Triply branched compound

Structurally related to these species are the triply branched compound 56+ and its rotaxanes 66+, 76+, and 86+ (Fig. 13.6)9, in which one, two, or three acceptor units are encircled by the electron donor macrocyclic compound 2. Although these rotaxanes cannot behave as degenerate molecular shuttles because of their branched topology, they are nevertheless interesting from the electrochemical viewpoint. 

The triply branched compound 56+ shows a reversible three-electron reduction process and, at more negative potential, a process strongly affected by electrode adsorption. A comparison with the behavior of dumbbell 34+ enables to assign these processes to the simultaneous first and second reduction of its three bipyridinium units. 

Figure 13.6 Structure formulas of triply branched compound 5b+ and of its rotaxanes 6, 7b, and 8b. ...

It is not necessary to have quite such a highly branched cross-linking agent to make a network of polymer chains. A triply branched compound is the basis for one of the strongest polymers known— one that we take for granted every time we use the kitchen. It is made by a very simple reaction. 

At the beginning of investigations on chiral dendrimers in our own group was the question of how to synthesize chiral, non-racemic derivatives of tris(hydroxymethyl)-methane [82], which we wanted to use as dendrimer center pieces. We have developed efficient diastereoselective syntheses of such triols [83-85] from ( R)-3-hydroxybutanoic acid, readily available from the biopolymer PHB [59,60] (cf. Sect. 2.4). To this end, the acid is converted to the dioxanone 52 [86, 87], from which various alkylation products and different aldol adducts of type 53 were obtained selectively, via the enolate (Fig. 20). These compounds have been reduced to give a variety of enantiopure chiral building blocks for dendrimers, such as the core unit 54, triply branching units 55a and 55b or doubly branching unit 56 [1,88]. 

The branched rotaxanes 66+, 76+, and 86+, besides the bipyridinium units of the triply branched backbone 56+, contain macrocycle 2 whose two DMB units are oxidized at distinct potentials the first oxidation process practically coincides with that of the model compound /i-dimethoxybenzene, whereas the second one is displaced to a slightly more positive potential. 

The use of dendronized silica as a stationary phase in capillary gas chromatography was pioneered in 2001 by Newkome et al.," who used the triply branched dendrons prepared in solution and immobilized on fused silica inside the capillary column. The dendritic architecture provides unique selectivity in separation of a Grob test mixture of compounds. The same dendron-silica hybrids were used by the authors for solventless capillary microextraction (CME) and preconcentration in chemical analysis. 

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