Homogeneous chiral dendritic catalysts

It is clear tliat the attachment of chiral catalysts to dendrimer supports offers a potential combination of the advantages of homogeneous and heterogeneous asymmetric catalysis, and provides a very promising solution to the catalyst-product separation problem. However, one major problem which limits the practical application of these complicated macromolecules is their tedious synthesis. Thus, the development of more efficient ways to access enantioselective dendritic catalysts with high activity and reusability remains a major challenge in the near future. 

Chiral manganese salen catalysts have been widely used for the asymmetric oxidation of unactivated olefins. The dendritic polyglycerol-supported Mn-salen catalyst (44) was developed for the asymmetric epoxidation of the chromene derivative in a continuous membrane fiow reactor. This fiow system involves the continuous removal of the product (and unreacted substrate) from the high-molecular-weight dendritic catalyst (44) by filtration through a nanomembrane (Scheme 7.33). Under optimal conditions, 70% conversion with up to 92% ee was achieved [133]. In this system, however, the dendritic catalyst (44) worked as a homogeneous catalyst rather than a heterogeneous one. 

Fig. 31. Selectivity comparison for the enantioselective addition of Et2Zn to benzaldehyde using different dendritic and non-dendritic homogeneous and heterogeneous Ti-TADDOLates as chiral catalysts [107,110], (S)-.(R) ratios refer to the 1-phenyl-propanol formed...

On use as homogeneous catalysts in the asymmetric reductive alkylation of benzaldehyde with diethylzinc to form secondary alcohols, the corresponding dendritic titanium-TADDOL complexes having either chiral or achiral dendrons gave enantiomeric excesses (ee) of up to 98.5 1.5 at a conversion of 98.7% (for the catalyst with GO dendrons). With larger dendrons the reduction of the ee to 94.5 5.5 (G4) remained within reasonable limits, while the drop in conversion to 46.8% (G4) proved to be drastic. In comparison, the unsubstituted Ti-TAD-DOL complex gave an ee of 99 1 with complete conversion. This negative den-... 

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