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

Subcritical CO₂/H₂O (30°C/80 bar) was employed as a renewable solvent mixture in a 1 dm³ ultrasound reactor. As a representative synthetic transformation, the metal-mediated Barbier allylation was used to demonstrate the facility of formation and separation of the homoallylic alcohol product. The chemoselectivity over the competing aldehyde reduction could be improved by deploying the biocompatible nonionic surfactant Tween 80, a saturated salt aqueous phase, or by carrying out the reaction at 60 °C/120 bar. All of these modifications led to an apparent rate increase in the desired allylation. A range of substituted benzaldehydes afforded the corresponding homoallylic alcohols in moderate to high yields. The presence of water constituted a necessary condition for efficient product formation, while CO₂ provided 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 that would typically require organic solvents.