Takeaki Iwamoto

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.

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.

A Stable Dialkylphosphinyl Radical

Dialkylphosphinyl radical 1 was synthesized as thermally stable yellow crystals and found to be monomeric both in solution and in the solid state. EPR spectrum showed that the spin density of 1 is mainly localized on the 3p orbital of the dicoordinated phosphorus atom. A distinct absorption band due to the electronic transition from nonbonding electron pair orbital to singly occupied 3p orbital on the phosphorus atom of 1 was observed at 445 nm in solution. Phosphinyl radical 1 underwent facile reaction with carbon tetrachloride, hydrogen abstraction, and a unique reaction with a persistent radical, galvinoxyl, giving a cyclic phosphaalkene and a silylether.

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.

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

A Stable 1,2-Disilacyclohexene and Its 14-Electron Palladium(0) Complex

A novel stable cyclic disilene, 1,2-diphenyl-3,3,6,6-tetrakis(trimethylsilyl)-1,2-disilacyclohexene (1), was synthesized via the reaction of the corresponding 1,4-dilithiobutane with phenylchlorosilane. This new strategy would be applicable to the synthesis of stable cyclic disilenes having rather small residual substituents on unsaturated silicon atoms. A stable tricoordinate Y-shaped disilene palladium complex, 2, that was synthesized by the reaction of 1 with bis(tricyclohexylphosphine)palladium was found to have the strongest pi-complex character among known disilene palladium complexes.

Concise Synthesis of Halogenated Chrysenes ([4]Phenacenes) that Favor π-Stack Packing in Single Crystals

A concise synthesis of halogenated chrysene derivatives from 2,2,2-trifluoroethyl tosylate and halostyrene was established. Friedel-Crafts type cyclization of a 1,1-difluoro-1-alkene bearing halogenated phenyl moieties involved skeletal rearrangement and dehydrogenation to afford the title compound in good to moderate yield. X-ray analysis revealed that the halogen substituents induce pi-stack packing of the molecules in the crystals.

Ligand dependence of π-complex character in disilene–palladium complexes

The synthesis and structures of new 16-electron disilene palladium complexes with 2,6-dimethylphenyl isocyanide and phenyldimethylphosphine ligands [L(1)L(2)Pd{(t-BuMe(2)Si)(2)Si=Si(SiMe(2)Bu-t)(2)}, where L(1) = L(2) = PhMe(2)P; L(1) = (cyclohexyl)(3)P, L(2) = 2,6-dimethylphenyl isocyanide; L(1) = L(2) = 2,6-dimethylphenyl isocyanide] are described. Comparison of the X-ray structural parameters around the disilene moiety among these complexes and related bis(trimethylphosphine)(disilene)palladium and 14-electron (tricyclohexylphosphine)(disilene)palladium revealed that the pi-complex character is sensitive to the residual ligands and increases with decreasing the strength of sigma-donation from the ligands.

Persistent Antimony‐ and Bismuth‐Centered Radicals in Solution

Stibinyl and bismuthinyl radicals are recognized as representative intermediates of antimony and bismuth compounds, but still elusive in the condensed phase. We successfully synthesized persistent stibinyl and bismuthinyl radicals in solution by facile dissociation of the corresponding dimers with bulky substituents. We characterized the radicals by NMR and UV/Vis spectroscopy and estimated the thermodynamic parameters for the dissociation equilibria. The radicals show n→p (HOMO→SOMO) transition bands at 497 nm (stibinyl) and 543 nm (bismuthinyl) in 3-methylpentane and react with a stable nitroxyl radical to give the cross-radical coupling products in good yields.