Nickel carbosilane dendrimer

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). 

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

Van Koterfs group used a chemically inert, lipophilic carbosilane dendrimer scaffold as support material for fixation of up to 12 transition metal complex fragments. The covalently fixed fragments with nickel as catalytic site acceler-... 

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). 

The first example of a catalytically active metallodendrimer, having catalytic groups at the periphery, was reported by van Koten, van Leeuwen and coworkers [20]. These authors prepared the nickel(II) complexes containing carbosilane dendrimers, which were successfully employed in the homogeneous regioselective Kharasch addition of polyhalogenoalkanes to the terminal C=C double bonds. Since these early studies there has been a steadily increasing number of dendrimer catalysts which have been synthesized and studied [15]. In this section, the details of peripherally modified chiral dendrimer catalysts for different asymmetric catalytic reactions will be summarized. 

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... 

The decrease in catalytic activity of the nickel-containing carbosilane dendri-mer shown in Fig. 6.28 was attributed to the formation of mixed complexes with nickel in both oxidation states II and III on the dendrimer surface, which competes with the reaction with substrate radicals occurring in Kharash reactions (Fig. 6.29). 

Their advantage over other types of dendrimers is their straightforward synthesis and, most importantly, their chemical and thermal stabilities. Two distinct steps characterize their synthesis a) an alkenylation reaction of a chlorosilane compound with an alkenyl Grignard reagent, and b) a Pt-cata-lyzed hydrosilylation reaction of a peripheral alkenyl moiety with an appropriate hydrosilane species. Scheme 2 shows the synthesis of catalysts Go-1 and Gi-1 via this methodology. In this case, the carbosilane synthesis was followed by the introduction of diamino-bromo-aryl groupings as the precursor for the arylnickel catalysts at the dendrimer periphery. The nickel centers of the so-called NCN-pincer nickel complexes were introduced by multiple oxidative addition reactions with Ni(PPh3)4. 

These cyclometalation compounds are van Koten s carbosilane pincer nickel dendrimer 9.54 [109, 111], pincer SCS palladium dendrimer 9.55 [110, 112], azobenzene platinum chloro-bridged liquid crytal 9.56 [113, 114], and N,N-dimethylnaphthylene palladium resolving agent 9.57 [117-119] as shown in Fig. 9.10, and photosensitizers for hydrogen production, bis(2-phenylpyridine-4-methyl,4 -fluoride) 9.59 and other photosensitizers with bis(2-phenylpyridine) derivatives 9.60 as shown in Fig. 9.11 [120,121]. 

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