Phosphines dendrimers

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. 

Mizugaki et al. 74) have recently utilized thermomorphic properties of Pd(0)-complexed phosphinated dendrimers for dendritic catalyst recycling. Using the method developed by Reetz 16), they prepared dendritic ligands containing, respectively, 2, 8, 16, and 32 chelating diphosphines. Palladium dichloride was com-plexed to the dendrimers, and a reduction in the presence of triphenylphosphine gave the Pd(0)-complexed dendrimers (80—83). The dendritic complexes were active... 

Palladium complexes with monocoordinated phosphine ligands were synthesized by mixing the corresponding phosphine dendrimer with [Pd(cod)MeCl], which afforded exclusively trans dendrimer(MePdCl) complexes (P/Pd = 2). Complexes in which the ligand is coordinated in a bidentate fashion were synthesized by reaction of the respective bisphosphine den-... 

Among many novel chiral phosphines, dendrimers containing chiral diphosphines may be of special practical importance. Why ... 

Water soluble polymers such as poly(enolato-co-vinyl alcohol-co-vinyl acetate) (PEW) act as polymeric ligands to Rh in the biphasic hydroformylation of olefins.170 Another type of polymeric ligands are phosphinated dendrimers, i.e., regularly branched polymers with an almost spherical morphology, which carry PPh2 substituents on the surface, suitable, e.g., for attaching ruthenium complexes.171... 

Given the simplicity of the above characterization of the reaction product by P NMR [13] and the excellent selectivity of this model reaction when excess [Ru3(CO)i2] was used, the same reaction between the phosphine dendrimers and [Ru3(CO)i2] could be more confidently envisaged. This reaction, catalyzed by 1 % equiv. [Fe Cp( / -C6Me6)] was carried out in THF at 20 °C. The dendrimer-cluster assembly was obtained in 50 % yield. This shows the selectivity and completion of the coordination of each of the 32 phosphino ligands of P-P to a Ru3(CO)h cluster fragment (Scheme 34). 

Finally, the 64-branch phosphine DAB-t e r-G4-[N(CH2PPh2)2]32 analogously reacts with [Ru3(CO)i2] and 1 % [Fe Cp( -C6Me6)] (20°C, THF, 20 min) to give the dark-red 192-Ru dendrimer. Characterization of the purity of these dendrimer-cluster assemblies is conveniently monitored by P NMR. This application should find extension to other metal-carbonyl clusters and other families of phosphine dendrimers. 

Scheme 11.17 Electron-transfer-chain catalyzed ligand substitution of one Ru-coordinated CO by dendritic phosphine termini in Reetz s 32-phosphine dendrimer under ambiant conditions leading to the 32-Ru3(CO)u dendrimer-cluster. The ETC mechanism41-42 proceeds for the introduction of the 32 cluster fragments in the dendrimer for ligation of the first Ru3(CO)n fragment to the dendritic phosphine. Then, this first complex [dendriphosphine.Ru3(CO)n] would undergo the same ETC cycle as [Ru3(CO)12] initially does to generate the bis-cluster complex [dendriphosphine. Ru3(CO)n 2], and so onto Scheme 11.18.

E. Alonso, D. Astruc - Introduction of the Cluster Fragment RusfCOjn at the periphery of Phosphine Dendrimers Catalyzed by the Electron-Reservoir Complex [Fe CpfCsMes)], J. Am. Chem. Soc. 2000, 122,3222... 

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

More recently, P-cored derivative (116) was prepared from a straightforward combination of a Heck coupling, to afford an intermediate functionalised stil-bene phosphine oxide (114),a Horner-Wittig reaction yielding the phosphine oxide (115), and finally trichlorosilane reduction (Scheme 31) [89]. Using similar strategies, both the valence isoelectronic N- (117) and C- (118) cored dendrimers have been prepared (Scheme 31). 

Figure 19 explains what in principle happens the cluster monolayer on the dendrimer film is mobilized by means of CH2CI2 vapour (a b). The phosphines are then removed by the SH functions (b->c). The bare AU55 nanoclusters move between the dendrimer molecules to form crystals (Auss) which finally appear on the surface (c d). The formation of crystalline superlattices of naked AU55 particles proves their stability which is founded in their perfect cuboctahedral shape. The (Auss) species is a novel modification of the element gold. 

Togni and co-workers have used the convergent methodology to link phosphine-containing chiral ferrocene ligands on the cyclophosphazene core to obtain dendrimeric structures of the type 37 (Fig. 21) (201). The reaction with the cyclophosphazene end occurs by the replacement of the P-Cl bond and by the formation of the P-0 bond. The dendrimers contain twelve and sixteen ferrocene moieties respectively. The phosphine units present can coordinate to Rh(I) to afford metallic dendrimers, which have been shown to be excellent catalysts for the enantioselective hydrogenation of dimethyl itaconate. The product... 

This methodology has been repeated to give dendrimers up to the third generation. A variation of this strategy consists of utilizing the phosphine Ph2PCH2OH in a Mannich-type reaction with the terminal NH group on the cyclophosphazene in the second step of the dendrimer... 

Other complexes with phosphine sulfides are the three coordinate [Au (S-PPh2)2C(PPh2S) (PPh2Bun)],302 the /i4-phospha-l-thiethane compound [Fe3(CO)9(/i4-PS)(AuPPh3)2] (558),3 27 and the gold(I) dendrimers containing P=NP=NP=S or P=NP=S groups as (559).3028... 

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... 

Dubois et al. [4] describe the synthesis of organophosphine dendrimers via the sequential addition of diethylvinyl phosphonate to primary phosphines followed by reduction with lithium aluminum hydride (Scheme 2). Metallation of... 

The selective and facile cleavage of the benzylic ether linkages of 1,2,3 or 4 is accomplished by treatment with iodotrimethylsilane to form the corresponding benzylic iodide. Further addition to these iodide derivatives of 1 affords dendrimers of generation 1 with phosphonium ion sites at the periphery. Such a strategy is conducted up to generation 3 with a phosphine or a phosphonium core (Scheme 3). 

Three generations of dendritic phosphines have been prepared from 3,5-diaminobenzoylglycine and 9-fluorenylmethoxycarbonyl-L-phenylalanine. The dendrimers were then attached to MBHA resin, treated with CH20 and Ph2PH, and converted to their Rh complexes. The polymer-supported complexes are excellent catalysts for the hydroformylation of alkenes, which could be recycled.283 The bidentate diphosphine A,A-bis-(P-(phosphabicyclo[3.3.1] nonan) methyl)aniline was prepared by phosphanomethylation of aniline. It forms a Rh-complex which is a highly regioselective catalyst in the hydroformylation of citronellene.284... 

---