Synthesis of Triborylalkenes from Terminal Alkynes by Iridium‐Catalyzed Tandem C?H Borylation and Diboration

A two‐step reaction to convert terminal alkynes into triborylalkenes is reported. In the first step, the terminal alkyne and pinacolborane (HBpin) are converted into an alkynylboronate, which is catalyzed by an iridium complex supported by a SiNN pincer ligand. In the second step, treatment of the reaction mixture with CO generates a new catalyst which mediates dehydrogenative diboration of alkynylboronate with pinacolborane. The mechanism of the diboration remains unclear but it does not proceed via intermediacy of hydroboration products or via B₂pin₂.     

Unexpected Chemistry from the Reaction of Naphthyl and Acetylene at Combustion‐Like Temperatures

The hydrogen abstraction/acetylene addition (HACA) mechanism has long been viewed as a key route to aromatic ring growth of polycyclic aromatic hydrocarbons (PAHs) in combustion systems. However, doubt has been drawn on the ubiquity of the mechanism by recent electronic structure calculations which predict that the HACA mechanism starting from the naphthyl radical preferentially forms acenaphthylene, thereby blocking cyclization to a third six‐membered ring. Here, by probing the products formed in the reaction of 1‐ and 2‐naphthyl radicals in excess acetylene under combustion‐like conditions with the help of photoionization mass spectrometry, we provide experimental evidence that this reaction produces 1‐ and 2‐ethynylnaphthalenes (C₁₂H₈), acenaphthylene (C₁₂H₈) and diethynylnaphthalenes (C₁₄H₈). Importantly, neither phenanthrene nor anthracene (C₁₄H₁₀) was found, which indicates that the HACA mechanism does not lead to cyclization of the third aromatic ring as expected but rather undergoes ethynyl substitution reactions instead.     

Exploring Route for Pyrophosphate-based Electrode Materials: Interplay between Synthesis and Structure

New sodium vanadium(III) hydrogenphosphate hydrate NaV(HPO₄)₂(H₂O)_0.5 and sodium vanadium(III) hydrogenphosphate β-NaV(HPO₄)₂ were prepared in mild hydrothermal conditions. The crystal structures of NaV(HPO₄)₂(H₂O)_0.5 [space group Cc, Z = 4, a = 8.46174(19) Å, b = 9.52583(19) Å, c = 8.69376(15) Å, β = 110.9553(11) °, V = 654.41(2) ų] and β-NaV(HPO₄)₂ [space group C2/c, Z = 4, a = 7.8681(3) Å, b = 9.8451(3) Å, c = 8.5180(2) Å, β = 107.626(2) °, V = 628.85(3) ų] were solved and refined from X-ray powder diffraction data. Both compounds were used as precursors in a new route for the preparation of attractive cathode material β-NaVP₂O₇. The formation of the structure motive providing fast sodium ion diffusion at the first synthesis stage and its further conservation upon stepwise dehydration was revealed. The oversized Na⁺-embedding channels are stabilized by site-coordinated water in NaV(HPO₄)₂(H₂O)_0.5 structure. The topology resemblance and difference in known sodium vanadium(III) complex phosphates are discussed.     

An Alternative Approach to Study Bifurcation from a Limit Cycle in Periodically Perturbed Autonomous Systems

The goal of this paper is to present a new method to prove bifurcation of a branch of asymptotically stable periodic solutions of a T-periodically perturbed autonomous system from a T-periodic limit cycle of the autonomous unperturbed system. The method is based on a linear scaling of the state variables to convert, under suitable conditions, the singular Poincaré map (with two singularity conditions) associated to the perturbed autonomous system into an equivalent non-singular equation to which the classical implicit function theorem applies directly. As a result we obtain the existence of a unique branch of T-periodic solutions (usually found for bifurcations of co-dimension 2) as well as a relevant property of the spectrum of their derivatives. Finally, by a suitable representation formula of the classical Malkin bifurcation function, we show that our conditions are equivalent to the existence of a non-degenerate simple zero of the Malkin function. The novelty of the method is that it permits to solve the problem without explicit reduction of the dimension of the state space as it is usually done in the literature by the Lyapunov–Schmidt method.     

Simulation of the diffusion of point defects in structures with local elastic stresses

The stress-mediated diffusion of nonequilibrium point defects into the bulk of a semiconductor is investigated by computer simulation. It is assumed that the point defects are generated on the surface of a semiconductor and that in the course of diffusion they pass through the local region of elastic stresses because the average length of defect migration is greater than the thickness and depth of the strained layer. Within the strained layer, point defect segregation or heavy defect depletion occurs if defect drift under stresses is directed in or out of the layer, respectively. The calculations also show that, in contrast to the case of local defect sink, the local region of elastic stresses practically does not change the distribution of defects beyond this region if there is no generation/absorption of point defects within the strained layer.     

Ultra high performance liquid chromatography-time of flight mass spectrometry for analysis of avocado fruit metabolites: method evaluation and applicability to the analysis of ripening degrees

We have developed an analytical method using UHPLC-UV/ESI-TOF MS for the comprehensive profiling of the metabolites found in the methanolic extracts of 13 different varieties of avocado at two different ripening degrees. Both chromatographic and detection parameters were optimized in order to maximize the number of compounds detected and the sensitivity. After achieving the optimum conditions, we performed a complete analytical validation of the method with respect to its linearity, sensitivity, precision, accuracy and possible matrix effects. The LODs ranged from 1.64 to 730.54 ppb (in negative polarity) for benzoic acid and chrysin, respectively, whilst they were found within the range from 0.51 to 310.23 ppb in positive polarity. The RSDs for repeatability test did not exceed 7.01% and the accuracy ranged from 97.2% to 102.0%. Our method was then applied to the analysis of real avocado samples and advanced data processing and multivariate statistical analysis (PCA, PLS-DA) were carried out to discriminate/classify the examined avocado varieties. About 200 compounds belonging to various structural classes were tentatively identified; we are certain about the identity of around 60 compounds, 20 of which have been quantified in terms of their own commercially available standard.     

Effect of monomer mixture composition on structure and chromatographic properties of poly(divinylbenzene-co-ethylvinylbenzene-co-2-hydroxyethyl methacrylate) monolithic rod columns for separation of small molecules

Porous poly(divinylbenzene-co-ethylvinylbenzene-co-2-hydroxyethyl methacrylate) monoliths were synthesized via thermally initiated free-radical polymerization in confines of surface-vinylized glass columns (150 mm × 3 mm i.d.) and applied to the reversed-phase separation of low-molecular-weight aromatic compounds. In order to compensate for the polymer shrinkage during the synthesis and prevent the monolith from detachment from the column wall, polymerization was conducted under nitrogen pressure. The reaction proceeded at 60°C for 22 h. 2,2'-Azo-bis-isobutironitrile was used as the initiator and 1-dodecanol was used as the porogen. A series of monoliths with different monomer ratios were obtained. All the monoliths had high specific surface areas ranging from 370 to 490 m(2)/g. In the studied range of monomer mixture compositions, the mechanical stability of the stationary phase in water/acetonitrile eluents was found to be high enough and practically insensitive to the fraction of 2-hydroxyethyl methacrylate (HEMA). Increasing the molar fraction of HEMA from 10.5% to 14.7% resulted in the decrease of column permeability by two orders of magnitude (from 1.1×10(-12) to 1.8×10(-14) m(2)) and led to weaker retention of alkylbenzenes. The higher HEMA content was shown to reduce the plate height of the columns in the separation of small molecules from 160-490 μm to 40-76 μm. This was attributed mainly to the decrease of the domain size of the monoliths leading to lower eddy dispersion and mass transfer resistance in the column.     

Reactions of 2-(trifluoromethyl)chromones with cyanoacetamides, ethyl cyanoacetate and diethyl malonate. Unexpected synthesis of benzo[c]coumarin derivatives

While 2-(trifluoromethyl)chromones react with cyanoacetamides in the presence of sodium ethoxide to produce 6-(2-hydroxyaryl)-4-(trifluoromethyl)-2-oxo-1,2-dihydropyridine-3-carbonitriles, their reactions with ethyl cyanoacetate and diethyl malonate under the same conditions took an entirely different course and gave novel functionalized derivatives of 6H-benzo[c]chromen-6-one.     

Enantioseparation of organic acids of pharmaceutical interest using eremomycin as a chiral selector

Strong adsorption of eremomycin on the fused-silica capillary wall was used for separation of enantiomers by CE. The capillary with adsorbed chiral selector was shown to be easily prepared and has reproducible properties. The effect of the chiral selector concentration, pH and composition of the BGE, and applied voltage on enantioseparation of acidic compounds, such as profens and aromatic carboxylic acids, was investigated. Two native α-amino acids, aspartic acid and glutamic acid, were enantioseparated. Fourteen tested compounds (including amino acids) were baseline resolved. Good selectivity of separation (α>1.09) was achieved. The migration order of ibuprofen and ketoprofen enantiomers was determined. The procedures were proposed for the analysis of flurbiprofen and warfarin in pharmaceuticals. Linearity was achieved in the concentration range of 4.0×10(-5)-2.0×10(-3) M for flurbiprofen and 3.2×10(-6)-4.9×10(-6) M for warfarin. The detection limits were found to be about 1×10(-5) M for flurbiprofen and 1×10(-6) M for warfarin.