Backbone structure dendrimers

Dendrimers, a relatively new class of macromolecules, differ from traditional Hnear, cross-Hnked, and branched polymers. The conventional way of introducing an active moiety into polymers is to Hnk it chemically into the polymeric backbone or a polymer branch. This synthetic approach results in a topologically complex material. Therefore, a significant effort has to be devoted to improve the structural complexities and functions of the polymers. 

There are reports of numerous examples of dendritic transition metal catalysts incorporating various dendritic backbones functionalized at various locations. Dendritic effects in catalysis include increased or decreased activity, selectivity, and stability. It is clear from the contributions of many research groups that dendrimers are suitable supports for recyclable transition metal catalysts. Separation and/or recycle of the catalysts are possible with these functionalized dendrimers for example, separation results from precipitation of the dendrimer from the product liquid two-phase catalysis allows separation and recycle of the catalyst when the products and catalyst are concentrated in two immiscible liquid phases and immobilization of the dendrimer in an insoluble support (such as crosslinked polystyrene or silica) allows use of a fixed-bed reactor holding the catalyst and excluding it from the product stream. Furthermore, the large size and the globular structure of the dendrimers enable efficient separation by nanofiltration techniques. Nanofiltration can be performed either batch wise or in a continuous-flow membrane reactor (CFMR). 

Fig. 4.12 Examples of different dendritic architectures (a) hyperbranched macromolecule, (b) chemical structure of classic PAMAM dendrimer, repeat unit, and one full branch of generation GI (c) schematic of a backbone for generation G4. (From ref. [106])...

The majority of studies into the catalytic behaviour of dendrimers with chiral catalytic centres at the periphery of the dendritic support have concerned non-phosphine-based catalysts. As has become apparent in these studies, the effect of the dendrimer fixation on the catalytic performance generally depends on the individual system. Factors such as the high local density of catalytic sites, the interaction of functional groups in the dendrimer backbone with the catalysts and the structural rigidity or flexibility of the dendrimers seem to play a role in many cases. 

In order to avoid this unfavourable effect of the functional groups in the dendrimer structure, a rigid hydrocarbon backbone without heteroatoms was synthesized. Dendrimers with poly(phenylethyne) backbones, bearing three and six ephedrine derivatives at the periphery, were studied in the alkylation of aldehydes and N-diphenylphosphinylimines and proved to be highly en-antioselective catalysts. For example, the system containing six catalytic sites catalyzed the addition of diisopropylzinc to aldehydes with enantioselectivi-ties of up to 86% ee. As a third backbone a polycarbosilane dendrimer was used, which is chemically inert and more flexible than the poly(phenylethyne)... 

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