Mechanism of Alkene, Alkane, and Alcohol Oxidation with H₂O₂ by an in Situ Prepared Mn^II/Pyridine-2-carboxylic Acid Catalyst

Abstract

The oxidation of alkenes, alkanes, and alcohols with H 2 O 2 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 H 2 O 2 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 H 2 O 2 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.

Oxidative Cleavage of Alkene C=C Bonds Using a Manganese Catalyzed Oxidation with H2O2 Combined with Periodate Oxidation

“…In the case of the epoxidation of diphenyl ethylene initially low conversion was observed under the standard reaction conditions. This was overcome by reducing the concentration of the starting material to 0.25 M (Scheme ).…”

“…The conditions used in the present study for the oxidation of alkenes to their corresponding epoxide/diol was reported earlier , . Briefly, alkene oxidation proceeds at room temperature using a combination of a Mn(II) salt, pyridine‐2‐carboxylic acid (PCA), (sub)stoichiometric butanedione and H 2 O 2 as terminal oxidant (Scheme ).…”

Origins of Catalyst Inhibition in the Manganese‐Catalysed Oxidation of Lignin Model Compounds with H2O2

“…The low conversion was not owing to loss of H 2 O 2 because it remained in solution unreacted. Notably, the conversion to ketone product, that is, the turnover number was approximately 100 regardless of whether the initial concentration of 1 was 0.05 or 0.5 m (Figure S1 in the Supporting Information), which is consistent with the zero‐order dependence on the substrate of the reaction observed previously …”

“…Recently, we reported a manganese(II) catalyst prepared in situ with pyridine‐2‐carboxylic acid (PCA) and sub‐stoichiometric ketones for the oxidation with H 2 O 2 of a broad range of organic compounds such as alkanes, olefins, aliphatic and benzylic alcohols under ambient conditions with high turnover numbers (up to 300 000 for the epoxidation of electron‐rich alkenes) and low catalyst loadings (Scheme ) . The simplicity of the catalyst in preparation and, importantly, its wide solvent scope make it a potential candidate for large‐scale application.…”

“…Recently,w er eported am anganese(II) catalyst prepared in situ with pyridine-2-carboxylic acid (PCA) and sub-stoichiometric ketonesf or the oxidation with H 2 O 2 of ab road range of organic compounds such as alkanes, olefins, aliphatic and benzylic alcohols under ambient conditions with high turnover numbers( up to 300 000 for the epoxidation of electron-rich alkenes) andlow catalystloadings (Scheme 1). [33][34][35][36][37][38][39] The simplicity of the catalysti np reparation and, importantly,i ts wide solvent scope make it ap otential candidate for large-scale application. In particular,t he good conversion shown in the oxidationo f secondary benzylic alcohols, such as p-X-1-phenylethanol (X = H, Br,O CH 3 )a nd aromatic vicinal diols to ketones, even if the benzylic alcohol positions were protected as ethers, encouraged us to consider the potential of this Mn system in catalytic lignin degradation, in particular for attack at the b-O-4 linkage.…”