Tuning Photoredox Catalysis of Ruthenium with Palladium: Synthesis of Heterobimetallic Ru-Pd Complexes That Enable Efficient Photochemical Reduction of CO2.

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.

Merging the Norrish type I reaction and transition metal catalysis: photo- and Rh-promoted borylation of C-C σ-bonds of aryl ketones.

Synthesis of arylboronates via borylation of C-C σ-bonds of aryl ketones was achieved by the combined use of photoenergy and a Rh catalyst. The cooperative system enables α-cleavage of photoexcited ketones to generate aroyl radicals via the Norrish type I reaction, which are successively decarbonylated and borylated with the rhodium catalyst. This work establishes a new catalytic cycle merging the Norrish type I reaction and Rh catalysis and demonstrates the new synthetic utility of aryl ketones as aryl sources for intermolecular arylation reactions.

Catalytic direct hydrocarboxylation of styrenes with CO2 and H2.

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.

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 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 and utilization of bis(o-phosphinophenyl)zinc as isolable PZnP-pincer ligands enabled by boron-zinc double transmetallation.

Bis(o-phosphinophenyl)zinc derivatives were successfully synthesized by the reaction of o-phosphinophenylboronates with dimethylzinc via boron-zinc double transmetallation. The transmetallation was significantly accelerated by the presence of the ortho PR2 substituent to give diarylzinc selectively. These organozinc compounds act as isolable phenylene-tethered PZnP-pincer ligands, affording PZnP-palladium and ruthenium complexes with a σ-accepting Zn-metalloligand.

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst.

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.

3D-Boronic Ester Architectures: Synthesis, Host-Guest Chemistry, Dynamic Behavior, and Supramolecular Catalysis.

Boronic esters are known to be formed simply by mixing boronic acids and alcohols under neutral conditions, and the equilibrium is in favor of the boronic esters when 1,2- or 1,3-diols are employed as alcohols. By utilizing the dynamic nature of the boronic ester formation, our group successfully constructed unique boron-containing 3D structures, such as ring-shaped macrocycles, cages, and tubes, based on the boronic ester formation of various aromatic di-, tri-, or hexaboronic acids with an originally designed tetrol 1 containing two sets of fixed 1,2-diol units oriented on the same face of an indacene framework. Various functions of the obtained boronates were further pursued to disclose the characteristic features of this system. This personal account describes our self-assembled boronate system using tetrol 1 including synthesis, host-guest chemistry, kinetic connection, characteristic dynamic behaviors, and supramolecular catalysis.

Visible-Light-Enabled Carboxylation of Benzyl Alcohol Derivatives with CO2 Using a Palladium/Iridium Dual Catalyst.

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.

(Z)-Selective Hydroboration of Terminal Alkynes Catalyzed by a PSP-Pincer Rhodium Complex.

A highly (Z)-selective hydroboration of terminal alkynes was achieved using a thioxanthene-based PSP-pincer rhodium catalyst. This hydroboration exhibited good chemoselectivity toward alkynes over carbonyl compounds such as ketones and aldehydes. The mechanistic studies indicated the involvement of rhodium-vinylidene intermediates, and the high (Z)-selectivity could be attributed to the rigid and electron-rich nature of the PSP-rhodium catalyst.

Rhodium-Catalyzed meta-Selective C-H Carboxylation Reaction of 1,1-Diarylethylenes via Hydrorhodation-Rhodium Migration.

A meta-selective C-H carboxylation reaction of 1,1-diarylethylene derivatives with CO2 by using a rhodium catalyst with NaOi Pr as a stoichiometric reductant has been achieved. Together with hydrogenation of the ethylene moiety, a carboxyl group was introduced to the meta-position of the aryl ring with high selectivity over the ortho-positions. Experimental and computational mechanistic studies indicate that this carboxylation reaction proceeds via hydrorhodation on the ethylene moiety, followed by 1,4-rhodium migration and successive 1,2-rhodium migration on the aryl ring. The use of a bulky phosphine ligand seems to be the key to this unusual aryl-to-aryl 1,2-rhodium shift.

Metal-Free Photoredox-Catalyzed Hydrodefluorination of Fluoroarenes Utilizing Amide Solvent as Reductant.

A metal-free photoredox-catalyzed hydrodefluorination of fluoroarenes was achieved by using N,N,N',N'-tetramethyl-para-phenylenediamine (1) as a strong photoreduction catalyst. This reaction was applicable not only to electron-rich monofluoroarenes but also to polyfluoroarenes to afford non-fluorinated arenes. The experimental mechanistic studies indicated that the amide solvent NMP plays an important role for regeneration of the photocatalyst, enabling additive-free photoreduction catalysis.

Ruthenium-catalyzed regio- and site-selective ortho C-H borylation of phenol derivatives.

Efficient synthesis of o-borylphenols is achieved through the Ru-catalyzed regio- and site-selective sp2 C-H borylation of aryl diphenylphosphinites followed by removal of the phosphorus directing group. A successful application to aryl phosphites enables practical one-pot borylation of phenols, demonstrating high synthetic utility of this protocol.

Rh-Catalyzed Direct Carboxylation of Alkenyl C-H Bonds of Alkenylpyrazoles.

The Rh-catalyzed direct carboxylation of alkenyl C-H bonds was achieved by using pyrazole as a removable directing group. In the presence of 5 mol% RhCl3 ⋅ 3H2 O, 6 mol% P(Mes)3 , and 2 equiv. of AlMe2 (OMe), the alkenyl C-H bond of various alkenylpyrazoles was directly carboxylated in good yields under CO2 atmosphere. Furthermore, several useful transformations of the pyrazole moiety of the product were achieved to afford synthetically useful carboxylic acid derivatives in good yields.

Photo-promoted Skeletal Rearrangement of Phosphine-Borane Frustrated Lewis Pairs Involving Cleavage of Unstrained C-C σ-Bonds.

An unprecedented photo-promoted skeletal rearrangement reaction of phosphine-borane frustrated Lewis pairs, o-(borylaryl)phosphines, involving cleavage of an unstrained sp2 C-sp3 C σ-bond is reported. The reaction realizes an efficient synthesis of cyclic phosphonium borate compounds. The reaction mechanism via a boranorcaradiene intermediate is proposed based on theoretical calculations. This work sheds light on the new photoreactivity of phosphine-borane FLPs.

Controlled release of the guest molecule via borate formation of a fluorinated boronic ester cage.

A boronic ester cage, which exhibits stimuli-responsive guest-release behavior, was constructed by self-assembly of tetrol with the indacene backbone and a fluorine-substituted benzenetriboronic acid derivative. The presence of fluorine substituents made it possible to control the guest release rate using simple amines by forming tetrahedral borates.

Supramolecular Photocatalysis by Utilizing the Host-Guest Charge-Transfer Interaction: Visible-Light-Induced Generation of Triplet Anthracenes for [4+2] Cycloaddition Reactions.

Supramolecular photocatalysis via charge-transfer excitation of a host-guest complex was developed by use of the macrocyclic boronic ester [2+2]BTH-F containing highly electron-deficient difluorobenzothiadiazole moieties. In the presence of a catalytic amount of [2+2]BTH-F , the triplet excited state of anthracene was generated from the charge-transfer excited state of anthracene@[2+2]BTH-F by visible-light irradiation, and cycloaddition of the excited anthracene with several dienes and alkenes proceeded in a [4+2] manner in high yields.

A K+-promoted Diels-Alder reaction by using a self-assembled macrocyclic boronic ester containing two crown ether moieties.

A K+-promoted Diels-Alder reaction of 1,4,9,10-anthradiquinone with various dienes is achieved in the presence of a self-assembled macrocyclic boronic ester [2+2]crown containing two crown ether moieties. The reaction rate is remarkably accelerated (up to 206-fold) compared to that in the absence of the promoter. Furthermore, the reaction proceeds regioselectively to yield an internal adduct. The self-assembly protocol was also demonstrated.

Synthesis, structure, and reactivity of pincer-type iridium complexes having gallyl- and indyl-metalloligands utilizing 2,5-bis(6-phosphino-2-pyridyl)pyrrolide as a new scaffold for metal-metal bonds.

The synthesis and structural analyses of pincer-type iridium complexes having gallyl- and indyl-metalloligands were achieved utilizing 2,5-bis(6-phosphino-2-pyridyl)pyrrolide as a new scaffold for metal-metal bonds. A BH3-coordinated PInP-Ir dihydride complex was also developed as an equivalent to an iridium dihydride complex, which could be a useful catalyst for synthetic reactions.

Rhodium-Catalyzed C-H Activation Enabled by an Indium Metalloligand.

Rhodium complexes with an indium metalloligand were successfully synthesized by utilizing a pyridine-tethered cyclopentadienyl ligand as a support for an In-Rh bond. The indium metalloligand dramatically changes the electronic and redox properties of the rhodium metal, thereby enabling catalysis of sp2 C-H bond activation.