Functional dendrimer periphery

A number of groups have reported the preparation and in situ application of several types of dendrimers with chiral auxiliaries at their periphery in asymmetric catalysis. These chiral dendrimer ligands can be subdivided into three different classes based on the specific position of the chiral auxiliary in the dendrimer structure. The chiral positions may be located at, (1) the periphery, (2) the dendritic core (in the case of a dendron), or (3) throughout the structure. An example of the first class was reported by Meijer et al. [22] who prepared different generations of polypropylene imine) dendrimers which were substituted at the periphery of the dendrimer with chiral aminoalcohols. These surface functionalities act as chiral ligand sites from which chiral alkylzinc aminoalcoholate catalysts can be generated in situ at the dendrimer periphery. These dendrimer systems were tested as catalyst precursors in the catalytic 1,2-addition of diethylzinc to benzaldehyde (see e.g. 13, Scheme 14). 

In order to functionalize the periphery of dendrimers of the type 69, van Koten and coworkers developed a strategy to cleanly obtain 4-lithioaryl substituted carbosi-lane dendrimers of the dendrimer generations zero (n = 4, 70a) and one (n = 12, 70b) (Scheme 24). The bromide-lithium exchange using n-butyllithium was successfully applied and the quantitative lithiation confirmed by hydrolysis or by conversion into functionalized dendrimers by reaction with various electrophiles. 

The dendrimer framework also plays an important role. The catalytic performance measured by activity, selectivity, stability, and recyclability depends on the dendritic architecture, and it is important to distinguish periphery-functionalized, core-functionalized, and focal point-functionalized dendrimers (Fig. 1). Periphery-functionalized dendrimers have catalytic groups located at the surface where they are directly available to the substrate. In contrast, when a dendrimer is functionalized at its core, the substrate has to penetrate the dendrimer support before it reaches the active center, and this transport process can limit the rate of a catalytic reaction if large and congested dendrimers are involved. 

The accessible peripheral catalytic groups enable reaction rates that are comparable to those of homogeneous systems, but the periphery-functionalized dendrimers contain multiple reaction sites and may have extremely high local catalyst concentrations, which can lead to cooperative effects in reactions that proceed via a... 

Another noteworthy difference between core- and periphery-functionalized dendrimers is that much higher costs are involved in the application of core-functionalized dendrimers due to their higher molecular weight per catalytic site. Furthermore, applications may be limited by the solubility of the dendrimer. (To dissolve 1 mmol of catalyst/L, 20 g/L of core-functionalized dendrimer is required (MW 20 000 Da, 1 active site) compared to 1 g/L of periphery-functionalized dendrimer (MW 20 000 Da, 20 active sites). On the other hand, for core-functionalized systems, the solubility of the dendritic catalyst can be optimized by changing the peripheral groups. 

The properties induced by the dendritic framework depend on the location of the functional groups within the structure. Periphery-functionalized dendrimers offer high accessibility of the metal complex, which allows reaction rates that are... 

A general trend observed in many of the reports concerning catalysis with periphery-functionalized dendrimers is that the activity of the catalysts decreases with the dendrimer generation, which is usually attributed to the increasing steric bulk around the metal centers as the dendrimer generation increases. Some of these negative effects have already been discussed in Section II. 

Bipyridinium-type units (also known as viologens) are well-known electron acceptors64 extensively used in chemical and electrochemical redox processes,65 since they can undergo two reversible one-electron reduction processes. Because of these peculiar properties such units can be profitably used to functionalize the periphery of dendrimers, but examples of dendrimers containing a bipyridinium-type unit as a core are also reported.66... 

In the case of surface-block dendrimers, the dendrimer periphery exhibits different functionalities in specific molecular segments. They are formed by coupling of dendrons differing in the nature of their terminal functionalities to a common core unit (cf. Section 3.2.1) [6, 10]. 

In the synthesis of functional dendrimers, interest has hitherto been focussed on variation of the functional core unit or peripheral groups and the resulting effects on the properties of the dendrimer. For a long time, the only function ascribed to the dendritic branches and their repeating units was that of a scaffold linking periphery and core. It was overlooked that, in the interior of the dendrimer scaffold, an individual characteristic (nano)environment can arise which is largely dependent upon the chemical characteristics and the polarity of the repeating units used to construct the dendrimer. Moreover, they can facilitate cascade processes and serve as a platform for cooperative effects between dendrimer branches [37]. 

Core-functionalized metallodendrimers have the advantage of creating isolated sites due to the environment of the dendritic framework. In the case of core-functionalized dendrimers, the molecular weight per catalytic site (ligand/catalyst) is higher than for periphery-functionalized dendrimers, which therefore involves higher costs from a commercial point of view. The... 

Fig. 14 BINAP core-functionalized dendrimers containing long alkyl chains in the periphery...

Because of their reversible electrochemical properties, ferrocene and its methyl derivatives are the most common electroactive units used to functionalize dendrimers. A recently reported example of this class of dendrimers is constituted by giant redox dendrimers (see e.g., the 81-Fc second generation compound 11 shown in Fig. 13) with ferrocene and pentamethylferrocene termini up to a theoretical number of 39 tethers (seventh generation), evidencing that lengthening of the tethers is a reliable strategy to overcome the bulk constraint at the dendrimer periphery [66]. 

We would like to report the synthesis of a star-shape poly(vinylmethyl-c6>-dimethyl)siloxane polymers functionalized in their exterior, which makes them especially suitable for application as catalytic supports. Similarly to catalysts bound to periphery-functionalized dendrimers [3] they offer regularly distributed and available catalytic sites. 

The electrochemical behavior of ferrocene is relatively simple, giving rise to a reversible monoelectronic oxidation process at a very accessible potential. Most commonly, ferrocene has been used to functionalize the periphery of dendrimers, along the scheme illustrated in Figure 2b. Dendrimers 1 [42] and 2 [55] exemplify the commonly observed electrochemical behavior ... 

In contrast to the core-functionalized systems, periphery-functionalized dendrimers have their catalytically active groups located at the surface of the den-... 

Carbosilane dendrimers containing titanium and zirconium complexes on their periphery have been prepared and used in olefin polymerization reactions.Generally, the best synthetic route to these materials involves the synthesis of generations of suitably functionalized dendrimers to which the metal is added in the final step giving products such as 276. Routes involving the incorporation of pre-metallated building blocks gave only low yields of the desired dendrimers. 

Dendrimers having phosphines as branching points (Figure 39 Rh = [Rh(cod)Cl]) were synthesized by acid-base hydrolysis of aminosilanes with alcohols. This organometallic compound could be considered as a periphery- and core-functionalized dendrimer. Interestingly, the complexes can be formed before or after the acid-base reaction that forms the dendrimer. The Rh(l) metallodendrimer of Figure 39 was catalytically active in the hydrogenation of... 

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