A Comparative Mechanistic Analysis of Palladium- and Nickel-Catalyzed α,β-Dehydrogenation of Carbonyls via Organozinc Intermediates

Introducing degrees of unsaturation into small molecules is a central transformation in organic synthesis. A strategically useful category of this reaction type is conversion of alkanes to alkenes with a conjugated electron withdrawing group. An efficient strategy for this conversion has been deprotonation to form a stabilized organozinc intermediate that can be subject to α,β-dehydrogenation through palladium and nickel catalysis. However, this general reactivity blueprint has not been studied with sufficient rigor to understand the complex mechanistic aspects of these pathways. This report details a comprehensive mechanistic study of allyl-Pd and allyl-Ni catalyzed dehydrogenation adjacent to electron-withdrawing groups, including identification of reversible and turnover limiting steps and the role of metal amides. One interesting finding is that β-hydride elimination can be preferred to a greater extent than C–C bond formation with Ni than with Pd, which juxtaposes the generally assumed trends that β-hydride elimination is more facile with Pd than Ni. Discussion of these findings are informed by KIE experiments, stoichiometric reactions, and rate studies. The knowledge gained from these studies is expected to lead to methodological advances in catalysis and greater understanding of nuanced reactivity differences between first- and second-row metals.