Hydrochloric Acid → 3-Fluoro-5-(trifluoromethyl)benzeneboronic acid

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In a second reaction vessel, 40 mL of tetrahydrofuran was cooled to -78° C., and 2.3 mL (0.021 mole) of trimethyl borate was added dropwise as the Grignard reagent of 3-fluoro-5-trifluoromethylphenyl bromide prepared above was transferred into the second reaction vessel using a cannula. The temperature of the reaction mixture was maintained below -60° C. during the additions. Upon completion of the additions, the reaction mixture was again cooled to -78° C., where it was stirred for about 45 minutes. After this time, the reaction mixture was allowed to warm to ambient temperature. The reaction mixture was then poured into about 200 mL of water and was made acidic with aqueous 5% hydrochloric acid. The mixture was extracted with four 100 mL portions of ethyl acetate. The combined extracts were dried with magnesium sulfate and filtered. The filtrate was concentrated under reduced pressure, yielding 3.3 grams of 3-fluoro-5-trifluoromethylphenylboronic acid, mp 167°-168° C. The NMR spectrum was consistent with the proposed structure.

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