1,3-Butadiene, nickel-catalyzed dimerization

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. 

Homogeneous nickel complexes proved to be versatile catalysts in dimerization and trimerization of dienes to yield different oligomeric products.46-55 Depending on the actual catalyst structure, nickel catalyzes the dimerization of 1,3-butadiene to yield isomeric octatrienes, and the cyclodimerization and cyclotrimerization to give 1,5-cyclooctadiene and all-trans-l,5,9-cyclododecatriene, respectively46 56 [Eq. (13.13)]. Ziegler-type complexes may be used to form cis,trans,trans-1,5,9-cyclododecatriene37,57 58 [Eq. (13.14)], which is an industrial intermediate ... 

The first indication, reported in 1971, for a nickel-catalyzed intramolecular allylation of an alkenic bond was the dimerization of butadiene to 2-methylvinylcyclopentane (263). This efficient transformation (90%) proceeded with 1.2 mol % of a Ni catalyst (prepared in situ from (Bu3P)2NiBr2 and BuLi) in the presence of methanol and presumably involves a nickel-ene reaction of (262) followed by a -elimi-nation of Ha (Scheme 55). 

Scheme 4 Products of Nickel-Catalyzed Butadiene Dimerization and TrimerizationP- 1...

Bis(acrylonitrile)-nickel catalyzes the dimerization of butadiene to cyclo-octa-1,5-diene in the presence of phosphites (89). Catalytic condensations of butadiene, however, will be discussed in Section IX. Of particular interest are reactions of bis(acrylonitrile)-nickel with norbornadiene. This highly... 

Despite being a less obvious starting material than a l,3-butadiene-2-yl coupling partner, l,2-butadien-4-yl precursors (such as 166 in Suzuki s pioneering example in Scheme 1.26) have seen the most use in dendralene synthesis [118, 131-136]. A couple of more recent examples include the palladium-catalyzed cross-coupling reaction of alkenyl bromides 179 with, for example, the organoindium derived from allenyl bromide 181, or 1,1-dimethyl allene (183) (via a Mizoroki-Heckreaction) (Scheme 1.28) [132,135]. Palladium(0)-catalyzed dimerizations or homocouplings can also furnish the C2-C3 bond [138-142], as can nickel(O)- [143,144] and rhodium(I)-catalyzed ones [137]. 

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 ... 

The solvent properties of alcohols with short carbon chains are similar to those of water and such alcohols could be used as the nonaqueous catalyst phase when the products are apolar in nature. The first commercial biphasic process, the Shell Higher Olefin Process (SHOP) developed by Keim et al. [4], is nonaqueous and uses butanediol as the catalyst phase and a nickel catalyst modified with a diol-soluble phosphine, R2PCH2COOH. While ethylene is highly soluble in butanediol, the higher olefins phase-separate from the catalyst phase (cf. Section 2.3.1.3). The dimerization of butadiene to 1,3,7-octatriene was studied using triphenylphosphine-modified palladium catalyst in acetonitrile/hexafluoro-2-phe-nyl-2-propanol solvent mixtures [5]. The reaction of butadiene with phthalic acid to give octyl phthalate can be catalyzed by a nonaqueous catalyst formed in-situ from Pd(acac)2 (acac, acetylacetonate) and P(0CeH40CH3)3 in dimethyl sulfoxide (DMSO). In both systems the products are extracted from the catalyst phase by isooctane, which is separated from the final products by distillation [5]. 

Unlike nickel catalysts which form cyclic dimers and trimers (1,5-cyclooctadiene and 1,5,9-cyclododecatriene), palladium compounds catalyze linear dimerization of conjugated dienes. 1,3-Butadiene itself is converted to 1,3,7-octatriene. The reaction most characteristic of palladium is the formation of various telomers. 1,3-Buta-diene dimerizes with incorporation of various nucleophiles to form telomers of the following type ... 

A chelating diphosphite prepared from biphenol and PCI3 provides a very stable Ni(0) complex that catalyzes the hydrocyanantion of butadiene without excess ligand. Although the stability of this catalyst is enhanced, the amount of butadiene dimerization byproducts is significant. A related nickel catalyst prepared using a chiral chelating diphosphite based on I -2,2 -binaphthol provides enantioselectivity in the hydrocyanation of norbomene. The major product, J -exo-2-cyanonorbomane, was obtained in an enantiomeric excess of up to 38%. 

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). 

The trans isomer of 1,4-hexadiene is one of the required monomers for EPDM rubber. Although iron-, cobalt-, and nickel-based Ziegler-type catalysts can codimerize butadiene and ethylene, the selectivity to the desired trans isomer is low. A soluble rhodium complex can, however, catalyze the dimerization with high selectivity to the trans isomer. 

Explain why (a) in butadiene dimerization with a nickel catalyst small amounts of vinyl cyclohexene and divinyl cyclobutane are formed (b) ROMP and RCM are thermodynamically favored reactions (c) Pd-catalyzed cross-coupling reactions can be carried out under ligandless conditions by adding tetralkyl ammonium salts (d) in CTOss-coupling reactions the choice of the added base may play a critical role. 

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