Enhanced catalyst performance through compartmentalization exemplified by colloidal L-proline modified microgel catalysts

Exploring and controlling chemical reactions in compartments opens new platforms for designing bioinspired catalysts and energy-autonomous systems. Aqueous polymer networks or hydrogels serve as a perfect model for biological tissues, allowing systematic investigations of chemical transformations in compartments. Herein, we report the synthesis of a versatile, colloidal microgel catalyst containing covalently bound L-proline as an organocatalyst. The key finding of our work is that the catalytic activity can be tuned by adjusting the distribution of the organocatalyst in the microgel network as well as the properties of the solvent. We demonstrate that L-proline groups integrated into microgels enable the reaction of 4-nitrobenzaldehyde and cyclohexanone in a heterogeneous reaction mixture in which free L-proline is not active. By controlling the localization of the L-proline groups within the microgel network (core or corona), the rate of the aldol reaction in homogenous and heterogeneous reaction mixtures can be modulated. Furthermore, microgels with covalently attached catalysts can be recycled and reused in sequential catalytic runs without deterioration of the catalyst performance in terms of activity and selectivity. The internal structure of the microgel in heterogeneous reaction mixtures was studied by computer simulations.