4-Methylthiophene-2-boronic acid is a thiophene derivative widely used in the synthesis of pesticides, pharmaceuticals, chemical reagents, dyes, and polymer auxiliaries.
3.0 g (8.85 mmol) of potassium phosphate heptahydrate, 1.50 g (5.90 mmol) of bis(boronic acid) pinacol ester B2(pin)2, 12 mg (0.015 mmol) of Xphos-Pd-G2, and 4 mg (0.008 mmol) of Xphos were added sequentially to a reaction vial with 6 mL of ethanol stirred to mix well, followed by 0.36 mL of (2.95 mmol) 4-methylthiophene-2-chloride was added and the reaction was carried out at room temperature for 1 hour. The reaction solution was diluted by adding 5mL of ethyl acetate, filtered through diatomaceous earth, washed with ethyl acetate, the filtrates were combined and concentrated under reduced pressure to obtain the crude product, which was separated by silica gel column chromatography, eluted with petroleum ether-ethyl acetate to obtain 4-methylthiophene-2-boronic acid pinacol ester. 4-Methyl thiophene-2-boronic acid pinacol ester was added to the reaction flask and hydrolyzed dropwise with dilute HCl. The solution first produced a precipitate, with the precipitate gradually disappeared, adjust the system pH value of 1. To the solution of NaOH solution with a mass fraction of 25% was added dropwise to pH 13, stirring for 1 h. Separation of the liquid, the organic phase was extracted with 15 mL of NaOH with a mass fraction of 10%, and the aqueous phase was combined, and the alkaline solution was extracted with 15 mL of THF for 2 times, respectively. The pH of the obtained base solution was adjusted with dilute HCl, turbidity was generated at the beginning, flocculent appeared slowly, and the pH was adjusted to 5.0. The aqueous phase was extracted with 70 mL of THF, and the organic phase was spin-dried and purified to obtain 4-methylthiophene-2-boronic acid.
