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

Nielsen, Robert J. and Goddard, William A., III (2006) Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes. Journal of the American Chemical Society, 128 (30). pp. 9651-9660. ISSN 0002-7863. https://resolver.caltech.edu/CaltechAUTHORS:20170512-144735702

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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.


Item Type:Article
Related URLs:
URLURL TypeDescription
http://dx.doi.org/10.1021/ja060915zDOIArticle
http://pubs.acs.org/doi/abs/10.1021/ja060915zPublisherArticle
http://pubs.acs.org/doi/suppl/10.1021/ja060915zPublisherSupporting Information
ORCID:
AuthorORCID
Nielsen, Robert J.0000-0002-7962-0186
Goddard, William A., III0000-0003-0097-5716
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
Funders:
Funding AgencyGrant Number
NSFCTS-0548774
Chevron-Texaco Energy Research and Technology Co.UNSPECIFIED
Army Research Office (ARO)UNSPECIFIED
Office of Naval Research (ONR)UNSPECIFIED
Issue or Number:30
Record Number:CaltechAUTHORS:20170512-144735702
Persistent URL:https://resolver.caltech.edu/CaltechAUTHORS:20170512-144735702
Official Citation:Mechanism of the Aerobic Oxidation of Alcohols by Palladium Complexes of N-Heterocyclic Carbenes Robert J. Nielsen and William A. Goddard III Journal of the American Chemical Society 2006 128 (30), 9651-9660 DOI: 10.1021/ja060915z
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:77428
Collection:CaltechAUTHORS
Deposited By: Ruth Sustaita
Deposited On:12 May 2017 23:50
Last Modified:03 Oct 2019 17:57

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