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

Over the past several decades, organometallic cross-coupling chemistry has developed into one of the most reliable approaches to assemble complex aromatic compounds from preoxidized starting materials. More recently, transition metal–catalyzed carbon-hydrogen activation has circumvented the need for preoxidized starting materials, but this approach is limited by a lack of practical amination protocols. Here, we present a blueprint for aromatic carbon-hydrogen functionalization via photoredox catalysis and describe the utility of this strategy for arene amination. An organic photoredox-based catalyst system, consisting of an acridinium photooxidant and a nitroxyl radical, promotes site-selective amination of a variety of simple and complex aromatics with heteroaromatic azoles of interest in pharmaceutical research. We also describe the atom-economical use of ammonia to form anilines, without the need for prefunctionalization of the aromatic component.

Site‐Selective Arene C—H Amination via Photoredox Catalysis.

Organic azides serve as synthetically useful surrogates for primary amines, a functional group which is ubiquitous in bioactive and medicinally relevant molecules. Historically, the formal hydroazidation of simple activated olefins and styrenes has proven difficult due to the inherent propensity of these compounds to oligomerize. Herein is disclosed a method for the anti-Markovnikov hydroazidation of activated olefins, catalyzed by an organic acridinium salt under irradiation from blue LEDs. This method is applicable to a variety of substituted and terminal styrenes and several vinyl ethers, yielding synthetically versatile hydroazidation products in moderate to excellent yield.

Anti-Markovnikov Hydroazidation of Activated Olefins via Organic Photoredox Catalysis

A pair of 9-mesityl-10-phenyl acridinium (Mes-Acr+ ) photoredox catalysts were synthesized with an iodoacetamide handle for cysteine bioconjugation. Covalently tethering of the synthetic Mes-Acr+ cofactors with a small panel of thermostable protein scaffolds resulted in 12 new artificial enzymes. The unique chemical and structural environment of the protein hosts had a measurable effect on the photophysical properties and photocatalytic activity of the cofactors. The constructed Mes-Acr+ hybrid enzymes were found to be active photoinduced electron-transfer catalysts, controllably oxidizing a variety of aryl sulfides when irradiated with visible light, and possessed activities that correlated with the photophysical characterization data. Their catalytic performance was found to depend on multiple factors including the Mes-Acr+ cofactor, the protein scaffold, the location of cofactor immobilization, and the substrate. This work provides a framework toward adapting synthetic photoredox catalysts into artificial cofactors and includes important considerations for future bioengineering efforts.

Design and Evaluation of Artificial Hybrid Photoredox Biocatalysts.

Efficient procedures are elaborated for the anti-Markovnikov addition of HCl, HF, phosphoric acids, and sulfonic acids, to alkenes in the presence of a photoredox catalytic system containing an acridinium sensitizer.

David A. Nicewicz

Papers

Site‐Selective Arene C—H Amination via Photoredox Catalysis.

Anti-Markovnikov Hydroazidation of Activated Olefins via Organic Photoredox Catalysis

Design and Evaluation of Artificial Hybrid Photoredox Biocatalysts.

The Direct anti‐Markovnikov Addition of Mineral Acids to Styrenes.

Direct Synthesis of Bicyclic Acetals via Visible Light Catalysis.