Ir-Allylnickel halides

The behavior of 3 toward ether or amines on the one hand and toward phosphines, carbon monoxide, and COD on the other (Scheme 2), can be qualitatively explained on the basis of the HSAB concept4 (58). The decomposition of 3 by ethers or amines is then seen as the displacement of the halide anion as a weak hard base from its acid-base complex (3). On the other hand, CO, PR3, and olefins are soft bases and do not decompose (3) instead, complexation to the nickel atom occurs. The behavior of complexes 3 and 4 toward different kinds of electron donors explains in part why they are highly active as catalysts for the oligomerization of olefins in contrast to the dimeric ir-allylnickel halides (1) which show low catalytic activity. One of the functions of the Lewis acid is to remove charge from the nickel, thereby increasing the affinity of the nickel atom for soft donors such as CO, PR3, etc., and for substrate olefin molecules. A second possibility, an increase in reactivity of the nickel-carbon and nickel-hydrogen bonds toward complexed olefins, has as yet found no direct experimental support. 

Examples of w-allylnickel-X compounds (X = anionic ligand) other than 77-allylnickel halides which have been used in combination with (alkyl)aluminum halides as olefin oligomerization catalysts are 7r-allyl-nickel acetylacetonate (11) (Section III), 7r-allylnickel aziridide (4, 56), and bis(7r-allyl)nickel (6) (59). In addition to ir-allylnickel halides, organo-nickel halides such as tritylnickel chloride (60, 61) and pentafluoro-phenylbis(triphenylphosphine)nickel bromide (62), or hydridonickel halides, e.g., trans-hydridobis(triisopropylphosphine)nickel chloride (12) (Section III), give active catalysts after activation with aluminum halides... 

Severe limitations on the usefulness of the classical Wurtz reaction in the production of cross-coupled products have led to the development of many more generally useful variants. In particular, the use of copper catalysis and of stoichiometric organocuprate species have proved very valuable. The reactions of ir-allylnickel halides with sp halides is also represented by equation (1), and the uses of these reagents are treated separately. In order to provide a balanced view of the value of ir-allylnickel halides, some additional reactions with centers other than sp are described. 

The dimeric ir-allylnickel halide complexes are most conveniently prepared from allylic halides and nickel(O) species, such as Ni(COD)2 and Ni(CO)4. Although other allylic systems have been used, the most common procedure involves treatment of an allylic bromide with Ni(CO)4 at 50-70 C. ° This gives rise to the moderately air-sensitive dimeric ir-allyl species (equation 46). In polar coordinating solvents, (DMF, HMPA, N-methylpyrrolidone, etc.) these dimeric species are converted into highly reactive monomeric species (equation 47). ... 

It has recently been suggested (19) that 7r-allylnickel complexes are intermediates in reactions involving allylic halides. Although ir-allylnickel chloride-triphenylphosphine (IX) is formed from allyl chloride and Ni(CO)3P(C6H5)3 without jrielding a carbonylation product (20), the dimeric 7r-allylnickel chloride (X) [prepared (13) by heating allyl chloride with nickel carbonyl in benzene solution] reacts rapidly with carbon monoxide to form butenoylnickel dicarbonyl chloride (XI) (Eq. 13). Moreover, this complex is converted by additional carbon monoxide into butenoyl chloride and nickel carbonyl (13), Eq. (14)... 

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