Combining photoredox catalysis and hydrogen atom transfer for dearomative functionalization of electron rich heteroarenes

Abstract

Reductive dearomatization has been a broadly explored means for rapid generation of sp³ complexity from simple planar arenes. Breaking the electron rich, stable aromatic systems requires strong reduction conditions. It has been notoriously challenging to dearomatize electron even richer heteroarenes. Herein we report an umpolung strategy enabling dearomatization of such structures under mild conditions. The reversal of the reactivity of these electron rich aromatics via photoredox mediated single electron transfer (SET) oxidation generates electrophilic radical cations, which can react with nucleophiles and break the aromatic structure to form a Birch type radical species. A crucial hydrogen atom transfer (HAT) is successfully engineered into the process to efficiently trap the dearomatic radical and minimize the formation of the overwhelmingly favorable, irreversible aromatization products. Particularly, a non-canonical dearomative ring-cleavage of thiophene/furan through selective C(sp²)–S bond breaking was first discovered. The preparing power of the protocol has been demonstrated for selective dearomatization and functionalization of various electron rich heteroarenes including thiophenes, furans, benzothiophenes and indoles. Furthermore, the process offers an unrivaled capacity for simultaneously introducing C–N/O/P bonds on these structures as exemplified by various “N”, “O” and “P” centered functional moieties with 96 examples.