Palladium-catalysed oxidation of alcohols to carbonyl compounds with 1,2-dichloroethane as the primary oxidant: a theoretical study

Abstract

Homogeneous palladium chloride catalyses the oxidation of alcohols to aldehydes or ketones in 1,2-dichloroethane (DCE). The active catalytic species is regenerated by the dechlorination of DCE to ethylene. This catalytic cycle is studied here using non-local density functional theory. The insertions of “naked” Pd, of HPdCl, and of PdCl₂ into the C–Cl bond of DCE are examined in detail, and it is shown that the most likely inserting species is Pd, then HPdCl and PdCl₂. Two distinct reaction pathways are proposed and investigated, namely insertion of HPdCl into the C–Cl bond, followed by HCl abstraction, and abstraction of HCl from HPdCl, followed by oxidative insertion of a “naked” Pd atom into the C–Cl bond. Both paths converge on the same species but the former is found to involve lower energies. Entropy effects, ligand effects and possible mechanisms for the formation of ethylene in this system are discussed. Based on these results, the complete oxidation cycle from methanol to formaldehyde is calculated as a model reaction for the dehydrogenation of alcohols in this system. It is shown that the original catalytic species does not necessarily need to be regenerated to complete a the catalytic cycle. Ligand effects and entropy effects are considered. The choice of methanol as a model of alcohol is discussed.