The Wacker process: inner- or outer-sphere nucleophilic addition? New insights from ab initio molecular dynamics.

The Wacker process consists of the oxidation of ethylene catalyzed by a Pd(II) complex. The reaction mechanism has been largely debated in the literature; two modes for the nucleophilic addition of water to a Pd-coordinated alkene have been proposed: syn-inner- and anti-outer-sphere mechanisms. These reaction steps have been theoretically evaluated by means of ab initio molecular dynamics combined with metadynamics by placing the [Pd(C(2)H(4))Cl(2)(H(2)O)] complex in a box of water molecules, thereby resembling experimental conditions at low [Cl(-)]. The nucleophilic addition has also been evaluated for the [Pd(C(2)H(4))Cl(3)](-) complex, thus revealing that the water by chloride ligand substitution trans to ethene is kinetically favored over the generally assumed cis species in water. Hence, the resulting trans species can only directly undertake the outer-sphere nucleophilic addition, whereas the inner-sphere mechanism is hindered since the attacking water is located trans to ethene. In addition, all the simulations from the [Pd(C(2)H(4))Cl(2)(H(2)O)] species (either cis or trans) support an outer-sphere mechanism with a free-energy barrier compatible with that obtained experimentally, whereas that for the inner-sphere mechanism is significantly higher. Moreover, additional processes for a global understanding of the Wacker process in solution have also been identified, such as ligand substitutions, proton transfers that involve the aquo ligand, and the importance of the trans effect of the ethylene in the nucleophilic addition attack.