Palladium mediated activation of molecular oxygen

Abstract

In developing environmentally benign chemistries, it is most important to use dioxygen directly in lieu of toxic and/or corrosive stoichiometric oxidants. Unfortunately, for many processes such as direct oxidations this has not yet become practical. To help develop such processes, we elucidate here the mechanism for the reaction of molecular oxygen with palladium-hydride complexes in nonpolar solvent using quantum mechanics for several PdII complexes, specifically focusing on the pathways proceeding through Pd0 and on the direct insertion of oxygen into the Pd-H bond. All the chapters presented herein focus on different aspects of the total problem as follows: Chapter 1: Activation of Molecular Oxygen by ((‒)-Sparteine)PdHCl: Direct Insertion presents the first proposal of a hydrogen atom abstraction from a palladium-hydride complex by triplet oxygen that demonstrates the feasibility of this mechanism. Chapter 2: Activation of Molecular Oxygen by [1,3-(CH2PtBu2)2-C6H3]PdH examines the reaction of an experimentally isolated palladium-hydride complex from which formation of Pd0 has been deemed unlikely, thus proving the previously predicted insertion mechanism to be active in a known system. Chapter 3: Activation of Molecular Oxygen by ((‒)-Sparteine)PdHCl: Pd0 presents a thorough investigation of possible base-assisted reductive elimination pathways that can lead to the formation of Pd0 in the ((‒)-Sparteine)PdHCl system, and it demonstrates that the direct insertion mechanism is in fact the process involved in this system. Chapter 4: Activation of Molecular Oxygen by ((‒)-Sparteine)Pd(OAc)H: Pd0 vs. Direct Insertion examines the substitution of the OAc ion for Cl in ((‒)-Sparteine)PdH system. The acetate ligand’s ability to act as a base while chelating the Pd significantly lowers the energy involved in the Pd0 pathway, switching the calculated preference to the Pd0 pathway. Chapter 5: Activation of Molecular Oxygen by (Pyridine)2Pd(OAc)H: Pd0 vs. Direct Insertion examines the reaction of (Pyridine)2Pd(OAc)H with O2. The calculated mechanisms present feasible cis/trans isomerisation and demonstrate that the Pd0 pathway is the favored pathway for both the cis and the trans cases. Appendix: Enantioselective Oxidations of Secondary Alcohols by (–)-Sparteine-PdII Complexes examines the mechanism of specific Pd oxidation catalysis focusing on alcohol binding, deprotonation to form the corresponding alkoxide and the subsequent beta-hydride elimination.