Periphery-functionalized dendrimers

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. 

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... 

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. 

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

Apparently, periphery-functionalized dendrimers have the ligands and/or catalysts at the surface of the dendrimer. The transition metals will be directly exposed... 

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). 

Synthesis of Periphery-Functionalized Dendritic Molecules Using Polylithiated Dendrimers as Starting Material, P. 

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. 

In periphery-functionalized dendritic catalysts, the functional groups at the surface determine the solubility and miscibility and thus the precipitation properties. Many dendrimers functionalized with organometallic complexes do not dissolve in apolar solvents, and the presence of multiple metal centers at the periphery facilitates precipitation upon addition of this type of solvent. It is emphasized that the use of dendrimer-immobilized catalysts with the goal of recovery through precipitation is worthwhile only if the tendency to precipitation of the dendritic system exceeds that of its non-dendritic equivalent. 

Percec V, Cho WD, Ungar G. Increasing the diameter of cylindrical and spherical supramolecular dendrimers by decreasing the solid angle of their monodendrons via periphery functionalization. J Am Chem Soc 2000 122 10273-10281. 

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 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]. 

In this system, the catalyst G3-I9 showed a similar reaction rate and turnover number as observed with the parent unsupported NCN-pincer nickel complex under the same conditions. This result is in contrast to the earlier observations for periphery-functionalized Ni-containing carbosilane dendrimers (Fig. 4), which suffer from a negative dendritic effect during catalysis due to the proximity of the peripheral catalytic sites. In G3-I9, the catalytic active center is ensconced in the core of the dendrimer, thus preventing catalyst deactivation by the previous described radical homocoupling formation (Scheme 4). 

Nonetheless, the transposition of homogeneous catalytic reactions from unsupported to dendrimer-supported catalysts is still not straightforward. Various dendritic effects , positive and negative ones, on the activity, selectivity, stability and solubility of metallodendrimer catalysts have been observed in this respect. In our own research we have found that a high concentration of metal centers at periphery-functionalized metallodendrimers may translate into a decrease in the catalytic performance due to undesirable side-reactions between the catalytic sites at the dendrimer surface (Fig. 4 and Scheme 4). In contrast, when the exact same catalyst is located at the focal point of a dendron, this matter is avoided by isolating the active site, thereby providing a more stable albeit less active catalyst (Scheme 13). 

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]. 

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 ... 

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