‘Oxo-hydroxo tautomerism’ as useful mechanistic tool in oxygenation reactions catalysed by water-soluble metalloporphyrins

Abstract

High valent metal–oxo species have been evoked as active intermediates in many different oxidation reactions using manganese or iron porphyrin complexes as catalysts and oxygen atom donors (H₂O₂, PhIO, NaOCl, KHSO₅, ... etc.) or dioxygen associated to a reductant as oxygen atom source. When these metalloporphyrin-catalysed oxidations are performed in water, such metal–oxo species are able to transfer an oxygen atom coming from either the oxygen source or from bulk water. This fact has been explained by the so-called oxo–hydroxo tautomerism, a mechanism involving a rapid shift of two electrons and one proton from a hydroxo ligand (electron-rich ligand formed by deprotonation of an aqua ligand) to the trans oxo species (electron-poor ligand) leading to the transformation of the hydroxo ligand into an electrophilic oxo entity on the opposite side of the initial oxo. This ‘oxo–hydroxo tautomerism’, evidenced by using ¹⁸O-labelled water, has been used as mechanistic tool to unambiguously characterize oxygen atom transfer mechanisms mediated by metal–oxo species in opposition to mechanisms related to free radical oxidation reactions.