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3,6-Dichloropyridazine-4-carboxamide (5.7 g, 29.69 mmol, 1.00 eq.) was mixed with phosphorus trichloride (50 mL) and heated to reflux for 1 hour. Upon completion of the reaction, the mixture was concentrated under reduced pressure to remove excess phosphorous trichloride. The concentrated residue was dissolved in dichloromethane and washed sequentially with water and saturated saline to remove water-soluble impurities. The organic phase was dried with anhydrous sodium sulfate, filtered and concentrated under reduced pressure to give the crude product. The crude product was purified by silica gel column chromatography with the eluent being a solvent mixture of ethyl acetate and petroleum ether (1:5, v/v). The final product was 3,6-dichloropyridazine-4-carbonitrile (5.1 g, 99% yield) as a light yellow solid.

[1] Patent: WO2015/25025, 2015, A1. Location in patent: Page/Page column 408
[2] Patent: WO2017/133665, 2017, A1. Location in patent: Page/Page column 40