GENERAL METHOD: Arylamine (10 mmol) was dissolved in a mixture of 5 mL of distilled water with 3.4 mL of 50% hydrofluoroboric acid. After cooling the reaction mixture to 0°C using an ice bath, sodium nitrite solution (0.69 g dissolved in 2 mL of distilled water) was added dropwise over 5 min. The resulting mixture was stirred for 1 h. The precipitate was collected by filtration and redissolved in a minimal amount of acetone. Subsequently, diethyl ether was added until the aryl diazonium tetrafluoroborate precipitate precipitated, the precipitate was filtered, washed several times with ether and dried under vacuum. TYPICAL REACTION PROCEDURE: The diazonium tetrafluoroborate (0.5 mmol) and pinacol ester of bisboronic acid (B2pin2, 1.5 mmol) were transferred to an oven-dried reaction tube under air atmosphere. Acetone/water (2:1 volume ratio, 4 mL) was added to the reaction tube via syringe. The sealed reaction tube was kept at 20°C and stirred for 1-2 hours. Upon completion of the reaction, dichloromethane was added for extraction, and the organic phases were combined and dried with anhydrous sodium sulfate (Na2SO4). Finally, the target product 4-methylthiophenylboronic acid pinacol ester was isolated and purified by silica gel column chromatography (eluent was petroleum ether/ethyl acetate).
