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A 3-chloro-1H-indole-2-carboxylic acid amide (28.05 g, 144 mmol) was mixed with phosphorous trichloride (67.36 mL, 722 mmol) in chloroform (525 mL) and the reaction was carried out at reflux for 3 hours. Upon completion of the reaction, the reaction mixture was cooled to 20 °C and slowly poured into a pre-prepared mixture of 40 mL of 37% hydrochloric acid and 1000 g of ice, and stirring was continued for 2 hours. Subsequently, the organic layer was separated and the aqueous phase was extracted with chloroform (500 mL). The organic layers were combined, washed sequentially with water and saturated sodium carbonate solution, dried over anhydrous sodium sulfate and concentrated under reduced pressure. The concentrated residue was dissolved in a 1:1 mixed solvent of 2-propanol and water and crystallized to finally obtain 3-chloro-1H-indole-2-carbonitrile (20.89 g, 82.1% yield). Melting point: 150-154°C.

[1] Patent: WO2012/59776, 2012, A1. Location in patent: Page/Page column 38
[2] Patent: US2013/217702, 2013, A1. Location in patent: Paragraph 0116