The microscopic investigation of structures of moving flux lines by neutron and muon techniques

We have used a variety of microscopic techniques to reveal the structure and motion of flux line arrangements, when the flux lines in low T _c type II superconductors are caused to move by a transport current. Using small-angle neutron scattering by the flux line lattice (FLL), we are able to demonstrate directly the alignment by motion of the nearest-neighbor FLL direction. This tends to be parallel to the direction of flux line motion, as had been suspected from two-dimensional simulations. We also see the destruction of the ordered FLL by plastic flow and the bending of flux lines. Another technique that our collaboration has employed is the direct measurement of flux line motion, using the ultra-high-resolution spectroscopy of the neutron spin-echo technique to observe the energy change of neutrons diffracted by moving flux lines. The muon spin rotation (μSR) technique gives the distribution of values of magnetic field within the FLL. We have recently succeeded in performing μSR measurements while the FLL is moving. Such measurements give complementary information about the local speed and orientation of the FLL motion. We conclude by discussing the possible application of this technique to thin film superconductors.     

Me2Zn‐Mediated Catalytic Enantio‐ and Diastereoselective Addition of TosMIC to Ketones

The first catalytic asymmetric addition of TosMIC to unactivated ketones is presented. A combination of Me₂Zn and aminoalcohol catalyst promoted the aldol addition/cyclization reaction to render oxazolines possessing a fully substituted stereocenter with excellent yields (up to 92 %), high enantioselectivities (up to 96 %), and complete diastereoselectivity. The chiral oxazolines were then used to give, after a straightforward acid hydrolysis, enantioenriched building blocks bearing tertiary alcohol motifs such as hydroxylaldehydes, hydroxylacids, and hydroxylesters without racemization.     

Enantio and diastereoselective addition of phenylacetylene to racemic α-chloroketones

In this report, we have presented the first diastereoselective addition of phenylacetylene to chiral racemic chloroketones. The addition is controlled by the reactivity of the chloroketones that allowed the stereoselective reaction to be performed at -20 °C. Chiral racemic chloroketones are used in the reaction. By carefully controlling the temperature and the reaction time we were able to isolate the corresponding products in moderate yields and with good, simple and predictable facial stereoselection. Our reaction is a rare example of the use of chiral ketones in an enantioselective alkynylation reaction and opens new perspectives for the formation of chiral quaternary stereocenters.     

Vitaminuntersuchungen

Ohne Zusammenfassung     

Topologically Irreducible Representations and Radicals in Banach Algebras

It is shown that the topologically irreducible representationsof a normed algebra define a certain topological radical inthe same way that the strictly irreducible representations definethe Jacobson radical and that this radical can be strictly smallerthan the Jacobson radical. An abstract theory of ‘topologicalradicals’ in topological algebras is developed and usedto relate this radical to the Baer radical (prime radical).The relations with topologically transitive representationsand standard representations in the sense of Meyer are alsoexplored. 1991 Mathematics Subject Classification: 46H15, 46H25,16Nxx.     

Die Oxydation des Germaniums (2) an der Quecksilbertropfelektrode und ihre analytische Anwendung

Zusammenfassung Das durch Reduktion von vierwertigem Germanium mit Natriumhypophosphit erhaltene zweiwertige Germanium gibt in saurer Lösung eine Anodenstufe, die der Konzentration vollkommen proportional ist. Die Messung der Stufenhöhe wird durch einen Überschuß von Chlorionen gestört. Um dieser Tatsache zu begegnen, werden cadmiumhaltige Grundlösungen vorgeschlagen. Die quantitative Bestimmung kann bereits bei Konzentrationen von 1.10^–6 Mol/l vorgenommen werden, ohne daß bei Einhaltung bestimmter Bedingungen die gewöhnlich vorkommenden Metalle stören.

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Summary The bivalent germanium obtained by reduction of quadrivalent germanium by means of sodium hypophosphite, gives, in acid solution, an anode stage, which is completely proportional to the concentration. The measurement of the height of the stage is interfered by an excess of chloride ions. To overcome this difficulty, it is advised that cadmium-bearing solutions be used. The quantitative determination can be made even at concentrations of 1 × 10^–6 mol/liter with no interference by the metals usually present, provided certain conditions are mainteined.

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Résumé Le germanium bivalent obtenu par réduction du germanium tétravalent avec l'hypophosphite de sodium donne, en solution acide, une vague anodique qui est rigoureusement proportionelle à la concentration. La mesure de la hauteur du palier est troublée par un excès d'ions chlore. Pour prévenir ce fait, on préconise l'emploi de solutions de base contenant du cadmium. La mesure quantitative peut déjà être effectuée pour des concentrations de 1 10^–6 Mol/l sans que les métaux habituellement rencontrés ne gênent, à condition d'observer des conditions précises.

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Mit 4 Abbildungen.     

A Dual Photoredox- and Cp2TiCl2-Catalyzed Approach for the Direct Access to α-Vinyl-β-hydroxy Esters

For the first time, a dual photoredox- and titanocene-catalyzed methodology for the regioselective access to α-vinyl-β-hydroxy esters towards aldehyde allylation with 4-bromobut-2-enoate is reported. The protocol is based on the Barbier-type properties of the inexpensive and available Cp₂TiCl₂ in catalytic amount (5 mol%). The developed mild reaction conditions gave access to a library of differently functionalized α-vinyl-β-hydroxy esters in moderate diastereoselectivity, employing the commercially available ethyl 4-bromobut-2-enoate and both aromatic and aliphatic aldehydes. The reaction was realized under visible light irradiation, in the presence of an organophotocatalyst (3DPAFIPN, 2 mol%) combined with Hantzsch's ester as the sacrificial reductant. In contrast to other Barbier-type reactions employing ethyl 4-bromobut-2-enoate, the photoredox system ensures a better regioselectivity. Moreover, the use of preformed organometallic nucleophilic species (e. g., dienolborinates), and the need of metal reductants or stoichiometric amount of transition metals in low oxidation state for Barbier-type reactions, is avoided. To support the experimental evidence, a detailed photophysical study shed light on the mechanism of the reaction.