Remote functionalization of organic molecules has been attracting increasing attention, because it provides a unique way to introduce or convert functional groups at unconventional positions. Chain walking of metal catalysts on alkyl chains is one of the powerful strategies to achieve effective remote functionalization by migrating reactive groups to positions distant from where the reactive functional groups in the substrates are originally present. Since we reported the use of nondissociative chain walking of the metal catalyst for selective conversion of remote unreactive C (sp³)-H bonds, by developing the palladium-catalyzed cycloisomerization of 1,n-dienes to form five-membered rings, we have been working on the use of nondissociative chain walking for various remote functionalization reactions such as hydrofunctionalizative and diborylative cyclizations of 1,n-dienes, asymmetric cycloisomerization, and selective long-distance isomerization of alkenes via chain walking. In this account, I would like to describe our efforts in the reaction developments as well as the features of the nondissociative chain walking mechanism beneficial for organic synthesis.

Chalcogen-bonding interaction has been recognized as a non-covalent interaction that plays a role in determining the conformation of pharmaceuticals and organic materials. This account describes a strategy to establish conformational control of axially chiral biaryls through chalcogen-bonding interaction and their application to dirhodium(II) paddlewheel complex. We prepared binaphtho- and bibenzocarcogenophene dicarboxylic acids bearing sulfur and selenium atoms in their fused aromatic rings as novel types of axially chiral functionalized dicarboxylic acid. The carboxy groups of these dicarboxylic acids were found to be conformationally locked in co-planar geometry toward the fused ring systems through chalcogen-bonding interaction (S…O or Se…O interaction). Contribution of the chalcogen-bonding interactions was proved by X-ray as well as NBO analyses in DFT calculation. Furthermore, N- and O-protected axially chiral binaphthothiophene δ-amino acids were synthesized and applied for derivatization to the artificial dipeptide. The crystal structure of the dipeptide also revealed that the carbonyl groups of both the amide and the ester moieties were conformationally fixed with naphthothiophene rings in co-planar geometry via chalcogen-bonding interaction. Finally, we employed the chalcogen-bonding interactions to lock the conformation of the dirhodium(II) carboxylate complex. This conformationally defined complex worked as an outstanding catalyst for highly stereoselective intramolecular C-H insertion reactions to α,β-diaryl γ-lactones. These results exemplified that the utility of chalcogen-bonding interaction for controlling the stereostructure of catalyst.

In recent years, electrochemical organic synthesis has attracted attention and has been re-evaluated as a powerful methodology in synthetic chemistry. Electrochemical organic reactions are classified into direct electrolysis, in which the substrate is directly oxidized or reduced at the electrodes, and indirect electrolysis, in which the substrate is oxidized or reduced by an active species, which is called “mediator”, generated by oxidation or reduction at the electrode. In indirect electrolysis, the reaction between the electrode and the mediator is direct electron transfer, but the reaction between the mediator and the substrate is a “chemical” reaction between the activated mediator and the substrate. Therefore, the use of the appropriate mediator enables uncommon molecular transformations that are usually difficult with high chemoselectivity. We describe here our developed electrochemical carbon-heteroatom bond formations for the synthesis of organic functional molecules with or without mediators. Electrochemical dehydrogenative C-S bond formation proceeded to afford thienoacenes. The use of Br⁻ as a mediator was essential for the reaction. This methodology could also be applicable for the synthesis of dibenzothiophene derivatives. We also developed electrochemical C-P bond formation for the synthesis of dibenzophosphole oxides (DPOs) using DABCO as a mediator. Sultones, sulfur analogs of lactones, could be synthesized by electrochemical C-O bond formation of sulfonates. The reaction is direct electrolysis which proceeds via radical pathway. We also found that direct electrolysis of biaryl sulfonyl hydrazides afforded dibenzothiophene dioxides via electrochemical C-S bond formation.

Organic polymer materials are utilized in a wide range of applications, making the extension of their lifespan crucial for resource conservation and cost reduction. Understanding molecular-level force transduction within bulk polymers is key to preventing material degradation. Mechanochromic mechanophores, molecular structures that exhibit changes in absorption and/or fluorescence in response to mechanical stimuli, have gained significant attention as reliable tools for probing small forces at the molecular scale. We have developed supramolecular mechanophores that alter their fluorescence properties through the disruption of intra- or intermolecular interactions upon mechanical stimulation. Unlike traditional mechanophores, these supramolecular mechanophores do not require covalent bond cleavage for activation, allowing pN-scale forces to be visualized instantly and reversibly. Among our motifs, “rotaxane-based supramolecular mechanophores” stand out for their versatile molecular architectures, enabling new photofunctions and distinctive mechanoresponsiveness. In this review, we show how replacing luminophores or varying stopper sizes alters the photophysical properties and activation behavior of rotaxane-based supramolecular mechanophores.

In coordination polymerization catalyzed by cationic transition-metal species, a counter anion exists close to the metal cation in a polymerization medium. Therefore, the counteranions with a designed structures may show unusual polymerization behaviors compared with conventional catalyst systems, although most of polymerization behaviors are determined by the steric and electronic effect from the ligand. Recently, we have found many examples that boron and aluminum-based activators play a crucial role in improving the activity and regio/stereospecificity, and this review summarizes these achievements in detail. Especially, modifications of methylaluminoxane (MAO), which is the most common activator for coordination olefin polymerization catalysts, often promoted unique polymerizations such as living polymerization, after the appropriate modifications. Besides, we newly designed a fluoroarylborane cocatalyst that successfully increased the propagation rate of olefin polymerization using a titanium catalyst, and the final molecular weight reached millions. This neutral fluoroarylborane has a strong Brønsted acidity and the resulting borataanthracenide counteranion would show an excellent non-coordinative character. The application of this borane to the cationic polymerization of beta-pinene, including the microstructure analyses, is also mentioned.

Frustrated Radical Pairs (FRPs) are two distinct radicals that do not annihilate each other due to their steric and/or electronic frustration, and they are analogous to Frustrated Lewis Pairs (FLPs). While FLPs have been extensively explored as catalysts and in material science, the study of FRPs remains less explored. Especially, the utilization of FRPs in organic synthesis is still limited. This short review highlights intriguing recent examples of molecular transformations in which neutral FRPs have been employed as a novel methodology to control radical reactions.