Haloarenes have been appreciated as one of the most fundamental and versatile compounds in the modern chemistry field because the halide functionality offers numerous possible transformations into valuable building blocks. Although classical halogenation using molecular halogens (mainly Cl₂ and Br₂) and acid activators is still a promising protocol, it suffers from handling difficulties, low selectivity, and limited practical application in fine-chemical synthesis. To address this issue, the development of highly active yet efficiently selective halogenation methods has long been desired. In this article, we introduce carborane-based sulfide catalysts for aromatic halogenation. Carboranes (C₂B₁₀H₁₂) are three-dimensional aromatic molecules with icosahedral geometry, high thermal and chemical stabilities, and unique electronic effects. We envisioned that these boron clusters would be a handle to modulate the electronic property of halonium species, whereas their spherical shape exerts a negligible change in steric factor. As a specific design, we synthesized a series of sulfur-substituted carboranes and systematically studied their activity. Density functional theory (DFT) calculations revealed that the m-carborane scaffold was most suitable for catalysis, and the possible fine-tuning by decorating the cluster vertices was important for modulating the electronic property of halonium species to maximize the catalytic performance.

The advancement of modern organic chemistry is inextricably linked to organic solvents. Organic solvents allow for the homogeneous dispersion of organic compounds, thereby enhancing reaction efficiency and enabling control over reaction rates and selectivity. Consequently, the principles of organic reactions have been cultivated and systematized based on their behavior in organic solvents, leading to high yields and selectivity in a wide array of molecular transformations. However, organic solvents pose significant environmental concerns, accounting for an estimated 80% of chemical waste in pharmaceutical manufacturing. Therefore, the development of sustainable organic synthesis methods that eliminate the need for organic solvents is a pressing need. Utilizing water as a reaction medium offers a potential solution. Historically, organic syntheses in the 19th century, such as Wöhler's urea synthesis, aldol condensation of acetone, and Kolbe's electrolysis, were performed in water. However, the increasing prevalence of water-insoluble organic compounds and the deactivation of organometallic compounds and reactive intermediates in water have necessitated the use of organic solvents. This presents challenges for organic synthesis in water, including the development of new reaction designs and catalysts. On the other hand, the unique properties of water offer opportunities for developing organic syntheses that differ fundamentally from those in organic solvents. In nature, enzymes possess unique strategies to efficiently carry out reactions in water. By emulating these natural processes and using them as a basis for catalyst and reaction design, new possibilities for organic synthesis in water are anticipated. This paper focuses on idiosyncratic organic synthesis specific to water, detailing recent achievements and future prospects.

Aromatic carboxylic acids have been recognized as important building blocks in organic synthesis field because of their wide availability and easy handling. Their carboxy group is known to act as a directing group in transition-metal-catalyzed C-H functionalization. For example, ortho-substituted benzoic acids undergo iridium-catalyzed dehydrogenative coupling with alkynes at the opposite ortho- and meta-positions accompanied by decarboxylation. Exceptionally, salicylic acid couples with alkynes at the ipso- and ortho-positions to produce 5,6,7,8-tetrasubstituted 1-naphthol derivatives. 1-Naphthoic acid reacts with diphenylacetylene under rhodium catalysis at the 2- and 3-positions selectively to afford 1,2,3,4-tetraphenylanthracene. In contrast, 9-anthracenecarboxylic acid exhibits unique reactivity. Treatment of this substrate with diphenylacetylene under rhodium catalysis does not give any intermolecular coupling products at all but gives an intramolecular C-O coupling product. 9-Anthracenecarboxylic acid also undergoes palladium-catalyzed dehydrogenative coupling with styrenes at the peri position and successive Wacker-type cyclization to produce (Z)-3-benzylidenedibenzo[de,h]isochromen-1-one derivatives.

C-H bond functionalization represents a powerful strategy for the selective introduction of functional groups at specific C-H bonds. This approach prevents the need for prefunctionalization of starting materials, offering superior atom- and step-economy compared to traditional cross-coupling reactions. However, many C-H bond functionalization reactions rely on stoichiometric amounts of additives, such as oxidants, bases, or acids, to effectively cleave an inert C-H bond. This dependency imposes significant challenges to achieving truly atom-economical transformations, such as additive-free C-H bond functionalization. Furthermore, these additives often compromise functional group compatibility, thereby limiting the applicability of these reactions. In this manuscript, I present a novel additive-free C(sp²)-H bond activation methodology employing carboxylic acid anhydrides. The in-situ generation of rhodium carboxylate or carbonate species through the decomposition of carboxylic acid anhydrides plays a pivotal role in facilitating a concerted metalation-deprotonation (CMD) mechanism without the need for external additives. This approach has been successfully applied to various additive-free and related transformations, including alkoxycarbonylation, acylation, alkylation, and arylation. Additionally, we explored 2,4,6-trimethylbenzoic acid-based carbonate anhydride as a novel alkoxycarbonylating reagent, enabling the introduction of alkoxycarbonyl groups with a diverse range of alkyl substituents. During our investigations, we identified the superior catalytic performance of rhodium iodide compared to previously reported rhodium complexes. This paper not only outlines these reactions but also explains their mechanism based on the experimental results.

Transition-metal complexes having metal-E triple bonds (E=heavy group 14 elements: Si, Ge, Sn, and Pb), which are generally called as tetrylyne complexes, have received great attention in their physical and chemical properties, in relation to the carbon congeners, i.e. well-known carbyne complexes. Thanks to the concept of kinetic stabilization using sterically hindered substituents, tetrylyne complexes have become accessible during the last two decades. However, despite significant progress, reactivity study of these complexes especially toward organic substrates remain limited. Our group has developed a new synthetic strategy, which allowed us to obtain neutral germylyne complexes that have metal-germanium triple bonds and silylyne complexes that have metal-silicon triple bonds of all group 6 metals (W, Mo, and Cr). Their molecular structures and bonding properties have been revealed by multiple NMR spectroscopies, single crystal-X-ray diffraction (SC-XRD) study as well as DFT calculations. Using these complexes, we found several new patterns of reactions including cycloaddition reactions ([2＋2], [2＋3], [2＋4] and combinations with intra-molecular bond activation) and activation reactions of external dihydrogen and benzene C-H bond, etc. These examples demonstrate high potential of metal-ligand cooperative activity of tetrylyne complexes at their strongly polarized triple bonds. This article describes these results on neutral germylyne and silylyne complexes.

It is well-known that the introduction of fluorine atom(s) into the organic frameworks significantly alters their chemical and biological properties. Several methods have been developed to introduce trifluoromethyl (CF₃) or difluoromethyl (CHF₂) moiety via the nucleophilic, electrophilic and radical intermediates. However, compared with trifluoro- and difluoromethylation reactions, monofluoromethylation is still limited. This review focuses on recent achievements in nucleophilic monofluoromethylation using carbenoid chemistry.