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

Abstract

Chiral\nmetal–organic frameworks (MOFs) with porous and tunable\nnatures have made them feasible for performing a variety of chemical\nreactions as heterogeneous asymmetric catalysts. By incorporating\nthe oxidation catalyst [BW₁₂O₄₀]^5– and the chiral group, l- or d-pyrrolidin-2-ylimidazole\n(<b>PYI</b>), into one single framework, the two enantiomorphs\nNi-<b>PYI</b>1 and Ni-<b>PYI</b>2 were obtained via self-assembly,\nrespectively. The channels of Ni-<b>PYI</b>s were enlarged through\na guest exchange reaction to remove the cationic chiral templates\nand were well modulated with hydrophilic/hydrophobic properties to\nallow molecules of both H₂O₂ and olefin ingress\nand egress. The coexistence of both the chiral directors and the oxidants\nwithin a confined space provided a special environment for the formation\nof reaction intermediates in a stereoselective fashion with high selectivity.\nThe resulting MOF acted as an amphipathic catalyst to prompt the asymmetric\ndihydroxylation of aryl olefins with excellent stereoselectivity.