A fundamental aim in the field of catalysis is the development of new modes of small molecule activation. One approach toward the catalytic activation of organic molecules that has received much attention recently is visible light photoredox catalysis. In a general sense, this approach relies on the ability of metal complexes and organic dyes to engage in single-electron-transfer (SET) processes with organic substrates upon photoexcitation with visible light.

Many of the most commonly employed visible light photocatalysts are polypyridyl complexes of ruthenium and iridium, and are typified by the complex tris(2,2′-bipyridine) ruthenium(II), or Ru(bpy)32+ (Figure 1). These complexes absorb light in the visible region of the electromagnetic spectrum to give stable, long-lived photoexcited states.1,2 The lifetime of the excited species is sufficiently long (1100 ns for Ru(bpy)32+) that it may engage in bimolecular electron-transfer reactions in competition with deactivation pathways.3 Although these species are poor single-electron oxidants and reductants in the ground state, excitation of an electron affords excited states that are very potent single-electron-transfer reagents. Importantly, the conversion of these bench stable, benign catalysts to redox-active species upon irradiation with simple household lightbulbs represents a remarkably chemoselective trigger to induce unique and valuable catalytic processes.






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Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

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Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

In this review, we highlight the use of organic photoredox catalysts in a myriad of synthetic transformations with a range of applications. This overview is arranged by catalyst class where the photophysics and electrochemical characteristics of each is discussed to underscore the differences and advantages to each type of single electron redox agent. We highlight both net reductive and oxidative as well as redox neutral transformations that can be accomplished using purely organic photoredox-active catalysts. An overview of the basic photophysics and electron transfer theory is presented in order to provide a comprehensive guide for employing this class of catalysts in photoredox manifolds.

Organic Photoredox Catalysis

In the investigation of efficient chemical transformations, the carbon-carbon bond-forming reactions play an outstanding role. In this context, organocatalytic processes have achieved considerable attention. 1 Beside their facile reaction course, selectivity, and environmental friendliness, new synthetic strategies are made possible. Particularly, the inversion of the classical reactivity (Umpolung) opens up new synthetic pathways. 2 In nature, the coenzyme thiamine (vitamin B1), a natural thiazolium salt, utilizes a catalytic variant of this concept in biochemical processes as nucleophilic acylations. 3 The catalytically active species is a nucleophilic carbene. 4

Organocatalysis by N-Heterocyclic Carbenes

Light can be considered an ideal reagent for environmentally friendly, 'green' chemical synthesis; unlike many conventional reagents, light is non-toxic, generates no waste, and can be obtained from renewable sources. Nevertheless, the need for high-energy ultraviolet radiation in most organic photochemical processes has limited both the practicality and environmental benefits of photochemical synthesis on industrially relevant scales. This perspective describes recent approaches to the use of metal polypyridyl photocatalysts in synthetic organic transformations. Given the remarkable photophysical properties of these complexes, these new transformations, which use Ru(bpy)(3)(2+) and related photocatalysts, can be conducted using almost any source of visible light, including both store-bought fluorescent light bulbs and ambient sunlight. Transition metal photocatalysis thus represents a promising strategy towards the development of practical, scalable industrial processes with great environmental benefits.

Visible light photocatalysis as a greener approach to photochemical synthesis

Yuming Yang,†,§ Qiang Zhao,‡,§ Wei Feng,† and Fuyou Li*,† †Department of Chemistry and State Key Laboratory of Molecular Engineering of Polymers and Institutes of Biomedical Sciences, Fudan University, Shanghai 200433, P. R. China ‡Key Laboratory for Organic Electronics and Information Displays (KLOEID) and Institute of Advanced Materials (IAM), Nanjing University of Posts and Telecommunications, Nanjing 210046, P. R. China.

Luminescent chemodosimeters for bioimaging.

The increasing need for more efficient synthetic methods and sustainable processes can be seen as a major driving force for new inventions; it also stimulates the creative rethinking of known concepts, which, in turn, will lead to the development of innovative chemistry. For example, starting from the challenge to mimic and understand enzymatic transformations, organocatalysis has now become an important tool in modern organic synthesis with new reactivity that complements that of enzyme and metal catalysis. As early as the beginning of the 20th century, photochemistry had already attracted the attention of (organic) chemists. But recent years have seen broader interest in photochemical transformations because of the generally mild conditions required for substrate activation—ideally light alone—and their suitability for “green reactions”. While classical photochemical steps have found notable applications in synthesis and the direct transformation of light into electric energy (photovoltaics) can already be considered a highly developed research field, efforts in photocatalysis have mainly targeted the development of artificial photosynthesis systems for the conversion of solar energy into storable chemical fuels. However, despite the obvious, practical advantages of visible light as an “infinitely available” promoter for chemical synthesis, the simple inability of most organic molecules to absorb light in the visible range of the spectrum has greatly limited the potential applications of photochemical reactions. One major strategy to address this drawback and to develop new efficient processes using visible light is the use of photosensitizers and photocatalysts. Upon irradiation, molecules are converted into their photoexcited states, which are chemically more reactive because of the significantly altered electronic distribution. Apart from following various common physical decay pathways, these photoexcited states can undergo chemical “deactivation” processes, which are either unimolecular and correspond to classical photochemical transformations (isomerizations, rearrangements, etc.) or proceed in a bimolecular manner. The interactions with other species range from bimolecular reactions such as photocycloadditions to quenching processes. Here, the most important pathways are energy-transfer reactions and electron-transfer reactions; both play a crucial role as indirect initiators for all types of photocatalytic reactions. Photoredox catalysis relies on the general property of excited states to be both more easily reduced as well as more easily oxidized than their corresponding ground states, and so the photocatalyst can serve either as an electron donor or an electron acceptor to be regenerated in the catalytic cycle (Scheme 1). The photocatalyst undergoes two distinct electron-transfer steps; both the “quenching” and the “regenerative” electron transfer can be productive with respect to a desired chemical transformation. Ideally, the two electrontransfer processes are connected by the substrates or intermediates of the catalyzed reaction and therefore do not require any sacrificial electron donor or acceptor.

Photoredox catalysis with visible light.

The dawn of old stars: Classic xanthene dyes like eosin Y (gr eoς=goddess of dawn) and green-light irradiation can replace precious metal complexes for the organocatalytic asymmetric -alkylation of aldehydes rendering the process purely organic

Metal‐Free, Cooperative Asymmetric Organophotoredox Catalysis with Visible Light

The dawn of old stars: Classic xanthene dyes like eosin Y (gr. eoς=goddess of dawn) and green-light irradiation can replace precious metal complexes for the organocatalytic asymmetric -alkylation of aldehydes rendering the process purely organic.

Metal-free, cooperative asymmetric organophotoredox catalysis with visible light

Extending mechanistic routes in heterazolium catalysis--promising concepts for versatile synthetic methods.

The targeted choice of specific photocatalysts has been shown to play a critical role for the successful realization of challenging photoredox catalytic transformations. Herein, we demonstrate the successful implementation of a rational design strategy for a series of deliberate structural manipulations of cyanoarene-based, purely organic donor–acceptor photocatalysts, using 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as a starting point. Systematic modifications of both the donor substituents as well as the acceptors’ molecular core allowed us to identify strongly oxidizing as well as strongly reducing catalysts (e.g., for an unprecedented detriflation of unactivated naphthol triflate), which additionally offer remarkably balanced redox potentials with predictable trends. Especially halogen arene core substitutions are instrumental for our targeted alterations of the catalysts’ redox properties. Based on their preeminent electrochemical and photophysical characteristics, all novel, purely...

A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor–Acceptor Cyanoarenes

Kirsten Zeitler

Papers

Photoredox catalysis with visible light.

Metal‐Free, Cooperative Asymmetric Organophotoredox Catalysis with Visible Light

Metal-free, cooperative asymmetric organophotoredox catalysis with visible light

Extending mechanistic routes in heterazolium catalysis--promising concepts for versatile synthetic methods.

A Toolbox Approach To Construct Broadly Applicable Metal-Free Catalysts for Photoredox Chemistry: Deliberate Tuning of Redox Potentials and Importance of Halogens in Donor–Acceptor Cyanoarenes