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Published August 2, 2006 | Supplemental Material
Journal Article Open

Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes

Abstract

Quantum mechanics (B3LYP density functional theory) combined with solvation (Poisson−Boltzmann polarizable continuum solvent model) was used to investigate six mechanisms for the aerobic oxidation of alcohols catalyzed by (NHC)Pd(carboxylate)_2(H_2O) complexes (NHC = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene). Of these, we find that "reductive β-hydride elimination", in which the β-hydrogen of a palladium-bound alkoxide is transferred directly to the free oxygen of the bound carboxylate, provides the lowest-energy route and explains the published kinetic isotope effect, activation enthalpy, reaction orders, and dependence of rate on carboxylate pK_a. The traditional β-hydride elimination mechanism cannot be responsible for the experimentally observed kinetic parameters, which we find could result from the subsequent reductive elimination of acetic acid, which yields a slightly higher calculated activation barrier. Reversible β-hydride elimination may provide a mechanism for the racemization of chiral alcohols, which would undermine attempts at an enantioselective oxidation. Competition among these pathways can be influenced by changing the electronic properties of the carboxylate and substrate.

Additional Information

© 2006 American Chemical Society. Received 7 February 2006. Published online 7 July 2006. Published in print 1 August 2006. We thank the group of Prof. Brian Stoltz at Caltech and Dr. Jonas Oxgaard for helpful discussions. This research was funded by the NSF (CTS-0548774) and by the Chevron-Texaco Energy Research and Technology Company. The facilities used for these studies were funded by grants from ARO-DURIP, ONR-DURIP, and NSF-MRI. All calculations were performed with Jaguar 5.0.49

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August 19, 2023
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