Dendrimeric catalysts

One of the main applications of dendrimers is in catalysis allowing easy recycling of the homogeneous catalyst by means of nanofiltration. Carbosilane dendrimers functionalized with diphenylphosphine groups at the periphery have been synthesized and characterized. Palladium complexes of these dendrimers have been used as catalysts in the allylic alkylation reaction. These dendrimeric catalysts can be used in a continuous process using a membrane reactor.509... 

The reduction of ketones or aldehydes with Pl SiFE/KF produces either the mono- or dialkoxydiphenylsilane depending on the stoichiometry of the reaction.75,319 The dendrimeric catalysts 70, 71, or 72 work with I t3Si H to give the silyl ether of acetophenone in excellent yield (Eq. 239).117... 

One approach that has overcome this dichotomy of substituent effects is the use of a combination of statistical effects and cooperativity in dendrimeric catalysts. 

Table 7 Results of the Pd-catalyzed allylic amination of crotyl acetate and piperidine, comparing the supramolecular dendrimeric catalyst with the corresponding monomer3...

Modified diaiyl prolinols at the aryl rings or the proline scaffold " were subsequently reported as successful promoters in the asymmetric epoxidation of disubstituted tra/zs-enones. Zhao and coworkers studied the possibility of using recyclable diaiyl prolinols in this process. Dendrimeric catalyst If and fluoro-derivative Ig/ were synthesised and used at 30 mol% loading in tetrachloromethane as solvent in the epoxidation of a variety of fra/zs-enones (Scheme 7.4). 

Dendrimeric catalysts can be trapped in poly(p-xylylene) nanotubes to make them reusable [108]. 

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. 

Dendrimers are well-defined hyperbranched macromolecules and have been applied to the preparation of the dendrimeric catalysts [37]. Activities of the den-drimeric catalysts are generally comparable to those of the parent low-molecular-weight catalyst. In some cases even higher activities can be observed with the dendrimeric catalyst by means of so called dendritic effects [38]. Piers et al. developed (perfluoroaryl)borane-functionalized carbosilane dendrimers (66), which were used as Lewis acid catalysts for the hydrosilation of acetophenone [39]. In the presence of (66) as a catalyst, the hydrosilylation of acetophenone cleanly proceeded to essentially 100% conversion over the course of several minutes at 5% catalyst loadings at room temperature as shown in Scheme 19.16. 

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... 

Figure 22 Carbosilane dendrimeric Lewis-acid catalyst (32) capped with perfhioroarylborane Lewis acids. (Adapted from ref. 53.)...

The expected contribution of catalysis in this area will derive both from the availability, at low processing costs, of new monomers obtained from biomasses and from the development of an optimized combination of biotechnology processes with classical and new biocatalytic processes. Research priorities for catalysis in the area of polymers from renewable materials for packaging, furniture, domestic water purification and recycling include the need to develop novel catalysts, e.g., for functionalization of polymeric and dendrimeric materials, with side-chain photoactive molecular switches (to be used as smart materials), or the development of multifunctional materials, combining, for example, nanofiltration with catalytic reactivity. 

Repeating the sequence shown in Figure 18.8 another five or six times (when congestion at the periphery occurs) is not an easy task without the introduction of Si-O-Si links and very few silane dendrimers of such a high generation have been prepared. The reaction is much more facile when HSi(CH3)Cl2 is used. Equipped with phosphine complexes either in the core or at the periphery the dendrimeric systems are used as catalysts [12], Controlling the state of platinum seems to be a key problem. 

In order to assess whether intramolecular cooperativity could occur within the dendrimeric [Co(salen)]catalyst the HKR of racemic l-cyclohexyl-l,2-ethenoxide was studied at low catalyst concentrations (2xl0 " M). Under these conditions the monomeric [Co(salen)] complex showed no conversion at all, while the dendritic [G2]-[Co(salen)]catalyst gave an impressive enantiomeric excess of 98% ee of the epoxide at 50% conversion. Further catalytic studies for the HKR with 1,2-hexen-oxide revealed that the dendritic catalysts are significantly more active than a dimeric model compound. However, the [Gl]-complex represents already the maximum (100%) in relative rate per Go-salen unit, which was lower for higher generations [G2] (66%) and [G3] (45%). 

Undoubtedly, the most notable feature of these new dendrimeric organometallic molecules is their ability to act successfully as effective homogeneous catalysts for the Kharasch addition reaction of polyhalogenoalkanes to olefinic C=C double bonds. Indeed, they show catalytic activity and clean regiospecific formation of 1 1 addition products in a similar way to that observed in the mononuclear compounds. Likewise, the nanoscopic size of these first examples of soluble dendritic catalysts allows the separation of such macromolecules from the solution of the products by ultrafiltration methods. 

The amino alcohol-dialkylzinc system can be applied to ehiral amine synthesis. Polymer-supported ephedrine was found to be an effective chiral ligand in the reaction of N-diphenylphosphinoylimines with diethylzinc (Eq. 19) [77-79]. The polymeric catalysts were, however, less efficient than monomeric model reactions. Several dendrimeric chiral ligands containing the ephedrine moiety (60, 61) have also been synthesized and used in the asymmetric alkylation of 7 -diphenylphosphinylimines by diethylzinc [80]. Both yield and enantioselectivity of the reaction were, however, lower when the dendrimeric ligands were used. 

A series of bis-Cp Ti derivatives where the Cp ring is attached to a dendrimeric unit have been obtained by reaction of the potassium salt of dendritic cyclopentadienides with CpTiCl3 or TiCl4 (Scheme 454). Cyclic voltam-mograms and catalytic behavior of these complexes in ethylene polymerization, using MAO as a co-catalyst, have been studied and compared to that of related non-dendritic complexes.1070... 

Ru(bipy)2(CO)2]+ 23 Ru colloids,111 or Pd complexes of dendrimeric phosphines112 were also found suitable. By far the most widely used soluble photosensitizer is [Ru(bipy)3]2 + however, [ReCl(bipy)(CO)3] and [ReBr(bipy)(CO)3] were also applied. The latter two Re complexes served as electron transfer catalysts, too, so the photoreduction of C02 proceeded without further additives.113,114 Heterogeneous photosensitizers, most of all Ti02, are subject to extensive studies. 

Dendrimers are oligomeric, ordered, modularly built, tree-like structures and have been found to have a wide range of applications, one of which is as supports for metal catalysts.Dendrimeric metal complexes can combine the advantages of both homogeneous and heterogeneous catalysts, namely a number of well-defined active sites as well as simple separation and reuse. The relative proximity of metal sites can be controlled by the nature and generation number of the dendrimer. Catalytically active metal complexes can be introduced into the core or the branches of dendrimer (Figure 4). 

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