Redox Inversion: A Radical Analog of Umpolung Reactivity for Base and Metal-Free Catalytic C(sp3)–C(sp3) Coupling

The construction of alkyl–alkyl bonds is a powerful tool in organic synthesis. Redox inversion–defined as the radical analog of polarity inversion–is used as a strategy for C(sp3)–C(sp3) coupling. Herein is reported a base and metal-free photocatalytic coupling of carboxylic acids to form biologically relevant bibenzyls through a radical-radical coupling. Mechanistic insight is gained through control reactions that implicate this new redox inversion strategy. In this work, the previously unexplored redox-opposite relationship between a carboxylic acid and its in situ activated redox active ester is implemented in catalysis.