Shining Visible Light on Reductive Elimination: Acridine-Pd-Catalyzed Cross-Coupling of Aryl Halides with Carboxylic Acids

Despite the recent tremendous progress on transition-metal/photoredox dual catalysis in organic synthesis, single transition-metal catalysis under visible-light irradiation, which can utilize light energy more efficiently, is still under-developed. Herein, we report the design of photosensitizing phosphinoacridine bidentate ligands for visible-light-induced transition-metal catalysis, expecting that the electron-accepting acridine moiety would create a highly reactive electron-deficient metal center toward reductive elimination via metal-to-ligand charge transfer (MLCT). Using these ligands, we have achieved a palladium-catalyzed cross-coupling reaction of aryl halides with carboxylic acids under visible-light irradiation. Electronic tuning of the phosphinoacridine ligands not only enabled the use of a variety of aryl halides as the coupling partner, including less reactive aryl chlorides, under blue light irradiation, but also realized the employment of lower-energy green and red light for the cross-coupling. Experimental mechanistic studies have proved that the reductive elimination of aryl esters is induced by photoirradiation of phosphinoacridine-ligated arylpalladium(II) carboxylate complexes. The theoretical calculation suggests that the reductive elimination in the excited state is promoted by decreasing the electron density of the Pd center through photoinduced intramolecular electron transfer, i.e., MLCT, in the transition state owing to the electron-deficient acridine scaffold. This is a very rare example of photoinduced reductive elimination on palladium(II) complexes.