Synthese
The general procedure for the synthesis of phenoxathiophene-4-boronic acid from phenoxathiophene and triisopropyl borate was as follows: phenoxathiophene (30 g, 145 mmol) was dissolved in 300 mL of tetrahydrofuran (THF) and the solution was cooled to -78 °C. Subsequently, n-butyllithium (n-BuLi) (72 mL, 180 mmol) was added slowly and dropwise. After stirring the reaction at -78 °C for 1 h, the reaction mixture was warmed to room temperature and the reaction was continued for 2 h. The reaction mixture was cooled to -78 °C for 1 h. The reaction mixture was then cooled to -78 °C for 2 h. The reaction mixture was again cooled to -78 °C and triisopropyl borate (33.8 g, 180 mmol) was slowly added. The reaction mixture continued to react at -78 °C for 2 hours before slowly warming to room temperature and reacting for 4 hours. Upon completion of the reaction, dilute hydrochloric acid (HCl) was added to terminate the reaction and the aqueous and organic layers were separated. The organic layer was concentrated by distillation under reduced pressure and extracted with chloroform (1 L) and water (1 L). The extracted organic layer was dried with magnesium sulfate and concentrated by distillation under reduced pressure again. Finally, phenoxathia-4-boronic acid (25 g, 68% yield) was obtained by recrystallization using hexane.
