Synthesis
3-bromo-4-fluoronitrobenzene (66 g, 300 mmol), potassium acetate (58.9 g, 600 mmol), bis(pinacolato)diboron (83.8 g, 330 mmol) and dichloro[[1,1'-bis(diphenylphosphino)ferrocene]palladium(II) dichloride dichloromethane adduct (7.35 g, 9 mmol) of 1,4-dioxane ( 900 ml, containing 18 ml DMSO) solution was degassed with nitrogen for 1 hour and then heated at 90°C for 14 hours. After completion of the reaction, it was cooled to room temperature, filtered, the filter cake was washed with ether and the filtrate was concentrated in vacuum. The residue was stirred with 2N aqueous sodium hydroxide solution (11 L) for 45 min and filtered. The filtrate was extracted with ether (2 x 750 ml) and the organic phase was discarded. The aqueous phase was cooled to 0°C, the pH was adjusted to 5 by dropwise addition of 36% hydrochloric acid (ca. 175 ml), allowed to stand at 0°C for 2 h, filtered and washed with cold water. The solid was dried under vacuum (300 mmHg) in the presence of phosphorus pentoxide to give 2-(2-fluoro-5-nitrophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (76.1 g, 95%).
A mixture of THF (600 ml) and water (30 ml) of 3-bromopyridine (19 g, 120 mmol), 2-(2-fluoro-5-nitrophenyl)-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (40 g, 150 mmol), and potassium fluoride (23 g, 396 mmol) was nitrogen deaerated for 10 min by the addition of tris(dibenzylidene acetone ) dipalladium(0) (2.2 g, 2.4 mmol) and tri-tert-butylphosphine (0.2 M solution of 1,4-dioxane; 2.4 ml, 0.48 mmol), mechanically stirred at room temperature for 30 min, heated at 50 °C for 1 h and cooled to room temperature. The reaction solution was poured into ice-cold 0.5N aqueous sodium hydroxide solution and stirred for 1 h. The reaction solution was filtered, washed with water, pumped to dryness, washed with isohexane and dried to give 3-(2-fluoro-5-nitrophenyl)pyridine (26.2 g).
A suspension of 3-(2-fluoro-5-nitrophenyl)pyridine (26 g, 119 mmol) in ethanol (200 ml) and ethyl acetate (200 ml) was added to palladium carbon (1.35 g, 6 mmol), and reacted under hydrogen pressure at 50 psi until the hydrogen absorption ceased (ca. 3 hr), filtered, and concentrated to give 3-(2-fluoro-5-aminophenyl)pyridine (22.4 g).
A 1,4-dioxane (40 ml) solution of 3-(2-fluoro-5-aminophenyl)pyridine (22.4 g, 119 mmol) was added to 48% aqueous hydrobromic acid (500 ml), cooled to 0 °C, aqueous sodium nitrite (9.5 g, 137 mmol) (30 ml) was added dropwise, the internal temperature was maintained at <5 °C, and the reaction was stirred at 0 °C for 2 hours. Add cooled aqueous solution of cuprous bromide (25.6 g, 179 mmol) 48% hydrobromide (150 ml), stir at 0°C for 10 min and heat at 50°C for 20 min. Cooled to room temperature, diluted with ice-cold water (500 ml), pH 8-9 adjusted with ice-cold 33% ammonia (ca. 750 ml) and stirred for 20 min. Extracted with ethyl acetate (600 ml), the organic phase was washed with water, brine (300 ml), dried with anhydrous magnesium sulfate, filtered and pre-adsorbed on silica gel. Column chromatography (silica gel; 20-50% ethyl acetate/isohexane with 1% methanol and 1% triethylamine) purified 3-(5-bromo-2-fluorophenyl)pyridine (22 g, 73%).
3-(5-Bromo-2-fluorophenyl)pyridine (21 g, 83 mmol), potassium acetate (16.4 g, 167 mmol) and bis(pinacolato)diboron (23.3 g, 92 mmol) were dissolved in 1,4-dioxane (250 ml) and DMSO (5 ml) and degassed with nitrogen for 1 hour. Dichloro[[1,1'-bis(diphenylphosphino)ferrocene]palladium(II) dichloride dichloromethane adduct (2 g, 2.5 mmol) was added and heated at 90°C for 18 hours. Cooled to room temperature, filtered and the filter cake was washed with ether. The filtrate was evaporated to dryness and the residue was treated with ice-cold 2N aqueous sodium hydroxide solution.
References
[1] Journal of Medicinal Chemistry, 2006, vol. 49, # 1, p. 35 - 38
[2] Bioorganic and Medicinal Chemistry Letters, 2006, vol. 16, # 6, p. 1582 - 1585
[3] Patent: WO2003/99816, 2003, A1. Location in patent: Page 45-46
[4] Organic Process Research and Development, 2006, vol. 10, # 3, p. 398 - 402
[5] Bioorganic and Medicinal Chemistry Letters, 2006, vol. 16, # 6, p. 1477 - 1480
