Agostic interactions, olefin insertion

The transition state was shown to have a four-centered nonplanar structure and the product showed a strong jS-agostic interaction.59 Molecular-mechanics (MM) calculations based on the structure of the transition state indicated that the regioselectivity is in good agreement with the steric energy of the transition state rather than the stability of the 7r-complex. The MM study also indicated that the substituents on the Cp rings determine the conformation of the polymer chain end, and the fixed polymer chain end conformation in turn determines the stereochemistry of olefin insertion at the transition state.59... 

Since molecular mechanics cannot be used to calculate the energy of transition states, suitable models were adopted. These models are extremely similar to the Jt-olefin complex with an orientation of the growing chain rather similar to that adopted when a a-agostic interaction is present. They were often called pre-insertion intermediates because the insertion transition state could be reached from these intermediates with a minimal displacement of the reacting atoms. 

The y-agostic interaction is such that the electron pair in the C, -He bond of the alkyl substituent at the metal is partly donated to the electron-poor metal this may play an important role in making more simple the insertion of a coordinated olefin into the metal-alkyl bond. This hypothesis has been tested in some instances [346-350], showing metal-hydrogen interaction via an oc-agostic bond. 

It has also been suggested that the experimental barrier of activation for propagation in olefin polymerisation is due to rearrangement of y-agostic species into those with a-agostic interactions in order to free the coordination site for the insertion of the next monomer molecule, rather than the actual insertion (Figure 3.19) [358],... 

In the first step, a cis 1,2-insertion of cyclopentene takes place the intermediate derived from this insertion is sterically too demanding to permit insertion of another monomer molecule and is isomerised before another cyclopentene molecule is inserted. Since only /1-hydrogen atoms are available for the stabilisation of the active species (jS-agostic interaction), /1-hydride transfer, followed by monomer rotation or migration, and then reinsertion occur, leading to 1,3-linked polymer chains. The rotation around the Zr olefin n bond results in the formation of cis-1,3-poly(cyclopentylene) [scheme (3a)] [20] while the n face migration of the olefin across the nodal plane of the double bond results in the formation of trans-1,3-poly(cyclopentylene) [scheme (3b)] [19]. 

The activation barriers for the insertion starting from 14c are visibly lowered compared to those obtained for 14. (Table 4-1). Decrease in the barriers demonstrates a role of the y-agostic interactions in stabilization of the TS for the olefin insertion reactions. However, for both metals the barriers are still substantially higher than the activation barriers for olefin insertion observed in the homopolymerization processes. 

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