Supported Au Nanoparticles-Catalyzed Regioselective Dehydrogenative Disilylation of Allenes by Dihydrosilane

Supported Au nanoparticles on TiO₂ catalyze the unprecedented dehydrogenative disilylation of monosubstituted and 1,1-disubstituted allenes by Et₂SiH₂ exclusively on the terminal double bond in a stereoselective manner. Treatment of the disilylation products with H₂O, in a one-pot operation also catalyzed by Au/TiO₂, leads to 3-alkylidene-1,2,5-oxadisilolanes, an unknown class of heterocyclic compounds, which are excellent scaffolds for the stereoselective synthesis of alkenes under Hiyama-type cross-coupling conditions.     

A distributed mass structural system for soil-structure-interaction and base isolation studies

A base isolation system placed within a structure for seismic protection will perform differently in the presence of soil-structure-interaction phenomena, as compared to the case where the structure is founded on competent soil and/or rock. This is so for two basic reasons: (a) there is filtering affecting the input ground motion signal at the base of the structure, and (b) the overall mechanical characteristics of the combined soil-structure system have changed as compared to the original structure resting on firm ground. Thus, a base isolation design has to account for soil-structure-interaction in order to have the system fine-tuned to the particular geological conditions and seismicity of the construction site in question. In this work, we introduce a distributed mass representation of the superstructure that has four possible dynamic response modes, namely flexure, shearing, torsion and axial vibrations. The foundation is treated as a rigid block, and the soil is represented by the equivalent spring-damper-virtual mass system, whereby these mechanical parameters may be frequency dependent. Typical base isolation systems used nowadays are lead rubber bearing designs whose mechanical behavior comprises a spring element, a damper and a hysteretic component. In terms of analysis, a substructuring methodology is employed that is cast in the frequency domain, with conditions of equilibrium and compatibility enforced at the common structure-foundation-soil boundaries. This approach is both efficient and adequate for investigating soil-structure-interaction effects, but cannot handle the hysteretic behavior of the base isolator, which is better captured by using time-stepping algorithms. In closing, some comments are made regarding possible ways for incorporating the base isolator in the present soil-structural system analysis, along with some preliminary results.     

Broadening the sense of ‘dynamic’: a microworld to support students’ mathematical generalisation

In this paper, we seek to broaden the sense in which the word ‘dynamic’ is applied to computational media. Focussing exclusively on the problem of design, the paper describes work in progress, which aims to build a computational system that supports students’ engagement with mathematical generalisation in a collaborative classroom environment by helping them to begin to see its power and to express it for themselves and for others. We present students’ strengths and challenges in appreciating structure and expressing generalities that inform our overall system design. We then describe the main features of the microworld that lies at the core of our system. In conclusion, we point to further steps in the design process to develop a system that is more adaptive to students’ and teachers’ actions and needs.     

Synthesis and study of a new diamidodipyrromethane macrocycle. An anion receptor with a high sulfate-to-nitrate binding selectivity

A new 2,6-diamidopyridinedipyrromethane hybrid macrocycle () has been synthesized that shows a high selectivity for dihydrogen phosphate and hydrogen sulfate relative to nitrate in acetonitrile solution as judged from UV-vis spectrophotometric titrations; this leads to the suggestion that this or related systems might find use in nuclear waste remediation applications requiring the selective removal of hydrogensulfate from nitrate-rich waste mixtures.     

Absolute vacuum ultraviolet absorption spectra of some gaseous azabenzenes

Absolute extinction coefficient and oscillator strength of pyridine, pyrimidine, pyrazine, and s-triazine were recorded with moderate resolution in the region of 3.5–9.5 eV. Analogous to ${}^1\text{A}{}_{\mathrm{<mtext>1</mtext><mtext>g</mtext>}}\text{→}{}^1\text{B}{}_{\mathrm{<mtext>2</mtext><mtext>u</mtext>}}$, ¹B_1u, ${}^1\text{E}{}_{\mathrm{<mtext>1</mtext><mtext>u</mtext>}}\text{π → π}{}^1$ transitions of benzene, several $\text{n → π}{}^1$ and Rydberg transitions are presented and discussed.     

Synchrotron radiation in archaeometry

January 2008 was the time of the official opening of SOLEIL's beamlines and guest house. This transition from construction to operational phase was made possible after commissioning of the 17°C water circuits of the beamlines. Users have access to eleven beamlines, from infrared to hard X-ray (AILES, SMIS, DESIRS, CASSIOPEE, TEMPO, DIFFABS, ODE, CRISTAL, SAMBA, PROXIMA1 and SWING). The LUCIA beamline, which has been operational on SLS since June 2004, will be installed at SOLEIL from mid-2008, and the XPEEM station of the future “soft X-ray microscopy” beamline is already producing fascinating XMCD and XMLD results at ELETTRA. Five beamlines are under construction.     

Reaction of hydrosilanes with alkynes catalyzed by gold nanoparticles supported on TiO2

Gold nanoparticles supported on TiO₂ (0.8–1.4 mol %) catalyze the β-(E) regioselective hydrosilylation of a variety of functionalized terminal alkynes with alkylhydrosilanes in 1,2-dichloroethane (70 °C). The product yields are excellent, and the reaction times relatively short, while almost equimolar amounts of alkynes and hydrosilanes can be used. Minor side-products in up to 35% relative yield of cis-oxidative (dehydrogenative) disilylation, an unprecedented reaction pathway, are formed in the cases of the less hindered hydrosilanes and alkynes. Triethoxysilane reacts faster and affords apart from β-(E) addition products, minor α-hydrosilylation regio-isomers in upto 15% relative yield. Internal alkynes are generally less reactive or even unreactive. It is proposed that cationic Au(I) species stabilized by the support are the reactive catalytic sites, forming in the presence of hydrosilanes either silyl–Au(III)–H (hydrosilylation pathway) or Au(III)–disilyl species (dehydrogenative disilylation pathway). Regarding the mechanism of hydrosilylation, kinetic experiments are in agreement with silyl carbometallation of the triple bond in the rate determining step of the reaction.     

Cyclization of 1,6-enynes catalyzed by gold nanoparticles supported on TiO2: significant changes in selectivity and mechanism, as compared to homogeneous Au-catalysis

Gold nanoparticles supported on TiO(2) (1.2 mol %) catalyze, for the first time under heterogeneous conditions, the cycloisomerization of a series of 1,6-enynes in high yields. In several cases, the product selectivity differs significantly as compared to homogeneous Au(I)-catalysis. Based on product analysis and stereoisotopic studies it is proposed that the major or exclusive pathway involves a 5-exo cyclization mode to form stereoselectively gold cyclopropyl carbenes that undergo a single cleavage pathway, in contrast to homogeneous Au-catalysis where the double cleavage pathway operates substantially.     

Photocycloaddition of biscyclopropyl alkenes to C60: an unprecedented approach toward cis-1 tricyclic-fused fullerenes

A novel, simple, and entirely regioselective tandem cycloaddition of biscyclopropyl-substituted alkenes to [60]fullerene has been revealed. This reaction affords cis-1 tricyclic-fused organofullerenes bearing the hitherto elusive 5-4-5 fused tricyclic ring system.