Visible Light-Mediated Radical-Polar Crossover ipso N–C Interconversion/para-Amination of Aniline Derivatives via “Evolved” Aryl Migration

Abstract

The ipso, para-C–H difunctionalization of substituted arenes remains a persistent challenge in synthetic chemistry. In this work, we address this problem by strategically introducing two new elementary steps—radical para-amination and deprotonation—into the well-established radical-polar crossover aryl migration framework. This maneuver elegantly diverts the reaction pathway from a simple aryl migration to a novel ipso/para-difunctionalization, illustrating how classic mechanistic paradigms can be “evolved” to address unmet synthetic needs. Employing oxime esters as bifunctional radical precursors under energy transfer (EnT) photocatalysis, the reaction enables ipso N–C interconversion coupled with para-amination of anilines bearing a tethered remote alkene. Mechanistically, the process involves EnT-promoted N–O bond homolysis, trapping of the alkyl radical by SO₂, sulfonyl radical addition to the alkene, intramolecular ipso-cyclization, radical–radical cross-coupling between a transient cyclohexadienyl radical and a persistent iminyl radical, and finally a deprotonation-driven aryl migration. The protocol demonstrates remarkable generality, accommodating a broad range of ortho- and meta-substituted aniline derivatives with diverse electronic and steric properties. This wide substrate tolerance, along with the observed specific ipso, para-selectivity, is governed primarily by the intrinsic nature of the transformation rather than conventional electronic or steric parameters. Mechanistic studies, including control experiments, radical-trapping assays, and quenching experiments, support the proposed EnT pathway and the radical-polar crossover cascade. DABSO plays a dual role: it serves as an SO₂ source to convert nucleophilic alkyl radicals into electrophilic sulfonyl radicals, while also releasing DABCO as a base to facilitate the final deprotonation step. This method provides efficient and modular access to valuable 4-aminated benzenepropanamide scaffolds, and offers a new disconnection strategy for remote C–H functionalization that synergistically merges radical and ionic processes. More broadly, this work highlights how strategically incorporating additional elementary steps into established mechanisms can significantly expand the synthetic toolbox.

Keywords

Energy transfer photocatalysis / ipso / para-C–H difunctionalization / DABSO / Amination / Evolution of radical–polar aryl migration / Bifunctional reagent / Radical-radical cross-coupling / 4-Aminated benzenepropanamide

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