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.






Open in a separate window


Figure 1


Ruthenium polypyridyl complexes: versatile visible light photocatalysts.

/pdf/visible-light-photoredox-catalysis-with-transition-metal-5d1oxrsczo.pdf

Visible Light Photoredox Catalysis with Transition Metal Complexes: Applications in Organic Synthesis

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

Over the last decade, substantial research has led to the introduction of an impressive number of efficient procedures which allow the selective construction of CC bonds by directly connecting two different CH bonds under oxidative conditions. Common to these methodologies is the generation of the reactive intermediates in situ by activation of both CH bonds. This strategy was introduced by the group of Li as cross-dehydrogenative coupling (CDC) and discloses waste-minimized synthetic alternatives to classic coupling procedures which rely on the use of prefunctionalized starting materials. This Review highlights the recent progress in the field of cross-dehydrogenative C sp 3C formations and provides a comprehensive overview on existing procedures and employed methodologies.

The Cross-Dehydrogenative Coupling of C sp 3H Bonds: A Versatile Strategy for CC Bond Formations

Asymmetric catalysis is a key feature of modern synthetic organic chemistry. Traditionally, different combinations of ligands and metals are used to perform highly enantioselective reactions. Since the renaissance of organocatalysis in the early 2000s, tremendous improvement in the field of metal-free catalysis has been achieved. Recently, the combination of transition metals and organocatalysts has allowed the development of new protocols enabling transformations that could not previously be realized. This article aims to present the latest contributions in the field of combined chiral Bronsted acid and metal catalyzed reactions, highlighting the advantages of these catalytic systems as well as describing the uncertainties regarding the molecular structure of the catalytically active species and the reaction mechanisms.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1002/chem.201001140

Unifying Metal and Brønsted Acid Catalysis—Concepts, Mechanisms, and Classifications

Dual catalysis: combining photoredox and Lewis base catalysis for direct Mannich reactions

Photoredox catalyzed C-P bond forming reactions-visible light mediated oxidative phosphonylations of amines.

A new dual catalytic system for efficient C[BOND]H functionalization was developed. The appropriate choice of two metal catalysts allows the oxidative alkynylation of tertiary amines under mild and sustainable reaction conditions.

Dual catalysis: combination of photocatalytic aerobic oxidation and metal catalyzed alkynylation reactions--C-C bond formation using visible light.

Let there be light: A heterogeneous photocatalytic system based on easily recyclable TiO(2) or ZnO allows cross dehydrogenative coupling reactions of tertiary amines. The newly developed protocols have successfully been applied to various C-C and C-P bond-forming reactions to provide nitro amines as well as amino ketones, nitriles and phosphonates.

Light-Mediated Heterogeneous Cross Dehydrogenative Coupling Reactions: Metal Oxides as Efficient, Recyclable, Photoredox Catalysts in CC Bond- Forming Reactions

Rene M. Koenigs

Papers

Unifying Metal and Brønsted Acid Catalysis—Concepts, Mechanisms, and Classifications

Dual catalysis: combining photoredox and Lewis base catalysis for direct Mannich reactions

Photoredox catalyzed C-P bond forming reactions-visible light mediated oxidative phosphonylations of amines.

Dual catalysis: combination of photocatalytic aerobic oxidation and metal catalyzed alkynylation reactions--C-C bond formation using visible light.

Light-Mediated Heterogeneous Cross Dehydrogenative Coupling Reactions: Metal Oxides as Efficient, Recyclable, Photoredox Catalysts in CC Bond- Forming Reactions