Heterogeneously functionalized dendrimers

ELECTRON PARAMAGNETIC RESONANCE (EPR) CHARACTERIZATION OF HETEROGENEOUSLY FUNCTIONALIZED DENDRIMERS... 

Han HJ, Sebby KB, Singel DJ, Cloninger MJ. EPR Characterization of heterogeneously-functionalized dendrimers. Macromolecules 2007 40 3030-3033. 

During the past two decades the homogeneous and heterogeneous catalytic enan-hoselective addition of organozinc compounds to aldehydes has attracted much attention because of its potential in the preparation of optically active secondary alcohols [69]. Chiral amino alcohols (such as prolinol) and titanium complexes of chiral diols (such as TADDOL and BINOL) have proved to be very effective chiral catalysts for such reactions. The important early examples included Bolm s flexible chiral pyridyl alcohol-cored dendrimers [70], Seebach s chiral TADDOL-cored Frechet-type dendrimers [28], Yoshida s BINOL-cored Frechet-type dendrimers [71] and Pu s structurally rigid and optically active BlNOL-functionalized dendrimers [72]. All of these dendrimers were used successfully in the asymmetric addition of diethylzinc (or allyltributylstannane) to aldehydes. 

The statistical functionalization of a fourth-generation PPI dendrimer was first published by Newkome et who used isocyanate-functional AB3 monomers for the end functionalization. As a result, dendrimers having a mixture of three chemically different end groups were synthesized. While similar statistically substituted structures have been proposed, examples of ideal and heterogeneously functionalized PPI dendrimers with an exact number of functionality have also been reported. PPI dendrimers are among the most widely studied families of dendrimers, most likely due to their commercial availability from DSM as Astramol . Unfortunately, DSM has terminated the production of PPI dendrimers and therefore their commercial availability is currently uncertain. 

A 50% functionalization evokes the interesting question, bearing in mind facile transesterification, of how the fluoroalkyl chains will be distributed over the molecules and how they will be distributed on one particular molecule This question has been examined in detail for dendrimers of the poly(propyleneimine) type functionalized with stearic acid [33]. It was proven that the compositional heterogeneity (distribution of degree of substitution) is random, but the positional heterogeneity (spatial distribution of the substituents over the dendrimer molecule) is not random. However, due to flexibility, no particular effect of the spatial distribution can be observed. Unlike the dendrimers, we expect the hyperbranched polyesteramides to be stiffer, so that spatial distribution could lead to interesting effects if the molecule were composed of a functionalized side and a non-func-tionalized side (Fig. 28), as shown possible for dendrimers via a convergent synthesis [34]. 

Based on this concept, Seebach et al. developed the first example of TADDOL-cored dendrimers (Figure 4.41) immobilized in a PS matrix [116]. The resultant internally dendrimer-functionalized polymer beads were loaded with Ti(OiPr)4, leading to a new class of supported Ti-TADDOLate catalysts for the enantioselective addition of diethylzinc to benzaldehyde. Compared to the conventional insoluble polymer-supported Ti-TADDOLate catalysts, these heterogeneous dendrimer catalysts gave much higher catalytic activities, with turnover rates close to those of the soluble analogues. The polymer-supported dendrimer TADDOLs were recovered by simple phase separation and reused for at least 20 runs, with similar catalytic efficiency. 

In recent years, much attention has been directed to highly branched oligosilanes as ligands [1] or as precursors for dendrimers [2] and other silicon-based networks [3]. We are especially interested in the stepwise building and functionalization of branched oligosilanes. Today, the formation of these materials can be obtained by salt elimination reactions of silylmetal derivatives with chlorosilanes or by heterogeneous catalytic disproportionation of methylchlorodisilanes [3]. 

Dendrimers have been investigated as a vector for nucleic acid-based therapies for nearly 20 years, and PAMAM and PPI dendrimers are the two most commonly used kinds that are commercially available.[113-115] Their unique chemical architecture with all primary, secondary, and tertiary amines enabling the proton-sponge effect described above make dendrimers one of the ideal platforms for gene delivery. Furthermore, the close-to-monodispersed chemical structure of dendrimers allows the precise control over their functionalities, which can minimize the unpredictable transfection efficiencies observed in the cases of heterogeneous liposomes and other polymer-based systems.[l 16]... 

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