The role of outer-sphere surface acidity in alkene epoxidation catalyzed by calixarene-Ti(IV) complexes

Cooperativity between Br?nsted acidic defect sites on oxide surfaces and Lewis acid catalyst sites consisting of grafted calixarene-Ti(IV) complexes is investigated for controlling epoxidation catalysis. Materials are synthesized that, regardless of the surface or calixarene substituent, demonstrate nearly identical UV-visible ligand-to-metal charge-transfer bands and Ti K-edge X-ray absorption near edge spectral features consistent with site-isolated, coordinatively unsaturated Ti(IV) atoms. Despite similar Ti frontier orbital energies demonstrated by these spectra, replacing a homogeneous triphenylsilanol ligand with a silanol on a SiO2 surface increases cyclohexene epoxidation rates with tert-butyl hydroperoxide 20-fold per Ti site. Supporting calixarene-Ti active sites on fully hydroxylated Al2O3 or TiO2, which possess lower average surface hydroxyl pKa than that of SiO2, reduces catalytic rates 50-fold relative to SiO2. These effects are consistent with SiO2 surfaces balancing two competing factors that control epoxidation rates-equilibrated hydroperoxide binding at Ti, disfavored by stronger surface Br?nsted acidity, and rate-limiting oxygen transfer from this intermediate to alkenes, favored by strongly H-bonding intermediates. These observations also imply that Ti-OSi rather than Ti-OCalix bonds are broken upon hydroperoxide binding to Ti in kinetically relevant steps, which is verified by the lack of a calixarene upper-rim substituent effect on epoxidation rate. The pronounced sensitivity of observed epoxidation rates to the support oxide, in the absence of changes to the Ti coordination environment, provides experimental evidence for the importance of outer-sphere H-bonding interactions for the exceptional epoxidation reactivity of titanium silicalite and related catalysts.     

Ultra-performance liquid chromatography for the determination of pesticide residues in foods by tandem quadrupole mass spectrometry with polarity switching

An ultra-performance liquid chromatographic (UPLC) electrospray ionisation tandem quadrupole mass spectrometric method has been developed for the determination of 52 pesticides in cereal-based baby foods, oranges and potatoes. The fast polarity switching capability of the mass spectrometer used enabled the determination of 44 of the compounds in the positive ionisation mode and 8 of the compounds in the negative ionisation mode in a single run. Prior to analysis, co-extractives were removed from acetonitrile extracts using dispersive solid-phase extraction with primary secondary amine (50 mg). The UPLC method separates all of the pesticides, resolves structural isomers (e.g. butocarboxim sulfoxide and aldicarb sulfoxide) and has a short (7 min) cycle time. Extracts spiked with pesticides at 0.10 and 0.01 mg kg(-1) yielded average recoveries in the range of 66-124% with relative standard deviations less than 19% for the majority of the analytes.     

Interaction of meso-tetrakis (4-sulfonatophenyl) porphyrin with cationic CTAC micelles investigated by small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR)

Small angle X-ray scattering (SAXS) and electron paramagnetic resonance (EPR) have been used to investigate the interaction of the water-soluble meso-tetrakis (4-sulfonatophenyl) porphyrin (TPPS(4)) with cationic cethyltrimethylammonium chloride (CTAC) micelles. To evaluate if the porphyrin protonation state affects its interaction with the micelle, both SAXS and EPR measurements were performed at pH 4.0 and 9.0. The best-fit SAXS curves were obtained assuming for CTAC micelle a prolate ellipsoidal shape in the absence and upon incorporation of 2-10 mM TPPS(4). SAXS results show that the presence of porphyrin impacts on micellar hydrophobic core, leading to a micellar reassembling into smaller micelles. Lineshapes of EPR spectra of 5- and 16-doxyl stearic acids (5- and 16-DSA, respectively) bound to 100 mM CTAC micelles exhibited slight changes as a function of porphyrin concentration. Spectral simulations revealed an increase of mobility restriction for both spin probes, especially at higher porphyrin concentration, where a small reduction of environment polarity was also observed for 16-DSA. The spin labels monitored only slight differences between pH 4.0 and 9.0, in agreement with the SAXS results.     

In situ AFM studies on self-assembled monolayers of adsorbed surfactant molecules on well-defined H-terminated Si(111) surfaces in aqueous solutions

The formation of self-assembled monolayers (SAMs) of adsorbed cationic or anionic surfactant molecules on atomically flat H-terminated Si(111) surfaces in aqueous solutions was investigated by in situ AFM measurements, using octyl trimethylammonium chloride (C8TAC), dodecyl trimethylammonium chloride (C12TAC), octadecyl trimethylammonium chloride (C18TAC)) sodium dodecyl sulfate (STS), and sodium tetradecyl sulfate (SDS). The adsorbed surfactant layer with well-ordered molecular arrangement was formed when the Si(111) surface was in contact with 1.0x10(-4) M C18TAC, whereas a slightly roughened layer was formed for 1.0x10(-4) M C8TAC and C12TAC. On the other hand, the addition of alcohols to solutions of 1.0x10(-4) M C8TAC, C12TAC, or SDS improved the molecular arrangement in the adsorbed surfactant layer. Similarly, the addition of a salt, KCl, also improved the molecular arrangement for both the cationic and anionic surfactant layers. Moreover, the adsorbed surfactant layer with a well-ordered structure was formed in a solution of mixed cationic (C12TAC) and anionic (SDS) surfactants, though each surfactant alone did not form the well-ordered layer. These results were all explained by taking into account electrostatic repulsion between ionic head groups of adsorbed surfactant molecules as well as hydrophobic interaction between their alkyl chains, which increases with the increasing chain length, together with the increase in the hydrophobic interaction or the decrease in the electrostatic repulsion by incorporating alcohol molecules into the adsorbed surfactant layer, the decrease in the electrostatic repulsion by increasing the concentration of counterions, and the decrease in the electrostatic repulsion by alternate arrangement of cationic and anionic surfactant molecules. The present results have revealed various factors to form the well-ordered adsorbed surfactant layers on the H-Si(111) surface, which have a possibility of realizing the third generation surfaces with flexible structures and functions easily adaptable to circumstances.     

A novel synthesis of benzo[c]phenanthridine skeleton and biological evaluation of isoquinoline derivatives.

Benzo[c]phenanthridine skeleton was synthesized from easily available starting N-methyl-o-toluamide 2 and o-methylbenzonitrile 5 in 7 steps. Radical cyclization of styrene 10 afforded 6,11-dimethyl-6,11-dihydro-5H-indeno[1,2-c]isoquinolin-5-one 13. Most 3-arylisoquinolines have displayed strong activities against human tumor cell lines. Especially, indenoisoquinolinone 13 exhibited excellent cytotoxicity (IC50 = 0.002 microgram/ml; HCT 15).     

Imaging of a pH-sensitive polymer brush on a porous membrane using atomic force microscopy in aqueous solution

Poly(methacrylic acid) was grafted on a nanoporous polycarbonate membrane using the glow-discharge method. Atomic force microscopy showed that the pore size is dependent upon pH in such a way that a low pH the graft chains are protonated and contracted to open the pore, whereas at high pH, the graft chains became extended, thus reducing the pore size. Water permeation through the graft membrane is regulated by the pH value. The permeation rate is low at high pH value and high at low pH. This phenomenon corresponds to the pore size.     

Kinetics and mechanism of the aminolysis of O‐phenyl 4‐nitrophenyl dithiocarbonate in aqueous ethanol

The reactions of the title substrate (1) with a series of secondary alicyclic amines are subjected to a kinetic investigation in 44 wt% ethanol‐water, at 25.0°C, ionic strength 0.2 M (KCl). Under amine excess over the substrate, pseudo‐first‐order rate coefficients (k_obs) are obtained. Plots of k_obs against [NH], where NH is the free amine, are nonlinear upwards, except the reactions of piperidine, which show linear plots. According to the kinetic results and the analysis of products, a reaction scheme is proposed with two tetrahedral intermediates, one zwitterionic (T^±) and another anionic (T⁻), with a kinetically significant proton transfer from T^± to an amine to yield T⁻ (k₃ step). By nonlinear least‐squares fitting of an equation derived from the scheme to the experimental points, the rate microcoefficients involved in the reactions are determined. Comparison of the kinetics of the title reactions with the linear k_obs vs. [NH] plots found in the same aminolysis of O‐ethyl 4‐nitrophenyl dithiocarbonate (2) in the same solvent shows that the rate coefficient for leaving group expulsion from T^± (k₂) is larger for 2 due to a stronger push by EtO than PhO. The k₃ value is the same for both reactions since both proton transfers are diffusion controlled. Comparison of the title reactions with the same aminolysis of phenyl 4‐nitrophenyl thionocarbonate (3) in water indicates that (i) the k₂ value is larger for the aminolysis of 1 due to the less basic nucleofuge involved and the small solvent effect on k₂, (ii) the k₃ value is smaller for the reactions of 1 due to the more viscous solvent, (iii) the rate coefficient for amine expulsion from T^± (k₋₁) is larger for the aminolysis of 1 than that of 3 due to a solvent effect, and (iv) the value of the rate coefficient for amine attack (k₁) is smaller for the aminolysis of 1 in aqueous ethanol, which can be explained by a predominant solvent effect relative to the electron‐withdrawing effect from the nucleofuge. © 1999 John Wiley & Sons, Inc. Int J Chem Kinet 31: 839–845, 1999     

Topology of Foliations and Decomposition of Stochastic Flows of Diffeomorphisms

Let M be a compact manifold equipped with a pair of complementary foliations, say horizontal and vertical. In Catuogno et al. (Stoch Dyn 13(4):1350009, 2013) it is shown that, up to a stopping time \(\tau \), a stochastic flow of local diffeomorphisms \(\varphi _t\) in M can be written as a Markovian process in the subgroup of diffeomorphisms which preserve the horizontal foliation composed with a process in the subgroup of diffeomorphisms which preserve the vertical foliation. Here, we discuss topological aspects of this decomposition. The main result guarantees the global decomposition of a flow if it preserves the orientation of a transversely orientable foliation. In the last section, we present an Itô-Liouville formula for subdeterminants of linearised flows. We use this formula to obtain sufficient conditions for the existence of the decomposition for all \(t\ge 0\).