H-Transfer steps of the Wacker process: A DFT study S. Ali Beyramabadi a , Hossein Eshtiagh-Hosseini a, * , Mohammad R. Housaindokht a , Ali Morsali b a Department of Chemistry, Ferdowsi University of Mashhad, Mashhad, Iran b Department of Chemistry, Faculty of Science, Islamic Azad University, Mashhad branch, Mashhad, Iran article info Article history: Received 17 June 2008 Accepted 2 December 2008 Available online 25 January 2009 Keywords: Wacker process DFT PCM Palladium b-Hydrogen elimination abstract The Wacker process involves a number of organometallic reactions, particularly the b-hydrogen elimina- tion, 1,2-insertion and the OAH 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 reac- tant of the b-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 b-hydrogen elimination and OAH bond cleavage steps. This water- assisted mechanism provides two possible pathways: one bearing the –OH group positioned on the opposite side of the coordinated H 2 O and the other the –OH group on the next to the coordinated H 2 O. 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 exper- imentally 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. Ó 2009 Elsevier B.V. All rights reserved. 1. Introduction In addition to the industrial importance in the synthesis of car- bonyl compounds from corresponding alkenes, the Wacker oxida- tion is interesting because its mechanism involves a number of organometallic reactions, such as ligand substitution, hydroxymet- allation, b-elimination and 1,2-insertion [1]. The main steps, and mostly suggested of the Wacker process (oxidation of ethylene to acetaldehyde) are as follows (Eqs. (1)–(8)) [1–14]: Recently, we [9] proposed that the nucleophilic attack of a water- chain model in the hydroxypalladation step (Eq. (4)) is the most appropriate mechanism for the rate-determining step (RDS) of the Wacker process. For the conversion of ethylene to acetaldehyde, this model is led to the anti attack of a chain with three water molecules, which is in good agreement with the experimental evidence espe- cially quantitative results obtained by Henry [14]. He demonstrated that the experimental activation enthalpy (DH z exp ) of the Wacker reaction, can be written as DH z exp ¼ DH 2 þ DH 3 þ E a ¼ 19:8 kcal=mol where DH 2 and DH 3 are the enthalpy changes related to Eqs. (2) and (3), respectively and E a is the energy barrier of the RDS (Eq. (4)) [14]. It has been predicted that the E a s of the RDS of the Wacker process are 14.72 and 12.66 kcal/mol, in the gas phase and PCM model, respectively [9]. Hence, the energy barriers of the steps occurring after the RDS should be lower than these calculated E a s. The suggestion of H-transfers sequence (Eqs. (6)–(8)) is based on the absence of any deuterium isotope incorporation when the 6 7 Pd Cl OH 2 H OH 7 8 Pd Cl OH 2 OH 8 9 CH 3 CH O H 2 O HCl Pd 0 6 7 8 Pd Cl Cl Cl Cl - 2 1 Pd Cl Cl Cl Cl 2- 2 H 2 O Pd Cl Cl OH 2 3 Cl - 3 H 2 O H 3 O + Pd Cl Cl OH 4 4 H 2 O Pd Cl Cl OH 2 5 OH - - - 5 6 Cl - Pd Cl OH 2 OH K 1 K 2 K 3 k´ C 2 H 4 1 2 3 4 5 0166-1280/$ - see front matter Ó 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.theochem.2008.12.049 * Corresponding author. Tel.: +98 511 8797022; fax: +98 511 8796416. E-mail address: heshtiagh@ferdowsi.um.ac.ir (H. Eshtiagh-Hosseini). Journal of Molecular Structure: THEOCHEM 903 (2009) 108–114 Contents lists available at ScienceDirect Journal of Molecular Structure: THEOCHEM journal homepage: www.elsevier.com/locate/theochem