Global and local electrophilicity patterns of diazonium ions and their reactivity toward pi-nucleophiles

The global and local electrophilicity patterns of a series of 15 diazonium ions have been evaluated using the absolute scale of electrophilicity proposed by Parr et al. (J. Am. Chem. Soc. 1999, 121, 1922). The predicted global electrophilicity pattern of the whole series of diazonium ions correctly compares with the experimental electrophilicity recently determined for these charged electrophiles. The global electrophilicity is then projected into the different potential active sites of the molecular ions using the electrophilic Fukui function. The highest regional electrophilicity power is found at the terminal nitrogen atom of the arenediazonium ions, yet the highest positive charge is located on the nitrogen atom directly attached to the aromatic ring. This result is consistent with the observed reactivity displayed by diazonium ions toward substituted alkenes, thereby suggesting that the formation of the azocarbocation intermediate is mostly orbital rather than charge controlled.     

Preliminary characterization of anhydrous ethanol used in Brazil as automotive fuel

This work reports preliminary studies on the characterisation of anhydrous ethanol (AEA) used as an automotive fuel mixed with gasoline in Porto Alegre (South Brazil). Pre-concentration of the impurities contained in 1000 ml of AEA was carried on through solid-phase extraction using XAD4 resin. The main compounds in the extract were identified by means of spectral data from the library of the equipment. The concentrate was then fractionated using a preparative liquid chromatographic column filled with activated silica gel and the elution procedure was carried out with, n-hexane, n-hexane-benzene (1:1, v/v) and dichloromethane. Prior to analysis by GC-MS, each fraction was reduced to 1 ml with a gentle stream of nitrogen. Saturated linear hydrocarbons and aromatic hydrocarbons eluted in the first fraction and oxygenated compounds such as aldehydes. ketones and alcohols, eluted in the second one. were the main compounds detected in the sample.     

Formation of a cyclobutylidene ring: intramolecular [2 + 2] cycloaddition of allyl and vinylidene C=C bonds under mild conditions

Intramolecular [2 + 2] cycloaddition of two C=C bonds in vinylidene complexes [Ru(eta5-C9H7){=C=C(R)H}(PPh3){kappa1-(P)-PPh2(C3H5)][BF4] affords cyclobutylidene complexes [Ru(eta5-C9H7){kappa2-(P,C)-(=CC(R)HCH2CHCH2PPh2)}(PPh3)][BF4], which can be also obtained by reaction of terminal alkynes with [Ru(eta5-C9H7)(PPh3){kappa3-(P,C,C)-PPh2(C3H5)}][PF6]. The reaction proceeds under mild conditions via vinylidene complexes, and the activation parameters were determined by kinetic studies.     

Simulation of aerated lagoon using artificial neural networks and multivariate regression techniques

The aim of this study was to develop an empirical model that provides accurate predictions of the biochemical oxygen demand of the output stream from the aerated lagoon at International Paper of Brazil, one of the major pulp and paper plants in Brazil. Predictive models were calculated from functional link neural networks (FLNNs), multiple linear regression, principal components regression, and partial least-squares regression (PLSR). Improvement in FLNN modeling capability was observed when the data were preprocessed using the PLSR technique. PLSR also proved to be a powerful linear regression technique for this problem, which presents operational data limitations.     

Quantitative characterization of the global electrophilicity pattern of some reagents involved in 1,3-dipolar cycloaddition reactions

The global electrophilicity power, ω, of a series of dipoles and dipolarophiles commonly used in 1,3-dipolar cycloadditions may be conveniently classified within a unique relative scale. The effects of chemical substitution on the electrophilicity of molecules have been evaluated using a representative set of electron-withdrawing and electron-releasing groups for a series of dipoles including nitrone, nitrile oxide and azide derivatives. The absolute scale of electrophilicity is used to rationalize the chemical reactivity of these species as compared to the static reactivity pattern of the reagents involved in the Diels-Alder reactions.     

Valine and phenylalanine as precursors in the biosynthesis of alkamides in Acmella radicans

Acmella radicans (Asteraceae) produces at least seven alkamides, most with either an isobutyl- or phenylethyl group as the amine moiety. These moieties suggest that the amino acids valine and phenylalanine are the biosynthetic precursors of these alkamides. On the basis of labeled feeding experiments using either L-[2H8]valine or L-[2H8]phenylalanine we present evidence for the involvement of these two amino acids in the biosynthesis of (2E,6Z,8E)-N-isobutyl-2,6,8-decatrienamide (affinin) (1), (2Z,4E)-N-(2-phenylethyl)-2,4-octadienamide (2), (2E)-N-(2-phenylethyl)-nona-2-en-6,8-diynamide (3), and 3-phenyl-N-(2-phenylethyl)-2-propenamide (4). Alkamides were isolated from young A. radicans plants and analyzed by gas chromatography-mass spectrometry (GC-MS). Additionally, in cell free in vitro experiments based on isobutyl and phenylethylamide biosynthesis, using a colorimetric assay and GC-MS, valine and phenylalanine decarboxylase activities were assayed in the soluble extract of A. radicans leaves.     

An investigation of the chemical stability of arsenosugars in basic environments using IC-ICP-MS and IC-ESI-MS/MS

This paper evaluates the chemical stability of four arsenosugars using tetramethylammonium hydroxide (TMAOH) as an extraction solvent. This solvent was chosen because of the near quantitative removal of these arsenicals from difficult to extract seafood (oysters and shellfish). Four arsenosugars (3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropylene glycol--As(328), 3-5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropanesulfonic acid--As(392), 3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropyl hydrogen sulfate--As(408), and 3-[5'-deoxy-5'-(dimethylarsinoyl)-beta-ribofuranosyloxy]-2-hydroxypropyl-2,3-hydroxypropyl phosphate--As(482)) were evaluated. The stability of these four arsenosugars were studied independently in a solution of 2.5% TMAOH at 60 degrees C over a period of up to 8 h. Two arsenosugars, As(328) and As(392), were found to be relatively stable in this solution for up to 8 h. However, As(408) and As(482) formed detectable quantities of dimethylarsinic acid (DMAA) and As(328) within 0.5 and 2 h, respectively. It was found that 97% of As(408) degrades after 8 h of treatment producing 3.4 times as much DMAA as As(328). This is contrary to As(482), which produces 13 times as much As(328) as DMAA and only 37% of the As(482) was converted by the 8 h treatment at 60 degrees C. These degradation products led to the investigation of weaker TMAOH extraction solvents. Three different concentrations (2.5%, 0.83% and 0.25%) were used to determine the effect of TMAOH concentration on the degradation rate of As(408). By reducing the TMAOH concentration to 0.83%, the conversion of the arsenosugar to As(328) and DMAA is nearly eliminated (less than 5% loss). Arsenosugars, As(408) and As(482), were also studied in 253 mM NaOH to verify the degradation products. The NaOH experiments were conducted to investigate a possible hydroxide based reaction mechanism. Similar degradation plots were found for each arsenosugar when compared to the 2.5% TMAOH data. A mechanism has been proposed for the formation of As(328) from As(408) and As(482) in base via an SN2 reaction (hydroxide attack) at the side chain carbon adjacent to the inorganic ester. The formation of DMAA is observed in all arsenosugars after prolonged exposure. This probably occurs via an SN2 attack at the arsenic atom.     

Zerovalent Rhodium and Iridium Silatranes Featuring Two‐Center,Three‐Electron Polar σ Bonds

Species with 2‐center, 3‐electron (2c/3e^?) σ bonds are of interest owing to their fascinating electronic structures and potential for interesting reactivity patterns. Report here is the synthesis and characterization of a pair of zerovalent (d⁹) trigonal pyramidal Rh and Ir complexes that feature 2c/3e^? σ bonds to the Si atom of a tripodal tris(phosphine)silatrane ligand. X‐ray diffraction, continuous wave and pulse electron paramagnetic resonance, density‐functional theory calculations, and reactivity studies have been used to characterize these electronically distinctive compounds. The data available highlight a 2c/3e^? bonding framework with a σ*‐SOMO of metal 4‐ or 5d_z² parentage that is partially stabilized by significant mixing with Si (3p_z) and metal (5‐ or 6p_z) orbitals. Metal‐ligand covalency thus buffers the expected destabilization of transition‐metal (TM)‐silyl σ*‐orbitals by d–p mixing, affording well‐characterized examples of TM–main group, and hence polar, 2c/3e^? σ “half‐bonds”.