Butadiene codimerization with ethylene

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. 

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. 

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. 

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... 

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. 

The results on Pd complexes as catalyst for the 1 1 codimerization are very limited at this time (85-87). Schneider (85, 86) found that palladous chloride in conjunction with dibutylaluminum chloride catalyzes the formation of exclusively trans-1,4-hexadiene as the 1 1 codimerization product of ethylene and butadiene. A vyn-7r-crotylpalladium complex was postulated as the key intermediate. The criterion for selectivity toward... 

Ziegler-type catalysts based upon Co, Ni, and Fe and in the presence of aluminum alkyls codimerize butadiene with olefins such as ethylene,... 

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. 

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