Ammonium Chloride → 4-(4,4,5,5-Tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole

Participants

Reaction Conditions

Reaction Steps

A flask equipped with a mechanical stirrer, a nitrogen inlet, an addition funnel and a thermowell was charged with 1-trimethylsilyl-4-iodopyrazole (225.1 g, 0.85 mol) and THF (2200 mL) at ambient temperature. This mixture was cooled to approximately −6° C. in an ice/salt/brine bath before a solution of isopropyl magnesium chloride in THF (2 M solution in THF, 510 mL, 1.02 mol, 1.2 equiv) was added at a rate such that the internal temperature did not exceed 0° C. The extent of metal/halogen exchange was monitored by GC and was found complete after about 10 min. To the orange brown solution was then added 2-isopropoxy-4,4,5,5-tetramethyl-1,3,2-dioxaborolane (isopropylpinacolborate, 347 mL, 1.7 mol, 2.0 equiv) slowly at first keeping the temperature below 0° C. and then fairly rapidly after about half of the compound was added allowing the temperature to reach 5° C. (the reaction becomes quite thick and then thins out slowly). The reaction is then stirred at 0° C. for 10 min before being warmed to ambient temperature over 1 h and stirred at ambient temperature for an additional 1 h. The reaction mixture was cooled to approximately 6° C. and the saturated aqueous ammonium chloride solution (NH4Cl, 2.2 L) was added with a temperature increase to 25° C. The mixture was stirred for 5 minutes before being diluted with toluene (10 L). The layers were separated (a large amount of solid is present in the aqueous layer) and the organic layer was sequentially washed with water (6×2.2 L) and brine (2×2.2 L) before being dried over sodium sulfate (Na2SO4). The drying reagent, sodium sulfate (Na2SO4), was removed by filtration and the solution was concentrated under reduced pressure. Residual toluene was co-evaporated with n-heptane to afford 4-(4,4,5,5-tetramethyl-1,3,2-dioxaborolan-2-yl)-1H-pyrazole (1, 90.3 g, 164.9 g theoretical, 54.8%) as a white solid. For 1: 1H NMR (400 MHz, DMSO-d6) δ 13.08 (bs, 1H), 7.94 (s, 1H), 7.62 (s, 1H), 1.23 (s, 12H) ppm; C9H15BN2O2 (MW, 194.04), LCMS (EI) m/e 195 (M++H).

Provenance & Evidence