Biomimetic metal-oxidant adducts as active oxidants in oxidation reactions

Abstract

• Heme and nonheme metal-oxidant adducts. • Structural and spectroscopic characterization of heme and nonheme metal-oxidant adducts. • Reactivities of heme and nonheme metal-oxidant adducts in oxidation reactions. • Mechanistic aspects inoxidation reactions by metal-oxidant adduct versus metal-oxo species. 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, H 2 O 2 , 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 (M n+ –OX) are firstly generated, followed by the O X (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 O O 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.