Functional dendrimers

C.A. Schalley and F. Vogtle (eds), Dendrimers Functional and Hyperbranched Building Blocks, Photophysical Properties, Applications in Materials and Life Sciences, Vol 5, Springer-Verlag GmbH, Berlin and Heidelberg, 2003. 

It has been demonstrated that dendrimers can be used also as fluorescent sensors for metal ions. Poly(propylene amine) dendrimers functionalized with dansyl units at the periphery like 34 can coordinate metal ions by the aliphatic amine units contained in the interior of the dendrimer [80]. The advantage of a dendrimer for this kind of application is related to the fact that a single analyte can interact with a great number of fluorescent units, which results in signal amplification. For example, when a Co ion enters dendrimer 34, the fluorescence of all the 32 dansyl units is quenched with a 32-fold increase in sensitivity with respect to a normal dansyl sensor. This concept is illustrated in Fig. 3. 

Figure 8.30 Synthesis of dendrimer-functionalized gold nanoparticles.

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... 

Fig. 9. 3rd-Generation PAMAM-dendrimer functionalized with disaccharide end groups [42]... 

Fig. 15. Optical activities of poly(propylene imine) dendrimers, functionalized at the periphery with protected phenylalanine or f-butoxy methoxy benzyl acetate groups, depend on the number of end groups [2]...

Dendrimers containing metal complexes as peripheral units. These dendrimers (Fig. lb) belong to the class of dendrimers functionalized on the surface. Dendrimers coated with up to 48 Ru(Cp)(CO)2R [3], 64 Fe(Cp)2 [4], and 3072 AuCl [5] units have been reported. 

A non-covalently functionalized dendrimer was also applied in a continuous allylic amination reaction.[33] PPI dendrimers functionalized with urea adamantyl groups can act as host molecules for phosphorus ligands equipped with acetyl urea groups (Figure 4.18). The so formed supramolecular complex was reacted with a palladium precursor... 

Figure 4.31. PPI and PAMAM dendrimers functionalized with Pyrphos.[41,42]...

Fig. 16.10 Reaction scheme for immobilization of DNA onto functionalized Si02 substrates. Plasma treated Si02 substrates are denoted as Si OH, APTMS functionalized substrates are denoted as Si NH2, dendrimer functionalized substrates are denoted as Si G(4.5)COOH, and substrates to which DNA capture probes have been immobilized are denoted as Si DNA. Inset Repeat unit of PAMAM dendrimer possessing terminal carboxylic acid functionality...

Figure 12. Way (3) Synthesis of dendrimers functionalized in the core, in the skeleton, and on the periphery.

Linear-dendritic star copolymers [5], most frequently obtained via processes in which dendrimers function as multifunctional initiator cores for the poly-... 

Several studies have been devoted to determine the localization of end-groups in modified polypropylene imine) dendrimers. Goddard et al. [ 157] and Cavallo and Fraternali [158] investigated the properties of the dendritic box, a fifth-generation polypropylene imine) dendrimer functionalized with (t-BOC)-pro-tected L-phenylalanine residues (Figure 16.12a) [159]. 

Baars et al. recently investigated the host-guest properties of polypropylene imine) dendrimers functionalized with tris-3,4,5-tri(tetraethyleneoxy)benzoyl units (Figure 16.15) [191]. These hosts are highly soluble in a broad range of solvents, from apolar solvents such as toluene to polar aqueous media. 

By manipulating the chemical properties of dendrimer functional groups, hydrophilic guest molecules can be dissolved in nonpolar solvents and hydro-phobic molecules can be dissolved in polar solution. This is possible because, independent of an encapsulated guest, dendrimers terminated in hydropho-... 

The convergent synthesis of a range of aryl ester dendrimers with peripheral tetrathiafulvalene units was also reported (Devonport et al. 1998). The dendrimers acquire some amount of the cation-radical tetrathiafulvalene tips on reacting with iodine in solutions. Another promising material is polyphenylene dendrimers functionalized with benzophenone units. On being transformed into the potassium ketyl derivatives, the dendrimers bring forth intermolecular biradicals. These technically attractive species contain four-membered cycles that coordinatively combine two potassium cations... 

In periphery-functionalized dendritic catalysts, the functional groups at the surface determine the solubility and miscibility and thus the precipitation properties. Many dendrimers functionalized with organometallic complexes do not dissolve in apolar solvents, and the presence of multiple metal centers at the periphery facilitates precipitation upon addition of this type of solvent. It is emphasized that the use of dendrimer-immobilized catalysts with the goal of recovery through precipitation is worthwhile only if the tendency to precipitation of the dendritic system exceeds that of its non-dendritic equivalent. 

Meijer et al. 84) investigated the asymmetric addition of Et2Zn to benzaldehyde catalyzed by PPI dendrimers functionalized with chiral amino alcohol ligands (94-95). 

Similar results were obtained by Suzuki et al. (55) in their investigation of enantioselective addition of Et2Zn to various N-diphenylphosphinylimines with PAMAM dendrimers functionalized with 4 and 8 chiral amino alcohol groups. They observed 92% ee for the monomeric ligand, 43% ee for GO, and 30—39% ee for G1 when using N-diphenylphosphinylbenzaldimine as the substrate. Again, a high local concentration of chiral active sites leads to a decrease in enantioselectivity. 

For example, PPI dendrimers functionalized with azobenzene chromophores appended with aliphatic side chains assemble into large spherical vesicles in water below pH 8 (Fig. 11.14 Tsuda et al. 2000). 

Scheme 6. van Koten s construction of a silane-based dendrimer functionalized at the periphery with arylnickel(ll) catalytically active sites. 

Combination of amines and dendrimers functionalized proton scavenger resins efficiently removes excess reagents from amide-formation reactions. Marsh et al.41 used a supported branched polyamine and... 

The rate and mechanism of water exchange have been found to be similar for different generations (gen. 3, 4 and 5) of PAM AM dendrimers functionalized with the same Gd(III) D03A-monoamide chelate and for the monomer Gd(III) D03A-monoamide complex itself [43]. This study gave the first experimental evidence that the attachment of a Gd(III) chelate to a dendrimer did not significantly influence the water exchange kinetics. 

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