2. Materials and Methods
Chemicals were purchased from Sigma-Aldrich Company (Chemie GmbH, Taufkirchen, Germany). 1H- and 13C-NMR spectra were recorded in CDCl3 and DMSO-d6 on a Jeol Spectrometer (Jeol, Tokyo, Japan) (400 MHz and 500 MHz). Infrared spectra were recorded on a Thermo Scientific Nicolet iS10 FT-IR spectrometer (Thermo Fisher Scientific, Waltham, MA, USA). X-ray diffraction data were collected on a Rigaku Oxford Diffraction Supernova diffractometer and processed with CrysAlisPro software v. 1.171.41.93a (Rigaku Oxford Diffraction, Yarnton, UK, 2020) using Cu Kα radiation.
Compounds
2a,
b and
3a,
b, which were required for this study, were prepared according to reported literature [
25]. The synthetic pathway for the desired compounds are depicted in
Scheme 1.
General procedure (GP1) for the synthesis of compounds 4a,b
For the synthesis of monohydrazino s-triazine derivatives, 4a,b excess hydrazine hydrate-80% (5.0 equiv.) was added dropwise to a solution of disubstituted s-triazine derivative 3a,b (1.0 equiv.) in refluxing ethanol overnight, the evolution of the reaction was monitored by TLC (ethyl acetate/hexane 3:7). After completion of the reaction, the solution was cooled, and the precipitate was filtered and washed with diethyl ether to afford monohydrazino s-triazine derivatives as white solids in a very good yield and purity. Spectroscopic data fully agreed with the proposed structures. All monohydrazino s-triazine derivatives were used directly without further purification in the next step to synthesize the corresponding pyrazolo s-triazene derivatives.
4-Hydrazinyl-N-phenyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine 4a
The reaction between 4-chloro-N-phenyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine 3a (1.45 g, 5 mmol) and excess hydrazine hydrate (1.26 mL, 25.0 mmol) was carried out in refluxing ethanol (30 mL) for 12 h according to the general procedure (GP1). TLC analysis (ethyl acetate/hexane 3:7) showed a single spot which indicated completion of the reaction and formation of the product 4a as a pure compound. Thereafter, the resulting white solid was filtered and used in the next step without further purification (1.10 g, 3.86 mmol, 77%), m.p. 136–140 °C; IR (KBr, cm−1): 3288, 3160 (2NH), 1586 (C=N), 1506–1440 (aromatic C-C); 1H-NMR (400 MHz, DMSO-d6) δ 9.00 (s, 1H, -NHNH2), 7.95 (s, 1H, -NH-), 7.74 (d, J = 8.0 Hz, 2H, H-2, H-6), 7.23 (t, J = 7.7 Hz, 2H, H-3, H-5), 6.90 (t, J = 7.3 Hz, 1H, H-4), 3.72 (s, 4H, 2NCH2-), 1.61 (q, J = 6.2 Hz, 2H, -CH2-), 1.52–1.45 (m, 4H, 2-CH2-); 13C-NMR (126 MHz, DMSO-d6) δ 168.19, 164.62, 164.40 (3C=N), 141.13 (C1), 128.85 (C3,5), 121.71 (C4), 119.91 (C2,6), 44.11 (2CH2N), 25.99 (2-CH2-), 24.92 (-CH2); Chemical Formula: C14H19N7.
N-(4-Bromophenyl)-4-hydrazinyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine
The reaction of N-(4-bromophenyl)-4-chloro-6-(piperidin-1-yl)-1,3,5-triazin-2-amine 3b (1.843 g, 5.00 mmol) and excess hydrazine hydrate (1.26 mL, 25.0 mmol) in refluxing ethanol (35 mL) was carried out according to the general procedure (GP1). After 12 h, the TLC analysis (ethyl acetate/hexane 3:7) provided evidence that the desired hydrazine derivative 4b was produced in its pure form. Ethanol and excess hydrazine were evaporated by vacuo as a part of work up procedure followed by an addition of 15 mL of n-hexane to the product as a white precipitate, which was collected by filtration (1.55 g, 4.75 mmol, 95%); 198–201 °C; IR (KBr, cm−1): 3224, 3230 (2NH), 1581 (C=N), 1506–1440 (aromatic C-C); 1H-NMR (400 MHz, DMSO-d6) δ 9.16 (s, 1H, -NHNH2), 7.97 (s, 1H, -NH-), 7.74 (d, J = 8.5 Hz, 2H, H-2, H-6), 7.38 (d, J = 8.6 Hz, 2H, H-3, H-5), 4.19 (s, 1H, -NH), 3.71 (s, 4H, 2NCH2-), 1.60 (q, J = 5.8 Hz, 2H, -CH2-), 1.51–1.44 (m, 4H, 4H, 2-CH2-); 13C-NMR (101 MHz, DMSO-d6) δ 168.35, 164.64 (3C=N), 140.71 (C1), 131.57 (C4), 121.80 (C3,5), 113.12 (C2,6), 44.18 (2CH2N-), 26.04 (2-CH2-), 24.96 (-CH2-); Chemical Formula: C14H18BrN7.
General procedure (GP2) for the synthesis of compounds 5a,b
For the synthesis of mono-pyrazole s-triazine derivatives 5a,b, a solution of acetyl acetone (1.2 equiv.) in DMF was added to a stirring solution of monohydrazino s-triazene derivative 4a,b (1 equivalent) in DMF at rt. The reaction mixture was treated with triethylamine (0.8 equiv.) and was allowed to reflux while stirring overnight. It was noticed that the reactivity of bromo-hydrazino s-triazine derivative 5b was relatively lower than that of the other hydrazino s-triazine derivatives, which required starting with excess amounts of acetylacetone (1.8 equiv.) and triethylamine (1.2 equiv.) for each equivalent of 4b. After completion of the reaction as identified by TLC analysis (ethyl acetate/hexane 5:5), it was allowed to cool down to rt., and then a small amount of ice-cold water was added with continuous stirring for 30 min. Thereafter, the reaction mixture was placed into an ice bath for 1 h and the precipitate of the product was separated by filtration and washed with several portions of ice-cold water and then dried overnight under vacuum to afford the product a very good yield. Some of the final products needed further purification by column chromatography on silica gel (gradient 20–60% EtOAc in hexane).
4-(3,5-Dimethyl-1H-pyrazol-1-yl)-N-phenyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine
According to the general procedure (GP2), 4-hydrazinyl-N-phenyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine 4a (428 mg, 1.50 mmol) was reacted with acetylacetone (180 mg, 1.80 mmol) in the presence of triethylamine (121 mg, 1.20 mmol) in refluxing DMF (10 mL). The reaction progress was monitored by TLC (ethyl acetate/hexane 5:5), which indicated the completion of the reaction after 8 h. Subsequently, the reaction mixture was stirred with ice water and placed in an ice bath for about 30 min to create the white precipitate of compound 5a, which was filtered, washed with water, and dried under vacuum (503 mg, 1.44 mmol, 96%); m.p. 195–197 °C; IR (KBr, cm−1): 3440 (NH), 1588 (C=N), 1512–1440 (aromatic C-C); 1H-NMR (400 MHz, CDCl3) δ 7.57 (s, 2H, H-2, H-6), 7.33 (s, 2H, H-3, H-5), 7.05 (s, 1H, H-4), 6.00 (s, 1H, H-4; Pyrazole), 3.87–3.78 (m, 4H, 2NCH2-), 2.66 (s, 3H, H-b), 2.32 (s, 3H, H-a), 1.71 (s, 2H, -CH2-), 1.66 (s, 4H, 2-CH2-); 13C-NMR (101 MHz, CDCl3) δ 164.75, 164.19, 162.63 (3C=N), 151.58 (C5; Pyraz.), 143.37 (C3; Pyraz.), 138.82 (C1, Ph), 128.94 (C3,5; Ph), 123.13 (C4, Ph), 119.99 (C2,6; Ph), 110.84 (C4, Pyraz.), 45.09 (2CH2N-), 25.83 (2-CH2-), 24.75 (-CH2-), 16.21, 14.10 (2CH3; Pyraz.); Chemical Formula: C19H23N7.
Compound 5a was recrystallized from ethanol:
About 15 mL of absolute ethanol was added to compound 5a and the mixture was heated gently until the entire amount dissolved completely. Thereafter, the solution was allowed to cool at rt. overnight to afford the product its crystalline form.
N-(4-Bromophenyl)-4-(3,5-dimethyl-1H-pyrazol-1-yl)-6-(piperidin-1-yl)-1,3,5-triazin-2-amine
The reaction of N-(4-bromophenyl)-4-hydrazinyl-6-(piperidin-1-yl)-1,3,5-triazin-2-amine 4b (728 mg, 2.00 mmol) and acetylacetone (360 mg, 3.6 mmol) in the presence of triethylamine (243 mg, 2.40 mmol) was carried out in refluxing DMF (15 mL) while stirring overnight. Once the reaction was complete based on TLC analysis (ethyl acetate/hexane 5:5), it was cooled in an ice bath while stirring with 15 mL of ice water to afford the product 5b as a white precipitate, which was separated by filtration, washed with water, and dried under vacuum (668 mg, 1.57 mmol, 79%); m.p. 198–201 °C; IR (KBr, cm−1): 3444 (NH), 1585 (C=N), 1530–1480 (aromatic C-C); 1H-NMR (400 MHz, CDCl3) δ 7.44–7.46 (d, J = 13.3 Hz, 4H, H-2, H-3, H-5, H-6), 6.00 (s, 1H, H-4; Pyrazole), 3.81 (s, 4H, 2NCH2-), 2.65 (s, 3H, H-b), 2.31 (s, 3H, H-a), 1.71 (s, 2H, -CH2-), 1.64 (s, 4H, 2-CH2-); 13C-NMR (126 MHz, CDCl3) δ 160.70–168.00 (3C=N), 151.02 (C5; Pyraz.), 142.37 (C3; Pyraz.), 136.56 (C1, Ph), 130.78 (C3,5; Ph), 120.65 (C2,6; Ph), 114.82 (C4; Ph), 110.10 (C4; Pyraz.), 44.22 (2CH2N-), 24.73 (2-CH2-), 23.58 (-CH2-), 15.06, 13.02 (2CH3; Pyraz.); Chemical Formula: C19H22BrN7.
Compound 5b was recrystallized from DCM/Diethyl ether:
Compound 5b was dissolved in about 10 mL of DCM with gentle heating until the dissolution was complete, then a layer of diethyl ether (50 mL) was allowed to slide through the flask. The crystals of compound 5b were collected after 48 h.
Crystal structure determination
The technical experimental method for the mounting data and solved the chemical structures [
30,
31,
32,
33] are amended in the
Supplementary Materials (SI). The crystallographic details are summarized in
Table 1.
Computational Study