Search results for "Bases"
showing 10 items of 1240 documents
Powered by public relations? Mutual perceptions of PR practitioners’ bases of power over journalism
2017
Public relations practitioners depend on journalists to report information, and journalists count on public relations practitioners to provide information. This mutual dependence gives the two parties a degree of power over each other that can be wielded if certain resources are available. However, there are many unanswered questions about how public relations exerts power over journalists and how these influence attempts may affect news coverage. We differentiate six bases of power that public relations practitioners may use to influence journalists. To test the use of these bases of power, we conducted a quantitative survey among German journalists and public relations practitioners. Our…
Estructura jerárquica de la colaboración entre autores. Análisis de los artículos sobre diagnóstico por la imagen publicados por los servicios hospit…
2005
Objetivo Analizar la distinta contribucion de las categorias jerarquicas en los articulos publicados por radiologos de hospitales pertenecientes a la Comunidad Valenciana en el periodo 1994-2001, asi como su distribucion en las revistas. Material y metodo Para la recuperacion de la muestra de articulos hemos disenado perfiles de busqueda especificos para las bases de datos IME y Medline. Se identifico la categoria jerarquica de los autores mediante entrevistas personales. Resultados Hemos analizado 417 trabajos, con 1.321 firmas distribuidas de la siguiente forma: jefe de servicio: 8,8%; jefe de seccion: 12,4%; adjunto: 50,7%, y residente: 28%. Las agrupaciones jerarquicas con participacion…
Water-Tolerant Trifloaluminate Ionic Liquids: New and Unique Lewis Acidic Catalysts for the Synthesis of Chromane
2018
The first example of triflometallate ionic liquids, named in analogy to chlorometallate ionic liquids, is reported. Trifloaluminate ionic liquids, synthesised from 1-alkyl-3-methylimidazolium triflates and aluminium triflate, were characterised by multinuclear NMR spectroscopy and FT-IR spectroscopy, revealing the existence of oligonuclear, multiply-charged trifloaluminate anions, with multiple bridging triflate modes. Acceptor numbers were determined to quantify their Lewis acidity, rendering trifloaluminate ionic liquids as medium-strength Lewis acids (AN = ca. 65). Used as acidic catalysts in the cycloaddition of 2,4-dimethylphenol and isoprene (molar ratio 2:1) to prepare chromane, trif…
Evolución de los recursos Documentales en adicciones
2001
La documentación científica en adicciones ha experimentado una evolución similar a la del resto de disciplinas científicas del momento, de ahí que las ventajas e inconvenien¬tes que aparecen en cualquier área sean asumibles por ésta.Actualmente podemos obtener información sobre aspectos adictivos a partir de dife¬rentes fuentes documentales que van siendo cada vez más accesibles tanto para los investigadores, los docentes como para cualquier persona en formación en este ámbi¬to. Si bien dada la diversidad existente es necesario que en los próximos años se haga un esfuerzo de clasificación e integración de la información en función de su utilidad, ya que en la mayoría de las ocasiones puede …
Theoretical study of the regio- and stereoselectivity of the intramolecular Povarov reactions yielding 5H-chromeno[2,3-c] acridine derivatives
2016
The intramolecular Povarov (IMP) reactions involved in the synthesis of 5H-chromeno[2,3-c] acridine derivatives [Tetrahedron Lett., 2010, 51, 3071–3074] have been studied using density functional theory (DFT) methods. The studied IMP reaction is a domino process that comprises two consecutive reactions: (i) a BF3 Lewis acid catalysed intramolecular aza-Diels–Alder (IMADA) reaction of an alkene tethered chromene imine (ATCI) giving a formal [4 + 2] cycloadduct, and (ii) a 1,3-hydrogen shift yielding the final chromeno product. The possible regio-(fused/bridged) and stereo-(cis/trans) isomeric channels associated with the IMADA reaction were thoroughly investigated and analysed. The activatio…
MoVReagents in Organic Synthesis
2016
The use of MoV reagents, and in particular MoCl5, in organic synthesis is surveyed. The oxidative treatment of aromatic substrates is the most common application. The unique properties of these reagents are due to their high oxidative power combined with exquisite Lewis acid properties. In several examples MoV reagents outperform other common oxidative coupling reagents. C–C bond formation through inter- and intramolecular oxidative coupling can lead to selective formation of five- to eight-membered ring systems. Mechanistic investigations of the courses of reactions involving MoV reagents and aromatic substrates indicate that radical cations are initially formed, entering the oxidative cou…
The mechanism of formation of 3H,9H-Pyrano[3,4-b]indol-3-ones from 3-indolalkanoic acids
1991
The mechanism of the formation of 1-methyl-3H,9H-pyrano[3,4-b]indol-3-one (4) from the corresponding 3-indolacetic acid 1 is discussed. The suggested mechanism is substantiated by the isolation of a stable intermediate 2 and its transformations in the presence of acetic anhydride and/or Lewis acids.
Catalytic Diastereo- and Enantioselective Synthesis of 2-Imidazolinones.
2019
Chiral cyclic ureas (2-imidazolinones) were prepared by the reaction of nitrones and isocyanoacetate esters using a multicatalytic system that combines a bifunctional Brønsted base-squaramide organocatalyst and Ag+ as a Lewis acid. The reaction could be achieved with a range of nitrones derived from aryl- and cycloalkylaldehydes with moderate diastereo- and good enantioselectivity. A plausible mechanism involving an initial formal [3 + 3] cycloaddition of the nitrone and isocyanoacetate ester, followed by rearrangement to an aminoisocyanate and cyclization to the imidazolinone, is proposed.
Counterion's Effect on the Catalytic Activity of Zn-Prolinamide Complexes in Aldol Condensations
2012
The catalytic activity of complexes involving organic ligands and Lewis acids can be modulated by changing any of their components. In this work we have studied the influence on the stereoselectivity and catalytic activity exerted by the counterion of zinc salts employed as cocatalysts of L-prolinamide in aldol condensations. The structures of the complexes have been determined both in solution and in the solid state.
Lewis Acid Induced [2+2] Cycloadditions of Silyl Enol Ethers with α,β-Unsaturated Esters: A DFT Analysis
2005
The Lewis acid (LA) induced cycloaddition of trimethysilyl vinyl ether with methyl acrylate has been studied by DFT methods at the B3LYP/6-31G* level. In the absence of an LA, a [4+2] cycloaddition between the silyl enol ether and methyl acrylate in the s-cis conformation takes place through an asynchronous, concerted bond-formation process. This cycloaddition presents a large activation enthalpy of 21.1 kcal mol–1. Coordination of the LA AlCl3 to the carbonyl oxygen atom of methyl acrylate yields a change of molecular mechanism from a concerted to a two-step mechanism and produces a drastic reduction of the activation energy. This stepwise mechanism is initialized by the nucleophilic attac…