Inner Workings of a Cinchona Alkaloid Catalyzed Oxa‐Michael Cyclization: Evidence for a Concerted Hydrogen‐Bond‐Network Mechanism

Cinchona alkaloids catalyze the oxa‐Michael cyclization of 4‐(2‐hydroxyphenyl)‐2‐butenoates to benzo‐2,3‐dihydrofuran‐2‐yl acetates and related substrates in up to 99 % yield and 91 % ee (ee=enantiomeric excess). Catalyst and substrate variation studies reveal an important role of the alkaloid hydroxy group in the reaction mechanism, but not in the sense of a hydrogen‐bonding activation of the carbonyl group of the substrate as assumed by the Hiemstra–Wynberg mechanism of bifunctional catalysis. Deuterium labeling at C‐2 of the substrate shows that addition of RO? H to the alkenoate occurs with syn diastereoselectivity of ≥99:1, suggesting a mechanism‐based specificity. A concerted hydrogen‐bond network mechanism is proposed, in which the alkaloid hydroxy group acts as a general acid in the protonation of the α‐carbanionic center of the product enolate. The importance of concerted hydrogen‐bond network mechanisms in organocatalytic reactions is discussed. The relative stereochemistry of protonation is proposed as analytical tool for detecting concerted addition mechanisms, as opposed to ionic 1,4‐additions.     

Parametric instability of a vertically driven magnetic pendulum with eddy-current braking by a flat plate

Nonlinear Dynamics - The vertically driven pendulum is one of the classical systems where parametric instability occurs. We study its behavior with an additional electromagnetic interaction caused...     

On the various impact of chemical composition and elastic strain in SiGe nanoscale islands to the diffuse X-ray scattering

High-resolution X-ray diffraction in combination with kinematical scattering simulations based on finite element calculations for the elastic strain field has been utilized to investigate the chemical composition of SiGe/Si(0 0 1) Stranski–Krastanov islands. Therefore, the intensity distribution in the vicinity of the symmetric (0 0 4) reflection has been three dimensionally measured and compared with respective simulations. Around an asymmetrical reflection, e.g. Si(1 1 3), the impact of lateral and vertical strain tensor components on the diffraction pattern decouples. This allows to distinguish between tetragonally distorted regions at the island bottom and elastically relaxed unit cells of cubic shape at the island apex in accordance with finite element calculations. Artificially switching off and on the various scattering abilities of silicon and germanium in otherwise equivalent simulations yields practically no difference in the resulting scattering. Thus, at X-ray energies far from absorption edges, the diffuse scattering is dominated by elastic strain which is caused by the chemical profile.     

Über die Einlagerung von Antimonfluoridchloriden in Graphit Versuche zur Darstellung von Metallfluorid-Graphitverbindungen

The reaction between graphite and SbF₃ in Cl₂ atmosphere or SbF₃Cl₂in inert atmosphere gives antimony fluoride-chloride graphite compounds. The first intercalation stage is deep blue and contains 67–68% SbF_xCl_y. The ideal composition is near [C₂₇]_GrSbF₃Cl₂ · 2 SbF₃Cl₂. SbF₃Cl₂ can be substituted partly by SbF₃ or SbF₅. The layer distance decreases with increasing F:Cl ratio from 8.87 to 8.36 Å. There exist also a second and third intercalation stage.     

Distortion of liquid metal flow in a square duct due to the influence of a magnetic point dipole

We consider liquid metal flow in a square duct with electrically insulating walls under the influence of a magnetic point dipole using three-dimensional direct numerical simulations with a finite-difference method. The dipole acts as a magnetic obstacle. The Lorentz force on the magnet is sensitive to the velocity distribution that is influenced by the magnetic field. The flow transformation by an inhomogeneous local magnetic field is essential for obtaining velocity information from the measured forces. In this paper we present a numerical simulation of a spatially developing flow in a duct with laminar inflow and periodic boundary conditions. (© 2012 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)