Photochemically generated silicon—carbon double-bonded intermediates : XII. Some properties of silacyclopropenes and silapropadienes produced by photolysis of ethynylpolysilanes

The photolysis of six ethynyl-substituted polysilanes has been investigated in the presence or absence of methanol. Irradiation of 1-ethynyl-2-phenyl- and 1-ethynyl-1-phenyltetramethyldisilane (I and II), 1-ethynyl-1,1-diphenyltrimethyldisilane (III) and 2-ethynylheptamethyltrisilane (V) in the presence of methanol produced methoxysilanes arising from the corresponding reactive silacyclopropene and silapropadiene intermediates. Irradiation of 1-enthynylheptamethyltrisilane (IV) led to the formation of dimethylsilylene, ethynylpentamethyldisilane and 1,1-dimethyl-2-pentamethyldisilanyl-1-silacyclopropene intermediate. The photolysis of tris(trimethylsilyl)ethynylsilane (VI) in the presence of methanol produced 1-trimethylsilyl-1-bis(trimethylsilyl)methoxysilylethene and hexamethyldisilane. Photolysis of II and III in the absence of a trapping agent gave the respective bis(silyl)acetylenes, via a 1,2-hydrogen shift, from the transient silacyclopropenes, in moderate yields, whereas the yields of the rearranged acetylenes from IV and V were low.     

Asymmetric synthesis,structures, and chiroptical properties of helical cycloparaphenylenes

Planar chiral carbon nanorings and nanobelts (CNRs and CNBs), the sidewall segment molecules of chiral-type carbon nanotubes (CNTs), have attracted attention owing to their characteristic chiroptical properties. From the appropriate CNTs, axially or planar chiral CNRs and CNBs have been designed and synthesized, but multiply helical sidewall segments were almost unexplored due to the difficulty in simultaneous control of multiple chiralities. In this article, we have succeeded in the perfectly diastereo- and enantiocontrolled catalytic synthesis of a cycloparaphenylene with four helical and two planar chiralities showing good chiroptical responses as chiral organic molecules. The perfectly stereocontrolled multiply helical structure was confirmed by a single-crystal X-ray diffraction analysis. The experimental and theoretical studies established the importance of the highly symmetric multiply helical structure in the cylindrical axis in obtaining good chiroptical responses.

The perfectly diastereo- and enantiocontrolled catalytic synthesis of a cycloparaphenylene with four helical and two planar chiralities showing good chiroptical responses was achieved by the rhodium-catalyzed alkyne cyclotrimerization.     

Experimental and computational study of the electronic structural changes in LiTi2O4 spinel compounds upon electrochemical Li insertion reactions

Electronic structural changes in LiTi(2)O(4) spinel compounds upon electrochemical lithium insertions were investigated by X-ray absorption spectroscopy (XAS) measurements and first principles calculations based on spin-polarized density functional theory. Ti K-edge, O K-edge XAS spectra and theoretical calculations indicate that oxide ions as well as titanium ions are involved in electronic structural changes caused by electrochemical lithium ion insertions. The considerable effect of the oxide ions in the early 3d transition metal (titanium) oxide system is discussed in this article.     

Creation and characteristics of phosphatidylcholine stationary phases for the chromatographic separation of inorganic anions

New stationary phases for chromatographic separation of anions, obtained by loading liposomes made from dimyristolyphosphatidylcholine (DMPC) onto reversed-phase packed columns (C18 and C30) are reported. Mono- and divalent anions were used as model analyte ions and retention data for these species were obtained using the DMPC stationary phases and used to elucidate the separation mechanisms involved in this chromatographic system. The DMPC stationary phases can separate anions by either a solvation-dependent mechanism or an electrostatic ion-exchange mechanism, depending upon the relative magnitudes of the negative electrostatic potential (Psi(-)) of the phosphate moiety (P-) and the positive electrostatic potential (Psi(+)) of the quaternary ammonium groups (N+) on the headgroup of DMPC. If Psi(+) > Psi(-), such as in case where Psi(-) has been reduced either by binding of eluent cations (e.g., H+ or divalent cations) onto the P- group of DMPC or by steric screening when a C30 reversed-phase material was used to support the DMPC, then the overall electrostatic surface potential (and hence also the effective anion-exchange capacity) was generally large and the anions were separated on the basis of an electrostatic mechanism. However, if Psi(+) was similar to Psi(-), such as in the case of using a C18 reversed-phase support and monovalent cations as eluent cations, then the overall electrostatic surface potential and the effective anion-exchange capacity were very small and the analyte anions were separated on the basis of a solvation-dependent mechanism. The DMPC stationary phases were found to be suitable for the direct determination of iodide and thiocyanate in highly saline water samples, such as seawater samples.     

Dodecylsulfate-coated monolithic octadecyl-bonded silica stationary phase for high-speed separation of hydrogen, magnesium and calcium in rainwater

The high-speed determination of hydrogen, magnesium and calcium ions by ion chromatography (IC) is demonstrated on a monolithic octadecyl-boned silica (ODS) column coated with lithium dodecylsulfate (Li-DS). This stationary phase, when used in conjunction with a 2 mM ethylenediamine and 0.1 mM Li-DS solution as eluent at pH 6.0, was found to be suitable for the rapid and efficient separation of hydrogen and magnesium and calcium in the order H+ < Mg2+ < Ca2+ within 4 min at a flow rate of 4.0 ml/min. Under the conditions, linear calibration plots of conductivity versus concentration were obtained for the cations over about three orders of magnitude, and the detection limits were 1 microM for H+, 2 microM for Mg2+ and Ca2+. Rainwater was analyzed directly using this IC system with satisfactory results.     

Characterization of geometrid sex pheromones by electrospray ionization time-of-flight mass spectrometry

The utility of liquid chromatography combined with time-of-flight mass spectrometry (LC/TOFMS) was demonstrated for studies on chiral unsaturated epoxy compounds, sex pheromones produced mainly by female moths in the family Geometridae. By electrospray ionization (ESI), each synthetic epoxyalkadiene derived from (Z,Z,Z)-3,6,9-triene with a C(18)-C(23) straight chain showed three ion series, [M + NH(4)](+), [M + H](+) and [M - OH](+), with high resolution and good sensitivity, indicating its molecular formula. In addition to these, characteristic fragment ions at m/z M - 57 and M - 71 for the 3,4-epoxides and at m/z M - 123 and 123 for the 9,10-epoxides were detected, whereas the 6,7-epoxides did not produce fragment ions that reflected their structures. Monitoring these diagnostic ions during the LC/MS analysis of a gland extract, the natural sex pheromone of the mulberry looper was confirmed to be (Z,Z)-cis-9,10-epoxy-3,6-octadecadiene, which was separable from the other positional isomers on an ODS column. Furthermore, (Z,Z)-cis-3,4-epoxy-6,9-nonadecadiene secreted by the Japanese giant looper was analyzed with a chiral column, and the stereochemistry was determined directly.     

Photolysis of organopolysilanes. Photochemical formation and reactions of 1-trimethylsilyl-1-phenyl-1-silacyclopropene derivatives

The photolysis of tris(trimethylsilyl)phenylsilane (I) in the presence of 1-hexyne, 3,3-dimethyl-1-butyne, trimethylsilylacetylene, 3-hexyne, 1-trimethylsilylpropyne, 1,2-bis(trimethylsilyl)acetylene and 2,2,5,5-tetramethyl-3-hexyne afforded the respective silacyclopropenes. The silacyclopropenes produced from monosubstituted acetylenes underwent photochemical isomerization to give disilanylacetylene derivatives, via a 1,2-hydrogen shift in the silacyclopropene ring. Irradiation of I in the presence of 3-hexyne, 1-trimethylsilylpropyne or 2,2,5,5-tetramethyl-3-hexyne, gave the corresponding silacyclopropenes which could be isolated by preparative GLC. The silacyclopropene from 1,2-bis(trimethylsilyl)acetylene, however, readily underwent thermal rearrangement to give [bis(trimethylsilyl)phenylsily] trimethylsilylacetylene via a 1,2-trimethylsilyl shift. This type of rearrangement was also found in the photochemical process.     

Photochromic isomerization of a dinuclear molybdenum complex with ethylene-1,2-dithiolate and disulfur ligands: x-ray structures of the two isomers

A photochromic complex with disulfur and dimethyl-ethylene-1,2-dithiolate ligands, [Mo(2)(mu-S(2))(mu-S(2)C(2)Me(2))(2)(S(2)C(2)Me(2))(2)] (3), was synthesized and characterized. Photoirradiation of 3 with visible light resulted in the formation of the isomer (3'). The electronic spectrum of 3' has a new intense peak in the near infrared region, and in the dark, the spectrum returns to that of 3. X-ray structural analyses of 3.C(6)H(6) and 3' revealed a large conformational change of the bridging dithiolate ligands: the two ligands in 3' come very close to each other compared to those in 3.C(6)H(6). Crystal data: 3.C(6)H(6), monoclinic, space group C2/c, a = 15.193(4) A, b = 14.287(3) A, c = 14.685(4) A, beta = 105.30(1) degrees, V = 3074(1) A(3), Z = 4; 3', monoclinic, space group C2/c, a = 21.5400(8) A, b = 9.5232(5) A, c = 13.9828(2) A, beta = 118.924(1) degrees, V = 2510.5(2) A(3), Z = 4. (1)H NMR spectra of 3 (3.06, 3.05, 1.66, and 1.31 ppm) and 3' (2.90, 2.75, 2.14, and 1.97 ppm) are also reported: each spectrum has four signals due to methyl groups, which accords well with the fact that each of the molecules, 3.C(6)H(6) and 3', has a crystallographic 2-fold axis.     

Electric-field-induced conductance switching in FeCo Prussian blue analogues

FeCo Prussian blue analogues, which are known as typical molecule-based magnets, exhibited abrupt conductance switching by applying a high electric field as well as by varying the temperature. The current density versus electric field (J-E) curves of FeCo Prussian blue with Rb cations in interstitial sites shows so-called negative resistance effects at electric fields higher than the threshold voltage. This means that the FeCo Prussian blue analogues are multiproperty materials in the sense that their conducting, magnetic, and optical properties can be reversibly controlled by certain external stimuli.     

Chemistry of a 1-silacyclopropene. Thermal production of 1,4-disilacyclohexa-2,5-dienes

Thermolysis of 1,1-dimethyl-2-phenyl-3-trimethylsilyl-1-silacyclopropene in the presence or absence of an alkyne such as diphenylacetylene or ethyl-dimethylsilylphenylacetylene gave a mixture of 1,1,4,4-tetramethyl-2,5-diphenyl-3,6-bis(trimethylsilyl- and 1,1,4,4-tetramethyl-2,6-diphenyl-3,5-bis(trimethylsilyl)-1,4-disilacyclohexa-2,5-diene in high yield. The formation of the 1,4-disilacyclohexa-2,5-diene system can be best rationalized in terms of a mechanism involving direct dimerization of the 1-silacyclopropene.