Browsing by Author "Domingo, LR"

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    Density functional theory study for the cycloaddition of 1,3-butadienes with dimethyl acetylenedicarboxylate. Polar stepwise vs concerted mechanisms
    (2002) Domingo, LR; Arnó, M; Contreras, R; Pérez, P
    The molecular mechanisms for the cycloaddition reactions of four low activated 1,3-butadiene systems (1,3-butadiene, (E)-1,3-pentadiene, (Z)-1,3-pentadiene, and 4-methyl-1,3-pentadiene) with dimethyl acetylenedicarboxylate (DMAD) have been studied using density functional theory method. For these cycloadditions, two competitive mechanisms have been characterized: the First one corresponds to a concerted C-C bond-formation where the asynchronicity depends on the methyl substitution. The second one corresponds to a stepwise process with a larger polar character where first a C-C bond is formed along the nucleophilic attack of 1,3-butadiene system to a conjugate position of the electron-poor substituted acetylene. Although the nonactivated 1,3-butadiene prefers the concerted process, substitution of hydrogen atoms by electron-releasing methyl groups favors the stepwise mechanism along with an increase of the charge-transfer process. A conformational analysis for DMAD reveals that both planar and perpendicular arrangements of the two-carboxylate groups have a decisive role on the dienophile/electrophile nature of this acetylene derivative. Thus, although the planar arrangement is preferred along the concerted process, the perpendicular favors the polar one along an increase of the electrophilicity of DMAD. The global and local electrophilicity power of these 1,3-butadienes and DMAD have been evaluated in order to rationalize these results. The study is completed with an analysis of the electrophilic/nucleophilic site activation, by probing the variations in local properties of DMAD perturbed by a model nucleophile with reference to a model transition structure. Inclusion of solvent effects, by means of a polarizable continuum model, does not modify these gas-phase results.
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    Quantitative characterization of the global electrophilicity power of common diene/dienophile pairs in Diels-Alder reactions
    (2002) Domingo, LR; Aurell, MJ; Pérez, P; Contreras, R
    The global electrophilicity power, omega, of a series of dienes and dienophiles commonly used in Diels-Alder reactions may be conveniently classified within a unique relative scale. Useful information about the polarity of transition state structures expected for a given reaction may be obtained from the difference in the global electrophilicity power, Deltaomega. of the diene/dienophile interacting pair. Thus the polarity of the process can be related with non-polar (Deltaomega small, pericyclic processes) and polar (Deltaomega big, ionic processes) mechanisms. (C) 2002 Elsevier Science Ltd. All rights reserved.