Phosphine-based dendrimers

Figure 24 Phosphine-based dendrimers prepared from T8[CH = CH2]8 and used in the hydroformylation reaction.

One of the first results on the use of phosphine dendrimers in catalysis was reported by Dubois and co-workers [16]. They prepared dendritic architectures containing phosphorus branching points which can also serve as binding sites for metal salts. These terdentate phosphine-based dendrimers were used to incorporate cationic Pd centers in the presence of PPh3. Such cationic metalloden-dritic compounds were successfully applied as catalysts for the electrochemical reduction of C02 to CO (e.g. 9, Scheme 9) with reaction rates and selectivities comparable to those found for analogous monomeric palladium-phosphine model complexes suggesting that this catalysis did not involve cooperative effects of the different metal sites. 

Complexation of gold ions, [Au(I)], with peripheral phosphine groups of a P-based dendrimer was reported by Majoral et al. [185]. Transmission electron microscopy (TEM) was used to analyze the large aggregates formed by the dendritic gold complexes and a direct correlation was observed between the size of the particles and the dendrimer generation number. In a recent report [186], Majoral et al. further demonstrated that up to 48 diphosphino groups could be anchored to the surface of dendrimers and various dendritic metal-complexes... 

The first examples of metallated phosphine-functionalized dendrimers that were suitable as homogeneous catalysts were reported by Reetz et al. [42], DAB-based phosphino dendrimers were synthesized from the commercially available DAB-polypropylene imine dendrimers (DAB = 1,4-diaminobutane) via a double phosphination of the amines with diphenylphosphine and formaldehyde. These dendrimers were loaded with Pd(II) centers by the reaction of the phosphine-functionalized dendrimer with [Pd(Me)2(tmeda)] (tmeda = tetramethylethylenediamine), in which the diphenylphosphino end groups act as a bidentate ligand (Scheme 6). 

The majority of studies into the catalytic behaviour of dendrimers with chiral catalytic centres at the periphery of the dendritic support have concerned non-phosphine-based catalysts. As has become apparent in these studies, the effect of the dendrimer fixation on the catalytic performance generally depends on the individual system. Factors such as the high local density of catalytic sites, the interaction of functional groups in the dendrimer backbone with the catalysts and the structural rigidity or flexibility of the dendrimers seem to play a role in many cases. 

Brunner et al. [26] synthesized and applied so-called dendrizymes in enan-tioselective catalysis. These catalysts are based on dendrimers which have a functionalized periphery that carries chiral subunits, (e.g. dendrons functionalized with chiral menthol or borneol ligands). The core phosphine donor atoms can be complexed to (transition) metal salts. The resultant dendron-enlarged 1,2-diphosphino-ethane (e.g. 16, see Scheme 17) Rh1 complexes were used as catalysts in the hydrogenation of acetamidocinnamic acid to yield iV-acetyl-phenylalanine (Scheme 17) [26]. A small retardation of the hydrogenation of the substrate was encountered, pointing to an effect of the meta-positioned dendron substituents. No significantly enantiomerically enriched products were isolated. However, a somewhat improved enantioselectivity (up to 10-11% e.e.) was... 

As this chapter covers two years of the literature relating to the above area, it has been necessary to be somewhat selective in the choice of publications cited. Nevertheless, it is hoped that most significant developments have been noted. The period under review has seen the publication of a considerable number of review articles, and most of these are cited in the relevant sections. In addition, several reviews having a broad relevance to the chemistry of phosphines have appeared, relating to the chemistry of silicon-based phosphines, and the specific contribution of phosphorus in dendrimer chemistry and that of related nanomaterials. ... 

Decomplexation of the P(III) moiety of the boronhydride complex (10) by the base l,4-diazabicyclo[2.2.2]octane (DABCO) followed by an internal Staudin-ger reaction has been reported to yield the linear polymer (12). Hyperbranched polymers are obtained by using the same procedure but starting from the diphosphine (13). Treatment of (11) with NaNs leads to an intramolecular Staudinger polycondensation affording polymer (12) as well. Compound (13) has proven to be a useful starting material for the synthesis of dendrimers with phosphine end groups. ... 

Dendrimers having phosphines as branching points (Figure 39 Rh = [Rh(cod)Cl]) were synthesized by acid-base hydrolysis of aminosilanes with alcohols. This organometallic compound could be considered as a periphery- and core-functionalized dendrimer. Interestingly, the complexes can be formed before or after the acid-base reaction that forms the dendrimer. The Rh(l) metallodendrimer of Figure 39 was catalytically active in the hydrogenation of... 

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