Regiospecific functionalization of methyl C-H bonds of alkyl groups in reagents with heteroatom functionality

We report the regiospecific, rhodium-catalyzed borylation of saturated terminal C-H bonds in molecules with existing functionality. Moderate to good yields were obtained with the organic substrate in excess and as limiting reagent. The borylations of trialkylamines, protected alcohols, protected ketones, and fluoroalkanes occurred regiospecifically at the methyl group that is least sterically hindered. Reactions were also conducted that probed electronic effects on the alkyl borylation. These reactions showed that the borylation occurred preferentially at the methyl group that is most electron-deficient. Methods to conduct tandem borylation of C-H bonds and conversion of the resulting boronate esters to alcohols, alkylarenes, and alkyltrifluoroborates were also developed.     

Defect chemistry of uranium oxides

Uranium oxides are known as nonstoichiometric compounds whose composition changes according to external conditions such as temperature and oxygen partial pressure. The change of composition caused by the formation of defect structure results in a change of their properties. In this paper, the compositional changes of UO₂ and doped UO₂ [(U, M)O₂; M=La, Ti, Pu, Th, Nb, Cr, etc.] and also those of other uranium oxides (U₄O₉, U₃O₈) are shown against oxygen partial pressure. From the results of doped UO₂, it is concluded that the valence control rule holds to a first approximation. The defect structures are estimated both from log x vs. log Po₂ (x: deviation from the stoichiometric composition and Po₂: oxygen partial pressure) and log vs. log Po₂ (: electrical conductivity) relations. The defect structures of UO₂ and doped UO₂ are derived based on the Willis model for UO_2+x. The detect structure of U₄O₉ phase is similar to that of UO_2+x, but the defect structures of U₃O₈ phase are complicated due to the existence of many higher-order phase transitions. The thermodynamic data such as the partial molar enthalpy and entropy and the heat capacity are important to characterize the defect structure. The high temperature heat capacities of UO₂ doped with Gd show pronounced increases at high temperatures the onset temperature decreases as the dopant content increases. The increase of heat capacity is interpreted to be due to the formation of lattice defects. The heat capacity measurements on U₄O₉ and U₃O₈ clucidate the presence of the phase transition. The mechanisms of these phase transitions are discussed.     

Folding Dynamics of 10‐Residue β‐Hairpin Peptide Chignolin

Short peptides that fold into β‐hairpins are ideal model systems for investigating the mechanism of protein folding because their folding process shows dynamics typical of proteins. We performed folding, unfolding, and refolding molecular dynamics simulations (total of 2.7 μs) of the 10‐residue β‐hairpin peptide chignolin, which is the smallest β‐hairpin structure known to be stable in solution. Our results revealed the folding mechanism of chignolin, which comprises three steps. First, the folding begins with hydrophobic assembly. It brings the main chain together; subsequently, a nascent turn structure is formed. The second step is the conversion of the nascent turn into a tight turn structure along with interconversion of the hydrophobic packing and interstrand hydrogen bonds. Finally, the formation of the hydrogen‐bond network and the complete hydrophobic core as well as the arrangement of side‐chain–side‐chain interactions occur at approximately the same time. This three‐step mechanism appropriately interprets the folding process as involving a combination of previous inconsistent explanations of the folding mechanism of the β‐hairpin, that the first event of the folding is formation of hydrogen bonds and the second is that of the hydrophobic core, or vice versa.     

Polymers having stable radicals. I. Synthesis of nitroxyl polymers from 4-methacryloyl derivatives of 2,2,6,6-tetramethylpiperidine

Polymers having stable nitroxyl free radicals, poly-4-methacryloylamino- and poly-4-methacryloyloxy-2,2,6,6-tetramethylpiperidine-1-oxyls, were synthesized from their precursor polymers by oxidizing them in a methanolic solution of hydrogen peroxide. The precursor polymers were prepared by radical polymerization of 4-methacryloyl-amino/oxy-2,2,6,6-tetramethylpiperidines in various solvents. These polymerizations in acetic acid were found to yield polymers of high molecular weight. The copolymers of the precursor monomers with styrene and methyl methacrylate were also prepared as precursor copolymers. These precursor polymers of a piperidine type were converted to the polymers having stable nitroxyl free radicals by the hydrogen peroxide method. In this report, it was assumed that the post-oxidation reaction introduced a nitroxyl group smoothly and quantitatively at room temperature. Elucidations of the stable radical formation and the electron spin behavior of the stable radical polymers were made in terms of elemental analyses, infrared, ultraviolet, and ESR spectroscopy.     

Effect of molecular packing on adsorption and micelle formation of a homologous cationic surfactant mixture of hexadecyltrimethylammonium bromide and dodecyltrimethylammonium bromide

The surface tension of an aqueous solution of a hexadecyltrimethylammonium bromide (HTAB) and dodecyltrimethylammonium bromide (DTAB) mixture was measured as a function of the total molality and the composition of DTAB at 298.15 K under atmospheric pressure. The phase diagrams of adsorption and micelle formation were constructed and the excess Gibbs energy was evaluated by analyzing the phase diagrams thermodynamically. Both the excess Gibbs energy in the adsorbed film and the excess surface area are negative; therefore the mutual interaction between HTAB and DTAB is said to be stronger than that between the same species and is enhanced with increasing adsorption. By combining the results with those obtained in previous studies, we claimed that DTAB molecules can use effectively the space among the hydrocarbon chains of HTAB molecules and their polar head groups take a staggered arrangement at the surface so as to reduce the electrostatic repulsion. Consequently the dispersion force between hydrophobic chains becomes stronger. Furthermore, the comparison of the excess Gibbs energy in the adsorbed film with that in the micelle shows that the staggered arrangement of molecules is not necessary in the spherical micelle.     

Flame retardancy and heat resistance of phenol-biphenylene-type epoxy resin compound modified with benzoguanamine

The flame retardancy and heat resistance of a phenol-biphenylene-type epoxy resin compound, which forms a self-extinguishing network structure, were increased by the inclusion of a benzoguanamine-modified phenol biphenylene resin. The benzoguanamine-modified phenol biphenylene resin contains a benzoguanamine unit to release non-flammable nitrogen substances during ignition and to increase the resin's reactivity toward epoxy resins, and biphenylene units to keep the resin's thermal degradation and water resistance. The addition of the benzoguanamine-modified phenol biphenylene resin in the epoxy resin compound improved the epoxy resin compound's flame retardancy and heat resistance, and also increased its glass transition temperature while maintaining its water resistance and mechanical properties. Copyright © 2003 John Wiley & Sons, Ltd.     

Solvent extraction and spectrophotometric determination of palladium(II) with some nitrogen-containing heterocyclic hydrazones in the presence of chloride ions

Three terdentate hydrazones, all containing the 1-phthalazino grouping in the hydrazine moiety but differing in the heterocyclic substituent in the aldehyde moiety, have been used as analytical reagents for palladium(II), the optimal conditions for the extractive spectrophotometric determination of palladium(II) in the presence of chloride ions being deduced. These compounds are highly selective and sensitive reagents for palladium(II), since they are not extracted into chloroform from sulfuric acid solutions and do not react with other platinum group metals. The desirable spectral properties of the palladium(II) complex of benzothiazole-2-aldehyde-1-phthalazinohydrazone (BAPhH) have also been discussed with respect to preference of the C  N structural form in the heterocyclic ring on the aldehyde moiety of the ligand.     

Heterodimerization of dye-modified cyclodextrins with native cyclodextrins

The heterodimerization behavior of dye-modified beta-cyclodextrins (1-6) with native cyclodextrins (CDs) was investigated by means of absorption and induced circular dichroism spectroscopy in an aqueous solution. Three types of azo dye-modified beta-CDs (1-3) show different association behaviors, depending on the positional difference and the electronic character of substituent connected to the CD unit in the dye moiety. p-Methyl red-modified beta-CD (1), which has a 4-(dimethylamino)azobenzene moiety connected to the CD unit at the 4' position by an amido linkage, forms an intramolecular self-complex, inserting the dye moiety in its beta-CD cavity. It also associates with the native alpha-CD by inserting the moiety of 1 into the alpha-CD cavity. The association constants for such heterodimerization are 198 M(-1) at pH 1.00 and 305 M(-1) at pH 6.59, which are larger than the association constant of 1 for beta-CD (43 M(-1) at pH 1.00). Methyl red-modified 2, which has the same dye moiety as that for 1 although its substituent position is different from that of 1, does not associate even with alpha-CD due to the stable self-intramolecular complex, in which the dye moiety is deeply included in its own cavity of beta-CD. Alizarin yellow-modified CD (3), which has an azo dye moiety different from that of 1 and 2, caused a slight spectral variation upon addition of alpha-CD, suggesting that the interaction between 3 and alpha-CD is weak. On the other hand, phenolphthalein-modified beta-CD (4), which forms an intermolecular association complex in its higher concentrations, binds with beta-CD with an association constant of 787 M(-1) at pH 10.80, where 4 exists as the dianion monomer in the absence of beta-CD. p-Nitorophenol-modified beta-CDs (5 and 6), each having p-nitorophenol moieties with a different connecting part with an amido and amidophenyl group, respectively, associated with alpha-CD with association constants of 66 and 16 M(-1) for 5 and 6, respectively. The phenyl unit in the connecting part of 6 may prevent the smooth binding with alpha-CD. All these results suggest that the dye-modified CDs, in which the dye part is not tightly included in its CD cavity, associate with the native CD to form heterodimer composed of two different CD units by inserting the dye moiety into the native CD unit. The resulting heterodimers have a cavity that can bind another appending moiety of host molecules. On this basis, more ordered molecular arrays or the supramolecular hereropolymers can be constructed.     

Self‐Assembled Aggregates and Molecular Bilayer Films of a Double‐Chain Fullerene Lipid: Structure and Electrochemistry

The self‐aggregation behavior of C₆₀ fullerenes that bear two octadecyl chains (lipid 1 ) as well as the structures and electrochemical properties of cast films of 1 are described. We also examined the self‐aggregation behavior in organic solvents of three previously reported compounds: C₆₀ with three each of hexadecyl (lipid 2 ), tetradecyl (lipid 3 ), or dodecyl (lipid 4 ) chains. The fullerene lipids in alcohols spontaneously formed spherical aggregates, whose diameters are related to the alkyl‐chain lengths, concentrations of the fullerene lipids, and the solvent polarity. The morphologies of the aggregates showed temperature dependence. Cast films of 1 formed multimolecular bilayer structures that undergo a phase transition typical of lipid bilayer membranes. The electrochemistry of cast films of 1 on an electrode in aqueous medium exhibits temperature dependence.     

Magnetic and calorimetric studies on rare-earth iron borates LnFe3(BO3)4 (Ln=Y, La-Nd, Sm-Ho)

A series of rare-earth iron borates having general formula LnFe₃(BO₃)₄ (Ln=Y, La-Nd, Sm-Ho) were prepared and their magnetic properties have been investigated by the magnetic susceptibility, specific heat, and ⁵⁷Fe Mössbauer spectrum measurements. These borates show antiferromagnetic transitions at low temperatures and their magnetic transition temperatures increase with decreasing Ln³⁺ ionic radius from 22 K for LaFe₃(BO₃)₄ to 40 K for TbFe₃(BO₃)₄. In addition, X-ray diffraction, specific heat, and differential thermal analysis (DTA) measurements indicate that the phase transition occurs for the LnFe₃(BO₃)₄ compounds with Ln=Eu-Ho, Y, and its transition temperature increases remarkably with decreasing Ln³⁺ ionic radius from 88 K for Ln=Eu to 445 K for Ln=Y.