Catalytic Codimerization of Ethylene and Butadiene

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

Butadiene could also be trimerized to give cyclododecatriene. The trimer is again used by Hulls to manufacture nylon 12 and Vestamid . The codimerization of butadiene and ethylene is used by DuPont to manufacture 1,4-hexadiene, one of the monomers of EPDM (ethylene, propylene, diene, monomers) rubber. The role of the diene monomer in EPDM rubber is to provide with two double bonds of different reactivities. The more reactive, terminal double bond takes part in the polymerization with ethylene and propylene. The less reactive internal one is used later on for cross-linking. These important catalytic reactions are shown in Fig. 7.6. 

With their delocalized 7i-electron system, allyl ligands can bond to metals in a manner similar to olefins. Allyl complexes have been detected as intermediates in catalytic processes involving propene or higher olefins and dienes. Examples include the cyclooligomerization of butadiene and the codimerization of butadiene with ethylene. 

Butadiene and ethylene are codimerized with a soluble rhodium-phosphine complex as the catalyst. Very little has been reported on the mechanistic evidence for this reaction. However, a catalytic cycle as shown in Fig. 7.9 involving a rhodium hydride seems likely. Reducing rhodium trichloride with ethanol in the presence of a tertiary phosphine generates the hydride complex 7.32. The 1,4-hydride attack on the coordinated butadiene gives an rf-allyl complex. This is shown by the conversion of 7.33 to 7.34. Ethylene coordination to 7.34 produces 7.35. 

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