We present here a review of the photochemical and electrochemical applications of multi-site proton-coupled electron transfer (MS-PCET) in organic synthesis. MS-PCETs are redox mechanisms in which both an electron and a proton are exchanged together, often in a concerted elementary step. As such, MS-PCET can function as a non-classical mechanism for homolytic bond activation, providing opportunities to generate synthetically useful free radical intermediates directly from a wide variety of common organic functional groups. We present an introduction to MS-PCET and a practitioner's guide to reaction design, with an emphasis on the unique energetic and selectivity features that are characteristic of this reaction class. We then present chapters on oxidative N-H, O-H, S-H, and C-H bond homolysis methods, for the generation of the corresponding neutral radical species. Then, chapters for reductive PCET activations involving carbonyl, imine, other X═Y π-systems, and heteroarenes, where neutral ketyl, α-amino, and heteroarene-derived radicals can be generated. Finally, we present chapters on the applications of MS-PCET in asymmetric catalysis and in materials and device applications. Within each chapter, we subdivide by the functional group undergoing homolysis, and thereafter by the type of transformation being promoted. Methods published prior to the end of December 2020 are presented.

Photochemical and Electrochemical Applications of Proton-Coupled Electron Transfer in Organic Synthesis.

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis.

Visible-light photoredox catalysis is a powerful and attractive strategy for organic molecule activation and new reaction design owing to its environmental-friendly characteristics and unique catalytic mechanisms, and has found wide applications in organic synthesis. This catalytic strategy enables controllable generation of diverse nitrogen-centered radicals (NCRs) under mild conditions, providing access to construction of diverse nitrogen-containing compounds. In this review, we critically illustrate the recent advances in the field of visible-light photoredox-catalyzed cyclization of nitrogen-centered radicals, based on the different radical precursors and activation modes. Wherever possible, particular emphasis is also put on working models and synthetic applications.

Recent advances in visible-light photoredox-catalyzed nitrogen radical cyclization

Photochemical reactions are a key method to generate radical intermediates. Often under these conditions no toxic reagents are necessary. During recent years, photo-redox catalytic reactions considerably push this research domain. These reaction conditions are particularly mild and safe which enables the transformation of poly-functional substrates into complex products. The synthesis of heterocyclic compounds is particularly important since they play an important role in the research of biologically active products. In this review, photochemical radical cyclization reactions of imines and related compounds such as oximes, hydrazones and chloroimines are presented. Reaction mechanisms are discussed and the structural diversity and complexity of the products are presented. Radical intermediates are mainly generated in two ways: (1) electronic excitation is achieved by light absorption of the substrates. (2) The application of photoredox catalysis is now systematically studied for these reactions. Recently, also excitation of charge transfer complexes has been studied in this context from many perspectives.

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Photochemical radical cyclization reactions with imines, hydrazones, oximes and related compounds

The development of ecofriendly methods for carbon-carbon (C-C) and carbon-heteroatom (C-Het) bond formation is of great significance in modern-day research. Metal-free cross-dehydrogenative coupling (CDC) has emerged as an important tool for organic and medicinal chemists as a means to form C-C and C-Het bonds, as it is atom economical and more efficient and greener than transition-metal catalyzed CDC reactions. Molecular iodine (I2 ) is recognized as an inexpensive, environmentally benign, and easy-to-handle catalyst or reagent to pursue CDCs under mild reaction conditions, with good regioselectivities and broad substrate compatibility. This review presents the recent developments of I2 -catalyzed C-C, C-N, C-O, and C-S/C-Se bond-forming reactions for the synthesis of various important organic molecules by cross-dehydrogenative coupling.

Metal-Free Cross-Dehydrogenative Coupling (CDC): Molecular Iodine as a Versatile Catalyst/Reagent for CDC Reactions.

Intermolecular Cyclopropanation of Styrenes Using Iodine and Visible Light via Carbon–Iodine Bond Cleavage

Visible-Light-Mediated Iminyl Radical Generation from Benzyl Oxime Ether: Synthesis of Pyrroline via Hydroimination Cyclization.

Transition-Metal-Free Synthesis of Phenanthridinones through Visible-Light-Driven Oxidative C–H Amidation

The intermolecular cyclopropanation of aromatic olefins with activated methylene compounds using iodine and visible light irradiation was described. This reaction proceeds under rare-metal-free conditions. Styrenes with various substituted groups (alkyl and electron-withdrawing groups) provided corresponding cyclopropanes in moderate to good yields.

Intermolecular Cyclopropanation of Styrenes Using Iodine and Visible Light via Carbon—Iodine Bond Cleavage.

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Intermolecular Cyclopropanation of Styrenes Using Iodine and Visible Light via Carbon–Iodine Bond Cleavage

Visible-Light-Mediated Iminyl Radical Generation from Benzyl Oxime Ether: Synthesis of Pyrroline via Hydroimination Cyclization.

Transition-Metal-Free Synthesis of Phenanthridinones through Visible-Light-Driven Oxidative C–H Amidation

Intermolecular Cyclopropanation of Styrenes Using Iodine and Visible Light via Carbon—Iodine Bond Cleavage.