Butadiene codimerization

In this codimerization reaction, the predominant complex is 3, which should lead to the ethylene-butadiene codimerization product. If the ethylene in complex 3 is displaced by butadiene to form 7 before the insertion reaction takes place, then a C8 or higher olefin could be formed... 

Using this catalyst norbomene (1) and butadiene codimerize to (2) in yields of about 90 %. Less strained olefins give lower yields of vinylcyclobutane products. 

The purpose of the ethylene-butadiene codimerization process is to produce rrmns-1,4-hexadiene from ethylene and butadiene, as shown in reaction (S.6S). The product is an important monomer in synthetic rubber production. 

Phenyl-1,4-hcxadicnc (122) is obtained as a major product by the codimerization of butadiene and styrene in the presence of a Lewis acid[110]. Pd(0)-catalyzed addition reaction of butadiene and aiiene (1 2) proceeds at 120 C to give a 3 1 mixture of trans- and c -2-methyl-3-methylene-l,5.7-octatriene (123)[lll]. 

Lastly, in perfluorobutadiene s codimerization reaction with butadiene, a significant amount of Diels-Alder adduct is obtained, with the perfluorodiene acting as the diene component [125] (equation 105)... 

Codimerization of butadiene with dicyclopentadiene (example 8, Table II) was shown to proceed via a crotyl-nickel complex (62). Ring contraction of cyclooctadiene (example 10, Table II) appears to be a hydride promoted reaction. The hydride-promoted dimerization of norbomadiene to -toly 1 norbornene (example 9, Table II) appears to be quite different from dimerization via a metallacycle (see Table I, example 16). 

The codimerization reaction we will discuss in this chapter belongs to the last type. The reaction and the reactants involved will be the simplest of them all, i.e., a 1 1 codimerization of ethylene and butadiene to form C dienes. 

Typical Isomer Distributions in 1 1 Codimerization of Ethylene and Butadiene by Various Transition Metal Catalysts... 

Scheme 2. Catalytic cycle for Rh catalyst for 1 1 codimerization of ethylene and butadiene to form 1,4-hexadiene.

The rate equation for the dimerization of ethylene (5) can be used to describe the codimerization in the presence of large excesses of butadiene. The rate of the addition reaction as measured by the disappearance of ethylene is represented in Eq. (5). It is first order in ethylene, proton, chloride, and rhodium. 

Hexadiene is the immediate product found in the codimerization reaction described above in a mixture of ethylene and butadiene. However, the reaction will not stop at this stage unless there is an overwhelming excess of butadiene and an adequate amount of ethylene present. As the conversion of butadiene increases, some catalyst begins to isomerize... 

Hexadiene which is formed by 1,4-addition of hydrogen and a vinyl group to butadiene, is the predominant product in the codimerization reaction. However, there is always a small amount (1-3%) of 3-methyl-... 

The essential steps in the nickel-catalyzed 1 1 codimerization reaction, which involve hydride addition to butadiene and ethylene coordination to the metal atom, were first proposed by Kealy, Miller, and Barney (35) and were later demonstrated by Tolman (40) using a model complex. Tolman prepared the complex H—Ni+L PFe [L = (EtO)3P] and showed that, after prior dissociation to form H—NiL3, it can react with butadiene to form a 7r-crotyl complex 19. 

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