alpha,beta,gamma-Trifluoroalkanes: a stereoselective synthesis placing three vicinal fluorines along a hydrocarbon chain

A regio- and stereoselective method for the synthesis of alpha,beta,gamma-trifluoroalkanes is described which allows the synthesis of single diastereoisomers of this structural motif. The methodology relied upon regiospecific and stereospecific hydrogen fluoride ring opening of allylic epoxides and then Sharpless cyclic sulfate methodology followed by nucleophilic fluoride attack to introduce the second fluorine. The resultant difluoro alcohol was converted to its triflate which was also displaced by fluoride ion to generate the alpha,beta,gamma-trifluoroalkanes.     

Electronic structure of ReO3Me by variable photon energy photoelectron spectroscopy,absorption spectroscopy and density functional calculations

Valence photoelectron (PE) spectra have been measured for ReO(3)Me using a synchrotron source for photon energies ranging between 20 and 110 eV. Derived branching ratios (BR) and relative partial photoionization cross sections (RPPICS) are interpreted in the context of a bonding model calculated using density functional theory (DFT). Agreement between calculated and observed ionization energies (IE) is excellent. The 5d character of the orbitals correlates with the 5p --> 5d resonances of the associated RPPICS; these resonances commence around 47 eV. Bands with 5d character also show a RPPICS maximum at 35 eV. The RPPICS associated with the totally symmetric 4a(1) orbital, which has s-like character, shows an additional shape resonance with an onset of 43 eV. The PE spectrum of the inner valence and core region measured with photon energies of 108 and 210 eV shows ionization associated with C 2s, O 2s, and Re 4f and 5p electrons. Absorption spectra measured in the region of the O1s edge showed structure assignable to excitation to the low lying empty "d" orbitals of this d(0) molecule. The separation of the absorption bands corresponded with the calculated orbital splitting and their intensity with the calculated O 2p character. Broad bands associated with Re 4d absorption were assigned to (2)D(5/2) and (2)D(3/2) hole states. Structure was observed associated with the C1s edge but instrumental factors prevented firm assignment. At the Re 5p edge, structure was observed on the (2)P(3/2) absorption band resulting from excitation to the empty "d" levels. The intensity ratios differed from that of the O 1s edge structure but were in good agreement with the calculated 5d character of these orbitals. An absorption was observed at 45 eV, which, in the light of the resonance in the 4a(1) RPPICS, is assigned to a 4a(1) --> ne, na(2) transition. The electronic structure established for ReO(3)Me differs substantially from that of TiCl(3)Me and accounts for the difference in chemical behavior found for the two complexes.     

Electronic structure of M(BH4)4, M = Zr, Hf, and U, by variable photon-energy photoelectron spectroscopy and density functional calculations

Photoelectron (PE) spectra have been obtained for the M(BH(4))(4) (M = Zr, Hf and U) molecules in the 20-60 eV photon-energy range, and for M = U, also in the 90-120 eV region. Derived branching ratios (BR) and relative partial-photoionization cross sections (RPPICS) of the valence bands are used to confirm band assignment and demonstrate d-orbital covalency for all three compounds and f-orbital covalency for U(BH(4))(4). Core ionizations are identified and used to confirm resonance features in the RPPICS. The absorption spectrum of Zr(BH(4))(4) between 20 and 60 eV shows 4p absorption at 35.5 eV, coincident with the 4p-4d resonance in the RPPICS of the 1e and 2t(2) ionization bands of Zr(BH(4))(4). Less intense absorption bands at 32.5 and 33.8 eV correspond with shape resonance features in the 1a(1) and 1t(2) PE bands. The RPPICS of the f band of U(BH(4))(4) shows two strong resonant features between 95 and 120 eV. Direct photoemission of the 5f electrons from U(BH(4))(4) results only in the observation of the (2)F(5/2) ion state of [U(BH(4))(4)](+), but in the 5d-5f resonant region, a weak band corresponding to the (2)F(7/2) ion state is also observed. The splitting of the 1t(2) band of U(BH(4))(4) is attributed to a small contribution of U 6p semi-core electrons to this MO. Density functional calculations give a good estimate of the pattern of ionization energies, although the calculated absolute values are lower than the experimental values, the first IE by 0.5 eV for Zr(BH(4))(4) and Hf(BH(4))(4) and 1.0 eV for U(BH(4))(4). The MO compositions are in very good agreement with the deductions made from the BR and RPPICS analyses.     

Electrophilic Azido Selenenylation of Alkenes. A Simple Synthetic Route to Racemic Taxol Side Chain

Simple alkenes react with PhSeOTf and NaN₃ in MeCN to afford β-phenylseleno azides as the result of a stereospecific trans addition. The regioselectivity of the process is determined by the structure of the alkene.     

Error bounds for suboptimal solutions to kernel principal component analysis

Suboptimal solutions to kernel principal component analysis are considered. Such solutions take on the form of linear combinations of all n-tuples of kernel functions centered on the data, where n is a positive integer smaller than the cardinality m of the data sample. Their accuracy in approximating the optimal solution, obtained in general for n = m, is estimated. The analysis made in Gnecco and Sanguineti (Comput Optim Appl 42:265–287, 2009) is extended. The estimates derived therein for the approximation of the first principal axis are improved and extensions to the successive principal axes are derived.     

High-field transport in semiconductor superlattices for interacting Wannier–Stark levels

We developed a microscopic theory of electron transport in superlattices within the Wannier–Stark approach by including the interaction associated with Zener tunneling between the energy levels pertaining to adjacent quantum wells. By using a Monte Carlo technique we have simulated the hopping motion associated with absorption and emission of polar optical phonons and determined the main transport parameters for the case of a GaAs/GaAlAs structure at room temperature. Interaction between the levels is found to be responsible for a systematic increase of the level energy with respect to the bottom of the quantum well at electric fields above about 20 kV/cm. When compared with the non-interacting case, at the highest fields the average carrier energy evidences a consistent increase, which leads to a significant softening of the negative slope of both the drift velocity and diffusivity versus electric field behavior.     

Structural analysis of lignin in chestnut wood by pyrolysis-gas chromatography/mass spectrometry

The goal of the study was to determine the percentage of lignin in chestnut wood and to characterize structurally this polymer by pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) according to a possible influence on ring shake defect. The sampling was performed in different sites of Lazio Region in Italy. Wood disks from trees with ring shake defect and without ring shake defect were selected and little amount of heartwood (1-2 g) sampled between 6th and 10th and between 11th and 15th annual growth ring were analyzed. The study allowed quantifying lignin content, which is in average 26% and syringyl/guaiacyl (S/G) ratio, which present an average value of 2.43. Belong the pyrolysis products of wood, the trees characterized by ring shake show a higher concentration of the phenol homosyringaldehyde respect to the healthy trees. There is not any evident difference in the products composition of pyrolysis in respect to trees’ age, especially the critical period of 12-14 years in which ring shake generally becomes more evident. The site of wood provenance is the higher factor of variability in the structure of lignin for many phenols.