近接するC-H接触を有するカルボカチオン種は、を形成する可能性があり興味がもたれる。ナフタレン-1,8-ジイル骨格に組み込まれた分子内トリアリールメタン―トリアリールメチリウム錯体は近接したC-H・・・C⁺接触を有する。組み込まれるアリレンスペーサーによってC-H・・・C⁺部位の距離や角度を変えることで、架橋性への影響を明らかにできると考えた。今回演者らはビフェニル骨格を中心として、四種の表題化合物を発生させ、分子内ヒドリド反応性の活性化障壁を実験的に求め、X線で決定したC-H・・・C+部分のジオメトリーとの相関について検討を行った。得られた知見から、三中心結合となることが期待される新たな表題化合物の合成を行っている。

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我々は剛直な縮環骨格からなるEind基を利用し、ジボラン(4)の新規異性体である二重–水素架橋ジボラン(4)の単離に成功している。このように、立体的に保護されたホウ素―水素化合物は新奇な結合様式や反応性を示すことが期待される。本発表では、我々が新たに開発したジボラン(6)の二電子還元によるB–B結合生成反応を利用した、様々な水素化ジボロン化合物の合成、構造、および反応性について報告する。

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Almost all conventional drug discovery research has been based on hydrocarbon-based frameworks and common chemical elements such as nitrogen, oxygen, sulfur, and the halogens. However, triggered by the approval of bortezomib, a boronic acid-containing pharmaceutical agent, the incorporation of functionalities that are not native in biological systems has been intensively investigated. Several other boron-containing pharmaceuticals have also been marketed. Therefore, the inclusion of various elements is one of the most promising strategies for the development of novel and distinctive drug candidates. In this symposium review, the author focused on the ‘elements chemistry’ approaches for the structural development of biologically active compounds, particularly those involving silicon and phosphorus. The isosteric exchange of Si and C (Si/C-exchange) is one of the most-investigated forms of substituting elements. We revealed the detailed physicochemical impact of Si/C-exchange, and we proposed several applications of silyl functionalities other than the simple Si/C-exchange. Regarding phosphorus, we recently revealed that the P-B substructure can function as the isostere of C-C or Si-C substructures. In addition to these isosteric exchanges, the development of biologically active compounds bearing unique substructures such as carboranes, hydrophobic boron clusters, and ferrocene is introduced. These novel strategies provide several options for structural development, offering great potential for expanding the chemical space of medicinal chemistry.

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The catalytic dehydrocoupling reactions of amine-boranes have attracted considerable interest from the perspectives of chemical hydrogen storage and the production of new inorganic polymers. In this review, significant progress made in coordination of amine-boranes at transition metals is described, and the elementary reactions in relation to the catalyzed dehydrocoupling process are also discussed. Highly unsaturated complexes as the precursors provide an efficient strategy to synthesize the complexes with amine-borane ligands. The amine-borane s-complexes with the h¹- or h²-coordination modes consist of electron-donation from the B-H bond to the metals exclusively and negligible back-donation to the B-H bond. The B-H activation (oxidative addition) of amine-boranes is achieved to the unsaturated Rh-PR₃ system, giving unique dimeric complexes with Me₃N-stabilized boryl ligands. The iridium amine-borane s-complexes convert thermally to the complexes with amino-borane ligands, together with elimination of hydrogen. X-ray crystallographic results of the ruthenium amino-borane s-complexes display shortening of the Ru-B and B-N bonds, indicating the B-N multiple-bond character. Dehydrocoupling reactions of primary and secondary amine-boranes promoted by a cationic iridium complex produce the linear diborazanes as kinetically stable products. The reaction pathway of the dehydrogenative oligomerization of amino-boranes is proposed.

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Recently, we have revealed that the hydrogen boride (HB) sheets can be formed by exfoliation and complete ion-exchange between protons and magnesium cations in magnesium diboride (MgB₂＋2H^＋ → Mg^2＋＋2HB) at room temperature and ambient pressure. We have then found that the hydrogen molecules (H₂) can be released from HB by the UV irradiation as much as 8 wt% at room temperature.

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Relative gas-phase stabilities (ΔE_X) of ring-substituted 2-phenyl-1,3-dehydroadamantane-5,7-diyliums were computationally determined as the energy difference of hydride transfer equilibria between ring-substituted and its parent dications. The generating process of the dications from the corresponding neutral species were divided into two steps: the generation step of the mono-cations from the neutral species and that of the dications from the mono-cations. The ΔE_X of these processes were also determined in the same manner. Obtained substituent effects were compared to those of α,α-dimethylbenzyl cations (σ⁺-reference) and 90˚-fixed α,α-dimethylbenzyl cations (σ⁰-reference). Statistical analyses revealed negligible through-resonance effects and a wide variety of saturation effects in these electron-deficient systems. The result is in contrast with those of 1-phenylcyclobutane-1,3-diyliums and 1-phenylcyclopentane-1,3-diyliums that show both significant through-resonance and saturation effects. The through-resonance and saturation effects were revealed to be independent of each other to confirm that the three-term extended Yukawa-Tsuno equation,-ΔE=ρ(σ⁰+r⁺Δσ_R⁺+S⁺Δσ_S⁺), is intrinsic for substituent-effect analyses in the electron-deficient species.

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Relative gas-phase stabilities (ΔE_X) of ring-substituted 2-phenyladamantane-5,7-diyliums were computationally determined as the energy difference of hydride transfer equilibria between ring-substituted and their parent dications. Obtained substituent effect was compared to those of α,α-dimethylbenzyl cations (σ⁺-reference) and 90˚-fixed α,α-dimethylbenzyl cations (σ⁰-reference). Statistical analyses revealed negligible through-resonance effect and significantly large saturation effect operating in the dication. The result is in contrast with those of 1-phenylcyclobutane-1,3-diyliums and 1-phenylcyclopentane-1,3-diyliums which show both significant through-resonance and saturation effects and that of 2-phenyl-1,3-dehydroadamantane-5,7-diyliums which shows both small through-resonance and saturation effects. The through-resonance and saturation effects have been shown to be independent of each other to confirm that the three-term extended Yukawa-Tsuno equation, -ΔE_X = ρ (σ⁰+r⁺Δσ_R⁺+s⁺Δσ_s⁺), is intrinsic in substituent-effect analyses for the dicationic systems.

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アルミニウム(Al)は,小さな比重と高い導電性などの優れた特性から工業用金属材料として欠くことのできない元素である。一方有機合成化学においても,有機金属利用の初期から現在に至るまで種々のアルミニウム化合物が有用な反応剤および触媒として用いられており,歴史的にも重要で興味深い応用例が数多く見られる。本講座では,はじめに「Alの化学」を理解する上でのキーポイントとなる元素としてのAlの特徴と,これに基づくAl化合物の一般的性質について述べ,これを軸として有機合成反応での利用(還元剤,ルイス酸触媒およびアルキル化試薬としての作用とその制御)について概説する。

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 Monocarba-closo-dodecaborate (1; [closo-CB₁₁H₁₂]⁻, or C1-carborane anion) is a symmetrical, stable anionic cluster, which possesses low nucleophilicity/basicity and exhibits three-dimensional aromaticity. In contrast to the rich applications of C2-carborane molecules (C₂B₁₀H₁₂), the chemistry of the C1-carborane anion as a platform for functional molecules has not been thoroughly studied thus far due to the lack of its efficient functionalization. In particular, no efficient general methods are available for the introduction of aryl and sp²/sp-carbon groups at the carbon vertex of the C1-carborane anion. The unique electronic structure and potential applications of the C1-carborane anion prompted us to investigate methods to functionalize it. We developed a general, efficient C-C cross-coupling reaction of 1 under palladium catalysis which yields a variety of 1-C-functionalized C1-carborane derivatives. The use of copper(I) or lithium species as a transmetalating partner facilitated the cross-coupling process of the sterically hindered C1-carborane anion. The potential application of 1-C-arylated C1-carborane anion derivatives thus obtained were explored, some of which showed potential as pharmacophores and ionic liquid crystal behavior. Furthermore, conjugation between σ- and π-aromatic moieties in 1-C-arylated monocarba-closo-dodecaborate anion derivatives was identified by means of kinetic experimental studies combined with theoretical calculations.

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Our cyclizations and ring transformations via two types of spirocyclic arenium ions are described in this article. A spiro [5.2] octadienyl cation, which is well known as a Cram’s arenium ion, has attracted considerable attention in the aspect from physical organic chemistry. Our effort to utilize the intermediate as a synthetic tool resulted in the development of a lactonization of β-aryltosylate with an internal ester, a lactone ring transformation, and an ether ring transformation via the Cram’s arenium ion. We also developed a 7-endo selective Friedel-Crafts cyclization of various epoxy unsaturated esters promoted by Lewis acids. Detailed experimental data showed that a spiro [5.5] undecadienyl cation is a key intermediate of this reaction. The reaction mechanism seems to involve ipso-cyclization and subsequent skeletal rearrangement of the resulting spirocyclic arenium ion. The Friedel-Crafts cyclization was also applied to the formation of tetrahydro-2-benzazepine.

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Polyhedral metal-main group clusters have attracted much attention, because of their structures and properties such as electron transfer and emission as well as a catalyst in organic syntheses. In this paper, we wish to report our recent results concerning the polyhedral platinum-silyl clusters. The reactions of zero-valent platinum complex bearing phosphine ligands with disilanes afforded the trigonal bipyramidal Pt₃Si₂ or octahedral Pt₄Si₂ clusters. Use of a sterically small disilane reduces the repulsion between the disilane and the phosphine ligand to induce the formation of higher dimensionally clusters. Each of the platinum-silyl clusters has a polyhedral skeletal structure compressed to Si-Si alignment with long Pt-Pt and short Si-Si distances, compared with those of the platinum clusters including other main group elements such as S, Se, and Ga. The liberation and rebinding of the phosphine ligand regulate a 1,2-migration on the platinum center in the formation process of clusters.

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A new type of rechargeable battery, the molecular cluster battery (MCB), has been developed by utilizing molecular clusters such as Mn12 and polyoxometalate (POM) clusters, as cathode active materials and a lithium metal as an anode. It was found that MCBs can exhibit higher battery capacities than conventional lithium ion batteries. In operando X-ray absorption fine structure analyses on MCBs of Mn12 and POMs revealed that they exhibit 8- and 24-electrons reduction during discharge, respectively. Such electron sponge behavior realized only in solid-state electrochemistry resulted in a high battery capacity. To improve the battery performances furthermore, nanohybrid materials of single-walled carbon nanotubes and POMs were prepared, and both charging/discharging rate and battery capacity of their MCBs were significantly better than those of the non-nanohybrid MCBs. Finally, in situ magnetic measurements under solid-state electrochemistry on a mixed-valent chromium Prussian blue analogue ferrimagnet revealed continuous changes in magnetization, transition temperature and coercive force. It is demonstrated that solid-state electrochemistry is a useful technique to explore various physical properties from fundamental sciences to applications.

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This account reviews our recent research on chemistry of “boryl anion”. Boryl anions can be classified with their metallic counterpart, such as Li, Mg, Cu, and Zn in a similar manner to the classification of carbanion. Syntheses and characterizations of boryllithium compounds have been achieved by using bulky substituents to prevent a dimerization of a boryl radical intermediate and nitrogen–containing heterocycle to gain an electronic stabilization. Transmetallation of boryllithium afforded a series of borylmetals having Mg, Zn, Cu, Ag, Au, Ti, or Hf. Some of these borylmetals can be regarded as a boryl anion species possessing a nucleophilic character towards organic electrophiles. Detailed analyses of group 11 borylmetal species showed a strong trans influence of boryl ligand. Borylhafnium complex could be utilized as a catalyst precursor for olefin polymerization.

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The molecular structures and chemical reactivity of boron compounds are characterized by the electron deficiency and low electronegativity of boron. Based on simple molecular design strategy taking advantage of the unique properties of boron, we have developed new chemical species and new organic transformations. This article describes our recent studies of (i) a two-coordinate boron cation (borinium ion) and (ii) a 9-borafluorene-mediated benzannulation reaction of alkyne derivatives. In the former, we successfully isolated a diarylborinium ion (Mes₂B⁺), which had been considered impossible to be isolated as a stable species without introducing particular electron-donating groups to boron. We found several unique small-molecule activation reactions mediated by Mes₂B⁺ such as C=O double bond cleavage of CO₂. Moreover, we revealed that Mes₂B⁺ serves as an excellent hole-doping reagent for carbon nanotube, graphene, and transition metal dichalcogenide. In the latter topic, we discovered a new efficient boron-mediated benzannulation reaction of alkyne derivatives, which enables the synthesis of a wide variety of extended π-conjugated molecules and polymers. The boron compounds used in these studies are simple but enable powerful organic transformations.

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ヘテロ元素を含む機能性ユニットを元素ブロックと呼ぶ．従来のホウ素錯体は高い剛直性と平面性に由来して，効率の良い発光特性を示すことから，発光性元素ブロックとして有用性が高い．ここで，一般的な有機色素が発光する希薄条件下において光らない柔軟性の高いホウ素錯体が見出されてきた．そして，これらの錯体の励起状態での挙動を制御すると，特異な刺激応答性の発光特性が得られることを筆者らは見出した．本稿では，これらの ｢柔軟な｣ ホウ素錯体が発見された経緯と，それらの錯体から成る元素ブロックを基盤としたユニークな発光特性について述べる．さらにそれらの特異な物性を用いた外部刺激·環境応答性の発光材料開発について説明する．主なトピックスとして，凝集誘起型性や発光クロミズム，それらの現象に基づくセンシング材料開発について最近の進捗を述べる．

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