General procedure for synthesis 3,5-dimethyl-1-aryl-1H-pyrazole derivatives (3a-b)
To a solution of acetylacetone (98 mmol) in ethanol (150 mL), aryl hydrazine (97.37 mmol) was slowly added. After cooling the reaction mixture to 0oC, 5 mL of concentrated hydrochloric acid was added. The reaction mixture was refluxed for 2 hours. After the reaction was complete, ethanol was evaporated, then the crude product was dissolved in DCM. The reaction mixture was treated with 10 % NaOH solution to slightly basic (pH around 9). After this mixture was extracted with dichloromethane (2x100 mL), the combined dichloromethane solution was washed with brine and dried over anhydrous sodium sulfate. The product was purified using column (EtOAc:hexane; 2:8)
3,5-dimethyl-1-phenyl-1H-pyrazole (3a): Yield 94%; red liquid;1H NMR (400 MHz, CDCl3) δ 7.38-7.23 (m, 5H), 5.94 (s, 1H), 2.25 (s, 3H), 2.24 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 148.92, 139.96, 139.34, 128.97, 127.20, 124.72, 106.93, 13.51, 12.36.
1-(2,4-dinitrophenyl)-3,5-dimethyl-1H-pyrazole (3b): Yield 90%; yellow solid; 1H NMR (400 MHz, CDCl3) δ: 8.80 (d, J = 4.0 Hz, 1H), 8.55 (dd,J = 8.0 Hz, 4.0 Hz,1H), 7.72 (d, J = 8.0 Hz, 1H), 6.12 (s, 1H), 2.29 (s, 3H), 2.27 (s, 3H). 13C NMR (101 MHz, CDCl3) δ 152.36, 146.22, 145.64, 140.90, 137.95, 129.33, 127.34, 120.99, 108.90, 77.38, 77.06, 76.74, 13.48, 11.61.