Highly Stable Zr(IV)-Based Metal–Organic Frameworks with Chiral Phosphoric Acids for Catalytic Asymmetric Tandem Reactions

Heterogeneous
Brønsted acid catalysts featuring high porosity,
crystallinity, and stability have been of great interest for both
fundamental studies and practical applications, but synthetically,
they still face a formidable challenge. Here, we illustrated a ligand
design strategy for directly installing chiral phosphoric acid catalysts
into highly stable Zr-MOFs by sterically protecting them from coordinating
with metal ions. A pair of chiral porous Zr­(IV)-MOFs with the framework
formula [Zr6O4(OH)8(H2O)4(L)2] were prepared from enantiopure
4,4′,6,6′-tetra­(benzoate) and -tetra­(2-naphthoate) ligands
of 1,1′-spirobiindane-7,7′-phosphoric acid. They share
the same topological structure but differ in channel sizes, and both
of them demonstrate excellent tolerance toward water, acid and base.
Significantly enhanced Brønsted acidity was observed for the
phosphoric acids that are uniformly distributed within the frameworks
in comparison with the nonimmobilized acids. This not only facilitates
the catalysis of asymmetric two-component tandem acetalization, Friedel–Crafts,
and iso-Pictet-Spengler reactions but also promotes the catalysis
of asymmetric three-component tandem deacetalization–acetalization
and Friedel–Crafts reactions benefiting from the synergy with
exposed Lewis acidic Zr­(IV) sites. The enantioselectivities are comparable
or favorable compared to those obtained from the corresponding homogeneous
systems. The features of high reactivity, selectivity, stability,
and recyclability for Zr­(IV)-MOFs make them hold promise as a new
type of heterogeneous acid catalyst for the eco-friendly synthesis
of fine chemicals.