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Akira Sekiguchi

Publications and source records attributed to Akira Sekiguchi.

36 records · Page 2Linked to original sources
PubMed2004

Cyclobutadiene dianions consisting of heavier group 14 elements: synthesis and characterization.

The first "heavy" cyclobutadiene dianions (CBD2-), tetrakis(di-tert-butylmethylsilyl)-1,2-disila-3,4-digermacyclobutadiene dianion 22- and tetrakis(di-tert-butylmethylsilyl)tetrasilacyclobutadiene dianion 42-, have been synthesized by the reductive dehalogenation of the corresponding precursors with potassium graphite in THF. Structural characterization of [K+(thf)2]2.22- and [K+(thf)2]2.42- has been achieved by X-ray crystallography, showing that the central four-membered ring is significantly folded and has two eta2-1,3-coordinated potassium cations accommodated above and below the ring. The "heavy" CBD2-'s 22- and 42- were found to be nonaromatic compounds, a conclusion that was supported by nucleus-independent chemical shift (NICS) calculations showing a lack of diatropic ring current effects.

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PubMed2003

Tetrasilatetrahedranide: a silicon cage anion.

The first silicon cage anion, tris{bis[bis(trimethylsilyl)methyl](methyl)silyl}tetrasilatetrahedranide (6-), has been synthesized by the reaction of tetrakis{bis[bis(trimethylsilylmethyl](methyl)silyl}tetrasilatetrahedrane with potassium graphite in diethyl ether by reductive cleavage of exocyclic Si-Si bond. The structural characterization of K+(18-crown-6).6- has been achieved by X-ray crystallography, showing that 6- is a separated ion pair and the tetrasilatetrahedranide moiety has a significantly distorted tetrahedrane skeleton containing one inverted tetrahedral (umbrella type) silicon atom. The four silicon atoms in the Si4 skeleton are equivalent on the NMR time scale due to the migration of the Dis2MeSi substituent.

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PubMed2003

Tetrahedranyllithium: synthesis, characterization, and reactivity.

The first representative of stable tetrahedranyl anion, tris(trimethylsilyl)tetrahedranyllithium (3), has been synthesized by the reaction of tetrakis(trimethylsilyl)tetrahedrane (2) with methyllithium in tetrahydrofuran. The structural characterization of the tetrahedranyllithium has been achieved by X-ray crystallography, showing that the structure of 3.(TMEDA)1.5 represents a stretched tetrahedron. The endocyclic C(Li)-C(SiMe3) bond lengths range from 1.5408(15) to 1.5441(15) A (av 1.5425(15) A), and are longer than the endocyclic C(SiMe3)-C(SiMe3) bond lengths, which range from 1.4961(15) to 1.5009(15) A (av 1.4986(15) A). Methyl- and hydrogen-substituted tetrahedranes have also been prepared by the reaction of 3 with dimethyl sulfate and cyclopentadiene, respectively.

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PubMed2003

1,4,5-trigermabicyclo[2.1.0]pent-2-en-5-ylium: the isolable bishomocyclopropenylium ion containing a heavier group 14 element.

The isolable bishomoaromatic cation containing a heavier Group 14 element, (2,3,5-deloc)-2-phenyl-1,4,5-tris(tri-tert-butylsilyl)-1,4,5-trigermabicyclo[2.1.0]pent-2-en-5-ylium TPFPB- (2+.TPFPB-; TPFPB- = tetrakis(pentafluorophenyl)borate) was synthesized by the reaction of 5-bromo-2-phenyl-1,4,5-tris(tri-tert-butylsilyl)-1,4,5-trigermabicyclo[2.1.0]pent-2-ene (1) with [Et3Si(benzene)]+.TPFPB- in benzene at room temperature. The characteristic bishomoaromatic structure was unambiguously determined by X-ray crystallographic analysis. NMR measurements on 2+.TPFPB- confirmed the nonclassical germyl cation structure in solution and showed strong shielding of homoconjugative carbon atoms. The bishomoaromaticity in 2+ was also investigated by theoretical calculations.

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PubMed2003

Tin-centered radical and cation: stable and free.

The first stable stannyl radical (tBu2MeSi)3Sn* (1) has been synthesized by the reaction of tBu2MeSiNa with SnCl2-dioxane in diethyl ether. The X-ray crystal structure and electron paramagnetic resonance (EPR) data of this radical show that 1 has a planar geometry, being a pi-radical in both the solid and the liquid states. One-electron oxidation of 1 with Ph3C+.B(C6F5)4- in benzene quantitatively produced the corresponding cation (tBu2MeSi)3Sn+.B(C6F5)4- (2), representing the stable free stannylium ion that has been fully characterized by X-ray analysis and NMR data. Being free, 2 features a record downfield shifted resonance for stannylium ions: +2653 ppm.

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PubMed2003

A chemical trick: how to make a digermene from a disilene, formation of 3delta-1,2,3,4-disiladigermetene.

The unexpected reaction of tetrakis[di-tert-butyl(methyl)silyl]disilagermirenes (1a and 1b) with GeCl2.dioxane results in the quantitative formation of trans-1,2-dichloro-1,2,3,4-tetrakis[di-tert-butyl(methyl)silyl]-3Delta-1,2,3,4-disiladigermetene, 2, representing the first cyclotetrametallene containing two different heavier group 14 elements and the first digermene incorporated in a four-membered ring. trans-1,2-Dichloro-1,2,3,4-tetrakis[di-tert-butyl(methyl)silyl]-3Delta-1,2,3,4-disilagermastannetene (8), with a Ge=Sn double bond, was also prepared by the reaction of 1 with SnCl2.dioxane. The crystal structure of 2 was established by X-ray crystallography, which showed a folded four-membered ring skeleton with a long Ge=Ge double bond (2.2911(4) A). The structural peculiarities of 2, as well as the possible mechanism of its formation, are also discussed.

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PubMed2003

Photochemical isomerization of 1,2,5-trisilabicyclo[3.2.0]hepta-3,6-diene to 1,4,7-trisilabicyclo[2.2.1]hepta-2,5-diene.

Upon irradiation of a benzene-d6 solution of 1,2,2,5-tetrakis[di-tert-butyl(methyl)silyl]-4,7-diaryl- 1,2,5-trisilabicyclo-[3.2.0]hepta-3,6-diene [1a: aryl = phenyl, b: aryl = 3,5-bis-(trimethylsilyl)phenyl], 1,4,7,7-tetrakis[di-tert-butyl-(methyl)silyl]-2,5-diaryl-1,4,7- trisilabicyclo[2.2.1]hepta-2,5-diene (2a,b) was formed via skeletal rearrangement.

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PubMed2003

Bis(tri-tert-butylsilyl)silylene: triplet ground state silylene.

Upon irradiation with lambda = 254 nm light over the temperature range of 9-80 K, methylcyclohexane glass matrixes of 1a and 1b gave a characteristic broad EPR signal at 845 mT (X-band, 9.4 GHz) due to bis(tri-tert-butylsilyl)silylene, (tBu3Si)2Si: (2). The signal intensity as a function of temperature (9-80 K) gave a linear relation, and the spin multiplicity of 2 in the ground state was established to be a triplet. Product analysis, from which disilacyclobutane derivative 3 and dihydrosilane (tBu3Si)2SiH2 (4) were formed, also supports the conclusion about the multiplicity of 2.

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PubMed2002

Nearly planar nonsolvated monomeric silyl- and germyllithiums as a result of an intramolecular CH-Li agostic interaction.

Pale-yellow crystals of the nonsolvated monomeric silyl- and germyllithiums, tris[di-tert-butyl(methyl)silyl]silyllithium 2a and tris[di-tert-butyl(methyl)silyl]germyllithium 2b, were obtained by one-electron reduction of the corresponding silyl and germyl radicals with lithium in hexane. The crystal structure analysis of both 2a and 2b showed almost planar geometry around the anionic centers, due to both intramolecular CH-Li agostic interactions and steric reasons. However, the free anions [(tBu2MeSi)3Si-][Li+(THF)4] 3a and [(tBu2MeSi)3Ge-][Li+(THF)n] (n = 3, 4) 3b no longer showed a planar geometry, because of the absence of the intramolecular CH-Li agostic interaction. A temperature-dependent 1H NMR study of 2a showed that the CH-Li interaction is weak.

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PubMed2002

The first silastannene >si=sn<: a new doubly-bonded system of heavier group 14 elements.

The reaction of bis[di-tert-butyl(methyl)silyl]dilithiosilane 1 with dichlorobis(2,4,6-triisopropylphenyl)stannane in THF at room temperature yielded highly air- and moisture-sensitive deep violet crystals of 1,1-bis[di-tert-butyl(methyl)silyl]-2,2-bis(2,4,6-triisopropylphenyl)-1-sila-2-stannaethene 2 with a Si=Sn double bond. The molecular structure of 2 was established by X-ray crystallography, which showed a trans-bent structure (bending angles of 26.2 degrees for the sp2 Si atom and 9.6 degrees for the sp2 Sn atom) with a >Si=Sn< bond length of 2.4188(14) A and a twisting angle of 34.6 degrees . The geometry and regioselectivity of the addition reaction of PhEH (E = O and S) corresponds to the polarity of the double bond Sidelta-=Sndelta+, which is also supported by theoretical calculations on the model silastannene (H3Si)2Si=SnPh2.

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PubMed2002

Tetrakis(trimethylsilyl)tetrahedrane.

Tetrakis(trimethylsilyl)tetrahedrane 3 has been synthesized upon irradiation of tetrakis(trimethylsilyl)cyclobutadiene 8, which can be prepared either by thermal nitrogen elimination from trimethylsilyl[1,2,3-tris(trimethylsilyl)-2-cycloprop-1-enyl]diazomethane 7 or by mild oxidation of cyclobutadiene dianion 9 with 1,2-dibromoethane. The structural characterization of tetrahedrane 3 has been achieved by X-ray crystallography. The surprising thermal stability of 3 - which is stable up to 300 degrees C - is discussed.

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PubMed2002

2,4-Disila-1-germatricyclo[2.1.0.0(2,5)]pentane: a new type of cage compound of group 14 elements with an extremely long Ge-C bridge bond and an "Umbrella"-type configuration of a Ge atom.

The thermolysis of 1,1,2,3-tetrakis[di-tert-butyl(methyl)silyl]-4-phenyl-1,2-disila-3-germacyclopenta-2,4-diene 1 at 175 degrees C results in the quantitative formation of 1,2,3,4-tetrakis[di-tert-butyl(methyl)silyl]-5-phenyl-2,4-disila-1-germatricyclo[2.1.0. 02,5]pentane 2, representing a new type of cage compound of group 14 elements. The crystal structure of 2 was established by X-ray crystallography, which showed an extremely long Ge-C bridge bond of 2.242(3) A and an "umbrella"-type configuration of the Ge atom. The compound 2 readily reacted with benzaldehyde to produce 1,4,5,7-tetrakis[di-tert-butyl(methyl)silyl]-3,6-diphenyl-2-oxa-1,4-disila-5-germabicyclo[2.2.1]hept-5-ene 3 with an endocyclic Ge=C double bond.

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PubMed2002

Isolable silyl and germyl radicals lacking conjugation with pi-bonds: synthesis, characterization, and reactivity.

The one-electron oxidation reaction of tris[di-tert-butyl(methyl)silyl]silyl and -germyl anions with dichlorogermylene-dioxane complex results in the formation of stable tris[di-tert-butyl(methyl)silyl]silyl and -germyl radicals 1 and 2, representing the first isolable radical species of heavier Group 14 elements lacking stabilization by conjugation with pi-bonds. The crystal structures of both silyl and germyl radicals 1 and 2 showed a completely planar geometry around the radical centers. The ESR spectra of 1 and 2 showed strong signals with characteristic satellites due to the coupling with the 29Si and 73Ge nuclei. The small values of the hyperfine coupling constants a(29Si) and a(73Ge) clearly indicate the pi-character of both radicals, corresponding to a planar geometry and sp2 hybridization of the radical centers. Both 1 and 2 easily undergo halogenation reactions with carbon tetrachloride, 1,2-dibromoethane, and benzyl bromide to form the corresponding halosilanes and halogermanes.

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PubMed2002

The cation cluster of heavier group 14 elements: a free germyl cation with trishomoaromaticity.

The heavier group 14 element cation cluster compound, [(tBu3Si)6Ge10I]+.TTFPB- (3+. TTFPB-; TTFPB- = tetrakis(2,3,5,6-tetrafluorophenyl)borate), was unexpectedly obtained by thermolysis of in situ prepared iodocyclotrigermene, (tBu3Si)3Ge3I, in toluene in the presence of KI and K+.TTFPB-. The molecular structure of 3+.TTFPB- has been unambiguously determined by X-ray crystallography, showing that 3+ is a free germyl cation. The three "naked" cationic germanium atoms in the framework constitute a three-center two-electron (3c-2e) bond with a trishomocyclotrigermenylium character, which is well-supported by a theoretical calculation on the model compound Ge10H7+.

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PubMed2002

The first halogen-substituted cyclotrigermenes: a unique halogen walk over the three-membered ring skeleton and facial stereoselectivity in the Diels-Alder reaction.

Dark red crystals of the halogen-substituted cyclotrigermenes [(tBu3Si)3Ge3X; X = Cl, Br, I] were obtained in good yields by the reaction of [(tBu3Si)3Ge3]+.TTFPB- (TTFPB- = tetrakis(2,3,5,6-tetrafluorophenyl)borate) with potassium halides (KCl, KBr, or KI) in diethyl ether. The crystal structures of the halogen-substituted cyclotrigermenes reveal a cis-bent Ge=Ge double bond, caused by the introduction of the electronegative halogen atom on the sp3 germanium atom of cyclotrigermene. In solution, an intramolecular halogen migration over the three-membered ring skeleton was observed. Facial stereoselectivity in the Diels-Alder reaction of new cyclotrigermenes with 2,3-dimethyl-1,3-butadiene is also reported.

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