Beta-hydride elimination mechanism

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

The mechanism of the Meerwein-Pondorf-Verley reaction is by coordination of a Lewis acid to isopropanol and the substrate ketone, followed by intermolecular hydride transfer, by beta elimination [41]. Initially, the mechanism of catalytic asymmetric transfer hydrogenation was thought to follow a similar course. Indeed, Backvall et al. have proposed this with the Shvo catalyst [42], though Casey et al. found evidence for a non-metal-activation of the carbonyl (i.e., concerted proton and hydride transfer [43]). This follows a similar mechanism to that proposed by Noyori [44] and Andersson [45], for the ruthenium arene-based catalysts. By the use of deuterium-labeling studies, Backvall has shown that different catalysts seem to be involved in different reaction mechanisms [46]. 

---