Beta hydride elimination

The active site is a cationic metallocene alkyl generated by reaction of a neutral metallocene formed from reaction with excess MAO or other suitable cocatalysts such as a borane Lewis acid. This sequence is shown in Figure 5.1 employing MAO with ethylene to form PE. Initiation and propagation occur through pre-coordination and insertion of the ethylene into the alkyl group polymer chain. Here, termination occurs through beta-hydride elimination... 

By considering the H-migration origin/destination, one may distinguish I, II and III/IV. On this basis, experiments (i) and (ii) with a type A catalyst as shown in Scheme 12.9 eliminated mechanisms I and II from consideration this left III and IV which were both fully consistent with the results. The outcome for (i) is obvious the allylic hydrogens (see Hb in mechanism I, Scheme 12.8) are not involved in the reaction. The outcome for (ii) is more subtle and relates to the stereochemistry attending fceta-carbopalladation and beta-hydride elimination which are both known to proceed with syn stereochemistry. Thus, mechanism II which does not involve a beta-hydride elimination would not affect the alkene stereochemistry (see Hc in II, Scheme 12.8), as was revealed by D-labelling, Scheme 12.9. In contrast, mechanisms III and IV should reverse the stereochemistry (see Hc in III and IV, Scheme 12.8), as was observed. 

If one now considers 16, the diastereoisomer of 17, it is evident that, due to the syn relationship between the Pd and the methyl group, syn beta-hydride elimination can only proceed in two directions, one to generate 11 (via 15) and one to generate 12. Here, then, was solution to the apparent paradox mechanism IV need merely be adapted so that intramolecular addition of the Pd-H to the alkene in intermediate 15 generates 16, and thus 12. 

Figure 2.8 Termination of the polyethylene chain by molecular hydrogen (top) and by Beta-hydride elimination to the titanium center (bottom) [32].

Chain Termination The polymerization chain termination step was the most readily characterized step in the polymerization sequence. Based on polymer end-group analyses, two basic steps have been written to explain the chain termination chemistry. One involves the interaction of molecular hydrogen with the active center, which creates a saturated end group on the polymer molecule, and the other a beta-hydride elimination involving the beta-carbon in the growing polymer chain attached to the active site, which produces an xmsaturated end group. These termination steps are illustrated in Figure 2.8 [32]. 

Figure 3.3 Beta-hydride elimination as a chain transfer mechanism for molecular weight control.

Spontaneous termlnatlon/Beta-hydride elimination - creates unsaturated chain end... 

Lemket, F. R., and Bullock, R. M. (1992). Insertion and Beta-Hydride Elimination Reactions of Ruthenium/Zirconimn Complexes Containing C2 Bridges with Bond Orders of 1,2, and 3. Organometallics, 11,4261-4267. 

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