The general procedure for the synthesis of 4-indoleboronic acid pinacol ester from 4-bromoindole and biboronic acid pinacol ester is as follows:
(a) Preparation of 4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-1H-indole. 4-Bromoindole (1.00 g, 5.10 mmol), bis(pinacolato)diboron (1.68 g, 6.63 mmol), potassium acetate (1.44 g, 15.3 mmol), and dichlorodi(diphenylphosphine)palladium(II) dichloride dichloromethane complex (206 mg, 0.26 mmol) were added to a 50 mL round-bottomed flask under argon gas protection. Anhydrous dimethyl sulfoxide (16 mL) was then added and the reaction mixture was heated at 90 °C with stirring for 4 hours. After completion of the reaction, it was cooled to room temperature, filtered through a short column of silica gel and the filter cake was washed with tert-butyl methyl ether (2 x 50 mL). The filtrates were combined, washed with saturated saline (3 x 50 mL), dried over anhydrous sodium sulfate and concentrated under reduced pressure. The crude product was purified by fast column chromatography (ethyl acetate/heptane, 1:4) to afford 4-(4,4,5,5-tetramethyl-[1,3,2]dioxaborolan-2-yl)-1H-indole as a white solid (1.24 g, quantitative yield).
