Codimerization of Butadiene and Ethylene

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 1,4-hexadiene is made by codimerization of ethylene and butadiene, with a RhCls/EtOH catalyst (Eq. 12.16). The catalyst is about 80% selective for the rnms-i,4-hexadiene, a remarkable figure considering all the different dimers Chat could have been formed. The catalyst is believed to be a rhodium hydride formed by reduction of the RhCls with the ethanol solvent (Section 3.2). This must react with the butadiene to give mostly the ann -methylallyl (crotyl) intennediate, which selectively inserts an ethylene at the unsubstituted end. The ds/trans ratio of the product Hobably depends on the ratio of the two isomers of the crotyl... 

Recently a codimerization of ethylene and butadiene using various organometallic catalyst systems to give cw-1,4 hexadiene was found, equation (7-5). Hexadiene is a potential third component (or termonomer) of ethylene-propylene-diene-rubber [sometimes called ethylene propylene-terpolymer, EPT or EPDM (according to the ASTM)]. 

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