Second generation dendrimers

When the preparation of the second-generation dendrimer was attempted by reacting trinuclear platinum-acetylide complex (27) (Fig. 5), which is also a first... 

Figure 13. Various cores (64, 65, 66, 67) and ball stick models of the second-generation dendrimers resulting from them.

Attempts to isolate the ferrocenyl dendrimers described above in a crystalline form suitable for X-ray structural determination have so far been unsuccessful. For this reason, we have used computer-generated molecular models in order to gain further information about the structural features of these materials. Figure 3 illustrates an energy-minimized structure determined from CAChe molecular mechanic calculations of the ferrocenyl dendrimer 2. From these studies, we have measured approximate diameters of 2 run for the first-generation dendrimers 1,3, and 5, and 3 nm for the second-generation dendrimers 2,4, and 6. 

Fig. 1. Structures of compounds used in this investigation 1 - first generation dendrimer, 2 - second generation dendrimer, 3 - terrylendiimide-only compound, 4 - peryleneimide-only compound, 5 -first generation dendrimer with tetracarbonyl core...

Fig. 2. Normalized steady state absorption (left part) and fluoresecence spectra (right part) of first generation dendrimer (solid line) and second generation dendrimer (dotted line).

A combination of hydrosilylation-ethynylation and hydrosilylation-reduction is successfully applied to the syntheses of carbosilane dendrimers bearing 4, 8 and 12 end-ethynyl groups as well as their Co2(CO)6 complexes263. The synthesis of a second generation dendrimer 268 possessing twelve Co2(CO)6(HC=C—) units is shown in Scheme 29 as an example. The structure of 268 is confirmed by X-ray crystallographic analysis263. 

In the present context, solid-phase synthesis has been used primarily for the preparation of peptide and glycopeptide dendrimers [23]. For example, a second-generation dendrimer could be prepared by successive addition of branched polyproline building blocks to a solid phase [24]. The divergent synthesis of polyamide dendrimers on polystyrenes was accomplished [25] by Frechet el al. 1991 (cf. Fig. 2.8) [26]. PAMAM dendrons could be grown up to the fourth-... 

With regard to structure/property relations, Tomalia et al. assumed that the diimide groups in larger dendrimers come into close proximity. The first-generation dendrimer with only six diimide units displayed a lower conductivity, yet the fifth generation with 96 diimide units has no particular advantage over the second-generation dendrimer with just 12 diimide functions. It was therefore postulated that low -stacks are formed in every dendrimer, with the result that... 

Chan et al. synthesized first- and second-generation dendrimers containing up to 12 chiral diamines at the periphery (Fig. 8) [29]. Their ruthe-nium(II) complexes displayed high catalytic activity and enantioselectivity in the asymmetric transfer hydrogenation of ketones and imines. Quantitative yields, and in some cases a slightly higher enantioselectivity compared to those of the monomeric systems (up to 98.7% ee), were obtained. 

The specific rotation [ ]d of the Oth and first generation dendrimers (65 and 67) was recorded at — 59.60 and — 69.70, respectively molar rotation was reported as - 5690 (65) and - 17690 (67), and the molar rotation per tartaric acid unit was determined to be — 1900 (65) and - 1970 (67), respectively. The use of bis(aryl ether) 64 allowed construction of tetrakis(aryl ether) 68, which, when treated with the bistosylate 66, afforded the corresponding dendron to be used for the preparation of the second generation dendrimers subsequent reaction with trisphenol 61 failed to give the desired material presumably due to steric and solubility problems. 

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