Covalently attached complexes material preparation

The preparation of solids with covalently attached POM complexes is a serious and worthwhile research target because these materials might be expected to be rather stable to POM leaching in solutions. Many new materials of this type have been reported in the literature [16,49,74,137-143] however, catalytic studies on covalently boimd POMs still remain a rare event. In 1992, Judeinstein reported the first POM-polymer hybrid where a lacunary Keggin POM cluster was covalently linked to polystyrene or polymethacrylate backbone through Si-0 bonds [137]. This approach has been further developed by several research groups. 

Covalent bonding refers to the materials made in which the transition metal is bonded directly to the resin through an organometallic bond. Two different approaches can be used to covalently attach metal complexes to polymer supports (i) synthesis of appropriate functional monomers and their (co)polymerization to form catalytically active polymers (Scheme 11.1) or (ii) attachment of metal complexes to preformed functional polymer supports by chemical reactions. Following these approaches, both soluble and cross-linked chiral polymeric metal complexes can be prepared. An example of an organometallic tin catalyst suitable for transesterification was reported by workers at Rohm and Haas Company [3]. 

In the dendritic [Co(salen)] complexes prepared by Breinbauer and Jacobsen the dendrimer again serves as - covalent - support material for the catalytic entities attached to the periphery [62]. These dendritic Jacobsen catalysts were obtained by reaction of the corresponding PAMAM dendrimers with active ester derivates of chiral ]Co(II)-(salen)] units according to standard peptide coupling methods. In hydrolytic kinetic resolution of vinylcyclohexane oxide the dendrimer 14 (Fig. 6.40) showed a dramatically increased reactivity compared to the commercially available monomeric Jacobsen catalyst [63-67]. Whereas the latter merely gave a conversion of less than 1% with an indeterminable ee, 14 afforded a conversion of 50% with an ee of 98 2. 

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