Mitsuo Kira

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

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. Carbons 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 atoms: stable germanium, tin and lead analogues of acetylene derivatives and a marginally stable tristannaallene have now been reported. However, no stable silicon compounds with formal sp-silicon atoms have been isolated. Evidence for the existence of a persistent disilaacetylene and trapping 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.

Progress in the Chemistry of Stable Disilenes

Publisher Summary Remarkable progress of the chemistry of disilenes seems to encourage building up the modern general theory of bonding, structure, and reactions applied for the heavier main-group element chemistry in the near future; the general theory should include the theory for organic chemistry as a special case. This chapter discusses present advances in novel synthetic methods for various types of stable disilenes. Stable disilenes and other unsaturated compounds of heavier group-14 elements show unique spectroscopic properties that cannot be easily obtained from organic compounds. Bonding and structures of disilenes have also been discussed in the chapter. For a detailed discussion on the structure of disilenes, it is helpful to understand the intrinsic difference in the bonding between disilene and ethylene. The chapter highlights reactions and mechanisms of disilenes. During the past decade, many new types of reactions have been found for novel types of disilenes such as cyclic and bicyclic disilenes and conjugated tetrasiladienes. Mechanistic studies have recently been performed on several fundamental reactions of disilenes both theoretically and experimentally, and have greatly deepened the understanding of their reaction pathways, their potential energy surfaces, the factors determining the rates and stereochemistry, and so on.

An isolable dialkylsilylene and its derivatives. A step toward comprehension of heavy unsaturated bonds

Synthesis and structural characteristics of an isolable dialkylsilylene and its application for the synthesis of unique silicon unsaturated compounds such as silanechalcogenones, silaketenimines, a tetrasila-1,3-diene and a trisilaallene are discussed. Because the silylene has no aromatic or alkenyl substituents, the derivatives are mostly free from the influence of the pi-conjugative substituents, and hence, are suitable for exploring the intrinsic or standard characteristics of the unsaturated bonds. Using perturbation molecular orbital (PMO) theory, the importance of pi-sigma* mixing is emphasized to figure out the key issues of heavy unsaturated bonds.

Regiospecific and highly stereoselective allylation of aldehydes with allyltrifluorosilane activated by fluoride ions

Abstract Allyltrifluorosilane/CsF systems form pentacoordinate allylsiliconates which undergo chemoselective, regiospecific and highly stereoselective allylation of aldehydes, presumably via six-membered cyclic transition states.

Anthryl-Substituted Trialkyldisilene Showing Distinct Intramolecular Charge-Transfer Transition

1-Naphthyl-, 9-phenanthryl-, and 9-anthryl-substituted trialkyldisilenes 1-3 were synthesized as the first stable disilenes with single polycyclic aromatic substituents, allowing elucidation of the unprecedented intramolecular charge transfer interaction between disilene pi and aromatic pi systems. Anthryl-substituted disilene 3 having a low-lying pi*(aryl) LUMO showed a distinct ICT absorption band due to the charge transfer from a disilene pi donor to an aromatic pi acceptor.

Stereohomogeneous synthesis of (E)- and (Z)-crotyltrifluorosilanes and highly stereoselective allylation of aldehydes☆

Abstract Stereohomogeneous (E)- and (Z)-crotyltrifluorosilanes were prepared and used for the highly diastereoselective synthesis of threo- and erythro-β-methylhomoallyl alcohols, respectively, from aldehydes in the presence of fluoride ions. The mechanism of the reaction was discussed.

Stable cyclic and acyclic persilyldisilenes

Abstract Synthesis, structure, and reactions of persilylated cyclic and acyclic disilenes that we have synthesized are described in detail. Due to the remarkable electronic and steric effects of trialkylsilyl substituents, tetrakis(trialkylsilyl)disilenes (RR′SiSiRR′: R, R′= i -Pr 2 MeSi ( 1a ), t -BuMe 2 Si ( 1b ), i -Pr 3 Si ( 1c ); R= i -Pr 2 MeSi, R′= t -BuMe 2 Si ( 1d )) showed interesting features in the geometry around SiSi bond, electronic spectra, 29 Si-NMR resonances of the unsaturated silicon nuclei, E , Z -isomerization, reduction by alkali metals, and various bimolecular reactions with alcohols, alkyl lithium, alkenes, alkynes, and haloalkanes. As the first cyclooligosilenes, 1-tris( t -butyldimethylsilyl)silyl-2,3,3-tris( t -butyldimethylsilyl)cyclotrisilene ( 2 ) and hexakis( t -butyldimethylsilyl)cyclotetrasilene ( 3 ) were synthesized typically by the reduction of the corresponding tris(trialkylsilyl)silyltrihalosilane with potassium graphite in THF and with sodium in toluene, respectively. Photochemical conversion of 3 to the corresponding bicyclo[1.1.0]butane ( 9 ) and its thermal reversion to 3 , as well as photochemical isomerization of 2 to 9 were observed.

Introduction of Clutch Function into a Molecular Gear System by Silane−Silicate Interconversion

Introduction of the clutch-declutch mechanism into a new gear system, bis(4-methyl-9-triptycyl)difluorosilane 1, is achieved by the reversible attachment of fluoride ion giving the corresponding fluorosilicate 2. Although the phase isomers of 1 (1(dl) and 1(meso)) cannot be separated because of the equilibrium via a slow gear slippage process (DeltaH(double dagger) = 17.2 +/- 0.2 kcal x mol(-1) and DeltaS(double dagger) = 0.9 +/- 0.9 cal x mol(-1) x K(-1)), 1 works as meshed molecular gears in solution at room temperature. On the other hand, silicate 2 in the solid state has quite an unusual TBP structure having two organic triptycyl groups at the apical positions and three electronegative fluorine atoms at the equatorial positions against the Muetterties rule. Rotation of the two triptycyl groups around Si-C bonds in 2 is facile and independent to each other in solution. Silicate 2 is reverted to the corresponding silane mixture by treating with excess water.

Organogermanium compounds VI. Electron spin resonance spectra of aryl-substituted germyl radicals.

Abstract Electron spin resonance spectra of five aryl-substituted germyl radicals are recorded. For phenyl-substituted radicals, Ph3Ge·, Ph2MeGe·, and PhMe2Ge·, π-spin populations are calculated by the McLachlan MO method. In Ph3Ge·, 82% of the spin is localized at germanium which reflects the small extent of 4p-2p conjugation. An excellent linear relation between g factors and spin densities at germanium was found. These observations provide the first evidence showing the spin delocalization through 4p-2p conjugation. The tri-p-tolylgermyl and trimesitylgermyl radicals also were studied by esr.

Fourteen‐Electron Bis(dialkylsilylene)palladium and Twelve‐Electron Bis(dialkylsilyl)palladium Complexes

Stable transition-metal complexes with divalent silicon ligands (silylene complexes) have been extensively studied because of their important role in many catalytic processes. Since the pioneering works by Zybill et al. and Tilley et al. , various base-stabilized and base-free silylene complexes have been synthesized and their versatile reactivity has been well explored. Although complexes with two or more silylene ligands are expected to show interesting bonding properties and reactivities that are not observed in monosilylene complexes, such complexes are still limited to donorbridged bis(silylene) complexes and complexes having cyclic diaminosilylenes as ligands. During the course of our study on the application of dialkylsilylene 1, which is the