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

Abstract

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.

Visible-light acridinium-based organophotoredox catalysis in late-stage synthetic applications

“…Iwasawa et al 60 reported a 4-phosphinoacridine ligands catalysed synthetic protocol for visible-light-induced transition-metal catalysis and developed a Pd-catalyzed cross-coupling reaction of aryl halides 50 with carboxylic acids 51 under irradiation with blue LEDs to give the aryl esters 52 ( Scheme 31 ).…”

Ligand-Enabled Oxidative Fluorination of Gold(I) and Light-Induced Aryl–F Coupling at Gold(III)

“…41 In addition, Iwasawa and co-workers demonstrated in 2022 that by using a photoactive acridine ligand instead of a photocatalyst, this challenging reductive elimination can also be achieved directly by irradiation of aryl Pd(II) carboxylate complexes (without external photosensitizer). 42 Considering the possibility to use light to promote Ar−F coupling at Au(III), complex 1e was analyzed by UV−vis absorption spectroscopy. It shows a broad absorption peak at λ max ∼ 327 nm (Figure 6a).…”

Red-light-mediated Barton decarboxylation reaction and one-pot wavelength-selective transformations

“…In addition to widely used singlet oxygen generation, 12–16 red-light-mediated organic transformation involving the photoredox mechanism or intramolecular charge transfer have been reported. 17–33 Very recently, we reported the red-light-mediated Barton–McCombie reaction. 34 In the presence of chlorophyll a as the catalyst 35 and tris(trimethylsilyl)silane or a Hantzsch ester as the hydrogen source, the methyl xanthate moiety was removed under red-light irradiation.…”