Convergent dendrimer

A dendrimer-based approach for the design of globular protein mimics using glutamic (Glu) and aspartic (Asp) acids as building blocks has been developed [151]. The preassembled Glu/Asp dendrones were attached to a 1,3-bifunctional adamantyl based on a convergent dendrimer synthesis strategy (see Fig. 28). 

Conventional sulfur vulcanization cure systems, 21 801 Convention du Mtre, 24 434 Convergent dendrimer synthesis method, 26 787-788... 

In general, convergent methods produce the most nearly isomolecular dendrimers. This is because the convergent growth process allows purification at each step of the synthesis and therefore no cumulative effects of failed couplings are found [85, 89]. Appropriately purified convergent dendrimers are probably the most precise synthetic macromolecules that exist today. 

Polymerization hybrids are obtained by the polymerization of a linear chain initiated from the focal point of a convergent dendrimer or the chain ends of a dendrimer, or by polymerization or copolymerization of a dendronized monomer. 

Branched phenylacetylene monomers can be used to construct dendrimers on supports. This was accomplished using a triazene tether as a focal point, the reaction triad outlined above, and an AB2 monomer, as seen in Scheme 7.16 As in similar solution-phase convergent dendrimer syntheses, the first step was to prime the periphery of the dendrimer with t-butyl groups by di-coupling of a triazene-tethered dibromoaryl monomer with (di-f-butyl-phenyl)acetylene. In this and all subsequent coupling steps, an excess amount of the monodendron acetylene was used to drive the reaction to completion. The reagents and catalysts were washed away and the excess monodendron was recovered. At the end of the first step, the tri-aryl dendron I-M3-(t-Bu)4 was cleaved from the support (M represents the dendritic monomer unit). Two equivalents of this product were then coupled with the triazene-tethered di-acetylene ary] monomer to produce the heptaaryl dendron I-M7-(r-Bu)8. 

Fig. 3.2 Molecular periphery on conclusion of a) divergent and b) convergent dendrimer growth (schematic). The step shown on the right generates the final periphery in each case...

The concept of postmodification of the molecule s periphery is thus not only appropriate for the introduction of simple functionalities determining the physical and chemical properties of the molecule (e.g. solubility), but also allows the introduction of complex function-bearing units (e.g. catalytically active units, anti-tumour agents), which could be too sensitive for convergent dendrimer synthesis. 

End group functionalisation on conclusion of divergent or convergent dendrimer assembly... 

Figure 5.27. Comparison of (a) divergent and (b) convergent dendrimer synthetic routes (adapted with permission from Grayson, S. M. Frechet, J. M. J. Chem. i ev.,2001,707,3819. Copyright 2001 American Chemical Society).

Convergent dendrimers, with their versatile three-dimensional scaffold, may be tailored to mimic, perhaps crudely, some elements of enzymatic structures. Numerous catalytic moieties, including manganese porphyrins,253,254 bis(oxazoline) copper complexes,304 305 tertiary amines,306 binaphthol titanium complexes,285 307 titanium taddolates,292,308 thiazolio-cyclophanes,309 and fullerene-bound bisoxazoline copper complexes,310 have been incorporated at the core of dendritic molecules to determine the effect of dendritic encapsulation on their catalytic activity. 

Financial support of our work on convergent dendrimers by the National Science Foundation (DMR 9816166) and the US Department of Energy (Basic Energy Sciences, Polymer Program of the Center for Advanced Materials, Lawrence Berkeley National Laboratory) is gratefully acknowledged. The authors also thank Dr. Stefan Hecht and Prof. Alex Adronov for invaluable discussions. 

Scheme 11.10 CpFe+-induced hexabenzylation of CgMeg applied to direct convergent dendrimer synthesis of a 54-allyl dendrimer.

The synthesis of triazole dendrimers starts from 3,5-dichloro-4-(4-methoxyphenyl)-4H-1,2,4-triazole, which can be obtained from methoxy-phenylisocyanate, as shown in Figure 9.4. This substance has an appropriate AB2-symmetry. Therefore, it is well-suited for convergent dendrimer synthesis. The reaction proceeds in the way of a Williamson type ether synthesis. To control the branching, 3,5-bis-(fen-butyl)-phenolis added. ... 

Soon after the introduction of convergent dendrimer synthesis, " hemispherical, aryl-ester-coated, benzyl ether-based dendrons were reported. Attachment to a divalent core and conversion to the polycarboxylate 25 (Fig. 5) yielded dendriiners that facilitated a 200-fold increase in pyrene (26) solubility in water eompared with that of pure water without the dendrimer. Notably, use of sodium dodecyl sulfate (SDS above cmc 9 X 10 M) for pyrene encapsulation resulted in only an 100-fold enhancement. 

The monodispersed nature of dendrimers has been verified extensively by mass spectroscopy, size exclusion chromatography, gel electrophoresis, and transmission electron microscopy (TEM) [55, 115]. As is always the case, the level of monodispersity is determined by the skill of the synthetic chemist, as well as the isolation or purification methods utilized. In general, convergent methods produce the most nearly isomolecular dendrimers. This is because the convergent growth process allows purification at each step of the synthesis and eliminates cumulative effects due to failed couplings [85, 116]. Appropriately purified, convergent dendrimers are probably the most precise synthetic macromolecules that exist today. 

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