Green-Rooney mechanism

Another possibility is that carbene species are generated via the dissociative adsorption of ethylene onto two adjacent chromium sites [71]. A second ethylene molecule then forms an alkyl chain bridge between the two chromium sites this can subsequently propagate via either the Cossee or the Green-Rooney mechanism. 

A key question remains how is the olefin formed in the overall process Molecular tantalum complexes are known to undergo facile a- and transfer processes, leading to tantalumalkylidene and tantalum tt-olefin complexes, respectively (mechanism 9, Scheme 29) [98]. Moreover, olefin polymerization with tantalum complexes belongs to the rare case in which the Green-Rooney mechanism seems to operate (Eq. 10, Scheme 29) [102]. Finally, intramolecular H-transfer between perhydrocarbyl ligands has been exemplified (Eq. 11, Scheme 29) [103,104]. 

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+. 

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. 

Green and Rooney81 proposed an alternative mechanism (Scheme 11.13b) that also accounted for Z-N catalysis. The mechanism resembles a metathesis-like pathway by starting with a-elimination to give a metal-carbene hydride followed by cycloaddition with the alkene monomer to form a metallacyclobutane. Reductive elimination finally yields a new metal alkyl with two more carbon atoms in the growing chain. The Green-Rooney mechanism, although plausible overall, requires an a-elimination, a process that is difficult to demonstrate. 

The Grubbs Stereochemical Isotope Experiment The 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. 

Schrock has found an ethylene oligomerization catalyst, Ta(=CH/-Bu)-Hl2(PMe3)3, which does appear to go via metalcycles (Eq. 11.61). After 20-50 ethylene units have been inserted, the chain -eliminates to give a 1-alkene. Since the alkyl form of the catalyst is d, the unmodified Green-Rooney mechanism is allowed. 

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. 

---