Focal point-functionalized dendrimers

The dendrimer framework also plays an important role. The catalytic performance measured by activity, selectivity, stability, and recyclability depends on the dendritic architecture, and it is important to distinguish periphery-functionalized, core-functionalized, and focal point-functionalized dendrimers (Fig. 1). Periphery-functionalized dendrimers have catalytic groups located at the surface where they are directly available to the substrate. In contrast, when a dendrimer is functionalized at its core, the substrate has to penetrate the dendrimer support before it reaches the active center, and this transport process can limit the rate of a catalytic reaction if large and congested dendrimers are involved. 

In core- (and focal point-) functionalized dendrimers, the catalyst may benefit from the site isolation created by the environment of the dendritic structure. Site-isolation effects in dendrimers can also be beneficial for other functionalities (a review of this topic has appeared in Reference (10)). When reactions are deactivated by excess ligand and when a bimetallic deactivation mechanism is operative, core-functionalized dendrimers can minimize the deactivation. 

Effect of Varying Focal Point Functionality on the Convergent Synthesis of Phenylacetylene Dendrimers... 

Supramolecular Assembly of Dendrimers Based on Focal Point Functionalized Dendrons... 

Very interesting antenna systems have been constructed by functionalizing the chain ends of a poly(arylether) convergent dendritic backbone with cou-marin-2 A =435 nm) and its focal point with coumarin-343 X =49Q nm) [33] dendrimer 8 represents the fourth generation. 

Figure 16.14 To investigate hydrogen bonding in dendrimers Zimmerman and Moore [187] prepared dendritic wedges (A-B = CH20 or C=C) functionalized with anthypyridine units at the focal point that function as hosts for benzamidinium guest molecules...

As the name impHes, the core unit, or focal point group, is located at the center of the dendritic macromolecule and, as such, constitutes only a small proportion of the overall dendrimer structure. However the role of functional groups at the central core can have a profound effect on a number of physical properties and be extremely useful in investigating the structure of dendrimers. The central role played by the focal point group in the success of the convergent growth approach has also been well documented in the above discussions. 

The functionality at the central core of the dendritic macromolecule can also be exploited as a unique reactive site for a variety of purposes. Perhaps the most dramatic example is the introduction of self-assembling units at the focal point to give dendritic fragments, 36, which undergo spontaneous self-assembly to give larger dendrimers [68] (Fig- 9). In this case the central core of the dendrimer is a self-as-... 

When the catalyst is located in the core of a dendrimer, its stability can also be increased by site-isolation effects. Core-functionalized dendritic catalysts supported on a carbosilane backbone were reported by Oosterom et al. 19). A novel route was developed to synthesize dendritic wedges with arylbromide as the focal point. These wedges were divergently coupled to a ferrocenyl diphosphine core to form dppf-like ligands (5). Other core-functionalized phosphine dendritic ligands have also been prepared by the same strategy 20). 

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