Methylenecyclobutanes are found to undergo Wacker oxidation via a semi-pinacol-type rearrangement. Key to a successful process is the use of tert-butyl nitrite as oxidant, which not only enables efficient catalyst turn-over but also ensures high Markovnikov-selectivity under mild conditions. Thus, cyclopentanones (26 examples) can be accessed in an overall good yield and excellent selectivity (up to 97 % yield, generally >99 : 1 ketone:aldehyde ratio). Stereochemical analysis of the reaction sequence reveals migration aptitudes in line with related 1,2-shifts. By introducing a pyox ligand to palladium, prochiral methylenecyclobutanes can be desymmetrized, thus realizing the first enantioselective Wacker oxidation. 相似文献
Large enhancements have been observed in the sub-barrier fusion cross sections for Ti+Ni systems in our previous studies.
Coupled channel calculations incorporating couplings to 2+ and 3− states failed to explain these enhancements completely. A possibilty of transfer channels contributing to the residual enhancements
had been suggested. In order to investigate the role of relevant transfer channels, measurements of one- and two-nucleon transfer
were carried out for 46,48Ti+61Ni systems. The present paper gives the results of these studies. 相似文献
Memories that utilize single-electron effects are an attempt at combining the discreteness observable in transport of electrons
on to very small capacitances (∼10-18 F) and into three-dimensionally quantum-confined states, with the reproducibility, architecture and integration of the field-effect
devices. We discuss the role size plays in the operation and its variability for such memories. In particular, we discuss
the implications of size effects through barriers on speed; through electrostatics on variability, acceptability and reproducibility
of properties desired; through random variations and of tunneling on limits in the use of the field-effect, and through interface-states
on the time-domain operation. For device properties and their variations, using silicon-on-insulator substrates, silicon and
back-insulator thicknesses matter through the linear variations introduced in the electrostatic potential and quadratic variations
introduced in the subband energies, the quantum-dots and nano-crystals matter secondarily through the electrostatics and the
linear dependence of capacitance on size and the quadratic dependence of the allowed eigen-energies on size. We also discuss
the implications of tunneling on time constants of charging of the confined states and in between the source and the drain
for the ultimate structure size limit.
Received: 14 April 2000 / Accepted: 17 April 2000 / Published online: 6 September 2000 相似文献
Water molecules doing time : Atomic‐resolution crystal structures of the PPIase domain of cyclophilin G, alone and in complex with cyclosporin A, and together with MD simulations and calorimetry, reveal how trapped water molecules influence the thermodynamic profile of a protein–ligand interaction.
“One‐pot” substitution of the twenty hydrogen atoms in pentagonal dodecahedrane (C20H20) by OH, F, Cl, and Br atoms is explored. Electrophilic insertion of oxygen atoms with DMDO and TFMDO as oxidizing reagents ended, far off the desired C20(OH)20, in complex polyol mixtures (up to C20H10(OH)10 decols, a trace of C20H(OH)19?). Perfluorination was successful in a NaF matrix but (nearly pure) C20F20 could be secured only in very low yield. “Brute‐force” photochlorination (heat, light, pressure, time) provided a mixture of hydrogen‐free, barely soluble C20Cl16 dienes in high yield and C20Cl20 as a trace component. Upon electron‐impact ionization of the C20Cl16 material sequential loss of the chlorine atoms was the major fragmentation pathway furnishing, however, only minor amounts of chlorine‐free C20+ ions. “Brute‐force” photobrominations delivered an extremely complex mixture of polybromides with C20HBr13 trienes as the highest masses. The MS spectra exhibited exclusive loss of the Br substituents ending in rather intense singly, doubly, and triply charged C20H4–0+(2+)(3+) ions. The insoluble ~C20HBr13 fraction (C20Br14 trienes as highest masses) obtained along a modified bromination protocol, ultimately allowed the neat mass selection of C20? ions. The C20Cl16 dienes and C20H0–3Br14–12 tri‐/tetraenes, in spite of their very high olefinic pyramidalization, proved resistant to oxygen and dimerization (polymerization) but added CH2N2 smoothly. Dehalogenation of the respective cycloaddition products through electron‐impact ionization resulted in C22–24H4–8+(2+) ions possibly constituting bis‐/tris‐/tetrakis‐methano‐C20 fullerenes or partly hydrogenated C22, C23, and C24 cages. 相似文献
It is well known that the water-wave problem with weak surface tension has small-amplitude line solitary-wave solutions which to leading order are described by the nonlinear Schrödinger equation. The present paper contains an existence theory for three-dimensional periodically modulated solitary-wave solutions which have a solitary-wave profile in the direction of propagation and are periodic in the transverse direction; they emanate from the line solitary waves in a dimension-breaking bifurcation. In addition, it is shown that the line solitary waves are linearly unstable to long-wavelength transverse perturbations. The key to these results is a formulation of the water wave problem as an evolutionary system in which the transverse horizontal variable plays the role of time, a careful study of the purely imaginary spectrum of the operator obtained by linearising the evolutionary system at a line solitary wave, and an application of an infinite-dimensional version of the classical Lyapunov centre theorem. 相似文献
The investigation of crimes involving chemical or biological agents is infrequent, but presents unique analytical challenges. The protein toxin ricin is encountered more frequently than other agents and is found in the seeds of Ricinus communis, commonly known as the castor plant. Typically, the toxin is extracted from castor seeds utilizing a variety of different recipes that result in varying purity of the toxin. Moreover, these various purification steps can also leave or differentially remove a variety of exogenous and endogenous residual components with the toxin that may indicate the type and number of purification steps involved. We have applied three gas chromatography-mass spectrometry (GC-MS) based analytical methods to measure the variation in seed carbohydrates and castor oil ricinoleic acid, as well as the presence of solvents used for purification. These methods were applied to the same samples prepared using four previously identified toxin preparation methods, starting from four varieties of castor seeds. The individual data sets for seed carbohydrate profiles, ricinoleic acid, or acetone amount each provided information capable of differentiating different types of toxin preparations across seed types. However, the integration of the data sets using multivariate factor analysis provided a clear distinction of all samples based on the preparation method, independent of the seed source. In particular, the abundance of mannose, arabinose, fucose, ricinoleic acid, and acetone were shown to be important differentiating factors. These complementary tools provide a more confident determination of the method of toxin preparation than would be possible using a single analytical method. 相似文献
The template-directed formation of regular nanoparticle arrays on two-dimensional crystalline protein layers after their treatment
with metal salt complexes was studied by transmission electron microscopy. For these investigations, bacterial surface layers
(S layers), recrystallized in vitro into sheets and tube-shaped protein crystals with typical dimensions in the micrometer range, were used as the template.
As identified by electron holography and scanning force microscopy, the S-layer tubes form alternating double layers when
deposited onto a solid substrate surface. Two distinct pathways for the metal particle formation at the templates have been
found: the site-specific growth of metal clusters by chemical reduction of the metal salt complexes, and the electron-beam
induced growth of nanoparticles in the transmission electron microscope. Both mechanisms lead to regular arrays with particle
densities > 6×1011cm-2. Nanoparticle formation by electron exposure takes exclusively place in the flat-lying double-layered protein tubes, where
a sufficient amount of metal complexes can be accumulated during sample preparation.
Received 6 December 2000 相似文献
Identification of an electron poor trifluoroacetophenone allows the formation of uniquely stable hemiketals from prochiral oxetanols. When exposed to a cobalt(ii) catalyst, efficient ring-opening to densely functionalized dioxolanes is observed. Mechanistic studies suggest an unprecedented redox process between the cobalt(ii) catalyst and the hemiketal that initiates the oxetane-opening. Based on this observation, a dynamic kinetic resolution of the transient hemiketals is explored that uses a Katsuki-type ligand for stereoinduction (up to 99 : 1 dr and 96 : 4 er) and allows a variety of 1,3-dioxolanes to be accessed (20 examples up to 98% yield).Desymmetrization of prochiral oxetanols via an electron-deficient hemiketal intermediate is achieved. Key to this process is the catalyst''s chiral recognition of one of the two hemiketal enantiomers enabling an efficient dynamic kinetic resolution.相似文献