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

The use of visible light sensitization as a means to initiate organic reactions is attractive due to the lack of visible light absorbance by organic compounds, reducing side reactions often associated with photochemical reactions conducted with high energy UV light. This tutorial review provides a historical overview of visible light photoredox catalysis in organic synthesis along with recent examples which underscore its vast potential to initiate organic transformations.

Visible light photoredox catalysis: applications in organic synthesis

Photoredox catalysis and organocatalysis represent two powerful fields of molecule activation that have found widespread application in the areas of inorganic and organic chemistry, respectively. We merged these two catalysis fields to solve problems in asymmetric chemical synthesis. Specifically, the enantioselective intermolecular α-alkylation of aldehydes has been accomplished using an interwoven activation pathway that combines both the photoredox catalyst Ru(bpy)3Cl2 (where bpy is 2,2′-bipyridine) and an imidazolidinone organocatalyst. This broadly applicable, yet previously elusive, alkylation reaction is now highly enantioselective and operationally trivial.

http://science.sciencemag.org/content/sci/322/5898/77.full.pdf

Merging Photoredox Catalysis with Organocatalysis: The Direct Asymmetric Alkylation of Aldehydes

The review covers the knowledge on photoremovable protecting
groups and includes all relevant chromophores studied in the
time period of 2000–2012; the most relevant earlier works are
also discussed.

/pdf/photoremovable-protecting-groups-in-chemistry-and-biology-2n33qs6p7b.pdf

Photoremovable Protecting Groups in Chemistry and Biology: Reaction Mechanisms and Efficacy

Fragmentation of substituted acetophenones and halobenzophenone ketyls. Calibration of a mechanistic probe

Positive Halogen Compounds. VIII. Structure and Reactivity in N-Bromosuccinimide Brominations

A free-radical chain reaction involving electron transfer. The replacement of the nitro group by hydrogen using trialkyltin hydride, a variation of the Kornblum reaction

On the mechanism of the reduction of .alpha.-haloketones by 1,3-dimethyl-2-phenylbenzimidazoline. Reduction by a SET (single electron transfer)-hydrogen atom abstraction chain mechanism

On donne trois methodes de preparation de ce compose: reaction de l'hypochlorite de t-butyle avec l'iode, reaction de l'hypochlorite de t-butyle avec les iodures metalliques et reaction du t-butylate de K avec l'iode. Reactivite de l'hypoiodite de t-butyle

Dennis D. Tanner

Papers

Fragmentation of substituted acetophenones and halobenzophenone ketyls. Calibration of a mechanistic probe

Positive Halogen Compounds. VIII. Structure and Reactivity in N-Bromosuccinimide Brominations

A free-radical chain reaction involving electron transfer. The replacement of the nitro group by hydrogen using trialkyltin hydride, a variation of the Kornblum reaction

On the mechanism of the reduction of .alpha.-haloketones by 1,3-dimethyl-2-phenylbenzimidazoline. Reduction by a SET (single electron transfer)-hydrogen atom abstraction chain mechanism

On the structure of tert-butyl hypoiodite