Mechanistic studies on metalloporphyrin epoxidation reactions with hydrogen peroxide: evidence for two active oxidative species

New evidence concerning the mechanism of oxidation reactions of alkenes and aromatic compounds catalyzed by metalloporphyrins with H2O2 were obtained based on the evaluation of results from structurally different substrates. A systematic change in metalloporphyrin structure and reaction conditions was accomplished with cis-cyclooctene as a model. Two maximum performance systems were obtained and then applied to the oxidation of selected substrates, namely 17β-acetoxy-4-androstene, (+)-3-carene, geraniol, and naphthalene. Substantial differences in reaction conversion and product selectivity were found. These systematic studies confirmed the evidence of two different active oxidation species, assigned as the hydroperoxy or oxo ones. The use of more electronegative porphyrins, iron as central metal, and protic solvent and the absence of co-catalyst favor the formation of a hydroperoxy active species. On the other hand, the oxo species was considered to be the main acting entity in the presence of less electronegative porphyrin ligands, manganese as central atom, aprotic solvent, and a buffering substance as co-catalyst. This state of knowledge makes it possible to modulate the reaction selectivity by the selection of the appropriate catalytic system.