1. In a 100 mL three-necked flask, 17 g (0.1 mol) of 4-bromophenol, 16.5 g (0.11 mol) of dimethyl tert-butyl chlorosilane, 30 mL of DMF, 20 mL of triethylamine, and 0.1 g of DMAP were added in this order. the reaction was allowed to run for 24 hours at room temperature. Upon completion of the reaction, equal volumes of water and petroleum ether were added and left to partition, the aqueous phase was discarded. The organic phase was washed to neutrality with water and the solvent was subsequently evaporated to give 1-bromo-4-[(1,1-dimethylethyl)dimethylmethylsilanethoxy]benzene.
2. To a 250 mL three-necked flask were added 2.6 g (0.11 mol) of magnesium fragments, a small amount of crystalline iodine and 50 mL of tetrahydrofuran. Subsequently, 28.7 g (0.1 mol) of a mixture of 1-bromo-4-[(1,1-dimethylethyl)dimethylmethylsilanyloxy]benzene prepared in Step 1 and 20 mL of tetrahydrofuran were added. A small amount of the above mixture was added to initiate the Grignard reaction (if no reaction occurs, it may be heated slightly to initiate the reaction), and then the remaining mixture was added slowly dropwise with stirring, with the dropwise acceleration being controlled to keep the reaction temperature below 35°C. After dropwise addition, stirring is continued for 1 hour at room temperature.
3. The reaction solution was cooled to -65 °C via an acetone-liquid nitrogen bath, followed by the slow dropwise addition of 11.44 g (0.11 mol) of trimethyl borate dissolved in 60 mL of tetrahydrofuran at -60 °C. The reaction solution was allowed to cool to -65 °C via an acetone-liquid nitrogen bath. After the dropwise addition, the reaction solution was allowed to warm naturally to -30 °C and then acidified with concentrated hydrochloric acid to pH=1. The reaction solution was transferred to a partition funnel, 100 mL of ethyl acetate was added, and left to partition. The organic phase was washed with saturated saline to neutral and the aqueous phase was separated.
4. The organic phase was transferred to a 250 mL three-necked flask, 28.71 g (0.1 mol) of tetrabutylammonium fluoride was added, and the reaction was carried out at room temperature for 24 hours. Upon completion of the reaction, the mixture was transferred to a dispensing funnel and washed with saturated saline to neutral. After separation of the aqueous phase, the organic phase was evaporated to remove the solvent and recrystallized with petroleum ether to give 4-hydroxyphenylboronic acid as a white solid.
