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. 2019 Apr;1(1):111-125.
doi: 10.1016/j.trechm.2019.01.008. Epub 2019 Feb 22.

Illuminating Photoredox Catalysis

Rory C McAtee  1 , Edward J McClain  1 , Corey R J Stephenson  1
Affiliations

Illuminating Photoredox Catalysis

Rory C McAtee et al. Trends Chem. 2019 Apr.

Abstract

Over the past decade, photoredox catalysis has risen to the forefront of synthetic organic chemistry as an indispensable tool for selective small-molecule activation and chemical-bond formation. This cutting-edge platform allows photosensitizers to convert visible light into chemical energy prompting generation of reactive radical intermediates. In this Review, we highlight some of the recent key contributions in the field, including: the impact of the chosen light arrays; promoting fundamental cross-coupling steps; selectively functionalizing aliphatic amines; engaging complementary mechanistic paradigms; and applications in industry. With such a wide breadth of reactivity already realized, the presence of photoredox catalysis in all sectors of organic chemistry is expected for years to come.

Keywords: chemical tool; photocatalysis; photoredox; radicals; synthesis; visible light.

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Conflict of interest statement

Conflicts of Interest The authors declare no conflicts of interest.

Figures

Figure 1.
Figure 1.
Photoredox catalysis as a modern approach to radical intermediate generation. (A) The classic (left) and modern (right; photoredox catalysis) approach to radical intermediates. (B) Commonly employed metal-centered and organic photocatalysts. (C) A general representation of the oxidative and reductive quenching cycle of Ru(bpy)32+. MLCT and ISC are metal-to-ligand charge transfer and intersystem coupling, respectively.
Figure 2.
Figure 2.
Evolution of light array designs for enhanced reaction efficiency and scalability. (A) Small library of commonly employed light array designs over the past decade. (B) The demonstration of enhanced reaction efficiency with the use of an integrated photoreactor. (C) The comparison of batch and flow processing for the trifluoromethylation of N-Boc pyrrole.
Figure 3.
Figure 3.
Impacting elementary cross-coupling steps with photoredox catalysis. (A) A general transition metal-mediated cross-coupling catalytic cycle. (B) The use of photoredox catalysis to overcome the challenges of oxidative addition to a CuI center. (C) Photoredox catalysis has had the greatest influence on the elementary transmetallation step. (D) The demonstration of triplet-triplet energy transfer from an excited state photocatalyst to an organometallic intermediate.
Figure 4.
Figure 4.
Selective oxidation of aliphatic amines enabled by photoredox catalysis and subsequent synthetic applications. (A) The resultant impact on the oxidation of aliphatic amines. (B) Initial efforts aimed at the direct oxidation of N-aryl tetrahydroquinoline and subsequent functionalization. (C) Amine oxidation to mimic the proposed biosynthesis of several catharanthine derived alkaloids. (D) The utility of photoredox catalysis in providing new tools for the synthesis of saturated building blocks of interest to the pharmaceutical sector.
Figure 5.
Figure 5.
Complementary mechanistic paradigms for anti-Markovnikov additions to alkenes. (A) Anti-Markovnikov selective hydrofunctionalization of alkenes via alkene radical cations. (B) Anti-Markovnikov selective hydrofunctionalization of alkenes via concerted proton-coupled electron transfer approach.
Figure 6.
Figure 6.
Recent applications of photoredox catalysis in total synthesis and the industrial sector. (A) The use of photoredox catalysis as a key step in the synthesis of natural products. (B) The use of photoredox catalysis in the direct late-stage C−H methylation, ethylation, and cyclopropanation of pharmaceutical and agrochemical agents. (C) Photocatalytic indoline dehydrogenation as a key step in the sustainable synthesis of elbasvir. (D) A practical photoredox-mediated hydrogen atom transfer protocol to selectively deuterate and tritiate α-amino sp3 C−H bonds of 18 pharmaceutical compounds.
Figure I.
Figure I.
Metal-to-ligand charge-transfer (MLCT) and intersystem crossing (ISC) events from the ground state to the triplet excited state for the prototypical transition-metal photocatalyst, Ru(bpy)32+, upon visible light irradiation.
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