In recent years, visible light-induced transition metal catalysis has emerged as a new paradigm in organic photocatalysis, which has led to the discovery of unprecedented transformations as well as the improvement of known reactions. In this subfield of photocatalysis, a transition metal complex serves a double duty by harvesting photon energy and then enabling bond forming/breaking events mostly via a single catalytic cycle, thus contrasting the established dual photocatalysis in which an exogenous photosensitizer is employed. In addition, this approach often synergistically combines catalyst-substrate interaction with photoinduced process, a feature that is uncommon in conventional photoredox chemistry. This Review describes the early development and recent advances of this emerging field.

Visible Light-Induced Transition Metal Catalysis.

The mainstream applications of visible-light photoredox catalysis predominately involve outer-sphere single-electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth-abundant metal-based Mn Ln -type (M=metal, Ln =polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner-sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited-state lifetimes for use as effective photocatalysts. The generic process, termed visible-light-induced homolysis (VLIH), entails the formation of suitable light-absorbing ligated metal-substrate complexes (Mn Ln -Z; Z=substrate) that can undergo homolytic cleavage to generate Mn-1 Ln and Z. for further transformations.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/anie.202100270

Visible-Light-Induced Homolysis of Earth-Abundant Metal-Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis.

Alkoxy radicals are highly reactive species that have long been recognized as versatile intermediates in organic synthesis. However, their development has long been impeded due to a lack of convenient methods for their generation. Thanks to advances in photoredox catalysis, enabling facile access to alkoxy radicals from bench-stable precursors and free alcohols under mild conditions, research interest in this field has been renewed. This review comprehensively summarizes the recent progress in alkoxy radical-mediated transformations under visible light irradiation. Elementary steps for alkoxy radical generation from either radical precursors or free alcohols are central to reaction development; thus, each section is categorized and discussed accordingly. Throughout this review, we have focused on the different mechanisms of alkoxy radical generation as well as their impact on synthetic utilizations. Notably, the catalytic generation of alkoxy radicals from abundant alcohols is still in the early stage, providing intriguing opportunities to exploit alkoxy radicals for diverse synthetic paradigms.

Alkoxy Radicals See the Light: New Paradigms of Photochemical Synthesis.

Decarboxylative C-H functionalization reactions are highly attractive methods for forging carbon-carbon bonds considering their inherent step- and atom-economical features and the pervasiveness of carboxylic acids and C-H bonds. An ideal approach to achieve these dehydrogenative transformations is through hydrogen evolution without using any chemical oxidants. However, effective couplings by decarboxylative carbon-carbon bond formation with proton reduction remain an unsolved challenge. Herein, we report an electrophotocatalytic approach that merges organic electrochemistry with photocatalysis to achieve the efficient direct decarboxylative C-H alkylation and carbamoylation of heteroaromatic compounds through hydrogen evolution. This electrophotocatalytic method, which combines the high efficiency and selectivity of photocatalysis in promoting decarboxylation with the superiority of electrochemistry in effecting proton reduction, enables the efficient coupling of a wide range of heteroaromatic bases with a variety of carboxylic acids and oxamic acids. Advantageously, this method is scalable to decagram amounts, and applicable to the late-stage functionalization of drug molecules.

Electrophotocatalytic Decarboxylative C−H Functionalization of Heteroarenes

Despite the rich photochemistry of 3d‐metal complexes, the utilization of excited‐state reactivity of these compounds in organic synthesis has been historically overlooked. The advent of photoredox catalysis has changed the perception of synthetic chemists towards photochemistry, and nowadays the potential of photoinduced, outer‐sphere single‐electron transfer events is widely recognized. More recently, an emerging new mode of photoactivation has taken the spotlight, based on an inner‐sphere mode of reactivity triggered by population of ligand‐to‐metal charge‐transfer (LMCT) excited states. Contrarily to photoredox, LMCT‐activation does not rely on matching redox potentials, offers unique reactivity profiles and is particularly well suited on Earth‐abundant metal complexes. Those appealing features are propelling the development of methods using this blueprint to generate highly reactive open‐shell species under mild conditions. The aim of this contribution is to provide a didactical tool for the comprehension of this emerging concept and facilitate the development of new synthetic methodologies to achieve sustainable chemical transformations.

Ligand‐to‐Metal Charge Transfer (LMCT) Photochemistry at 3d‐Metal Complexes: An Emerging Tool for Sustainable Organic Synthesis

Ligand-Accelerated Iron Photocatalysis Enabling Decarboxylative Alkylation of Heteroarenes

Thioether-Assisted Cu-Catalyzed C5-H Arylation of Imidazo[1,5-a]pyridines.

A visible light-mediated “green” protocol for metal-free oxidative coupling of heteroarenes and aliphatic C–H components has been achieved via a radical pathway. This cross-dehydrogenative coupling method features a broad scope of substrates. The green oxidant H2O2 is used as a hydrogen atom transfer reagent, which is environmentally benign, low cost, and atom economical.

Green oxidant H2O2 as a hydrogen atom transfer reagent for visible light-mediated Minisci reaction

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Ligand-Accelerated Iron Photocatalysis Enabling Decarboxylative Alkylation of Heteroarenes

Thioether-Assisted Cu-Catalyzed C5-H Arylation of Imidazo[1,5-a]pyridines.

Green oxidant H2O2 as a hydrogen atom transfer reagent for visible light-mediated Minisci reaction