Angewandte
Chemie
DOI: 10.1002/anie.201204530
Asymmetric Catalysis
Homochiral Crystallization of Metal–Organic Silver Frameworks:
Asymmetric [3+2] Cycloaddition of an Azomethine Ylide**
Xu Jing, Cheng He, Dapeng Dong, Linlin Yang, and Chunying Duan*
Metal–organic frameworks (MOFs) are hybrid solids with
infinite network structures built from organic bridging ligands
and inorganic connecting nodes. Besides the potential appli-
cations in many diverse areas,[1] MOFs are ideally suited for
catalytic conversions, because they can impose size- and form-
selective restriction through readily fine-tuned channels and
pores, thus providing precise knowledge about the pore
structure and the nature and distribution of catalytically
active sites.[2,3] In particular, analogues of homogeneous
asymmetric catalysts can be synthetically incorporated into
MOFs, thus resulting in the incorporation of the selectivity of
these single-site catalysts into micropores, and thereby
enhancing the shape-, size-, and enantioselectivities of
catalytic reactions in comparison to those performed in
homogeneous solution.[4] While recent progress in MOF-
based asymmetric catalysis has proved that these emerging
catalysts provide a new exciting opportunity[5] for the syn-
thesis of enantiopure compounds, including chiral drugs and
fine chemicals, privileged asymmetric metal catalysts or
organocatalysts that are incorporated into the nodes of
frameworks are still quite limited.[6]
The 1,3-dipolar cycloaddition of azomethine ylides with
electron-deficient olefins is an extremely versatile process to
form highly substituted chiral pyrrolidines, which provide an
important motif with widespread applications to the synthesis
of biologically active compounds and natural products.[7] The
cycloaddition reaction is also one of the most fascinating
transformations and has inspired much research interest in
the development of asymmetric catalytic variants. The con-
figuration of the four new stereogenic centers of the product
could be established in one step with complete atom
economy.[8] Recently, elegant studies in this field have
shown that chiral silver(I) and copper(I) bisdentate imine
complexes are adequate homogeneous catalysts to afford the
corresponding cycloadducts in good yields and high enantio-
selectivities.[9] Like for other precious-metal-catalyzed reac-
tions, it is highly desirable to incorporate chiral metal
complexes that are able to generate a metallodipole within
the nodes of frameworks, thus resulting in efficient catalytic
activity with the catalysts being recyclable and reusable to
minimize the metal trace in the product.
By incorporating three pyridine–imine bidentate chela-
tors into a triphenylamine fragment, we realized the homo-
chiral crystallization of silver-based MOFs by using cincho-
nine or cinchonidine as chiral templates. We envisioned that
the distorted tetrahedral silver(I) centers within the frame-
work would not only act as chiral nodes to connect these
ligands, but also be asymmetric catalytic sites for the 1,3-
dipolar cycloaddition reactions. We also postulated that the
twist configurations of the three phenyl rings attached to one
nitrogen atom might exhibit atropisomeric chirality in the
solid state,[10] thus facilitating the chiral transfer between the
silver centers and finally leading to the formation of chiral
MOFs. In the meantime, the coordination intermediate that
corresponds to cinchonine moieties is expected to be useful
for controlling the absolute chirality of the silver centers, such
as those observed in the asymmetric catalytic reactions.[11]
The ligand tris(4-(1-(2-pyridin-2-ylhydrazono)ethyl)-
phenyl) amine (TPHA; Scheme 1), was readily prepared by
reaction of tris(4-acetylphenyl)amine with 2-hydrazinylpyri-
dine in a molar ratio of 1:3. Reaction of TPHA with AgBF4 in
methanol afforded new compound Ag-TPHA. Single-crystal
structure analysis showed that Ag-TPHA crystallizes in the
chiral space group I213 (Figure 1).[12] The silver(I) atom is
positioned at a 21 fold axis and is coordinated by two identical
bidentate chelators from two different ligands in a distorted
tetrahedral geometry. The dihedral angle between the biden-
tate chelating rings is 49.4(6)8. The ligand is positioned at
a three-fold axis to three bridged silver ions through its
bidentate chelators. It adopts an atropisomeric chirality with
a torsion angle of about 74.5(5)8 between the pair of the
phenyl rings. In this case, the Schiff base ligands could be
viewed as three connecting nodes, the silver atoms worked as
directional connectors. Accordingly, the homochiral frame-
[*] X. Jing, Dr. C. He, D. Dong, L. Yang, Prof. C. Duan
State Key Laboratory of Fine Chemicals
Dalian University of Technology
Dalian, 116023 (P.R. China)
E-mail: cyduan@dlut.edu.cn
[**] We gratefully acknowledge financial support from the NSFC
(21171029 and 21025102).
Scheme 1. Homochiral crystallization of the ligand TPHA, which com-
prises three bidentate chelators, with silver(I) ions in the presence of
chiral adduct in order to assembly the silver-based MOF.
Supporting information for this article is available on the WWW
Angew. Chem. Int. Ed. 2012, 51, 1 – 6
ꢀ 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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