TADDOL, dendronized

A dendritic catalyst 9, which like 7 and 8 has a chiral metal complex as core, was synthesised by Seebach et al. [54, 55]. The core building block was a,a,a ,a -tetraaryl-l,3-dioxolan-4,5-dimethanof (TADDOL), to which both chiral and achiral dendrons and those with peripheral octyl groups can be linked (Fig. 6.36, cf. Section 4.2.3, Fig. 4.62). 

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-... 

Scheme 9 Dendronized TADDOL ligands which are cross-linked to form a dendritic network [58]...

The field of catalytic metallodendrimers has seen many advances, particularly in enantioselective catalysis. " Thanks to the early work of Seebach and coworkers, the synthesis of core-functionalised TADDOL dendrimers 40 has been achieved by using four Frechet dendrons (Figure 7.2). These dendrimers were used as ligands in the synthesis of titanium TADDOLates, which were evaluated as catalysts in the asymmetric addition of diethylzinc to benzaldehyde. In a general manner, the good stereoselectivities (89-97% enantiomeric excesses) detected were found to be comparable to those observed with the monomeric TADDOL ligand (98% enantiomeric excess). No attempt to recycle the catalyst or the ligand was mentioned by the authors. 

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