Recent Advances in Employing Catalytic Donors and Acceptors in Electron Donor–Acceptor Complex Photochemistry

Abstract

Electron donor–acceptor (EDA) complexes provide a means to initiate radical reactions under visible light irradiation using substrates that do not absorb visible light individually. Catalytic approaches to complex formation are vital for advancing this synthetic strategy as it decouples the complexation and photogeneration of radicals from substrate functionalization, a limitation inherent to stoichiometric approaches that restricts structural diversity. This Synopsis highlights recent developments in EDA complex photochemistry in which either the donor or acceptor are employed catalytically.

Overcoming Back Electron Transfer in the Electron Donor–Acceptor Complex-Mediated Visible Light-Driven Generation of α-Aminoalkyl Radicals from Secondary Anilines

“…Compound 11 was afforded as a white solid after purification on the silica column using a gradient of 0−10% ethyl acetate in petroleum ethers (17.4 mg, 67%). Spectroscopic data are in accordance with the literature; 23 (3aR,9bS)-2-Propyl-3a,4,5,9b-tetrahydro-1H-pyrrolo [3,4-c]quinoline-1,3(2H)-dione (12). Performed on a 0.19 mmol scale.…”

“…An EDA complex is a weak molecular aggregate that can form between a nucleophilic donor and an electrophilic acceptor. , Associated with the formed complex is the emergence of a new electronic transition in the electromagnetic spectrum, absent in either the donor or acceptor. Excitation of the complex at this energy induces a net SET from the donor to the acceptor. , The radicals formed after the SET can subsequently be used in a range of reactions. ,− The fact that the new electronic transition is of lower energy than needed to excite any of the individual species separately very often results in that visible light can be used to excite the EDA complex which is attractive as UV light can be circumvented.…”

“…The use of amines as potent electron donors in EDA complexes has been widely documented. ,− Recently, we have shown that tertiary anilines can act as donors in EDA complexes in combination with activated alkenes for the visible light-driven synthesis of tetrahydroquinolines (THQs), circumventing the use of complex and expensive photoredox catalysts. − However, whereas this catalyst-free approach works well for N , N -dialkylated anilines, secondary anilines have not been suitable substrates most likely due to back electron transfer (BET). Thus, a major challenge for applications of EDA complexes in organic synthesis is to overcome the BET that can occur after the photoinitiated SET, suppressing the formation of radical ions. , This is especially prominent for the generation of secondary α-aminoalkyl radicals as the BET from the amine radical cation back to the oxidant is faster than any forward process (Scheme A) .…”

Selective C–S Bond Constructions Using Inorganic Sulfurs via Photoinduced Electron Donor–Acceptor Activation

“…The mixture of aryl halide 1f or PC3 with Li 2 S in the presence of Na 2 CO 3 showed significant visible-light absorptions ( IV and V ). It is believed that Li 2 S could trigger this reaction through the formation of the EDA complex, which induces the single-electron transfer (SET) upon the irradiation of visible light to produce radical ion pairs . In addition, the hypothetical benzenethiol intermediate 4-nitrobenzenethiol 1f ′ ( III ) shows a small absorption band, while the mixture of PC3 - 1f ′-Na 2 CO 3 ( VI ) or 1f - 1f ′-Na 2 CO 3 ( VII ) exhibited an obvious increase in visible-light absorption.…”

A Decarboxylative Cross-Coupling Platform To Access 2-Heteroaryl Azetidines: Building Blocks with Application in Medicinal Chemistry

“…Within the context of developing highly selective kinase inhibitors constructed from sp 3 -rich building blocks, we were interested in preparing 2-heteroaryl azetidines. We envisaged that recent advancements by the groups of Fu, , Molander, − Aggarwal, Overman and Pitre, , Melchiorre, , and others − in photomediated chemistry using electron donor–acceptor (EDA) complexes might provide a platform for more direct access to these compounds that have heretofore been difficult to prepare. Here, we report a photomediated nickel-catalyzed cross-coupling of commercially available or easily prepared azetidine-2- N -hydroxyphthalimide (NHP) esters and heteroaryl iodides to access 2-heteroaryl azetidines (Figure d).…”