Mallik Sumitava, Sfreddo Enrico, Wang Hailong, Melchiorre Paolo
University of Bologna, Department of Industrial Chemistry 'Toso Montanari' Via Piero Gobetti 85 Bologna 40129 Italy
Chem Sci. 2024 Nov 25;16(1):124-129. doi: 10.1039/d4sc07495e. eCollection 2024 Dec 18.
4-Hydroxycoumarins are well-known for their ground-state nucleophilic behavior, which has been widely exploited for their functionalization. Herein, we reveal a previously unexplored photochemical reactivity: upon deprotonation and excitation with purple light, 3-substituted 4-hydroxycoumarins reach an excited state and act as single-electron transfer (SET) reductants, generating radicals from stable substrates. This newfound reactivity enables the direct synthesis of 3,3-disubstituted 2,4-chromandiones a radical dearomatization process. By enabling the incorporation of alkyl and perfluoroalkyl fragments, this protocol offers a straightforward and mild route to access synthetically valuable chromanone scaffolds featuring a quaternary stereocenter. Comprehensive photophysical studies confirmed that deprotonated 4-hydroxycoumarins are potent SET reductants in their excited state, making them suitable for initiating radical-based transformations.
4-羟基香豆素因其基态亲核行为而闻名，这种行为已被广泛用于其功能化反应。在此，我们揭示了一种此前未被探索的光化学反应性：3-取代的4-羟基香豆素在去质子化并用紫光激发后，会达到激发态并作为单电子转移（SET）还原剂，从稳定底物生成自由基。这种新发现的反应性使得能够直接合成3,3-二取代的2,4-色满二酮——一种自由基去芳构化过程。通过能够引入烷基和全氟烷基片段，该方案提供了一条直接且温和的途径来获得具有季碳立体中心的具有合成价值的色满酮骨架。全面的光物理研究证实，去质子化的4-羟基香豆素在其激发态是有效的SET还原剂，使其适合引发基于自由基的转化反应。
