Keiichiro Ogawa

Aggregation controlled proton tautomerization in salicylideneanilines

Abstract Proton tautomerization of salicylideneanilines in the solid state and in solution is reviewed, mainly focusing on the zwitterionic character of the NH form and its stabilization in the solid state and in solution at low temperature. The essential role of aggregation of molecules in proton tautomerization is emphasized.

X-ray Diffraction Analysis of Photochromic Reaction of Fulgides: Crystalline State Reaction Induced by Two-Photon Excitation

Although most fulgides show their photochromism in the solid state, crystal structure changes accompanying the photochromism have not been previously observed. The photochromic reactions have been so far considered to take place on surfaces or at defects of the crystals or to proceed with destruction of the crystals. In this study we have succeeded in observing crystal structure changes accompanying the photochromism of fulgides using X-ray diffraction analysis. Detection of the photoproducts in the crystal structures was not possible when the single crystals of the fulgides were irradiated with steady UV light. Two-photon excitation by pulsed laser light was essential to produce a sufficient amount of the photoproducts without significant deterioration in the quality of the crystals.

Terpenoids from six lophoziaceae liverworts

Abstract Six liverworts belonging to the Lophoziaceae were investigated and six new compounds isolated and their structures determined by application of 2D NMR techniques or X-ray analysis. Setiformenol, isolated from Tetralophozia setiformis, is the first example of a cembrane-type diterpenoid from a liverwort.

Torsional vibration and central bond length of N‐benzylideneanilines

The crystal structures of N-benzylideneaniline (1), N-benzylidene-4-carboxyaniline (2), N-(4-methylbenzylidene)-4-nitroaniline (3), N-(4-nitrobenzylidene)-4-methoxyaniline (4), N-(4-nitrobenzylidene)-4-methylaniline (5), N-(4-methoxybenzylidene)aniline (6) and N-(4-methoxybenzylidene)-4-methylaniline (7) were determined by X-ray diffraction analyses at various temperatures. In the crystal structures of all the compounds, an apparent shortening of the central C=N bond was observed at room temperature. As the temperature was lowered, the observed bond lengths increased to approximately 1.28 A at 90 K, irrespective of substituents in the molecules. The shortening and the temperature dependence of the C=N bond length are interpreted in terms of an artifact caused by the torsional vibration of the C-Ph and N-Ph bonds in the crystals. In the crystal structures of (1) and (7), a static disorder around the C=N bond was observed, which is also responsible for the apparent shortening of the C=N bond.

Synthesis and structure of tris[(2-alkoxymethyl)phenyl]germanes and tris[(2-methylthiomethyl)phenyl]germane

Abstract Tris[(2-alkoxymethyl)phenyl]germanes and tris[(2-methylthiomethyl)phenyl]germane were prepared and their structures were investigated by means of X-ray crystallographic analysis and 73Ge NMR spectra, which indicated incipient dative bonding between germanium and sulfur or oxygen atoms to make the germanium atom hexavalent and heptavalent, respectively.

Electronic absorption spectra and geometry of (E)-stilbene and ‘stiff’ stilbenes

Examination of the electronic absorption spectra of (E)-stilbene (1), indeno[2,1-a]indene (2), 5,6,11,12-tetrahydrochrysene (3), (E)-1,1′-bi-indanylidene (4), and (E)-1,1′-bitetralinylidene (5) in fluid solution at room temperature and in glassy solution at 77 K led to the following conclusions. (a) The torsion angles of the C–Ph bonds in (1) deviate from 0° to some extent in the fluid solution and are nearly 0° in the glassy solution. (b) The (E)-stilbene skeleton in (2)—(4) is planar or nearly planar in the fluid as well as in the glassy solution. (c) The torsion angles of the C–Ph bonds in (5) substantially deviate from 0° in the fluid solution and become smaller in the glassy solution. However, they still deviate from 0° to a considerable extent. The observed spectral change is explained by the use of the Frank–Condon principle.

Conformational change of N-benzylideneanilines in crystals

The crystal structures of N-(4-nitrobenzylidene)aniline (1), N-(4-chlorobenzylidene)-4-methylaniline (2) and N-(4-methylbenzylidene)-4-methylaniline (3) were determined by X-ray diffraction analyses at various temperatures. A dynamic disorder was observed in the crystal structures of all compounds. The dynamic disorder is accounted for in terms of a conformational change involving a pedal motion in the crystals.

1-(3-methyl-2-indolyl)pyridinium bromide: Synthesis and auto-oxidation of its catalytic hydrogenation product

Abstract 1-(3-Methyl-2-indolyl)pyridinium bromie (I) was prepared by the bromination of skatole with dioxan-dibromide in dioxan in the presence of pyridine. The compound (I) gave II on catalytic hydrogenation. The free base of II was unstable and easily autoxidized to the 3-hydroxy compound IV. The mechanism of the formation of I is discussed.

Conformational change of all-trans-1,6-diphenyl-1,3,5-hexatriene in two crystalline forms

The crystal structures of all-trans-1,6-diphenyl-1,3,5-hexatriene in two polymorphic forms were examined by X-ray diffraction analysis at various temperatures. In both crystal forms, a misoriented minor conformer was observed at higher temperatures, and it disappeared at lower temperatures. The temperature dependence of the crystal structures was accounted for in terms of a conformational change through pedal motions in crystals.

Ethane Bond Length in 1,2-Diphenylethanes

Crystal structures of 1,2-diphenylethane (1), 1,2-bis(4-dimethoxyphenyl)ethane (3), 1,2-bis(2,3-dimethylphenyl)ethane (4), 1,2-bis(2,4-dimethylphenyl)ethane (5) and 1,2-bis(2,4,6-trimethylphenyl)ethane (6) were determined by X-ray diffraction analysis at various temperatures. An apparent shortening of the ethane bond and its temperature dependence were observed and it was interpreted as an artifact caused by the torsional vibration of the C—Ph bonds in crystals. The extent of the shortening was dependent on the compounds and was explained in terms of the difference in the amplitude of the torsional vibration controlled by ortho methyl groups. In the crystal structures of 1, a so far unrecognized orientational disorder was observed at room temperature and 315 K. The disorder, which disappeared at lower temperatures, proved to be dynamic and was ascribed to a conformational interconversion through a very large amplitude torsional vibration. The disorder might be another reason for the exceptionally short ethane bond in 1.