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.     

Neutron diffraction structure determination of the high-temperature form of lithium tritantalate,HLiTa3O8

The crystal structure of HLiTa₃O₈ has been reexamined by electron and neutron diffraction techniques. Neutron Weissenberg and electron diffraction photographs show that the space group of the compound isPmmn and notPmma as determined previously by X-ray diffraction techniques. There are eight molecules in the unit cell of lattice parametersa = 16.718(2)A?,b = 7.696(1)A?,c = 8.931(1)A?. These values show that thebaxis of the new cell is doubled with respect to the parameter measured by X-rays. The structural refinement was based on 1074 independent reflections measured on a single crystal with a four-circle neutron diffractometer. The positions of all atoms, including the lithium atoms have been determined. The finalR andwRfactors were 0.036 and 0.035, respectively. The eight lithium cations occupy two sets of4fpositions(x,¼, z)of thePmmn space group. The ordering of four lithium ions over two sets of possible positions (4j) of space groupPmma is responsible for the doubling of theb axis. The other four Li⁺ occupy two sets of positions (2d) of space groupPmma. All lithium ions are surrounded by 12 oxygen atoms arranged as cuboctahedra. The large thermal vibrations found for the lithium atoms and the ionic conductivity of HLiTa₃O₈ at high temperatures are consistent with weak LiO bonding.     

Structural aspects of the metal-insulator transitions in (Ti0.9975V0.0025)4O7

The structure of (Ti_0.9975V_0.0025)₄O₇ has been refined at 298, 135, and 100°K from single-crystal X-ray diffraction data. This sample belongs to the region of the $\text{Ti}{}_4\text{O}{}_7\text{V}{}_4\text{O}{}_7$ phase diagram where still two electrical transitions are observed. The three structures of the V-doped sample are almost identical to the corresponding structures of pure Ti₄O₇. The charges are disordered in the metallic phase, they order very slightly in the intermediate phase, and they are almost completely ordered along the 3113 and the 4224 chains in the low-temperature phase. There is no evidence of cation pair-bond formation in the intermediate phase, whereas the Ti³⁺ cations are all paired in the low-temperature phase. The cationic Debye-Waller factors in the intermediate phase are anomalously large, which is indicative of a disorder. Thus, in order to explain the physical properties of the intermediate phase, as in the case of the pure sample, all the Ti³⁺ cations are thought to form pair bonds with no long-range order. The TiTi distances along the pseudorutile c-axis are shorter in the V-doped sample than in the pure one. This indicates that the relative bond strengths increase with the incorporation of vanadium. The main difference between pure Ti₄O₇ and (Ti_0.9975V_0.0025)₄O₇ is in the lattice parameters and unit-cell volume variations with temperature. In the latter sample two discontinuities are observed, each corresponding to one transition, whereas in the former only one transition is observed, which corresponds to the higher transition. In the low-temperature phase the V cations are either 3+ or 4+ and are either on the 3113 chains or on the 4224 chains. The V doping gives rise to four different patterns. Each one is discussed in order to explain the differences between V-doped sample and pure Ti₄O₇.     

(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.     

The crystal structure of SnYb3Rh4Sn12, a new ternary superconducting stannide

The crystal structure of superconducting SnYb₃Rh₄Sn₁₂ has been determined from single-crystal X-ray diffraction data. This compound is cubic, space group Pm3n, $\text{a}{}_{\mathrm{o}}\text{= 9.676}\text{A}\text{?}$ (1) and has two formulae per unit cell. The structure was solved from Patterson and subsequent Fourier synthesis. The least squares refinement was based on 375 independent reflections. The final R and wR factors were 0.015 and 0.014, respectively. The two Sn(1) atoms occupy the 2a (000) positions, the six Yb atoms the 6d ($\text{1}\text{4}$$\text{1}\text{2}$ 0) positions, the eight Rh atoms the 8e ($\text{1}\text{4}$$\text{1}\text{4}$$\text{1}\text{4}$) positions and the twenty-four Sn(2) atoms the 24k (Oyz) positions (y ～ 0.31, z ～ 0.15). The Sn(2) atoms form a tridimensional array of corner-sharing trigonal prisms whose centers are occupied by the rhodium atoms. The Sn(1) and the Yb atoms occupy the icosahedral and cuboctahedral holes of this array, respectively. They form a sublattice which has the arrangement found in the structure of the A15 compounds. The structure of SnYb₃Rh₄Sn₁₂ can be described as containing two interpenetrated structures, namely Yb₃Sn and RhSn₃, or as having an A15 arrangement of clusters of atoms such as (SnSn₁₂) and (YbSn₁₂). These clusters are bound together by face-sharing among them; and by the rhodium atoms. An analogy is drawn between SnYb₃Rh₄Sn₁₂ and the perovskite-like ternary oxides A′A″₃B₄O₁₂.     

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.     

Classification of lead white pigments using synchrotron radiation micro X-ray diffraction

Lead white pigment was used and synthesised for cosmetic and artistic purposes since the antiquity. Ancient texts describe the various recipes, and preparation processes as well as locations of production. In this study, we describe the results achieved on several paint samples taken from Matthias Grünewald’s works. Grünewald, who was active between 1503 and 1524, was a major painter at the beginning of the German Renaissance. Thanks to X-ray diffraction analysis using synchrotron radiation, it is possible to associate the composition of the paint samples with the masters ancient recipes. Different approaches were used, in reflection and transmission modes, directly on minute samples or on paint cross-sections embedded in resin. Characterisation of lead white pigments reveals variations in terms of composition, graininess and proportion of mineral phases. The present work enlightens the presence of lead white as differentiable main composition groups, which could be specific of a period, a know-how or a geographical origin. In this way, we aim at understanding the choices and the trading of pigments used to realise paintings during northern European Renaissance. PACS 61.10.Nz; 07.85.Qe; 61.43.Gt