Hydroxypalladation precedes the rate-determining step in the wacker oxidation of ethene

The complete reaction mechanism and kinetics of the Wacker oxidation of ethene in water under low [Cl^-], [Pd^II], and [Cu ^II] conditions are investigated in this work by using ab initio molecular dynamics. These extensive simulations shed light on the molecular details of the associated individual steps, along two different reaction routes, starting from a series of ligand-exchange processes in the catalyst precursor PdCl₄^2- to the final aldehyde-formation step and the reduction of Pd^II. Herein, we report that hydroxylpalladation is not the rate-determining step and is, in fact, in equilibrium. The newly proposed rate-determining step involves isomerization and follows the hydroxypalladation step. The mechanism proposed herein is shown to be in excellent agreement with the experimentally observed rate law and rate. Moreover, this mechanism is in consensus with the observed kinetic isotope effects. This report further confirms the outer-sphere (anti) hydroxypalladation mechanism. Our calculations also ratify that the final product formation proceeds through a reductive elimination, assisted by solvent molecules, rather than through β-hydride elimination. Isomerization matters: Isomerization, involving a cis-to-trans ligand rotation, is the rate-determining step in the Wacker oxidation of ethene. Ab initio molecular-dynamics simulations explore the full mechanism and kinetics of this reaction (see figure).