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

Background Interest in photochemical synthesis has been motivated in part by the realization that sunlight is effectively an inexhaustible energy source.Chemists have also long recognized distinctive patterns of reactivity that are uniquely accessible via photochemical activation. However, most simple organic molecules absorb only ultraviolet (UV) light and cannot be activated by the visible wavelengths that comprise most of the solar energy that reaches Earth’s surface. Consequently, organic photochemistry has generally required the use of UV light sources. Visible light photocatalysis. ( A ) Transition metal photocatalysts, such as Ru(bpy) 3 2+ , readily absorb visible light to access reactive excited states. ( B ) Photoexcited Ru*(bpy) 3 2+ can act as an electron shuttle, interacting with sacrificial electron donors D (path i) or acceptors A (path ii) to yield either a strongly reducing or oxidizing catalyst toward organic substrates S. Ru*(bpy) 3 2+ can also directly transfer energy to an organic substrate to yield electronically excited species (path iii). bpy, 2,29-bipyridine; MLCT, metal-to-ligand charge transfer. Advances Over the past several years, there has been a resurgence of interest in synthetic photochemistry, based on the recognition that the transition metal chromophores that have been so productively exploited in the design of technologies for solar energy conversion can also convert visible light energy into useful chemical potential for synthetic purposes. Visible light enables productive photoreactions of compounds possessing weak bonds that are sensitive toward UV photodegradation. Furthermore, visible light photoreactions can be conducted by using essentially any source of white light, including sunlight, which obviates the need for specialized UV photoreactors. This feature has expanded the accessibility of photochemical reactions to a broader range of synthetic organic chemists. A variety of reaction types have now been shown to be amenable to visible light photocatalysis via photoinduced electron transfer to or from the transition metal chromophore, as well as energy-transfer processes. The predictable reactivity of the intermediates generated and the tolerance of the reaction conditions to a wide range of functional groups have enabled the application of these reactions to the synthesis of increasingly complex target molecules. Outlook This general strategy for the use of visible light in organic synthesis is already being adopted by a growing community of synthetic chemists. Much of the current research in this emerging area is geared toward the discovery of photochemical solutions for increasingly ambitious synthetic goals. Visible light photocatalysis is also attracting the attention of researchers in chemical biology, materials science, and drug discovery, who recognize that these reactions offer opportunities for innovation in areas beyond traditional organic synthesis. The long-term goals of this emerging area are to continue to improve efficiency and synthetic utility and to realize the long-standing goal of performing chemical synthesis using the sun.

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Solar Synthesis: Prospects in Visible Light Photocatalysis

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.

Metal-catalyzed enantioselective allylation, which involves the substitution of allylic metal intermediates with a diverse range of different nucleophiles or S(N)2'-type allylic substitution, leads to the formation of C-H, -C, -O, -N, -S, and other bonds with very high levels of asymmetric induction. The reaction may tolerate a broad range of functional groups and has been applied successfully to the synthesis of many natural products and new chiral compounds.

Metal-catalyzed enantioselective allylation in asymmetric synthesis.

Photochemical reactions are remarkable for their ability to easily assemble cyclobutanes and other strained ring systems that are difficult to construct using other conventional synthetic methods. We have previously shown that Ru(bpy)32+ is an efficient photocatalyst that promotes the [2+2] cycloadditions of electron-deficient olefins with visible light. Here, we show that Ru(bpy)32+ is also an effective photocatalyst for the [2+2] cycloaddition of electron-rich olefins. This transformation is enabled by the versatile photoelectrochemical properties of Ru(bpy)32+, which enables either one-electron reduction or one-electron oxidation of interesting organic substrates under appropriate conditions.

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[2+2] Cycloadditions by Oxidative Visible Light Photocatalysis

Durch metallkatalysierte enantioselektive Allylierungen, die unter Substitution von Allylmetall-Zwischenstufen mit zahlreichen Nucleophilen oder als allylische SN2′-Substitution verlaufen, werden in Gegenwart ausgewahlter Kombinationen aus Metallen und chiralen Liganden C-H-, C-C-, C-O-, C-N-, C-S- und andere Bindungen mit sehr hoher asymmetrischer Induktion geknupft. Die Reaktionen sind mit einer Vielzahl funktioneller Gruppen vereinbar und wurden erfolgreich in der Synthese vieler Naturstoffe und neuer chiraler Verbindungen verwendet.

Metallkatalysierte enantioselektive Allylierungen in der asymmetrischen Synthese

Hip to be square: Styrenes participate in [2+2] cycloadditions upon irradiation with visible light in the presence of an iridium(III) polypyridyl complex. In contrast to previous reports of visible light photoredox catalysis, the mechanism of this process involves photosensitization by energy transfer and not electron transfer.

Zhan Lu

Papers

Metal-catalyzed enantioselective allylation in asymmetric synthesis.

[2+2] Cycloadditions by Oxidative Visible Light Photocatalysis

Metallkatalysierte enantioselektive Allylierungen in der asymmetrischen Synthese

Visible light photocatalysis of [2+2] styrene cycloadditions by energy transfer.

Recent Advances in Hydrometallation of Alkenes and Alkynes via the First Row Transition Metal Catalysis