Allylation of exocyclic N-Acyliminium ions generated from chiral N-[1-(phenylsulfonyl)alkyl]oxazolidin-2-ones

N-[1-(Phenylsulfonyl)alkyl]oxazolidin-2-ones are successfully prepared by condensation of the corresponding optically active oxazolidin-2-ones with aldehydes and benzenesulfinic acid. At low temperature, in the presence of titanium tetrachloride, these sulfones are converted into N-acyliminium ions, which react with allyltrimethylsilane with a variable degree of stereoselectivity. The best results are obtained with (R)-5,5-dimethyl-4-phenyloxazolidin-2-one as a chiral auxiliary. Cleavage of the oxazolidin-2-one ring with lithium/ammonia affords the corresponding homoallylamines, which reveal an absolute configuration opposite that expected on the basis of the usual steric effects. A complexation of the Lewis acid with the N-acyliminium ion may be responsible of this rather unusual stereochemical outcome.     

Bulky trialkylsilyl acetylenes in the Cadiot-Chodkiewicz cross-coupling reaction

Bulky trialkylsilyl-protected alkynes such as triethylsilyl (TES), tert-butyldimethylsilyl (TBS), and triisopropylsilyl (TIPS) acetylenes underwent the Cadiot-Chodkiewicz cross-coupling reaction with different bromoalkynes to form a variety of synthetically useful unsymmetrical diynes in good yields. The diyne alcohol 10 was transformed regio- and stereoselectively into enynes by hydrotelluration, carbometalation, and reduction reactions.     

Interaction of Cu(+) and Cu(2+) ions with alpha-alanine. A density functional study

The Cu(+) and Cu(2+) preferred binding sites on alpha-alanine and their affinity values for this amino acid were determined at the density functional level using three different hybrid exchange correlation potentials and the 6-311++G** basis set. The results demonstrated that the two ions both give stable complexes with alpha-alanine but the stability order of the metalated species and the coordination sites are different depending on the nature of the cation. In particular, the Cu(+)-alpha-alanine ground-state structure is characterized by an eta(2)-N,O coordination with the nitrogen and oxygen atoms belonging to the amino and carbonyl groups, respectively. In contrast, the most stable complex of the Cu(2+)-alpha-alanine system has an eta(2)-O,O coordination with the cation bonded to the -CO(2) (-) moiety of the zwitterionic form of the amino acid. Comparison with the Cu(+) and Cu(2+) affinity values for glycine, computed at the same levels of theory, demonstrated that the relative values do not change significantly as different hybrid functionals are used, although the absolute affinities are strongly influenced by the choice of the hybrid potential.     

Honeycomb pattern formation by laser-beam filamentation in atomic sodium vapor

We have observed transverse pattern formation leading to highly regular structures in both the near and far fields when a near-resonant laser beam propagates without feedback through an atomic sodium vapor. One example is a regular far-field honeycomb pattern, which results from the transformation of the laser beam within the vapor into a stable three-lobed structure with a uniform phase distribution and highly correlated power fluctuations. The predictions of a theoretical model of the filamentation process are in good agreement with these observations.     

TEM Imaging of Mass-selected Polymer Molecules

Polyethylene glycol (PEG) molecules with masses below 1300amu are electrosprayed (ES) from solution, mobility-selected at high resolution in a differential mobility analyzer (DMA), collected on a grid and imaged by transmission electron microscopy (ES–DMA–TEM). The DMA resolves individual n-mers, and selects only one out of the many present in the original sample. Ion identity is established from parallel mass spectra (ES-MS). The images reveal spherical particles 1.46nm in diameter, in good agreement with the known ion mass and bulk density. The DMA-selection technique opens new paths for the study of very small particles.     

Photoelectron diffraction study of a catalytically active overlayer: C2H2 on Pd{111}

A quantitative structure determination of a newly discovered (2×2) adsorption phase of acetylene chemisorbed on Pd{111} has been performed by scanned-energy mode photoelectron diffraction: this phase corresponds to the threshold coverage for the catalytic conversion of acetylene to benzene. The carbon atoms in the C₂H₂ molecule are located almost over bridge sites with a C–C bond length of 1.34+0.10 Å, the centre of the molecule being positioned almost over a hollow site. Of the two hollow sites the hcp site (directly above a second layer Pd atom) is favoured, particularly by a subset of the data most sensitive to this aspect of the structure, but the full analysis indicates that the fcc site (above a third layer Pd atom) cannot formally be excluded. The adsorption site adopted by acetylene in the higher coverage phase on Pd{111} is essentially identical. This is the dominant structure in the coverage regime which is catalytically active for the conversion of acetylene to benzene. The implications of these findings for acetylene coupling reactions over Pd{111} are discussed.     

A multimode small signal analysis of the single pass free electron laser

We analyze the small signal multimode aspects of the single pass free electron laser and study the operating conditions and the relevant laser parameters.     

The diameter of random regular graphs

We give asymptotic upper and lower bounds for the diameter of almost everyr-regular graph onn vertices (n → ∞).