Homochiral crystallization of metal-organic silver frameworks: asymmetric [3+2] cycloaddition of an azomethine ylide.

Abstract

Metal–organic frameworks (MOFs) are hybrid solids with infinite network structures built from organic bridging ligands and inorganic connecting nodes. Besides the potential applications in many diverse areas, MOFs are ideally suited for catalytic conversions, because they can impose sizeand formselective 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. 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. While recent progress in MOFbased asymmetric catalysis has proved that these emerging catalysts provide a new exciting opportunity for the synthesis 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. 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. 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 configuration of the four new stereogenic centers of the product could be established in one step with complete atom economy. 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 enantioselectivities. Like for other precious-metal-catalyzed reactions, 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 chelators into a triphenylamine fragment, we realized the homochiral crystallization of silver-based MOFs by using cinchonine or cinchonidine as chiral templates. We envisioned that the distorted tetrahedral silver(I) centers within the framework would not only act as chiral nodes to connect these ligands, but also be asymmetric catalytic sites for the 1,3dipolar 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, 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. 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-hydrazinylpyridine in a molar ratio of 1:3. Reaction of TPHAwith 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 bidentate 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-

DOI: 10.1002/anie.201204530

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Cite this paper

@article{Jing2012HomochiralCO, title={Homochiral crystallization of metal-organic silver frameworks: asymmetric [3+2] cycloaddition of an azomethine ylide.}, author={Xu Jing and Chunying Duan and Dapeng Dong and Linlin Yang}, journal={Angewandte Chemie}, year={2012}, volume={51 40}, pages={10127-31} }