Synthesis and use of alpha-silyl-substituted alpha-hydroxyacetic acids

[reaction: see text] Rhodium-catalyzed oxygen transfer was used to generate benzyl 2-silyl-2-oxoacetates in good yields. The hydrogenation of these compounds led to chiral alpha-silyl-substituted alpha-hydroxyacetic acids. Resolution by means of HPLC using a chiral stationary phase afforded an enantiomerically pure representative of this class of compounds, which was successfully applied as a chiral ligand in an asymmetric aldol-type reaction.     

Die Verwendung des elektrischen Stromes als W?rmequelle

Ohne Zusammenfassung     

Nuclear shapes of the transitional nuclei 186, 188, 190, 192Os

Excitation functions at θ_lab = 130° have been measured for inelastic scattering of 13–24 MeV alpha particles from ^{186,188,190,192}Os. Data have been obtained for 0⁺, 2⁺, 4⁺, 2⁺' states in ^186,188,192Os and for 0⁺, 2⁺ states in ¹⁹⁰Os. Charge and mass quadrupole deformations, β₂^c and β₂^N, were deduced from coupled-channels analysis of the 2⁺ data. No simple model could provide good fits to the 4⁺ data over the entire energy range but the very deep interference minima observed establish the charge and mass hexadecapole moments, β₄^c and β₄^N, to be negative. The energies of the interference minima for the 4⁺ states could be reproduced by coupled-channels calculations only if large differences in β₄^c and β₄^N were allowed. Data for the (J, K)^π = (2, 2)⁺ states are inconsistent with an asymmetric rotational model.     

Intermolecular rhodium(II)-catalyzed reactions with silicon-substituted carbonyl ylides

[reaction: see text] The Rh(II)-catalyzed reaction of benzyl 2-trialkylsilyl-2-diazoacetates with various acyclic and cyclic ketones affords novel dioxolanones via silicon-substituted carbonyl ylides in up to 98% yield.     

Reorientation measurements on tungsten isotopes

In a particle-γ coincidence experiment, a thick tungsten target, of natural isotopic abundance, was bombarded with a and ¹⁶O beams. From analysis of the deexcitation γ-rays following Coulomb excitation, the spectroscopic quadrupole moment of the second 2⁺ state (the 22' state) was determined for ¹⁸⁶W and ¹⁸⁴W. In a separate Coulomb excitation experiment a thin, isotopically enriched ¹⁸⁶W target was bombarded with ¹⁶O ions. From analysis of projectiles scattered elastically and inelastically the quadrupole moment of the 2^+' state of ¹⁸⁶W was extracted. The results of the two experiments are in good agreement. The quadrupole moment of the 2^+' state is found to be opposite in sign to that of the first 2⁺ state for both isotopes studied. However, its magnitude decreases rapidly in going from ¹⁸⁶W to ¹⁸⁶W, in contrast to the predictions of the rotation-vibration or asymmetric rotor models. The microscopic theory of Kumar and Baranger does predict the experimental trend, qualitatively. Thus the present results are interpreted as being evidence of strong coupling between β and γ degrees of freedom in the tungsten isotopes, which, according to the theory of Kumar and Baranger, is the source of the reduced value of the quadrupole moment.