O-H 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 O-H 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 beta-hydrogen elimination from -OH group (beta HE) or through direct reductive hydrogen elimination (RHE). As regards beta HE, two different pathways were assumed, for both of which the test calculations showed high energy barriers. Therefore, beta HE 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 (E(a)s), which are considerably lower than the E(a)s 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 C-H bond cleavage step of the Wacker process. (C) 2009 Elsevier B.V. All rights reserved.