1H-NMR investigations of the conformation of aryl-(hydroxynaphthyl)-methylpiperidines. Intramolecular interactions,IV

Summary The¹H-NMR spectra of aryl-(hydroxynaphthyl)-methylpiperidines, which are model compounds for intramolecular hydrogen bonding, have been analyzed in order to investigate their conformations in solution. As dynamic phenomena can be assumed from line broadening, low temperature spectra have been measured to evaluate the coalescence temperatures and the energy barriers. The latter have been discussed with respect to the size and position of selected substituents. It can be shown that the molecules exist in one energetically favorable conformation with the aryl ring perpendicular to the plane of the naphthol ring system. The interaction between the naphthol ring and the aryl ring influences the conformation at the piperidine ring moiety. This effect leads to an increase of the inversion barrier of the piperidine residue.On Sabbatical Leave from Department of Chemistry, Quaid-i-Azam University, Islamabad, Pakistan     

On-line size exclusion chromatography-pyrolysis-gas chromatography-mass spectrometry for copolymer characterization and additive analysis

On-line coupled size exclusion chromatography-pyrolysis gas chromatography mass spectrometry (SEC-Py-GC-MS) is studied as a novel tool for the characterization of complex polymer samples. An automated system for on-line SEC-Py-GC-MS allowing transfer of multiple fractions was developed based on stop-flow operation of the SEC dimension, syringe-based transfer of the SEC fraction to the GC instrument and solvent elimination with subsequent pyrolysis in a programmed temperature vaporization (PTV) injector. After optimization the system was applied to the characterization of a complex terpolymer composed of very similar monomers. The use of the system for combined pyrolysis and additive analyses in polycarbonate was also demonstrated. Results obtained with the new method indicate the interesting potentials of the method for detailed characterization of polymeric materials.     

Nitrilases catalyze key step to conformationally constrained GABA analogous gamma-amino acids in high optical purity

Five- and six-membered carbocyclic gamma-amino acids were prepared in high enantiomeric purity by nitrilase-mediated transformation of hitherto unreported gamma-amino nitriles. The nitrilases investigated reveal a strong enantiopreference for cis-isomers (up to 99% ee), whereas trans-isomers were available in up to 86% ee. The biocatalytic enantioselective syntheses of cis-3-aminocyclohexanecarboxylic acid (3b), trans-3-aminocyclohexanecarboxylic acids (4b, 6b, 8b) as well as trans-3-aminocylopentanecarboxylic acid (2b) are hereby reported for the first time.     

Zur Ringumwandlung von 2-Amino-thiophen-3-carbonsäureestern: Pyridon- und Pyridazinon-Derivate

Whereas treatment of the ethyl 5-acetyl-2-amino-4-methyl-thiophene-3-carboxylate (1) with potassium hydroxide yields the 2-hydroxy-thiophene-3-carbonitrile4 its hydrazone2 is converted into the 1-amino-5-mercapto-2-pyridone derivative6. The transformation of the 2-amino-5-phenylazo-thiophene derivative9 by potassium hydroxide yields the substituted 3-mercapto-pyridazin-6(1H)one10, with sodium ethoxide the 5-phenylhydrazono-2-oxo-thiolen-3-carbonitrile11 is formed. From6 the 5-mercapto-2-pyridone derivatives7 d,e can be obtained.

     

Silver and gold complexes with a new 1,10-phenanthroline analogue N-heterocyclic carbene: a combined structural, theoretical, and photophysical study

Unusual coordination for gold: an imidazolium salt was synthesized and used as a precursor for an N-heterocyclic carbene, which can be considered as the carbene analogue of 1,10-phenanthroline. Like the diimine congener, this ligand gives luminescent metal complexes. Remarkably, the Au(III) complex features a gold atom in an unusual environment: it is surrounded by six donor atoms, two of which interact electrostatically with the Au atom.     

Conformational energies of silacyclohexanes C5H10SiHMe,C5H10SiH(CF3) and C5H10SiCl(SiCl3) from variable temperature Raman spectra

From variable temperature vibrational Raman spectra, the axial/equatorial enthalpy differences for the substituted silacyclohexanes C₅H₁₀SiHMe, C₅H₁₀SiH(CF₃) and C₅H₁₀SiCl(SiCl₃) were determined. The pure liquids and solutions in various solvents were investigated. Preferred conformations are equatorial for methylsilacyclohexane and axial for trifluoromethylsilacyclohexane, consistent with earlier results from nuclear magnetic resonance experiments and ab initio calculations. For C₅H₁₀SiCl(SiCl₃) an enthalpy difference close to zero was found, which is supported by high‐level which is supported by high‐level quantum chemical calculations at the second‐order Møller‐Plesset (MP2) and coupled cluster with single, double, and perturbative triple excitations (CCSD(T)) levels, which employed various basis sets. A novel synthesis for C₅H₁₀SiCl(SiCl₃) was developed using ClMg(CH₂)₅MgCl instead of BrMg(CH₂)₅MgBr as a starting material. The procedure avoids the formation of partially brominated products, facilitating the purification of the compound. ¹H, ¹³C and ²⁹Si nuclear magnetic resonance data are reported. Copyright © 2012 John Wiley & Sons, Ltd.     

Central European journal of operations research (CJOR) “operations research applied to health services (ORAHS) in Europe: general trends and ORAHS 2020 conference in Vienna,Austria”

Central European Journal of Operations Research - This articles provides a short summary of the research topics and latest research results of the European Working Group “Operations Research...