Alkene Insertions with Nickel-Allyl Complexes

Alkene Insertions with Nickel-Allyl Complexes 

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Alkene Insertions with Nickel-Allyl Complexes... 

Insertion of dienes into M-H bond or M-alkyl bond affords r -allylic complexes or its )7 -alken-J7 -yl resonance form. The allylic complex may further undergo insertion of other unsaturated compounds such as alkene or diene into the unsubstituted or substituted terminal of the allyhc ligand. If successive butadiene insertion takes place, polymers with internal unsaturated bonds are produced as will be described later. A nickel-catalyzed reaction of butadiene with 2 mol of HCN affords adiponitrile, an important feedstock in polymer synthesis (Eq. 1.15). 

The cyclization of allylic acetates tethered with alkenes has been extensively investigated by Oppolzer, employing a number of transition metals including nickel. The process has been termed a metallo-ene reaction however, the mechanism likely involves a sequence of oxidative addition to generate a jt-aUyl complex, alkene insertion, and P-hy-dride elimination. The process is quite general in scope and provides a very useful method for the preparation of 1,4-dienes such as 46 (Scheme 28). 

The reaction of norbornene with A -methylallylnickel bromide or iodide gives linear polymers of the alkene via cw-insertion. Evidence for the stereochemistry of insertion is provided by the reaction of norbornene with h -methylallylnickel chloride, which gives the alkene complex (6). Various types of reaction of (6) then result in cis-insertion of the norbornene into the nickel-allyl bond. Thus treatment of (6) with sodium acetate gives (7), which is shown by A"-ray studies to be formed by an ejcu-c/s-insertion of the alkene. A similar cw-insertion occurs with the corresponding palladium complex. ... 

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