Understanding the reaction mechanism is one of the keys to achieving controllable processes in chemistry, biology, and some applied sciences. Obviously, this refers to oxidation processes that are so widespread in nature, including living cells, and in manmade systems, including the chemical industry. Oxidation processes in the chemical industry are mainly catalytic reactions, in general, using metals, their oxides or organometallic compounds as catalysts. In living systems, enzymes, natural complex catalysts, also containing metallic elements, most of which are organometallic complexes of transition metals play a similar role. From the viewpoint of the reaction mechanism, the reactions occurring via the transfer of oxygen atoms to the substrate are one of the widespread types of oxidation processes, observed both in manmade chemical and natural biological systems. Currently, the high efficiency and selectivity of enzymatic oxidation under very mild conditions are not yet available in manmade chemical systems. What and how can we learn from Nature? Two very close and, at the same time, different approaches may serve this purpose. They are known as bio-inspiration and biomimicry. In this regard, the present volume, discussing different catalytic strategies, also involves certain achievements obtained both in bio-inspired and biomimetic systems in comparison with the application of traditional organometallic catalysts of transition metal elements in oxidation reactions.
The intended audience of this book may comprise not only researchers in the fields of chemistry, physics, and biology, but also practitioners in the fields of chemical and biological engineering, pharmaceutical industry, medicine, as well as students at different learning levels. For this reason, the first chapter is written mainly for scientists and engineers, as well as other interested specialists, undergraduates, and postgraduates, who are not familiar with the problems of oxidation processes occurring by the mechanism of oxygen atom transfer reactions to substrates. This chapter acquaints the reader with some fundamentals related to the kinetic peculiarities of these reactions, which may be useful for understanding the state-of-the-art in this area of investigation. Present developments in at least two main branches of catalysis are based on achievements in this area of investigation. One of them is catalytic or enzymatic oxidation of organic substrates by the participation of transition metal-oxo compounds in the presence of different oxidants, including dioxygen. The second important area is considered the catalytic or photocatalytic oxidation of water using transition metal-oxo complexes. Both of these branches are of fundamental importance in biology. As a part of the biological evolution of life, Nature carries out these chemical transformations using enzymes, through the reactions of oxo-atom transfer in photochemical formation of oxygen (oxygenic photosynthesis), on the one hand, and its reduction in the respiration processes, on the other hand. In this chapter, the main types of oxidation reactions and the place of oxygen atom transfer reactions in their general classification, from the point of view of the mechanisms, have been discussed. Modern perceptions of the mechanism of oxygen atom transfer reactions in oxidation processes by the participation of transition metal-oxo complexes permit to distinguish at least two main types of reactions. Here, a brief description of these has been presented. The first group of mechanisms involves inner sphere reactions of transition metal-oxo complexes forming an intermediate complex with the substrate with the direct participation of the metallic centers. Then, this intermediate decomposes into an oxygenated product and a reduced form of the initial metal in a complex compound. For the second type of mechanism, named the outer sphere reaction mechanism, it has been considered that the intermediate complex is formed due to the interaction between the oxo-ligand of transition metal complex and the substrate. This chapter addresses the different aspects of the problems of the functionalization of C-H bonds of organic compounds in oxidative catalysis by transition metal-oxo complexes. According to the accepted mechanism, the catalytic cycle involves either the direct transfer of oxygen-atom from the catalyst to the substrate or the hydrogen atom abstraction from the substrate, hydroxylation of metal-ion and subsequent formation of oxygenated products. To perform this catalytic cycle, the reduced metal-ion returns to its initial state being oxidized by another oxidant in the reaction medium. Thermodynamic and kinetic analyses of the catalytic cycles indicate that the major factors determining the reaction mechanism are the energy required to rupture the C-H bonds in oxo-atom transfer reactions and the energy of metal-oxygen bond in re-oxidation of metal. For a successful catalysis, these two energy values must be comparable. These problems are briefly discussed in this chapter. In the last section of the mentioned chapter, the mechanism of oxo atom transfer reactions has been discussed in light of the phenomenon of multiple spin-state reactivity. It has been exemplified by the reactions of “bare” transition metal-oxo cations (MO)+, where M is a transition metal, with inorganic (H2) and organic (CH4) compounds. A great number of theoretical calculations and experimental results indicate that the relationships between the spin states of transition metal-oxo complexes and their reactivity are common for the majority of oxo-atom transfer reactions in the catalysis. In chemical or biological systems, changes in the spin state in transition metal-oxo complexes and, consequently, changes in the reaction pathways permit to explain some of the unusual kinetic features observed in oxo atom transfer reactions.
The following two chapters of the present volume are scientific reviews devoted to the different aspects of some modern problems of the mechanisms in oxygen atom transfer reactions mainly related to the biological systems. Chapter 2 discusses the mechanisms of oxygen atom transfer reactions related to the bio-inspired activation of dioxygen and its subsequent reactions. The mechanisms of enzymatic oxidation are compared with the schemes of catalytic cycles in oxidation by transition metalorganic complexes as synthetic models of enzymes. In general, this chapter, to some extent, summarizes different catalytic strategies (bio –inspired, biomimetic, synthetic models of enzymes, industrial catalysts) in the activation of dioxygen and its further reactions, including oxygen atom transfer reactions from transition metal complexes to substrates. The bio-inspired activation of dioxygen is exhibited in examples of substrate oxidation by some popular enzymes, such as P450s, monooxygenases, and dioxygenases. Here, the catalytic cycle for P450 is based on the heme-Fe(III) complex, which forms the key intermediate Fe(IV)=O+∙ and carries out the hydrogen atom abstraction from RH and further transfer of OH to the substrate. This is a classic example of the oxygen rebound mechanism activating the C-H bonds via the radical pathway. A number of other examples demonstrate the widespread importance of oxygen rebound mechanisms in biomimetic chemistry. The analogies and differences of the catalytic cycles of monooxygenases and dioxygenases in bio-inspired oxidation of substrates have been discussed using numerous examples. Here, the discussion is also centered on comparable descriptions of the differences in the enzymatic cycles of dioxygenases with respect to the structural and chemical peculiarities of substrates. For example, according to the proposed schemes, when the pyrrole ring of L-tryptophan is cleaved and insertion of two oxygen atoms are inserted into the structure, in the case of tryptophan 2,3-dioxygenase (TDO) and indoleamine 2,3 dioxygenase (IDO), the supplier of four electrons to the oxygen atoms is the same substrate, but in schemes for intradiol ring-cleaving dioxygenases and extradiol dioxygenases, the activation of oxygen requires two electrons from external donor(s) other than the substrate. Special attention has been paid to oxidation systems which are of interest to the chemical and pharmaceutical industries. Among them, the cleavage of C=C bond and stereoselective or asymmetric epoxidation of olefins catalyzed by synthetic transition metalorganic complexes is one of the important areas in modern catalysis. The final section of this chapter covers new catalytic strategies for the activation of dioxygen in oxidation reactions. Among the numerous factors influencing the catalytic activity, the structure of the first coordination sphere of the metal-ions and the surrounding hydrogen bond network is crucial for the successful oxidation of substrates. Apparently, hydrogen bonds play a stabilizing role in the generation of superoxo radicals and promote the cleavage of the O-O bond via the formation of metal-oxo moieties. An analogous role in the reaction media plays Lewis acids in chemical systems. These perceptions have been demonstrated by the example of vanadium(IV) complexes oxidation schemes. Summarizing the literature data presented in Chapter 2, the authors remark that the creation of efficient industrial catalysts, particularly, in olefin epoxidation, may be achieved using dioxygenase-type enzymes that do not require extra electron suppliers.
Unlike the previous chapter, the third chapter is a review highlighting the peculiarities of oxygen atom transfer reactions from the viewpoint of biomimetic chemistry on the examples of only nickel organometallic complexes. On the occasion of the preparation of this chapter, one of the authors, pr. Sankaralingam, wrote: In synthetic biomimetic model chemistry, iron and manganese complexes are the most exploited catalysts in the realm of organic transformations reactions. In contrast to a large number of high level and comprehensive reviews reported based on Mn, Fe and Cu oxygen species in various oxidation reactions, relatively less emphasis has been put on nickel oxygen species in oxo-atom transfer reactions. This chapter aims at summarizing the noteworthy attempts in oxo atom transfer reactions catalyzed by nickel complexes. In this regard, thorough data are available involving the methods of synthesis, characterization, and revelation of the electronic and geometric structural features of the nickel organometallic complexes, as well as reaction intermediates in the activation of dioxygen and further oxygen-atom transfer reactions to substrates. Considerable attention has been paid to the effects of the stereoelectronic properties of the ligand structure on the catalytic efficiency in oxo atom transfer reactions. Chapter 3 consists of three main paragraphs involving the reactions of oxygen atom transfer and hydrogen atom abstraction catalyzed by nickel organic complexes separately, as well as reactions exhibiting both oxygen atom transfer and hydrogen atom abstraction reactivity jointly. The catalytic role of Ni ions of enzymes, such as glyoxylase I, nickel superoxide dismutase, urease, NiFe hydrogenase, CO dehydrogenase, acetyl-CoA synthase and, methyl-CoM reductase, among others, was the subject of a great number of investigations in biomimetic chemistry. In oxidation processes, involving oxo atom transfer reactions, as has been shown in this chapter, the active forms of complexes mainly contain Ni(I) and Ni(III), and often Ni(0) and Ni(II) species. In the activation of dioxygen, different nickel oxo, peroxo, superoxo intermediates may be formed, the majority of which are active in oxygen atom transfer or hydrogen abstraction reactions. Of particular interest is the section of Chapter 3 devoted to the discussion of the Ni-complexes exhibiting both the oxygen atom transfer and the hydrogen atom abstraction reactivities. Apparently, these observations are related to the electromeric states of Ni-complexes, (i) NiII-O• and (ii) NiIII=O, exhibiting different reactivity depending on the nature of substrates (for example, the electrophilicity with PPh3 or CO and nucleophilicity with ArCHO). This review also emphasizes the importance of the ligand architecture in the reactivity of organometallic oxo, dioxo, peroxo superoxo, and hydoperoxo Ni-organic complexes. Usually, their reactivity in oxo-atom transfer reactions correlates with the stereo-electronic properties of the ligands.
The aim of Chapter 4 is to acquaint the reader with the reactions of oxygen atom transfer in the oxidation of organic compounds with dioxygen or other oxidants that occur under visible light or UV irradiation in heterogeneous catalytic systems. Usually, heterogeneous photocatalytic redox reactions occur in multicomponent systems consisting of at least a substrate, oxidant, catalyst, catalyst support, solvent, often also sensitizer. Visible light or UV irradiation may be adsorbed by one or more component(s) of the system, which become electronically excited species. Subsequently, they may enter different physical and chemical interactions, transferring energy or electrons to other components involving the catalyst or nominal catalyst. Often, the photochemically generated intermediates, active oxygen species, act as catalysts, for example metal-oxo moieties in transition metal complexes in oxidative catalysis. Two main classes of reactions, namely the photogenerated and catalyzed photolysis, are known in heterogeneous photoredox systems depending on the type of catalyst functionality. Examples of heterogeneous photocatalytic redox reactions, given in this chapter, involving mainly the reactions of organic compounds on TiO2 or TiO2-based semiconductor catalysts, demonstrate the predominant role of oxygen atom transfer reactions in the mechanisms of a great number of oxidation or oxidative decomposition processes. Discussing some aspects of the determination of the type of heterogeneous photocatalytic systems, it was concluded that, seemingly, the majority of known heterogeneous photocatalytic reactions on TiO2, in particular, oxidation through oxygen atom transfer mechanisms, are photoassisted (catalyzed photolysis) processes. Among the oxygen atom transfer agents, transition metal-oxo complexes constitute the main class of compounds widespread in living nature and synthetic chemical systems. Some peculiarities of the photoassisted transfer of oxygen atom in oxidation reactions are discussed in this chapter. Using molybdenum metal-oxo complexes as an example, a significant enhancement of the catalytic activity in oxygen atom transfer on the heterogenization of the homogeneous catalyst was observed. Mo-oxo complexes anchored on TiO2 with covalent chemical bonds, exhibit improved photocatalytic activity in selective oxidation and oxidative destruction reactions, such as the interaction of O2 with DDT (dichlorodiphenyltrichloroethane) or other chlorophenyl substituted alkanes, which may not be oxidized at so mild conditions even other strong oxidants. All the examples of photocatalytic reactions mentioned in this chapter also indicate that oxidation occurring by oxygen atom transfer is one of the effective pathways for the creation of new catalytic systems that are economically advantageous and environmentally benign.
Generally, all chapters of this volume introduce not only some fundamentals and state-of-the -art, but also the main directions of development in investigations leading to the revelation of the reaction mechanisms in oxygen atom transfer reactions. For obvious reasons, a separate volume cannot address most of the problems in this field. However, I hope that this volume will be of interest to a wide range of readers, from researchers to students. On the other hand, the discussion of certain problems will apparently give rise to new problems and new interests. This is one of the main aims of creating such a volume.
I am very grateful to academician Jincai Zhao for the foreword for this volume. I would also like to acknowledge the valuable contributions of all authors preparing this volume during a very difficult time for humanity, caused by Covid-19 in the world.
Institute of Chemical Physics
National Academy of Sciences of the Republic of Armenia
Republic of Armenia