Dendrimers centers

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]. 

Figure 14.5 Dependence of the average distance between dendrimer centers on dendrimer solution concentration, . Interpenetration of neighboring den-drimers occurs when this distance becomes shorter than 2/ H (according to ref. [27])...

A luminescent unit extensively used to functionalize dendrimers is the so-called dansyl (5-dimethylamino-l-naphthalenesulphonamido group). Dendrimers (up to the third generation, compound 9) containing a single dansyl unit attached off center [39] show that this fluorescent unit, which is very sensitive to environment polarity, is progressively shielded from interaction with water molecules as the dendrimer generation increases. 

Dendrimers with Metal Centers as Linkers and Branching Points... 

In a recent report [171] Newkome and He extended this concept and described the use of two ruthenium centers per appendage [—(Ru)—(x)—(Ru)—] towards construction of a four-directional dendrimer (e.g., 81, Fig. 36). A combination of convergent and divergent approaches, hence, allowed the stepwise construction of metallodendrimers via controlled metal complexation. 

There is a plethora of examples representing dendrimers with metal-binding sites on the periphery. Polycarboxylates were the simplest, early examples [179], although dendrimers possessing more complicated ligand complexing centers have invariably been built via a convergent approach. 

Fig. 1. Introduction of one or more than one stereogenic elements (center, axis, plane or helix) leads to different types of chiral dendrimers...

The stereogenic centers of chiral dendrimers synthesized so far are either generated by asymmetric synthesis, or they are derived from molecules of the pool of chiral building blocks. The only investigation on chiral dendrimers, consisting of achiral building blocks exclusively, was published by Meijer et al., who synthesized dendrimers such as 31 [61] (Fig. 14). This compound ows its chiral-... 

Fig. 14. Chiral dendrimers 31 (prepared as racemic mixture) with a core chirality center. Compounds 32 and 34 are derived from (S)-solketal 33 as enantiopure precursor [61,64,66]...

Fig. 16. Chiral 4th-generation dendrimers from para- or meto-(chloromethyl)-phenyl substituted acetonides and a 1,3,5-benzenetricarboxylic acid center piece [72]... 

When branch bromide 61, consisting of four type-55a building blocks of (S)-configuration at the benzylic stereogenic centers, was coupled with the chiral triol 54, dendrimer 63 was obtained in 51 % yield after purification. With the... 

Fig. 23. Formation of various 2nd-generation dendrimers or dendritic compounds using branch building blocks of (S)- or (.Reconfiguration at the benzylic centers [90], Three of the eight combinations tested by us led to dendrimer formation (products 63,65,66) while five halted at the stage of the doubly coupled products 64,70-73, cf. Fig. 22. Core molecule 69 has less hindered OH groups and is, of course, not an isomer of 54 and ent-54...

Another possibility to find out more about the structure of these dendrimers was chosen by incorporating fluorine atoms. The use of 19F-NMR spectroscopy offered an additional tool to study the conformation of the dendrimer, especially with the fluorines attached close to the stereogenic centers [91 ]. Following our previously developed methods [92], fluorine-containing 1st- and 2nd-genera-tion chiral dendrimers such as 76 were synthesized (Fig. 24). 

Fig. la-d. Different kinds of metal-containing dendrimers a a metal complex as a core b metal complexes only as peripheral units c metal complexes only in the branches d metals as branching centers... 

When the metal complexes constitute the peripheral units (Fig. lb) and/or belong to the branches (Fig. 1 c) of a dendrimer, a number of equivalent metal-based centers are present since dendrimers are usually highly symmetric species by their own nature. The metal-based centers may or may not interact, depending on distance and nature of the connector units. Multielectron redox processes can therefore be observed, whose specific patterns are related to the degree of interaction among the various units. 

In dendrimers based on metals as branching centers (Fig. 1 d), the electrochemical behavior is even more complex since (i) each unit of the dendrimer is electro active, (ii) the chemical nature of the metal-based units constituting the dendrimer may be different, (iii) chemically equivalent units can be different from the topological viewpoint, and (iv) the degree of interaction among the moieties depends on their chemical nature and distance. 

Larger dendrimers based on a Ru(bpy)2+ core and containing up to 54 peripheral methylester units (12) have recently been obtained [29a]. Both the metal-centered oxidation and ligand-centered reduction processes become less reversible on increasing dendrimer size [29b]. 

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