Nickel dendritic carbosilane

Fig. 6.27 Nickel-loaded carbosilane dendrimers 1-3 of increasing generation number (GO—C2) and a corresponding non-dendritic reference substance 4...

Fig. 6.29 Dendritic carbosilane-nickel complexes with decreasing catalytic activity owing to mixed complex formation...

An early example of a dendritic catalyst was reported by Knapen et al. 24), who functionalized GO and G1 carbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a) and applied them as catalysts in the Kharasch addition of organic halides to alkenes (Scheme 3). 

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). 

Ni-Based Dendritic Catalysts. One of the first reported examples of a metalloden-drimer catalyst32 resulted from the attachment of monoanionic chelating N-C-N -type pincer moieties to the periphery of a carbosilane dendrimer, followed by the complexation of nickel(II) ions.22,23,33,34 These pincer-based carbosilane metallodendrimers (Figure 10.2) have been used... 

A soluble dendritic Ni catalyst for the atom-transfer radical addition (ATRA, i.e., polyhalogenated alkane addition to olefins, the Kharasch addition) was described by van Leeuwen and van Koten et al. in 1994 [17]. GO and G1 carbosilane den-drimers, fimctionalized with NGN pincer-nickel(II) groups, were synthesized and applied as homogeneous catalysts for the addihon of organic halides to alkenes [Eq. (7)]. 

---