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, Volume: 17( 1)

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Photoredox Catalysis in Organic Chemistry

*Correspondence:

Lauren Perez
Department of Organic Chemistry, University of Bonn, Bonn, Germany
E-mail: lauren.P@yahoo.com

Abstract

In organic chemistry, photoredox catalysis has emerged as a potent method for activating tiny compounds. These methods in general depend on the ability of metal complexes and organic dyes to transform visible light into chemical energy by engaging in single electron transfer with organic substrates and producing reactive intermediates. The ability of photo redox catalysis to quickly build novel reaction pathways is highlighted in this perspective, with a focus on multicatalytic techniques that make it possible to create difficult carbon and carbon-heteroatom bonds.

Keywords

Organozides; Photoredox catalysis; Lithium alkylamides; Dehydrogenative; Tetrasubstituted alkenes

Introduction

Numerous novel bond structures in organic chemistry have been created over the past century thanks to the discovery, advancement and application of light mediated catalysis. In more recent years, the study of photo catalysis has experienced a substantial revival and once more, a number of novel activation modes have given rise to a wide range of novel bond forming protocols and synthetic techniques.

The field of photocatalysis includes an expanding array of generic activation modes in which photonic energy is selectively directed to a specially crafted photon absorbing catalyst (a photocatalyst) that, upon excitation, is able to motivate an auxiliary substrate, reagent or secondary catalyst to take part in novel reaction pathways that were previously impractical under thermal control.

The most frequent ways for photocatalysts to activate specific molecules while also converting light into chemical energy are I energy transfer, organometallic excitation, light induced atom transfer and photo redox catalysis, all of which will be covered in depth in this special issue. But for the sake of this perspective, we shall limit our discussion to the significance and usefulness of photo redox catalysis as used in organic synthesis. Before discussing the amazing contemporary impact of this area on the field of organic reaction innovation and its use in both industrial and academic settings, we will first analyze the historical value of photoredox catalysis. Particular focus has been placed on papers that show the distinctive features of photoredox catalysis and at the same time the truly original nature of the accompanying reactions it can produce. The coverage of this paper cannot and is not intended to be complete.

Although photo redox catalysis has been widely used over the past 40 years in the domains of water splitting, carbon dioxide reduction and the creation of innovative solar cell materials, its potential to be used for organic synthesis has only lately began to be fully understood. An important element in this industry’s recent, brisk expansion is activation platform has received quick recognition.

Accessible organic dyes and metal polypyridyl complexes make it easier for visible light to be converted into chemical energy under unusually moderate circumstances. When stimulated, these Single Electron Transfer (SET) events can take place between molecules. Utilizing simple organic (and organometallic) substrates access to reactive organisms with open shells. Here, the photoredox catalyst is selectively excited by visible light exposure at wavelengths where ordinary organic molecules do not absorb. The resulting excited species can function as a powerful oxidant and a powerful reluctant at the same time, opening up a reaction environment that is specific to organic chemistry. The fact that the reaction medium can be either oxidative or reductive (but not both) in this electronic duality clearly contrasts with the classic redox reaction manifolds (such as electrochemistry) and opens up previously closed redox neutral reaction platforms. Numerous novel synthetic techniques have been developed thanks to recent advances in photoredox catalysis. The frequent observation of novel forms of reactivity not seen with conventional reaction manifolds is due in part to the properties of photo redox catalysts, which include their capacity to act as both an oxidant and a reluctant simultaneously in their excited states (for example, the triplet state of Ir(ppy) 3 is 56 kcal mol^-1 above the ground state). They can also convert visible light into significant amounts of chemical energy. the combination of these abilities creates the opportunity to obtain previously elusive.

Since nitrogen heterocycles are a favored motif in biologically active chemicals and natural products, synthetic chemists have long concentrated on creating and functionalizing these significant ring structures. This is because advances in photoredox catalysis have produced a wide range of synthetic methods for the site specific functionalization of amines. A broad oxidative activation method for N-arylamines has emerged as a result of early work by the Stephenson group, which revealed a photoredox catalyzed Aza-Henry reaction in 2010.

It has also been possible to combine this N-arylamine activation mode in a beneficial way with other catalytic platforms. In these techniques, the second catalytic system often mediates in situ production of the nucleophilic coupling partner. For instance, rueping and colleagues described a photoredox proline catalyzed mannish reaction in which the creation of the required carbon bond is preceded by the catalytic generation of the iminium ion and an enamine. Similar to this, rovis published an organocatalytic asymmetric iminium acylation technique in which a chiral N-heterocyclic carbene catalyst was used to generate a breslow type intermediate in situ.

High degrees of enantioselectivity in the amino ketone products were produced as a result of trapping the electrophilic iminium ion with this catalytic intermediate. The N-arylamine activation mechanism described here can also be employed in conjunction with anion binding catalysis, as Stephenson and Jacobsen showed.

The authors achieved a two step asymmetric Mukaiyama Mannich reaction using chiral thiourea 5 and a ruthenium photoredox catalyst. 28 a highly enantioselective transformation in this reaction depended on the identity of the oxidant and the photocatalyst counterion. In order to achieve the requisite enantiofacial discrimination in the nucleophilic addition, the prochiral iminium ion’s firmly coupled halide counterion was found to be tightly bound to the thiourea catalyst.