Mechanism of alkene, alkane, and alcohol oxidation with H2O2 by an in situ prepared MnII/pyridine-2-carboxylic acid catalyst

Article abstract of DOI:10.1021/acscatal.6b00320

The oxidation of alkenes, alkanes, and alcohols with H₂O₂ is catalyzed efficiently using an in situ prepared catalyst comprised of a Mn^II salt and pyridine-2-carboxylic acid (PCA) together with a ketone in a wide range of solvents. The mechanism by which these reactions proceed is elucidated, with a particular focus on the role played by each reaction component: i.e., ketone, PCA, Mn^II salt, solvent, etc. It is shown that the equilibrium between the ketone cocatalysts, in particular butanedione, and H₂O₂ is central to the catalytic activity observed and that a gem-hydroxyl-hydroperoxy species is responsible for generating the active form of the manganese catalyst. Furthermore, the oxidation of the ketone to a carboxylic acid is shown to antecede the onset of substrate conversion. Indeed, addition of acetic acid either prior to or after addition of H₂O₂ eliminates a lag period observed at low catalyst loading. Carboxylic acids are shown to affect both the activity of the catalyst and the formation of the gem-hydroxyl-hydroperoxy species. The molecular nature of the catalyst itself is explored through the effect of variation of Mn^II and PCA concentration, with the data indicating that a Mn^II:PCA ratio of 1:2 is necessary for activity. A remarkable feature of the catalytic system is that the apparent order in substrate is 0, indicating that the formation of highly reactive manganese species is rate limiting.