Additions of enantiopure alpha-sulfinyl carbanions to (S)-N-sulfinimines: asymmetric synthesis of beta-amino sulfoxides and beta-alphamino alcohols

The addition of the lithium anions derived from (R)- and (S)-methyl and -ethyl p-tolyl sulfoxides to (S)-N-benzylidene-p-toluenesulfinamide provides an easy access route to enantiomerically pure beta-(N-sulfinyl)amino sulfoxides. Stereoselectivity can be achieved when the configurations at the sulfur atoms of the two reagents are opposite (matched pair), thus resulting in only one diastereoisomer, even for the case in which two new chiral centers are created. The N-sulfinyl group primarily controls the configuration of the carbon bonded to the nitrogen, whereas the configuration of the alpha-sulfinyl carbanion seems to be responsible for the level of asymmetric induction, as well as for the configuration of the new stereogenic C-SO carbon in the reactions with ethyl p-tolyl sulfoxides. An efficient method for transforming the obtained beta-(N-sulfinyl)amino sulfoxides into optically pure beta-amino alcohols, based on the stereoselective non-oxidative Pummerer reaction, is also reported.     

(Z)-3-p-tolylsulfinylacrylonitrile as a chiral dienophile: diels-alder reactions with furan and acyclic dienes

The behavior of (Z)-3-p-tolylsulfinylacrylonitrile (1) as a chiral dienophile has been evaluated from its reactions with furan and acyclic dienes. Electrostatic interactions of the cyano group with the sulfinyl one restrict the conformational mobility around the C-S bond, thus controlling the pi-facial selectivity, which is almost complete in all cases, the approach of the diene from the less-hindered face of the dienophile (that bearing the lone electron pair) in the predominant rotamer being the favored one. The regioselectivity is also completely controlled by the cyano group. Additionally, the reactivity of compound 1 as well as its endo-selectivity are both higher than those observed for the corresponding (Z)-3-sulfinylacrylates, thus proving the potential of sulfinylnitriles as chiral dienophiles.     

Spin-singlet clusters in the ladder compound NaV2O5

The space group of alpha(')-NaV2O5 turns below T(c) = 34 K from Pmmn with all V sites equivalent, into Fmm2 with three independent vanadium sites per layer. This is incompatible with models of charge ordering into V4+ and V5+. Our structure determination indicates that the phase transition consists of a charge ordering with three distinct valence states, formally V4+, V4.5+, and V5+. The singlet formation is not associated with dimerization on the spin ladder, but with the formation of spin clusters. Finally, we ascribe the quadrupling of the c axis to the large polarizability of the V2O5 skeleton.     

Dependence of pulser driving responses on electrical and motional characteristics of NDE ultrasonic probes

Acoustic performance in ultrasonic transmitters can be improved by means of a suitable electrical driving response and matching/tuning networks. It is important to predict this electrical response, but doing so is not easy because it departs notably from the nominal pattern with the loading probes. In practice, the analysis of HV pulser spikes in NDE applications requires fairly complex models in the transient regime and, in addition, non-linear problems could arise, especially in the case of tuned transmitters. In this paper, the most relevant influences of loading characteristics of NDT ultrasonic probes on the pulser electrical driving responses are evaluated in time and frequency domains. Conventional pulse generators and typical NDE pulsers are considered. Driving responses are analysed across commercial ultrasonic probes and, alternatively, across similar purely electrical loads. Distinct influences on pulser responses from electrical and motional sections of the probes are identified. All these aspects are studied on the basis of experimental and computer results.     

Magnetic structures determined by neutron diffraction in the EuBϵ−xCx system

Magnetic structures of pure and carbon-doped europium hexaboride EuB_6?xC_x, were determined by neutron diffraction on powders prepared from ¹¹B and ¹⁵³Eu. EuB₆ is a simple ferromagnet, whereas the x = 0.20 compound has an incommensurate spiral structure with propagation vector τ = (0.28, 0, 0). Data for a magnetically inhomogeneous intermediate composition, x = 0.05, indicate a mixture of ferromagnetic and helimagnetic domains with τ = (0, 0, 0.104). Helimagnetism in EuB_6?xC_x arises from a competition between ferromagnetic near-neighbour exchange and antiferromagnetic interactions due to conduction electrons.     

Etude par diffraction de neutrons de la solution solide Pb1−1xThxF2+2x

Bragg neutron diffraction studies have been carried out on the fluorite type solid solutions Pb_1?xTh_xF_2+2x (0 < × < 0.25). The distribution of fluorine atoms between normal and interstitial sites as a function of the substitution rate is given. A volume of presence has been determined for interstitial anions. Electrical and structural data are correlated. Two conductivity mechanisms are proposed according to the fact that the pathway is free or blocked up. The best electrical performances are obtained for Pb_0.875Th_0.125F_2.25, which corresponds to a maximum of vacancies.     

Etude des proprietes structurales et electriques de la solution solide Pb1−xBixOxF2−x

An investigation of the PbF₂BiOF system at 600°C has allowed to isolate a Pb_1-xBi_xO_xF_2-x solid solution for x ? 0.67 the structure is of fluorite type and for 0.67 < × ? 0.80 it presents a fluorite derived rhombohedral distortion. The ionic conductivity has been examined. For × ? 0.50 Pb_1-xBi_xO_xF_2-x has electrical performances of the same order than those of the best so far known fluorides. Bragg neutron diffraction investigations have been carried out in order to account for the electrical behavior of the cubic solid solution (0 < x ? 0.67). The distribution of the anions between normal and interstitial sites is given as a function of substitution rate. The structure has been described on the basis of local BiOF-type clusters. The formation of isolated antiprisms BiX₈ (X = O,F) allows to understand the evolution of the electrical properties.     

Neutron diffraction investigation of the cationic distribution in the structure of the spinel-type solid solutions Li2−2xM1+xCl4 (M = Mg, V): Correlation with the ionic conductivity and NMR data

The structure of the spinel-type solid solutions Li_2?2xMin1+xCl₄ M = Mg, V has been studied by neutron diffraction. The cationic distribution in the stoichiometric phases Li₂MgC1₄ and Li₂ VCl₄ is inverse at room temperature, but some vacancies tend to appear in the tetrahedral 8(a) sites. With increasing temperature a gradual migration of the Li⁺ ions from the 8(a) positions into the octahedral 16(c) ones has been established for Li₂MgCl₄. The extra vacancies in the highly deficient spinels LiMgCl₃ and LiVCl₃ ($\text{x =}\text{1}\text{3}$) have been found to be located in the tetrahedral 8(a) sites only. The correlation between the thermal evolutiomn of the cationic dustribution and the corresponding conductivity and NMR data gives a satisfying interpretation of the jump mechanisms responsible for the high ionic mobility in these materials.     

Etude de la structure magnetique et de la transition de typr “spin-flop” de Rb2FeF5

The nature of the magnetic interactions in the chain compound Rb₂FeF₅ has been investigated using neutron diffraction and magnetic measurements under high applied fields. Rb₂FeF₅ orders antiferromagnetically at T_N = 8.0 ± 0.5 K; the magnetic structure is of the A_Z + G_X mode and the moment of the Fe³⁺ ion extrapoled to 0K is 3.5 ± 0.2 μ_B, this low value being due to zero-point spin reduction. Within a chain the Fe³⁺ ions are antiferromagnetically coupled with an exchange constant of J/k = ?8.8 K. A spin-flop behavior has been observed and interpreted on the basis of the molecular field theory. The critical field was found to be H_C = 65 kOe at 1.7 K.