Modulating the Performance of an Asymmetric Organocatalyst by Tuning Its Spatial Environment in a Metal-Organic Framework.

Systematically tuning the spatial environment around the active sites of synthetic catalysts is a difficult challenge. Here, we show how this can be accomplished in the pores of multicomponent metal-organic frameworks. This relies on embedding a catalytic unit in a pore of the MUF-77 framework and then tuning its environment by introducing different functional groups to the surrounding linkers. This approach benefits from the structural regularity of MUF-77, which places each component in a precise location to circumvent disorder. Prolinyl groups, which are catalytically competent toward asymmetric aldol reactions, were selected as the catalytic unit. Since every prolinyl group is positioned in an identical environment, correlations between the pore architecture and the activity of these single-site catalysts can be elucidated. Systematic engineering of the pore structure, which is achieved by installing modulator groups on the framework linkers, impacts on the reaction rate and the enantiomeric excess of the aldol products. Furthermore, the spatial environment around the proline catalyst can override its innate stereochemical preference to dictate the preferred enantiomer of the reaction product. These results offer a new way to design three-dimensional active site environments for synthetic catalysts.