Homogeneous organocatalysts are small, metal-free organic molecules, which have been widely used in laboratory-scale research due to their low cost, low toxicity, and ease of modification. However, due to the requirement of high loading of such catalysts and their low recovery efficiency, they do not have a wide range of applications in the industrial setting. In comparison, the recovery and separation of heterogeneous catalysts are easier, but their activity and selectivity are generally lower. Therefore, developing recoverable homogeneous organocatalysts is an important task. Herein, we explored a novel photochemical method that enables the recovery of homogeneous organocatalysts. We designed a series of photoswitchable homogeneous organocatalysts that exhibit photo-controlled solubility change. There are three essential moieties that compose these organocatalysts. The first is light-responsive azobenzene moiety, which displays strong pi-pi interactions in the trans configuration, but upon photoisomerization, the cis isomer weakens the intermolecular interactions and disrupts their packing manner. The second is the linker moiety, which modifies the strength of H-bonding, pi-pi interactions, and London dispersion forces, all of which influence the solubility of isomers in common solvents. The third moiety is the active catalyst site: a tertiary amine that catalyzes Henry reaction. The organocatalyst in the cis form exhibits a solubility of ~30 mM, which is 10 times greater than the ~3 mM solubility of trans form. Accordingly, the UV-activated organocatalyst in the cis form can catalyze the reaction homogeneously, while the trans form, generated by visible light irradiation, can be precipitated and recovered efficiently after the reaction.

Tertiary amine Lewis base organocatalysts: Photo-controlled solubility for recovery

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