Previously, we accidentally found bidentate nitrogen-based ligands, in particular, 1,10-phenanthroline well work in palladium-catalyzed direct C-H bond arylation reaction of 5-membered heteroarenes. With this finding, we then explored catalytic utilities of the palladium/bidentate nitrogen-based ligand systems. Firstly, we extensively examined the utility for the direct C-H bond arylation of heteroarenes, and revealed that the catalytic system showed high activity for the reaction. A wide variety of C-H bonds on heteroarenes were arylated with the catalytic system. Importantly, the arylation at C4-position of azoles proceeded with high efficiency without any additives except base. A variety of multi-functionalized azole syntheses were also investigated, and we achieved a synthetic method of three different aryl groups introduced azoles from unfunctionalized azoles via simple sequential arylation reactions with a sole catalytic system. We also investigated the utilities of this catalytic system to oxidative coupling of terminal alkynes and azoles to result in good efficiency. One of the important features of this catalytic system is non-use of a phosphine ligand. With this feature, we achieved direct C-H bond arylation of thiocarbonylated compounds, that was hardly achieved because of the direct reaction of thiocarbonyl groups and phosphine ligands to result in loss of catalytic activity. To gain the insight of the reaction mechanism of this reaction with thiocarbonylated compounds, several control experiments were carried out.

Carbon dioxide (CO₂) is recognized as an unfavorable industrial waste due to its greenhouse effect, and the reduction of CO₂ emissions has become integral to the creation of a sustainable society. From a different point of view, however, CO₂ is also recognized as an ideal C1 source for chemical synthesis because it is inexpensive, abundant, and nontoxic. In this context, the development of chemical transformations to useful compounds via the use of CO₂ as a reactant has become a hot topic in the field of green sustainable chemistry. Among these transformations, the synthesis of cyclic carbonates from epoxides and CO₂ has been the most extensively studied due to their high utility in industrial processes. Although a wide variety of metal catalysts and organocatalysts such as quaternary onium salts have been developed to promote this reaction, high temperature (>100 °C) and pressure (>10 atm) are often required to produce the products in satisfactory yields. Therefore, the development of catalysts that can be effective for this reaction under mild conditions continues to be highly desired. Herein, we summarize our catalytic approaches for the CO₂ fixation under mild reaction conditions. Design concepts of our catalytic systems were discussed based on experimental results. Our catalytic system could also be applied to cyclic thiocarbonate synthesis by the use of carbon disulfide (CS₂). The mechanisms for these two reactions are discussed based on the stereochemistry of cyclic carbonate and thiocarbonate products. Although these two reactions are similar, the reaction mechanisms are totally different.

The positive muon (µ⁺) is an elementary subatomic particle that is assigned as a member of lepton (spin I=1/2). Whereas muons are present in cosmic radiation, high-energy beam of proton from accelerators such as cyclotron and synchrotron produce almost fully polarized muons. The muon has one ninth of the proton mass (0.1126m_p), and the polarized “ultralight proton” is applicable to similar spectroscopic analyses to electron-spin resonance (ESR) and nuclear magnetic resonance (NMR) without the alternative magnetic resonance techniques. In contact with matter, the positive muon captures an electron and become a muonium (Mu= [µ⁺e^-]) that is a light isotope of hydrogen atom. The muoniums have been utilized for monitoring radical reactions of usual unsaturated organic molecules including benzene, alkene, alkyne, and ketone, and the corresponding radicals via muonium addition can be characterized by muon spin rotation/resonance/relaxation (µSR) spectroscopy. In this article, principles and usefulness of µSR for study of functional organic compounds are overviewed, and our recent findings on µSR measurements for the sterically encumbered 1,3-diphosphacyclobutane-2,4-diyl of singlet biradical are presented. A regioselective addition of muonium to the skeletal phosphorus atom in the 1,3-diphosphacyclobutane-2,4-diyl unit was characterized, which would be useful to understand the 4-membered heterocyclic molecular skeleton promoting the low-voltage p-type organic semiconductor behavior.

Rotation of the part of the molecules has been of interest in terms of artificial molecular rotors, which are one of the classes of molecular machines. We have been investigated macrocage molecules with a bridged π-electron system have been synthesized as “crystalline molecular gyrotops”, because the interior π-electron system can be rotate even in a crystal. These molecules were easily synthesized by applying silicon junctions that connect the π-electron rotor and three alkyl spokes. In this paper, chemistry of molecular gyrotops having phenylene, biphenylene, naphthalene-diyl, thiophene-diyl, and thiophenedioxide-diyl rotors have been described. Their cage structures were successfully confirmed by X-ray crystallography. Rotation of the rotor was investigated by solution and solid-state NMR spectroscopy. Some of them showed crystal-to-crystal phase transitions between static and dynamic states of the rotor, and their mechanisms were discussed.

17H-Tetrabenzo [a, c, g, i] fluorene (TBF) as one of π-extended fluorene derivatives was first prepared about 60 years ago. Although π-extended fluorenes have been received vast attention from the field of organic electronics, TBF derivatives have not been well explored from the physicochemical points of view. Six 17,17- dialkylTBFs (methyl to n-hexyl) prepared from TBF show intense blue fluorescence in solutions. On the other hand, in the solid state, the fluorescent spectra and the quantum yields (Φ_SD) vary with their alkyl chains. In particular, the Φ_SD values of dialkylTBFs bearing n-propyl to n-hexyl groups are close to unity, while the fluorescence of parent TBF is almost quenched. Crystallographic analyses revealed that the π-π interactions of the TBF moieties probably play an important role in quenching the solid-state fluorescence. Quinone methide, and unsymmetrical and symmetrical fulvalene involving a TBF unit were synthesized. The electrochemical analyses reveal that these compounds exhibit high electron affinities. The substituent effect on optical properties of 3,14-diphenyl-17,17-di(n-pentyl)TBF derivatives bearing various substituents (H, CN, COOCH₃, CHO, NO₂, CH=C(CN)₂) at 4-position of the phenyl groups were also synthesized and characterized. The diformyl, dinitro, and bis(dicyanoethenyl) derivatives show large solvatofluorochromism (CHO; Δλ_FL=67 nm, NO₂; Δλ_FL=96 nm, CH=C(CN)₂; Δλ_FL=170 nm). Moreover, the diformyl derivative exhibits crystallochromism in fluorescence, and the dinitro and bis(dicyanoethenyl) derivatives display aggregation induced emission. The bis(dicyanoethenyl) derivative can also serve as a highly sensitive fluorescence “turn-on” probe for cyanide ion. The findings suggest that the ground-to-excited state electronic transition of these compounds should have intramolecular charge-transfer (ICT) character.

Marine macrolide natural products are an important source of new chemotherapeutics and their lead compounds. Although in recent years significant advances of NMR instruments have enabled structure determination of natural products at sub-microgram scale, complex macrolides are still challenging targets for structure assignment because many of them contain multiple stereogenic centers along conformationally flexible macrocyclic backbone. Accordingly, total synthesis plays a conclusive role in structure determination of complex macrolides. Furthermore, biological assessment of synthetic material is indispensable for validating the biological activity/potency of natural products. In this paper, we delineate our total synthesis of a marine macrolide iriomoteolide-2a as a case study that demonstrates the significance of total synthesis in natural product chemistry.

Biological activity and toxicity of nitro compounds in therapeutics have been well studied. However, an attempt for using versatile reactivity of the nitro group for bioconjugation is limited. This review describes i) nitrofuryl compounds that form a covalent bonding with a thiol-containing protein at the adjacent position of the nitro group, and ii) dual reactivity of dinitroimidazoles toward a thiol and an amine, and its application for cyclization of peptides.

Diketopyrrolopyrroles (DPPs) are pigments consisting of a condensed lactam core flanked by aromatic rings. DPPs have been widely explored in various electronic devices and fluorescence probe because of their high photostability and fluorescence quantum yields and conjugated structure. As a typical electron-deficient core, DPP can attach electron-donating substitutes to form a donor-acceptor-donor (D-A-D) structure, which enhances NIR-absorption and semiconductive property.

Nodulisporic acids A-F, reported by Merck Research Laboratories, show unique biological activity against insects. For the synthesis of Nodulisporic acid F, Smith’s group has developed a new method to construct the indole skeleton and has achieved its first total synthesis. To construct the more advanced Nodulisporic acids, they have developed a new strategy exploiting a palladium-mediated cross-coupling/indole construction tactic based on the Barluenga’s chemistry and have achieved the first total syntheses of Nodulisporic acids D, C, and B via a unified synthetic strategy. In this review, synthetic studies on Nodulisporic acids by Smith’s group are described.