The o-bromophenylboronic acid trimer was dissolved in 600 mL of anhydrous tetrahydrofuran under nitrogen protection. After homogenization, the solution was transferred to a 2 L three-neck flask and cooled to -10 °C. Under the condition of maintaining temperature, 1.2 L of 1 M isopropylmagnesium chloride solution (1.2 moles) was added slowly and dropwise. After the dropwise addition, the reaction was continued to be stirred at -10 °C for 1-3 hours. Subsequently, the reaction system was allowed to warm up naturally to room temperature and stirring was continued for 2-5 hours. The reaction system was cooled to 0 °C again, dry ice was added in three batches, the reaction was stirred and the reaction progress was monitored by TLC. Upon completion of the reaction, the reaction was quenched with 15% aqueous hydrochloric acid, the pH was adjusted to 1-2, and stirring was continued for 3-5 hours at room temperature to ensure complete hydrolysis of the trimer. After precipitation of the solid, it was filtered and recrystallized from a mixed solvent of water and ethanol to give 81.4 g of white solid o-carboxyphenylboronic acid with HPLC purity of 98.4%. The overall yield of the two-step reaction was 49%.
