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

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

Visible-light photocatalysis has evolved over the last decade into a widely used method in organic synthesis. Photocatalytic variants have been reported for many important transformations, such as cross-coupling reactions, α-amino functionalizations, cycloadditions, ATRA reactions, or fluorinations. To help chemists select photocatalytic methods for their synthesis, we compare in this Review classical and photocatalytic procedures for selected classes of reactions and highlight their advantages and limitations. In many cases, the photocatalytic reactions proceed under milder reaction conditions, typically at room temperature, and stoichiometric reagents are replaced by simple oxidants or reductants, such as air, oxygen, or amines. Does visible-light photocatalysis make a difference in organic synthesis? The prospect of shuttling electrons back and forth to substrates and intermediates or to selectively transfer energy through a visible-light-absorbing photocatalyst holds the promise to improve current procedures in radical chemistry and to open up new avenues by accessing reactive species hitherto unknown, especially by merging photocatalysis with organo- or metal catalysis.

Visible-Light Photocatalysis: Does It Make a Difference in Organic Synthesis?

The present review offers an overview of the current approaches for the photochemical and photocatalytic generation of reactive intermediates and their application in the formation of carbon–carbon bonds. Valuable synthetic targets are accessible, including arylation processes, formation of both carbo- and heterocycles, alpha- and beta-functionalization of carbonyls, and addition reactions onto double and triple bonds. According to the recent advancements in the field of visible/solar light catalysis, a significant part of the literature reported herein involves radical ions and radicals as key intermediates, with particular attention to the most recent examples. Synthetic application of carbenes, biradicals/radical pairs and carbocations have been also reported.

Carbon-Carbon Bond Forming Reactions via Photogenerated Intermediates.

The direct decarboxylative arylation of α-oxo acids has been achieved by synergistic visible-light-mediated photoredox and nickel catalysis. This method offers rapid entry to aryl and alkyl ketone architectures from simple α-oxo acid precursors via an acyl radical intermediate. Significant substrate scope is observed with respect to both the oxo acid and arene coupling partners. This mild decarboxylative arylation can also be utilized to efficiently access medicinal agents, as demonstrated by the rapid synthesis of fenofibrate.

/pdf/merging-photoredox-and-nickel-catalysis-the-direct-synthesis-fauv04ibq5.pdf

Merging Photoredox and Nickel Catalysis: The Direct Synthesis of Ketones by the Decarboxylative Arylation of α-Oxo Acids

In nature, many organisms generate large families of natural product metabolites that have related molecular structures as a means to increase functional diversity and gain an evolutionary advantage against competing systems within the same environment. One pathway commonly employed by living systems to generate these large classes of structurally related families is oligomerization, wherein a series of enzymatically catalysed reactions is employed to generate secondary metabolites by iteratively appending monomers to a growing serial oligomer chain. The polypyrroloindolines are an interesting class of oligomeric natural products that consist of multiple cyclotryptamine subunits. Herein we describe an iterative application of asymmetric copper catalysis towards the synthesis of six distinct oligomeric polypyrroloindoline natural products: hodgkinsine, hodgkinsine B, idiospermuline, quadrigemine H and isopsychotridine B and C. Given the customizable nature of the small-molecule catalysts employed, we demonstrate that this strategy is further amenable to the construction of quadrigemine H-type alkaloids not isolated previously from natural sources.

Jeffrey M. Lipshultz

Papers

Merging Photoredox and Nickel Catalysis: The Direct Synthesis of Ketones by the Decarboxylative Arylation of α-Oxo Acids

Catalyst-controlled oligomerization for the collective synthesis of polypyrroloindoline natural products.

Merging Photoredox and Nickel Catalysis: The Direct Synthesis of Ketones by the Decarboxylative Arylation of α-Oxo Acids