Redox Inversion: A Radical Analogue of Umpolung Reactivity for Base- and Metal-Free Catalytic C(sp 3 )-C(sp 3 ) Coupling.

The construction of alkyl-alkyl bonds is a powerful tool in organic synthesis. Redox inversion, defined as switching the donor/acceptor profile of a functional group to its acceptor/donor profile, is used for C(sp 3 )-C(sp 3 ) coupling. We report a photocatalytic coupling of carboxylic acids to form bibenzyls through a radical-radical coupling. Mechanistic insight is gained through control reactions. This unexplored redox-opposite relationship between a carboxylic acid and its redox-active ester is implemented in catalysis.