Boosting Enantioselectivity of Chiral Organocatalysts with Ultrathin Two-Dimensional Metal-Organic Framework Nanosheets

The development of methodologies for inducing and tailoring enantioselectivities of catalysts is an important issue in asymmetric catalysis. In this work, we demonstrate for the first time that chiral molecular catalysts can be boosted from completely nonselective to highly enantioselective when installed in nanostructured metal-organic frameworks (MOFs). Exfoliation of layered crystals is one of the most direct synthetic routes to unitrathin nanosheets, but its use in MOFs is limited by the availability of layered MOFs. We illustrate that layered MOFs can be designed using ligand-capped metal clusters and angular organic linkers. This leads to the synthesis of two three-dimensional (3D) layered porous MOFs from Zn-4-p-tert-butylsulfonyl calix[4]arene and chiral angular 1,1'-binaphthol/-biphenol dicarboxylic acids, which can be ultrasonic exfoliated into one- and two-layer nanosheets. The obtained MOF materials are efficient catalysts for asymmetric cascade condensation and cyclization of 2-aminobenzamide and aldehydes to produce 2,3-dihyroquinazolinones. While both binaphthol and biphenol display no enantioselectivity, restriction of their freedom in the MOFs leads to 56-90% and 46-72% ee, respectively, which are increased to 72-94% and 64-82% ee after exposure to external surfaces of the flexible nanosheets. Moreover, the MOF crystals and nanosheets exhibit highly sensitive fluorescent enhancement in the presence of chiral amino alcohols with enantioselectivity factors being, respectively, increased up to 1.4 and 2.3 times of the values of the diols, allowing them to be utilized in chiral sensing. Therefore, the observed enantioselectivities increase in the order organocatalyst < MOF crystals < MOF nanosheets in both catalysis and sensing. This work not only provides a strategy to make 3D layered MOFs and their untrathin nanosheets but also paves the way to utilize nanostructured MOFs to manipulate enantioselectivities of molecular catalysts.