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 unltrathin 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 Zn4-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.