Kinetics and mechanism of the aminolysis of 4-methylphenyl and 4-chlorophenyl 4-nitrophenyl carbonates in aqueous ethanol

Reactions of 4-methylphenyl 4-nitrophenyl carbonate (MPNPC) and 4-chlorophenyl 4-nitrophenyl carbonate (ClPNPC) with a series of quinuclidines (QUIN) and the latter carbonate with a series of secondary alicyclic amines (SAA) are subjected to a kinetic investigation in 44 wt % ethanol-water, at 25.0 degrees C and an ionic strength of 0.2 M. The reactions were followed spectrophotometrically at 330 or 400 nm (4-nitrophenol or 4-nitrophenoxide anion appearance, respectively). Under excess amine, pseudo-first-order rate coefficients (k(obsd)) are found. For all these reactions, plots of k(obsd) vs free amine concentration at constant pH are linear, the slope (k(N)) being independent of pH. The Br?nsted-type plots (log k(N) vs pK(a) of the conjugate acids of the amines) for the reactions of the series of QUIN with MPNPC and ClPNPC are linear with slopes (beta(N)) 0.88 and 0.87, respectively, which are explained by a stepwise process where breakdown of a zwitterionic tetrahedral intermediate (T(+/-)) to products is rate limiting. The Br?nsted-type plot for the reactions of the series of SAA with ClPNPC is biphasic with slopes beta(1) = 0.2 (high pK(a) region) and beta(2) = 0.9 (low pK(a) region) and a curvature center at pK(a)(0) = 10.6. This plot is in accordance with a stepwise mechanism through T(+/-) and a change in the rate-determining step, from T(+/-) breakdown to T(+/-) formation as the basicity of the SAA increases. Two conclusions arise from these results: (i) QUIN are better leaving groups from T(+/-) than isobasic SAA, and (ii) the non-leaving group effect on k(N) for these reactions is small, since beta(nlg) ranges from -0.2 to - 0.3. From these values, it is deduced that ClPNPC is ca. 70% more reactive than MPNPC toward SAA and QUIN, when expulsion of the leaving group from T(+/-) is the rate determining step.     

The interaction parameter of crosslinked networks and star polymers

Recent results on blends containing star polymers have revived the interest on the interaction parameters of structures that contain junctions between chains, a matter which can be connected with the earlier studies on the influence of crosslinks on the interaction parameters of polymer networks and gels. Here, we review results on crosslinked networks and star polymer solutions together with the more recent work on star polymer blends. The review covers swelling and elastic deformation of gels, differential vapour sorption between crosslinked and uncrosslinked polymers, osmotic equilibrium of gels and of star polymer solutions, and neutron scattering of polymer blends containing star polymers. In the systems reviewed, the interaction parameters of stars and networks differ from those of linear chains, and the difference is attributed mainly to entropic effects.     

Stable Luminescent [Cu(NN)(PP)]+ Complexes Incorporating a β-Cyclodextrin-Based Diphosphane Ligand with Metal-Confining Properties

A β-cyclodextrin-based diphosphane with metal-confining properties was efficiently synthesized thanks to an unprecedented Smiles-like rearrangement of diphenyl-(2-phosphanylphenyl)phosphane in the presence of excess n-BuLi. The cis-chelating bidentate ligand is capable of forming very stable heteroleptic [Cu(NN)(PP)]⁺ complexes in which a metal-bound diimine ligand (bpy, phen, or mmp) is located within the cyclodextrin cavity. As a result of ligand encapsulation, flattening of the metal tetrahedral geometry in the excited state is disfavored, thereby resulting in enhanced luminescent properties.     

Methanolysis of 4-bromobenzenediazonium ions. Effects of acidity, [MeOH] and temperature on the formation and decomposition of diazo ethers that initiate homolytic dediazoniation

We have investigated the effects of solvent composition, acidity and temperature on the dediazoniation of 4-bromobenzenediazonium (4BrBD) ions in MeOH-H(2)O mixtures by employing a combination of spectrometric and chromatographic techniques. The kinetic behaviour is quite complex; in the absence of MeOH, dediazoniations follow first-order kinetics with a half-life t(1/2) approximately 3000 min (T = 45 degrees C), but addition of small concentrations of MeOH lead to more rapid but non-first-order kinetics, suggestive of a radical mechanism, with t(1/2) approximately 125 min at 25% MeOH. Further increases in the MeOH concentration slow down the rate of dediazoniation and reactions progressively revert to first-order behaviour, and at percentages of MeOH higher than 90%, t(1/2) approximately 1080 min. Analyses of reaction mixtures by HPLC indicate that three main dediazoniation products are formed depending on the particular experimental conditions. These are 4-bromophenol (ArOH), 4-bromoanisole (ArOMe), and bromobenzene (ArH). At acidities (defined as -log[HCl]) < 2, the main dediazoniation products are the substitution products ArOH and ArOMe but, upon decreasing the acidity, the reduction product ArH becomes predominant at the expense of ArOH and ArOMe, indicating that a turnover in the reaction mechanism takes place under acidic conditions. At any given MeOH content, the plot of k(obs) or t(1/2) values against acidity is S-shaped, the inflexion point depending upon the MeOH concentration and the temperature. Similar S-shaped variations are found when plotting the dediazoniation product distribution against the acidity. The acid-dependence of the switch between the homolytic and heterolytic mechanisms suggests the homolytic dediazoniation proceeds via transient diazo ethers. The complex kinetic behaviour can be rationalized by assuming two competitive mechanisms: (i) the spontaneous heterolytic dediazoniation of 4BrBD, and (ii) an O-coupling mechanism in which the MeOH molecules capture ArN(2)(+) to yield a highly unstable Z-adduct which undergoes homolytic fragmentation initiating a radical process. Analyses of the effects of temperature on the equilibrium constant for the formation of the diazo ether and on the rate of splitting of the diazo ether allowed, for the first time, estimation of relevant thermodynamic parameters for the formation of diazo ethers under acidic conditions.     

Synthesis, structure, and magnetic properties of [(S)-[PhCH(CH3)N(CH3)3]][Mn(CH3CN)2/3Cr(ox)3] x (CH3CN)_(solvate), a 2D chiral magnet containing a quaternary ammonium chiral cation

The synthesis, structure, and magnetic properties of a novel oxalate-based bimetallic magnet obtained by using the chiral (S)-trimethyl-(1-phenyl-ethyl)-ammonium, ((S)-[PhCH(CH3)N(CH3)3](+)), cation as template is reported. This compound can be formulated as [(S)-[PhCH(CH3)N(CH3)3]][Mn(CH3CN)2/3Cr(ox)3] x (CH3CN)_(solvate), and it crystallizes in the chiral trigonal space group P3. It shows a distorted two-dimensional honeycomb structure formed by Mn(II) and Cr(III) ions connected through oxalate anions with [(S)-[PhCH(CH3)N(CH3)3](+) cations and solvent molecules intercalated between the oxalate layers. Two-thirds of the Mn(II) ions of the honeycomb anionic network are heptacoordinated. This compound behaves as a soft ferromagnet with an ordering temperature of 5.6 K.     

Pentacyclic Triterpene Profile and Its Biosynthetic Pathway in Cecropia telenitida as a Prospective Dietary Supplement

Promising research over the past decades has shown that some types of pentacyclic triterpenes (PTs) are associated with the prevention of type 2 diabetes (T2D), especially those found in foods. The most abundant edible sources of PTs are those belonging to the ursane and oleanane scaffold. The principal finding is that Cecropia telenitida contains abundant oleanane and ursane PT types with similar oxygenation patterns to those found in food matrices. We studied the compositional profile of a rich PT fraction (DE16-R) and carried out a viability test over different cell lines. The biosynthetic pathway connected to the isolated PTs in C. telenitida offers a specific medicinal benefit related to the modulation of T2D. This current study suggests that this plant can assemble isobaric, positional isomers or epimeric PT. Ursane or oleanane scaffolds with the same oxygenation pattern are always shared by the PTs in C. telenitida, as demonstrated by its biosynthetic pathway. Local communities have long used this plant in traditional medicine, and humans have consumed ursane and oleanane PTs in fruits since ancient times, two key points we believe useful in considering the medicinal benefits of C. telenitida and explaining how a group of molecules sharing a closely related scaffold can express effectiveness.     

Effects of FeCl3 as oxidizing agent on the conduction mechanisms in polypyrrole (PPy)/pc–ZnO hybrid heterojunctions grown by oxidative chemical vapor deposition

P‐type polypyrrole (PPy) films are deposited on glass and on n‐type polycrystalline ZnO (pc–ZnO) substrates by oxidative chemical vapor deposition under three different amounts of FeCl₃ used as oxidizing agents to form hybrid heterojunctions. Their microstructure, morphology, and electrical characteristics are studied. Particularly, current–voltage characteristics of the PPy/pc–ZnO heterojunctions are analyzed by defining an electrical equivalent circuit. The extracted parameters, together with the estimated heterojunction barrier height and the HOMO energy level of the PPy, indicate that a thermionic emission of holes at the heterojunction determines the saturation current of the diode at low voltage. For larger FeCl₃ amounts, the diode ideality factor increases indicating an increment of recombination by tunneling of charge carriers occurring at the heterojunction. This is attributed to a narrowing of the space charge region due to an increment of the number of charge carriers with a growing amount of FeCl₃. At high voltages, the PPy thickness influences the ohmic and space–charge limited current mechanisms at the bulk region. © 2016 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2016 , 54, 1537–1544