Steric interactions dendrimers

Bidentate palladium complexes of 5 were formed upon the addition of PdCl2, as shown by P-NMR spectroscopy, and even for the largest dendrimers steric interactions between the dendritic branches did not prevent the facile formation of the bidentate complex. Crotylpalladium chloride complexes were prepared in situ from these dendrimers and used as catalysts for allylic alkylation reactions. The regio-selectivity for the branched product for the alkylation of 3-phenylallyl acetate with diethyl sodio-2-methylmalonate increased when the higher-generation dendrimers were used, albeit at the cost of a lower activity. 

The synthesis described by Lambert and coworkers145 is outlined in Scheme 10. The final step afforded the first generation dendrimer in 85% yield after recrystallization. The structure of the dendrimer was confirmed by mass spectroscopy, NMR spectroscopy and X-ray crystallography, which indicated significant steric interaction between the three dendritic wedges. There exist twenty-seven pathways with seven silicon atoms within the dendrimer, but none of these pathways corresponds to an all-anti configuration which is optimal for electron delocalization. In spite of this, the maximum absorbance in the UV spectrum is red-shifted with respect to linear heptasilane and very similar to that of linear... 

The different lengths of these paths mandate that some terminal groups will be tethered closer to the center of the branched assembly than others and be subject to more steric interactions. This suggests that this dendrimer class has surfaces with higher fractal dimensions [13, 97-99] and interior branching zones which are less ordered than those of the symmetrical Starburst dendrimers (Fig. 17). 

A number of workers have established that encapsulation can be quite efficient. Meijer has shown that if, after encapsulation, the surface groups of the dendrimer are functionalized with a bulky group, the steric interactions on the surface become so severe that the guest is permanently entrapped in the dendrimer. This is another example of a supramolec-ular container compound of the sort discussed in Chapter 4. Hence, dendrimers have been proposed as receptors for use in sensing and catalysis, due to their interior pockets and tailorability. 

The term generation describes the number of times arms have been extended. The nature of each generation can be varied so that mixtures of steric requirements and hydro-phobic or hydrophilic character can be introduced offering materials with varying structures and properties. By varying the hydrophobic and hydrophilic interactions and steric nature of the arms secondary and tertiary structural preferences can be imposed on the dendrimer. 

In contrast, the diastereoselectivity of the dendritic host increases. This indicates that on shielding of the chiral core unit with sterically more demanding dendrons in higher-generation dendroclefts the monosaccharide guests are no longer bound in the immediate vicinity of the chiral core unit instead, a less specific host/guest interaction takes place with the dendrimer scaffold. 

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