Radical-Electron Transfer Coupling in Aromatic Systems

Single electron transfer within aromatic systems drives a diverse array of radical-mediated coupling reactions. By transient generation of arene radical anions, these processes circumvent the need for transition metal catalysts, forging C–C bonds through radical propagation and rearomatisation steps. Mechanistic variants include SRN1 pathways, base-promoted homolytic aromatic substitution (BHAS) and photochemical initiation, each harnessing organic electron donors to trigger substrate reduction. Control over radical recombination and electron flow has expanded the scope of biaryl synthesis, direct C–H functionalisation and heterocycle arylation, with emerging examples demonstrating gram-scale reliability and broad functional group tolerance. This convergence of radical and electron-transfer chemistry promises more sustainable routes to complex aromatic architectures, underpinning advances in pharmaceuticals, agrochemicals and electronic materials.

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Research from all publishers

Investigations into the role of potassium tert-butoxide have revealed its capacity to act not only as a base but also as a mediator of single electron transfer, often generating organic super-electron donors in situ that initiate aryl–aryl coupling under metal-free conditions. Complementary studies have identified heterocycle-derived organic super-electron donors that selectively trigger haloarene–arene couplings, elucidating the structures and electronic requirements of effective initiators. Further insights have emerged from analyses of intramolecular versus intermolecular electron transfer in radical nucleophilic aromatic substitution of dihalogenated π-systems, demonstrating that π-conjugation governs the efficiency of intramolecular SET and can be qualitatively assessed via the ratio of mono- to double substitution products.

Radical-Electron Transfer Coupling in Aromatic Systems publication trend

The graph below shows the total number of articles in radical-electron transfer coupling in aromatic systems across all publications each year (not limited to Nature Index journals).

Technical terms

Single Electron Transfer (SET): A mechanism in which one electron moves from donor to acceptor, generating radical ions.

Radical Anion: An anionic species bearing an unpaired electron, typically reactive toward bond formation or cleavage.

SRN1 (Radical Nucleophilic Aromatic Substitution): A radical-mediated substitution process where an aryl halide undergoes nucleophilic attack following SET reduction.

BHAS (Base-Promoted Homolytic Aromatic Substitution): A coupling pathway in which a base induces homolytic bond cleavage in an aromatic substrate, forming radicals that propagate substitution.

Organic Super-Electron Donor: A highly reducing organic species that transfers electrons readily to substrates, initiating radical coupling cascades.

References

1. KOtBu: A Privileged Reagent for Electron Transfer Reactions?. Journal of the American Chemical Society (2016).
2. Organic super-electron-donors: initiators in transition metal-free haloarene–arene coupling. Chemical Science (2014).
3. Intra- versus intermolecular electron transfer in radical nucleophilic aromatic substitution of dihalo(hetero)arenes – a tool for estimating π-conjugation in aromatic systems. Chemical Science (2017).