Butadiene, nickel complex-catalyzed reactions dimerization

These telomerization reactions of butadiene with nucleophiles are also catalyzed by nickel complexes. For example, amines (18-23), active methylene compounds (23, 24), alcohols (25, 26), and phenol (27) react with butadiene. However, the selectivity and catalytic activity of nickel catalysts are lower than those of palladium catalysts. In addition, a mixture of monomeric and dimeric telomers is usually formed with nickel catalysts ... 

We had established in previous catalytic reactions involving complex 24 that this precatalyst was activated by the removal of the cod (1,5-cyclooctadiene) from the ruthenium by its reaction with the alkyne substrate via a [2 + 2 + 2] cydization as illustrated in Equation 1.64 [57]. Thus, not only does this reaction constitute an activation of the Ru complex 24 by reacting off the cod, it also serves as a novel atom economic reaction in its own right. Both internal and terminal alkynes participate. The overall atom economy of this process is outstanding since cod itself is simply available by the nickel-catalyzed dimerization of butadiene. Thus, the tricyclic product is available by the simple addition to two molecules of butadiene and an alkyne with anything else only needed catalytically. 

The telomerization reaction is catalyzed by various transition metal complexes as nickel, palladium or platinum, but among them, palladium catalysts proved to be the most efficient. With palladium catalysts, linear cis, trans and branched octadienyl ethers are the major products and side products that can arise from the linear dimerization or the degenerative telomerization of butadiene are formed in marginal amounts (Scheme 6). 

Structural characterization of these species has revealed that complex 69 adopts a trigonal-bipyramidal geometry in which the methylallyl moiety occupies the apical position (Ni-GN 189 pm), whereas complex 71 adopts a square-pyramidal structure with the cyanide ligand at the apical position at a relatively long distance from the Ni center (Ni-CN 199 pm). The latter structure is similar to the bromo analog NiBr(allyl)(dippe), prepared from the reaction of the nickel(i) hydrido dimer [NiH(dippe)]2 and allyl bromide. The involvement of allyl cyano species such as 69 and 71 in the catalytic hydrocyanation of butadiene is supported by the following observations (i) complex 69 catalyzes the isomerization of 2-methyl-3-butenenitrile to 3-pentenenitrile (ca. 100 turnovers at 100 °G), (ii) complex 71 decomposes slowly to give Ni(0) complexes of cis- and // 77i--crotonitrile (Scheme 20). 

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