Tt-allylnickel complexes

The TT-allylnickel complex (372), obtained by the action of nickel tetracarbonyl on a 6)5-chloro-4-en-3-one (371), can be methylated at C-4, though not very efficiently, by methyl iodide. The 7r-allylpalladium complex was unreactive to alkylation. 

Even as a toluene emulsion, these complexes show catalytic activity towards ethylene and propylene which is several orders higher than that of TT-allylnickel halides. Paralleling the increase in catalytic activity, the selectivity of this catalyst is also increased—i.e., the products are mainly ethylene or propylene dimers. The most active catalytic systems for dimerizing ethylene and propylene are obtained by replacing toluene with halogenated hydrocarbons such as chlorobenzene since in these more polar solvents, the complexes XIII are soluble. 

Tt-Allylnickel halides. Billington has reviewed the preparation of these complexes from allylic halides using Ni(CO)4 or Ni(COD)2, and their use in synthesis, mainly of natural products (54 references). These complexes react with a wide range of both aliphatic and aryl bromides or iodides as well as aldehydes, ketones, epoxides, and quinones. One advantage is that both allyl ligands react. They do not react with acid chlorides, esters, ethers, nitriles, or acetals. 

Dienes or olefins are polymerized in the presence of a 7r-allylnickel complex and a phosphate as catalysts to afford the polymers having a Tt-allyl group at the end position as shown in eq. (19.50). Then they are able to easily copolymerize with styrene or isocyanates [61,72,92-96]. Further, the polymerization of isocyanate and cyclohexadiene has been tried in the presence of nickel catalysts [94,95]. 

That this comparison seems to have validity (neglecting the legitimated analogy between bis-JT-allylnickel and symmetry-reduced benzene) may be atMboted to the fact that open-chain TT-systems may display some residual aromaticity (see Ref. ). Metal complexes with C3 symmetry must have three identical ligands (S) in the same state. When we change only one l and (S versus L) symmetry is reduced. The structural consequence is the change to a T- or Y-shaped moiety (see Fig. 1 in Scheme 2.5-4 and similar discussions in Refs. Within thcK arran ... 

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