Elucidating the Significance of β-Hydride Elimination and the Dynamic Role of Acid/Base Chemistry in a Palladium-Catalyzed Aerobic Oxidation of Alcohols

摘要

The mechanistic details of aerobic alcohol oxidation with catalytic Pd(IiPr)(OAc)_2(H_2O) (IiPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) are disclosed. Under optimal conditions, β-hydride elimination is rate-limiting supported by kinetic studies including a high primary kinetic isotope effect (KIE) value of 5.5 ± 0.1 and a Hammett ρ value of -0.48 ± 0.04. On the basis of these studies, a late transition state is proposed for β-hydride elimination, which is further corroborated by theoretical calculations using density functional theory. Additive acetic acid modulates the rates of both the alcohol oxidation sequence and regeneration of the Pd catalyst. With no additive [HOAc], turnover-limiting reprotonation of intermediate palladium peroxo is kinetically competitive with β-hydride elimination, allowing for reversible oxygenation and decomposition of Pd(0). With additive [HOAc] (>2 mol %), reprotonation of the palladium peroxo is fast and β-hydride elimination is the single rate-controlling step. This proposal is supported by an apparent decomposition pathway modulated by [HOAc], a change in alcohol concentration dependence, a lack of [O_2] dependence at high [HOAc], and significant changes in the KIE values at different HOAc concentrations.