Synthesis and characterization of the first stable α,ω-bis(silanetriols) containing phenylene, biphenylene, α,ω-bis(phenylene)vinylene, α,ω-bis(phenylene)ethynylene, and α,ω-bis(phenylene)buta-1,3-diynylene organic bridging groups are described. They were obtained by controlled hydrolysis of the corresponding bis(trialkoxysilyl) precursors. An alternative route involving phase-transfer catalysis is also reported. These solids were organized as layers. The triol end groups formed hydrophilic layers, whereas the body of the sheets corresponded to hydrophobic layers. The distance between two neighboring hydrophilic layers was directly related to the length of the organic group; nevertheless, the packing was induced by the hydrogen-bond network.
Methods for enantioselective transfer of carbenes starting from precursors carrying two carbonyl groups have been elaborated. A one-pot procedure for olefin cyclopropanation with CHacidic precursors via intermediate phenyliodonium ylides has been developed. The structure of the [Rh 2 {(S)-nttl} 4 ] catalyst was optimized to produce up to 98% ee in olefin cyclopropanations with dimethyl malonate or Meldrum's acid. Highly selective Rh(II)-catalyzed olefin cyclopropanations could be observed upon replacement of methyl diazoacetoacetate by methyl (silyloxyvinyl)diazoacetate. Enantioselective dipolar cycloadditions of diazopyruvate to polar olefins have been realized with Ru(II)-pybox catalysts.Asymmetric carbene transfer has seen spectacular development during the recent years thanks to the development of highly efficient and selective catalysts. 1 The reaction involves the transition-metal-induced decomposition of a diazo compound 2 to afford an intermediate metallocarbene, which subsequently transfers the carbene moiety to an appropriate substrate. The enantioselectivity of the reaction may be controlled by the chiral ligands surrounding the metal. Exceptional enantioselectivities have been reported for asymmetric transfer of diazoacetate esters 1a and amides 1b, having a single substituent on the diazo group. In contrast, the enantioselectivity of diazo compounds carrying two substituents of type 2 and 3 is more difficult to control (Figure 1). A notable exception to this is provided by vinyl-and aryl-substituted diazo esters 2c and 2d, respectively, with which impressive selectivities have been achieved. 3 Davies proposed that the unusual selectivity of the metallocarbenes derived from these latter precursors should be ascribed to a transition state occurring late on the reaction coordinate, as evidenced by the high negative r-value of -1.0 for cyclopropanation of styrene with Rh(II) catalysts. 4In contrast, diazo esters having a second electron-withdrawing substituent such as diazomalonate (2a) or 2-diazodimedone (3a) exhibit low selectivity. The corresponding metallocarbenes are highly electrophilic, and the transition state for carbene transfer occurs early on the reaction coordinate. Typically, a r-value of -0.3 has been reported for cyclopropanation of styrene with diazomalonate 2a. In addition, the steric requirements of disubstituted carbenes differ obviously from those of their mono-substituted counterparts, so that the need for different catalysts is not surprising. This paper summarizes efforts which we have carried out during the past years with the objective to tune Rh(II) catalysts in order to achieve enantioselective carbene transfer with diazo esters carrying a second electron-attracting substituent 2a, 2b, 3 and 4 (Figure 1) or the corresponding phenyl iodonium ylides. Alternatively, vinyl diazoacetates were used as synthetic equivalents of diazo acetoacetates in cyclopropanations and formal cycloadditions.
This paper concerns the polycondensation of the lamellar organized a,v-organo-bis-silanetriols, 4,4'-bis(trihydroxysilyl)stilbene 1H, 4,4'-bis(trihydroxysilyl)diphenylacetylene 2H, 4,4'bis(trihydroxysilyl)diphenylbuta-1,3-diyne 3H. A mild thermal polycondensation (180 uC) in the solid state leads to highly polycondensed (90%) non-porous solids (Ht) having a high density. As evidenced by X-ray powder diffraction analysis the initial organization of the starting bis-silanetriols on the nanometric scale is retained during the process. Moreover lamellar ordering into plates, observed in TEM, corresponds to a micrometric scale organization. An average size of the crystallites has been determined, corresponding to, respectively, 18 (1Ht) and 40 (2Ht and 3Ht) stacked layers per crystallite. These results show that it is possible, using the architecture of the molecular precursor as a scaffold, to prepare highly organized silicon based hybrid materials by a mild thermal polycondensation in the solid state.
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