Immobilisation strategy

Figure 2,6 Identification of AMP and an immobilisation strategy suitable for a covalent surface coating with biocompatible properties. Reproduced with permission from K. Rapsch, EE Bier, M. Tadros and M. von Nickisch-Rosenegk, Bioconjugate Chemistry, 2014, 25, 2, 308. 2014, American Chemical Society...

In this section several examples of [Mn(saZen)] and iAn porpK) complexes immobilised on solid inorganic supports, with major predominance for the former complex, will be presented and their catalytic activity in epoxidation reactions will be discussed. The section is subdivided by type of catalyst support and immobilisation strategy. 

Fig. 38 Different immobilisation strategies for the preparation of silica-supported Rh catalysts...

A non-covalent immobilisation of chiral trans-N,N-dipTopyl diaminocyclo-hexane was also investigated by the same authors on the basis of acid-base interactions. This immobilisation strategy employed solid acids, such as polystyrene (PS)/sulfonic acids. The involvement of these heterogeneous catalysts in the aldolisation of cyclohexanone with benzaldehydes allowed the aldol products to be synthesised in excellent yields and enantioselectivities of up to 99% ee eombined with moderate to high nnti-diastereoselectivities of up to 92% de, as shown in Seheme 2.33. Furthermore, these polymeric sulfonic acids could be easily reeovered by filtration and reused for six eyeles with a similar stereoseleetivity but a slightly decreased activity. 

An alternative strategy to obtain silica immobilised catalysts, pioneered by Panster [23], is via the polycondensation or co-condensation of ligand functionalised alkoxysilanes. This co-condensation, later also referred to as the sol-gel process [24], appeared to be a very mild technique to immobilise catalysts and is also used for enzyme immobilisation. Several novel functional polymeric materials have been reported that enable transition metal complexation. 3-Chloropropyltrialkoxysilanes were converted into functionalised propyltrialkoxysilanes such as diphenylphosphine propyltrialkoxysilane. These compounds can be used to prepare surface modified inorganic materials. Two different routes towards these functional polymers can be envisioned (Figure 3.4). One can first prepare the metal complex and then proceed with the co-condensation reaction (route I), or one can prepare the metal complex after the... 

An alternative strategy for catalyst immobilisation uses ion-pair interactions between ionic catalyst complexes and polymeric ion exchange resins. Since all the rhodium complexes in the catalytic methanol carbonylation cycle are anionic, this is an attractive candidate for ionic attachment. In 1981, Drago et al. described the effective immobilisation of the rhodium catalyst on polymeric supports based on methylated polyvinylpyridines [48]. The activity was reported to be equal to the homogeneous system at 120 °C with minimal leaching of the supported catalyst. The ionically bound complex [Rh(CO)2l2] was identified by infrared spectroscopic analysis of the impregnated resin. 

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