Immobilized chiral metal complexe

Another path of manufacture of practical catalysts is using immobilized chiral metal complexes. Thus, the complex [Rh-BESIAP] was occluded in an elastomeric t5T)e polydimethylsiloxane membrane, which gave a re-generable active membrane-catalyst with the same enantioselectivity as the homogeneous catalyst in the hydrogenation of acetoacetic acid ester into methyl (7 )-(-)-3-hydroxybutyrate, that can be pol5mierized into polyester (Scheme 7.17.). 

Spectacular achievements in catalytic asymmetric epoxidation of olefins using chiral Mnm-salen complexes have stimulated a great deal of interest in designing polymeric analogs of these complexes and in their use as recyclable chiral catalysts. Techniques of copolymerization of appropriate functional monomers have been utilized to prepare these polymers, and both organic and inorganic polymers have been used as the carriers to immobilize these metal complexes.103... 

Experimental evidence of the —S03" H0Si— interaction have been obtained from IR, Rh K-edge EXAFS, and CP MAS 3 IP NMR studies. These supported catalysts have been tested for the hydrogenation and hydroformylation of alkenes. No Rh leaching was observed.128-130 An extension to the immobilization of chiral metal complexes for asymmetric hydrogenation is reported below. 

Chiral ligand exchange chromatography utilizes immobilized tremsition metal complexes that selectively bind one enantiomer of the analyte, which is usually an amino acid. 

Although packed columns have been used in earlier studies, capillary columns containing wall-coated or immobilized chiral selectors are more widely used at present. The chiral selector may be of different nature, such as amino acid derivatives, chiral metal complexes, etc. [90], At present, cyclodextrins (CD) and their derivatives dominate as chiral stationary phases (CSP) in GC enantioseparations [90, 91], CD-based CSPs for GC are thermally stable, widely applicable, and easy to use. 

In this context, however, asymmetric transformations with chiral metal complexes or enzymes are particularly important. The demand for enantiopure building blocks in the fine chemical and pharmaceutical industries is still hampered by simple asymmetric processes which can be scaled up, as well as by the stability, recyclability, and hence the price of most chiral catalysts (refer also to the chapter by K.-U. Schoning in this volume). Immobilization of effective and robust catalytic systems and their application in continuous flow reactors is regarded as a key for success in this field. 

Ionic liquids can efficiently immobilize transition metal complexes used as catalysts for epoxidation. This phenomenon is encountered not only when specially designed, polar ligands are used, but also for unmodified catalysts. For instance, several unsuccessful attempts have been made to immobilize Jacobsen s chiral Mn(iii)salen epoxidation catalyst 81. Ionic liquids are the first medium to immobilize this complex efficiently without requiring additional modification of the ligand. 

Asymmetric Hydrogenations over Chiral Metal Complexes Immobilized in SILCAs I 257... 

Fontecave M, Hamelin O, Menage S (2005) Chiral-at-Metal Complexes as Asymmetric Catalysts. 15 271-288 FraUe JM, Garcia JI, Mayoral JA (2005) Non-covalent Immobilization of Catalysts Based on Chiral Diazaligands. 15 149-190 Frenking G, see Deubel D (2005) 12 109-144 Fu GC, see Netherton M (2005) 14 85-108... 

The most well-developed recent examples of catalysis concern catalysts for oxidation reactions these are essentially achiral or chiral metal-salen complexes. Taking into account a number of results suggesting the importance of a degree of mobility of the bound complex, Sherrington et al. synthesized a series of polymer-supported complexes in which [Mn(salen)Cl] units are immobilized in a pendant fashion by only one of the aromatic rings, to polystyrene or poly(methacrylate) resin beads of various morphology (Figure 6).78,79... 

Lee et al. [103] synthesized a chiral Rh-complex with a bisphosphine-contain-ing cation as ligand (Fig. 41.8, 2) to improve the immobilization of the transition-metal complex within the ionic liquid. 

The preparation of this type of catalyst is quite simple. HPAs such as phos-photungstic acid were adsorbed onto inorganic supports such as clays, alumina, and active carbon. Subsequently, the metal complex was added to form the immobilized catalyst. If necessary, the catalyst can be pre-reduced. These types of catalysts were developed mainly for enantioselective hydrogenations. For instance, a supported chiral catalyst that was based on a cationic Rh(DIPAMP) complex, phosphotungstic acid and alumina showed an ee-value of 93% with a TOF of about 100 IT1 in the hydrogenation of 2-acetamidoacrylic acid methyl ester (Fig. 42.4 Table 42.2). 

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