Enantioselective encapsulation, immobilization

Immobilization of chiral complexes in PDMS membranes offers a method for the generation of new chiral catalytic membranes. The heterogenization of the Jacobsen catalyst is difficult because the catalyst loses its enantioselectivity during immobilization on silica or carbon surfaces whereas the encapsulation in zeolites needs large cages. However, the occlusion of this complex in a PDMS matrix was successful.212 The complex is held sterically within the PDMS chains. The Jacobsen catalyst occluded in the membrane has activity and selectivity for the epoxidation of alkenes similar to that of the homogeneous one, but the immobilized catalyst is recyclable and stable. 

Yilmaz, E., and M. Sezgin. 2012. Enhancement of the Activity and Enantioselectivity of Lipase by Sol-Gel Encapsulation Immobilization onto B-Cyclodextrin-Based Polymer. Applied Biochemistry and Biotechnology 166 (8) 1927-1940. 

Very few examples have been described for the non-covalent immobilization of chiral porphyrin complexes (Fig. 26). In the first case, the porphyrin-dichlororutheninm complex was encapsulated in silica, which was prepared around the complex by a sol-gel method [78], in an attempt to prevent deactivation observed in solution in the epoxidation of different alkenes with 2,6-dichloropyridine N-oxide. In fact, the heterogeneous catalyst is much more active, with TON up to 10 800 in the case of styrene compared to a maximum of 2190 in solution. Enantioselectivities were about the same imder both sets of conditions, with values aroimd 70% ee. 

Chiral (salen) Mn(III) complexes have been found to be highly enantioselective for the asymmetric epoxidation of conjugated cis-disubstituted and trisubstituted oleftns[10]. The increasing interest towards this reaction brought some authors to develop the heterogeneous chiral salen catalysts. However, to date three kinds of approach have been adopted for the immobilization of chiral salens (1) Chiral Mn salen complexes were supported on polymers[l 1], (2) The encapsulation of salen complex using ship-in-bottle method was... 

In another recent paper (Sawae et al., 2005), polyethylene glycol microspheres were prepared using an oil-in-water-in-oil double-emulsification method to encapsulate lipase complexes. The PEG microspheres exhibited heat-resistant properties that are advantageous when compared with the lipase complex itself dissolved in organic solvent. It was found that PEG microspheres retained high enantioselectivity in the esterification of phenyl ethyl alcohol and lauric acid in isooctane. The immobilized lipase complexes in the PEG microspheres can be easily recovered, and their activity remained preserved up to 10 reuses. 

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