Tri-and Tetramerization of Ethylene

Tri- and tetramerization of ethylene to give 1-hexene and 1-octene with high selectivity can be achieved with Cr-based catalysts. For the industrial manufacture of 1-hexene, 7.8 is used as the precatalyst. In this catalytic system 2,6-dimethyl pyrrole, 7.9, is used as the stabilizing ligand and a mix of EtjAl and Et AlCl as cocatalysts. 

Complex 7.8 is an analogue of chromium triacetate where, instead of acetate, 2-ethyl hexanoate groups are present. The main function of 2-ethyl hexanoate is to make the precatalyst soluble in relatively nonpolar organic solvents. Under catalytic conditions, 2-ethyl hexanoate groups are lost, and 7.8 is converted to 7.10. The formation of 7.10 from 7.8 can be explained by the following reactions. 

In reaction 7.2.2.1, the reduction of Cr to Cr by AlEtj is accompanied by the production of ethane and ethylene. Reaction 1.2.22 is basically a ligand exchange reaction where the remaining ethyl hexanoate ligands are replaced by the chlorides. Finally, the catalytic intermediate 7.10 is produced according to reaction 7.2.2.3. In this reaction 2,6-dimethyl pyrrole (LH) loses a proton and coordinates to the metal. 

Note that apart from reducing Cr , AIEtj also acts as a Lewis acid and interacts with the chloride ligand to create more positive charge and room around chromium. This helps in forming the metallacyclic catalytic intermediate 7.11. Computational studies show that in both the catalytic intermediates, 7.10 and 7.11, the pyrrole derivative coordinates to the metal in the fj -mode. 

Apart from 2,6-dimethyl pyrrole, another stabihzing ligand that works well is 7.12. With 7.12, high selectivity to 1-hexene is obtained when partially hydrolyzed AKBu is used as the cocatalyst. 

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Second, a five-membered metallacycle may be formed by pseudo-OA involving the metal atom and two olefinic double bonds. This is shown by reaction 2.3.3.2. As shown by reaction 2.3.3.3, the five-membered metallacycle can also become a seven-membered one by the insertion of an ethylene in the metal carbon bond. This type of reaction is encountered in the Cr-catalyzed tri- and tetramerization of ethylene (Section 7.2.4). 

As we will see in Chapter 7, such metallacyclic intermediates are especially important for the selective tri- and tetramerization of ethylene. 

In the previous chapters we discussed alkene-based homogeneous catalytic reactions such as hydrocarboxylation, hydroformylation, polymerization, and oligomerization. In this chapter we discuss a number of other homogeneous cataljTic reactions where an alkene is the only or one of the principal reactants. Some of the industrially important reactions that fall under the former category are selective di-, tri-, and tetramerization of ethylene, dimerization of propylene, and di-and trimerization of butadiene. 

As shown by reaction 7.2.1, selective di-, tri-, and tetramerization of ethylene produces isomers of butene, hexene, and octene, respectively. Homogeneous catalysts with high selectivity for the linear isomers with the double bonds at the terminal positions are known and industrially used. 

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