General procedure for the synthesis of 2-bromo-4-chloro-3-fluoropyridine from 4-chloro-3-fluoropyridine:
1. Intermediate 101A. 2-Bromo-4-chloro-3-fluoropyridine was prepared by slowly adding 1.6 M hexane solution of n-butyllithium (n-BuLi) (5.23 mL, 8.36 mmol) dropwise to a tetrahydrofuran (THF, 40 mL) solution of 2,2,6,6-tetramethylpiperidine (1.54 mL, 9.12 mmol) at -78 °C.
2. The reaction solution was warmed to 0 °C and stirred for 0.5 hours.
3. The reaction solution was again cooled to -78 °C and a solution of 4-chloro-3-fluoropyridine (0.769 mL, 7.60 mmol) in THF (5 mL) was added dropwise over 30 min.
4. Stirring was continued at -78 °C for 30 min.
5. A solution of N-bromosuccinimide (NBS, 1.624 g, 9.12 mmol) in THF (25 mL) was added dropwise to the reaction solution.
6. The reaction solution was stirred at -78 °C for 1 hour, then brought to room temperature and stirred for 12 hours.
7. Upon completion of the reaction, the reaction mixture was diluted with ethyl acetate (EtOAc) and water.
8. The organic layer was separated, washed with brine, concentrated and purified by silica gel column chromatography to afford the target product 2-bromo-4-chloro-3-fluoropyridine (0.541 g, 34%) as an orange oil (volatile). 9.
9. The structure of the product was confirmed by 1H NMR (400 MHz, CDCl3): δ 8.13 (d, J = 5.0 Hz, 1H), 7.35 (t, J = 5.1 Hz, 1H).
