Hexadiene from Butadiene and Ethylene

4-Hexadiene is utilized as a third co-monomer in ethylene/propylene elastomers. While the terminal double bond is incorporated into the polymeric chain, the internal one is not and can be cross-linked (vulcanized) to confer 

4-Hexadiene in its E form is manufactured from butadiene and ethylene (Equation 20). The Z isomer must be kept to a minimum owing to its adverse effect on polymerization. The reaction, first reported in 1961, was commercialized by DuPont. The present world production is estimated around 2.5-3 kt/a. Various metal salts (Ni, Co, Fe) were tested, but the best results were obtained with RhCls.hydrate. 

Optimal conversion to 1,4-hexadiene is favoured by a large excess of ethanol over RhCla. 3-Methyl-1,4-pentadiene and 2,4-hexadiene are also formed in low yields. Butadiene and ethylene are used in excess to restrict isomerization to 2, 4-hexadiene as they compete with the product (1,4-hexadiene) for the coordination sites involved. 

The reaction is stereoselective, 1,4-hexadiene being mainly obtained in the E configuration the Z-isomer is present only to a small extent and its formation can be further decreased by donor ligands such as Bu3P=0 and (Me2N)3P=0. The reaction is also regioselective the positional isomer CH2=CHCH(CH3)CH=CH2, derived from ethylene insertion into the more substituted carbon-rhodium bond of the allylic system, is present to less than 1 % when ethanol is in large excess. 

As in the other cases of C-C bond formation catalyzed by transition metal complexes, the critical step is to form the initial metal-carbon bond, which is butadiene insertion into a Rh-H bond to form a Rh-allylic species. 

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Figure 9 Representation of the Rh-catalyzed formation of E 1,4-hexadiene from butadiene and ethylene (other ligands on Rh omitted for simplicity).

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