Electrochemical Formation and Activation of Hydrogen Peroxide from Water on Fluorinated Tin Oxide for Baeyer–Villiger Oxidation Reactions

Abstract

The two-electron oxidation of water (2e-WOR) has been studied in the past as a possible method for the alternative preparation of hydrogen peroxide. Often, fluorinated tin oxide (FTO) is used as an anode and FTO itself was found also to be active for 2e-WOR. Because one use of H2O2 is as an oxygen donor for Baeyer–Villiger oxidation of ketones catalyzed by tin compounds and materials, presently we were interested in studying the use of in situ formed H2O2 for these reactions. First, the formation of H2O2 was verified in an acetonitrile/water solvent in a 2e-WOR reaction, which is more efficient than a comparable reaction in water in terms of the H2O2 concentration attained and faradaic efficiency at comparable potentials, that is, ∼3 V vs SHE. Second, initial studies on oxygenation of reactive substrates such as sulfides showed normalized reaction rates (NRRs) for two-electron oxidation reactions that were about 3 times higher than the NRR for H2O2 formation, indicating the formation of an active oxygen-donating or oxidizing species on the electrode surface prior to the formation and release of H2O2 into solution. Third, the Baeyer–Villiger oxygenation of 2-adamantanone at 2.1 V versus SHE in acetonitrile/water showed both the formation of the expected lactone product and hydroxylation at both tertiary and secondary C–H bonds. Hydroxylation is most easily explained by the presence of hydroxyl radical species as supported by the formation of a spin adduct and its identification by electron paramagnetic resonance. However, the potential used, 2.1 V versus SHE, is an underpotential for the formation of a solvated hydroxyl radical in solution, thereby leading to the conclusion that surface-bound hydroxyl species, OH*, are those that are reactive for the apparent one-electron water oxygenation reaction. Fourth, it was shown that although H2O2 can be thermally activated on FTO as a catalyst to a minor degree, electrochemical activation is by far more significant, leading to the use of FTO as an electrochemical catalyst for activation of H2O2 for the Baeyer–Villiger oxygenation and also alkene epoxidation.