Synthesis
The general procedure for the synthesis of 3,4-dimethylphenylboronic acid from trifluoromethanesulfonic acid-3,4-dimethylphenyl ester is as follows: in a 50 mL round-bottomed flask (equipped with a side arm, condenser, and stirring bar) triphenylphosphine (0.131 g, 0.5 mmol, 20 mol%), p-iodoiodobenzene (0.585 g, 2.5 mmol), and triethylamine (1.78 mL, 12.5 mmol). The system was degassed by alternating vacuum and argon filling three times. Palladium dichloride (0.023 g, 0.13 mmol, 5 mol%) was added under positive argon atmosphere. After stirring at room temperature for 15 minutes, diisopropylaminoborane (5 mL, 1 M THF solution, 5 mmol) was added and again degassed by alternating vacuum and argon filling three times. The reaction mixture was heated to reflux and maintained at reflux for 12 hours. Upon completion of the reaction, it was cooled to 0 °C and methanol (6 mL, note: exothermic reaction accompanied by hydrogen release) was slowly added. Stirring was continued for 15 minutes, followed by concentration under reduced pressure to remove all solvent to give a black solid. The solid was dissolved in 3M sodium hydroxide solution (8 mL) and washed with hexane (3 x 10 mL). The aqueous phase was cooled to 0 °C (ice bath) and acidified with concentrated hydrochloric acid to pH ≤ 1, at which point 3,4-dimethylphenylboronic acid precipitated as a white solid. The aqueous phase was extracted with ether (3 x 10 mL), the organic phases were combined, dried with magnesium sulfate and filtered. Finally, the solvent was removed by concentration under reduced pressure to give the white solid product 3,4-dimethylphenylboronic acid.
References
[1] Tetrahedron, 2011, vol. 67, # 3, p. 576 - 583
