Pore asymmetric epoxidation

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

For investigating the pore confinement effect, the chiral Mn(Salen) catalyst was immobilized in MCM-41 and MCM-48 with different pore sizes [84]. In the asymmetric epoxidation of unfunctionalized olefins with m-chloroperoxybenzoic acid as oxidant, it was found that the conversion and enantioselectivity were closely correlated with the pore size of the supports. The catalysts immobilized on MS with large pore sizes exhibited higher conversion. For the MCM-41-supported catalyst, the enantioselectivity increased with increasing pore size. However, for MCM-48-supported catalysts, the compatible pore size of the support with the substrate was found to be beneficial for obtaining higher enantioselectivity in olefin epoxidation. 

This chapter focuses on several recent topics of novel catalyst design with metal complexes on oxide surfaces for selective catalysis, such as stQbene epoxidation, asymmetric BINOL synthesis, shape-selective aUcene hydrogenation and selective benzene-to-phenol synthesis, which have been achieved by novel strategies for the creation of active structures at oxide surfaces such as surface isolation and creation of unsaturated Ru complexes, chiral self-dimerization of supported V complexes, molecular imprinting of supported Rh complexes, and in situ synthesis of Re clusters in zeolite pores (Figure 10.1). 

In the presence of the immobilized (2S,4S)-4-hydroxyproline Mo catalyst (13b), nerol and geraniol react selectively with r-BuOOH to form the 2,3-epoxide with ee values of 64 and 47%, respectively. Surprisingly, when Mo is complexed by the diastereomeric (2S,4R) form (13a), racemic epoxidation is observed. The enantioselective catalysis appears to be promoted by immobilization in the zeolite USY pores. Indeed, in the epoxidation of nerol, an ee of 10% was found for the homogeneous asymmetric Mo complex, whereas the supported catalyst favors the selective production of the (2S,3R)-epoxide (64% ee). 

Long-range electron transfer is postulated to occur from ferrocene to tris(bipyridine)iron(III) constructed within the pores of a NaY zeolite. The iron bipyridine complex is too large to move throughout the faujasite pores to the surface, thus requiring the long-range transfer. The asymmetric catalyst, titanium tartrate, has been prepared inside NaY and used as an immobilized catalyst for the epoxidation of cinnamyl alcohol. ... 

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