Controlled Exchange of Achiral Linkers with Chiral Linkers in Zr‑Based UiO-68 Metal–Organic Framework

The
development of highly robust heterogeneous catalysts for broad
asymmetric reactions has always been a subject of interest, but it
remains a synthetic challenge. Here we demonstrated that highly stable
metal–organic frameworks (MOFs) with potentially acid-labile
chiral catalysts can be synthesized via postsynthetic exchange. Through
a one- or two-step ligand exchange, a series of asymmetric metallosalen
catalysts with the same or different metal centers are incorporated
into a Zr-based UiO-68 MOF to form single- and mixed-M­(salen)
linker crystals, which cannot be accomplished by direct solvothermal
synthesis. The resulting MOFs have been characterized by a variety
of techniques including single-crystal X-ray diffraction, N2 sorption, CD, and SEM/TEM-EDS mapping. The single-M­(salen)
linker MOFs are active and efficient catalysts for asymmetric cyanosilylation
of aldehydes, ring-opening of epoxides, oxidative kinetic resolution
of secondary alcohols, and aminolysis of stilbene oxide, and the mixed-M­(salen)
linker variants are active for sequential asymmetric alkene epoxidation/epoxide
ring-opening reactions. The chiral MOF catalysts are highly enantioselective
and completely heterogeneous and recyclable, making them attractive
catalysts for eco-friendly synthesis of fine chemicals. This work
not only advances UiO-type MOFs as a new platform for
heterogeneous asymmetric catalysis in a variety of syntheses but also
provides an attractive strategy for designing robust and versatile
heterogeneous catalysts.