Achiral Trisubstituted Thioureas as Secondary Ligands to CuI Catalysts: Direct Catalytic Asymmetric Addition of α‐Fluoronitriles to Imines

Thioureas have emerged as effective hydrogen‐bonding catalysts over the last two decades, and they are broadly utilized in asymmetric catalysis. We report that achiral trisubstituted thioureas function as beneficial secondary ligands to Cu^I catalysts, thereby enabling highly diastereo‐ and enantioselective addition of α‐fluoronitriles to imines. The structure of the thiourea significantly affects the reaction outcome, and kinetic experiments indicate that the thioureas enhance the stereocontrol by binding to the Cu^I complex. The reaction products can be readily transformed into valuable β‐amino acid derivatives bearing a fluorinated tetrasubstituted stereogenic center.     

Electronic absorption and emission spectra of Alq3 in solution with special attention to a delayed fluorescence

Tris(8-quinolinolato)aluminum(III) (Alq₃) shows electronic absorption bands at 378, 360 (in a 1:1 mixed solvent of methanol and ethanol (ME) at 77 K), 334, 316, 300, 263, 255.8, and 233 nm in ethanol at room temperature. According to the polarized fluorescence excitation spectrum together with MO calculations, for instance, the 360 nm band is assigned to an LL CT transition (an intramolecular charge transfer transition between two ligands), and the 378 nm band to an LM/ML CT one (an intramolecular charge transfer transition between ligand and metal). Alq₃ shows a broad fluorescence band peaking at around 478 nm in the ME matrix at 77 K. The emission spectrum measured with a phosphoroscope has two emission bands at 567 and 478 nm. The 567 nm band accompanies vibronic bands at 578 and 605 nm, being safely assigned to a phosphorescence of Alq₃. The lifetimes of the 478 and 567 nm bands are both 5.4 ms. The lifetime of the 478 nm band together with the band position and its band shape indicate that this band can be assigned to a delayed fluorescence.     

Topological aspects of excitons in artificial structure

The exciton properties of thin nanotube structures are investigated theoretically. Anisotropic size dependencies have been found in the exciton binding energy, the kinetic energy for the relative motions of an electron and a hole, and the wavefunction. These anisotropies arise from the different boundary conditions in the tube-length and circumferential directions, namely, the topological features of nanotubes. We also found that it is possible to change the topology of exciton wavefunctions by varying the tube-length and the tube-radius. These findings suggest that the optical properties of nanotubes such as oscillator strength or nonlinear susceptibilities can be controlled by tuning the structural parameters, thus yielding a novel guiding principle for designing optical functional materials.     

Thermal extrusion of cellulose using hydroxypropyl methylcellulose

Cellulose - Hydroxypropyl methylcellulose (HPMC), a cellulose derivative, is highly water soluble, viscoelastic, and thermoplastic. However, the thermoplasticity of HPMC has not yet been studied in...     

Synergistic Biophysical Techniques Reveal Structural Mechanisms of Engineered Cationic Antimicrobial Peptides in Lipid Model Membranes

In the quest for new antibiotics, two novel engineered cationic antimicrobial peptides (eCAPs) have been rationally designed. WLBU2 and D8 (all 8 valines are the d -enantiomer) efficiently kill both Gram-negative and -positive bacteria, but WLBU2 is toxic and D8 nontoxic to eukaryotic cells. We explore protein secondary structure, location of peptides in six lipid model membranes, changes in membrane structure and pore evidence. We suggest that protein secondary structure is not a critical determinant of bactericidal activity, but that membrane thinning and dual location of WLBU2 and D8 in the membrane headgroup and hydrocarbon region may be important. While neither peptide thins the Gram-negative lipopolysaccharide outer membrane model, both locate deep into its hydrocarbon region where they are primed for self-promoted uptake into the periplasm. The partially α-helical secondary structure of WLBU2 in a red blood cell (RBC) membrane model containing 50 % cholesterol, could play a role in destabilizing this RBC membrane model causing pore formation that is not observed with the D8 random coil, which correlates with RBC hemolysis caused by WLBU2 but not by D8.     

Nucleophilic and Electrophilic Activation of Non‐Heteroaromatic Amides in Atom‐Economical Asymmetric Catalysis

Recent advances in catalytic asymmetric carbon–carbon bond‐forming reactions of non‐heteroaromatic amide substrates are highlighted. Among carbonyl compounds, amides have received limited attention in catalytic asymmetric transformations mainly owing to their lower reactivity. Amides are reluctant to form enolates for nucleophilic addition, and α,β‐unsaturated amides exhibit diminished electrophilicity at the β‐carbon. Recent advances in asymmetric catalysis rendered these amides amenable to enantioselective reactions with perfect atom economy, producing synthetically useful chiral building blocks. This Minireview summarizes recent developments in the field.     

Effects of meniscus on the directional growth of potassium niobate single crystals

The study of meniscus effects on the directional growth of single crystals of potassium niobate (KNbO₃) with a [110]_pc oriented seed by the top-seeded-solution-growth technique has been described. The directional growth of KNbO₃ single crystals has been illustrated for different crystallographic orientations of the crystal. Experimental results show that the shape of the solution flux meniscus, which is determined by a number of growth parameters, has a significant effect on growth rates on different 100_pc, 010_pc and 001_pc type surfaces. Findings based on this work enable one to select a dominant growth direction and hence to engineer the geometrical shaping of resultant KNbO₃ single crystals through careful control of growth parameters. Maximum crystal dimensions for the grown KNbO₃ along [100]_pc and [001]_pc achieved in this work are 34.0 and 31.2 mm, respectively.     

Separation of strontium ions from a simulated highly active liquid waste using a composite of silica-crown ether in a polymer

To partition Sr(II) from highly active liquid waste (HLW), a macroporous silica-based 4,4',(5')-di(t-butylcyclohexano)-18-crown-6 (DtBuCH18C6)-tri-n-butyl phosphate (TBP) polymeric composite, (DtBuCH18C6+TBP)/SiO(2)-P, was synthesized. It was done by impregnation and immobilization of DtBuCH18C6 and TBP into the pores of the SiO(2)-P particles, where DtBuCH18C6 was modified with TBP. The sorption of Sr(II) and some co-existent elements contained in a simulated HLW onto (DtBuCH18C6+TBP)/SiO(2)-P was investigated at 323 K. It was found that in 2.0 M HNO(3), Sr(II) exhibited strong sorption ability and high selectivity over all the tested metals except Ba(II). Chromatographic partitioning of Sr(II) from 2.0 M HNO(3) containing 5 mM of the tested elements was performed by (DtBuCH18C6+TBP)/SiO(2)-P packed column. La(III), Y(III), Na(I), K(I), Cs(I), Ru(III), Mo(VI), and Pd(II) had almost no sorption and flowed into effluent along with 2.0 M HNO(3). Sr(II) adsorbed strongly by (DtBuCH18C6+TBP)/SiO(2)-P was then eluted effectively by water, while Ba(II) flowed into effluent along with Sr(II) due to the similar chemical properties. In addition, the bleeding of total organic carbon in aqueous phase was evaluated. The results demonstrated that in 2.0 M HNO(3), application of the macroporous silica-based DtBuCH18C6 polymeric composite in chromatographic partitioning of Sr(II) from the simulated HLW is feasible.     

γ‐Cyclodextrin as Inhibitor of the Precipitation Reaction between Berberine and Glycyrrhizin in Decoctions of Natural Medicines: Interaction Studies of Cyclodextrins with Glycyrrhizin and Glycyrrhetic Acid by 1H‐NMR Spectroscopy and Molecular‐Dynamics Calculation

To prevent the precipitation reaction between glycyrrhizin ( 1 ) and berberine ( 3 ) in the decoctions of Glycyrrhiza/Coptis rhizome or Glycyrrhiza/Phellodendron bark, the presence of cyclodextrin (CD) in the mixture was proven to be effective. The preventing effect decreased in the order γ‐CD>β‐CD, and no effect was observed for α‐CD. On the other hand, the extraction degree of 1 from the natural medicine Glycyrrhia was considerably increased in the presence of γ‐CD, γ‐CD being much more effective than α‐ or β‐CD. Thus, the blocking effect of CD on the precipitate formation between 1 and 3 is suggested to be primarily dependent on the stability of the inclusion complex of the CD with 1 . To establish the structure of such a preferred inclusion complex, the interactions of 1 with β‐ and γ‐CDs were investigated by ¹H‐NMR spectroscopy and molecular‐dynamics (MD) calculations. The ¹H‐NMR measurements showed that the increase in solubility of 1 in H₂O is dependent on the degree of its inclusion into the CD, which depends on the molecular size of the CD. The MD calculations suggested that the H‐bond interactions are sufficiently strong to form a stable [ 1 /γ‐CD] complex, in which the lipophilic rings C, D, and E of 1 are fully inserted into the molecular cavity of γ‐CD, thus forming a kind of structure covered by a hydrophilic molecular capsule, while such an interaction mode is impossible for α‐ or β‐CD.     

Highly beta-selective O-glucosidation due to the restricted twist-boat conformation

[reaction: see text] Ethyl 1-thio-2,3,4,6-tetrakis-O-triisopropylsilyl-beta-d-glucopyranoside, ethyl 6-O-benzyl-1-thio-2,3,4-tris-O-triisopropylsilyl-beta-d-glucopyranoside, and ethyl 6-O-pivaloyl-1-thio-2,3,4-tris-O-triisopropylsilyl-beta-d-glucopyranoside induced highly beta-selective O-glucosidations. Among them, the 6-O-pivaloylated substrate provided the best selectivity up to alpha/beta = 3:97 with cyclohexylmethanol, and the substrate was used for glucosidations with secondary and tertiary alcohols in a highly beta-selective manner. The selectivity would be caused by the twist-boat conformation of the pyranose; this is the first beta-selective O-glucosidation based on conformational control of the pyranose ring.