Engineering Chiral Polyoxometalate Hybrid Metal–Organic Frameworks for Asymmetric Dihydroxylation of Olefins

Chiral
metal–organic frameworks (MOFs) with porous and tunable
natures have made them feasible for performing a variety of chemical
reactions as heterogeneous asymmetric catalysts. By incorporating
the oxidation catalyst [BW12O40]5– and the chiral group, l- or d-pyrrolidin-2-ylimidazole
(PYI), into one single framework, the two enantiomorphs
Ni-PYI1 and Ni-PYI2 were obtained via self-assembly,
respectively. The channels of Ni-PYIs were enlarged through
a guest exchange reaction to remove the cationic chiral templates
and were well modulated with hydrophilic/hydrophobic properties to
allow molecules of both H2O2 and olefin ingress
and egress. The coexistence of both the chiral directors and the oxidants
within a confined space provided a special environment for the formation
of reaction intermediates in a stereoselective fashion with high selectivity.
The resulting MOF acted as an amphipathic catalyst to prompt the asymmetric
dihydroxylation of aryl olefins with excellent stereoselectivity.