A Well-Defined Complex for Palladium-Catalyzed Aerobic Oxidation of Alcohols: Design, Synthesis, and Mechanistic Considerations
David R. Jensen, Mitchell J. Schultz, Jaime A. Mueller, Matthew S. Sigman*
*Department of Chemistry, University of Utah, 315 S. 1400 East, Salt Lake
City, UT 84112, USA, Email: sigman
chem.utah.edu
D. R. Jensen, M. J. Schultz, J. A. Mueller, M. S. Sigman, Angew. Chem. Int. Ed., 2003, 42, 3810-3813.
DOI: 10.1002/anie.200351997
Abstract
A robust and effective Pd catalyst for the aerobic oxidation of various alcohols has been discovered. Using a slightly higher concentration of acetic acid as additive and extending the reaction times, the oxidation can be carried out under ambient atmosphere of air.
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Details
The document discusses the development of a well-defined palladium(II) complex for the aerobic oxidation of alcohols using molecular oxygen. Traditional Pd-catalyzed oxidations often require high catalyst loadings, excess ligands, or high oxygen pressures. The new Pd complex, featuring a single N-heterocyclic carbene (NHC) ligand, stabilizes Pd intermediates without excess ligand, enhancing efficiency. The acetate ligands serve dual roles: stabilizing the Pd center and acting as a base for deprotonation. The complex [Pd(IiPr)(OAc)2(H2O)] (1) was synthesized and characterized, showing high conversion rates (>99%) for various alcohols under mild conditions. The addition of acetic acid improved consistency at lower catalyst loadings. The catalyst demonstrated up to 1000 turnovers and could operate under ambient air, eliminating the need for pure oxygen. Mechanistic studies suggest intramolecular deprotonation and a readily accessible site for β-hydride elimination. The proposed catalytic cycle involves alcohol binding, intramolecular deprotonation, β-hydride elimination, and regeneration of the Pd complex. Future work aims to explore chiral NHC ligands for asymmetric oxidation and apply the system to other Pd-catalyzed reactions. The study highlights the robustness and efficiency of the new Pd catalyst for aerobic alcohol oxidation.
Key Words
alcohols, oxidation, aldehydes, ketones, oxygen, palladium, reaction mechanisms
ID: J06-Y2003-550
