The general procedure for the synthesis of benzo[b]furan-5-boronic acid from 5-bromobenzo[b]furan and trimethyl borate is as follows:
1. a hexane solution of n-butyllithium (1.5 M; 3.72 ml, 5.6 mmol) was slowly added dropwise to a solution of tetrahydrofuran (10 ml) of 5-bromobenzo[b]furan (1.0 g, 5.0 mmol) under argon protection, keeping the reaction temperature at -60°C.
2. stirring was continued at the same temperature for 30 min.
3. trimethyl borate (0.69 ml, 6.0 mmol) was added to the reaction system, followed by slow warming of the reaction mixture to room temperature over a period of 4 hours.
4. Water (5 ml) was added to the reaction mixture and cooled to 5 °C, followed by distillation under reduced pressure to remove the tetrahydrofuran.
5. The pH was adjusted to 1 by adding 1N hydrochloric acid to the residue and then extracted with ethyl acetate.
6. The organic layer was washed with brine, dried and the solvent was removed by distillation.
7. The residue was ground with hexane/ether mixture, filtered and dried to give benzo[b]furan-5-boronic acid (551 mg) in light brown solid form.
