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.

手性金属有机框架（chiral metal–organic frameworks, MOFs）凭借其多孔性与可调控性，可作为多相不对称催化剂用于多种化学反应。将氧化催化剂[BW12O40]5–与手性基团L-或D-吡咯烷-2-基咪唑（pyrrolidin-2-ylimidazole, PYI）引入单一骨架，分别通过自组装得到两种对映异构体Ni-PYI1与Ni-PYI2。通过客体交换反应放大Ni-PYIs的孔道以移除阳离子手性模板，并对其亲疏水性质进行精准调控，使过氧化氢（H2O2）与烯烃分子均可自由进出。手性导向剂与氧化剂在限域空间中共存，为以高立体选择性方式生成反应中间体提供了特殊环境。所得金属有机框架作为两亲性催化剂，可高效催化芳基烯烃的不对称双羟化反应，展现出优异的立体选择性。