Cycloaddition of CO2 to epoxides “around water”: a strategy to apply and recycle efficient water-soluble bio-based organocatalysts in biphasic media

Abstract

The synthesis of cyclic carbonates by cycloaddition of CO₂ to epoxides is a convenient non-reductive strategy to valorize carbon dioxide. Biphasic chemistry is an emerging and convenient approach to carry out catalytic reactions; processes where water constitutes one of the distinct layers of the reaction have led to significant enhancements of reaction rates and/or allowed the facile recovery of the catalyst via phase separation. Nevertheless, such an approach remains very rare for the target cycloaddition reaction. Herein, we show that purposely designed water-soluble organocatalysts can be used to efficiently carry out the synthesis of cyclic carbonates from CO₂ and a broad range of epoxides by acting at the interface between water and the surrounding organic layer (i.e., “around water” biphasic systems) and are easily recovered as aqueous solutions at the end of the reaction and reused as such. Importantly, the addition of simple inorganic salts to the aqueous layers accelerates the catalytic cycloaddition reaction in a way that generally correlates with their salting-out potential. Such an effect was exploited to enhance the reaction rates and to promote the carbonation of challenging bio-based epoxidized fatty acid esters by using simple seawater as the aqueous layer. Control experiments and DFT calculations were employed to gain insights into the catalytic process that takes place in a heterogeneous fashion at the interface between the surface of small water droplets containing the catalyst and the continuous epoxide phase.