This review describes our investigations into bent anthracene dimers as building blocks for various nanoarchitectures since 2008. Using these building blocks, we prepared novel polyaromatic capsules, bowls, and tubes through coordination and covalent bonds, hydrophobic effect, and π-stacking interactions. Among them, the coordinative capsule is formed by the quantitative self-assembly of metal ions and bispyridine ligands embedding the bent building block. The capsule possesses a spherical polyaromatic nanocavity, capable of binding various molecules in aqueous solutions. For example, reactive radical initiators and oligosulfurs are greatly stabilized in the cavity upon encapsulation. Biomolecules such as hydrophilic sucrose, hydrophobic androgens, and volatile monoterpenes are bound by the capsule with high selectivity. In addition, an analogous capsule, with a spheroidal cavity, displays unique binding properties toward bowl-shaped molecules. We are convinced that continued investigations will render the present capsules useful molecular nanotools for synthetic, material, and biological applications.
Regiocontrol is very important to obtain the desired products efficiently in synthetic organic chemistry. Because there are many C-H bonds with similar reactivities in organic molecules, it was difficult to promote regioselective C-H transformations without using directing groups. In this account, I describe a new strategy to control regioselectivities in C-H transformations using noncovalent interactions, such as hydrogen bond and Lewis acid-base interaction, between the catalyst and the substrate. As a result, highly regioselective C-H transformations have been achieved. By using the catalytic system, the rate of the C-H transformation was accelerated and the yield of the product was improved dramatically. In addition, the substrate and functional group specificities were expressed.
π-Conjugated compounds have been extensively investigated as potentially useful organic materials, but accessible structures are currently still limited mainly due to the lack of efficient synthetic methods. In particular, preparation of extended π-conjugation systems often requires tedious synthetic approaches. In this regard, we recently devised a new synthetic strategy “stitching reaction” that allows for a facile access to new types of bridged π-conjugated compounds in a convergent manner under rhodium catalysis. For example, we achieved the first synthesis of ‘quinoidal’ fused oligosiloles from two different oligo(silylene-ethynylene)s, and found that these new π-conjugated compounds exhibit unique electronic properties. In addition, structurally related π-conjugated compounds with other bridging groups such as CO could also be synthesized in a similar fashion through the use of appropriate precursors. We could further expand the utility of this process by developing a new and efficient synthesis of useful π-conjugated compounds such as dibenzo[a,e]pentalenes and fluorene derivatives. Furthermore, this synthetic strategy was successfully applied to the preparation of π-conjugated polymers which cannot be synthesized by using other existing methods. Herein we describe an overview including physical properties of newly synthesized π-conjugated compounds.
Due to the recent remarkable progress in both theory and hardware, integrating computational analysis into experimental studies improves our understanding of observed phenomena and guides future project planning. In the discipline of synthetic organic chemistry, this two-way approach is a powerful tactic for the clever design of new reactions and transition-metal catalysis. Summarizing our recent achievement, we herein report the development of a methodology based on the unique chemical properties of silver catalyst. An asymmetric O-H insertion reaction into phenols using a chiral homobinuclear complex created in situ from AgNTf2 and (S)-XylylBINAP is presented along with computational analysis. By taking advantage of the high electrophilicity of silver-carbenes, chemoselectivity and enantioselectivity were controlled in phenol dearomatization reactions. Site- and chemoselective C-H functionalization using nitrene species, i.e., an isoelectronic nitrogen analog of carbene species, were achieved, leading to the synthesis of an array of spiroaminals. The theoretical investigation supports the origin of the chemoselectivity and the composition of an elementary hydrogen transfer/radical recombination mechanism involving triplet ground states of silver-nitrene species. We end by describing our newly developed method for diazo-free generation of silver-carbene, and its application for the dearomatization of nonactivated arenes.
Polyfunctional ligands containing Lewis acid (LA) moieties have attracted great interest over the last decade on account of their interesting properties. One of the most notable features is their ability to function as σ-electron acceptor (Z-type) ligands for transition metals (TMs). The presence of LA moiety around a TM strongly influences the reactivity of the TM complex and allows unconventional metal-ligand cooperation, leading to versatile applications in catalysis. Z-type ligands are typically used to generate electrophilic catalysts by withdrawing electrons from the TM via TM→Z-type ligand interactions. The cooperative activation of σ-bonds in TM→LA interactions has been observed in a number of catalytic systems. In addition, the interconversion between X- and Z-type ligands has been found to promote efficient cross-coupling reactions. This article describes our recent results on Z-type ligand chemistry. First, a new type of Pd-borane cooperation is described, in which Pd cross-coupling reactions operate via an unconventional mechanism involving anionic Pd(0) species. This cooperation enables the hydro- / deutero-dehalogenation of a variety of (hetero)aryl chlorides. Next, the first cross-coupling reactions of the fluoro-silanes and -germanes as coupling partners are introduced. The coordination of the fluoro-silanes and -germanes with the TM as Z-type ligands presents a new approach for bond activation. In this case, the TM acts as a Lewis base and efficiently weakens the Si-F and Ge-F bonds via TM→σ*(E-F) interactions (E＝Si and Ge). The synergistic action of an external LA allows for the cleavage of strong Si-F and Ge-F bonds and converts the Z-type silane and germane ligands into X-type silyl and germyl moieties, respectively. This could be employed in the Si-C and Ge-C cross-coupling reactions of fluoro-silanes and -germanes.
Metal-ligand cooperation, in which both metal and ligand participate in bond cleavage and formation, is gathering great attention in recent years. While many types of complexes have been employed in metal-ligand cooperative heterolytic cleavage of H-H bond, a cyclopentadienone-hydroxycyclopentadienyl-based metal complex, originally reported by Shvo, has a unique character, that is, red/ox activity of the metal center along with the heterolytic bond cleavage. To date, various transition-metal complexes (M＝Ru, Fe, Re, Mo, Ir, Rh, Pt, etc.) having cyclopentadienone/hydroxycyclopentadienyl ligands are reported to show catalytic reactions utilizing the heterolytic cleavage/formation of H-H bond. On the other hand, there are substantial efforts to expand the scope of the bond to be cleaved other than the H-H bond. This article summarizes our recent progress in the metal-ligand cooperative cleavage reactions of C-H and B-H bonds by cyclopentadienone metal complexes with introduction of the historical background.
Biological systems demonstrate sophisticated functions such as accurate recognition, efficient transfer/storage, and catalysis via fine control of multi-component systems using flexible skeletons such as peptides. Thus, the construction and control of multi-component systems using flexible units is a promising approach to mimic sophisticated functions in artificial systems. Peptides are also useful framework for developing such artificial systems because they have suitable flexibilities and enable design of ordered structures through molecular interactions between the functional groups in their main and side chains. However, due to the difficulty on constructing stable structures with peptides, limited studies have been reported for solid materials, especially crystalline materials, using short peptides. To overcome this issue, we have been developing basic motives for artificial multi-component systems in crystals using metal complexes of short, flexible peptides. This review describes our recent advances in the construction of multi-component systems.
Lewis acidity-diminished boron functionalities, B(dan) (dan＝naphthalene-1,8-diaminato) and B(aam) (aam＝anthranilamidato), have efficiently been installed into organic frameworks in a straightforward manner by a variety of catalytic borylation reactions, where chemoselective σ-bond metathesis between a transition metal catalyst and an unsymmetrical diboron [(pin)B-B-(dan) or (pin)B-B(aam)] is a key elementary step. The dan/aam-substituted organoboranes obtained therefrom have been found to be utilized for direct cross-coupling without prior acidic deprotection, regardless of their diminished boron-Lewis acidity that usually retards transmetalation. In addition, we have also succeeded in developing copper-catalyzed aryl- and cyano-stannylation of arynes, in which Lewis acidity increment of a tin center facilitates their progress.
Utilization of light energy, especially visible-light energy, which can be gained inevitably from sunlight, has now attracted emerging attention due to the recent serious environmental issues. In the field of homogeneous catalysis, various efficient photoredox reactions have been developed, in which mainly ruthenium and iridium visible-light sensitizers play the key roles in the catalysis to promote single electron transfer to generate reactive radical species under mild conditions. Apart from these reactions, utilization of the excited-state of the catalyst itself may lead to the new type light-controlled reactions. In order to realize such reactions, precise design of the photocatalysts is inevitable for the efficient formation of the desired excited-state. Here, we will introduce our intensive studies on developing the visible-light driven catalysts.
Chiral metal complexes have been widely used as enantioselective catalysts and chiroptical materials. In recent years, “chiral-at-metal” complexes, in which the ligand is achiral but the coordination geometry is chiral, have appeared, expanding the variety of chiral metal complexes. However, functional chiral-at-metal complexes are so far limited to those with octahedral or half-sandwich geometry. Since tetrahedral chiral-at-metal complexes usually racemize instantaneously, optically pure complexes were not available, especially in solution. Here, we present the first example of a tetrahedral chiral-at-metal complex with high configurational stability and enantioselective catalytic function. In this complex, an elaborately designed tridentate ligand imparted stability against stereoinversion while leaving a labile coordination site. We have also developed a method for the asymmetric synthesis of this chiral-at-metal complex using an acidic chiral auxiliary. The emergence of tetrahedral chiral-at-metal complexes is expected to make a significant contribution to catalytic chemistry and materials science.
Hydroxylation of hydrocarbons is a simple chemical transformation reaction. However, this is not an easier task than it looks in the chemical equations. The reactions generally require harsh (high temperature, high pressure, strongly acidic, or basic) conditions, which induce over oxidation giving a complicated mixture of products. In contrast, living organisms perform such difficult tasks like selective oxidation of methane to methanol and benzene to phenol by employing appropriate oxidizing species generated using earth abundant transition metals (Fe and Cu) in the well-organized reaction centers of metallo-monooxygenases. We have been trying to understand the essence of the chemical functions of such metalloenzymes to construct selective and efficient oxidation reactions of inert aliphatic and aromatic compounds. In this article, our recent accomplishments in this endeavor are summarized together with the backgrounds of the related chemistry.