Dendrimers as Support for Phosphines

In contrast to other areas of hydroformylation chemistry, in dendrimer-based hydroformylation phosphorus ligands play a role not only as ligating groups for rhodium but also as scaffolds with amine- or amino alcohol functionahty [120]. Moreover, nanoparticles, colloids, or mesoporous silica have been used to stabilize unmodified [121] or phosphine (e.g., PPhj, diphenylphosphinophenylsulfonate)-modified rhodium complexes [122]. 

Two main strategies for the synthesis of dendrimers can be differentiated divergent and convergent methods [123]. The divergent method is particularly suited for the large-scale preparation of dendrimers. However, products often suffer from purification problems and imperfection. In contrast, the convergent 

In a similar approach, water-soluble dendrimers were constructed by the reaction of PAMAM with varying amounts of Ph2P(CH20H)2Cl [131]. Additional sulfonate groups improved the water solubility further. 

A similar synthesis strategy was employed to construct high-weight catalysts carrying multiple catalytic centers in the outer core of the dendrimer [137]. The catalyst depicted below is characterized by 16 catalytic centers. It was found to be more active in the hydroformylation of vinyl arenes than its synthetic precursor carrying only four catalytic units. The ratio of branched to linear aldehydes ranged from 36 1 to 39 1 at 99% conversion. By simple filtration, the dendrimeric catalyst was separated from the product. Even the 10th hydroformylation cycle proceeded without loss of activity and selectivity. 

FMOC = Fluorenylmethyloxycarbonyl HBTU = 2-(1H-Benzotriazol-1-yl)-1,1,3,3-tetramethyluroniumhexafluorophosphate DIPEA = Diisopropylethylamine 

---