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 underdeveloped. 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.

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Cationic Au(I) complex-catalyzed double cyclizations of 1,2-dialkynylbenzene derivatives are valuable for the straightforward synthesis of ladder-type π-conjugated compounds. We have previously developed a method for synthesizing ladder-type π-conjugated molecules with spiro skeleton (spiro fluorene-phenylene vinylenes) by the triple cyclization of 1-biphenylethynyl-2-phenylethynylbenzenes using an AuCl(SMe2)/tBuXPhos/AgNTf2 catalyst system. Experimental mechanistic studies revealed that an Ag(I) complex or HNTf2, as well as a cationic Au(I) complex, catalyzes different reaction steps. However, detailed insight into which of the Ag(I) complex or HNTf2 functions as the actual cocatalyst and the origin of the selectivity of the reaction have not been elucidated. Here we report a detailed reaction mechanism for this triple cyclization by dual catalysis based on computational studies. A cationic Au(I) complex catalyzes the first and second cyclization steps, whose regioselectivity is thermodynamically and kinetically controlled. The third spirocyclization step to construct a spirocenter is catalyzed by in situ generated HNTf2 rather than an Au(I) or Ag(I) complex via a benzyl cation intermediate. The present Au(I)/Brønsted acid dual-catalyzed mechanism of the triple cyclization of 1-biphenylethynyl-2-phenylethynylbenzenes provides complimentary activation modes to the conventionally used single Au(I)-catalyzed one.