OH bond cleavage step of the Wacker process: A DFT study

The Wacker process consists of several important organometallic reactions. The termination step of this process, i.e., the OH bond cleavage reaction, was investigated by employing density functional theory (DFT) with modeling the solvent (water) as a polarizable continuum (PCM) and also assuming that the water molecules will directly involve in this reaction. The termination step can proceed either through β-hydrogen elimination from OH group (βHE) or through direct reductive hydrogen elimination (RHE). As regards βHE, two different pathways were assumed, for both of which the test calculations showed high energy barriers. Therefore, βHE turns out to be implausible. As for the RHE mechanism, in which the solvent molecules have a significant effect on the reduction of the activation energies, four different pathways were assumed. Here, the lowest energy barriers (Eas), which are considerably lower than the Eas of rate-determining step, belong to the pathway in which a “chain” of one water molecule assists the H-transfer from OH group to the chloride ligand, (pathway D). This pathway, therefore, can be accepted as the most appropriate mechanism for the OH bond cleavage step of the Wacker process.