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

Catalytic dehydrogenative cross-coupling: forming carbon-carbon bonds by oxidizing two carbon-hydrogen bonds

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

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Principles Of Fluorescence Spectroscopy

Disclosed herein is a general catalytic system for the intermolecular anti-Markovnikov hydroamination of alkenes. By using an organocatalytic photoredox system, α- and β-substituted styrenes as well as aliphatic alkenes undergo anti-Markovnikov hydroamination. Heterocyclic amines were also successfully employed as nitrogen nucleophiles, thus providing a direct route to heterocyclic motifs common in medicinal agents.

anti-Markovnikov hydroamination of alkenes catalyzed by a two-component organic photoredox system: direct access to phenethylamine derivatives.

Methods for the direct C–H functionalization of aromatic compounds are in demand for a variety of applications, including the synthesis of agrochemicals, pharmaceuticals, and materials. Herein, we disclose the construction of aromatic nitriles via direct C–H functionalization using an acridinium photoredox catalyst and trimethylsilyl cyanide under an aerobic atmosphere. The reaction proceeds at room temperature under mild conditions and has proven to be compatible with a variety of electron-donating and -withdrawing groups, halogens, and nitrogen- and oxygen-containing heterocycles, as well as aromatic-containing pharmaceutical agents.

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Direct C-H Cyanation of Arenes via Organic Photoredox Catalysis.

A direct catalytic anti-Markovnikov addition of carboxylic acids to alkenes is reported. The catalyst system is comprised of the Fukuzumi acridinium photooxidant (1) and a substoichiometric quantity of a hydrogen-atom donor. Oxidizable olefins, such as styrenes, trisubstituted aliphatic alkenes, and enamides, can be employed along with a variety of carboxylic acids to afford the anti-Markovnikov addition adducts exclusively. A deuterium-labeling experiment lends insight to the potential mechanism.

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Direct Catalytic Anti-Markovnikov Addition of Carboxylic Acids to Alkenes

Synthetic transformations that functionalize unactivated aliphatic C–H bonds in an intermolecular fashion offer unique strategies for the synthesis and late-stage derivatization of complex molecules. Herein we report a general approach to the intermolecular functionalization of aliphatic C–H bonds using an acridinium photoredox catalyst and phosphate salt under blue LED irradiation. This strategy encompasses a range of valuable C–H transformations, including the direct conversions of a C–H bond to C–N, C–F, C–Br, C–Cl, C–S, and C–C bonds, in all cases using the alkane substrate as the limiting reagent. Detailed mechanistic studies are consistent with the intermediacy of a putative oxygen-centered radical as the hydrogen atom-abstracting species in these processes.

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A General Strategy for Aliphatic C-H Functionalization Enabled by Organic Photoredox Catalysis.

Metal-free, visible light-initiated, living cationic polymerization of 4-methoxystyrene using 2,4,6-tri(p-tolyl)pyrylium tetrafluoroborate and methanol is demonstrated. Molecular weight and dispersity are controlled by the concentration of methanol. Initial mechanistic analysis suggests that methanol likely serves to regulate propagation of the cation chain end via reversible chain transfer in a manner analogous to reversible addition–fragmentation chain transfer polymerization.

David A. Nicewicz

Papers

anti-Markovnikov hydroamination of alkenes catalyzed by a two-component organic photoredox system: direct access to phenethylamine derivatives.

Direct C-H Cyanation of Arenes via Organic Photoredox Catalysis.

Direct Catalytic Anti-Markovnikov Addition of Carboxylic Acids to Alkenes

A General Strategy for Aliphatic C-H Functionalization Enabled by Organic Photoredox Catalysis.

Visible Light Photoinitiated Metal-Free Living Cationic Polymerization of 4-Methoxystyrene.