Functional dendrimer scaffold

In the synthesis of functional dendrimers, interest has hitherto been focussed on variation of the functional core unit or peripheral groups and the resulting effects on the properties of the dendrimer. For a long time, the only function ascribed to the dendritic branches and their repeating units was that of a scaffold linking periphery and core. It was overlooked that, in the interior of the dendrimer scaffold, an individual characteristic (nano)environment can arise which is largely dependent upon the chemical characteristics and the polarity of the repeating units used to construct the dendrimer. Moreover, they can facilitate cascade processes and serve as a platform for cooperative effects between dendrimer branches [37]. 

Fig. 3.9 On b ifunctionalisation according to Majoral et ai. daughter dendrons are produced divergently starting from specific functions in the interior of the dendrimer scaffold...

However, these bifunctionalisation methods are comparatively laborious and applicable only in special cases, since the monofunctionalisation step is limited to substrates possessing an additional coupling site in protected form for the second functional unit. A more versatile method of local bifunctionalisation, which has no need of a deprotection step and also utilises commercially available dendrimer scaffolds, consists in the functionalisation of POPAM dendrimers bearing amine terminal groups with sulphonyl chlorides and subsequent substitution of the sulphonamide proton with other sulphonyl chlorides [63] or with alkyl- or (dendritic) benzyl bromides [64] (see Fig. 3.13). 

Whereas the bifunctionalisation strategies presented so far are relatively straightforward, the synthesis of multifunctional dendrimers with more than two kinds of functional units requires considerable synthetic effort. Preparation of dendrimers with a functional core and additional functional units in the dendrimer scaffold and in the periphery requires de novo synthesis of the entire dendrimer scaffold, with the synthesis conditions having to be tolerable for all groups (Fig. 3.15). 

De novo synthesis of dendrimer scaffold necessary a) Convergent synthesis Dendron assembly using appropriately prefunctionalised monomeric building blocks (e.g. AB2FG) starting from the peripheral functional units of the dendrimer to be... 

Specific attractive interactions between the dendrimer building blocks (e.g. hydrogen bonds between amino acid building blocks) or complexation of metal ions [86] by suitable functional groups in the dendrimer scaffold can favour the formation of chiral substructures and effect distortion or folding of the overall structure. Interactions with solvent molecules can also have a pronounced effect on the overall structure. 

Baigude, H. Katsuraya, K. Okuyama, K. Yachi, Y. Sato, S. Uryu, T. S3mthesis of dicarboxylate oligosaccharide multilayer terminal functionality upon poly(lysine) dendrimer scaffolding. J. Polymer Sci. Part A 2002, 40, 3622-3633. 

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