Hypervalent organoiodine(III/V) reagents (λ³-/λ⁵-iodane) have enjoyed widespread application as oxidant/electrophile in synthetic organic chemistry. In contrast, the development of the chemistry of hypervalent organo-λ³-bromane and λ³-chlorane is still just beginning. We have developed a versatile, high-yield synthesis of λ³-chloranes and λ³-bromanes through the reaction of various haloarenes/chloroalkenes with readily prepared arenediazonium tetrakis(pentafluorophenyl)borate under mild conditions. The improved accessibility enabled us to investigate the unique physico-chemical characters and unprecedented reactivity of these hypervalent organo-λ³-haloganes (e.g. onium structure, enhanced electrophilicity, etc.). The leaving group ability of λ³-haloganyl groups was also evaluated as it governs the rate of a variety of chemical transformations.

Isosterism and bioisosterism are concepts in molecular design of specialty materials and drugs compounds. Fluorine and fluorine-containing functional groups are often used as isosteres and bioisosteres. Fluorine atom (F) is the most suitable isostere of hydrogen atom (H). With regard to electronegativity, fluorine is also used as a good substitute for hydroxyl (OH) group. Trifluoromethyl (CF₃) group can be used as an equivalent for methyl or isopropyl groups, and difluoromethyl (-CF₂H) group is an alcohol (-OH) equivalent. Pentafluoro-λ⁶-sulfanyl (SF₅) group has recently attracted attention as a super CF₃ group. This review introduces an undeveloped fluorine-containing functional group, tetrafluoro-λ⁶-sulfanyl (SF₄) moiety. The SF₄ is a unit consisting of four fluorine atoms bonded around an octahedral sulfur (VI) atom, with strong electron-withdrawing property and high lipophilicity. Characteristic of the SF₄ unit are the cis- and trans-geometries connecting the two fragments. Interestingly, the trans-SF₄ unit can connect two separated parts in a linear (180°) configuration via an octahedral sulfur center. Although organic compounds having an SF₄ unit were known as early as the 1950s, their synthetic studies have been largely underdeveloped. We herein describe the structural and chemical characteristics of SF₄-containing compounds, the background of the synthetic study, and their state-of-the-art synthetic constructions.

In chemical species containing of oxygen-halogens and nitrogen-halogens bonds, the halogen substituents show versatile reactivities, because oxygen and nitrogen atoms have higher electronegativity than halogen elements except for fluorine. I have focused on such monovalent halogen species and developed various synthetic methods utilizing O-X and N-X bonds which involves ionic and radical reactions. In this account, I describe such practical synthetic methodologies, which are CO₂ fixation to unsaturated alcohols and amines, oxidative homo- and cross-coupling reactions of aromatic amines, novel generation of nitrile oxides from aldoximes, synthesis of oxazole-fused C₆₀, iodoamination of alkenes in water, diaziridination of secondary amines, diastereodivergent diamination of unactivated alkenes, practical aziridination of electron-deficient alkenes, selective synthesis of nitrogen-bridged fullerenes, and benzylic C-H amidation.

Effective use of the light that surrounds us is the hottest research topic in all fields, including the energy field. Light is the means by which reactions occur, and can be thought of as a “type of reagent.” Since light has no shape or weight, it is a clean substance that does not leave any residue, and can be one of the most important methods for investigating processes that reduce environmental impact. Against this background, the authors have been conducting research aiming at the development of novel molecular transformation reactions driven by photochemical activation of representative elements. In this study, we focused on iodine, which is easy to handle, has low toxicity, and is highly responsive to visible light. That is to say, iodine and carbon-iodine bonds readily respond to light energy as small as visible light and undergo bond cleavage to generate radical species. By applying this phenomenon to fine organic synthesis reactions, the authors have succeeded in developing a methodology for easily synthesizing compounds such as phenacyl halides, cyclopropanes, and lactones, which have high industrial value, from light energy and inexpensive raw materials.

Diaryliodonium(III) salts (Ar¹Ar²I⁺X⁻) are hypervalent iodine(III) compounds that function as useful arylation agents forming aryl-carbon and aryl-heteroatom bonds with or without transition metal catalysts. The iodine(III) center of diaryliodonium salts has electrophilic properties, which induces the arylation reactions of nucleophiles. For the cases with Ar¹≠Ar² in diaryliodonium salts, the chemoselectivity of aryl-group transfers depends on the nature of the aryl ligands, including electronic and steric effects. This article describes ligand control strategy leading to unified aryl selectivity and enhanced reactivity for the arylation of various nucleophiles.

Diaryliodonium(III) salts bearing trimethoxyphenyl (TMP) or mesityl (Mes) groups underwent the selective arylations, wherein TMP and Mes groups serve as “dummy ligand” to form aryl-carbon or aryl-heteroatom bonds with the other aryl groups. TMP-iodonium salts were employed for various types of arylation via single electron transfer process, metal-free ligand coupling, and metal-catalyzed coupling to afford the corresponding arylation products with unified selectivity. Furthermore, we found that TMP-iodonium acetates (Ar(TMP)I⁺AcO⁻) exhibit extremely high reactivities for metal-free arylation of phenols and hydroxylamines, wherein the interaction between the ortho-methoxy groups of TMP and the iodine(III) center presumably enhances the basicity of the acetate anion to cooperatively activate the nucleophiles. These effective metal-free arylations would provide practical methods for the synthesis of organic functional aromatic molecules.

Enantioselective carbon-halogen (F, Cl, Br) bond formations are particularly important due to their potential as synthetic intermediates as well as marine natural products and pharmaceuticals. Among the various methods available to build carbon-halogen bonds, the enantioselective electrophilic α-halogenation of carbonyl compounds is one of the most common. Over the past few decades, α-halogenation reactions using 1,3-dicarbonyl compounds, aldehydes, and ketones have been well established. However, few reports are available on catalytic enantioselective α-chlorination for carboxylic acid derivatives with pK_a that are relatively high, and hence it has been considered to be challenging to generate enolate species catalytically. Here, chiral π-copper(II) complex-catalyzed enantioselective α-fluorination and π-copper(II)-π complex-catalyzed enantioselective α-chlorination and α-bromination reactions of N-acyl-3,5-dimethylpyrazoles are described. The π-copper(II) complex of Cu(OTf)₂ with 3-(2-naphthyl)-L-alanine-derived amides greatly accelerate the α-fluorination in the presence of an external base such as 2,6-lutidine. In sharp contrast, the π-copper(II)-π complexation of Cu(OTf)₂ with 3-(2-naphthyl)-L-alanine-derived amides further increases the Lewis acidity, and triggers the in situ generation of enolate species without an external base, which has a suppressing effect for α-chlorination and α-bromination due to undesired halogen bonding. This strategy provides facile access to α-halogenated compounds in high yield with excellent enantioselectivity. X-ray crystallographic and ESR analyses of the catalyst complexes suggest that the release of two counter anions (2TfO⁻) from the copper(II)center might be crucial for the efficient activation of N-acyl-3,5-dimethylpyrazoles.

In recent years, halogen bonding (XB) interaction has attracted much attention in organic synthesis, especially for the activation of substrates. On the other hand, the use of XB interaction for the “stabilization” of molecules has been less explored. In this manuscript, the authors described our recent work on the development of new class of N-acyliminoiodinanes and iodonium ylides, which can be used as nitrene and carbene equivalents, respectively. The intramolecular XB interactions were confirmed by X-ray structure and natural bond orbital (NBO) analyses. The synthesized N-acyliminoiodinanes were found to be activated by photo-irradiation to react with silyl enol ethers, glycals, alkynes, and(hetero)aromatic compounds to afford the corresponding aminated adducts. In addition, the iodonium ylides baring intramolecular XB interaction underwent additive-free chemoselective coupling reaction of thioamides, even in protic polar solvents. The intramolecular XB could also control the coordination mode of the nucleophile via σ-holes, suppressing undesired side reactions. These findings would pave new avenues chemoselective coupling reactions using hypervalent iodine compounds.

Phosphorescence is a unique luminescence property with a number of advantages that are complementary to those of fluorescence. However, achieving efficient room-temperature phosphorescence (RTP) using metal-free organic molecules is a challenging task owing to the spin-forbidden nature of phosphorescence. Extensive research in the last decade has revealed that decent RTP can be obtained from organic crystals based on the suppression of nonradiative decay. Nonetheless, once the rigid crystalline lattice is destroyed by applying mechanical stimuli, such RTP often disappears. We have recently developed heteroaromatic 1,2-diketones as a novel scaffold of organic RTP. Unlike conventional phosphors, the diketones exhibit efficient RTP in various noncrystalline states/environments. This article describes the RTP of halogenated thienyl diketone derivatives, focusing on their response to mechanical stimuli and the multiple and indispensable roles of halogens therein.

Stable radicals possess open-shell electronic states and exhibit vivid physical properties such as electrical conductivity and magnetism attributed to unpaired electrons. Their luminescence properties, however, have yet to be well examined, mainly due to the rarity of emissive radicals and chemical reactivity in their excited states. I have developed highly-photostable luminescent organic radicals and luminescent open-shell metal complexes with luminescent radicals as ligands, and investigated photofunctions based on their doublet or multiplet states. The studies suggest that the radicals can demonstrate photofunctions unique to their open-shell electronic states, such as magnetic-field-controlled luminescence, which are unexpected or impossible to achieve for conventional closed-shell molecules. Such functions will expand the scope of the radicals in molecular photonics.