[Synthesis]
General procedure for the synthesis of 5-bromoindole and triisopropyl borate as raw materials for the synthesis of 5-bromoindoleboronic acid: In a dry and nitrogen-protected three-necked flask, anhydrous THF (80 mL) solution of NaH (1.02 g, 42.5 mmol) was added, equipped with a magnetic stirrer and a septum. The reaction system was cooled to 0 °C under N2 atmosphere and 5-bromoindole (5.0 g, 25.5 mmol) was added slowly dropwise, and stirring was continued for 15 min after completion of the dropwise addition. Subsequently, n-BuLi (2.5 M hexane solution, 15 mL, 37.5 mmol) was added slowly and stirred for 10 min. Then, triisopropyl borate (22 mL, 51 mmol) was added dropwise. After completion of the dropwise addition, the reaction mixture was gradually warmed to room temperature and stirred overnight, and stirring was continued for 2 hours. After completion of the reaction, the reaction was quenched with saturated NH4Cl solution and stirred for 30 minutes. The organic layer was separated and the aqueous layer was extracted with EtOAc. The organic layers were combined, washed sequentially with water and brine and dried over Na2SO4. After complete removal of the solvent, the residue was treated with EtOAc/petroleum ether and filtered to afford the target product 1H-indol-5-ylboronic acid (b1) as a colorless solid (1.02 g, 25% yield). The structure of the product was determined by 1H-NMR (DMSO-d6) δ 11.1 (s, 1H), 8.30 (s, 1H), 7.81 (d, 1H, J = 8.1 Hz), 7.47 (d, 1H, J = 8.1 Hz), 7.34 (s, 1H) and 13C-NMR (100 MHz, DMSO-d6) δ 138.0, 128.0, 127.5, 127.4, 127.3, 125.4, 111.0, 102.0 confirmed. |