Chiral amine catalysts are widely utilized in various asymmetric reactions. Among them, binaphthyl-based secondary amine catalysts show unique reactivity and unusual stereoselectivity in comparison with frequently used ʟ-proline and its derivatives. In spite of their utility, however, asymmetric reactions employing binaphthyl-based amines are rare because of their synthetic inefficiency. In this context, we have developed chiral secondary amine catalysts based on a phenylcyclopropane scaffold as a novel chiral motif. Newly synthesized amine catalysts function as effective catalysts for several asymmetric reactions. In addition, the phenylcyclopropane-based amino sulfonamide was found to be an effective catalyst for the syn-selective Mannich reaction and conjugate addition with alkynyl Z-ketimines.

Allylpalladium intermediates usually act as an electrophile due to the property of the η³-complex, which is much more stable than the nucleophilic η¹-complex. We have developed type-I and -II umpolung cyclizations of aldehyde-containing allylpalladium intermediates, where the unstable η¹-complexes are effectively captured by the intramolecular electrophile. The diastereoselectivities of the type-I cyclization highly depend on how to generate the allylpalladium intermediates either by oxidative addition of allyl acetate or carbopalladation of allenes. In the former case, the constituent elements and length of the tether also affect the diastereoselectivities. The enantioselectivities achieved by using SEGPHOS or DM-SEGPHOS ligand are useful to rationale the reaction mechanism via common cationic Zimmerman-Traxler transition states consisting of the cationic η¹-allylpalladium intermediates. A sterically demanding substituent at the center of the allyl moiety is necessary for high enantioselectivity. On the other hand, the diastereoselectivities of the type-II cyclization using an achiral diphosphine ligand show that the reaction proceeds through a chair-chair transition state, based on which we have designed substrates applicable to asymmetric variant. In addition, we have developed one-pot type-II cyclizations by combining palladium-catalyzed reactions using allylic and propargylic carbonates, which can provide a solution to the complexity of substrate preparation.

Lewis-acid catalyst is one of the most fundamental catalysts in organic chemical reactions. In this paper, our recent quantum chemical studies on Lewis-acid catalyzed organic chemical reactions have been presented. First, we focused on the mechanism of Lewis-acid activated Lewis acid catalysts. Then, the normal-electron-demand Diels-Alder reaction between cyclopentadiene and maleic anhydride was discussed in order to clarify the origin of the stereoselectivity. The endo selectivity is attained when dienophile carries a group (or groups) involving highly polarized bonds. We show that the well-known donor-acceptor relationship in the normal-electron-demand Diels-Alder reaction is linked to the endo selectivity. On the basis of the results, the Diels-Alder reaction of cyclopentadiene with methyl acrylate catalyzed by a simple Lewis acid, AlCl₃, has been investigated. The Lewis-acid catalyst facilitates the cycloaddition and brings a higher endo selectivity in the highly asynchronous process. Finally, we discussed two reactions catalyzed by tris-(pentafluorophenyl)borane, B(C₆F₅)₃.

π-Expanded alkynes and alkenes are promising as organic materials like semiconductors, light-emitters, and dyes for photoelectric conversion. So far a number of π-expanded compounds were synthesized, and their optical and physical properties were investigated for future reference. We describe herein synthetic protocols of functionalized diarylethynes and ethenes by using benzyl sulfones and aldehydes as starting compounds. Both diarylethynes and ethenes were synthesized through diarylethenyl sulfones which were prepared by the consecutive base-assisted aldol-type reaction of benzyl sulfones with arylaldehydes, phosphorylation (acetylation), and base-assisted elimination of phosphoric (acetic) acid. When the ethenyl sulfones were treated with sterically hindered base like lithium hexamethyldisilazide (LiHMDS), sulfinic acid eliminated to give the desired ethynes. This synthetic protocol could be applied to the syntheses of highly strained cyclic diynes (Sondheimer-Wong diynes). The strained cyclic diynes were successfully transformed to diaminopentalenes through amination and transannulation by treatment with an excess amount of lithium amide/amine. On the other hand, when the ethenyl sulfones were subjected to perylene/blue LEDs-promoted reductive desulfonylation, the desired diarylethenes (stilbenes) were successfully synthesized. We also developed 1,3,6,8-tetra(4-alkoxyphenylethynyl)pyrene as visible-light driven photocatalyst. The pyrene photocatalyst accelerated the desulfonylation of ethenyl sulfones by irradiation of green light with LEDs, and afterwards the catalyst was successfully separated from the desired ethynes by column chromatography on silica gel.

Recently, optical imaging technology in the second near-infrared (NIR-II, λ =1000—1400 nm) window has attracted attention due to the merits of high S/N ratio as well as real-time detection at deeper penetration in biological tissues compared to the light energy in the traditionally-used visible and first near-infrared window (NIR-I, λ=700—1000 nm) regions. As potential small organic NIR-II chromophores, various polymethines and phthalocyanine-based dyes have shown superior biocompatibility, structure designability, and optical wavelength tunability. However, dye instability arising from the small HOMO-LUMO band gap is a common technical drawback that limits the scope of current applications.

In this regard, porphyrins and their analogues could be potential NIR chromophore scaffolds offering new opportunities for optical materials to be used in the fields ranging from light-harvesting to sensing and therapeutic applications. Several skeletal modifications of the porphyrins, such as 1) core expansion, 2) peripheral fusion, and 3) chromophore array, could downshift the wavelength to the NIR-II window along with high photo- and chemical stability. This article describes the synthesis of a series of large π-conjugated “N-confused” porphyrin analogues with inverted pyrrole rings linked at the α and β-positions as key structures to furnish the desired optical properties in the use of optical applications. Furthermore, metal complexation-induced molecular orbital mixing/tuning of the macrocyclic porphyrin scaffolds enabled the fundamental optical and excited-state dynamics.

Polyoxometalates, which are anionic metal oxide clusters, have recently attracted considerable attention as photocatalysts because of their unique photoinduced charge-transfer properties, multielectron redox properties, acid-base properties, and reactivities of the photoexcited species. In particular, polyoxometalate photocatalysis has been utilized for liquid-phase organic reactions. However, typical polyoxometalate photocatalysts, including decatungstate, require irradiation with UV light because of the large energy gaps between the O²⁻-based highest occupied molecular orbitals(HOMOs)and the W⁶⁺-based lowest unoccupied molecular orbitals(LUMOs)therein. We have developed a new precise synthesis method of polyoxometalates by controlled reaction of lacunary polyoxometalates and metal species in organic solvents. Based on the design of the structures and electronic states of polyoxometalates, we have developed visible-light responsive polyoxometalate photocatalysts. This article describes the precise synthesis of polyoxometalates and their applications to visible-light-responsive photocatalysis for organic synthesis.

Continuous-flow syntheses have been paid attention to green sustainable chemistry (GSC). Several flow reactors have been investigated using catalysts and biocatalysts. Especially, the redox reactions using immobilized biocatalysts with cofactors are much attention and attractive. Regeneration or reuse of cofactors is a big issue to be solved for effectiveness, such as increasing total turnover numbers. Herein, oxidation and asymmetric reduction reactions using immobilized biocatalysts with reusable cofactors are highlighted.