Transition Metal and Photocatalyst Free Arylation via Photoexcitable Electron Donor Acceptor Complexes:Mediation and Catalysis

Electron donor acceptor (EDA) complexes formed between arylating agents and nucleophilic substrates or additives, undergo photoexcitation to produce aryl radicals. Two main modes of reactivity for such complexes exist: mediation, where EDA complex is formed between two reagents, or catalysis, in which the electron donor additive is used as an organocatalyst.

Abstract

Visible-light-activated organic reactions unlock novel avenues for complex molecular transformations, impossible under standard “thermal” conditions, which makes them powerful tools in the arsenal of synthetic chemistry. However, transition metal-based or organic photoredox catalysts are often used to ensure productive absorption of visible light, which might be not desirable to medicinal chemistry and industry due to toxicity, low sustainability, and high cost of most photocatalysts. A more environmentally and economically benign approach is based on the formation of transient electron donor-acceptor (EDA) complexes between two reagents or a reagent and an additive, that readily absorb visible light, acting as internal photosensitizers. Within the EDA complex-based arylation strategies, chemical transformations are mediated by noncovalent interaction between two molecules, namely between electron-poor aryl halides or their synthetic equivalents and electron-rich nucleophilic reagents or additives. Moreover, besides stoichiometric EDA complexes between two molecules, EDA complex based organocatalysis can be achieved in certain cases through regeneration of the donor molecules in the course of the reaction. Photoexcitation of the EDA complexes induces a single electron transfer (SET) process to generate aryl radical species for the arylation step. This Review will focus on the state-of-the-art EDA complex-based arylation strategies utilizing aryl halides, aryldiazonium, diaryliodonium, arylsulfonium and arylphosphonium salts as reactants, published mainly in the last five years.

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