Triisopropyl borate → 5-Chloro-2-methoxyphenylboronic acid

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A stirred solution of 5.8 grams (0.026 mole) of 3-bromo-4-methoxychlorobenzene in 150 mL of tetrahydrofuran was cooled to -80° C., and 11.5 mL of n-butyllithium in hexanes (2.5 Molar-0.029 mole) was added dropwise during a 15 minute period, while maintaining the reaction mixture temperature at about -70° C. The initial reaction was very exothermic, which required cooling the reaction mixture to about -100° C. Upon completion of the addition, the reaction mixture was stirred at -80° C. for 15 minutes. After this time, 17.5 mL (0.076 mole) triisopropyl borate was added during a 1 minute period. The reaction mixture was then allowed to warm slowly to ambient temperature during a 3 hour period, where it was stirred for an additional 1 hour. After this time, the reaction mixture was concentrated under reduced pressure to a volume of about 50 mL. The concentrate was then poured into 500 mL of ice-water. The mixture was then made acidic with about 26 mL of aqueous 2N hydrochloric acid. The mixture was then filtered to collect a solid, which was dried, yielding 3.9 grams of 5-chloro-2-methoxyphenylboronic acid. The NMR spectrum was consistent with the proposed structure.

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