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 recent years, photoredox catalysis has come to the forefront in organic chemistry as a powerful strategy for the activation of small molecules. In a general sense, these approaches rely on the ability of metal complexes and organic dyes to convert visible light into chemical energy by engaging in single-electron transfer with organic substrates, thereby generating reactive intermediates. In this Perspective, we highlight the unique ability of photoredox catalysis to expedite the development of completely new reaction mechanisms, with particular emphasis placed on multicatalytic strategies that enable the construction of challenging carbon–carbon and carbon–heteroatom bonds.

http://pubs.acs.org/doi/pdf/10.1021/acs.joc.6b01449

Photoredox Catalysis in Organic Chemistry

In the last few years, visible-light initiated organic transformations have attracted increasing attention. The development of visible-light-promoted photocatalytic reactions, which enable rapid and efficient synthesis of fine chemicals, is highly desirable from the viewpoint of cost, safety, availability, and environmental friendliness. In this Minireview, recent advances made in this fast developing area of research are discussed.

Visible-light photoredox catalysis.

The interaction between an electronically excited photocatalyst and an organic molecule can result in the genertion of a diverse array of reactive intermediates that can be manipulated in a variety of ways to result in synthetically useful bond constructions. This Review summarizes dual-catalyst strategies that have been applied to synthetic photochemistry. Mechanistically distinct modes of photocatalysis are discussed, including photoinduced electron transfer, hydrogen atom transfer, and energy transfer. We focus upon the cooperative interactions of photocatalysts with redox mediators, Lewis and Bronsted acids, organocatalysts, enzymes, and transition metal complexes.

http://pubs.acs.org/doi/pdf/10.1021/acs.chemrev.6b00018

Dual Catalysis Strategies in Photochemical Synthesis

Redox-photosensitized reactions. 7. Aromatic hydrocarbon-photosensitized electron-transfer reactions of furan, methylated furans, 1,1-diphenylethylene, and indene with p-dicyanobenzene

Tris(2,2'-bipyridine)ruthenium(2+)-mediated photoreduction of olefins with 1-benzyl-1,4-dihydronicotinamide: a mechanistic probe for electron-transfer reactions of NAD(P)H-model compounds

Vapor-phase beckmann rearrangement over alumina-supported boria catalyst prepared by vapor decomposition method

Reductions photosensibilisees par Ru(bpy) 3 2+ , d'enones, de cinnamates et cinnamonitriles par BNAH. Etude des processus de transfert d'electron

Redox-photosensitized reactions. 11. Ru(bpy)32+-photosensitized reactions of 1-benzyl-1,4-dihydronicotinamide with aryl-substituted enones, derivatives of methyl cinnamate, and substituted cinnamonitriles: electron-transfer mechanism and structure-reactivity relationships

Die UV-Bestrahlung einer Losung von Dicyanobenzol (I) und Furan (II) ergibt in Gegenwart von Phenanthren das Produkt (III), das durch Saulenchromatographie z.B. an basischem Al-oxid in das Furanylbenzonitril (IV) ubergeht.

Photochemical reactions of aromatic compounds. 28. Photosensitized electron-transfer reaction of electron donor-acceptor pairs by aromatic hydrocarbons

Hiroshi Sakurai

Papers

Redox-photosensitized reactions. 7. Aromatic hydrocarbon-photosensitized electron-transfer reactions of furan, methylated furans, 1,1-diphenylethylene, and indene with p-dicyanobenzene

Tris(2,2'-bipyridine)ruthenium(2+)-mediated photoreduction of olefins with 1-benzyl-1,4-dihydronicotinamide: a mechanistic probe for electron-transfer reactions of NAD(P)H-model compounds

Vapor-phase beckmann rearrangement over alumina-supported boria catalyst prepared by vapor decomposition method

Redox-photosensitized reactions. 11. Ru(bpy)32+-photosensitized reactions of 1-benzyl-1,4-dihydronicotinamide with aryl-substituted enones, derivatives of methyl cinnamate, and substituted cinnamonitriles: electron-transfer mechanism and structure-reactivity relationships

Photochemical reactions of aromatic compounds. 28. Photosensitized electron-transfer reaction of electron donor-acceptor pairs by aromatic hydrocarbons