Bidentate Boron Lewis Acids: Selectivity in Host–Guest Complex Formation

Bidentate boron Lewis acids based on 1,8‐diethynylanthracene were synthesised in two steps by initial stannylation of the terminal alkynes and subsequent tin–boron exchange with different chloroboranes. The reactions were very selective, and the target compounds were obtained in high purity and good to excellent yields. Complexation experiments of 1,8‐bis[(diphenylboranyl)ethynyl]anthracene with nitrogen bases (pyridine, pyrimidine, TMEDA) afforded three stable adducts, which were structurally characterised by X‐ray diffraction. Competition experiments demonstrated the selective exchange of guests, and quantum‐chemical calculations provided information on their energetics. NMR experiments at low temperature gave insight into the dynamic behaviour of the TMEDA adduct.     

Assessment of the nature of interactions of cations with cycloheptatriene derivatives using change in the aromaticity: Comparison with electron density and NBO results

ABSTRACT

Interactions of cycloheptatriene derivatives, C₇H₆X, (X?=?NH, PH, AsH, O, S, Se) with the cations H⁺, CH₃⁺, Cu⁺, Al⁺, Li⁺, Na⁺, and K⁺ are studied using B3LYP functional and 6-311++G(d,p) basis set. The calculated gas-phase cation affinities (CA) and cation basicities (CB) for all molecules decrease as H⁺ > CH₃⁺ > Cu⁺ > Al⁺ > Li⁺ > Na⁺ > K⁺. We used the induced aromaticity in the 7-membered ring of C₇H₆X upon interaction with the cations, M⁺, as a measure of C₇H₆X/M⁺ interaction. Nucleus-independent chemical shift (NICS) and harmonic oscillator model of aromaticity (HOMA) were used as two indices of aromaticity. The highest and lowest induced aromaticities were observed for interactions of H⁺ and K⁺, respectively. Also, the aromaticity induced by interaction with a cation in C₇H₆AsH and C₇H₆PH was larger than that in C₇H₆NH and C₇H₆O. Hence, the aromaticity was considered as a measure of covalency for the C₇H₆X/M⁺ interactions showing a rational dependence on both the molecule and cation. The nature of the interactions was also assessed using electron density, charge distribution analysis and NBO calculations. The results of the aromaticity indices, NICS and HOMA, were compared with the electron density and NBO results.     

Sulfonic acid–functionalized magnetic Fe3-xTixO4 nanoparticles: New recyclable heterogeneous catalyst for one-pot synthesis of tetrahydrobenzo[b]pyrans and dihydropyrano[2,3-c]pyrazole derivatives

In this work, titanomagnetite nanoparticles (Fe_3-xTi_xO₄) have been used as a novel suppport for the synthesis of a magnetic acidic catalyst. These nanoparticles were functionalized with sulfonic acid groups to prepare the Fe_3-xTi_xO₄@SO₃H nanoparticles. The synthesized acidic nanoparticles have been explored as new and efficient recyclable heterogeneous catalyst for a one-pot, three-component synthesis of tetrahydrobenzo[b]pyrans known as 4H-chromenes and 1,4-dihydropyrano[2,3-c]pyrazoles. The structure of the catalyst was established by infrared, energy dispersive x-ray (EDX), and scanning electron microscopy analyses. The reactions proceed smoothly to furnish the respective products in excellent yields and short reaction times. The facile reaction conditions, easy isolation of the products, versatility, and easy magnetic separation and reusability of the catalyst with no significant loss of activity are the main merits of the present method.     

Ab initio molecular dynamics simulation of ionic liquids

Ab initio Car-Parinnello molecular dynamics is used to simulate the structure and the dynamics of 1-butyl-3-methylimidazolium iodide ([bmim]I) ionic liquid at 300 K. Site-site pair correlation functions reveal that the anion has a strong interaction with any three C-H's of the imidazolium ring. The ring bends over and wraps around the anion such that the two nitrogen atoms take a distance to the anion. Electron donating butyl group contributes the electronic polarization in addition to geometrical (out-of-plane) polarization of the ring due to the liquid environment. This facilitates bending of the ring along the axis passing through nitrogen atoms. The average bending angle depends largely on the alkyl chain length and slightly on the anion type. Redistribution of electron density over the ring caused by the electron donating alkyl group provides additional independent evidence to the instability of lattice structure, hence the low melting point of the ionic liquid. Simulated viscosity and diffusion coefficients of [bmim]I are in quite agreement with the experiments.     

C-20 ketone reduction of hydrocortisone by rice field microalga <Emphasis Type="Italic">Chlorella vulgaris</Emphasis> MCCS 013

A unicellular microalga, Chlorella vulgaris, was isolated from rice field and applied in the biotransformation experiment of hydrocortisone (1). This strain has not been previously tested for hydrocortisone bioconversion. Fermentation was carried out in BG-11 medium supplemented with 0.05% substrate at 25°C for 14 days incubation. The products obtained were chromatographically purified followed by their characterization using spectroscopic methods. 11β,17α,20β,21-Tetrahydroxypregn-4-en-3-one (2), 11β,17β-dihydroxyandrost-4-en-3-one (3), and 11β-hydroxyandrost-4-ene-3,17-dione (4) were the main bioproducts in the hydrocortisone bioconversion. Bioreaction characteristics observed were 20-ketone reduction for accumulation of compound 2 and side chain degradation of the substrate to prepare compounds 3 and 4. Time course study showed the accumulation of the product 2 from the second day of the fermentation and 3 as well as 4 from the third day. All the metabolites reached their maximum concentration in seven days. Microalgal 18S rRNA gene was also amplified by PCR. PCR products were sequenced to confirm their authenticity as 18S rRNA gene of microalgae. The result of PCR blasted with other sequenced microalgae in NCBI showed 100% homology to the 18S small subunit rRNA of six strains of Chlorella vulgaris.     

Tunable Dual Fluorescence of 3‐(2,2′‐Bipyridyl)‐Substituted Iminocoumarin

3‐(2,2′‐Bipyridyl)‐substituted iminocoumarin molecules (compounds 1 and 2 ) exhibit dual fluorescence. Each molecule has one electron donor and two electron acceptors that are in conjugation, which leads to fluorescence from two independent charge transfer (CT) states. To account for the dual fluorescence, we subscribe to a kinetic model in which both CT states form after rapid decays from the directly accessed S₁ and S₂ excited states. Due to the slow internal conversion from S₂ to S₁, or more likely the slow interconversion between the two subsequently formed CT states, dual emission is allowed to occur. This hypothesis is supported by the following evidence: 1) the emission at short and long ends of the spectrum originates from two different excitation spectra, which eliminates the possibility that dual emission occurs after an adiabatic reaction at the S₁ level. 2) The fluorescence quantum yield of compound 2 grows with increasing excitation wavelength, which indicates that the high‐energy excitation elevates the molecule to a weakly emissive state that does not internally convert to the low‐energy, highly emissive state. The intensity of the two emission bands of 1 is tunable through the specific interactions between either of the two electron acceptors with another species, such as Zn²⁺ in the current demonstration. Therefore, the development of ratiometric fluorescent indicators based on the dual‐emitting iminocoumarin system is conceivable. Further fundamental studies on this series of compounds using time‐resolved spectroscopic techniques, and explorations of their applications will be carried out in the near future.     

Preparation of novel and unique nonspherical particles with almond-shell-like shape via dual-seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this study, the preparation of micron-sized polymer particles having a novel and unique nonspherical shape which we called almond-shell-like by dual-seeded dispersion polymerization (DSDP) of 2-ethylhexyl methacrylate (EHMA) with polystyrene (PS) and poly(methyl methacrylate) (PMMA) seed particles in the presence of decane droplets and evaporation of decane after the polymerization was discussed. The experimental results showed that mushroom-like morphology which is a precursor of the almond-shell-like shape was obtained from DSDP of EHMA. It was found that with changing the PS/PMMA seed particles' weight ratio, the size of the dents on the surface of the particles can be controlled. Furthermore, it was observed that various nonspherical particles can be produced using different methacrylic seed particles and initiators.     

Effect of second monomer and initiator type, mixing method, and stabilizer content on the shape of the particles produced by seeded dispersion polymerization in the presence of saturated hydrocarbon droplets

In this work, the preparation of micron-sized polymer particles with nonspherical shapes via seeded dispersion polymerization of 2-ethylhexyl methacrylate with polystyrene seed particles in the presence of hydrocarbon droplets and evaporation of hydrocarbon after the polymerization under various polymerization conditions was discussed. The effect of second monomer and initiator type, mixing method, and stabilizer content on the shape of the obtained particles was investigated. It was observed that particles with more nonspherical shapes were obtained with increasing the alkyl chain length of ester group of the methacrylate of the second monomer which is because of increasing the absorption amounts of hydrocarbon by second polymer domains. Moreover, the experimental results showed that shape of the particles which was produced by shaking is more nonspherical than the shape of the particles which was obtained from tumbling. Furthermore, particles with different nonspherical shapes were prepared by changing the initiator type and stabilizer content.