Catalyst self-assembling supramolecular

Costantino et al. [103] reported the direct aza-Diels-Alder reaction between 2-cyclohexen-l-one and benzaldimines in water, catalyzed by layered a-zirconium hydrogen phosphate (a-ZrP) in the presence of sodium dodecyl sulfate (SDS) at 30 ° C (Table 6.2). The yield of the product was good, and the reaction was faster and the exo-diastereoselectivity was higher than when an organic solvent was used. A Cr(III)(salen)Cl catalyst embedded into a self-assembled supramolecular aggregate displays substrate selectivity with an up to 3.5 fold increase in activity in favor of longer over shorter dienophiles in the reaction with cyclopentadiene under aqueous micellar conditions [104]. 

Recently, a new class of supramolecular CpRu-containing catalysts for hydration of alkynes has emerged. These catalysts are based on the supramolecular self-assembly of monodentate ligands through hydrogen bonding association, as shown in Scheme 10.9 [41-43]. The remarkable activity of catalytic systems such as 15-17... 

Figure 1.13 Generation of rhodium-based supramolecularcatalysts by assembly of pyridine/hydroxypyridine pairs (a) Self-assembly modes of pyridine-based phosphines, (b) Alkene hydroformylation with supramolecular rhodium-diphosphine catalysts (c) CAChe minimized 3D structure ofthe rhodium-diphosphine complex (other ligands from the metal omitted for clarity).

Hydrogen bonding is not the only supramolecular strategy for efficiently generating the catalyst backbone by assembly of suitable constituent blocks metal-directed self assembly has also been employed. Reek, van Leeuwen and coworkers (Chapter 8)... 

Figure 1.16 Transition metal Rh-catalysts formed by self-assembly (a) Schematic representation of the self-assembly process, (b) Hydroformylation of styrene with supramolecular Rh-catalysts.

In the last Section 6.4 new supramolecular approaches to construct synthetic biohybrid catalysts are described. So-called giant amphiphiles composed of a (hydrophilic) enzyme headgroup and a synthetic apolar tail have been prepared. These biohybrid amphiphilic compounds self-assemble in water to yield enzyme fibers and enzyme reaction vessels, which have been studied with respect to their catalytic properties. As part of this project, catalytic studies on single enzyme molecules have also been carried out, providing information on how enzymes really work. These latter studies have the potential to allow us to investigate in precise detail how slight modifications ofthe enzyme, e.g., by attaching a polymer tail, or a specific mutation, actually infiuence the catalytic activity. 

New Supramolecular Approaches in Transition Metal Catalysis Template-Ligand Assisted Catalyst Encapsulation, Self-Assembled Ligands and Supramolecular Catalyst Immobilization... 

A networked supramolecular POM-based hybrid catalyst has been synthesized by Ikegami and coworkers via the self-assembly of [PWi2O40]3 and non-crosslinked copolymer based on N-isopropylacrylamide (NIPAM) and ammonium cations (Figure 6.7) [124, 125]. This heterogeneous NIPAM-POM hybrid catalyst showed very high catalytic activity for the epoxidation of allylic alcohols with H202 (e.g., up to 35 000 TON for phytol). In addition, the oxygenation of amines and sulfide could be... 

In this synthetic strategy, supramolecular interactions are used to construct the catalyst backbone. The approach can be used for a wide variety of catalysts, and mechanistic effects include catalytic supramolecular assemblies, self-assembly, metal-mediated catalysis and organocatalysis, and chiral induction. 

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