Editorial Cite This: Acc. Chem. Res. 2018, 51, 2601−2602
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Uncovering the Complexity of the Simplest Atom Transfer Reaction Guest Editorial for the Accounts of Chemical Research special issue on “Hydrogen Atom Transfer”.
Acc. Chem. Res. 2018.51:2601-2602. Downloaded from pubs.acs.org by 185.251.71.60 on 11/20/18. For personal use only.
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metal−oxygen compounds and in the study of their reactivity in the oxidation of organic substrates, as well as in energy production and in their use as models for the study of important biological processes. These aspects are thoroughly discussed in two Accounts that summarize recent developments in HAT reactions involving synthetic mononuclear metal−oxygen intermediates, focusing on the mechanistic dichotomy of concerted vs stepwise pathways and on the role of acidic additives; and in the synthesis of high-valent corrole and corrolazine based metal-oxo and metal-hydroxo porphyrinoid complexes and the study of their HAT reactivity. An additional contribution analyzes kinetic isotope effects for HAT reactions from hydrocarbons to oxoiron(IV) species, showing how the interplay between experiment and theory can provide information on spin states, geometry, and ligand sphere constitution of the reactive transition state. HAT steps are also at the basis of the development of synthetically useful procedures. Within this framework, the development, application, and synthetic utility of metal hydride hydrogen atom transfer (MHAT) reactions is described in one Account, uncovering recent applications in dual-catalytic crosscouplings based on the merger of MHAT and Ni catalysis. Another contribution discusses intramolecular 1,4-HAT in 1amino-1-hydroxyallyl radicals generated by carbonylation of alkenyl radicals in the presence of amines and the ensuing transformations of these intermediates. HAT reactions have emerged moreover as a useful tool for the development of selective aliphatic C−H bond functionalization procedures. The broad issue of selectivity is discussed in two Accounts dealing with copper-catalyzed asymmetric radical transformations via a radical relay process, where in particular the development of procedures for the enantioselective cyanation and arylation of benzylic C−H bonds is described, and with HAT based aliphatic C−H bond functionalization, where the role of electronic, stereoelectronic, torsional, and medium effects on selectivity is highlighted, drawing a parallelism between alkoxyl radicals and metal-oxo species. Two Accounts discuss in detail the autoxidation of hydrocarbons and polymers and the mechanistic features behind the development of highly efficient RTAs. In one of them, the critical role of peroxyl radicals in polymer degradation is discussed, uncovering possible strategies to enhance or prevent these detrimental processes. The other contribution provides a mechanistic understanding of the RTA activity of phenoxazines and organosulfur compounds in the inhibition of hydrocarbon autoxidation, highlighting moreover the potency of the former compounds as inhibitors of ferroptosis.
ver a century of research has highlighted the importance of hydrogen atom transfer (HAT) as a key step in a wide variety of chemical and biological processes. HAT represents one of the most fundamental chemical reactions, involved in processes as diverse as hydrocarbon combustion, smallmolecule activation, and atmospheric chemistry as well as in the oxidative damage to biomolecules and polymers. HAT steps are moreover at the basis of the mechanism of action of radical-trapping antioxidants (RTAs), of several enzymatic and biomimetic reactions, and are increasingly represented in the development of synthetically useful procedures. This special issue on “Hydrogen Atom Transfer” provides, through a collection of 16 Accounts, a detailed and updated overview of some of the most important aspects and most recent developments of HAT processes. The mechanistic description and definition of HAT reactions in the broader context of proton-coupled electron transfer (PCET) has attracted great interest in the last decades from both the experimental and theoretical points of view. Two Accounts deal with this broad topic providing, through a continuum of PCET reactivity, a common framework for the description of reactions that lie at opposite ends of the reactivity spectrum such as HAT and multiple-site concerted proton−electron transfer (MS-CPET), and discussing the role played by theory in the design of both molecular and heterogeneous catalysts to control the movement and coupling of electrons and protons through the study of PCET reactions relevant to energy conversion processes. Activation and conversion of highly abundant hydrocarbons such as methane and ethane to added-value products under mild conditions is a fundamental transformation of enormous importance for the energy industry. This topic is addressed by two Accounts dealing in particular with the computational elucidation of the catalytic mechanisms of methane hydroxylation by copper sites in particulate methane monooxygenase and copper-exchanged zeolites, and with the thermal activation of methane by metal oxide clusters doped with noble-metal atoms and by carbides and borides of base metals, where in particular the latter contribution discusses the implications of these molecule-level mechanisms for the development of new catalysts. Representative examples described in two Accounts illustrate different aspects of HAT involved in biologically relevant reactions. In one of them, fundamental mechanistic information on enzymatic C−H bond cleavage via HAT through the study of the temperature dependence of the kinetic isotope effects in enzymatic systems such as taurine dioxygenase, the dual copper β-monooxygenases, and soybean lipoxygenase is provided. The other contribution gives a detailed description of the most recent findings on the mechanism of radical initiation in the radical S-adenosylmethionine (SAM) enzyme superfamily. Several research groups have been involved in the preparation and structural characterization of biomimetic © 2018 American Chemical Society
Received: October 19, 2018 Published: November 20, 2018 2601
DOI: 10.1021/acs.accounts.8b00525 Acc. Chem. Res. 2018, 51, 2601−2602
Accounts of Chemical Research
Editorial
Finally, in the framework of gas-phase HAT reactions, the role of weakly bound hydrogen bonded complexes and tunneling in reactions at very low temperatures involving the hydroxyl radical is described, discussing the implications of these findings for gas-phase chemistry in space. The Accounts presented in this special issue highlight the heightened development of HAT processes in recent years, hopefully representing a reference tool for both the practitioner and the newcomer to the field. The diversity of the topics discussed and the associated combination of experimental and theoretical approaches point toward multidisciplinarity as a strong driver for the rapid future advancement of the field.
Miquel Costas
Universitat de Girona
Massimo Bietti
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Università di Roma “Tor Vergata”
AUTHOR INFORMATION
ORCID
Miquel Costas: 0000-0001-6326-8299 Massimo Bietti: 0000-0001-5880-7614 Notes
Views expressed in this editorial are those of the authors and not necessarily the views of the ACS.
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DOI: 10.1021/acs.accounts.8b00525 Acc. Chem. Res. 2018, 51, 2601−2602
