Nobuharu Iwasawa

New Ru-Pd heterobimetallic complexes were synthesized and structurally characterized utilizing 6,6″-bis(phosphino)-2,2':6',2″-terpyridine as a scaffold for the metal-metal bond. The dicationic Ru-Pd complex was found to exhibit high catalytic activity as a photocatalyst for photochemical reduction of CO₂ to CO under visible light irradiation. This study established a new design of transition metal catalysts that tune photoredox catalysis with metalloligands.

Hemilabile ligands have been widely utilized for transition metal catalysis, but hemilability upon excitation, termed as “photohemilability”, is still elusive despite the recent progress of organometallic photocatalysis. In this study, it was discovered that Thioxantphos‐based PSP−Rh pincer complexes undergo dehydrogenative C−H bond activation of arenes under irradiation with visible light. Theoretical calculations indicate that a transient photodissociation of the strong S−Rh bond brought about by d‐d excitation on Rh is the key to realize oxidative addition of arenes to the Rh(I) center, which can be regarded as a photohemilabile behavior. This C−H bond activation process was successfully applied to visible light‐driven C−H borylation of simple arenes.

A three-component hydrocarboxylation of an olefin with CO₂ and H₂ could be regarded as a dream reaction, since it would provide a straightforward approach for the synthesis of aliphatic carboxylic acids in perfect atom economy. However, this transformation has not been realized in a direct manner under mild conditions, because boosting the carboxylation with thermodynamically stable CO₂ while suppressing the rapid hydrogenation of olefin remains a challenging task. Here, we report a rhodium-catalysed reductive hydrocarboxylation of styrene derivatives with CO₂ and H₂ under mild conditions, in which H₂ served as the terminal reductant. In this approach, the carboxylation process was largely accelerated by visible light irradiation, which was proved both experimentally and by computational studies. Hydrocarboxylation of various kinds of styrene derivatives was achieved in good yields without additional base under ambient pressure of CO₂/H₂ at room temperature. Mechanistic investigations revealed that use of a cationic rhodium complex was critical to achieve high hydrocarboxylation selectivity.

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.

Facile synthesis, isolation, and complexation behavior of bis(o-phosphinophenyl)zinc derivatives as phenylene-tethered PZnP-pincer type ligands were demonstrated.

Invited for this month′s cover is the group of Nobuharu Iwasawa at Tokyo Institute of Technology. The image shows a switchable site‐selective carboxylation between benzylic C−O and aryl C−Cl moieties enabled by a palladium/iridium dual catalyst under visible light irradiation. The Communication itself is available at 10.1002/cssc.202102095.

A highly efficient carboxylation of benzyl alcohol derivatives with CO₂ using a palladium/iridium dual catalyst under visible‐light irradiation was developed. A wide range of benzyl alcohol derivatives could be employed to provide benzylic carboxylic acids in moderate to high yields. Mechanistic studies indicated that the oxidative addition of benzyl alcohol derivatives was possibly the rate‐determining‐step. It was also found that a switchable site‐selective carboxylation between benzylic C−O and aryl C−Cl moieties could be achieved simply by changing the palladium catalyst.