4-Methyl-1,2-dihydropyridazine-3,6-dione (9.48 g, 75.2 mmol) was suspended in phosphoryl chloride (70 mL, 750 mmol) under nitrogen protection and stirred at room temperature. Subsequently, the reaction mixture was heated to a mild reflux condition and maintained for 4 hours until a golden yellow homogeneous solution was formed. Upon completion of the reaction, the mixture was cooled and the excess trichlorophosphorus was removed by distillation under reduced pressure (14 mbar, 50-70°C). The resulting viscous brown oil was slowly added dropwise to an ice-cooled saturated sodium bicarbonate solution (200 mL) while stirring vigorously. The pH of the mixture was adjusted to 6 by batchwise addition of solid sodium bicarbonate, followed by extraction with ethyl acetate (2 x 60 mL). The organic phases were combined, washed with saturated sodium bicarbonate solution (30 mL), dried over anhydrous magnesium sulfate, and concentrated under reduced pressure to afford 3,6-dichloro-4-methylpyridazine (11.5 g, 70.8 mmol, 94% yield) as a yellow powder. The product was recrystallized from light petroleum ether/ether and had a melting point of 87-88°C. IR spectrum (KBr, cm^-1): 3054, 1567, 1434, 1351, 1326, 1145, 1121, 914, 720. NMR hydrogen spectrum (200 MHz, CDCl3): δ 2.42 (3H, d, J = 1.0 Hz), 7.41 ( 1H, q, J = 0.9 Hz). NMR carbon spectrum (50 MHz, CDCl3): δ 19.2 (CH3), 130.1 (CH-5), 140.7 (C-4), 155.6 (C-6), 157.3 (C-3). Low resolution mass spectrum (EI): m/z 162 ([M+]). Calculated elemental analysis (C5H4Cl2N2): C, 36.84; H, 2.47; N, 17.19. Measured values: C, 36.93; H, 2.57; N, 17.48.
