Effect of the Polyanion Structure on the Mechanism of Alcohol Oxidation with HO Catalyzed by Zr-Substituted Polyoxotungstates.

Zr-monosubstituted polyoxometalates (Zr-POMs) of the Lindqvist (BuN)[{WOZr(μ-OH)}] (), Keggin (BuN)[{PWOZr(μ-OH)}] (), and Wells-Dawson (BuN)KH[{PWOZr}(μ-OH)] () structures catalyze oxidation of alcohols using aqueous hydrogen peroxide as an oxidant. With 1 equiv of HO and 1 mol % of Zr-POM, selectivity toward aldehydes and ketones varied from good to excellent, depending on the alcohol nature. Catalytic activity and attainable substrate conversions strongly depended on the Zr-POM structure and most often decreased in the order > ≫ . The reaction mechanism was probed using a test substrate, cyclobutanol, radical and O scavengers, and kinetic and spectroscopic (attenuated total reflectance-Fourier transform infrared (ATR-FT-IR), P NMR and electrospray ionization-mass spectrometry (ESI-MS)) tools. The results point to heterolytic alcohol oxidation in the presence of and and homolytic alcohol oxidation in the presence of . Kinetic and spectroscopic studies implicated an oxidation mechanism that involves both alcohol and peroxide binding to followed by an inner-sphere heterolytic H-abstraction from the α-C-H bond by the Zr-hydroperoxo group, leading to a carbonyl compound. The unique capability of to generate O upon interaction with HO complicates the reaction kinetics and improves the product yield. Spectroscopic studies coupled with stoichiometric experiments unveiled that dimeric monoperoxo {Zr(μ-η:η-O)} and monomeric hydroperoxo {Zr(η-OOH)} species accomplish the transformation of alcohols to carbonyl compounds.