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

• Published in: Chemistry : a European journal Vol. 18; no. 47; pp. 14981 - 14988
• Main Authors: Wigington, Bethany N., Drummond, Michael L., Cundari, Thomas R., Thorn, David L., Hanson, Susan K., Scott, Susannah L.
• Format: Journal Article
• Language: English
• Published: Weinheim WILEY-VCH Verlag 19.11.2012; WILEY‐VCH Verlag; Wiley; Wiley Subscription Services, Inc
• Subjects: aerobic oxidation; Alcohols - chemistry; Atoms & subatomic particles; base-assisted catalysis; Biomimetic Materials - chemistry; biomimetic synthesis; Catalysis; Catalytic oxidation; Chemistry; Chemistry, Multidisciplinary; Hammett correlation; Hydrogenation; Ligands; Molecular Structure; Organometallic Compounds - chemical synthesis; Organometallic Compounds - chemistry; Oxidation-Reduction; Oxygen - chemistry; Physical Sciences; Quantum Theory; Science & Technology; Thermodynamics; vanadium; Vanadium - chemistry; DIOXYGEN ACTIVATION; SELECTIVE OXIDATION; MOLECULAR CALCULATIONS; base-assisted catalysis; HOMOGENEOUS CATALYSTS; ALPHA-HYDROXY ESTERS; aerobic oxidation; QUINQUEVALENT VANADIUM; DECARBOXYLATION-DEOXYGENATION; Hammett correlation; KINETIC RESOLUTION; ORGANIC-COMPOUNDS; vanadium; biomimetic synthesis; GAUSSIAN-BASIS SETS
• ISSN: 0947-6539; 1521-3765
• DOI: 10.1002/chem.201202499

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. Biomimetic aerobic oxidation of alcohols, as catalyzed by an (8‐hydroxyquinolinato)vanadium complex, is promoted by Brønsted bases, which deprotonate the alcoholato ligand and thereby induce electronic reorganization (see scheme). This base‐assisted dehydrogenation resembles certain metalloenzyme‐catalyzed oxidations, in which a base situated near the active site participates directly in the redox reaction.