Gold(I)-catalyzed intramolecular hydroamination of unactivated C==C bonds with alkyl ammonium salts

A mixture of (5)AuCl [ = PCy2[2-(2,6-C6H3(OMe)2)C6H4]] and AgOTf catalyzes the intramolecular hydroamination of unactivated C==C bonds with primary and secondary ammonium salts.     

One‐ or Two‐Electron Transfer? The Ambiguous Nature of the Discharge Products in Sodium–Oxygen Batteries

Rechargeable lithium–oxygen and sodium–oxygen cells have been considered as challenging concepts for next‐generation batteries, both scientifically and technologically. Whereas in the case of non‐aqueous Li/O₂ batteries, the occurring cell reaction has been unequivocally determined (Li₂O₂ formation), the situation is much less clear in the case of non‐aqueous Na/O₂ cells. Two discharge products, with almost equal free enthalpies of formation but different numbers of transferred electrons and completely different kinetics, appear to compete, namely NaO₂ and Na₂O₂. Cells forming either the superoxide or the peroxide have been reported, but it is unclear how the cell reaction can be influenced for selective one‐ or two‐electron transfer to occur. In this Minireview, we summarize available data, discuss important control parameters, and offer perspectives for further research. Water and proton sources appear to play major roles.     

Synthesis of the antifungal beta-1,3-glucan synthase inhibitor CANCIDAS (caspofungin acetate) from pneumocandin B0

A novel three-step synthesis of the highly functionalized antifungal agent CANCIDAS (caspofungin acetate, 2) is described, starting from the natural product pneumocandin B0 (1). The highlights of the synthesis include a stereoselective formation of a phenylthioaminal, a remarkable chemoselective, high-yielding, one-step borane reduction of a primary amide, and a stereoselective substitution of the phenylthioaminal with ethylenediamine producing 2 in a 45% overall yield.     

Consequences of the center–of–mass correction in nuclear mean–field models

We study the influence of the scheme for the correction for spurious center–of–mass motion on the fit of effective interactions for self–consistent nuclear mean–field calculations. We find that interactions with very simple center–of–mass correction have significantly larger surface coefficients than interactions for which the center–of–mass correction was calculated for the actual many–body state during the fit. The reason for that is that the effective interaction has to counteract the wrong trends with nucleon number of all simplified schemes for center–of–mass correction which puts a wrong trend with mass number into the effective interaction itself. The effect becomes clearly visible when looking at the deformation energy of largely deformed systems, e.g. superdeformed states or fission barriers of heavy nuclei. Received: 6 September 1999     

Effective Rheology of Two-Phase Flow in Three-Dimensional Porous Media: Experiment and Simulation

We present an experimental and numerical study of immiscible two-phase flow of Newtonian fluids in three-dimensional (3D) porous media to find the relationship between the volumetric flow rate (Q) and the total pressure difference (\(\Delta P\)) in the steady state. We show that in the regime where capillary forces compete with the viscous forces, the distribution of capillary barriers at the interfaces effectively creates a yield threshold (\(P_t\)), making the fluids reminiscent of a Bingham viscoplastic fluid in the porous medium. In this regime, Q depends quadratically on an excess pressure drop (\(\Delta P-P_t\)). While increasing the flow rate, there is a transition, beyond which the overall flow is Newtonian and the relationship is linear. In our experiments, we build a model porous medium using a column of glass beads transporting two fluids, deionized water and air. For the numerical study, reconstructed 3D pore networks from real core samples are considered and the transport of wetting and non-wetting fluids through the network is modeled by tracking the fluid interfaces with time. We find agreement between our numerical and experimental results. Our results match with the mean-field results reported earlier.     

Towards surface quantum optics with Bose–Einstein condensates in evanescent waves

We present a surface trap which enables the study of coherent interactions between ultracold atoms and evanescent waves. The trap combines a magnetic Joffe trap with a repulsive evanescent dipole potential. Exploiting the advantages of both approaches this technique improves recent surfaces traps, which are based either on magnetic or optical traps alone. On the one hand, the position of the magnetic trap can be controlled with high precision which makes it possible to move ultracold atoms to the surface of a glass prism or to withdraw the atoms from the surface in a controlled way. On the other hand, the optical potential of the evanescent wave partially compensates for strong attractive surface forces and generates a potential barrier at only a few hundred nanometers from the surface. This barrier prevents the surface potentials from limiting the trap depth of the magnetic trap. The surface trap is probed with ⁸⁷Rb Bose–Einstein condensates (BECs), which are stably positioned at distances from the surfaces below one micrometer.