4-p-Tolylazo-5-trifluoromethyl-2,4-dihydropyrazol-3-one

To a cold solution of 5-trifluoromethyl-2,4-dihydropyrazol-3-one [1,2] (0.76 g, 5 mmol) in ethanol (25 ml) containing sodium acetate (0.82 g, 10 mol), p-tolyldiazonium chloride [ca. 5 mmol; prepared by adding a solution of sodium nitrite (5 mmol / 2 ml H₂O) to an ice cold solution of p-toluidine in 2 ml conc. HCl] was added dropwise with stirring at 0-5°C. The reaction mixture was stirred at room temperature for three hours and the precipitated crude product was filtered, washed with water, dried and crystallized from ethanol to give 1.0 g (74%) of 2 as orange crystals.

M.p. 180-181°C

IR (KBr, cm^-1): 3210 (NH), 1670 (CO pyrazolone).

MS (m/z): 270.

¹H-NMR (250 MHz, DMSO-d₆): 2.31 (s, 3H, CH₃); 2.45 (s, 1H, CH); 7.21 (dd, 4H, aromatic CH); 12.58 (s, 1H, NH).

¹³C-NMR (75 MHz, CDCl₃): 24.2 (CH₃); 51.5 (CH); 122.2 (2 aromatic carbons); 129.7 (2 aromatic carbons); 134.3, 143.7 (quaternary aromatic carbons); 135.7 (q, CF₃); 153.6 (C=N); 173.1 (CO).