Progress in Photoinduced Radical Reactions using Electron Donor‐Acceptor Complexes

Abstract

Photocatalyzed organic synthesis transformation is a remarkable green synthetic strategy because of the advantages of operational simplicity, high chemoselectivities, cheap, and environmental benignancy, along with the extensive applications in the fields of organic, pharmaceutical and functional material chemistry. Generally, photoredox catalysts or photosensitizers are necessary for the generation of their excited states to perform the successive oxidative or reductive reactions through the single electron transfer (SET) or energy transfer (ET) process. Furthermore, the exploration of a colored electron donor-acceptor (EDA) complex or a charge transfer (CT) complex between an electron-rich and an electron-poor substrate provides the chance to deliver the excited intermediate under the irradiation of light, resulting in the formation of radical activate species through a single electron transfer to induce successive various radical reactions. These reactions were performed without the need of any external photocatalysts under mild reaction conditions. Herein, this review focuses on the recent progress on photoinduced radical addition reactions, radical borylations, reductive reactions, radical-radical cross-coupling reactions, degradation reactions and radical cascade cyclization via EDA complexes. We highlight these novel green synthetic methodologies and applications, as well as the mechanisms. This review will help to provide references for organic and medicinal chemists who are charmed by these green organic photochemical transformations based on EDA complexes.

Visible-light-induced cross-coupling of aryl iodides with hydrazones via an EDA-complex

“…The major pathway was that the base abstracts a proton from hydrazone and generates hydrazone anion A, which forms EDA complex B with iodobenzene 2b. The EDA complex 13 is irradiated with blue light, affording benzylic radical C and aryl radical D via single electron transfer with the release of iodide. Then, cross-coupling of C and D forms a new C–C bond and generates intermediate E. Subsequent N 2 -extrusion under base condition yields 3v, 14 which abstracts a proton from H 2 O or solvent affording the product 3v (the source of the hydrogen atom has been studied by several deuterium labelling experiments, see Scheme S2 in the ESI † ).…”

Photocatalytic Three‐Component Tandem Annulation Access to Multiply Substituted 1,2,4‐Triazole‐3,5‐diamines

“…When compound 6 was mixed with 3 , the color of the mixture turned from colorless into orange very fast. Meanwhile, an obvious red shift in UV‐vis absorption spectra, which was mainly attrituted to the formation of an electron donor‐acceptor (EDA) complex from 6 and 3 [12b–e] . In addition, a Job's plot of the EDA complexes was builded to test the stoichiometry between 6 and 3 .…”

A practical and sustainable two-component Minisci alkylation via photo-induced EDA-complex activation

“…As an alternative, the photoinduced electron donor-acceptor (EDA) complex strategy is gaining considerable attention because of its ability to manifest photocatalyst-free, oxidant-free reaction conditions. [26][27][28][29][30] The radicals are generated via a synergistic non-covalent interaction between an electron-rich donor and an electron-decient acceptor. Although there are already many transformations reported based on this paradigm to construct new C-C, C-N, C-S, and C-B bonds efficiently, only a handful of reports are available in the realm of Minisci reactions (Fig.…”