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.

Manganese-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 as PowerPoint slide