Step 1: Preparation of 4-methylthiophenylboronic acid
To a tetrahydrofuran (THF) (15 mL) solution of 4-bromoanisothioether (10) (3 g, 14.8 mmol) cooled to -78 °C and stirred under nitrogen protection, n-butyllithium (n-BuLi) (10 mL) was slowly added. The mixture was warmed to room temperature and stirring was continued for 20 minutes. Subsequently, the mixture was cooled again to -78 °C and a solution of triisopropyl borate (10 mL, 17.7 mmol) in THF (10 mL) was slowly added. After stirring at -78 °C for 1 hour, the mixture was warmed to room temperature and stirring was continued for 2 hours. Upon completion of the reaction, it was acidified with cold 5% hydrochloric acid (HCl), diluted with water (25 mL) and extracted with ethyl acetate. The organic layers were combined, washed sequentially with brine solution (20 mL) and water (20 mL), dried over anhydrous sodium sulfate (Na2SO4) and concentrated. The resulting residue was purified by column chromatography (eluent: ethyl acetate-hexane) to afford the target product 4-methylthiophenylboronic acid (11) (170 mg, 10% yield).
1H NMR (200 MHz, CDCl3): δ 8.10 (d, J=7.9 Hz, 2H), 7.33 (d, J=7.9 Hz, 2H), 2.55 (s, 3H, SCH3), 1.56 (bs, D2O exchangeable, OH), 1.25 (s, exchangeable, OH).
IR: vmax (KBr, cm-1): 3406,1594.
