Mechanistic Studies and Metallacycles

The following points about the catalytic cycle and the intermediates should be noted. First, the conversion of 6.46 to 6.47 is accompanied by the formation of styrene, which takes place by the insertion of ethylene 

There is competition between ethylene and PPh for occupying the vacant coordination sites of 6.49 and 6.50. For selective a-alkene formation, the Ni-P bond must therefore be of optimum strength. If it is too strong then there may be no reaction. On the other hand, if it is too weak then substitution of a-alkene by phosphine, i.e., 6.51 to 6.47, would be difficult. In such a situation, PE rather than a-alkenes may be the main product. 

The steric and electronic characteristics of the chelating (P, O) ligand exercise considerable influence on the stabilities of all the intermediates and transition states. Thus modifications of the chelating (P, O) ligand, as in 6.54 and 6.55, have notable effects on the product profile and the activity. 

With 6.46 as the precatalyst, solid products that consist of high-molecular-weight oligomers are not formed at all. However, with 6.54 as the precatalyst about 75% of the total product is sohd. Introduction of a polar sulfonyl group as in 6.55 increases the activity of the catalyst by about 100 times as that of 6.46. 

In the Ni-catalyzed oligomerization reaction, chain growth therefore takes place by the Cossee mechanism. However, there is another 

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