Aryl Boroxines via Rotary Evaporation

Aryl boroxines are typically prepared from aryl boronic acids by lengthy thermal-vacuum or azeotropic dehydration methods. However, the boronic acid/boroxine equilibrium is rapid in solution at ambient temperature and readily achieved in the amorphous solid state on gentle warming. Rotary evaporation (220 rpm, 30 min, <50 mbar, 50 °C) of THF solutions efficiently drives the equilibrium to the boroxine (>95-99%) and provides a simple pretreatment strategy for control of speciation.