Matched/Mismatched interaction of a cyclic hexapeptide with ion pairs containing chiral cations and chiral anions

The binding of a chiral quaternary ammonium ion to a cyclopeptide containing aromatic amino acid subunits is affected not only by the configuration of the cation but also by the configuration of the chiral counterion. Analysis of the binding equilibria shows that complex formation involves interaction of the whole ion pair with the host indicating that steric requirements of the anion influence complex geometry and stability.     

Application of divergent multi-component reactions in the synthesis of a library of peptidomimetics based on γ-amino-α,β-cyclopropyl acids

The multi-component condensation of organozirconocene, aldimine and zinc carbenoid was applied to the stereoselective synthesis of cyclopropane amino acid derivatives. These compounds served as scaffolds for the preparation of a 46-member library. The C- and N-termini of the cyclopropane amino acid derivatives were diversified by condensations with ten amines and ten acylating agents, respectively. To improve yields and accelerate library synthesis, most products were prepared under microwave irradiation and purified by polymer-bound scavengers and SPE methodology. All compounds were analyzed by LC-MS and a representative selection was fully characterized.     

Structural and magnetic properties of hexagonal perovskites La1.2Sr2.7MO7.33 (M = Ru, Ir) containing peroxide ions

The structures of the new compound La(1.2)Sr(2.7)IrO(7.33) and the recently discovered La(1.2)Sr(2.7)RuO(7.33) have been solved using a combination of X-ray and neutron diffraction. Both compounds crystallize in the trigonal space group Rm and consist of isolated MO6 (M = Ru, Ir) octahedra, which are arranged in well-defined hexagonal perovskite slabs. These slabs are separated by (Sr2O(1+delta)) layers containing both O2- and (O2)2- ions. The composition can therefore be written as La(1.2)Sr(2.7)MO(7-delta)(O2)delta with delta = 0.33. Results of the magnetic susceptibility and XANES measurements show that the transition metal cations are in a pentavalent state. While in La(1.2)Sr(2.7)RuO(7.33) an antiferromagnetic interaction between the Ru5+ ions is found, La(1.2)Sr(2.7)IrO(7.33) shows a very small temperature-independent paramagnetism down to 1.8 K due to the strong spin-orbit coupling characteristic for the 5d element iridium.     

Influence ofpH and concentration of complexing agents on fluorescence of samarium(III), gadolinium(III), and terbium(III)—Ethylenediaminetetraacetic acid or nitrilotriacetic acid complexes in aqueous solutions

The fluorescence intensity of aqueous solutions of Sm(III), Gd(III) and Tb(III) complexes withEDTA orNTA was investigated as a function of complexone concentration over a widepH range. For Sm(III) and Tb(III) complexes the ratio of intensities of hypersensitive and allowed bands was calculated in order to obtain information about intensity borrowing of the fluorescence bands as a result of vibronic coupling.     

ansa-Metallocene derivatives XXXIII. 2-Dimethylamino-substituted bis-indenyl zirconium dichloride complexes with and without a dimethylsilyl bridge: syntheses, crystal structures and properties in propene polymerization catalysis

Bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (2-(CH₃)₂N-C₉H₆)₂ZrCl₂, and dimethylsilyl-bridged bis(2-N,N-dimethylamino-indenyl) zirconium dichloride, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂, were prepared by reaction of the corresponding ligand lithium salts with ZrCl₄ in toluene. Diffractometric structure determinations reveal C₂-symmetric complex geometries for both complexes. An increased electron density at the Zr center of the dimethylamino-substituted complexes is indicated by reduction potentials which are 0.3–0.4 V more negative than those of their unsubstituted analogs. When activated with methyl aluminoxane in toluene solution, (CH₃)₂Si(2-(CH₃)₂N-C₉H₅)₂ZrCl₂ catalyzes the polymerization of propene to polymers with a microstructure comparable with that of polymers produced with other Me₂Si-bridged bis(indenyl)ZrCl₂ complexes, but with a substantially increased fraction of i-propyl end groups derived from alkyl exchange between Zr-polymer and Al---Me species.     

Präparative chromatographische Enantiomerentrennung von synthetisch nützlichen cyclischen Acetalen

Preparative Chromatographic Resolution of Synthetically Useful Cyclic Acetals Racemic cyclic acetals derived from aldehydes and glycine, glycolic acid, thioglycolic acid, formylacetic acid, and acetoacetic acid (oxazolidinones 4 – 13 , dioxolanones 14 , 15 , oxathiolanone 16 , dioxinones 17 – 23 ) are resolved by preparative high-pressure liquid chromatography on silica gel coated with the polymer from N-acryloylphenylalanine ethyl ester (Chiraspher®). The separation factors α range from1,1 to 2,4. Use of a Prepbar®-chromatography system allows injection of several grams at a time. The enantiomeric acetals thus obtained are fully characterized. First application to amino-acid synthesis are mentioned.     

Practical asymmetric enzymatic reduction through discovery of a dehydrogenase-compatible biphasic reaction media

[reaction: see text] An enzyme-compatible biphasic reaction media for the asymmetric biocatalytic reduction of ketones with in situ cofactor regeneration has been developed. In this biphasic reaction media, which is advantageous for reactions at higher substrate concentrations, both enzymes (alcohol dehydrogenase and FDH from Candida boidinii) remain stable. The reductions with poorly water-soluble ketones were carried out at substrate concentrations of 10-200 mM, and the optically active (S)-alcohols were formed with moderate to good conversions and with up to >99% ee.     

An on-line potentiometric sequential injection titration process analyser for the determination of acetic acid

An on-line potentiometric sequential injection titration process analyser for the determination of acetic acid is proposed. A solution of 0.1 mol L(-1) sodium chloride is used as carrier. Titration is achieved by aspirating acetic acid samples between two strong base-zone volumes into a holding coil and by channelling the stack of well-defined zones with flow reversal through a reaction coil to a potentiometric sensor where the peak widths were measured. A linear relationship between peak width and logarithm of the acid concentration was obtained in the range 1-9 g/100 mL. Vinegar samples were analysed without any sample pre-treatment. The method has a relative standard deviation of 0.4% with a sample frequency of 28 samples per hour. The results revealed good agreement between the proposed sequential injection and an automated batch titration method.     

Catalysts for the living insertion polymerization of alkenes: access to new polyolefin architectures using Ziegler-Natta chemistry

Coordination-insertion polymerization systems have long been superior to their anionic, cationic, and radical polymerization counterparts with regard to stereochemical control. However, until five years ago, these metal-based insertion methods were inferior to ionic and radical mechanisms in the category of living polymerization, which is simply a polymerization that occurs with rapid initiation and negligible chain termination or transfer. In the last half decade, the living insertion polymerization of unactivated olefins has emerged as a powerful tool for the synthesis of new polymer architectures. Materials available today by this route range from simple homopolymers such as linear and branched polyethylene, to atactic or tactic poly(alpha-olefins), to end-functionalized polymers and block copolymers. This review article summarizes recent developments in this rapidly growing research area at the interface of synthetic and mechanistic organometallic chemistry, polymer chemistry, and materials science. While special emphasis is placed on polymer properties and novel polymeric architectures, most of which were inaccessible just a decade ago, important achievements with respect to ligand and catalyst design are also highlighted.     

Boron implantation in Si: Channeling effects studied by SIMS and simulation

The axial channeling behaviour of boron implants in <100>, <110> and <111> silicon wafers is investigated by SIMS. Large differences of channeling characteristics such as channeled projected range (the projected range of channeled ions or channeling peak) and the fraction of channeled to implanted ions are observed among the three major crystal orientations. Within the critical angle, the channeling behaviour is very sensitive to the incidence beam angle with respect to crystal orientations. SIMS measurements are performed at different positions along several critical directions over a whole wafer. Well channeled profiles with an incidence beam angle to crystal orientations of 0 ° are obtained for each ion implantation energy and orientation. The results are used to test various models of ion implantation by simulation. A 3-parameter model for electronic stopping power of boron in silicon was proposed.