Near-Enantiopure Trimerization of 9-Ethynylphenanthrene on a Chiral Metal Surface

Enantioselectivity in heterogeneous catalysis strongly depends on the chirality transfer between catalyst surface and all reactants, intermediates, and the product along the reaction pathway. Herein we report the first enantioselective on-surface synthesis of molecular structures from an initial racemic mixture and without the need of enantiopure modifier molecules. The reaction consists of a trimerization via an unidentified bonding motif of prochiral 9-ethynylphenanthrene (9-EP) upon annealing to 500 K on the chiral Pd₃-terminated PdGa{111} surfaces into essentially enantiopure, homochiral 9-EP propellers. The observed behavior strongly contrasts the reaction of 9-EP on the chiral Pd₁-terminated PdGa{111} surfaces, where 9-EP monomers that are in nearly enantiopure configuration, dimerize without enantiomeric excess. Our findings demonstrate strong chiral recognition and a significant ensemble effect in the PdGa system, hence highlighting the huge potential of chiral intermetallic compounds for enantioselective synthesis and underlining the importance to control the catalytically active sites at the atomic level.     

Rare-earth Metal Borosilicides R9Si15–xB3 (R = Tb,Yb): New Ordered Structures Derived from the AlB2 Structure Type

Two novel ternary borosilicides R₉Si_15–xB₃ (R = Tb, x = 1.80, R = Yb, x = 1.17) were synthesized from the initial elements using tin flux method. Their crystal structures were determined by means of X-ray single crystal diffraction. Both refer to space group R32, Z = 1: a = 6.668(2) Å, c = 12.405(4) Å [R₁ = 0.027, wR₂ = 0.031 for 1832 reflections with I_o > 2σ (I_o)] for Tb₉Si_15–xB₃, and a = 6.5796(3) Å, c = 12.2599(5) Å [R_F = 0.052, wR = 0.090 for 1369 reflections with I_o > 2σ (I_o)] for Yb₉Si_15–xB₃. The structures represent a new structure type, derived from that of AlB₂, with ordering in the metalloid sublattice resulting in distorted [Si₅B] hexagons. The presence or absence of boron in this ordered structure is discussed on the basis of difference Fourier syntheses, interatomic distances, structural analysis, and theoretical calculations in relation with the parent structures of the binaries AlB₂ and Yb₃Si₅ (Th₃Pd₅ type of structure). Theoretical calculations show substantial covalent interactions between the metal and nonmetal elements. The small percentage of silicon atoms, which are missing in these nonstoichiometric compounds, probably allows strengthening boron-metal and boron-silicon bonding.     

Reactive polymers incorporating silyl enol ether groups. I. synthesis by radical polymerization of 2-(trimethylsiloxy)-butadiene and 2-(tert)-butyldimethylsiloxy butadiene

2-(Trimethylsiloxy)butadiene (TMSBD) and 2-(tert-butyldimethylsiloxy)butadiene (TBMSBD) were copolymerized with styrene (St) and methyl methacrylate (MMA) under free-radical conditions. The obtained polymers were found to contain reactive silyl enol ether groups in a ratio identical to the TMSBD or TBMSBD molar fraction in the copolymer. All investigated samples displayed only 1,4- and 3,4-microstructures. The influence of several experimental factors on the yields, rates of polymerization, microstructures, and copolymer compositions were examined. Monomer reactivity ratios r₁ and r₂ at 60°C were determined from copolymer composition curves at low conversions. The homopolymerization of TBMSBD was also investigated and results were compared with those previously obtained for TMSBD. A slight increase in rates was observed and was rationalized on the basis of the higher viscosity resulting from the structural change in the monomer. Thermal stabilities of the synthesized polymers were investigated by TGA and their glass transition temperatures were determined by DSC. All measurements are compatible with a possible use of TMSBD and TBMSBD copolymers as reactive polymers. © 1996 John Wiley & Sons, Inc.     

Lichtblicke – Petrischalenexperimente in der Overhead-Projektion

Overhead projection with supplementary lenses provides „wallfilling big screen”︁ detailed pictures of physical and chemical phenomena and thus helps to train chemists to make accurate observations, comparisons, and interpretations. By using divided petri dishes, it is possible to present parallel and control experiments with minimal amounts of substances in one field of vision, and thus to establish the links between substance and reaction, and formula and equation. In particular, for the demonstration of electrochemical processes, the divided petri dish proves to be a projectable minimal system, which is characterized by maximum economy and ecology. Great recognition with little substance?.     

Dedicated polymers for future progress in microelectronics

The continuous miniaturization in microelectronics requires advanced materials and processes. Novel specifically functionalized anhydride-containing filmforming polymers which spontaneously react with bisaminoalkylsiloxanes were synthesized for use in photolithographic processes. This allows the modelling of photoresist patterns and thus enhancement of practical resolution. In addition, efficient polymeric dielectrics with improved solubility in environmentally safe solvents were developed. The latter can be patterned using the above mentioned photolithographic process.     

Gas hydrate single-crystal structure analyses

The first single-crystal diffraction studies on methane, propane, methane/propane, and adamantane gas hydrates SI, SII, and SH have been performed. To circumvent the problem of very slow crystal growth, a novel technique of in situ cocrystallization of gases and liquids resulting in oligocrystalline material in a capillary has been developed. With special data treatment, termed oligo diffractometry, structural data of the gas hydrates of methane, acetylene, propane, a propane/ethanol/methane-mixture and an adamantane/methane-mixture were obtained. Cell parameters are in accord with reported values. Host network and guest are subject to extensive disorder, reducing the reliability of structural information. It was found that most cages are fully occupied by a guest molecule with the exception of the dodecahedral cage in the acetylene hydrate which is only filled to 60%. For adamantane in the icosahedral cage a disordered model is proposed.     

Ultraviolet radiation as a limiting factor in leaf expansion and development

Reductions in leaf growth are a commonly observed response to ultraviolet radiation, but the underlying mechanisms remain poorly defined. This study examined the response of leaves exposed to a UV environment across a range of organizational scales, including leaf expansion rate, epidermal cell size and number, biomechanical properties, leaf–water relations and activity of cell-wall peroxidases. Two experimental approaches were used; Lettuce ( Lactuca sativa L.) plants were propagated under (a) supplementary UV-B (9 kJ m⁻² day⁻¹) in controlled environment (CE) conditions, and (b) field conditions, where plants were placed under three horticultural films with differing UV transmissions. In both experiments, UV-B caused the greatest reductions in leaf expansion and final leaf size, with some reductions attributable to UV-A wavelengths. In supplementary UV-B conditions, adaxial cell size was reduced, while in field plants, both cell size and cell number were lower in an increased UV environment, as was the case with abaxial cells in CE plants. Although leaf turgor and leaf extensibility were not affected by UV wavelengths, breaking strain of leaf tissue was decreased under supplementary UV-B. Cell-wall peroxidase activity was increased in both supplementary UV conditions and in the field, where only a zero UV environment showed no upregulation of cell-wall peroxidase.