H-Transfer steps of the Wacker process: A DFT study

The Wacker process involves a number of organometallic reactions, particularly the beta-hydrogen elimination, 1.2-insertion and the O-H bond cleavage reactions. This work seeks to fully explore the mechanism of these H-transfer steps, employing density functional theory, and handling the solvent effects with the PCM model. This approach ruled Out the simple mechanism bearing the cis and trans isomers of the reactant of the beta-hydrogen elimination step. Therefore, a more improved model will be used, in which one water molecule is assumed to be in the second-coordination sphere. The water molecule will highly reduce the energy barriers of the beta-hydrogen elimination and O-H bond cleavage steps. This water-assisted mechanism provides two possible pathways: one bearing the -OH group positioned on the opposite side of the coordinated H2O and the other the -OH group on the next to the coordinated H2O. Involving the high energy barrier (for the last step), the first pathway is ruled out. The second pathway, in which the third step is the reductive elimination of hydrogen from the -OH group, leads to the experimentally confirmed energy barriers for these H-transfer steps. Thus, this pathway would be accepted as the most appropriate mechanism for this part of the Wacker process. (C) 2009 Elsevier B.V. All rights reserved.