Synthesis
General procedure for the synthesis of 2-bromopyridine-5-boronic acid from 2,5-dibromopyridine and triisopropyl borate:
1. 2,5-dibromopyridine (2.37 g, 10 mmol) was dissolved in 90 mL of ether/THF (8:1) at -78 °C and 7.5 mL of n-BuLi (1.6 M hexane solution) was added dropwise slowly. After the dropwise addition was completed, the reaction mixture was continued to be stirred at -78°C for 2 hours.
2. Triisopropyl borate (4.49 g, 24 mmol) was added and the reaction mixture was stirred at -78 °C for 2 h, followed by slow warming to room temperature.
3. The reaction was quenched with 10 mL of water and stirring was continued overnight.
4. The organic solvent was evaporated and the remaining aqueous layer was adjusted to pH 10 with 5% NaOH and washed with ether (30 mL x 3).
5. The aqueous layer was carefully acidified to pH 4 with 48% HBr to precipitate the target product 2-bromopyridine-5-boronic acid (1.46 g, 72%).
NMR data (300 MHz, DMSO-d6): δ 8.68 (dd, J = 2.1, 0.7 Hz, 1H), 8.53 (br.s, 2H), 8.05 (dd, J = 2.1, 7.8 Hz, 1H), 7.67 (dd, J = 7.8, 0.7 Hz, 1H).
