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.本研究采用亚临界二氧化碳/水（Subcritical CO₂/H₂O，30 ℃/80 bar）作为可再生混合溶剂，于1 dm³超声反应器中开展实验。以金属介导的巴比耶烯丙基化反应（Barbier allylation）作为代表性合成转化模型，验证该体系中高烯丙醇（homoallylic alcohol）产物的高效合成与分离可行性。针对竞争性的醛还原副反应，通过添加生物相容性非离子表面活性剂吐温80（Tween 80）、采用饱和盐水相，或是将反应条件调整为60 ℃/120 bar，可提升目标反应的化学选择性。上述所有优化策略均可显著提升目标烯丙基化反应的表观反应速率。一系列取代苯甲醛底物均可转化为对应高烯丙醇产物，收率处于中等至较高水平。水的存在是高效合成目标产物的必要条件，而二氧化碳相则可通过对高烯丙醇的高效富集，为产物的简洁分离提供适宜的相环境。以此方式，可在1小时内从二氧化碳相中分离得到0.025 mol的高烯丙醇产物，无需使用有机溶剂的常规后续萃取步骤。