Photoinitiated anti-Hydropentafluorosulfanylation of Terminal Alkynes

A photoinitiated anti-hydropentafluorosulfanylation of terminal alkynes using SF₅Cl and (TMS)₃SiH as the hydrogen atom donor is reported. This transformation generates selectively (Z)-(1-alken-1-yl)pentafluoro-λ⁶-sulfanes (Z:E : >85:15), thus allowing the preparation of this previously unknown geometrical isomer. DFT calculations highlight that the selectivity is due to the intrinsic preference of SF₅-substituted vinylic radicals to adopt a cis geometry, and to increased steric contacts during the transition structures leading to the minor (E)-products.     

One‐Minute Wear‐Rate Measurement

Summary: Single‐asperity wear measurements were performed on small quantities of polyethylene with different molecular weights. Experiments could be performed on a surface area of 25 × 3000 μm². A quantitative value of wear rate could be obtained that was proportional to macroscopic results relating the wear rate to the molecular weight. Material transfer limited the maximum sliding velocity to 1 μm · s⁻¹. Possible solutions for this problem have been identified and will be a subject of further investigation.

Indentation profile for high‐molecular‐weight polyethylene (PE‐H).     

An unexpected result in the condensation of 2-amino-6-bromopyridine with ethyl 4-chloroacetoacetate: Four dihalo-substituted pyrido[1,2-a]pyrimidin-4-ones as reaction products

We report here the condensation of 2-amino-6-bromopyridine with ethyl 4-chloroacetoacetate in polyphosphoric acid. In this reaction, a mixture of the four possible dihalo-4H-pyrido[1,2-a]pyrimidin-4-ones 1–4 was obtained.     

A facile synthesis of 3,5-diaminopyrazole-4-carbothioamides and 3,5-diaminopyrazole-4-carboxylates

Simple and convenient synthesis of hitherto unknown 3,5-diaminopyrazole-4-carbothioamides 3 as well as new ethyl 3,5-diaminopyrazole-4-carboxylates 7 is reported. The key intermediates were 2-cyano-propenethioamides 2 and 2-(ethoxycarbonyl)propenethioamides 5 which were readily obtained by reaction of phenyl isothiocyanate with 3-(2-acylhydrazino)-3-aminopropenenitriles 1 and ethyl 3-(2-acylhydrazino)-3-aminopropenoates 4 respectively. Intramolecular cyclization of compounds 2 afforded pyrazole-4-carbothioamides 3 while propenethioamides 5 gave pyrazole-4-carboxylates 7 .     

Electro-oxidation of CO on Pd black in phosphotungstic acid

Electro-oxidation of CO by Pd black in phosphotungstic acid (PWA) has been studied on porous gas-diffusion electrodes. Galvanostatic steady-state polarization data show that the reaction proceeds under strong activation control at low current densities. The current density at 0.5 V_NHE is found to be three times higher than that in a H₂SO₄ reference electrolyte. X-ray photoelectron spectroscopic data on various electrochemically tested electrodes show lower oxidized Pd species in PWA than in H₂SO₄. It is conjectured that an increase in the electron density around Pd sites is ameliorated by PWA in relation to H₂SO_4, with a consequent promoting effect of the heteropolyacid towards CO electro-oxidation. Received: 29 July 1998 / Accepted: 15 September 1998     

Identifying the true origins of selectivity in chiral phosphoric acid catalyzed N-acyl-azetidine desymmetrizations

The first catalytic intermolecular desymmetrization of azetidines was reported by Sun and coworkers in 2015 using a BINOL-derived phosphoric acid catalyst (J. Am. Chem. Soc. 2015, 137, 5895–5898). To uncover the mechanism of the reaction and the origins of the high enantioselectivity, Density Functional Theory (DFT) calculations were performed at the B97D3/6-311+G(2d,2p)/SMD(toluene)//B97D3/6-31G(d,p)/CPCM(toluene) level of theory. Comparison of four possible activation modes confirms that this reaction proceeds through the bifunctional activation of the azetidine nitrogen and the thione tautomer of the 2-mercaptobenzothiazole nucleophile. Upon thorough conformational sampling of the enantiodetermining transition structures (TSs), a free energy difference of 2.0 kcal mol⁻¹ is obtained, accurately reproducing the experimentally measured 88% e.e. at 80 °C. This energy difference is due to both decreased distortion and increased non-covalent interactions in the pro-(S) TS. To uncover the true origins of selectivity, the TSs optimized with the full catalyst were compared to those optimized with a model catalyst through steric maps. It is found that the arrangements displayed by the substrates are controlled by strict primary orbital interaction requirements at the transition complex, and their ability to fit into the catalyst pocket drives the selectivity. A general model of selectivity for phosphoric acid-catalyzed azetidine desymmetrizations is proposed, which is based on the preference of the nucleophile and benzoyl group to occupy empty quadrants of the chiral catalyst pocket.

The origins of selectivity in azetidine desymmetrizations have been determined computationally. Comparison of structures with model and full catalysts provided key details missed by typical analyses of the stereodetermining transition structures.     

Structural and Electrical Properties of Sol–Gel-processed CdTiO3 Powders and Films

Cadmium titanate, CdTiO₃, was prepared by the sol–gel technique in bulk and in thin film form. The thermal evolution of the gels and the phase changes were studied by thermo- gravimetric analysis (TGA), X-ray diffractometry (XRD) and Raman and energy-dispersive (EDS) spectroscopies. The morphology of the samples was observed using scanning electron microscopy (SEM). Gels heated to 800 °C gave rise to powders with only the ilmenite-like phase. The orthorhombic perovskite phase is the only crystalline phase observed after a 4 h heat-treatment at 1100 °C. With respect to the conventional preparation method by solid-state reaction, by the sol–gel method it is possible to prepare the ilmenite phase at lower temperatures and the perovskite phase in a shorter time. Clear, homogeneous thin films were obtained by the dip-coating method. The refraction index and the thickness of the films were measured using ellipsometry. The humidity-sensitive electrical properties were measured for thin films deposited on alumina substrates with comb-type gold electrodes, heated to 200 °C and 450 °C. The films heated to 200 °C, which still contained organics, showed a variation of the resistance of six orders of magnitude in the relative humidity (RH) range tested (4–87% RH). The films heated to 450 °C, made of ilmenite-type CdTiO₃, were nearly insensitive to RH. © 1997 by John Wiley & Sons, Ltd.     

A highly accurate GPU Lattice Boltzmann method with directional interpolation for the probability distribution functions

In this article, a highly accurate and graphics processing unit (GPU)-accelerated Lattice Boltzmann Method (LBM) is presented. The methodology is derived from a combination of conventional and recent LBM algorithms, mainly focusing on reducing the computational time, memory allocation, and complexity of existing algorithms. The general implementation focuses on accelerating the overall methodology using GPGPU technology based on Compute Shaders from OpenGL and avoids the storage of the distribution function components to reduce the memory allocation size. Furthermore, an efficient spatial interpolation of the probability distribution function components is described, based on a directional interpolation, without unnecessary control points for the reconstruction of virtual nodes data. The present methodology, tested for spatial accuracy via two- and three-dimensional Lid-Driven Cavity benchmark cases, shows excellent agreement with the results reported in the literature. Additionally, time efficiency is analyzed by comparing different configurations for the construction of virtual streaming points.