Yoshiyuki Mizuhata

Stable Heavier Carbene Analogues

In recent decades, it has generally been recognized that carbenes play an important role as transient intermediates. As a result of a number of stable carbenes having been isolated and investigated in detail, it is not an exaggeration to say that the chemistry of carbenes has been thoroughly investigated and is now well-understood.1 In addition, much attention has also been paid to the heavier analogues of carbenes, i.e., silylenes (R2Si:), germylenes (R2Ge:), stannylenes (R2Sn:), and plumbylenes (R2Pb:). These so-called metallylenes are monomeric species of the polymetallanes. This is especially true of the silylenes, which are believed to be monomers of polysilane. The metallylenes could be expected to be of great importance in fundamental and applied chemistry as a result of their many differences and similarities to carbenes. The valency of the central atom of the heavier carbene analogues (R2M:, M ) Si, Ge, Sn, Pb) is two. That is, its oxidation state is MII and its stability increases as the principal quantum number (n) increases. In fact, dichloroplumbylene and dichlorostannylene, PbCl2 and SnCl2, respectively, are very stable ionic compounds. However, these dihalides exist as polymers or ion pairs both in solution and in the solid state. The dichlorogermylene complex GeCl2 · (dioxane)3 is also known to be stable and isolable, whereas the dihalosilylenes are barely isolable compounds.2 The early silylene research was concerned largely with comparing the chemistry of the dihalosilylenes with that of carbenes. Hence, the chemistry of the metallylenes has been considered mainly from the molecular chemistry point of view.4 In contrast to the carbon atom, the heavier group 14 atoms have a low ability to form hybrid orbitals. They therefore prefer the (ns)2(np)2 valence electron configurations in their divalent species.5 Since two electrons remain as a singlet pair in the ns orbital, the ground state of H2M: (M ) Si, Ge, Sn, Pb) is a singlet, unlike the case of H2C:, where the ground state is a triplet (Figure 1).1a On the basis of theoretical calculations, the singlet-triplet energy differences ∆EST for H2M, [∆EST ) E(triplet) E(singlet)], are found to be 16.7 (M ) Si), 21.8 (M ) Ge), 24.8 (M ) Sn), and 34.8 (M ) Pb) kcal/mol, respectively. That of H2C: is estimated as -14.0 kcal/mol.6 Furthermore, the relative stabilities of the singlet species of R2M: (M ) C, Si, Ge, Sn, Pb; R ) alkyl or aryl) compared to the corresponding dimer, R2MdMR2, are estimated to increase as the element row descends, C < Si < Ge < Sn < Pb. It follows, therefore, that one can expect that a divalent organolead compound such as plumbylene should be isolable as a stable compound. However, some plumbylenes, without any electronic or steric stabilization effects, are known to be thermally unstable and undergo facile disproportionation reactions, giving rise to elemental lead and the corresponding tetravalent organolead compounds.7 On this basis, it could be concluded that it might be difficult to isolate metallylenes as stable compounds under ambient conditions, since they generally exhibit extremely high reactivity toward other molecules as well as themselves. Metallylenes have a singlet ground state with a vacant p-orbital and a lone pair of valence orbitals. This extremely high reactivity must be due to their vacant p-orbitals, since 6 valence electrons is less than the 8 electrons of the “octet rule”. Their lone pair is expected to be inert due to its high s-character. In order to stabilize metallylenes enough to be isolated, either some thermodynamic and/or kinetic stabilization of the reactive vacant p-orbital is required (Figure 2). A range of “isolable” metallylenes have been synthesized through the thermodynamic stabilization of coordinating Cp* ligands, the inclusion of heteroatoms such as N, O, and P, * To whom correspondence should be addressed. Phone: +81-774-38-3200. Fax: +81-774-38-3209. E-mail: tokitoh@boc.kuicr.kyoto-u.ac.jp. Chem. Rev. 2009, 109, 3479–3511 3479

Reactivity of an aryl-substituted silicon–silicon triple bond: 1,2-disilabenzenes from the reactions of a 1,2-diaryldisilyne with alkynes

The reactivity of a diaryl-substituted disilyne, Ar-Si[triple bond, length as m-dash]Si-Ar, with alkynes was examined. Reaction of the disilyne with acetylene yielded a 1,2-disilabenzene as the sole product.

Reactivity of an Aryl-Substituted Silicon―Silicon Triple Bond: Reactions of a 1,2-Diaryldisilyne with Alkenes

The reactivity of a bulky, diaryl-substituted disilyne, Ar-Si identical withSi-Ar, was examined for the first time. Reaction of the disilyne with ethylene yielded an ethylene-bridged bis(silacyclopropane), which is interpreted as a further reaction product of the initially formed 1,2-disilacyclobutene species with ethylene. A cyclohexane fused with a 1,2-disilacyclobutene was obtained in the reaction with cyclohexene. In the reaction with 2,3-dimethyl-1,3-butadiene, a tricyclo derivative was isolated from the complex product mixture.

Synthesis and properties of stable 2-metallanaphthalenes of heavier group 14 elements

The first stable neutral stannaaromatic compound, 2-stannanaphthalene , was synthesized by taking advantage of an extremely bulky and efficient steric protection group, 2,4,6-tris[bis(trimethylsilyl)methyl]phenyl (Tbt). The molecular structure and aromaticity of were discussed on the basis of X-ray crystallographic analysis, NMR, UV-vis, and Raman spectroscopy, cyclic voltammetry, and theoretical calculations. 2-Germanaphthalene , which has a framework similar to that of , was synthesized for comparison, and systematic elucidation was made for the properties of 2-metallanaphthalene systems containing a heavier group 14 element (Si, Ge, or Sn).

Germabenzenylpotassium: A Germanium Analogue of a Phenyl Anion

Reduction of the stable germabenzene, 1-Tbt-2-tert-butyl-germabenzene (Tbt=2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), with KC8 resulted in the formation of 2-tert-butylgermabenzenylpotassium, that is, the germanium analogue of phenylpotassium, under concomitant elimination of the aryl group from the Ge atom. Under an inert atmosphere, this germabenzenylpotassium could be isolated in the form of stable yellow crystals. In the crystalline state, as well as in solution, the germabenzenyl moiety adopts a monomeric form, even though the X-ray diffraction analysis suggests the presence of highly reactive Ge=C double bonds. The spectroscopic and X-ray crystallographic analyses, in combination with theoretical calculations indicate an ambident character for this germabenzenyl anion, with contributions from aromatic and germylene resonance structures.

Small-molecule-induced clustering of heparan sulfate promotes cell adhesion

Adhesamine is an organic small molecule that promotes adhesion and growth of cultured human cells by binding selectively to heparan sulfate on the cell surface. The present study combined chemical, physicochemical, and cell biological experiments, using adhesamine and its analogues, to examine the mechanism by which this dumbbell-shaped, non-peptidic molecule induces physiologically relevant cell adhesion. The results suggest that multiple adhesamine molecules cooperatively bind to heparan sulfate and induce its assembly, promoting clustering of heparan sulfate-bound syndecan-4 on the cell surface. A pilot study showed that adhesamine improved the viability and attachment of transplanted cells in mice. Further studies of adhesamine and other small molecules could lead to the design of assembly-inducing molecules for use in cell biology and cell therapy.

Synthesis and properties of a stable 6-stannapentafulvene.

The first donor-free 6-stannapentafulvene stable at ambient temperature was synthesized and isolated, exhibiting the shortest tin-carbon bond length among those previously reported.

Novel silacyclohexadienyl chromium and iron complexes bearing a bulky substituent on the central silicon atom

Novel hydrido(silacyclohexadienyl)chromium complexes bearing a bulky substituent, Tbt (2,4,6-tris[bis(trimethylsilyl)methyl]phenyl), were synthesized using formal insertion reactions of Cr(0) into C–H or Si–H bond. When the reactions of 1-silacyclohexa-2,4-dienes bearing a hydroxy or chloro group on the silicon atom with [Cr(CH3CN)3(CO)3] were performed, the corresponding hydrido(1-silacyclohexa-2,4-dienyl) complexes were obtained as the sole product. A similar reaction of a hydrosilane having a similar skeleton with [Cr(CH3CN)3(CO)3] gave an unprecedented type of silacyclohexadienyl complex, hydrido(1-silacyclohexa-1,3-dienyl)chromium bearing a three-center bonding interaction among the silicon, hydrogen, and chromium atoms. These are the first syntheses of silacyclohexadienyl complexes by the formal insertion reaction toward the corresponding silacyclohexadienes. Furthermore, the isolation of the silacyclohexadienyl anion was achieved; it was applied to synthesis of the corresponding sandwich-type iron complex. Structures of the newly obtained complexes were revealed based on their NMR and IR spectroscopic data and X-ray diffraction study.

Reactions of a kinetically stabilized 2-stannanaphthalene with elemental sulfur and selenium: synthesis of novel cyclic chalcogenides containing a tin atom

Novel five-membered cyclic trisulfide and 1,3-dihydrobenzo[c]selenophene derivatives containing a tin atom were synthesized by the reaction of a kinetically stabilized 2-stannanaphthalene with elemental sulfur and selenium, and their molecular structures were determined by X-ray crystallographic analysis. In addition, the results obtained for 2-stannanaphthalene were systematically compared with those obtained for germanium and silicon analogs.

Dodeca­allyl­hexa­sila­cyclo­hexa­ne

The molxadecule of the title compound, C36H60Si6, exhibits point group symmetry Ci, with the centre of inversion located at the centre of the Si6 ring. The Si6 ring has a chair conformation. In the crystal, molxadecules are linked via C—H⋯π(allxadyl) interxadactions.