Synthesis
A suspension was prepared in dioxane (10 mL) with 5-bromo-2-cyclopropylpyridine (0.21 g, 1.1 mmol), bis(pinacolato)diboron (0.31 g, 1.2 mmol) and potassium acetate (0.32 g, 3.2 mmol). The suspension was degassed with argon for 10 min and then PdCl2-dppf (0.026 g) was added. The reaction mixture was heated to 80 °C and kept for 15 hours. After completion of the reaction, it was cooled to room temperature and filtered through a diatomaceous earth plug. The filtrate was concentrated to give a black oily substance. The oily substance was dissolved in ether and extracted four times with 1.0 M sodium hydroxide solution. The combined yellow aqueous phases were cooled to 10°C and the pH was adjusted to 6.5 with 2.5 M hydrochloric acid, followed by repeated extraction with ether. The combined organic phases were dried with anhydrous magnesium sulfate, filtered and concentrated to give 0.27 g (103%) of 6-cyclopropylpyridine-3-boronic acid pinacol ester as a yellow oil, which solidified slowly. 1H-NMR analysis showed the product to be about 60-65% pure, with the major impurity being pinacol borane. The crude product can be directly used in the subsequent reaction without further purification.GC-MS m/z 245.2 (M+), 244.2 (M-I); 1H-NMR (CDCl3) δ 8.78 (br s, 1H), 7.93 (dd, 1H), 7.10 (d, 1H), 2.10 (m, 1H), 1.34 (s, 12H), 1.10- 1.00 (m, 4H) ppm.
References
[1] Patent: WO2006/65215, 2006, A1. Location in patent: Page/Page column 29
[2] Patent: WO2015/52264, 2015, A1. Location in patent: Paragraph 0556; 0557
[3] Patent: US2018/296543, 2018, A1. Location in patent: Page/Page column 0731; 0732
