A Biomimetic Pathway for Vanadium-Catalyzed Aerobic Oxidation of Alcohols: Evidence for a Base-Assisted Dehydrogenation Mechanism

Authors

  • Bethany N. Wigington,

    1. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510 (USA)
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  • Dr. Michael L. Drummond,

    1. Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, TX 76201 (USA)
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  • Prof. Dr. Thomas R. Cundari,

    Corresponding author
    1. Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, TX 76201 (USA)
    • Department of Chemistry, Center for Advanced Scientific Computing and Modeling (CASCaM), University of North Texas, Denton, TX 76201 (USA)
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  • Dr. David L. Thorn,

    1. Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (USA)
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  • Dr. Susan K. Hanson,

    Corresponding author
    1. Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (USA)
    • Chemistry Division, Los Alamos National Laboratory, Los Alamos, New Mexico 87545 (USA)
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  • Prof. Dr. Susannah L. Scott

    Corresponding author
    1. Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510 (USA)
    2. Department of Chemical Engineering, University of California, Santa Barbara 93106-5080 (USA), Fax: (+1) 805-893-4731
    • Department of Chemistry and Biochemistry, University of California, Santa Barbara, CA 93106-9510 (USA)
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Abstract

The first step in the catalytic oxidation of alcohols by molecular O2, mediated by homogeneous vanadium(V) complexes [LVV(O)(OR)], is ligand exchange. The unusual mechanism of the subsequent intramolecular oxidation of benzyl alcoholate ligands in the 8-hydroxyquinolinato (HQ) complexes [(HQ)2VV(O)(OCH2C6H4-p-X)] involves intermolecular deprotonation. In the presence of triethylamine, complex 3 (X=H) reacts within an hour at room temperature to generate, quantitatively, [(HQ)2VIV(O)], benzaldehyde (0.5 equivalents), and benzyl alcohol (0.5 equivalents). The base plays a key role in the reaction: in its absence, less than 12 % conversion was observed after 72 hours. The reaction is first order in both 3 and NEt3, with activation parameters ΔH=(28±4) kJ mol−1 and ΔS=(−169±4) J K−1 mol−1. A large kinetic isotope effect, 10.2±0.6, was observed when the benzylic hydrogen atoms were replaced by deuterium atoms. The effect of the para substituent of the benzyl alcoholate ligand on the reaction rate was investigated using a Hammett plot, which was constructed using σp. From the slope of the Hammett plot, ρ=+(1.34±0.18), a significant buildup of negative charge on the benzylic carbon atom in the transition state is inferred. These experimental findings, in combination with computational studies, support an unusual bimolecular pathway for the intramolecular redox reaction, in which the rate-limiting step is deprotonation at the benzylic position. This mechanism, that is, base-assisted dehydrogenation (BAD), represents a biomimetic pathway for transition-metal-mediated alcohol oxidations, differing from the previously identified hydride-transfer and radical pathways. It suggests a new way to enhance the activity and selectivity of vanadium catalysts in a wide range of redox reactions, through control of the outer coordination sphere.

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