Modified Green-Rooney mechanism

Modified Green-Rooney mechanism (ground and transition state a-agostic interaction)... 

An example of an alkyne insertion involving an early metal complex is shown in Equation 9.72. The insertion of dimethylacetylene into the permethylscandocene-alkyl complex occurs in a manner similar to that for the insertion of olefins into d metal-alkyl complexes. This reaction gave the product of a cis addition. The competition experiment shows that there is no measurable isotope effect of the a-hydrogen, implying that the modified Green-Rooney mechanism is not followed in this case. 

FIGURE 11.4 The Grubbs experiment. Since tbe a-CH bonds of the metal alkyl are not involved in the Cossee mechanism (Eq. 11.58), we expect a 50 50 mixture of isotopomers, as observed in some situations. On the modified Green-Rooney mechanism shown here, we would expect a preferential binding of C—H over C—in the agostic intermediate, which leads to a non-50 50 ratio as observed for certain systems. 

Two major mechanisms have been proposed for alkene polymerization. These are the Cossee-Arlman mechanism and the Green-Rooney mechanism. A modified version of the latter has also been considered to explain the behavior of homogeneous, metallocene catalysts. The original Cossee-Arlman mechanism was proposed for the TiCl3 based heterogeneous catalyst. In the following sections we discuss these different mechanisms in some detail. In the following discussion in accordance with the results obtained from the metallocene systems, the oxidation states of the active surface sites are assumed to be 4+. 

On the cover is a space-filling model of an active rpolypropylene chain. This represents one of the first discoveries of an onsa-metallocene catalyst capable of producing isotactic polyprop ene. The transition structure of the modified Green-Rooney propylene insertion mechanism is also depicted. The enantioselectivity of the propylene insertion step requires a catalyst with both appropriate symmetry and stereorigidity. 

Figure 3. Modified Cossee mechanism for the polymerization of olefins with early transition metals. Green, Rooney and Brookhart introduced the presence of the adjuvant a-agostic interaction in the transition state.

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