Simulation of a crossover from the precipitation wave to moving Liesegang pattern formation

Model simulations to investigate the precipitation wave phenomenon and a crossover from the precipitation wave to moving Liesegang patterns were performed. The chemical scheme contains four chemical species via the interaction of precipitation and redissolution (complex formation), in which the precipitation reaction term was based on Ostwald's supersaturation theory. In this article, for the first time, all the features and behaviors of the heterogeneous traveling waves are reproduced, which were observed experimentally in the work of Zrínyi et al. (Zrínyi, et al. J. Phys. Chem. 1991, 95, 1618.). The detailed investigation of the pattern formation showed three possible states of the system, which depend on the initial concentration of the inner and outer electrolytes, respectively. These are the precipitation wave (single moving precipitation band), the moving Liesegang pattern (moving precipitation bands), and the state where these two patterns coexist.     

A triangle-square equilibrium of metallosupramolecular assemblies based on pd(II) and pt(II) corners and diazadibenzoperylene bridging ligands

Tetraaryloxy-substituted diazadibenzoperylene bridging ligands 1a,b were employed in transition metal-directed self-assembly with Pd(II) and Pt(II) phosphane triflates 2a,b which resulted in complex dynamic equilibria between molecular triangles 3a-d and molecular squares 4a-d in solution. Characterization of the equilibria and assignment of the metallacycles was accomplished by (1)H and (31)P[(1)H] NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI-FTICR-MS). It was found that the equilibria depend on several factors, such as the metal ion (Pd(2+) or Pt(2+)), the solvent, and the steric demand of the phenoxy substituents of the diazadibenzoperylene ligands 1a,b. Introduction of bulky tert-butyl groups in 1b shifts the equilibrium significantly in the direction of the molecular squares. Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and diazadibenzoperylene ligands that help explain the distinct dynamic behavior observed in variable-temperature NMR studies. Concentration-dependent UV/vis and fluorescence spectroscopy revealed the limited stability of the assemblies and confirmed the reversible nature of the dynamic equilibria.     

Determination of phenoxyalkanoic acids and other herbicides at the ng/ml level in water by solid-phase extraction with poly(divinylbenzene-co-N-vinylpyrrolidone) sorbent and high-performance liquid chromatography--diode-array detection

A method for the determination of phenoxyalkanoic acids and other polar compounds in environmental water samples without pH adjustment before extraction has been developed. Recoveries were calculated from 500 ml of milliQ water spiked at the level of 0.5 ng/ml using solid-phase extraction (SPE) and HPLC-DAD. Different SPE materials (RP-C18, ENV+, ENV+-C8, SAX and Oasis HLB) were tested. After method optimization, 15 of the 16 compounds studied could be extracted with recoveries better than 70% on the most suitable copolymeric poly(divinylbenzene-co-N-vinylpyrrolidone) material (Oasis HLB cartridges).     

Cascade alkenyl amination/Heck reaction promoted by a bifunctional palladium catalyst: a novel one-pot synthesis of indoles from o-haloanilines and alkenyl halides

A novel approach for the synthesis of the important indole ring is described. Indoles are obtained from o-bromoanilines and alkenyl halides in a Pd-catalyzed cascade process that involves an alkenyl amination followed by an intramolecular Heck reaction. The overall process represents the first example of the participation of alkenyl amination reactions in Pd-catalyzed cascade reactions. Initially, the relative reactivity of aryl and alkenyl bromides and chlorides towards Pd-catalyzed amination was investigated. Competition experiments were carried out in the presence of primary and secondary amines, and these revealed the reactivity order alkenyl bromides > aryl bromides > alkenyl chlorides > aryl chlorides, as well as very high chemoselectivity; the more reactive halide was always favored. Thereafter, optimized reaction conditions for the sequential alkenyl amination/Heck cyclization to give indoles were investigated with the model reaction of o-bromoaniline with alpha-bromostyrene. An extensive screening of ligands, bases, and reaction conditions revealed that the [Pd2(dba)3]/DavePhos, NaOtBu, toluene combination at 100 degrees C were the optimized reaction conditions to carry out the cascade process (dba=dibenzylideneacetone, DavePhos=2-dicyclohexylphosphino-2'-N,N-dimethylaminobiphenyl). The reaction proceeds with aryl, alkyl, and functionalized substitutents in both starting reactants. The cyclization was also studied with N-substituted o-bromoanilines (which would give rise to N-substituted indoles); however, in this case, indole formation occurred only with 1-substituted-2-bromoalkenes. Finally, the application of this methodology to o-chloroanilines required further optimization. Although the catalyst based on DavePhos failed to promote the cascade process, a catalytic combination based on [Pd2(dba)3]/X-Phos promoted the formation of the indole ring also from the less reactive chloroanilines.     

Preparation and characterisation of thermoresponsive poly[(N-isopropylacrylamide-co-acrylamide-co-(hydroxyethyl acrylate)] microspheres as a matrix for the pulsed release of drugs

Despite the large number of publications and patents concerning pH/thermoresponsive polymers, few data are available concerning the preparation of thermoresponsive cross-linked microspheres from preformed polymers. Therefore, N-isopropylacrylamide-co-acrylamide-co-(2-hydroxyethyl acrylate) copolymers were obtained as a new thermoresponsive material with a lower critical solution temperature (LCST) around 36 degrees C, in phosphate buffer at pH 7.4, and with a cross-linkable OH group in their structure. The LCST value was determined both by UV spectroscopy and microcalorimetric analysis. These copolymers were solubilised in acidified aqueous solution below their LCST, dispersed in mineral oil, and transformed into stable microspheres by cross-linking with glutaraldehyde. The thermoresponsive microspheres were characterised by optical and scanning electron microscopy, degree of swelling, and water retention. The pore dimensions of the microspheres and the retention volumes of some drugs and typical compounds were evaluated at different temperatures by liquid chromatography. Indomethacin, as a model drug, was included in the microspheres by the solvent evaporation method. Finally, the influence of temperature and of temperature cycling on drug release was investigated.     

A multinuclear NMR study of [Pt0(PPh3)2(alkene)] compounds containing asymmetric olefins

The 1H, 13C, 31P, and 195Pt NMR spectra of [Pt0(PPh3)2(eta-ABC(1) = C(2)XY)] compounds (ABC(1)= C(2)XY (1) A = B = X = Y = H; (3) A = B = X = H, Y = CN; (4) A = H, B = p-NO2-Ph, X = COOCH3, Y = CN; (5) A = H, B = Ph, X = COOCH3, Y = CN; (6) A = H, B = Ph, X = Y = CN; (7) A = H, B = OEt, X = Y = CN), where X and Y are electronacceptor substituents, and the 1H spectrum of [Pt0(PPh3)2(eta2-C60)] (2) are reported together with extended analyses and assignments, based also on the ring current effect of the olefin phenyl in (4-6). Deviations from first order in the 13C spectra allowed the determination of the relative signs of the coupling constants J(P(1), C) and J(P(2), C) of the alkene and of the triphenylphosphine carbons. Best fit simulation of the phosphine C ipso spectrum provided also the 13C isotopic shift on phosphorus for (1). These compounds are characterised by strong differences between the two platinum-phosphorus coupling constants in the case of asymmetric olefins (3-7). The chemical shifts of alkene C(1) and C(2) indicate notable polarisation of the olefin after complexation, while the 1J(Pt, C(1)) and 1J(Pt, C(2)) values are in agreement with a stronger interaction of Pt with C(1) than with C(2). These findings together with the trend of 195Pt chemical shifts confirm the important role played by back-donation in the bonding of platinum(0)-olefin compounds.     

Derivatives of carbohydrazide, thiocarbohydrazide and diaminoguanidine as photometric analytical reagents-I 1,3-bis[(2-pyridyl)methyleneamino]thiourea and 1,3-bis[(2-pyridyl)methyleneamino]guanidine

1,3-Bis[(2-pyridyl)methyleneamino]thiourea (PMAT) and 1,3-bis[(2-pyridyl)methyleneamino]-guanidine (PMAG) have been prepared. They have been examined and characterized by infrared and ultraviolet spectroscopy. A spectrophotometric method has been used for determination of the protonation constants of the reagents. Finally, a spectrophotometric survey has been made of the reactions of various cations with PMAT and PMAG.     

Characterisation of flavonols in broccoli (Brassica oleracea L. var. italica) by liquid chromatography-uV diode-array detection-electrospray ionisation mass spectrometry

The flavonoid composition of broccoli inflorescences has been studied by LC/UV-DAD/ESI-MSn. A large number of hydroxycinnamic acid esters of kaempferol and quercetin glucosides has been characterised. The structures of the flavonoid glycosides were analysed after alkaline hydrolysis, and were identified as 3-sophoroside/sophorotrioside-7-glucoside/sophoroside of kaempferol, quercetin and isorhamnetin (this last found in trace amount). These complex quercetin and isorhamnetin glucosides have not been previously characterised in nature. In addition, several less complex glucosides based on the same aglycones have been identified. The effect of sugar substitution and acylation on chromatographic mobility and ESI ionisation and fragmentation are discussed.     

Synthesis and photoluminescent properties of 1,1'-binaphthyl-based chiral phenylenevinylene dendrimers

New chiral, soluble binaphthyl derivatives that incorporate stilbenoid dendrons at the 6,6'-positions have been prepared. The synthesis of the new enantiopure dendrimers was performed in a convergent manner by Horner-Wadsworth-Emmons (HWE) reaction of the appropriately functionalized 1,1'-binaphthyl derivative (R)-1 and the appropriate dendrons (R)(2n)G(n)-CHO. Different electroactive units were incorporated in the peripheral positions of the dendrons in order to tune both the optical and electrochemical behavior of these systems. Fluorescence measurements on the chiral dendrimers reveal a strong emission with maxima between 409 and 508 nm depending upon the substitution pattern. Finally, the redox properties of the dendrimers were determined by cyclic voltammetry, showing the influence of the functional groups at the peripheral positions of the dendrimer on the redox behavior of these systems.     

Palladium-catalyzed reaction of aryl iodides with acetic anhydride. A carbon monoxide-free synthesis of acetophenones

[reaction: see text] The palladium-catalyzed reaction of aryl iodides with acetic anhydride provides a straightforward and experimentally simple carbon monoxide-free route to acetophenones. The reaction tolerates a wide range of functionalized aryl iodides. Acetophenones are isolated in excellent yield with a variety of neutral, slightly electron-rich, and slightly electron-poor aryl iodides, whereas moderate yields are obtained with aryl iodides containing strongly electron-withdrawing substituents.