,4-oxadiazole
(4e)
is
prepared from the acid chloride 1 (0.43 g, 1.9 mmol) and tetrazole 3e (0.37 g, 1.6 mmol) in toluene (5 ml). Yield
0.26 g (42%); mp 105°C (ethanol). 13C NMR spectrum (DMSO), δ, ppm: 40.45 (CH2 at triazole); 44.45 (CH2 at
tetrazole); 136.1 (C-4 triazole); 142.5 (C-5 triazole); 158.8 (C tetrazole); 160.5 (C-2 oxadiazole); 162.7 (C-5
oxadiazole); 120.3-130.9 (Ph at triazole and Ph at tetrazole, assignments ambiguous). Found, % C 59.41; H
3.78; N 32.47; C19H15N9O. Calculated, %: C 59.22; H 3.9; N 32.73.
5-Methyl-2-(2-phenyl-1,2,3-triazol-4-yl)-1,3,4-oxadiazole (5a) is prepared from the acid chloride 2
(0.5 g, 2.4 mmol) and tetrazole 3a (0.35 g, 4.1 mmol) in toluene (5 ml). Yield 0.6 g (67%); mp 132°C (ethanol).
13C NMR spectrum (acetone), δ, ppm: 10 (CH3); 119.2 (Co Ph); 128.8 (Cp Ph); 129.8 (Cm Ph); 135.6 (Ci Ph);
135.7 (C-4 triazole); 139.5 (C-5 triazole); 157.9 (C-2 oxadiazole); 164.4 (C-5 oxadiazole). Found, %: C 58.23;
H 3.54; N 30.42. C11H9N5O. Calculated, %: C 58.15; H 3.96; N 30.84.
5-Phenyl-2-(2-phenyl-1,2,3-triazol-4-yl)-1,3,4-oxadiazole (5b) is prepared from the acid chloride 2
(0.54 g, 2.6 mmol) and tetrazole 3b (0.25 g, 1.7 mmol) in toluene (10 ml). Yield 0.26 g (53%); mp 195°C
(mixture of ethanol and DMF). 13C NMR spectrum (HMPA), δ, ppm: 164.04 (C-5 oxadiazole); 157.1 (C-2
oxadiazole); 136.6 (C-5 triazole); 132.4 (C-4 triazole); 118.6 (Co Ph at triazole); 126.5 (Cp Ph at triazole); 127.4 (Cm
Ph at triazole); 135.2 (Ci Ph at triazole); 122.1 (Co Ph at oxadiazole); 129.4 (Cp Ph at oxadiazole); 129.8 (Cm Ph
oxadiazole). 138.4 (Ci Ph at oxadiazole). Found, %: C 66.12; H 3.85; N 24.05. C16H11N5O. Calculated, %: C 66.44;
H 3.81; N 24.22.
2,5-Bis(2-phenyl-1,2,3-triazol-4-yl)-1,3,4-oxadiazole (5c) is prepared from the acid chloride 2 (1 g,
4.8 mmol) and tetrazole 3c (0.68 g, 3.2 mmol) in toluene (7 ml). Yield 0.93 g (84.5%); mp 217-219ºC (mixture
of benzene and DMF). 13C NMR spectrum HMPA), δ, ppm: 117.2 (Co Ph); 127.5 (Cp Ph); 128.5 (Cm Ph); 135.5
(Ci Ph); 132.9 (C-4 triazole); 137.2 (C-5 triazole); 156.3 (C-2 oxadiazole); 162.6 (C-5 oxadiazole). Found, %:
C 60.84; H 3.47; N 31.66. C18H12N8O. Calculated, %: C 60.67; H 3.37; N 31.46.
Diethyl bis[2-(2-phenyl-1,3,4-oxadiazol-5-yl)ethyl]malonate (7a) is obtained from the bistetrazole 6a
(1 g, 2.8 mmol) and benzoyl chloride (1.2 g, 8.5 mmol) in toluene (10 ml). Yield 0.65 g (45%); mp 118-120°C
(ethanol). 13C NMR spectrum (acetone), δ, ppm: 13.6 (CH3); 20.8 (CH2Cquat); 30.15 (CH2 at oxadiazole); 61.7
1161

(CH2O); 56.7 (Cquat); 124.4 (Ci Ph); 126.6 (Co Ph); 129.3 (Cm Ph); 131.7 (Cp Ph); 164.6 (C-2 oxadiazole); 166.2
(C-5 oxadiazole); 170.3 (CO). Found, %: C 64.21; H 5.25; N 11.03. C27H28N4O6. Calculated, %: C 64.29; H
5.56; N 11.11.
Tris[2-(2-phenyl-1,3,4-oxadiazol-5-yl)ethyl]nitromethane (7b) is prepared from the tristetrazole 6b
(1 g, 2.9 mmol) and benzoyl chloride (1.8 g, 12.9 mmol) in toluene (10 ml). Yield 0.57 g (35%); mp 184-186°C
(ethanol). 13C NMR spectrum (HMPA), δ, ppm: 19.6 (CH2 at oxadiazole); 30.5 (CH2CH2 at oxadiazole); 92.8
(CNO2); 123.4 (Ci Ph); 125.8 (Co Ph); 129 (Cm Ph); 131.4 (Cp Ph); 163.7 (C-2 oxadiazole); 165.3 (C-5
oxadiazole). Found, %: C 64.29; H 4.75; N 16.52. C31H27N7O5. Calculated, %: C 64.47; H 4.68; N 16.98.
Tris[2{2-(2-phenyl-1,2,3-triazol-4-yl)-1,3,4-oxadiazol-5-yl}ethyl]nitromethane (7c) is prepared from
the tristetrazole 6b (0.5 g, 1.4 mmol) and the acid chloride 2 (1.04 g, 5 mmol) in toluene (10 ml). Yield 0.5 g
(46%); mp 223-225°C (decomp, mixture of ethanol and DMF). 13C NMR spectrum (HMPA), δ, ppm: 19.6 (CH2
oxadiazole); 30.4 (CH2CH2 at oxadiazole); 92.8 (Ctert); 118.2 (Co Ph); 128.5 (Cp Ph); 129.7 (Cm Ph); 134.9 (Ci
Ph); 135.8 (C-4 triazole); 138.5 (C-5 triazole); 157.1 (C-2 oxadiazole); 165.9 (C-5 oxadiazole). Found, %:
C 56.84; H 3.65; N 28.44. C37H30N16O5. Calculated, %: C 57.07; H 3.86; N 28.79.
Diethyl Bis[2-(5-trifluromethyl-1,3,4-oxadiazol-2-yl)ethyl]malonate (8a). Trifluoroacetic acid
anhydride (5.4 g, 25.6 mmol) was added dropwise at room temperature to the bistetrazole 6a (3 g, 8.5 mmol) in
methylene chloride (10 ml). After gas evolution had ceased the reaction product was neutralized with a saturated
solution of Na2CO3. The aqueous layer was extracted with methylene chloride and the combined extract was dried
over magnesium sulphate. Solvent was removed in air and the dry residue was recrystallized to give product 8a
1
(2.74 g, 87%) with mp 64-65°C (ethanol). H NMR spectrum (acetone-d6), δ, ppm (J, Hz): 2.04 (6H, t, J = 7,
2CH3CH2); 2.56 (4H, 2CH2 at oxadiazole); 3.15 (4H, 2CH2 Cquat); 4.24 (4H, q, J = 7, 2CH3CH2). 13C NMR
spectrum, δ, ppm (J, Hz): 14.3 (CH3); 21.6 (CH2 at oxadiazole); 30.3 (CH2CH2 at oxadiazole); 57.2 (Cquat); 62.6
13 19
(CH2O); 116.6 (q, J
F = 270.2, CF3); 156 (q, J = 43.6, C-5 oxadiazole); 169.9 (C-2 oxadiazole); 170.8 (C=O).
C
Found, %: C 41.73; H 3.57; N 11.41. C17H18F6N4O6. Calculated, %: C 41.80; H 3.69; N 11.48.
Tris[2-(5-trifluoromethyl-1,3,4-oxadiazol-2-yl)ethyl]nitromethane (8b) is prepared similarly to
compound 8a from the tristetrazole 6b (0.5 g, 1.47 mmol) and trifluoroacetic acid anhydride (1.5 g, 7.2 mmol)
in methylene chloride (10 ml). Yield 0.65 g (80%); mp 98-100°C (ethanol). 13C NMR spectrum (acetone-d6), δ,
ppm (J, Hz): 20.1 (CH2 at oxadiazole); 30.7 (C, CH2CH2 at oxadiazole); 92.01 (CNO2); 116.7 (q,
J13 F = 267.7, CF3); 155 (q, J13
= 42.5, C-5 oxadiazole); 168.3 (C-2 oxadiazole). Found, %: C 34.97;
19
19
C,
C,
F
H 2.24; N 17.58. C16H12F9N7O5. Calculated, %: C 34.72; H 2.17; N 17.72.
2,2,6,6-Tetrakis[2-(5-trifluoromethyl-1,3,4-oxadiazol-2-yl)ethyl]cyclohexanone (8c) is prepared
similarly to compound 8a from the tetrakistetrazole (1.4 g, 2.9 mmol) and trifluoroacetic acid anhydride (3.6 g,
17.3 mmol) in methylene chloride (10 ml). Yield 1.17 g (54%) as colorless crystals; mp 75-77°C (ethanol). 13C
NMR spectrum (DMSO), δ, ppm (J, Hz): 19.3 (CH2 at oxadiazole); 30.9 (CH2CH2 at oxadiazole); 115.6 (q,
13 19
J
F = 271.8, CF3); 153.6 (q, J13
= 43.7, C-5); 168.8 (C-5 oxadiazole); 215.2 (C=O); 15.4 (γ-CH2
19
C,
C
F
cyclohexane); 31.7 (β-CH2 cyclohexane); 49.2 (α-CH2 cyclohexane). Found, %: C 41.13; H 2.77; N 14.46.
C26H22F12N8O5. Calculated, %: C 41.38; H 2.92; N 14.85.
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