General procedure for the synthesis of 2-chloropyridine-5-sulfonyl chloride from 2-chloro-5-aminopyridine: 10 g (79.2 mmol) of 2-chloro-5-aminopyridine was dissolved in a mixture of concentrated hydrochloric acid (16 mL) and glacial acetic acid (89 mL), and cooled to 10 °C in an ice bath. 5.46 g (79.2 mmol) of sodium nitrite was added in batches and the reaction temperature was controlled to be below 15 °C. After stirring the reaction mixture for 1 hour, it was slowly added dropwise to a solution containing sulfur dioxide (carrier 27 mL), 2.77 g (16.2 mmol) of sulfur dioxide, copper(II) chloride dihydrate, and a solution consisting of acetic acid (59 mL) and water (11 mL), with the dropwise addition time being controlled to be between 5 and 10 minutes. The reaction mixture was gradually warmed to room temperature and subsequently poured into an ice-water mixture (300 mL) and stirring was continued for 15 minutes. The precipitate was isolated by filtration, washed with water (2 x 100 mL) and dried under reduced pressure to give 12.99 g of 2-chloropyridine-5-sulfonyl chloride. The product was characterized by 1H NMR (400 MHz, chloroform-d): δ 7.62 (1H, d, J = 8.31 Hz), 8.26 (1H, dd, J = 8.56,2.69 Hz), 9.04 (1H, d, J = 2.69 Hz).
