One-pot, microwave-assisted synthesis of polymethylene-bridged bis(1H-1,2,4-triazol-5(3)-amines) and their tautomerism

Article abstract of DOI:10.1016/j.tetlet.2018.09.018

A highly selective and efficient method for the synthesis of 3,3′(5,5′)-polymethylene-bis(1H-1,2,4-triazol-5(3)-amines) was developed using the reaction of dicarboxylic acids and aminoguanidine in an aqueous medium. This one-pot, microwave-promoted method was proved to be scalable affording the desired products in good yields and purity. The scope of the method was successfully explored by the preparation of a small library of polymethylene-bis(1H-1,2,4-triazol-5(3)-amines) with different alkyl chain linkers. The annular prototropic tautomerism in the prepared compounds was studied using NMR spectroscopy and X-ray crystallography.

Full text of DOI:10.1016/j.tetlet.2018.09.018

Accepted Manuscript
One-pot, microwave-assisted synthesis of polymethylene-bridged bis(1H-1,2,4-
triazol-5(3)-amines) and their tautomerism
Felicia Phei Lin Lim, Lee Ming Hu, Edward R.T. Tiekink, Anton V. Dolzhenko
PII:
S0040-4039(18)31099-2
https://doi.org/10.1016/j.tetlet.2018.09.018
TETL 50257
DOI:
Reference:
Tetrahedron Letters
To appear in:
Received Date:
Revised Date:
Accepted Date:
18 July 2018
30 August 2018
6 September 2018
Please cite this article as: Lim, F.P.L., Hu, L.M., Tiekink, E.R.T., Dolzhenko, A.V., One-pot, microwave-assisted
synthesis of polymethylene-bridged bis(1H-1,2,4-triazol-5(3)-amines) and their tautomerism, Tetrahedron Letters
(2018), doi: https://doi.org/10.1016/j.tetlet.2018.09.018
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1
Tetrahedron Letters
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One-pot, microwave-assisted synthesis of polymethylene-bridged bis(1H-1,2,4-
triazol-5(3)-amines)and their tautomerism
Felicia Phei Lin Lima, Lee Ming Hua, Edward R. T. Tiekinkb and Anton V. Dolzhenkoa,c∗
a
School of Pharmacy, Monash University Malaysia, Jalan Lagoon Selatan, Bandar Sunway, Selangor Darul Ehsan 47500, Malaysia
b Research Centre for Crystalline Materials, School of Science and Technology, Sunway University, Bandar Sunway, Selangor Darul Ehsan 47500, Malaysia
c School of Pharmacy and Biomedical Sciences, Curtin Health Innovation Research Institute, Faculty of Health Sciences, Curtin University, GPO Box U1987
Perth, Western Australia 6845, Australia
ARTICLE INFO
ABSTRACT
Article history:
Received
Received in revised form
Accepted
A highly selective and efficient method for the synthesis of 3,3’(5,5’)-polymethylene-bis(1H-
1,2,4-triazol-5(3)-amines) was developed using the reaction of dicarboxylic acids and
aminoguanidine in an aqueous medium. This one-pot, microwave-promoted method was proved
to be scalable affording the desired products in good yields and purity. The scope of the method
was successfully explored by the preparation of a small library of polymethylene-bis(1H-1,2,4-
triazol-5(3)-amines) with different alkyl chain linkers. The annular prototropic tautomerism in
the prepared compounds was studied using NMR spectroscopy and X-ray crystallography.
Available online
Keywords:
Triazole
Dicarboxylic acid
Microwave-assisted synthesis
One-pot synthesis
Tautomerism
2018 Elsevier Ltd. All rights reserved.
The twin-drugs strategy involves linking two identical
pharmacophores in one molecule by chemically and
selectivity of all steps is questionable as confirmed by the low
purity of products (by reported elemental analysis data for 6, 8,
and 9).3f
a
pharmacologically inert bridge. It is an efficient approach to
significantly improve the effectiveness of bioactive agents. A
polymethylene chain is often used as a linker between the
pharmacophores in twin-drugs and the length of this chain is
adjusted to achieve optimal therapeutic properties.
Many symmetrical polymethylene-bridged diamines, their
derivatives and analogues, including bis(N-heterocycles), have
been found to possess antiparasitic,1 antibacterial,2 and
anticancer3 properties. Some notable recent examples of these
bioactive compounds include highly potent antimalarial diamine
1;1d bis-thiourea 2 inhibiting soluble epoxide hydrolase activity at
a low nanomolar concentration range;4 bis-imidazoles 3 and 4
possessing strong anti-biofilm activity against Staphylococcus
aureus and Pseudomonas aeruginosa, respectively;2c compound
5 effectively bis-intercalating into the tatelomeric DNA/RNA
duplex;3e and bis(1H-1,2,4-triazol-5-amine)
6
possessing
anticancer properties3f (Fig. 1).
Bis-triazole 6 was previously synthesized using the reaction of
bis-urea 8 or bis-thiourea 9, which were prepared from sebacoyl
chloride (7), with hydrazine (Scheme 1).3f This two-step process
produced 6 in 59-61% overall yield. However, using a relatively
unstable acid chloride and extended reaction times (4-8 h for
e—ac—h s—tep) makes this process less practical. Moreover, the
Figure 1. Selected bioactive symmetrical polymethylene-bridged
compounds.
* Corresponding author. Tel.: +60 3 5514 5867; fax: +60 3 5514 6364; e-mail addresses: dolzhenkoav@gmail.com,
anton.dolzhenko@monash.edu (A.V. Dolzhenko).

2
Tetrahedron Letters
Scheme 1. Synthesis of octamethylene-bis(1H-1,2,4-triazol-5-amine) (6).3f
To our satisfaction, we observed the formation of ethylene-
bis(1H-1,2,4-triazol-5-amine) 12, which was isolated via
filtration (Entry 1). To further improve the reaction yield, we
continued the optimization process and found that increasing the
amount of potassium carbonate led to a slight increase in product
yield (Entry 2). However, further optimization of the reaction
conditions, particularly increasing the temperature, was limited
due to exessive pressure generated in the reaction vessel.
Screening other bases for the reaction (Entries 3-4), we identified
that under similar conditions sodium hydroxide was equally
efficient as a base in terms of yield. Hence, the optimization was
continued using sodium hydroxide as a base. Further variations in
the reaction parameters revealed that 12 could be obtained in
good yield by heating succinic acid and aminoguanidine
hydrochloride in water at 220 °C for 15 min followed by the
addition of an aqueous solution of sodium hydroxide and further
microwave irradiation at 200 °C for 5 min (Entry 16). It should
be noted that the exclusive presence of ethylene-bis(1H-1,2,4-
triazol-5-amine) 12 was observed in the crude product isolated
from the reactions.
Scheme 2. Synthesis of decamethylene-bis(1H-1,2,4-triazol-5-
amine) (11).5
A homologue of compound 6, decamethylene-bis(1H-1,2,4-
triazol-5-amine) (11) was prepared via the cyclization of bis-
amidoguanidine 10 (Scheme 2).5 This reaction required heating at
high temperature (160-180 °C) for 3 h but afforded 11 in rather
modest yield (26%). Previously, we developed an effective
method for the cyclization of amidoguanidines in water under
To explore the scope of the method, various dicarboxylic
acids were used as substrates in the reaction with aminoguanidine
hydrochloride to produce polymethylene-bis(1H-1,2,4-triazol-
5(3)-amines). The optimized conditions9 were found to be
sufficiently general and suitable for the synthesis of compounds
6, 11-17 in good yields (Table 2). In all cases, the reaction was
selective and only polymethylene-bis(1H-1,2,4-triazol-5(3)-
amines) were isolated in high purity via filtration. The reaction
was also effectively performed on 3.4 mmol and 17 mmol scales
with similar outcomes.
microwave
irradiation
affording
the
corresponding
aminotriazoles in excellent yields.6 This approach was further
elaborated for the synthesis of more complex compounds in a
one-pot manner.7 Therefore, we decided to adopt a similar one-
pot, microwave-promoted methodology to prepare 3,3’(5,5’)-
polymethylene-bis(1H-1,2,4-triazol-5(3)-amines) from readily
available dicarboxylic acids.
As the model reaction, we decided to examine the
condensation of succinic acid with aminoguanidine. This reaction
under conventional heating was reported to suffer from a lack of
selectivity resulting in the formation of a mixture of products,
from which pure ethylene-bis(1H-1,2,4-triazol-5-amine) 12 was
isolated in 8-11% yield only.8 To overcome this problem, we
optimized the reaction conditions under microwave irradiation.
It should also be noted that the developed method was
successfully tested using a Monowave 450 (Anton Paar)
microwave synthesizer for the synthesis of 12 under the
conditions optimized using the Discover SP (CEM). The
outcome was satisfactory affording 12 in 91% yield, therefore
confirming the reproducibility of the method in different
microwave reactors.
Initially, using the Discover SP (CEM) microwave
synthesizer, succinic acid was heated with aminoguanidine
hydrochloride in water at 200 °C for 15 min. After cooling, an
aqueous solution of potassium carbonate was added to the
reaction mixture and heating was continued at 200 °C for 5 min.

3
Table 1. Optimization of the reaction conditions for the synthesis of ethylene-bis(1H-1,2,4-triazol-5(3)-amine) 12a
Molar ratio of succinic acid:
Entry
Conditions
Isolated yield 12 (%)
aminoguanidine
1
2
1:2
1:2
1:2
1:2
1:3
1:4
1:5
1:6
1:7
1:5
1:5
1:5
1:5
1:5
1:5
1:5
1:5
i: water, 200 °C, 15 min; ii: K2CO3 (0.5 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: K2CO3 (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaHCO3 (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min,
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 200 °C, 10 min; ii: NaOH (1.0 equiv.), water, 200 °C, 5 min
i: water, 180 °C, 30 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
i: water, 200 °C, 15 min; ii: NaOH (1.0 equiv.), 180 °C, 5 min
i: water, 180 °C, 15 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
i: water, 190 °C, 15 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
i: water, 210 °C, 15 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
i: water, 220 °C, 15 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
i: water, 230 °C, 15 min; ii: NaOH (1.0 equiv.), 200 °C, 5 min
33
37
27
36
58
66
79
74
71
58
58
68
46
54
79
85
77
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
a Reaction was performed using a Discover SP (CEM) microwave synthesizer on a 3.4 mmol scale (succinic acid) using 2 mL of water in the first step, followed
by the addition of 1 mL of the specified aqueous base in the second step.

4
Tetrahedron Letters
Table 2. One-pot synthesis of polymethylene-bis(1H-1,2,4-triazol-5(3)-amines).a
Compound
Structure
Scale (mmol)
Yield (%)
12
3.4
17
85
88
13
14
15
16
17
6
3.4
17
58
63
3.4
17
77
66
3.4
17
52
56
3.4
17
72
65
3.4
17
57
54
3.4
17
63
63
11
3.4
17
72
67
a Reaction was performed in water using a Discover SP CEM microwave synthesizer at 220 °C for 15 min, followed by the addition of aq. NaOH and heating at
200 °C for 5 min.
The annular prototropic tautomerism, possible in both triazole
rings of the prepared compounds, might result in six different
tautomers representing different combinations of the 5-amino-
1H-1,2,4-triazole, 3-amino-1H-1,2,4-triazole, and 5-amino-4H-
1,2,4-triazole rings (Scheme 3). These three principal forms of
the 1,2,4-triazole ring can be distinguished on the basis of NMR
spectroscopy data (though without allowing assigning the
products in DMSO-d6 gave two sets of signals for the protons of
the primary amino group and the triazole ring. These signals
together with two sets of signals for the triazole ring carbon
atoms in the 13C NMR spectra served for the assignment of two
main forms (A and B) of the triazole ring (three potential
tautomers). The assignments were confirmed with the literature
data,6-8,10 and the equilibrium constant (KT) and corresponding
Gibbs free energy (∆G300) values were estimated (Table 3).
1
presence of individual tautomers). The H NMR spectra of the
Scheme 3. Theoretically possible tautomers of polymethylene-bis(1H-1,2,4-triazol-5(3)-amines).

5
Table 3. Tautomeric composition polymethylene-bis(1,2,4-triazol-5(3)-amines) in DMSO-d6.a
Composition (%)
Compounds
n
KT
-∆G300 (kJmol-1)
A
B
12
13
14
15
16
17
6
2
3
72
68
67
66
66
65
65
65
28
32
33
34
34
35
35
35
2.57
2.13
2.03
1.94
1.94
1.86
1.86
1.86
2.35
1.89
1.77
1.65
1.65
1.55
1.55
1.55
4
5
6
7
8
11
10
a Assigned by NMR spectroscopy performed at 27 °C.
In DMSO-d6 solution, the tautomeric equilibria was shifted
mainly towards the 5-amino-1H-form (A) of the triazole ring
with the 3-amino-1H-form (B) being less prominent and the 4H-
form (C) undetectable. The expansion of the polymethylene link
between the triazole rings led to an increase in the 3-amino-1H-
form (B) in the equilibrium. This trend can be explained by the
diminishing distance electron-withdrawing effect of one triazole
ring on another. However, for compounds possessing seven or
more methylenic groups in the linking chain, the equilibrium
constant remained the same. It should be noted that the tautomer
signals were indistinguishable when the NMR spectra of the
compounds were recorded in CDCl3.
In conclusion, we have developed an efficient and practical
one-pot, microwave-promoted synthesis of polymethylene-
bis(1H-1,2,4-triazol-5(3)-amines)
from
readily available
dicarboxylic acids. The method was found to be scalable and
transferable to another microwave reactor. The tautomeric
preferences in the products were determined in solution (by
NMR spectroscopy) and solid state (X-ray crystallography). The
tautomeric equilibrium in the solution depended on the length of
the chain connecting the triazole rings. A unique crystallisation
of the 5-substituted 3-amino-1H-1,2,4-triazole tautomeric form
was also observed.
Acknowledgments
In crystals, the tautomerizable 5(3)-amino-1,2,4-triazoles
usually appear in the 5-amino-1H-tautomer form.11 An example
of 5-amino-1H- and 3-amino-1H-tautomers crystallising together
has also been reported.12 With the objective to investigate the
tautomeric preference in crystals of the prepared compounds, we
performed single crystal X-ray crystallography for representative
compound 14.13 The molecular structure of 14, shown in Fig.
2(a), revealed that this compound crystallised with the 3-amino-
1H-1,2,4-triazole rings on both sides of the molecule. This
tautomeric form is not typical for crystals of tautomerizable 5(3)-
amino-1,2,4-triazoles and represents a rather unique case of
finding 3-amino-1H-1,2,4-triazoles in crystals.
The Research Centre for Crystalline Materials (Sunway
University) is thanked for the X-ray intensity data. This work is
supported by the Ministry of Higher Education, Malaysia under
Fundamental Research Grant Scheme, grant number
FRGS/1/2015/SG01/MUSM/03/1.
References and notes
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S.; Van der Eycken, E. V.; Steenackers, H. P. L. Bioorg. Med.
Chem. 2018, 26, 1470-1480.
Figure 2. Molecular structure of 14 showing atom labelling and
70% probability displacement ellipsoids; unlabelled atoms are
related by the symmetry operation -x, 2-y, -z.

6
Tetrahedron Letters
3.
(a) Kumar, A.; Banerjee, S.; Roy, P.; Sondhi, S. M.; Sharma, A.
Bioorg. Med. Chem. Lett. 2017, 27, 501-504; (b) Kopsida, M.;
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9. General method for the microwave-assisted synthesis of bis(5(3)-
amino-1,2,4-triazol-3(5)-yl)polymethylenes (6, 11-17). A mixture
of the corresponding dicarboxylic acids (3.4 mmol) and
aminoguanidine hydrochloride (1.88 g, 17 mmol) in water (2 mL)
was irradiated in a 10 mL seamless pressure vial using the
microwave system operating at maximum microwave power (up
to 300 W) at 220 °C for 15 min. After cooling to ambient
temperature, of 17 M aq. NaOH (1 mL) was added to the vial and
the reaction mixture was irradiated again at 200 °C for 10 min.
After cooling to ambient temperature, the precipitated solid was
filtered, washed with cold water and recrystalised from water to
give desired product. The reaction was also replicated on an
increased scale of the dicarboxylic acids (17 mmol) and
aminoguanidine hydrochloride (9.40 g, 85 mmol) in water
(10 mL).
10. Buzykin, B. I.; Mironova, E. V.; Nabiullin, V. N.; Gubaidullin, A.
T.; Litvinov, I. A. Russ. J. Gen. Chem. 2006, 76, 1471-1486.
11. (a) Garcia, E.; Lee, K. Y. Acta Crystallogr., Sect. C: Cryst. Struct.
Commun. 1992, C48, 1682-1683; (b) Borbulevych, O. Y.;
Shishkin, O. V.; Desenko, S. M.; Chernenko, V. N.; Orlov, V. D.
Acta Crystallogr., Sect. C: Cryst. Struct. Commun. 1998, C54,
442-444; (c) Dolzhenko, A. V.; Tan, G. K.; Koh, L. L.;
Dolzhenko, A. V.; Chui, W. K. Acta Crystallogr., Sect. E: Struct.
Rep. Online 2009, 65, o125; (d) Zhang, S.; Liu, P.-J.; Ma, D.-S.;
Hou, G.-F. Acta Crystallogr., Sect. E: Struct. Rep. Online 2012,
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Paz, F. A. Acta Crystallogr., Sect. E: Struct. Rep. Online 2012, 68,
o2700-o2701; (f) Hernandez-Gil, J.; Ferrer, S.; Ballesteros, R.;
Castineiras, A. Acta Crystallogr., Sect. E: Struct. Rep. Online
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Chui, W. K. Acta Crystallogr., Sect. E: Struct. Rep. Online 2009,
65, o126.
13. Crystal data for C8H14N8 (14): M = 222.27, monoclinic, P21/n, a =
5.00860(10), b = 5.1190(2), c = 19.5646(6) Å, β= 93.306(3)˚, V =
-3
500.78(3) Å3, Z = 2, Dx = 1.474 g cm , F(000) = 236, µ = 0.844
mm-1. CCDC 1851291.
Supplementary Material
Supplementary data associated with this article can be found,
in
the
online
version,
at
http://dx.doi.org/10.1016/j.tetlet.2018.XX.XXX. These data
include experimental details, copies of H and 13C NMR spectra
1
of the prepared compounds, and X-ray crystallography data for
compound 14.

7
-
-
-
-
Efficient triazole synthesis under microwave
irradiation
Practical approach to polymethylene-bis(1H-
1,2,4-triazol-5(3)-amines)
One-pot synthesis of 1,2,4-triazoles in aqueous
media
Tautomerism in 1,2,4-triazoles