3. Chiral Zeolite Mimics for Enantioselective Processes.

Zeolites have been extensively used in the size-selective separation and catalysis of commodity chemicals. The design of chiral zeolitic materials will expand their utility into the areas of enantioselective separations and syntheses. Despite extensive research over the past few decades, enantiomerically pure chiral zeolites have not been synthesized to date. All previous attempts to chiral porous materials relied on templating with chiral surfactants. Calcinations at high temperatures (in order to remove the organic templates) inevitably destroys the chiral environment conferred by chiral surfactant templates.

Porous polymeric metal-organic frameworks (MOFs) have attracted much interest in recent years owing to their potential applications in many areas. Given that most asymmetric catalyses and chiral separations are done at room temperature (or even lower temperatures), chiral porous materials based on MOFs should have adequate stability and represent promising candidates for enantioselective separations and syntheses. The shapes, sizes, functions, and chirality of the voids within these MOFs can be rationally tuned using the molecular building block approach. Such chiral zeolitic materials will have well-defined nanoscale chiral pockets that represent heterogeneous (and reusable) artificial enzymes.

We have taken a multi-disciplinary approach toward chiral, porous solids based on MOFs. In the first approach, we have sought to synthesize single-crystalline chiral MOFs using weaker metal-ligand ligation. A variety of chiral bipyridyl and dicarboxylate bridging ligands have been synthesized via multi-step sequences. By combining with metal connectors of appropriate binding geometries, numerous single-crystalline chiral MOFs have thus far been obtained.

Homochiral solids based on 2D coordination networks using enantiopure dicarboxylate ligands and dimetal secondary building units (SBUs) as the building blocks were synthesized and single-crystal X-ray diffraction studies reveal their infinite 2D coordination network structures (Fig.5, Angew. Chem. 2002, 41, 1159). It is clear from Fig 5 that ethoxy-protected BINOL functionalities are pointing toward the open cavities of these MOFs.

Homochiral MOFs based on chiral bipyridines have also been synthesized (JACS, 2003, 125, 6014). The coordination of C2 symmetric 1,1-binaphthyl-6,6’-bipyridine ligands and linear metal-connecting points Ni(acac)₂ led to helical chains, which associate in parallel to form nanotubes of 2´2 nm in dimensions (Fig 6). These nanotubes intertwine to form periodically-ordered, interlocked nanotubular architectures that possess nanometer-scale open channels and have high affinity for aromatic molecules. Chiral crown ethers have also been successfully incorporated into the walls of these nanotubes, which promises to lead to novel chiral zeolitic materials applicable in enantioselective processes.

We have also obtained single-crystalline MOFs based on metal phosphonates (JACS, 2001, 123, 10395; 2002, 124, 14298). Homochiral lanthanide bisphosphonates were synthesized and adopt lamellar structures with large asymmetric channels with a dimension of ~12 Å (Fig 7). The framework structures of these lanthanide bisphosphonates are stable toward the removal of included guest molecules and have been used for heterogeneous asymmetric catalysis and bulk chiral separations with modest e.e. Similar lanthanide phosphonate lamellar solids containing chiral crown ethers have recently been synthesized, and are expected to be more efficient enantioselective sorbents.

In the second approach, we have synthesized amorphous chiral porous solids that contain catalytically more appropriate functionalities. Chiral porous zirconium phosphonates with Ru-BINAP moieties in the framework were synthesized via a molecular building block approach (Scheme VI), and characterized by a variety of techniques including TGA, adsorption isotherms, XRD, SEM, IR, and microanalysis (Angew. Chem., in press). These highly porous hybrid solids were used for enantioselective heterogeneous asymmetric hydrogenation of b-keto esters with e.e. values of up to 95%. The solid catalysts can be readily recovered by simple filtration and reused five times without loss of activity and enantioselectivity. More recently, we have prepared amorphous chiral porous solids containing Ru-BINAP-DPEN moieties as shown in Scheme VII (JACS, submitted). Remarkably, these solid catalysts catalyze the hydrogenation of aromatic ketones with enantioselectivity much higher than the parent Ru-BINAP-DPEN homogenous catalyst reported by Noyori et al. In the presence of KO^tBu, a wide range of aromatic ketones have been hydrogenated with e.e.’s in the range of 91.0-99.2% and turnover frequency as high as 700 h^-1. This catalytic process can be carried out with only 0.005 mol% solid loading. The solid catalysts have been recovered and reused for 8 times without the loss of activity and enantioselectivity.

Representative Publications:

1 “Chiral Porous Hybrid Solids for Practical Heterogeneous Asymmetric Hydrogenation of Aromatic Ketones.” Hu, A.; Ngo, H.L.; Lin, W. submitted to J. Am. Chem. Soc.

2 “Interlocked Chiral Nanotubes Assembled from Quintuple Helices.” Cui, Y.; Lee, S.J.; Lin, W. J. Am. Chem. Soc. 2003, 125, 6014-6015 [pdf].

3 “Chiral Porous Hybrid Solids for Highly Enantioselective Heterogeneous Asymmetric Hydrogenation of b-Keto Esters.” Hu, A.; Ngo, H.L.; Lin, W. Angew. Chem., Int. Ed. in press.

4 “A Homochiral Triple Helix Constructed from an Axially Chiral Bipyridine.” Cui, Y.; Ngo, H.L.; Lin, W. Chem. Commun. 2003, 1388-1389 [pdf].

5 “Homochiral 3D Porous Frameworks Assembled from 1- and 2-D Coordination Polymers.” Cui, Y.; Ngo, H.L.; Lin, W. Chem. Commun. 2003, 994-995 [pdf].

6 “Hierarchical Assembly of Homochiral Porous Solids Using Coordination and Hydrogen Bonds.” Cui, Y.; Ngo, H.L.; White, P.S.; Lin, W. Inorg. Chem. 2003, 42, 652-654 [pdf].

7 “Chiral Crown Ether Pillared Lamellar Lanthanide Phosphonates.” Ngo, H.L.; Lin, W. J. Am. Chem. Soc. 2002, 124, 14298-14299 [pdf].

8 “Rational Design of Homochiral Solids Based on 2D Metal Carboxylates.”Cui, Y.; Evans, O.R.; Ngo, H.L.; White, P.S.; Lin, W. Angew. Chem., Int. Ed. 2002, 41, 1159-1162 [pdf].

9 “Homochiral 3D Lanthanide Coordination Networks with An Unprecedented 4⁹6⁶ Topology.” Cui, Y.; Ngo, H.L.; White, P.S.; Lin, W. Chem. Commun. 2002, 1666-1667 [pdf].

10 “Homochiral Metal-Organic Frameworks Based on Transition Metal Bisphosphonates.” Evans, O.R.; Manke, D.R.; Lin, W. Chem. Mater. 2002, 14, 3866-3874 [pdf].

11 “Chiral Porous Solids Based on Lamellar Lanthanide Phosphonates.” Evans, O.R.; Ngo, H.L.; Lin, W. J. Am. Chem. Soc. 2001, 123, 10395-10396 [pdf]. Also see highlight in Science.