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

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

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

C–H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C–H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C–H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C–H activation until summer 2018.

3d Transition Metals for C-H Activation.

The merger of transition metal catalysis and photocatalysis, termed metallaphotocatalysis, has recently emerged as a versatile platform for the development of new, highly enabling synthetic methodologies. Photoredox catalysis provides access to reactive radical species under mild conditions from abundant, native functional groups, and, when combined with transition metal catalysis, this feature allows direct coupling of non-traditional nucleophile partners. In addition, photocatalysis can aid fundamental organometallic steps through modulation of the oxidation state of transition metal complexes or through energy-transfer-mediated excitation of intermediate catalytic species. Metallaphotocatalysis provides access to distinct activation modes, which are complementary to those traditionally used in the field of transition metal catalysis, thereby enabling reaction development through entirely new mechanistic paradigms. This Review discusses key advances in the field of metallaphotocatalysis over the past decade and demonstrates how the unique mechanistic features permit challenging, or previously elusive, transformations to be accomplished. Transition metal catalysis is well established as an enabling tool in synthetic organic chemistry. Photoredox catalysis has recently emerged as a method to effect reactions that occur through single-electron-transfer pathways. Here we review the combination of the two to show how this provides access to highly reactive oxidation states of transition metals and distinct activation modes that further enable the synthetic chemist.

The merger of transition metal and photocatalysis

Cross-dehydrogenative-coupling reaction has long been recognized as a powerful tool to form a C–C bond directly from two different C–H bonds. Most current processes are performed by making use of stoichiometric amounts of oxidizing agents. We describe here a new type of reaction, namely cross-coupling hydrogen evolution (CCHE), with no use of any sacrificial oxidants, and only hydrogen (H2) is generated as a side product. By combining eosin Y and a graphene-supported RuO2 nanocomposite (G-RuO2) as a photosensitizer and a catalyst, the desired cross-coupling products and H2 are achieved in quantitative yields under visible light irradiation at room temperature.

A cascade cross-coupling hydrogen evolution reaction by visible light catalysis.

Cross-coupling hydrogen evolution reaction in homogeneous solution without noble metals.

Visible light catalysis assisted site-specific modification of α-amino acids by C–H bond functionalization without the use of any oxidant or base is described. Using Ru(bpy)3(PF6)2 and Co(dmgH)2pyCl as a photosensitizer and a catalyst, respectively, a variety of glycine esters with β-keto esters or indole derivatives can be quantitatively converted into the desired cross-coupling products and hydrogen (H2) in good to excellent yields under visible light irradiation. A mechanistic study reveals that the cascade electron transfer processes from glycine ester to the photoexcited Ru(bpy)3(PF6)2 and then to Co(dmgH)2pyCl catalyst, together with the capture of protons delivered by substrates, are crucial for the cross-coupling hydrogen evolution reaction of secondary amines in organic solvents.

Visible Light Catalysis Assisted Site-Specific Functionalization of Amino Acid Derivatives by C–H Bond Activation without Oxidant: Cross-Coupling Hydrogen Evolution Reaction

Cobalt-Catalyzed Cross-Dehydrogenative Coupling Reaction in Water by Visible Light

[2+2] Photocycloaddition, for example, the dimerization of chalcones and cinnamic acid derivatives, is a unique strategy to construct cyclobutanes, which are building blocks for a variety of biologically active molecules and natural products. However, most attempts at the above [2+2] addition have focused on solid-state, molten-state, or host-guest systems under ultraviolet-light irradiation in order to overcome the competition of facile geometric isomerization of nonrigid olefins. We report a general and simple method to realize the intermolecular [2+2] dimerization reaction of these acyclic olefins to construct cyclobutanes in a highly regio- and diastereoselective manner in solution under visible light, which provides an efficient solution to a long-standing problem.

Tao Lei

Papers

A cascade cross-coupling hydrogen evolution reaction by visible light catalysis.

Cross-coupling hydrogen evolution reaction in homogeneous solution without noble metals.

Visible Light Catalysis Assisted Site-Specific Functionalization of Amino Acid Derivatives by C–H Bond Activation without Oxidant: Cross-Coupling Hydrogen Evolution Reaction

Cobalt-Catalyzed Cross-Dehydrogenative Coupling Reaction in Water by Visible Light

General and Efficient Intermolecular [2+2] Photodimerization of Chalcones and Cinnamic Acid Derivatives in Solution through Visible-Light Catalysis.