Microwave-assisted synthesis of novel 8-chloro-[1,2,4]triazolo[4,3-a]pyridine derivatives

Article abstract of DOI:10.3906/kim-1408-78

A series of novel 1,2,4-triazolo[4,3- a]pyridine derivatives were synthesized from 2,3-dichloropyridine and hydrazine hydrate as starting materials by multistep reactions under microwave assistance, and their structures were characterized by ¹H

Full text of DOI:10.3906/kim-1408-78

Turk J Chem
Turkish Journal of Chemistry
(2015) 39: 867 – 873
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http://journals.tubitak.gov.tr/chem/
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c
⃝ TUBITAK
doi:10.3906/kim-1408-78
Research Article
Microwave-assisted synthesis of novel 8-chloro-[1,2,4]triazolo[4,3-a]pyridine
derivatives
Lin-Jiong ZHANG1, Ming-Yan YANG1, Bei-Zhen HU2, Zhao-Hui SUN1,
Xing-Hai LIU1,∗, Jian-Quan WENG1, Cheng-Xia TAN1
1College of Chemical Engineering, Zhejiang University of Technology, Hangzhou, P.R. China
2Shaoxing Entry Exit Inspect & Quarantine Bur PRC, Shaoxing, P.R. China
Received: 29.08.2014
•
Accepted/Published Online: 04.02.2015
•
Printed: 28.08.2015
Abstract: A series of novel 1,2,4-triazolo[4,3-a]pyridine derivatives were synthesized from 2,3-dichloropyridine and
hydrazine hydrate as starting materials by multistep reactions under microwave assistance, and their structures were
characterized by 1 H NMR, MS, and elemental analysis. This method provides several advantages such as high yields,
facile work-up, and environmental friendliness.
Key words: 1,2,4-Triazolo[4,3-a]pyridine, urea, microwave-assisted synthesis
1. Introduction
At present, nitrogen-containing compounds are a hot topic due to their diverse function in the field of organic
synthesis, medicinal chemistry, pesticide chemistry, and industrial chemistry.1−3 1,2,4-Triazole and pyridine
derivatives often displayed broad and excellent activities.4,5 On the other hand, fused heterocycles generally
exhibit properties of the single heterocyclic. The incorporation of a pyridine ring into a triazole ring was proved
to be a good way to produce novel active compounds.6 Furthermore, various derivatives of ureas including
simple ureas,7 arylureas,8 aminourea,9 and thioureas10 are considered privileged scaffolds in drug discovery
with a wide array of biological activities.11−13 In the literature, aminourea derivatives have been utilized for
their herbicidal,14 antibacterial,15 and insecticidal activity.16
Generally, green chemistry has been attracting great interest from chemists because of its environmental
benefits.17 Many green methods, such as catalyst-free,18 supercritical fluids,19 ionic liquids,20 solvent-free
reactions,21 and ultrasound22 or microwave23 irradiation as energy sources were applied in organic synthesis.
Recently, microwave irradiation has become an effective tool in organic synthesis, because of its short reaction
time and higher yields.
In view of all these facts and in continuation of our research on bioactive compounds,24−26 a series of novel
urea derivatives containing 1,2,4-triazolo[4,3-a]pyridine moiety were synthesized under microwave irradiation.
2. Results and discussion
2.1. Synthesis
Microwave technology was applied to the synthetic reaction to shorten the reaction time and increase the yields
of urea derivatives 4. The one pot synthesis of intermediate 1 under microwave irradiation was conducted, but
∗Correspondence: xhliu@zjut.edu.cn
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the result was not better than that of conventional conditions. Intermediate 1 and diethyl oxalate led to the
intermediate 2 by nucleophile substitution reaction. All the reaction mechanisms are nucleophile substitution
reactions (Scheme 1). The mixture of acylhydrazine and RNCO in different solvents was irradiated with
microwaves. This reaction was completed with higher yields compared with the conventional mode of heating.
The reaction parameters were optimized for the synthesis of the title compounds. Compound 4b was chosen
as a model reaction under different conditions. Several key reaction conditions were investigated including the
reaction times, reaction temperatures with or without microwave irradiation, and the reaction solvent. Table
1 indicates that the microwave irradiation can accelerate the rate of the reaction. The effect of three different
solvents was studied. The best solvent of this reaction is CH3 CN; the yield of 4b was significantly higher than
that of other solvents. The reaction temperature, reaction time, and reaction molar ratio were also studied.
When they were reacted at 90 ◦ C under a stoichiometric ratio of 1:1.3 for 1 min under microwave irradiation,
the yield of compound 4b was higher (Table 2).
Scheme 1. The reaction mechanism of the title compounds.
2.2. Spectrum
In the title compounds, there are three NH groups (Figure). The signals of NH protons were observed at around
δ8.00∼10.41 ppm. The order of chemical shift of NH protons is δ(Hc)(10.10∼10.41) > δ(Ha) (8.15∼9.24)
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> δ(Hb) (8.00∼9.20). Of these, the chemical shift of Ha and Hb is difficult to distinguish, due to the hydrogen
bonds that are formed between the two O atoms and NH (Figure). The ESI-MS spectrum showed that the
m/z of molecular ion is in agreement with its molecular formula. The measured elemental analyses were also
consistent with the corresponding calculated values.
Table 1. Comparison of yields of 4b through methods with or without microwave irradiation.
Entry Solvent Method Time Temperature/◦C Yield/%
1
2
3
4
5
6
7
8
CH3CN No-MW 3 h
r.t.
1 min 90
1 min 50
90
91
60
65
75
81
91
89
CH3CN MW
CH2Cl2 MW
Toluene MW
CH3CN MW
CH3CN MW
CH3CN MW
CH3CN MW
1 min 115
1 min 80
1 min 85
1 min 90
2 min 90
Table 2. Comparison of yields of 4b in different stoichiometric ratio under microwave irradiation.
Entry Solvent Method Time Temperature/◦C Stoichiometric ratio Yield/%
1
2
3
4
CH3CN MW
CH3CN MW
CH3CN MW
CH3CN MW
1 min 90
1 min 90
1 min 90
1 min 90
1:1.1
1:1.2
1:1.3
1:1.4
72
78
84
83
Figure. Plausible hydrogen bonding in 8-chloro-[1,2,4]triazolo[4,3-a]pyridine derivatives.
3. Experimental
3.1. Materials and methods
All reagents were analytical grade. Melting points were determined using an X-4 apparatus and were uncor-
rected. 1 H NMR spectra were measured on a Bruker Avance 400 MHz spectrometer using TMS as an internal
standard and DMSO-d6 as solvent. A CEM Discover Focused Synthesizer was used to carry out the microwave
reaction. Elemental analysis was performed with a PerkinElmer 240C analyzer.
3.2. Synthesis
3.2.1. General procedure
2,3-Dichloropyridine (7.50 mmol) was dissolved in ethanol (300 mL); then hydrazine hydrate (30 mmol) was
added dropwise under refluxing over 72 h to give 3-chloro-2-hydrazinylpyridine 1. A CEM designed 10-mL
pressure-rated vial was charged with 3-chloro-2-hydrazinylpyridine 1 (143 mg, 1 mmol) and diethyloxalate (1
mmol). The mixture was irradiated in a CEM Discover Focused Synthesizer (150 w, 140 ◦ C, 200 psi, 15 min).
The mixture was cooled to room temperature by passing compressed air through the microwave cavity for 2
min. It was poured into cold ice (40 mL) and the formed precipitate filtered. The crude solid was recrystallized
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from ethanol to give the title compound 2. Then compound 2 (20 mmol) was reacted with hydrazine hydrate
(30 mmol) under microwave irradiation (150 W, 100 ◦ C, 200 psi, 10 min), to afford compound 3. Last, the
title compound 4 was synthesized from compound 3 and isocyanate under microwave conditions. All the other
compounds are synthesized according to the procedure (Scheme 2).
Scheme 2. 4a: R = phenyl; 4b: R = 2,5-dimethylphenyl; 4c: R = 2-methoxyphenyl; 4d: R = 3-chloro-2-methylphenyl;
4e: R = 3-phenoxyphenyl; 4f: R = m-tolyl; 4g: R = 2,6-dichloro-4-(trifluoromethyl)phenyl; 4h: R = 3,5-dichlorophenyl;
4i: R = 2,3-dimethylphenyl; 4j: R = 2-methyl-4-nitrophenyl; 4k: R = 2-methyl-3-(trifluoromethyl)phenyl; 4l: R = 2-
(trifluoromethoxy)phenyl; 4m: R = naphthalen-1-yl; 4n: R = 2-chloro-5-(trifluoromethyl)phenyl.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-phenylsemicarbazide 4a: white crys-
tal, yield 50.51%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 6.97 (d, J = 7.6 Hz, 1H, Ph-H),
7.26 (m, 5H, 1Py-H, 4Ph-H), 7.90 (d,J = 7.6 Hz, 1H, Py-H), 8.43 (s, 1H, NH), 8.80 (s, 1H, NH), 9.14 (d,J
= 7.2 Hz, 1H, Py-H), 10.99 (s, 1H, NH). MS (ESI), m/z: 331 (M+1)+ . Elemental anal. (%), calculated: C,
50.84; H, 3.35; N, 25.41; found: C, 50.99; H, 3.44; N, 25.34.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2,5-dimethylphenyl)semicarbazide
4b: light yellow crystal, yield 78.21%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 2.18 (s, 3H, –CH3),
2.24 (s, 3H, –CH3), 6.80 (d, J = 7.6 Hz, 1H, Ph-H), 7.05 (d,J = 7.6 Hz, 1H, Ph-H), 7.25 (t,J = 7.2 Hz, 1H,
Py-H), 7.50 (s, 1H, Ph-H), 7.83 (d,J = 7.6 Hz, 1H, Py-H), 8.06 (s, 1H, NH), 8.57 (s, 1H, NH), 9.14 (d,J = 7.2
Hz, 1H, Py-H), 11.02 (s, 1H, NH). MS (ESI), m/z: 359 (M+1)+ . Elemental anal. (%), calculated: C, 53.56;
H, 4.21; N, 23.42; found: C, 53.65; H, 4.53; N, 23.65.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2-methoxyphenyl)semicarbazide 4c:
white crystal, yield 59.25%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 3.87 (s, 3H, –CH3), 6.85–7.26
(m, 4H, 1Py-H, 3Ph-H), 7.82–8.26 (m, 3H, 1Py-H, 1Ph-H, 1NH), 8.90–9.16 (m, 2H, 1NH, 1Py-H), 11.06 (s, 1H,
NH). 13 C NMR (DMSO-d6 , 100 MHz) δ: 56.25, 111.27, 116.49, 118.81, 120.80, 121.01, 122.54, 125.60, 128.82,
129.26, 139.99, 148.19, 149.03, 155.17, 157.86. MS (ESI), m/z: 361 (M+1)+ . Elemental anal. (%), calculated:
C, 49.94; H, 3.63; N, 23.30; found: C, 50.21; H, 3.76; N, 23.45.
4-(3-chloro-2-methylphenyl)-1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)semicar-
bazide 4d: light yellow crystal, yield 75.83%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 2.27 (s,
3H, –CH3), 7.17–7.26 (m, 3H, 1Py-H, 2Ph-H), 7.83 (d, J = 7.0 Hz, 1H, 1Py-H), 8.43 (s, 1H, NH), 8.66 (s, 1H,
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NH), 9.14 (d, J = 6.8 Hz, 1H, 1Py-H), 11.08 (s, 1H, NH). MS (ESI), m/z: 379 (M+1)+ . Elemental anal. (%),
calculated: C, 47.51; H, 3.19; N, 22.16; found: C, 47.55; H, 3.23; N, 22.32.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(3-phenoxyphenyl)semicarbazide 4e:
white crystal, yield 83.72%, mp 268–272 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 6.96 (t, J = 6.8 Hz, 4H,
Ph-H), 7.09 (t, J = 6.8 Hz, 1H, Ph-H), 7.25 (t, J = 7.2 Hz, 1H, Py-H), 7.36 (t,J = 8.0 Hz, 2H, Ph-H), 7.51
(d,J = 8.8 Hz, 2H, Ph-H), 7.84 (d,J = 7.2 Hz, 1H, Py-H), 8.45 (s, 1H, NH), 8.87 (s, 1H, NH), 9.14 (d,J = 6.8
Hz, 1H, Py-H), 11.01 (s, 1H, NH). MS (ESI), m/z: 423 (M+1)+ . Elemental anal. (%), calculated: C, 56.81;
H, 3.58; N, 19.88; found: C, 56.75; H, 3.45; N, 20.01.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-m-tolylsemicarbazide 4f: white crys-
tal, yield 84.00%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 2.25 (s, 3H, CH3), 6.78 (d, J = 7.3 Hz,
1H, Ph-H), 7.13 (t, J = 8.5 Hz, 1H, Ph-H), 7.22–7.31 (m, 3H, 1Py-H, 2Ph-H), 7.83 (d,J = 7.3 Hz, 1H, Py-H),
8.40 (s, 1H, NH), 8.75 (s, 1H, NH), 9.13 (d,J = 6.8 Hz, 1H, Py-H), 10.99 (s, 1H, NH). MS (ESI), m/z: 345
(M+1)+ . Elemental anal. (%), calculated: C, 52.26; H, 3.80; N, 24.38; found: C, 52.34; H, 3.99; N, 24.45.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2,6-dichloro-4-(trifluoromethyl)
phenyl)semicarbazide 4g: white crystal, yield 91.5%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ:
7.19 (t, J = 7.0 Hz, 1H, Py-H), 7.77 (m, 2H, 1Py-H, 1Ph-H), 8.01 (s, 2H, 1Ph-H, 1NH), 8.89 (s, 1H, NH), 9.15
(d,J = 7.0 Hz, 1H, Py-H), 10.56 (s, 1H, NH). MS (ESI), m/z: 489 (M+Na)− . Elemental anal. (%), calculated:
C, 38.53; H, 1.72; N, 17.97; found: C, 38.73; H, 1.97; N, 18.21.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(3,5-dichlorophenyl)semicarbazide 4h:
yellow green crystal, yield 72.03%, mp 260–265 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 7.25 (t, J = 7.2 Hz,
1H, Py-H), 7.6 (m, 1H, Ph-H), 7.77–7.88 (m, 3H, 1Py-H, 2Ph-H), 8.56 (s, 1H, NH), 9.15 (d,J = 4.0 Hz, 1H,
Py-H), 9.46 (s, 1H, NH), 11.11 (s, 1H, NH). MS (ESI), m/z: 398 (M–1)− . Elemental anal. (%), calculated: C,
42.08; H, 2.27; N, 21.03; found: C, 42.22; H, 2.35; N, 21.22.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2,3-dimethylphenyl)semicarbazide 4i:
light yellow crystal, yield 85.66%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 2.12 (s, 3H, CH3), 2.25
(s, 3H, CH3), 6.92 (m, 1H, Ph-H), 7.04 (t, J = 7.6 Hz, 1H, Ph-H), 7.25 (t, J = 7.2 Hz, 1H, 1Py-H), 7.83 (d,J
= 7.3 Hz, 1H, Py-H), 8.05 (s, 1H, Ph-H), 8.15 (s, 1H, NH), 8.51 (s, 1H, NH), 9.15 (d,J = 7.0 Hz, 1H, Py-H),
11.01 (s, 1H, NH). MS (ESI), m/z: 359 (M+1)+ . Elemental anal. (%), calculated: C, 53.56; H, 4.21; N, 23.42;
found: C, 53.45; H, 4.45; N, 23.65.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2-methyl-4-nitrophenyl)semicarba-
zide 4j: light yellow crystal, yield 94.82%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 2.38 (s, 3H,
CH3), 7.26 (t, J = 7.1 Hz, 1H, Py-H), 7.84 (d,J = 7.3 Hz, 1H, Py-H), 7.99 (m, 3H, Ph-H) 8.50 (s, 1H, NH),
9.05 (s, 1H, NH), 9.14 (d,J = 6.8 Hz, 1H, Py-H), 11.19 (s, 1H, NH). MS (ESI), m/z: 390 (M+1)+ . Elemental
anal. (%), calculated: C, 46.22; H, 3.10; N, 25.16; found: C, 46.13; H, 3.43; N, 25.25.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2-methyl-3-(trifluoromethyl)phenyl)
semicarbazide 4k: light yellow crystal, yield 89.00%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ:
2.32 (s, 3H, CH3), 7.26 (t, J = 7.1 Hz, 1H, Py-H), 7.38 (t, J = 7.5 Hz, 1H, Ph-H), 7.48 (t, J = 7.8 Hz, 2H,
Ph-H), 7.84 (d,J = 7.3 Hz, 1H, Py-H), 8.47 (s, 1H, NH), 8.72 (s, 1H, NH), 9.15 (d,J = 6.8 Hz, 1H, Py-H),
11.10 (s, 1H, NH). MS (ESI), m/z: 413 (M+1)+ . Elemental anal. (%), calculated: C, 46.56; H, 2.93; N, 20.36;
found: C, 46.76; H, 2.79; N, 20.51.
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1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(2-(trifluoromethoxy)phenyl)semi-
carbazide 4l: white crystal, yield 86.96%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 7.12 (t, J =
7.1 Hz, 1H, Ph-H), 7.25 (t, J = 7.1 Hz, 1H, Py-H), 7.35 (m, 2H, Ph-H), 7.83 (d,J = 7.2 Hz, 1H, Py-H), 8.16
(d,J = 9.6 Hz, 1H, Ph-H), 8.58 (s, 1H, NH), 8.94 (s, 1H, NH), 9.14 (d,J = 6.9 Hz, 1H, Py-H), 11.13 (s, 1H,
NH). MS (ESI), m/z: 415 (M+1)+ . Elemental anal. (%), calculated: C, 43.44; H, 2.43; N, 20.26; found: C,
43.65; H, 2.62; N, 20.34.
1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)-4-(naphthalen-1-yl)semicarbazide 4m:
light yellow crystal, yield 93.00%, mp > 300 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ: 7.26 (t, J = 7.2
Hz, 1H, Py-H), 7.50 (m, 4H, Ph-H), 7.70 (t, J = 8.4 Hz, 1H, Ph-H), 7.83 (d,J = 7.3 Hz, 1H, Py-H), 7.93 (d,J
= 8.2 Hz, 1H, Ph-H), 8.13 (d,J = 8.0 Hz, 1H, Ph-H), 8.67 (s, 1H, NH), 8.91 (s, 1H, NH), 9.17 (d,J = 7.2 Hz,
1H, Py-H), 11.15 (s, 1H, NH). MS (ESI), m/z: 381 (M+1)+ . Elemental anal. (%), calculated: C, 56.78; H,
3.44; N, 22.07; found: C, 56.96; H, 3.53; N, 22.25.
4-(2-chloro-5-(trifluoromethyl)phenyl)-1-(8-chloro-[1,2,4]triazolo[4,3-a]pyridine-3-carbonyl)
semicarbazide 4n: light yellow crystal, yield 57.10%, mp 252–255 ◦ C; 1 H NMR (DMSO-d6 , 400 MHz), δ:
4.72 (d, J = 6.4 Hz, 2H, NH), 7.20 (t, J = 7.2 Hz, 1H, Py-H), 7.46 (m, 1H, Ph-H), 7.77 (d,J = 7.2 Hz, 1H,
Py-H), 8.54 (s, 1H, Ph-H), 9.15 (d,J = 7.2 Hz, 1H, Py-H), 9.50 (s, 1H, Py-H), 10.54 (s, 1H, NH). MS (ESI),
m/z: 434 (M+1)+ . Elemental anal. (%), calculated: C, 41.59; H, 2.09; N, 19.40; found: C, 41.55; H, 2.23; N,
19.54.
Acknowledgments
This work was supported financially by the National Natural Science Foundation of China (No. 21002090), the
Key Innovation Team of Science and Technology in Zhejiang Province (2010R50018-06), and the National Key
Technologies R&D Program (2011BAE06B03-01).
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