Phosphorus-based dendrimers

The first neutral phosphorus-containing dendrimers ( phosphodendrimers ) were described in 1994 by Majoral et al., who subsequently substantially expanded the range of compounds. The phosphorus was bonded to other heteroatoms such as N and O [104] and also to carbon [105]. However, dendrimers are also known in which phosphorus atoms are located in the core unit or in the periphery, and also both in the core and at the branching points as well as the periphery [106]. An example with pentavalent phosphorus atoms in the core unit and at the branching points is shown in Fig. 4.54. 

In an initial assembly step, trichlorothiophosphorus is allowed to react with the sodium salt of 4-hydroxybenzaldehyde. Subsequent activation by addition of a hydrazine derivative yields a first-generation phospho-dendrimer (Fig. 4.54). Iteration of this synthetic sequence ultimately leads to the fourth generation (molar mass 11 269 Da). 

The periphery changes its functionality according to the generation (formyl groups or dichlorothiophosphoryl units). The resulting dendrimers are very 

Reaction of dichlorothiophosphoryl units with allylamines gave corresponding phosphodendrimers with terminal allyl functions while reaction with propargyl-amine afforded molecules with terminal alkyne groups. Peripheral tri- and tetra-functionality could thus be attained [108]. 

Phosphodendrimers are of relevance as modifiers of material surfaces in the development of DNA chips [109]. Organic phosphodendrimers also have potential applications as new gelators in small quantities they permit gelling of organic and organometallic substances in water (hydrogels) under mild conditions [110]. 

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Dubois et al. [184] reported the synthesis of phosphorus based dendrimers terminated with multivalent palladium complexes. Iterative free radical addition... 

The ability to transport nucleic acid into cells depends upon the size of the dendrimer. Thus phosphorus-based dendrimers of the third, fourth, and fifth generation are more efficient than those of lower generations (Fig. 4.35). The nature of the quaternary ammonium end groups also has an influence on transfection ability A dendrimer that is methylated at the terminal nitrogens is less effective than a protonated dendrimer. 

Chemoselective polyalkylation of phosphorus-based dendrimers offers an opportunity to produce cationic dendrimers from neutral ones [66]. 

Fig. 8 Hydrogelators based on ammonium-functionalised phosphorus-based dendrimers...

Figure 8.11 The gold complexing phosphorus-based dendrimer with differentially branched arms that was described by Majoral and co-workers that has potential applications in catalysis [58].

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. 

Scheme 4.49. Construction of P-based dendrimers with neutral trivalent phosphorus cores.

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. ... 

Since the first synthesis of a polypropylene inline) (PPI) dendrimer [1], several families of dendrimers have been prepared including the poly(amidoamines) (PAMAM), polyethers, polyphenylenes), polyphenylacetylenes), polysilanes, silicon, phosphorus and nitrogen-based dendrimers... 

A new divergent synthesis of tribranched phosphorus-containing dendrimers and their metal-containing counterparts was recently proposed [7]. It is based on the acid-base hydrolysis of aminosilanes with derivatives containing OH groups. The preparation involved the reaction of a bifunctionalized silane such... 

Other interests in the area of phosphorus acids and their derivatives has centered on the following subjects synthesis of new ligands and catalysts for known reactions, synthesis of supramolecular logic gates, use of phosphates as electrophiles in palladium-catalyzed cross-coupling reactions, synthesis of phosphate based dendrimers and conjugates with oligosaccharides. 

The ligands 7 with R = 4-hydroxyphenyl have been grafted to the periphery of phosphorus-based dendruners [67-71] by reaction of the phenol function with the surface P-Cl bonds as exemplified for the first generation in Scheme 5.7 [72]. Dendrimers of generation 1—4 (G -7, n — 1-4) have been synthesized, the structure of G2-7 being shown in the Scheme 5.8. 

Especially, the divergent and convergent strategies have been applied to many dendrimer syntheses. Nowadays, there are dendrimers with branching units which are based on silicon, phosphorus and other heteroatoms,1101 dendrimers which are built up supramolecularly (rather than covalently)1111 and even pure hydrocarbon dendrimers.112,131 A dendrimer of the tenth generation with 3072 gold ions at its periphery, which is 15 nanometers in diameter, has been prepared successfully by Majoral et al.1101... 

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