Ein Beitrag zur darstellenden Geometrie des R4

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Crown-containing styryl derivatives of naphthopyrans: Complexation with alkaline-earth metal cations and photochemistry

The photochemistry and complexation with alkaline-earth metal cations of two crown-containing naphthopyrans and their crownless analogues are reported. Two types of photoreactions occur in the system, namely, the reversible formation of the open form, which is responsible for the photochromism of these compounds, and the geometric isomerization of the closed form. The photochromic properties (UV spectrum and lifetime) of the naphthopyrans do not change upon complexation with metal cations. This is explained by the specific features of the structure of the open form and by the kinetic parameters of the processes.     

Preparation and ESR-Spectroscopical Investigation of Remarkably Persistent Oxoisobenzofuranyl Radicals

The synthesis of the diastereoisomeric 1,1′-diaryl-1,1′-bi(isobenzofuran)-3,3′(1H,1′H)-diones 3a–d starting from the readily available 2-aroylbenzoic acids 1a–d is described (Scheme 1). Of the colourless dimers 3a–d , only the sterically congested 3a and 3b dissociate at ambient temperature in solution to the deep red free 3-oxoisobenzofuran-1-yl radicals 4a and 4b , respectively. The radicals 4a, b are extremely persistent in the absence of O₂. The structures of these radicals are confirmed and the coupling constants assigned by ESR and ENDOR spectroscopy and computer simulation of their ESR spectra. The dissociation equilibrium constant at 20° in toluene for 3a is determined to be 1.18 · 10^?5 M . By studying the steady-state radical concentration as a function of temperature, the enthalpy and entropy changes for the homolytic dissociation of 3a are determined.     

Effect of glycyrrhizic acid on lappaconitine phototransformation

1H NMR and CIDNP methods were used to demonstrate that triterpene glycoside (glycyrrhizic acid, GA) can substantially change the efficiency and direction of phototransformation of alkaloid lappaconitine (LA) due to both its solubilization in GA micelles and protonation of LA amine nitrogen in water-alcohol solutions. The LA solubilization in the GA micelle suppresses the process of deacylation.     

On the Hydrogen Oxalate Binding Motifs onto Dinuclear Cu and Ag Metal Phosphine Complexes

We report the binding geometries of the isomers that are formed when the hydrogen oxalate ((CO₂)₂H=HOx) anion attaches to dinuclear coinage metal phosphine complexes of the form [M₁M₂dcpm₂(HOx)]⁺ with M=Cu, Ag and dcpm=bis(dicyclohexylphosphino)methane, abbreviated [MM]⁺ . These structures are established by comparison of isomer-selective experimental vibrational band patterns displayed by the cryogenically cooled and N₂-tagged cations with DFT calculations of the predicted spectra for various local minima. Two isomeric classes are identified that feature either attachment of the carboxylate oxygen atoms to the two metal centers (end-on docking) or attachment of oxygen atoms on different carbon atoms asymmetrically to the metal ions (side-on docking). Within each class, there are additional isomeric variations according to the orientation of the OH group. This behavior indicates that HOx undergoes strong and directional coordination to [CuCu]⁺ but adopts a more flexible coordination to [AgAg]⁺ . Infrared spectra of the bare ions, fragmentation thresholds and ion mobility measurements are reported to explore the behaviors of the complexes at ambient temperature.