Dendrimer framework

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 the first part of this overview, we focus on the recycling of dendritic catalysts. This part of the review is divided according to the various recycling approaches, and the sections are organized by way of the reactions catalyzed. In the second part, we describe examples in which attachment of the catalyst to the dendrimer framework results in modified performance. (Although we attempted to make a clear division between catalyst recycling and dendritic effects, these two properties cannot always be addressed separately.)... 

A range of more elaborate functional groups have recently been appended to, or embedded within, the dendrimer framework. These derivatives are designed to... 

Schliiter et al. successfully accomplished generation-specific incorporation of an individual aryl bromide functionality into poly(benzyl ether) dendrimers. Whatever its position in the dendrimer framework, this bromide function could also be utilised for further chemical modification because it can react with aryl-boronic acid via Suzuki cross-coupling [42]. 

Subsequent modification of the branching units in the interior of a dendrimer framework has so far received comparatively tittle attention. From a synthetic standpoint, however, this strategy offers significantly more design scope than its counterpart. Thus dendrimers also become accessible whose branching units are hard to synthesise or are either not sufficiently reactive or too sensitive to be used in iterative dendrimer synthesis. However, restricted accessibility of internal groups compared to peripheral functionalities can prove problematic. 

The design of metallodendrimers involves considering the position and repetition of the (catalytically active) metal site in the dendrimer framework, such as on the periphery (A) or at the core (B). Figure 2 shows schematic representations of different types of metallodendrimers. 

Different approaches have been developed for binding a metal or organo-metallic moiety to these dendrimer frameworks. In numerous coordination compounds, the dendrimer and the metal are linked through a dative metal-heteroatom bond [32], while in organometallic compounds the linkage between the metal and the dendritic framework is realized via a or n metal-carbon bonding [11]. 

Intramolecular reactions occurring with both partners being part of the dendrimer framework. At present, this is the most uncommon case, presumably due to synthetic difficulties. 

In the case of the core-functionalized dendrimers, it is expected that a steric shielding or blocking effect of the specific microenvironment created by the dendritic structure might modulate the catalytic behavior of the core [11, 26]. This site-isolahon effects in dendrimer catalysts may be beneficial for some reactions, whereby the catalysts often suffer from deactivahon caused by coordination saturation of the metal centers, or by the irreversible formation of an inactive metallic dimer under conventional homogenous reaction conditions. The encapsulation of such an organometallic catalyst into a dendrimer framework can specifically prevent the deachvahon pathways and consequently enhance the stability and... 

DENs are typically named by the dendrimer from which they are prepared along with the metal dendrimer stoichiometry used in the nanopartide synthesis, e.g. G5-OH(Pt5o). With careful synthetic techniques, nanopartides with very narrow particle size distributions (1.3 0.3 nm) can be selectively prepared inside dendrimer interior cavities [14]. Provided that metal reoxidation is prevented, DENs are stable for long periods of time and do not agglomerate, since the nanopartides are trapped within the dendrimer framework [14, 25]. Further, this general synthetic methodology is quite flexible, having been used to prepare monometallic Pt, Au, Cu, Pd, and Ru nanopartides [14] as well as a number of bimetalUc systems (see below). 

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