Citation:
Cenci, S. M., Cox, L. R., Leeke, G. A. (2014) Ultrasound-induced CO2/H2O emulsions as a medium for clean product formation and separation: The barbier reaction as a synthetic example, ACS Sustainable Chemistry and Engineering, Vol. 2, Iss. 5, pp. 1280 - 1288
Abstract:
: 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.