General procedure for synthesis
4-bromo-3,5-dimethyl-1-aryl-1H-pyrazole derivatives (4a-b):
A mixture of 3,5-dimethyl-1-aryl-1H-pyrazole (23 mmol) and NBS (132
mmol) in acetone (100 mL) was irradiated under an ultrasonic wave in a
25-30oC bath for 2 h. The solution was concentrated
and reaction mixture was extracted with EtOAc. After the resulting
residue was purified using column (EtOAc:hexane; 2:8)
4-bromo-3,5-dimethyl-1-phenyl-1H-pyrazole (4a): Yield 75%,
brown solid, 1H NMR (400 MHz, CDCl3) δ
7.40-7.29 (m, 5H), 2.28 (s, 3H), 2.25 (s,3H). 13C NMR
(101 MHz, CDCl3) δ 147.47, 139.76, 137.43, 129.13,
127.75, 124.58, 96.43, 12.35, 11.74.
4-bromo-1-(2,4-dinitrophenyl)-3,5-dimethyl-1H-pyrazole (4b):Yield 70%, bright yellow solid 1H NMR (400 MHz,
CDCl3) δ 8.85 (d, J = 4Hz, 1H), 8.58 (dd,J = 8.0, 4.0 Hz, 1H), 7.71 (d, J = 8.0 Hz, 1H), 2.29 (s,
6H). 13C NMR (101 MHz, CDCl3) δ
152.78, 144.94, 138.31, 130.18, 129.56, 123.45, 116.31, 77.33, 77.22,
77.01, 76.70, 48.27, 16.01.