Nickel migratory catalysts

Nickel(II) complexes obtained from nickel(0) or nickel(II) compounds and the (Me3Si)2NP(=NSiMe3)2 phosphorane component, which promote the isomerisation 2, co-polymerisation of a-olefins, are referred to as nickel migratory catalysts. It must be emphasised that this is a peculiarity of the (Me3Si)2NP(=NSiMe3)2 phosphorane other phosphoranes of this type in which only one silicon atom is substituted by a carbon atom give an inactive catalyst [183],... 

In the last decade an enormous revival of late transition catalysts for the polymerisation of alkenes has taken place [45] (remember that the first discovery of Ziegler for ethene polymerisation also concerned nickel and not titanium). The development of these catalysts is due to Brookhart in collaboration with DuPont (Figure 10.28) [46], Detailed low-temperature NMR studies have revealed the mechanism of the reaction [47], Interestingly, the resting state of the catalyst is the ethene-metal-alkyl complex and not the metal-alkyl complex as is the case for the ETM catalysts. For ETM catalysts the alkene complex intermediates are never observed. Thus, the migratory insertion is the rate-determining step (the turnover limiting step , in Brookhart s words) and the reaction rate is independent of the ethene concentration. 

Tamao and Ito proposed a mechanism for the nickel-catalyzed cyclization/hydrosilylation of 1,7-diynes initiated by oxidative addition of the silane to an Ni(0) species to form an Ni(ii) silyl hydride complex. Gomplexation of the diyne could then form the nickel(ii) diyne complex la (Scheme 1). Silylmetallation of the less-substituted G=C bond of la, followed by intramolecular / -migratory insertion of the coordinated G=G bond into the Ni-G bond of alkenyl alkyne intermediate Ila, could form dienylnickel hydride intermediate Ilia. Sequential G-H reductive elimination and Si-H oxidative addition would release the silylated dialkylidene cyclohexane and regenerate the silylnickel hydride catalyst (Scheme 1). 

Acetyl CoA synthetase (ACS) is capable of forming acetyl coenzyme A (Ac-SCoA) from carbon monoxide, CHj, and HSCoA. The so-called A cluster of ACS features a tetradentate tripeptide motif Cys-Gly-Cys accommodating two nickel centers [42] for oxidative addition of CHj, migratory insertion of CO, and, finally, reductive elimination of C0A-SCOCH3. Interestingly, this mechanism strongly resembles the Monsanto acetic acid process using cis-[Rh l2(CO)2] as catalyst. 

Some nitriles also add across 1,2-dienes (Scheme 28). Alkynylcyanation takes place predominantly across the internal double bond of 1,2-dienes to give selectively cyanoalkyl-substituted enynes [83, 84]. Cyanoformates also add across 1,2-dienes in a similar manner in the presence of nickel catalyst alone to give cyanoalkyl-substituted acrylates [85, 86], whereas carbocyanatitm of 1,2-dienes with other nitriles remains unexplored. The 1,2-diene-carbocyanation can be initiated by oxidative addition followed by the coordination of 1,2-dienes at the terminal double bond, and subsequent migratory insertion into the C-Ni bond... 

When the same reaction was carried out with the nickel analogue, complete decomposition to compound 43, in which one of the NHC rings had opened, took place (Scheme 3.15). ° A similar migratory insertion of a ligand on nickel to the NHC was proposed by Hall et al. based on computational studies of the dehydrogenation of ammonia borane by NHC-Ni-NHC based catalysts. 

The study of ethylene polymerization with diimine catalysts has been examined both experimentally and theoretically. Theoretically, it was shown that the migratory insertion barrier of ethylene on the model nickel catalyst [Ni(CH3)(HN=CHCH=NH)]+ is 9.9 kcal mol"... 

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