Photoredox catalysis has attracted the increasing attention of chemists in various fields from the viewpoints of green chemistry, solar chemistry, clean redox processes, and radical chemistry. Discussion of the principles and features of photoredox catalysis will be followed by illustration using typical examples with emphasis on the reactions developed in the authors’ laboratory which proceed via an oxidative quenching cycle accompanying reduction of external substrates in the first stage. Photoredox catalysis turns out to be quite effective for trifluoromethylation of olefinic substrates with electrophilic trifluoromethylating reagents such as Umemoto’s and Togni’s reagents, 1e-reduction of which generates the key intermediate, trifluoromethyl radical. Key features of photoredox catalysis will be discussed in terms of visible light-driven reactions, radical reactions, redox-neutral system and atom economy. The concept can be extended to other radical systems such as N- and C-centered radicals. This review article will be closed with an overview of future prospects for photoredox catalysis.
This account describes our studies on the transition-metal mediated transformation of tetrafluoroethylene into a variety of polyfluorinated organic compounds. We have divided our account into three main parts. First, we describe a series of palladium-catalyzed cross-coupling reactions that involve a C-F bond cleavage of tetrafluoroethylene as a key elementary reaction step. Also described are reactions that enable the construction of tetrafluoroethylene-bridging structures, such as R-CF2CF2-R’ and R-CF2CF2-H, from tetrafluoroethylene using either copper(I) or nickel(0) species. This account emphasizes the merits of a derivatization of readily available perfluorinated compounds to valuable highly fluorinated organic compounds.
This personal account summarizes our recent progress in the development of C-H transformations. We achieved ortho-selective C-H borylation and silylation using Lewis acid-base interaction between two substrates, and meta-selective C-H borylation using hydrogen bonding between a hydrogen donor unit of a ligand and a substrate functional group. Regioselective C-H trifluoromethylation and related reactions of 6-membered heteroaromatic compounds were realized at the 2-, 4-, and benzylic-positions of the heteroaromatic rings. In addition, we developed C-H transformations directed towards the synthesis of organic functional materials, such as highly soluble polyimides and π-conjugated molecules containing either heteroatom(s) or a Lewis acid-base interaction.
Phenalenyl radical is an odd-alternant hydrocarbon radical with a highly symmetric (D3h) structure. The high stability and amphoteric redox nature of this radical have triggered studies on its chemical reactivity, physical properties, and functionalities. The main problem hindering the use of phenalenyl radicals as the molecular components of functional materials is their propensity to σ-dimerization. Challenging issue will lie in understanding the self-association character and also in controlling the molecular overlap motif. This article summarizes our studies on the self-association behavior of phenalenyl radicals.
The ruthenium (Ru) precatalyst complex dichlorobis[2-[(dicyclohexylphosphino-κP)methyl]pyridine-κN]ruthenium(II) (RuPCY), first developed by us, was shown to be useful not only for the catalytic hydrogenation of inert carboxylic acid derivatives including amides and esters, but also for the catalytic dehydrogenation of primary (1°) and secondary (2°) alcohols and subsequent reactions. RuPCY is versatile, and different Ru catalysts derived therefrom are quite useful for realizing a variety of indiscrete/discrete catalytic hydrogen management (HyMA) reactions. The sequential use of different starting materials and different catalysts (all derived from RuPCY) permits otherwise challenging orthogonal HyMA processes. This accommodates, for example, a salt-free access and/or shortcut to synthetic intermediates of pharmaceutically important substances such as donepezil (Aricept®) and atorvastatin (Lipitor®).
In situ click chemistry is a target-guided synthesis technique for discovering highly potent enzyme inhibitors by assembling azides and alkynes into triazoles inside the affinity site of a target enzyme. We here review research aimed at the rapid discovery of a novel and potent inhibitor of bacterial chitinases using in situ click chemistry. Chitinase inhibitors have chemotherapeutic potential as fungicides, pesticides and anti-asthmatics. Argifin, which has been isolated and characterized as a cyclopentapeptide natural product by our research group, shows strong inhibitory activity against chitinases. Via a combination of efforts to develop a useful chitinase inhibitor from an azide-bearing argifin fragment and application of the chitinase template in situ click chemistry with a library of alkynes, we rapidly obtained a very potent and novel 1,5-disubstituted triazole inhibitor of Serratia marcescens chitinase (SmChi) B. The new inhibitor expressed a 300-fold increase of inhibition against SmChiB compared to that of argifin. We also succeeded in obtaining crystal structures of a chitinase complexed with an azide inhibitor and an O-allyl oxime fragment as a mimic of a click partner, revealing an elegant mechanism for accelerating syn-triazole formation of ‘in situ click chemistry’, thereby allowing generation of its own distinct inhibitor. This represents the first example of expression of a pre-triazole state of ‘in situ click chemistry’. This work exemplifies the benefits of ‘in situ click chemistry’-approach in efforts to produce novel and reliable inhibitors.
Extensive investigation of functional organosilicon and organogermanium compounds has identified various valuable applications in organic chemistry and advanced materials. This review summarizes the major developments of metal-mediated coupling reactions between group 14 hydrides and organic halides during the last decade with an emphasis on our own studies. High reactivity and selectivity have been achieved for C-Si and C-Ge bond formations under mild conditions. This transformation shows good functional group compatibility, and can serve as a powerful tool for the synthesis of medicinal, pharmaceutical, agrochemical, electrical, and photoluminescent compounds. Ground- and excited-state properties of fluorescence materials have been investigated by DFT and TD-DFT calculations, and several important aspects of the experimental observations have been validated. Direct functionalization of H-terminated Si and Ge surfaces has been demonstrated utilizing Pd-mediated arylation reactions, illustrating the potential for further development of the Pd-catalyzed reactions for the organic modification of semiconductor surfaces.
Cycloaddition and cycloisomerization based on the interaction between an allene and other π-components has enabled us to build a variety of useful cyclic frameworks. This account describes our recent research on the RhI-catalyzed cycloaddition and cycloisomerization of allene-alkyne derivatives.
Cross-coupling reactions of aryl sulfides are more difficult than they look. We have disclosed that transition metal-NHC complexes allow us to efficiently use a wide range of aryl sulfides as electrophilic coupling partners in various cross-coupling reactions and have thus significantly expanded the scope of aryl sulfides available for the cross-coupling technology. Newly introduced nucleophilic partners include arylzinc reagents, Grignard reagents such as alkynylmagnesium species, amines, ketimines, and diborons to achieve Negishi-type coupling, Kumada-Tamao-Corriu-type coupling, Buchwald-Hartwig-type amination, Buchwald-Hartwig-Miura-type carbonyl α-arylation, and Miyaura-type borylation, respectively. These cross-coupling reactions are particularly advantageous when combined with sulfur-specific transformations to prepare organosulfur substrates. Through the development of catalytic transformations of C-S bonds, sulfur-based organic synthesis has come to complement conventional halogen-based organic methodology.
Organic electronics have attracted much attention as they promise great potential for applications as lightweight and flexible electronic devices. In order to improve the performance of such devices, the development of improved charge-transporting materials is of paramount importance. The molecular design of such materials should aim to control not only the electronic structure, but also the molecular orientation of these compounds in the solid state. For the development of materials with improved charge-transporting properties, we focused on compounds with a quasiplanar structure, i.e. a structure that is slightly twisted from planarity. As a model skeleton, we designed and synthesized partially oxygen-bridged triarylamines. On the basis of their quasiplanar skeletons as key units, we have developed hole-transporting materials (HTMs) that were subsequently used in organic electronics devices, such as OLEDs and perovskite solar cells, and led to improved performances. This account article illustrates our endeavors regarding the development of such functional π-conjugated materials, with a focus on facile and efficient synthetic strategies.