Carbosilane based dendrimer catalysts

Fig. 4 Schematic structure of parts of Van Koten s carbosilane-based catalysts (note that the structures have four times the number of branches shown here for each dendrimer)...

Ni-Based Dendritic Catalysts. One of the first reported examples of a metalloden-drimer catalyst32 resulted from the attachment of monoanionic chelating N-C-N -type pincer moieties to the periphery of a carbosilane dendrimer, followed by the complexation of nickel(II) ions.22,23,33,34 These pincer-based carbosilane metallodendrimers (Figure 10.2) have been used... 

Recently, Soai et al. reported the synthesis of series of chiral dendrimer amino alcohol ligands based on PAMAM, hydrocarbon and carbosilane dendritic backbones (Figure 4.31) [99-102]. These chiral dendrimers were used as catalysts for the enantioselective addition of dialkylzincs to aldehydes and N-diphenylphosphi-nylimines (Scheme 4.25). The molecular structures of the dendrimer supports were shown to have a significant influence on the catalytic properties. The negative dendrimer effect for the PAMAM-bound catalysts was considered due to the fact that the nitrogen and oxygen atoms on the dendrimer skeleton could coordinate to zinc. 

In contrast to the PAMAM-bound catalysts, chiral dendrimers based on rigid poly(phenylethyne) and flexible carbosilane backbones without heteroatoms proved to be highly efficient catalysts in such alkylation reactions. Excellent enantioselectivities in the asymmetric addition of dialkylzincs to aldehydes (up to 93% ee) and N-diphenylphosphinyli mines (up to 94% ee) were achieved. 

By encapsulation of the trialkylphosphine-modified rhodium catalyst into zeolites, the high chemoselectivity was maintained but the Hb ratio in the product alcohol could be increased by as much as 10 times [65]. Anchoring of trialkylphos-phines to carbosilane dendrimers based on polyhedral silsesquioxane (POSS) produced dendrimeric ligands, which in the hydroformylation-hydrogenation of 1-octene preferentially produced linear alcohols. Regioselectivity exceeded that achieved with the low molecular ligand [66]. 

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