Shuhei Higashibayashi

Asymmetric synthesis of a chiral buckybowl, trimethylsumanene.

The first asymmetric synthesis of a chiral buckybowl, a C3 symmetric (C)-8,13,18-trimethylsumanene (1), was achieved by employing a synthetic strategy that translates chirality at sp3 centers into bowl chirality. The synthesis features a syn selective cyclotrimerization of an enantiopure halonorbornene derivative, tandem ring-opening/closing olefin metathesis reactions, and DDQ oxidation at low temperature. The bowl-to-bowl inversion energy of 1 was determined as 21.6 kcal/mol by circular dichroism spectra measurement.

Enantioselective synthesis of a chiral nitrogen-doped buckybowl

Bowl-shaped aromatic compounds, namely buckybowls constitute a family of curved polycyclic aromatic carbons along with fullerenes and carbon nanotubes. Doping of heteroatoms to the carbon frameworks of such aromatic compounds drastically modulates their physical and chemical properties. In contrast to nitrogen-doped azafullerenes or carbon nanotubes, synthesis of azabuckybowls, nitrogen-doped buckybowls, remains an unsolved challenging task. Here we report the first enantioselective synthesis of a chiral azabuckybowl, triazasumanene. X-ray crystallographic analysis confirmed that the doping of nitrogen induces a more curved and deeper bowl structure than in all-carbon buckybowls. As a result of the deeper bowl structure, the activation energy for the bowl inversion (thermal flipping of the bowl structure) reaches an extraordinarily high value (42.2u2009kcal per mol). As the bowl inversion corresponds to the racemization process for chiral buckybowls, this high bowl inversion energy leads to very stable chirality of triazasumanene.

Fluorinated and Trifluoromethylated Corannulenes

The syntheses and properties of corannulenes carrying electron-withdrawing groups (F, CF3 , C6 F5 ) are reported. Direct fluorination of corannulene (C20 H10 ) was carried out with xenon difluoride, and the crystal structure of the product was confirmed by the X-ray analysis. Novel trifluoromethylated corannulenes, including the versatile 4,9-dibromo-1,2-bis(trifluoromethyl)corannulene, were obtained by various established ring-closing reactions. Besides the use of hexafluorobutyne for the construction of fluoranthenes by Diels-Alder reaction as precursor molecules to form 1,2-disubstituted corannulenes, bis(pentafluorophenyl)acetylene was employed as dienophile. The molecular structure and crystal packing of a trifluoromethylated corannulene was determined by single-crystal X-ray analysis and compared with those known brominated and trifluoromethylated corannulenes. The general electron-acceptor properties of corannulenes bearing substituents introduced in particular positions by liquid-phase synthesis are discussed together with published computational results.

Preparation of C3‐Symmetric Homochiral syn‐Trisnorbornabenzenes through Regioselective Cyclotrimerization of Enantiopure Iodonorbornenes

C(3)-symmetric homochiral (-)-syn-trisoxonorbornabenzene 1 possessing a rigid cup-shaped structure was synthesized through a novel regioselective cyclotrimerization of enantiopure iodonorbornenes catalyzed by palladium nanoclusters. The yield of the cyclotrimerization was dependent on the stability of the palladium clusters, which was ascertained from the appearance and TEM images of the reaction mixtures. The efficient preparation of (-)-syn-1 was established in short steps, including the newly developed cyclotrimerization reaction. The thus-prepared homochiral (-)-syn-1 can serve as a key intermediate for the synthesis of C(3)-symmetric homochiral cup-shaped molecules with a helical arrangement of substituents. Introduction of several types of substituents was well demonstrated through palladium-catalyzed coupling reactions with the corresponding phosphate and triflate of (-)-syn-1.

Correlation between bowl-inversion energy and bowl depth in substituted sumanenes

Abstract The correlation between the bowl-inversion energy and the bowl depth for sumanenes monosubstituted with an iodo, formyl, or nitro group was investigated experimentally and by theoretical calculations. The bowl-inversion energies of the substituted sumanenes were determined experimentally by two-dimensional NMR exchange spectroscopy measurements. Various density functional theory methods were examined for the calculation of the structure and the bowl-inversion energy of sumanene, and it was found that PBE0, ωB97XD, and M06-2X gave better fits of the experimental value than did B3LYP. The experimental value was well reproduced at these levels of theory. The bowl structures and bowl-inversion energies of monosubstituted sumanenes were therefore calculated at the ωB97XD/6-311+G(d,p) level of theory. In both the experiments and the calculations, the correlation followed the equation ΔE = acos4 θ, where a is a coefficient, ΔE is the bowl-inversion energy, and cos θ is the normalized bowl depth, indicating that the bowl inversion follows a double-well potential energy diagram.

Emission amplification by sumanene nanocrystals in an onigiri-type organic-organic assembly

Core-shell assemblies of nanocrystals of sumanene (shell) and sumanenetrione (core) resembling onigiri (filled Japanese rice balls) were fabricated by stepwise reprecipitation. Optical properties of the onigiri-type assemblies showed that emissions from the core sumanenetrione nanocrystals were amplified through energy transfer from the shell sumanene nanocrystals through the nanocrystalline interface.

Columnar/herringbone dual crystal packing of pyrenylsumanene and its photophysical properties

Summary A single crystal of pyrenylsumanene was found to exhibit both columnar and herringbone crystal packing. The sumanene moieties form unidirectional columnar structures based on π–π stacking while the pyrene moieties generate herringbone structures due to CH–π interactions. The absorption and emission maxima of pyrenylsumanene were both red-shifted relative to those of sumanene and pyrene, owing to the extension of π-conjugation. Monomer emission with high quantum yield (0.82) was observed for pyrenylsumanene in solution, while excimer-type red-shifted emission was evident in the crystalline phase.

Beam-induced graphitic carbon cage transformation from sumanene aggregates

We found that electron-beam irradiation of sumanene aggregates strongly enhanced their transformation into a graphitic carbon cage, having a diameter of about 20u2009nm. The threshold electron dose was about 32 mC/cm2 at 200u2009keV, but the transformation is still induced at 20u2009keV. The transformation sequence suggested that the cage was constructed accompanied by the dynamical movement of the transiently linked sumanene molecules in order to pile up inside the shell. Thus, bond excitation in the sumanene molecules rather than a knock-on of carbon atoms seems to be the main cause of the cage transformation.