A novel one-pot approach to oxidative aromatization and bromination of pyrazolidin-3-one with HBr-H2O2 system
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Shanguang Yang
Abstract
An efficient and green one-pot method for the oxidative aromatization and bromination of pyrazolidin-3-ones under mild conditions with a HBr-H2O2 system was developed. A mechanism was proposed.
Introduction
Pyrazole derivatives attract attention due to their excellent pharmacological properties [1], [2], [3], [4]. Halogenated pyrazolols [5], [6], especially 4-bromo derivatives, are precursors for the synthesis of functionalized pyrazoles [7], [8], [9], such as pharmaceuticals [10], multifunctional materials [11], [12], fused- [13] and spiro-heterocyclic compounds [14]. Various protocols for the synthesis of brominated pyrazolols have been reported. However, bromination of pyrazolols with Br2 in acetic acid is carried out under harsh conditions and requires careful manipulation [15]. Efforts have been made by Ahmed et al. [16] to use the photolysis of N-bromosuccinimide as the source of bromine [16]. Due to the formation of the coupling by-products, bromopyrazolols are afforded in low yields. Although N-bromobenzamide [17] and dibromoisocyanuric acid [18] have also been explored as mild brominating agents for pyrazolols, these approaches have met with limited success.
Recently, pyrazolidinones have been reported as substrates for the synthesis of pyrazolols [7], [11], [12], [19], [20], [21], [22], [23]. Unfortunately, few efficient methods for direct access to 4-bromopyrazolol from pyrazolidinone have been reported. Traditionally, there are two steps in the synthesis of 4-bromopyrazolol from pyrazolidinone [7], [11], [12] that involve aromatization of pyrazolidin-3-ones to give pyrazol-3-ols and bromination of pyrazol-3-ols [7]. This protocol requires the use of a transition-metal catalyst, extremely toxic liquid bromine, a long reaction time, multi-step manipulations and the final yield is only moderate. In recent years, brominations with safe bromination reagents and green co-oxidants have received growing attention because of their greenness and high efficiency [24]. Herein, we report a novel one-pot, metal-free, atom-economic and highly effective method for the preparation of 4-bromopyrazol-3-ol from pyrazolidin-3-one using a HBr-H2O2 system under mild conditions.
Results and discussion
The starting compounds 1 (Scheme 1) were commercially available or easily prepared according to previously published procedures [7], [11], [12], [19], [20], [21], [22], [23]. Initially, 1-(4-chlorophenyl)-pyrazolidin-3-one (1a) was treated with various amounts of hydrogen peroxide and hydrobromic acid using various solvents. It was found that the use of different solvents including carbon tetrachloride, chloroform, dichloromethane, methanol and N,N-dimethylformamide had little effect on the outcome, affording the yield of the product 2a in the range from 72% to 90%. Nevertheless, the highest yield of 90% was obtained for the reaction conducted in chloroform. Under optimized conditions, the synthesis of 2a was conducted in chloroform at 60°C using 3 equivalents of H2O2 and 1 equivalent of HBr.
Decreasing the amount of hydrogen peroxide to 1 equivalent or 2 equivalents resulted in a decrease of the yield of the product 2a. On the other hand, an increase in the amount of hydrogen peroxide from 3 equivalents to 4 equivalents did not affect the yield. With the optimized reaction conditions for 2a, a wide range of 4-bromo-3-hydroxypyrazolidines were synthesized (Scheme 1). The reaction proceeds well with pyrazolidin-3-ones containing a phenyl group, a substituted phenyl group or a pyridinyl group. With an increase in the electron-donating capability of the substituent at the 5-position of pyrazolidin-3-ones (MeO>CH3>H), the efficiency of the reaction is increased in the same order.
Interestingly, when the reaction with 5 equivalents of HBr was carried out for 3 h (Scheme 2), the corresponding dibrominated products 3b,j were obtained accompanied by the corresponding monobrominated products 2b,j in moderate yields. However, in the case of compound 1i only monobrominated product 2i was acquired in high yield. For further confirmation of the structure of the series, a single crystal of 3j was obtained and subjected to X-ray diffraction analysis (Figure 1).
Molecular structure of 3j with atom labeling.
To gain access to the mechanism, the experiments were carried out as shown in Scheme 3. When the reaction under optimized conditions was performed in the presence of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) as a radical scavenger, only a trace amount of 1-(4-chlorophenyl)-1H-pyrazol-3-ol (4) was detected, suggesting a radical pathway. This finding and previous reports [25], [26], [27], are consistent with the mechanism suggested in Scheme 4. First, compound 1 undergoes a reaction with a bromine radical to generate the corresponding radical intermediate I, which is subsequently converted to the substituted pyrazol-3-ol II by oxidation with molecular bromine. Then, in the presence of the bromine radical, the corresponding radical species III is generated from the intermediate compound II. Finally, the reaction of the intermediate product III with the bromide radical furnishes the observed 4-bromopyrazol-3-ol 2.
Conclusions
A green and efficient protocol to prepare substituted 4-bromo-pyrazol-3-ols 2 from pyrazolidin-3-ones 1 under mild conditions in excellent yields was described. The corresponding dibromination products 3b,j were also obtained in moderate yields under harsh conditions.
Experimental
Proton nuclear magnetic resonance (1H NMR) spectra were obtained on a Bruker spectrometer operating at 600 MHz with dimethyl sulfoxide-d6 as solvent and tetramethylsilane as internal standard. Infrared (IR) spectra were obtained using KBr disks on a Bomem Michelson Series Fourier-transform infrared spectrometer. The mass spectrometry (MS) data were recorded with an Agilent 7890A-5975C instrument. Melting points were obtained on an X-4 microscope electrothermal apparatus (Taike, China) and are uncorrected. Elemental analyses were carried out on a Vario EL III analyzer. Spectral data for compounds 2b, 3b and 4 are virtually identical with the reported values [9], [20], [28].
Synthesis of 4-bromopyrazol-3-ones 2a–l
A mixture of 1a–l (0.01 mol) in CHCl3 (10 mL) and 40% hydrobromic acid (40%, 0.01 mol) was stirred at 60°C and treated dropwise for 10 min with an aqueous solution of H2O2 (30%, 0.03 mol), and stirring was continued for an additional 20 min. The resultant precipitate of 2a–l was filtered off and crystallized from ethanol. The filtrate was concentrated and the residue was subjected to silica gel chromatography eluting with ethyl acetate/petroleum ether (10:1) to give an additional amount of 2a–l.
4-Bromo-1-(4-chlorophenyl)-1H-pyrazole-3-ol (2a)
White solid; mp 192–194°C; yield 90%; 1H NMR: δ 11.13 (s, 1H, OH), 8.58 (s, 1H, CH), 7.71 (d, J=9 Hz, 2H, Ar), 7.51 (d, J=9 Hz, 2H, Ar); IR: 3440, 2971, 1617, 1563, 1494, 1392, 1306, 1105 cm−1; MS: m/z 274.3 [(M+1)+, 100%]. Anal. Calcd for C9H6BrClN2O: C, 39.52; H, 2.21; N, 10.24. Found: C, 39.81; H, 2.03; N, 10.02.
4-Bromo-1-phenyl-1H-pyrazole-3-ol (2b)
Yellow solid; mp 188–190°C; yield 91%; 1H NMR: δ 11.02 (s, 1H, OH), 8.54 (s, 1H, CH), 7.69 (d, J=8 Hz, 2H, Ar), 7.45 (t, J=8 Hz, 2H, Ar), 7.23 (t, J=8 Hz, 1H, Ar); IR: 3449, 2927, 1620, 1550, 1493, 1309, 1102 cm−1; MS: m/z 240.1 [(M+1)+, 100%].
4-Bromo-1-(4-fluorophenyl)-1H-pyrazole-3-ol (2c)
White solid; mp 219–221°C; yield 89%; 1H NMR: δ 11.01 (s, 1H, OH), 8.51 (s, 1H, CH), 7.70 (m, 2H, Ar), 7.30 (t, J=9 Hz, 2H, Ar); IR: 3451, 2971, 1622, 1568, 1519, 1404, 1311, 1240, 1087 cm−1; MS: m/z 258.1 [(M+1)+, 100%]. Anal. Calcd for C9H6BrFN2O: C, 42.05; H, 2.35; N, 10.90. Found: C, 42.22; H, 2.21; N, 10.68.
4-Bromo-1-(4-methylphenyl)-1H-pyrazole-3-ol (2d)
Yellow solid; mp 183–185°C; yield 92%; 1H NMR: δ 10.93 (s, 1H, OH), 8.47 (s, 1H, CH), 7.56 (d, J=8 Hz, 2H, Ar), 7.24 (d, J=8 Hz, 2H, Ar), 2.31 (s, 3H, CH3); IR: 3454, 2964, 1625, 1548, 1381, 1327, 1082 cm−1; MS: m/z 254.2 [(M+1)+, 100%]. Anal. Calcd for C10H9BrN2O: C, 47.46; H, 3.58; N, 11.07. Found: C, 47.31; H, 3.73; N, 10.88.
4-Bromo-1-(3-chloropyridin-2-yl)-1H-pyrazole-3-ol (2e)
Yellow solid; mp 233–235°C; yield 91%; 1H NMR: δ 11.48 (s, 1H, OH), 8.49 (s, 1H, CH), 7.98 (d, J=8 Hz, 1H, Ar), 7.60 (d, J=8 Hz, 1H, Ar), 7.37 (d, J=8 Hz, 1H, Ar); IR: 3451, 2965, 1626, 1582, 1444, 1389, 1305, 1261, 1068 cm−1; MS: m/z 273.1 [(M+1)+, 100%]. Anal. Calcd for C8H5BrClN3O: C, 35.00; H, 1.84; N, 15.31. Found: C, 34.83; H, 2.02; N, 15.50.
4-Bromo-5-methyl-1-(4-chlorophenyl)-1H-pyrazol-3-ol (2f)
Yellow solid; mp 221–223°C; yield 92%; 1H NMR: δ 10.80 (s, 1H, OH), 7.55 (m, 2H, Ar), 7.51 (m, 2H, Ar), 2.28 (s, 3H, CH3); IR: 3455, 2978, 1617, 1545, 1510, 1393, 1102 cm−1; MS: m/z 288.2 [(M+1)+, 81%]. Anal. Calcd for C10H8BrClN2O: C, 41.77; H, 2.80; N, 9.74. Found: C, 41.61; H, 2.93; N, 9.89.
4-Bromo-5-phenyl-1-(4-chlorophenyl)-1H-pyrazol-3-ol (2g)
White solid; mp 253–255°C; yield 94%; 1H NMR: δ 11.07 (s, 1H, OH), 7.43 (m, 3H, Ar), 7.39 (d, J=9 Hz, 2H, Ar), 7.30 (m, 2H, Ar), 7.21 (d, J=9 Hz, 2H, Ar); IR: 3462, 2973, 1552, 1496, 1331, 1075 cm−1; MS: m/z 350.3 [(M+1)+, 100%]. Anal. Calcd for C15H10BrClN2O: C, 51.53; H, 2.88; N, 8.01. Found: C, 51.34; H, 3.06; N, 8.22.
4-Bromo-5-methoxy-1-(4-chlorophenyl)-1H-pyrazol-3-ol (2h)
Yellow solid; mp 228–230°C; yield 95%; 1H NMR: δ 11.37 (s, 1H, OH), 7.65 (m, 2H, Ar), 7.38 (m, 2H, Ar), 3.74 (s, 3H, OCH3); IR: 3452, 2961, 1632, 1560, 1503, 1402, 1314, 1207, 1054 cm−1; MS: m/z 300.4 [(M+1)+, 100%]. Anal. Calcd for C10H8BrClN2O2: C, 39.57; H, 2.66; N, 9.23. Found: C, 39.48; H, 2.79; N, 9.38.
4-Bromo-1,5-diphenyl-1H-pyrazol-3-ol (2i)
Yellow solid; mp 242–244°C; yield 93%; 1H NMR: δ 10.98 (s, 1H, OH), 7.41 (m, 3H, Ar), 7.29 (m, 5H, Ar), 7.14 (d, J=8 Hz, 2H, Ar); IR: 3453, 2965, 1624, 1541, 1444, 1335, 1252, 1085 cm−1; MS: m/z 314.1 [(M+1)+, 100%]. Anal. Calcd for C15H11BrN2O: C, 57.16; H, 3.52; N, 8.89. Found: C, 57.37; H, 3.38; N, 8.71.
4-Bromo-5-methyl-1-phenyl-1H-pyrazol-3-ol (2j)
White solid; mp 218–220°C; yield 91%; 1H NMR: δ 10.71 (s, 1H, OH), 7.49 (m, 4H, Ar), 7.36 (t, J=7 Hz, 1H, Ar), 2.27 (s, 3H, CH3); IR: 3450, 2969, 1623, 1549, 1496, 1409, 1328, 1270, 1103 cm−1; MS: m/z 254.1 [(M+1)+, 100%]. Anal. Calcd for C10H9BrN2O: C, 47.46; H, 3.58; N, 11.07. Found: C, 47.63; H, 3.81; N, 10.86.
4-Bromo-5-methyl-1-(4-methylphenyl)-1H-pyrazol-3-ol (2k)
White solid; mp 232–234°C; yield 92%; 1H NMR: δ 10.64 (s, 1H, OH), 7.35–7.28 (m, 4H, Ar), 2.35 (s, 3H, CH3), 2.24 (s, 3H, CH3); IR: 3452, 2974, 1627, 1553, 1515, 1406, 1323, 1257, 1103 cm−1; MS: m/z 266.1 [(M+1)+, 100%]. Anal. Calcd for C11H11BrN2O: C, 49.46; H, 4.15; N, 10.49. Found: C, 49.63; H, 4.36; N, 10.06.
4-Bromo-5-methyl-1H-pyrazol-3-ol (2l)
White solid; mp 192–194°C; yield 85%; 1H NMR: δ 2.09 (s, 3H, CH); IR: 2060–3270, 1587, 1253, 1172, 1053 cm−1; MS: m/z 176.9659 [(M+1)+, 100%). Anal. Calcd for C4H5BrN2O: C, 27.14; H, 2.85; N, 15.83. Found: C, 27.26; H, 2.96; N, 15.66.
Synthesis of 4-bromo-1-(4-bromophenyl)pyrazol-3-ones 3b,j
A mixture of 1b,j (0.01 mol), CCl4 (10 mL) and hydrobromic acid (40%, 0.05 mol) was stirred, heated under reflux and treated dropwise with an aqueous solution of H2O2 (30%, 0.08 mol) for 3 h. After concentration under reduced pressure, the residue was chromatographed on silica gel eluting with ethyl acetate/petroleum ether (10:1) to give 3b,j.
4-Bromo-1-(4-bromophenyl)-1H-pyrazole-3-ol (3b)
Yellow solid; mp 199–201°C; yield 50%; 1H NMR: δ 11.17 (s, 1H, OH), 8.62 (s, 1H, CH), 7.69 (m, 4H, Ar); IR: 3458, 2962, 1624, 1556, 1494, 1390, 1302, 1209, 1048 cm−1; MS: m/z 318.9 [(M+1)+, 100%].
4-Bromo-5-methyl-1-(4-bromophenyl)-1H-pyrazol-3-ol (3j)
White solid; mp 243–245°C; yield 58%; 1H NMR: δ 10.82 (s, 1H, OH), 7.68 (d, J=9 Hz, 2H, Ar), 7.45 (d, J=9 Hz, 2H, Ar), 2.28 (s, 3H, CH3); IR: 3446, 2971, 1628, 1547, 1510, 1395, 1322, 1261, 1108 cm−1; MS: m/z 330.3 [(M+1)+, 100%]. Anal. Calcd for C10H8Br2N2O: C, 36.18; H, 2.43; N, 8.44. Found: C, 36.33; H, 2.61; N, 8.28.
1-(4-Chlorophenyl)-1H-pyrazol-3-ol (4)
White solid; mp 189–191°C; 1H NMR: δ 10.32 (s, 1H, OH), 8.24 (d, J=3 Hz, 1H, CH), 7.70 (d, J=9 Hz, 2H, Ar), 7.48 (d, J=9 Hz, 2H, Ar), 5.84 (d, J=3 Hz, 1H, CH); IR: 3438, 2976, 1630, 1545, 1489, 1382, 1245, 1057, 945, 757 cm−1; MS: m/z 195.7 [(M+1)+, 100%].
Acknowledgment
This project is supported by the Priority Academic Program Development of the Jiangsu higher education institutions (1107047002).
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Articles in the same Issue
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- Preliminary Communications
- Microwave-assisted synthesis of γ-thiolactams from ethyl isocyanoacetate
- Microwave-assisted synthesis of isoindolinones via Pd-mediated tandem coupling reactions
- Research Articles
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