Mechanism of Alkene Insertion

In the Cossee-Arlman mechanism insertion is considered to be direct. The transition state of 6.5 to 6.6 by the Cossee-Arlman mechanism is therefore as designated by 6.8. In 6.8, for clarity the Cl ligands are not shown and represents the growing polymer chain. 

As we saw in Section 2.2.2, an interaction between metal and the a-hydro-gen of an alkyl group is called an agostic interaction. In an extreme situation, where the interaction between the metal atom and the a-hydrogen leads to formal cleavage of the carbon-hydrogen bond, a mechanism involving a metal-carbene intermediate may be invoked. This proposal is known as the Green and Rooney mechanism, and two of the proposed catalytic intermediates are shown by 6.9 and 6.10. 

The heterogeneous character of the conventional Ziegler-Natta catalyst makes studies directed towards mechanistic and structural elucidation at a molecular level extremely difficult. Experimental evidence is therefore sought from homogeneous metallocene and other related catalysts (see Section 6.5). Such evidence does not support the Green-Rooney mechanism. 

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Shmidt, A. F., Khalaika, A., Nindakova, L. O., Shmidt, E. Y. Mechanism of alkene insertion into the Pd-Ar bond in the Heck reaction. Kinetics and Catalysis (Translation ofKinetika i Kataliz) 1998, 39, 200-206. 

The mechanisms of alkene insertions are known for some square-planar Pt(II) complexes , but it is difficult to distinguish between pathways in which a four-coordinated alkene/hydride species collapses by migratory insertion to a three-coordinated (perhaps solvated) species, and pathways in which a five-coordinated alkene/hydride collapses to a four-coordinated product. Both pathways occur, depending on the ligands. The rates of insertion of ethylene into the neutral and cationic hydrides trans-PtHX(PEt3)2 and trans-[PtHL(PEt3)2] + (X = Cl , NOj", CN L = acetone, CO, PEtj, AsPha, P(OMe)3 and P(OPh)3) leads to Scheme 1 °. 

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