NCN-pincer nickel

An early example of a dendritie eatalyst was reported by Knapen et al. 24), who functionalized GO (generation zero) and G1 earbosilane dendrimers with up to 12 NCN pincer-nickel(II) groups (7a). These dendrimers were applied as eatalysts in the Kharasch addition of organic halides to alkenes (Seheme 3). 

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. 

Based on these preliminary results, a small library of NCN-pincer nickel-containing metallodendrimers was prepared by Van Koten et al. in order to investigate the factors that can affect the catalyst performance and their applicability in nanofiltration membrane reactors [35,36]. The strategy in this... 

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). 

Chelating pincer NCN ligands are known to stabilize Ni(iii) complexes,which play a key role in the nickel-catalyzed Kharasch addition of GGI4 and other halogeno compounds to olefinic double bonds. The Ni(ii) pincer complexes display very low oxidation potentials and are readily oxidized by mild reagents such as GGI4... 

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