Showing papers by "Chizuko Kabuto published in 2003"

A stable silicon-based allene analogue with a formally sp -hybridized silicon atom

Abstract: Carbon chemistry exhibits a rich variety in bonding patterns, with homo- or heteronuclear multiple bonds involving sp-hybridized carbon atoms as found in molecules such as acetylenes, nitriles, allenes and carbon dioxide. Carbon's heavier homologues in group 14 of the periodic table—including silicon, germanium and tin—were long thought incapable of forming multiple bonds because of the less effective pπ–pπ orbital overlap involved in the multiple bonds. However, bulky substituents can protect unsaturated bonds and stabilize compounds with formally sp-hybridized heavy group-14 atoms1,2: stable germanium2, tin3 and lead4 analogues of acetylene derivatives and a marginally stable tristannaallene5 have now been reported. However, no stable silicon compounds with formal sp-silicon atoms have been isolated. Evidence for the existence of a persistent disilaacetylene6 and trapping7 of transient 2-silaallenes and other X = Si = X′ type compounds (X, X′ = O, CR2, NR, and so on) are also known, but stable silicon compounds with formally sp-hybridized silicon atoms have not yet been isolated. Here we report the synthesis of a thermally stable, crystalline trisilaallene derivative containing a formally sp-hybridized silicon atom. We find that, in contrast to linear carbon allenes, the trisilaallene is significantly bent. The central silicon in the molecule is dynamically disordered, which we ascribe to ready rotation of the central silicon atom around the molecular axis.

Addition of stable nitroxide radical to stable divalent compounds of heavier group 14 elements.

Abstract: The reactions of stable cyclic dialkylgermylene 2 and dialkylstannylene 3 with 2,2,6,6-tetramethylpiperidinyl-1-oxy (TEMPO) radical (2 equiv) gave the corresponding 1:2 adducts 4 and 5, respectively, which were characterized by NMR, MS, and X-ray analyses. The kinetics of the stepwise addition of two TEMPO molecules to germylene 2 revealed that the initial addition of TEMPO to 2 was 1010 times slower than the second TEMPO addition to the resulting germyl radical. The origin of the rate difference was discussed on the basis of the qualitative perturbation theory. In contrast to the reactions of 2 and 3, the reaction of dialkylsilylene 1 with TEMPO gave an interesting 1,3-dioxadisiletane derivative.

Synthesis of all stereoisomers of sulfinylcalix[4]arenes.

Abstract: All four stereoisomers of p-tert-butylsulfinylcalix[4]arene [4(rccc), 4(rcct), 4(rctt), and 4(rtct)], arising from the disposition of the four sulfinyl groups with respect to the mean plane of the four bridging sulfur atoms, have been prepared via oxidation of p-tert-butylthiacalix[4]arene (2) or its tetra-O-benzyl ethers 5 of defined conformations. Thus, treatment of 2 with 4.4 molar equiv of NaBO3·4H2O gave the rtct and rctt isomers in 27% and 17% yields, respectively, while oxidation of cone 5 (5C) and partial cone 5 (5PC) proceeded stereoselectively to give, after debenzylation of the resulting tetrasulfoxides 12 and 15, the rccc and rcct isomers in 56% and 28% yields, respectively, based on 5. The sulfinylcalix[4]arenes 4 were treated with iodomethane in the presence of a base to give the corresponding tetramethyl ethers 16, the structures of which in regard to the disposition of the sulfinyl groups and the conformation of the phenol units were determined by X-ray crystallography. Also reported is th...

Novel η3-1-Silaallyl Tungsten Complexes via Si−H Bond Activation of Hydrovinylsilanes: Structure and Reactivity toward Methanol

Abstract: Reactions of cis-Cp*(CO)2W(MeCN)Me (1) with HSiMe2(CHCR2) (R = H, Me) afford the novel η3-1-silaallyl complexes Cp*(CO)2W(η3-Me2SiCHCR2) [R = H (2), Me (3)] accompanied by liberation of MeCN and CH4 via thermal Si−H bond activation. η3-Coordination and exo conformation of the 1-silaallyl ligand in 3 are shown by X-ray crystal analysis, which reveals the partial double bond character of the Si−C bond (1.800(4) A) in the silaallyl moiety. Complexes 2 and 3 show extremely high reactivity toward MeOH to give the hydrido-(methoxysilyl)alkene complex trans-Cp*(CO)2WH(η2-MeOMe2SiCHCH2) (4) and the four-membered metallacycle (6), respectively.

Comparison of structures between platinum and palladium complexes of a tetrasilyldisilene

Abstract: The first η2-disilene–palladium complexes were synthesized using two different reactions; the reactions of bis(phosphine)dichloropalladiums with a 1,2-dilithiotetrakis(trialkylsilyl)disilane, which...

Pd(II) Complexes with thiacalix[4]-arene and -aniline; subtle, but distinct influences of phenol and aniline units on the 3-D structure

Abstract: The Pd(II) complexes of thiacalix[4]aniline (TCAn) and thiacalix[4]arene (TCAr), in which four aniline and phenol units are linked by four sulfides at the o,o′-positions, respectively, were prepared by simply heating the ligands with Pd(II)(OAc)2 in CHCl3 or benzene at reflux and structurally characterized by X-ray diffraction methods. In TCAn, two thiacalix ligands adopting a pinched-cone conformation are fused at the narrower rims to coordinate to two Pd(II) ions by amide NH− and the adjacent S atoms. In contrast for TCAr, two ligands coordinate to two Pd(II) ions with phenolate O− and thioether S atoms at the narrower rims. Four sets of hydrogen bonding between the O− and free OH cause a deviation of the Pd(II) centers from the square coordination plane and also lead the conformation of the calix to be more cone-like to provide enough space to accommodate guest CH3CN molecules.

Insertion of a stable dialkylsilylene into silicon-chlorine bonds

Abstract: The reactions of a stable dialkylsilylene with chlorosilanes such as tetrachlorosilane, dimethyldichlorosilane, and dichlorosilane occurred smoothly at room temperature in hydrocarbon solvents to give the corresponding Si-Cl bond insertion products. In the reaction of the silylene with dichlorosilane, only the Si-Cl bond insertion product was obtained, while a similar reaction with dimethylchlorosilane gave only the Si-H insertion product, emphasizing the remarkable difference in the steric requirements between these two insertion reactions. No reaction took place during the treatment of the silylene with trimethylchlorosilane. The Si-Cl insertion reactions are expected to be applied in the synthesis of new organosilicon frameworks that cannot be obtained by conventional methods.

A stable silicon congener of highly strained bicyclo[3.2.0]hepta-1,3,6-triene.

Abstract: A silicon-containing fused bicyclic compound with a highly strained bridgehead double bond, 2,3,6,7-tetra-tert-butyl-4-(tert-butyldimethylsilyl)-5-(tert-butyldimethylsiloxy)-5-silabicyclo[3.2.0]hepta-1,3,6-triene (2), was synthesized quantitatively by the reaction of 1,2-bis-tert-butyl-4,4-bis(tert-butyldimethylsilyl)-4-silatriafulvene (3) with di-tert-butylcyclopropenone (4) at 80 degrees C. An X-ray crystallographic analysis for 2 not only confirmed a bicyclic structure having a silacyclopentadiene (silole) ring fused with a silacyclobutene ring but also the remarkable deformation around the double bonds; the sum of the bond angles around the unsaturated bridgehead carbon was 333 degrees . The strain energy of a model 5-silabicyclo[3.2.0]hepta-1,3,6-triene was calculated at the MP2/6-31+G(d,p)//B3LYP/6-31+G(d) level (30.2 kcal/mol) to be comparable to that for parent bicyclo[3.2.0]hepta-1,3,6-triene (30.7 kcal/mol). Despite the high steric strain, 2 was stable enough to be kept intact for several months in the air. The high stability is ascribed to the effective steric protection of the ring system by the bulky substituents.

Synthesis, stability and bonding situation of tris-, bis- and mono[9-(azuleno[1,2-b]thienyl)]methyl cations.

Abstract: Stable tris-, bis- and mono[9-(azuleno[1,2-b]thienyl)]methyl cations (7a, 8a and 9a) and their derivatives, with a 6-isopropyl substituent on each azuleno[1,2-b]thiophene ring (7b, 8b and 9b) were prepared by the hydride abstraction reaction of the corresponding methane derivatives. The bonding situation of these compounds including the methane derivatives was examined by analysis of the 3J(H,H) values for the seven-membered ring from the 1H NMR spectra. The methane derivatives exhibited a significant alternating pattern in the 3J(H,H) values, which indicated that the π-system of the azulene core is perturbed by the fused thiophene ring, showing a tendency towards a localized heptafulvene substructure. The 3J(H,H) values of 7b and 8b in the seven-membered ring revealed that the alternating C–C bond lengths in the azulene core still existed. The cations 9a and 9b, which exhibited nearly equal 3J(H,H) values in the seven-membered ring, exhibit the development of a delocalized tropylium substructure in the azulene core. X-ray crystal analysis of 6-isopropylazuleno[1,2-b]thiophene revealed substantial bond-length alternation in the seven-membered ring. Significant bond-length equalization in the seven-membered ring was also confirmed by the X-ray crystal analysis of 9b. The stability of these carbocations was examined by measurement of the pKR+ values and the redox potentials, which revealed that the bond-length alternation in the azulene core does not significantly affect the stability of the carbocations.

A New Chiral Pyrrolidine- and Pyrrolidi- noneethanols for Enantioselective Addition of Diethylzinc to Arylaldehydes

Abstract: New chiral pyrrolidineethanols and unusual type of pyrrolidinoneethanol fused bicycle[2.2.2]octane ring systems as backbones were synthesized and were used to the addition of diethylzinc to aromatic aldehydes to afford the secondary aryl alcohols leading to enantioselectivities up to 94% ee.

Synthesis of Tungsten Thienyl Complexes via C–H Bond Activation of Thiophenes

Abstract: The reactions of cis-Cp∗W(CO)2(MeCN)Me (1) with thiophene or 3-methylthiophene resulted in selective α-C–H bond activation to give cis-Cp∗W(CO)2(MeCN)R [R = 2-C4H3S (2a), 2-C4H2S-4-Me (2b)]. 2a reacted with 3-methylthiophene to afford thienyl exchange products 2b and thiophene in addition to decomposition products. Reversibility of the exchange reaction was shown by the reaction of 2b with thiophene.

Sila‐metalation Route to Hydrido(trialkylsilyl)silyllithiums.

Abstract: The proton abstraction (sila-metalation) of trialkylsilyl-substituted dihydridosilanes with t-BuLi or LDA in THF was found to be a convenient route to the corresponding silyllithiums (RR‘SiHLi; 1a, R, R‘ = t-BuMe2Si; 1b, R, R‘ = Me3Si; 1c, R, R‘ = i-Pr2MeSi; 1d, R = t-BuMe2Si, R‘ = 4-methylphenyl). Hydridosilylithium 1a was isolated as air- and moisture-sensitive, but thermally stable, colorless crystals. X-ray analysis has shown that 1a is dimeric in the solid state, where two lithium atoms bridge between anionic silicon atoms forming a parallelogram, each lithium atom is coordinated by one THF molecule, and the Si−H hydrogen atoms are in the plane of the parallelogram. X-ray analysis has shown that (t-BuMe2Si)2GeHLi (5) has a dimeric structure similar to that of 1a.

Synthesis of Optically Active Bihelicenols.

Abstract: All six stereoisomers of dimethyl 5,5′-dihydroxy-1,1′,12,12′-tetramethyl-[6,6′]bi(benzo[c]phenanthrenyl)-8,8′-dicarboxylate (bihelicenol) were synthesized by the oxidative coupling of methyl 8-hydroxy-1,12-dimethylbenzo[c]phenanthrene-5-carboxylate (helicenol), and their structures were determined by X-ray analysis.

“Thiacalix[4]aniline” as a Highly Specific Extractant for Au(III) and Pd(II) Ions.

Abstract: Thiacalix[4]aniline (4), a cyclic tetramer of p-tert-butylaniline bridged with four sulfides, extracted Au(III) and Pd(II) ions specifically from acidic solutions among 41 metal ions including soft metal ions such as Hg(II), Cd(II), Zn(II), Pb(II), and Cu(II).

Dinuclear Titanium(IV) Complex of p-tert-Butylthiacalix[4]arene as a Novel Bidentate Lewis Acid Catalyst.

Abstract: Treatment of p - tert -butylthiacalix[4]arene (H 4 L, 2 ) with TiCl 4 in dichloromethane gave two novel dinuclear titanium(IV) complexes formulated as [Ti 2 LCl 4 ]. One of the complexes was subjected to X-ray crystallographic analysis to show that the calixarene ligand (L 4− ) adopted a cone conformation, forcing the two metal centers to reside in close vicinity with syn arrangement with respect to the mean plane defined by the macrocycle ( 3 ). The other one was assigned to be an anti titanium(IV) complex ( 4 ) based on 1 H NMR spectroscopy. The syn complex showed high catalytic activity in the Mukaiyama–aldol reaction of aromatic aldehydes with silyl enol ethers, indicating the double-activation ability of the bidentate Lewis acid toward the aldehydes.