The general procedure for the synthesis of 4-indoleboronic acid from 4-bromoindole was as follows: a mixture of potassium hydride (35% w/w, dispersed in mineral oil, 4.5 g, 39.3 mmol) and ether (105 mL) was added to a round-bottomed flask at 0 °C, followed by the slow addition of a solution of 4-bromo-1H-indole (7.0 g, 35.7 mmol) in ether (35 mL). The reaction mixture was transferred to a cold bath at -78 °C with continuous stirring for 45 min. A solution of tert-butyllithium (1.7 M in pentane, 46.2 mL, 78.5 mmol) pre-cooled to -78 °C was added to the reaction system via cannula. At -78 °C, tributyl borate (29 mL, 107.1 mmol) was added to the reaction slurry. The reaction system was gradually warmed to room temperature for 21 hours. Subsequently, the reaction mixture was placed in a 0 °C bath and phosphoric acid solution (350 mL, 1 M) was added. The mixture was stirred for 30 min and then extracted with ether, the organic phase was dried with anhydrous sodium sulfate and concentrated in vacuum to afford the target product 4-indoleboronic acid (4.01 g, 70% yield) as a beige solid. The product was characterized by 1H NMR ((CD3)2CO/15% D2O, 400 MHz) and mass spectrometry (MS).
