Biomimetic metal-oxidant adducts as active oxidants in oxidation reactions

Catalytic oxidation reactions are pivotal transformations in living creatures and chemical industries, in which metal-oxygen intermediates, including high-valent metal-oxo species, have been invoked as the key intermediates that effect the chemical functionalization of organic substrates. In nature, metalloenzymes utilize various metal-oxygen intermediates, such as metal-oxo, -hydroperoxo, -peroxo, and -superoxo species, in the catalytic oxidation of organic substrates by activating dioxygen. In biomimetic and synthetic oxidation reactions, various terminal oxidants, such as iodosylarenes, H2O2, alkyl peroxides, peracids, and NaClO, have been employed to generate such metal-oxygen intermediates. Indeed, in the reactions of biomimetic compounds and the terminal oxidants, metal-oxidant adducts (Mn+–OX) are firstly generated, followed by the OX (X = PhI, OR, OC(O)R, and Cl) bond scission to form high-valent metal-oxo complexes as the ultimate oxidants for the functionalization of organic substrates. Although high-valent metal-oxo species have been investigated intensively over the past several decades in the communities of biological and bioinorganic/biomimetic chemistry due to their important chemical properties in the oxidation reactions, the metal-oxidant adduct complexes also exhibit fascinating structural and reactivity features. This review is intended to focus on the synthesis, characterization, and reactivity studies of metal-oxidant adducts, such as metal-iodosylarene, -hydro(alkyl)peroxo, -acylperoxo, and -hypochlorite complexes, in heme and nonheme systems. The mechanisms of the conversion of metal-oxidant adducts to their corresponding high-valent metal-oxo species, the factors tuning the OO bond cleavage modes to form high-valent metal-oxo species, and the comparison of the reactivities of the metal-oxidant adducts and high-valent metal-oxo species are also discussed in this review.