Photo‐induced Desulfurative Processes for Carbon Radical Generation
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Abstract:
Abstract Thiols constitute an important family among sulfur‐containing compounds, with well‐established applications in various fields ranging from medicine to material science. For instance, thiol residues are good hydrogen donors which reduce radical species in biological or chemical processes. However, even though the S−H bond activation of thiols for providing access to thiyl radicals has been largely studied, desulfurative processes affording carbon‐based radicals by C−S bond activation have been less explored. In recent years, photoredox catalysis has become the prevalent method for the generation of radicals under soft reaction conditions from readily available starting materials under visible light. In this context, recent studies have been devoted to the development of photocatalytic procedures aiming at the desulfurization of thiol derivatives leading to new C−H, C−C or C‐Het bond formation reactions. This review will cover the synthetic methodologies and strategies for photo‐mediated desulfurization of native thiols, thioethers, sulfonium salts and xanthates to access new organic compounds. This emerging field is especially interesting for new transformations of cysteine and peptide derivatives.Keywords:
Photoredox catalysis
Sulfonium
Thiol
Organic Synthesis
ConspectusPhotoredox catalysis has emerged as a powerful tool for the utilization of visible light to drive chemical reactions between organic molecules that exhibit two rather ubiquitous properties: colorlessness and redox-activity. The photocatalyst, however, requires significant absorption in the visible spectrum and reversible redox activity. This very general framework has led to the development of several new modes of reactivity based on electron and energy transfer steps between photoexcited catalyst states and various organic molecules. In the past years, major effort has been devoted to photoredox-catalytic aromatic substitutions involving an initial reductive activation of various aryl electrophiles by the photocatalyst, which opens a new entry into selective arene functionalizations within organic synthesis endeavors. This, however, has led to a unilateral emphasis of synthetic developments including catalyst modifications, substrate scope studies, and combinations with other chemical processes.This Account summarizes recent reports of new protocols for the synthesis of aromatic esters, thioethers, boronates, sulfonates, heterobiaryls, deuteroarenes, and other functionalized arenes under mild photoredox conditions with organic dyes. On the other hand, mechanistic studies were largely neglected. This Account emphasizes the most relevant experiments and techniques, which can greatly assist in the exploration of the mechanistic foundation of aromatic photoredox substitutions and the design of new chemical reactivities. The nature and physicochemical properties of the employed organic dyes, the control of its acid–base chemistry, the choice of the irradiation sources, and the concentrations of substrates and dyes are demonstrated to decisively affect the activity of organic photocatalysts, the chemo- and regioselectivities of reactions, and the operating mechanisms. Several methods of distinction between photocatalytic and radical chain processes are being discussed such as the determination of quantum yields by conventional actinometric studies or modern photon counter devices. Careful analyses of key thermodynamic and kinetic data of the single electron transfer steps involved in aromatic photoredox substitutions by experimental and theoretical techniques are being exemplified with recent examples from the literature including the determination of redox potentials by DFT and CV, fluorescence quenching studies, and transient absorption/emission spectroscopy. This Account provides the uninitiated reader with an overview of the potential of organic photoredox catalysis for aromatic substitution reactions and encourages the practitioners to consult highly instructive synthetic, mechanistic, theoretical, and spectroscopic tools that are available in research laboratories.
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Inorganic radicals have so far led a shadowy existence in synthetic organic radical chemistry. This article briefly reviews the synthetic applications of the most important inorganic radicals. In addition, a new synthetic concept is presented, which should demonstrate that with inorganic, oxygen-centered radicals of the type X-O*, in which X is NO2, SO3-, and H, respectively, novel oxidative radical reactions could be performed, which in turn are difficult or impossible with their organic counterparts, the alkoxyl radicals R-O*.
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Radicals are an important class of versatile and highly reactive species. Compared with the wide applications of various C-centred radicals, however, the N-radical species including N-centred radicals and radical ions remain largely unexplored due to the lack of convenient methods for their generation. In recent years, visible light photoredox catalysis has emerged as a powerful platform for the generation of various N-radical species and methodology development towards the synthesis of diverse N-containing compounds. In this tutorial review, we highlight recent advances in this rapidly developing area with particular emphases put on the working models and new reaction design.
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A fundamental challenge in the field of catalysis is the development of sustainable and efficient methods for the activation of molecules. One approach for the activation of organic molecules that has recently received much attention is photoredox catalysis with visible light. On the other hand, the application of aryl radicals in organic synthesis is challenging, but very useful. This thesis describes the generation of aryl radicals and their application in organic synthesis, such as the direct arylation of heteroarenes, the synthesis of benzothiophenes, phenanthrenes and the amino arylation of alkenes using visible light photoredox catalysis. In addition, the synthesis of substituted tetrahydroisoquinolines is described.
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Photoredox catalysis and electrochemical organic synthesis are two emerging fields in organic chemistry. Inspired by the similarity between single electron transfer events in photoredox catalysis and half-reactions in electrochemistry, we developed a new electrochemical approach that mimics the photoredox catalysis for organic synthesis. We demonstrated using this approach for electrochemical trifluoromethylation of arenes and heteroarenes. We also established a theory for guiding the rational design of reaction conditions. Our approach will open a new avenue at the intersection of photoredox catalysis and electrochemical organic synthesis.
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