Ultrasound-induced CO2/H2O emulsions as a medium for clean product formation and separation: The barbier reaction as a synthetic example

摘要

: Subcritical CO2/H2O (30 °C/80 bar) wasemployed as a renewable solvent mixture in a 1 dm3ultrasound reactor. As a representative synthetic transformation,the metal-mediated Barbier allylation was used todemonstrate the facility of formation and separation of thehomoallylic alcohol product. The chemoselectivity over thecompeting aldehyde reduction could be improved bydeploying the biocompatible nonionic surfactant Tween 80,a saturated salt aqueous phase, or by carrying out the reactionat 60 °C/120 bar. All of these modifications led to an apparentrate increase in the desired allylation. A range of substituted benzaldehydes afforded the corresponding homoallylic alcohols inmoderate to high yields. The presence of water constituted a necessary condition for efficient product formation, while CO2provided an appropriate phase for clean product separation by exploiting a favorable homoallylic alcohol enrichment. In this way,0.025 mol of homoallylic alcohol product could be isolated from the CO2 phase in 1 h, avoiding further extraction stages thatwould typically require organic solvents.