General procedure for the synthesis of 2-formylthiophene-4-boronic acid pinacol ester from 4-bromo-2-thiophenecarboxaldehyde and bis(pinacolato)diboron (865 mg, 3.403 mmol), potassium acetate (667 mg, 6.81 mmol) and PdCl2(dppf) (96 mg, 0.131 mmol). The reaction mixture was placed in a microwave reactor and heated at 150 °C for 20 min. After completion of the reaction, all solvents were removed by evaporation. The residue was partitioned between ethyl acetate (100 mL) and water (100 mL). The aqueous layer was extracted with ethyl acetate (2 x 100 mL). The combined organic phases were washed with brine (100 mL), dried over anhydrous MgSO4 and concentrated. The crude product was purified by Combiflash fast column (elution gradient: hexane/ethyl acetate, 1%-30% ethyl acetate, 20 min) to afford pinacol ester of 2-formylthiophene-4-boronic acid (550 mg, 88% yield).LC/MS analysis: m/z 238.2 ([M+H]+), retention time 1.88 min.
