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

Abstract

The first step in the catalytic oxidation of alcohols by molecular O(2), mediated by homogeneous vanadium(V) complexes [LV(V)(O)(OR)], is ligand exchange. The unusual mechanism of the subsequent intramolecular oxidation of benzyl alcoholate ligands in the 8-hydroxyquinolinato (HQ) complexes [(HQ)(2)V(V)(O)(OCH(2)C(6)H(4)-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)(2)V(IV)(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 NEt(3), 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 σ(p). 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.

Knocking on Wood: Base Metal Complexes as Catalysts for Selective Oxidation of Lignin Models and Extracts

“…Finally, a Hammett study performed using a series of vanadium complexes with parasubstituted benzyl alcoholate ligands suggested that negative charge develops on the benzylic carbon atom in the transition state. 43 Together, these results were consistent with a pathway for oxidation that involved deprotonation of the C−H bond of the benzyl alcoholate ligand by the base (NEt 3 ) in concert with 2e-reduction of the vanadium center (Scheme 5b). 43 Computational studies successfully located this novel base-assisted dehydrogenation pathway and were able to independently replicate the experimentally observed electronic trends.…”

“…43 Together, these results were consistent with a pathway for oxidation that involved deprotonation of the C−H bond of the benzyl alcoholate ligand by the base (NEt 3 ) in concert with 2e-reduction of the vanadium center (Scheme 5b). 43 Computational studies successfully located this novel base-assisted dehydrogenation pathway and were able to independently replicate the experimentally observed electronic trends. In addition, these studies suggested that the base shuttles a proton from the benzylic C−H to the vanadium-oxo, generating the vanadium(III) hydroxide intermediate [(HQ) 2 V III (OH)(O CHPh)].…”

“…Kinetic studies were performed, and the reaction was found to be first order in both vanadium and triethylamine with activation parameters (ΔH ‡ = 28 ± 4 kJ/mol and ΔS ‡ = −169 ± 4 J/mol·K) consistent with a bimolecular reaction. 43 A large primary kinetic isotope effect (10.2 ± 0.6) was determined, indicating that C−H bond breaking occurs at or before the rate-limiting step. Finally, a Hammett study performed using a series of vanadium complexes with parasubstituted benzyl alcoholate ligands suggested that negative charge develops on the benzylic carbon atom in the transition state.…”

An Overview of Selective Oxidation of Alcohols: Catalysts, Oxidants and Reaction Mechanisms

“…Heterogeneous catalysts [42,[88][89][90][91][92][93] for AD reactions are also highly dependent on the support acting as a base or acid. There are many examples of homogeneous rhenium deoxygenation catalysts, but a limited number of rhenium heterogeneous nanoparticle catalysts have been reported [94][95][96].…”

A Reusable Unsupported Rhenium Nanocrystalline Catalyst for Acceptorless Dehydrogenation of Alcohols through γ‐C–H Activation

“…Until now, chemists have focused on AD using homogeneous catalysts; however, most homogeneous catalysts require an acid or base additive, extensive synthesis of ligands, and are difficult to separate and recycle. [1][2][3][4][5][6] Heterogeneous catalysts [7][8][9][10][11][12][13][14] for AD reactions are also highly dependent on the support acting as a base or acid. Our work and that of others on deoxydehydration (DODH) of biomass-derived polyols illustrate the use of discrete oxorhenium complexes as homogeneous catalysts.…”