Inaccessibility of β-hydride elimination from -OH functional groups in Wacker-type oxidation

Quantum mechanics calculations (B3LYP and MPW1K density functional theory) on mechanisms relevant to the Wacker process for dehydrogenation of alcohol to ketone show that the commonly accepted mechanism for product formation (β-hydride elimination (BHE) leading to Pd-H formation) is not energetically feasible (36.2 kcal/mol). An alterative pathway involving a five-bodied reductive elimination (RE) leads to an activation enthalpy of 18.8 kcal/mol, which is just half that of the BHE from the -OH group usually assumed for the Wacker process. We find that a water molecule catalyzes both processes, reducing the barrier to 17.2 for RE and 25.0 for BHE, but will not change the relative ordering of the two mechanisms. This suggests that assumptions of BHE mechanisms should be reexamined for cases in which the β atom is not an alkyl group.