Manganese 1,4,7-trimethyl-1,4,7-triazacyclononane complexes: Versatile catalysts for the oxidation of organic compounds with hydrogen peroxide

The oxidation of phenols, cinnamic acids and methyl aryl sulfides by hydrogen peroxide, using three catalyst systems, [L2Mn2IV(μ-O)3](PF6)2, L = 1,4,7-trimethyl-1,4,7-triazacyclononane; [LMnIV(OMe)3(PF6); and MnII/L/H2O2, have been studied. The results from a combination of spectroscopic and kinetic studies, coupled with Hammett correlations and 18O labelling experiments, suggest that with each system the active oxidant is an electrophilic, mononuclear oxo-manganese (V) species. The influence of additives that can act as co-ligands for the manganese species has been investigated, with a view to controlling the activity/selectivity of the active oxidant. The two-step, sulfide–sulfoxide–sulfone, oxidation shows an unusual switch in the philicity of the active oxidant from electrophilic in the first step to nucleophilic in the second. Mechanisms for the oxidations are proposed.

Graphical abstractManganese-1,4,7-trimethyl-1,4,7-triazacyclononane complexes catalyse the oxidation of phenolate and cinnamate anions and sulfides by hydrogen peroxide. The data are compatible with the active oxidant being an electrophilic oxo-manganese(V) species. In contrast, the comparable oxidation of sulfoxides involves a nucleophilic oxidant. A mechanism in which hydrogen peroxide reacts with a manganese-sulfoxide complex is proposed. Figure optionsDownload full-size imageDownload as PowerPoint slide