A recyclable bifunctional acid–base organocatalyst with ionic liquid character. The role of site separation and spatial configuration on different condensation reactions

Abstract

A series of bifunctional organic catalysts containing acid and basic sites with ionic liquid characteristics have been prepared and their catalytic activity and reaction coordinate for aldol and Knoevenagel condensations have been compared. While the only factor controlling catalyst activity for the Knoevenagel condensation was the distance between the acid and base sites, the spatial orientation of the organocatalyst is also key to achieve high activity and selectivity in the Claisen–Schmidt condensation. Mechanistic studies based on theoretical DFT calculations show that the acid–base bifunctional organocatalyst follows a mechanism inspired in natural aldolases for the synthesis of trans-chalcones, being able to produce a large variety of these compounds of industrial interest. The combination of the acid–base pairs within the proper geometry and the ionic liquid nature makes this catalyst active, selective and recyclable.