Regiodivergent synthesis of pyrazino-indolines vs. triazocines via α-imino carbenes addition to imidazolidines

Hexahydropyrazinoindoles were prepared in a single step from N-sulfonyl triazoles and imidazolidines. Under dirhodium catalysis, α-imino carbenes were generated and formed nitrogen ylide intermediates that, after subsequent aminal opening, afforded the pyrazinoindoles predominantly via formal [1,2]-Stevens and tandem Friedel–Crafts cyclizations. Of mechanistic importance, a regiodivergent reactivity was engineered through the use of a specific unsymmetrically substituted imidazolidine that promoted the exclusive formation of 8-membered ring 1,3,6-triazocines. Based on DFT calculations, an original Curtin–Hammett-like situation was demonstrated for the mechanism. Further derivatizations led to functionalized tetrahydropyrazinoindoles in high yields.

Hexahydropyrazinoindoles are prepared in a single step from N-sulfonyl triazoles and imidazolidines. Of mechanistic importance, a regiodivergent reactivity can be engineered towards the exclusive formation of 8-membered ring 1,3,6-triazocines.     

Framework composition effects on the performance of steam-activated FeMFI zeolites in the N2O-mediated propane oxidative dehydrogenation to propylene

The N(2)O-mediated oxidative dehydrogenation of propane (ODHP) to propylene has been investigated in the temperature range of 673-773 K over steam-activated FeMFI zeolites with different framework compositions (Si-Al, Si-Ga, Si-Ge, and pure Si). The catalysts, which were characterized by ICP-OES, XRD, SEM, N(2) adsorption, NH(3)-TPD, and FTIR of pyridine adsorbed, HRTEM, and UV/vis, have a very similar iron content (0.6-0.7 wt %). A tapered element oscillating microbalance (TEOM) coupled to on-line analysis of products has been applied to simultaneously monitor activity and mass changes during the ODHP reaction. FeAlMFI and FeGaMFI zeolites display higher propylene yields (up to 25%) and a much slower deactivation due to coking than do FeGeMFI and FeMFI zeolites. The higher density and strength of acid sites in the former samples did not induce a faster catalyst deactivation. In general, catalyst deactivation was accelerated upon decreasing the reaction temperature. The total amount of coke formed in the samples during 400 min on stream ranged from 6 to 23 wt %, increasing linearly with the amount of propylene produced. The distinct performance of both groups of zeolites is likely related to the different iron speciation, which is influenced by the composition of the framework. The smaller primary crystallites of FeAlMFI and FeGaMFI, as well as the presence of mesopores in these samples, which are created by dislodgement of aluminum and gallium to extraframework positions, appear to be also beneficial for higher catalyst effectiveness and a reduced deactivation.     

Study of the 1-chloro-2,4-dinitrobenzene + oh- reaction in tetradecyltrimethylammonium bromide aqueous micellar solutions

The 1-chloro-2,4-dinitrobenzene + OH^- reaction was studied in tetradecyltrimethylammonium bromide aqueous micellar solutions. Influence of changes in [surfactant] as well as in [NaOH] on the observed rate constant were rationalized by using pseudophase kinetic models. At high hydroxide ions concentration an additional pathway across the micellar boundary had to be considered. This revised version was published online in June 2006 with corrections to the Cover Date.     

Micellar medium effects on the hydrolysis of phenyl chloroformate in ionic,zwitterionic, nonionic,and mixed micellar solutions

The spontaneous hydrolysis of phenyl chloroformate was studied in various anionic, nonionic, zwitterionic, and cationic aqueous micellar solutions, as well as in mixed anionic–nonionic micellar solutions. In all cases, an increase in the surfactant concentration results in a decrease in the reaction rate and micellar effects were quantitatively explained in terms of distribution of the substrate between water and micelles and the first‐order rate constants in the aqueous and micellar pseudophases. A comparison of the kinetic data in nonionic micellar solutions to those in anionic and zwiterionic micellar solutions makes clear that charge effects of micelles is not the only factor responsible for the variations in the reaction rate. Depletion of water in the interfacial region and its different characteristics as compared to bulk water, the presence of high ionic concentration in the Stern layer of ionic micelles, and differences in the stabilization of the initial state and the transition state by hydrophobic interactions with surfactant tails can also influence reactivity. The different deceleration of the reaction observed in the various micellar solutions studied was discussed by considering these factors. Synergism in mixed‐micellar solutions is shown through the kinetic data obtained in these media. © 2002 Wiley Periodicals, Inc. Int J Chem Kinet 34: 445–451, 2002     

Preparation and protonation of 2-pyrimidyl- and 2-pyrazylpalladium(II) complexes

The oxidative addition of 2-chloropyrimidine or 2-chloropyrazine to [Pd(PPh₃)₄] yields a mixture of trans-[PdCl(C₄H₃N₂-C²)(PPh₃)₂] (I) and [PdCl(μ-C₄H₃N₂-C²,N¹)(PPh₃ (II) (C₄H₃N₂ = 2-pyrimidyl or 2-pyrazyl group). The mononuclear complexes I are quantitatively converted into the binuclear species II upon treatment with H₂O₂. The reaction of II with HCl gives the N-monoprotonated derivatives cis-[PdCl₂(C₄H₄N₂-C²)(PPh₃)] (III), from which the cationic complexes trans-[PdCl(C₄H₄N₂-C²)(L) (L = PPh₃, IV; PMe₂Ph, V; PEt₃, VI) can be prepared by ligand substitution reactions. Reversible proton dissociation occurs in solution for III–VI. The low-temperature ¹H NMR spectra of trans-[PdCl(C₄H₄N₂-C²)(PMe₂Ph)₂]ClO₄ show that the heterocyclic moiety undergoes restricted rotation around the PdC² bond and that the 2-pyrazyl group is protonated predominantly at the N¹ atom. These results and the ¹³C NMR data for the PEt₃ derivatives are interpreted on the basis of a significant d_π → π^★ back-bonding contribution to the palladium—carbon bond of the protonated ligands.     

Decanoic acid reverse micelle-based coacervates for the microextraction of bisphenol A from canned vegetables and fruits

Decanoic acid reverse micelle-based coacervates were proposed for the extraction of bisphenol A (BPA) from canned vegetables and fruits prior to its determination by liquid chromatography and fluorescence detection at lambda(exc) = 276 nm and lambda(em) = 306 nm. The procedure involved the extraction of minute quantities (300-700 mg) of homogenized food sample with an aqueous solution containing 10% of THF and 0.5% of decanoic acid, conditions under which the coacervate (around 340 microL) formed in situ and instantaneously. The overall sample treatment, which included extraction and centrifugation, took about 25-30 min, and several samples could be simultaneously treated using conventional lab equipment. No clean-up or solvent evaporation were required. Extraction efficiencies mainly depended on the decanoic acid and THF concentration in the aqueous solution and were not affected by the pH or the temperature in the ranges studied (1-4 and 20-60 degrees C, respectively). Recoveries in samples ranged between about 81 and 96%. The precision of the method, expressed as relative standard deviation, was about 3% and the quantitation limit was around 9 ng g(-1), which was far below the current specific migration limit (SML) set for BPA by the EU Commission (600 ng g(-1)). The method was successfully applied to the determination of BPA in the solid content of canned fruit salad, peaches in syrup, mango slices, red peppers, sweetcorn, green beans and peas. BPA was present at concentrations in the range from 7.8 to 24.4 ng g(-1) in canned fruits and from 55 to 103 ng g(-1) in canned vegetables.     

Simple and double pyridinium salts with cyanobiphenyl groups as ionic liquids and ionic liquid crystals: synthesis and investigation of thermal behavior

Research on Chemical Intermediates - A series of mono- and di-cationic pyridinium salts with one or two promesogenic cyanobiphenyl groups and different counterions, bromide, dodecylsulfate or...     

Time‐Dependent Free‐Particle Salpeter Equation: Numerical and Asymptotic Analysis in the Light of the Fundamental Solution

A detailed study of the spinless (1+1)D free‐particle Salpeter equation is presented. It involves several aspects of the topic: from the analysis of the behavior of solutions of the equation, both numerically evaluated and asymptotically approximated for definite initial conditions, to the comparison with the behavior of the corresponding solutions of the Schrödinger equation in order to both highlight the differences and to possibly understand how the latter “flow” in the former. Interesting analogies with other fields of physics, in particular with optics, are suggested.     

Different field distributions obtained with an axicon and an amplitude mask

A combination of an axicon and an amplitude mask is used to obtain an intensity distribution that differs from the typical Bessel one. Experimental and numerical characterization of the field is made for different propagation distances and types of amplitude masks.