Metallacycle Mechanism for Selective Ethylene Oligomerization

Companies using the Ziegler Aufbaureaktion, the SHOP technology, or the a-SABLIN process for the production of 1-alkenes get into trouble if the market for one specific 1-alkene grows very much faster than the market for all other chain lengths (Wass, 2007). Therefore, it is of obvious interest to develop alternative catalyst systems for the selective synthesis of a specific 1-aIkene from ethylene. 

Why is the metallacycle mechanism able to give selective trimerization while the insertion/elimination mechanism found for A1-, Zr-, or Ni-catalyzed ethylene 

At the end of this subchapter it should be briefly noted that ethylene oligomerization is not the only way to produce 1-alkenes. 

Dow Chemical Company operates a 1-octene plant in Tarragona, Spain, in which butadiene is reacted with methanol (2 1 molar ratio) in a homogeneous, Pd-catalyzed telomerization reaction. 1-Octene is formed after hydrogenation of the initial telomerization product by abstraction of methanol (van Leeuwen et al, 2010). 

Section 2.3 introduced the fundamentals of catalysis. There, the fact that both homogeneous and heterogeneous catalysts follow a similar type of catalytic cycle has been highlighted. Indeed, substrates coordination/adsorption, reaction at the catalytic active center and, finally, product elimination/desorption are common features for all catalytic processes. However, a closer look reveals that different metals can react in the cycle in a very different manner with the same substrate. An illustrative example for this fact is the ethylene oligomerization discussed here. 

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