Oxygen Atom Transfer Reactions

Investigations of the mechanisms of oxygen atom transfer reactions of transition metal organometallic complexes are mainly related to their abundance in chemical syntheses and biological oxidation processes. They are important stages in the catalytic and enzymatic oxidation cycles of substrates, as well as in the catalytic oxidation of water.

These brief notes on the mechanisms of oxygen atom transfer reactions involve certain fundamentals (geometric and electronic structures, spin states and reactivity of oxocomplexes), as well as some specific peculiarities of the oxo-atom transfer reactions of transition metal complexes (hydrogen atom abstraction and oxygen rebound mechanisms, intra- and intermolecular types of oxo-atom transfer, multistate reactivity). This chapter introduces readers to the categorization and place of oxo-atom transfer reactions in the classification of catalytic oxidation processes in the context of general problems of the mechanisms in this area. The chapter also provides readers with certain data on the activation of dioxygen and the functionalization of C-H bonds in oxidation processes via the oxo-atom transfer reactions of transition metal complexes. The role of the two and multiple spin states reactivity in the mechanisms of these reactions has also been discussed.

This chapter is written mainly for non-specialist readers in this area and serves as a general introduction to the next chapters of this collection of works.

In nature, redox enzymes mediated dioxygen activation with oxidations proceeds smoothly and highly selectively under ambient temperature, whereas in the chemical industry, versatile oxidations are commonly performed at elevated temperature, which leads to the occurrence of radical chain process, thus causing low product selectivity and environmental pollution. This chapter will first introduce the strategies of enzymes including P450s, methane monooxygenase, dioxygenases in dioxygen activation and catalysis, thus illustrating how enzymes activate dioxygen and selectively transfer the resulting active oxygen to their substrates. Then, inspired by enzymatic dioxygen activation, the progress in biomimetic dioxygen activation with related catalytic oxidations by synthetic redox metal complexes will be presented, and its current challenges will be discussed as well. Finally, a recent new strategy for dioxygen activation and catalysis, that is, Lewis acid promoted dioxygen activation by redox metal complexes, will be introduced; this new strategy may have more closely biomimicked enzymatic dioxygen activation than those traditional strategies, thus shedding new light on catalyst design for industrial oxidations.

Oxygen atom transfer (OAT) reactions catalyzed by metal complexes have been a subject of intensive research over the century, owing to the prevalent involvement of OAT in organic transformations mediated by several biologically important enzymes such as methane monooxygenases, cytochrome P450, etc as well as in synthetic chemical reactions. In biomimetic model chemistry, iron and manganese complexes are the most exploited catalysts in the realm of oxygen atom transfer reactions and many of these metal complexes produce very short-lived reactive metaloxygen intermediates during the catalytic reactions. Characterization of such reactive intermediates of numerous heme and non-heme iron and manganese complexes and comparing them with their natural enzyme analogous have emerged as a promising approach toward understanding several intricate enzymatic mechanisms. Considerable research advancements in the studies of OAT reactions involving late transition metal complexes such as cobalt, nickel, and copper have also been recognized in the past few years. In this account, various reports have been published, demonstrating catalytic oxidation of organic substrates by the active nickel-oxygen species generated either via heterolysis or homolysis of O-O bond of oxidant bound nickel complexes. This book chapter aims at a comprehensive summary of noteworthy attempts contributed towards nickel catalyzed OAT reactions and various implicated or well-characterized nickeloxygen active intermediates. The effect of stereoelectronic properties of ligand architecture on catalytic efficiency and various characterization techniques used to identify the catalytically active nickel-oxygen species are also discussed.

Insights into the mechanisms of oxygen atom transfer in the photooxidation of organic compounds for heterogeneous photocatalytic systems have been presented. These reactions have a wide variety of practical applications in chemistry, biology, and applied sciences. The role of the oxygen atom transfer mechanism in homogeneous photocatalysis has been investigated for nearly a century. Relatively little attention has been paid to the disclosure of oxygen atom transfer reactions in heterogeneous photocatalytic systems. This chapter discusses some problems related to the catalytic oxygen atom transfer in the oxidation of organic compounds, mainly with dioxygen, under UV irradiation or visible light, in heterogeneous reaction systems. Various active oxygen species, including oxygen atom transfer agents, as reaction intermediates can be generated in these systems. Depending on the nature of the active oxygen species, including metal-oxo compounds, the photoassisted catalytic oxygen atom transfer can occur mainly by the primary photoexcitation of either the catalyst or its photosensitive solid support, in rare cases, also the catalyst/support complex. The peculiarities of the mechanism of photo-driven oxygen atom transfer were mainly exemplified by the reactions occurring in heterogeneous catalytic systems containing transition metal oxides, their metalorganic complexes, and other photosensitive solid materials, including heterogenized homogeneous photocatalysts on the different supports, such as the transition metal-oxo complexes on the semiconductor materials. Special attention has been paid to the chemistry of TiO2 and TiO2-based semiconductor photocatalysis from the point of view of the reaction mechanisms, including oxygen atom transfer reactions.