Oscillator strength and polarization of the forbidden n-->pi* band of trans-azobenzene: a computational study

The trans-azobenzene molecule is thought to prefer a planar C2h geometry, in gas phase as well as in solution, according to the most recent computational studies. As a consequence, the weak n-->pi* absorption band is forbidden by symmetry at the equilibrium geometry, and its intensity depends on the effect of the vibrational motions on the electronic structure. In this computational study, we determine the contribution of the vibrational modes to the oscillator strength, taking into account the anharmonicity, the thermal distributions, and the solvent effects. The good agreement of our results with the measured absorption spectrum confirms the C2h equilibrium structure of trans-azobenzene, with a relatively easy torsion of the phenyl groups around the N--C bonds. We also address the question of the polarization of this transition, which is a preliminary step to interpret the time-resolved fluorescence anisotropy measurements [C.-W. Chang et al., J. Am. Chem. Soc., 126, 10109 (2004)], a very sensitive probe of solvent effects on the excited state dynamics.     

Synthesis of carbapyochelins via diastereoselective azidation of 5-(ethoxycarbonyl)methylproline derivatives

Two configurationally stable carbon-based analogues of pyochelin have been prepared from Boc-pyroglutamic acid-tert-butyl ester in 11 and 13 steps. Introduction of the amino group was achieved by a highly diastereoselective electrophilic azidation reaction to afford novel bis-alpha-amino acid proline derivatives.     

Synthesis of [1,2,4]triazoloquinazoline and [1,2,4]-triazolo-1,4-benzodiazepine derivatives

Some [1,2,4]triazolo[1,5-c]quinazolin-5(6H)-ones 7 , the corresponding isomers [1,2,4]triazolo[4,3-c]quinazo-lin-5(6H)-ones and the 5-amino derivatives 8, 9 and 11 have been synthesized starting from the acylamidrazones 5 . The preparation of 5H-[1,2,4]triazolo[1,5-d]-1,4-benzodiazepin-6(7H)-ones 15 and of 5-cyclicaminomethyl-[1,2,4]triazolo[1,5-c]quinazolines 16 and 17 is also reported.     

Reactions of Diazo Compounds with Alkenes Catalysed by [RuCl(cod)(Cp)]: Effect of the Substituents in the Formation of Cyclopropanation or Metathesis Products

The reaction of diazo compounds with alkenes catalysed by complex [RuCl(cod)(Cp)] (cod=1,5‐cyclooctadiene, Cp=cyclopentadienyl) has been studied. The catalytic cycle involves in the first step the decomposition of the diazo derivative to afford the reactive [RuCl(Cp){?C(R¹)R²}] intermediate and a mechanism is proposed for this step based on a kinetic study of the simple coupling reaction of ethyl diazoacetate. The evolution of the Ru–carbene intermediate in the presence of alkenes depends on the nature of the substituents at both the diazo N₂?C(R¹)R² (R¹, R²=Ph, H; Ph, CO₂Me; Ph, Ph; C(R¹)R²=fluorene) and the olefin substrates R³(H)C?C(H)R⁴ (R³, R⁴=CO₂Et, CO₂Et; Ph, Ph; Ph, Me; Ph, H; Me, Br; Me, CN; Ph, CN; H, CN; CN, CN). A remarkable reactivity of the complex was recorded, especially towards unstable aryldiazo compounds and electron‐poor olefins. The results obtained indicate that either cyclopropanation or metathesis products can be formed: the first products are favoured by the presence of a cyano substituent at the double bond and the second ones by a phenyl.     

Carbon nanotube field-effect transistor detector associated to gas chromatography for speciation of benzene,toluene, ethylbenzene, (o-, m- and p-)xylene

An analytical methodology based on a field-effect transistor detector using carbon nanotubes (NTFET) coupled to a gas chromatograph has been developed for the speciation of the following aromatic compounds: benzene, toluene, ethylbenzene, m-xylene, p-xylene and o-xylene (BTEX). This methodology combines the proven separation capability of gas chromatography (GC) with the potential for detection of a NTFET. The developed analyzer shows a high and stable analytical response upon repeated analysis of BTEX during 4 weeks, with detection limit less than 4 μg/L. The GC–NTFET system also shows a great suitability for actual monitoring of indoor atmospheres and no significant difference was observed between the results obtained by the developed analyzer and a more classical analytical methodology, namely gas chromatography–flame ionization detection (GC–FID).     

Diels-Alder reactions of 2-azadienes derived from cysteine and serine methyl esters and aldehydes

The Diels-Alder reactions of N-benzylidenedehydroalanine methyl ester 1a with but-3-en-2-one and with other electron deficient dienophiles have been found to give new dihydro- and tetrahydropyridines. The cycloaddition reactions are regioselective but not stereoselective. Cycloaddition reactions between 1a and enamines have also been observed. The [4 + 2] cycloaddition reactions of other N-arylidenedehydroalanine methyl esters are also reported. Two new types of azadiene were prepared, namely N-(benzoylmethylene)dehydroalanine methyl ester 1e and N-(ethoxycarbonylmethylene)dehydroalanine methyl ester 1f. Their reactions with N-cyclohexen-1-ylpyrrolidine and with N-cyclopenten-1-ylpyrrolidine have led to the isolation of the dihydropyridine and pyridine esters.     

Fast liquid chromatography–quadrupole-linear ion trap mass spectrometry for the analysis of pharmaceuticals and hormones in water resources

This paper presents the development of a fast multi-residue method for the determination of 49 pharmaceuticals and 6 metabolites from different therapeutic classes in water resources by means of Ultra-performance™ liquid chromatography (UPLC) coupled to tandem mass spectrometry. The use of the UPLC technology enabled all the 55 compounds to be separated chromatographically in less than 9 min (6.3 min positive mode and 2.7 min negative mode) and with a total analysis time of 18 min when considering column conditioning. Improved resolution, sensitivity and a reduction of matrix effects were obtained under these conditions. Unequivocal identification and quantification of the target compounds was also performed by using the dual acquisition modes of the hybrid triple quadrupole-linear ion trap (QqLIT) system. Triple quadrupole mode by means of selected reaction monitoring (SRM) was used for quantification, whilst a second SRM transition together with information-dependent analysis (IDA) experiments was used for confirmation. Additionally, one general, single solid-phase extraction (SPE) method was developed by using Oasis HLB cartridges. Quality parameters of the method in wastewaters were established obtaining a fast, robust, reproducible and cost-effective method for all the target pharmaceuticals. Finally, the optimized SPE-UPLC/QqLIT method was used for the analysis of the target compounds in wastewaters from Spain. Thirty-one out of fifty-five compounds were identified in the samples collected.     

The first enantioselective [3+2] cycloaddition of epoxides to arylisocyanates: asymmetric synthesis of chiral oxazolidinone phosphonates

A [3+2] cycloaddition of diethyl 1,2-oxiranephosphonate to aryl isocyanates catalyzed by lanthanide cations is described. The reaction is highly regioselective and 5-substituted 2-oxazolidinone phosphonates are obtained with a regioselectivity greater than 95:5 with respect to the 4-substituted regioisomer, and in up to 84% yield. When 20 mol % of Pybox-Yb³⁺ is used as a catalyst, enantiomerically enriched products are obtained in up to 75% ee, depending on the reaction conditions, and the nature of the isocyanate. Low temperatures benefit asymmetric induction, but have an adverse effect on the regioselectivity for para-substituted aryl isocyanates.     

Theoretical characterization of H2 adsorption on AuPt clusters

Linear and triangular AuPt_n cluster reactions with H₂ are studied theoretically, using ab initio multiconfiguration self-consistent field (MC-SCF) calculations, followed by extensive multireference configuration interaction (MR-CI) variational and perturbative. Both the linear dimer and the triangular trimers capture the hydrogen molecule by the Pt cluster side and by the Au cluster side. Gold has an electronic effect on the Pt activity, more important than a geometrical one, poisoning the Pt activity to dissociate H₂ and lowering the adsorption heats. This effect is stronger in the AuPt dimer, where the H₂ capture occurs only at the molecular level, without showing hydrogen bond dissociation. The trimers look more active, relaxing the H(SINGLE BOND)H bond until breakage. No activation barriers are observed in all the cases considered. The hydrogen molecule is not able to cross the clusters due to the large barriers present. © 1998 John Wiley & Sons, Inc. Int J Quant Chem 67: 399–409, 1998