Synthesis
General procedure for the synthesis of 6-bromo-1H-indazole from 4-bromo-2-fluorobenzaldehyde: Hydrazine hydrate (30 mL, 832 mmol) and 4-bromo-2-fluorobenzaldehyde (4.69 g, 23 mmol) were added to a 100 mL round bottom flask. The reaction mixture was stirred at 125 °C for 3 hours. Upon completion of the reaction, it was cooled to room temperature and the reaction solution was subsequently concentrated under reduced pressure. The concentrated reaction solution was quenched by pouring it into an ice-water mixture (100 mL) and then extracted with ethyl acetate (3 × 100 mL). The organic phases were combined, dried over anhydrous sodium sulfate, and filtered to remove the desiccant. The filtrate was concentrated to dryness under reduced pressure, and the resulting crude product was adsorbed on silica gel. The crude product was purified by column chromatography using a Redi-Sep pre-populated silica gel column (40 g) with hexane/ethyl acetate (0 to 100% gradient) as eluent to give the final 6-bromo-1H-indazole (4.6 g, 18 mmol, 76% yield). The product was detected by LCMS, [M + H]+ m/z 197.9 (calculated value C7H5BrN2 197.0).1H NMR (400 MHz, CD3OD): δ 8.03 (s, 1H), 7.67-7.72 (m, 2H), 7.24-7.26 (m, 1H).
References
[1] Patent: CN106146401, 2016, A. Location in patent: Paragraph 0117; 0190; 0191
[2] Patent: US2015/368278, 2015, A1. Location in patent: Paragraph 1273; 1274
[3] Journal of Organic Chemistry, 2006, vol. 71, # 21, p. 8166 - 8172
[4] Patent: US2007/173506, 2007, A1. Location in patent: Page/Page column 48
[5] Organic Process Research and Development, 2011, vol. 15, # 3, p. 565 - 569
