Nickel complexes metallacyclopentanes

Recently, other pathways where the C—C bond is postulated to form via metalla-cyclopentanes (Scheme 3) have been established for the catalytic dimerization of ethylene and higher a-olefins by zirconium , tantalum, and nickel complexes. At least for the tantalum systems, the mechanisms of conversion of the metallacyclopentane to the product olefin differ significantly with the specific catalyst. With some variations, the metal-... 

Nickel complexes of alkenes are involved in many catal)dic transformations. Many of the reaction classes of alkenes involve migratoiy insertion of an alkylnickel or a hydridonick-el species. Alternatively, some transformations are initiated by the oxidative cyclization of nickel-alkene complexes with a second unsaturated component to produce five-mem-bered metallacycles. Several examples of nickel-bis(alkene) complexes and nickel metallacyclopentanes are known, and the interconversion of these two structural classes has been studied (Scheme SS). " ... 

The [2 + 2] reaction also occurs for stained olefins 49 and 50, giving only the exo cycloadducts (Scheme 25). It was thought that the reaction proceeds via the exo metallacyclopentane intermediate Y arising from monodentate coordination to the exo face of the bicyclic olefins. The similarity in the electronic spectra of nickel complexes of these olefins to those of the complex of norbomene, which was known to coordinate on the exo face, supports this mechanism. 

The chemistry of group 10 metallacyclopentanes is well established, with metalla-cyclic complexes of nickel and platinum having been extensively investigated, particularly by the groups of Grubbs74 79 and Whitesides80 82 in the 1970s. 

Norbornene or norbornadiene-type substrates with nickel, ruthenium or cobalt catalysts undergo stereoselective dimerization or codimerizations with other substrates. Numerous examples have been reported15. Stoichiometric use of an iridium complex in the cyclodimerization of norbornadiene results in an isolable c.vo-rrans-c.ro-metallacyclopentane 7, formed from two norbornadiene units18. Upon heating in the presence of triphenylphosphanc, the exo-trans-e.YO-dimer 8 is liberated from complex 7. 

The formation of a metallacyclopentane from two moles of ethylene has been observed with nickel phosphine complexes. Metallacyclopen-tanes are often included in mechanisms for the dimerization of olefins. Table 8 contains four Pt complexes, as well as one each of Co, Ni, and Ir. 

Many crystal structures of nickel-alkene complexes have been reported. As demonstrated in Scheme 55, bis(alkene) complexes may exist in equilibrium with the corresponding metallacyclopentane complex. However, several alkene complexes which have the potential to undergo oxidative cyclization to a metallacycle have been fuUy characterized. The substitution chemistry of bis(iY -cycloocta-l,5-diene)nickel(0) (2) is representative of most nickel(0)-alkene complexes, which are readily substituted by a variety of ligands. Bis(q -ethene)(tricyclohexylphosphine)nickel(0) has been prepared and fully character-ized,l " l and a variety of complexes of electron-deficient alkenes such as 69 have been prepared which tend to be more stable than the complexes of ethene (Scheme 56).l" " The alkene complexes may be prepared directly from bis(q -cycloocta-l,5-diene)nickel(0) (2)l" l or from nickel(ll) chloride " that is reduced by zinc metal. 

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