Cossee mechanism

Here we discuss more details of the Cossee mechanism using recent evidence obtained by modern organometallic chemistry, together with relevant results of theoretical calculations. 

The Cossee mechanism has been demonstrated by direct observation of organometallic complexes where a C = C bond inserts itself into an M-C bond as shown in Eq. (7)-(9). A labeling experiment on a cationic platinum complex 111 indicated the reversible insertion of the coordinated alkene into the Pt-C bond as shown in Eq. (7) [142]. 

Polymerization occurs by repeated migratory insertion of olefin into the (Tv-oriented metal-carbon bond by the generally accepted Cossee mechanism [5, 60]. This mechanism is believed to be shared by all transition metal coordination polymerization... 

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.

Site control versus chain-end control. Over the years two mechanisms have been put forward as being responsible for the stereo-control of the growing polymer chain firstly the site-control mechanism and secondly the chain-end control mechanism. In the site control mechanism the structure of the catalytic site determines the way the molecule of 1-alkene will insert (enantiomorphic site control). Obviously, the Cossee mechanism belongs to this class. As we have seen previously, propene is prochiral and a catalyst may attack either the re-face or the, v/-facc. If the catalyst itself is chiral as the one drawn in Figure 10.2, a diastereomeric complex forms and there may be a preference for the... 

Calculations confirm the Cossee mechanism for olefin insertion [7, 26-28]. For the simple Me2AlEt model, the ethene complexation energy is only a few kcal/mol. The activation energy calculated at the highest theoretical level [7] agrees well with the experimental estimate. 

Parrinello studies, Meier suggested the possibility that in dimeric complexes dynamic effects might favour insertion (at a single metal centre, e.g., in 5) over chain transfer, but no free-energy data were provided to support this hypothesis [35], Insertion over two metal centres (as in 6), as an alternative to the standard Cossee mechanism, was shown to have a comparable barrier to the standard mechanism [36, 37]. However, this alternative mechanism does not have an improved propagation/chain transfer balance and hence does not offer a better explanation for the observed polymerization activity. It appears... 

The Ziegler-Natta polymerization of ethylene and propylene is among the most significant industrial processes. Current processes use heterogeneous catalysts formed from Ti(IH)Cl3 or MgCl2-supported Ti(IV)Cl4 and some otganoaluminum compounds. The widely accepted Cossee mechanism of ethylene polymerization is illustrated in Scheme 62. 

Figure 6.6 a Propagation cycle in the synthesis of polyethylene according to the Cossee mechanism b calculated energy/reaction coordinate diagram for the ethene insertion step. 

The catalytic cycle for alkene polymerization, the Cossee mechanism, is extremely simple. Coordination of the alkene to Cp2ZrR+ is followed by insertion to give a new complex Cp2ZrR+. No change in the oxidation state of Zr occurs in either of these steps. 

Figure 4 (a) Modified Cossee mechanism for olefin polymerizations with group 4 transition metals (b) modified mechanism in the presence of an anionic counterion. 

Although the Cossee mechanism accounted for much of the experimental data related to Z-N polymerization, it was a bold proposal because 1,2-inser-tions into M-C bonds of early transition metal complexes were unknown at the time. Metal alkyls, should they form by 1,2-insertion into an M-C bond, ought to have a high propensity to undergo loss of a (3-hydrogen by 1,2-elimination. Thus,... 

Nevertheless, the paucity of evidence for direct M-C alkene insertion was a nagging problem associated with the Cossee mechanism. Several years later,... 

The experiments just described point out the feasibility of the 1,2-M-C insertion described by the Cossee mechanism, but they fail to distinguish between it and the Green-Rooney pathway. Grubbs85 reported definitive evidence in support of the Cossee mechanism when he measured the rate of polymerization (in the presence of catalyst 42) of a 1 1 mixture of H2C=CH2and D2C=CD2 (equation 11.27). There was no kinetic isotope effect, thus supporting the Cossee mechanism. 

In all cases, Grubbs found that the cis-to-trans ratio was always 1 1, thus demonstrating that a-activation does not influence the rate or stereochemistry of alkene insertion. The result of the experiment was the key piece of evidence supporting the Cossee mechanism for Z-N polymerization long sought after by chemists. The experiment allowed researchers to make a clear distinction between metathesis and Z-N polymerization, the former involving the chemistry of the M=C bond and the latter that of the M-C bond.87... 

Since this work, Grubbs and others have suggested that the evidence shows that there is a modification of the Cossee mechanism that may occur, depending on the catalyst system.88 Further work indicates an agostic interaction of an... 

In at least one case, there is an example of ethene polymerization using a Ta-carbene complex in which there is strong evidence for a metathesis-based mechanism. See H. W. Turner and R. R. Schrock, J. Am. Chem. Soc., 1982,104, 2331, a paper that describes oligomerization of up to 35 ethene units in the presence of Ta[=CH(f-Bu)](H)(PMe3)3l2. It seems clear that although the Cossee mechanism is operative when polymerization occurs in the presence of Z-N-type catalysts, some polymerizations may involve metathesis, especially when hydrido-metal carbenes can form readily. 

Modified Cossee Mechanism Showing an a-Agostic Interaction... 

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