Francisco Méndez

Local softness and chemical reactivity of maleimide: nucleophilic addition

Abstract The local softness and the Fukui function of maleimide are determined through a finite differences scheme in order to show the usefulness of these concepts for rationalizing the inherent chemical reactivity of a given molecule. The local softness surface diagram of maleimide indicates that soft nucleophiles interact with the a carbon atoms, whereas hard nucleophiles interact with the carbonyl carbon atoms, in agreement with the experimental evidence.

Oxazole as an Electron-Deficient Diene in the Diels–Alder Reaction

The Diels-Alder cycloaddition reaction of oxazole with ethylene is facilitated by addition of an alkyl group or Brønsted or Lewis acids to the oxazole nitrogen atom. The efficacy consists of stabilizing the transition state, lowering the activation barrier and the HOMO(dienophile)-LUMO(diene) gap, and increasing the reaction exothermicity.

Chemical reactivity of the imidazole: a semblance of pyridine and pyrrole?

It has been suggested that pyridine and pyrrole could be patterns for imidazole reactivity studies due to the amine (-NH-) and aza (-N═) nitrogen atoms. The analyses of the local and global electronic indexes prove and quantify that imidazole has an intermediate analogy between pyrrole and pyridine.

Influence of fluorine atoms and aromatic rings on the acidity of ethanol.

Absolute gas-phase acidities Delta(acid)G(0)(OH) and Delta(acid)G(0)(CH) were calculated at the B3LYP and MP2 levels using six different standard basis sets for the OH and CH heterolytic bond cleavage of ethanol and twelve derivatives of the type CH(3-n)F(n)CHX(r)OH, where n ranges from zero to three and represents the number of fluorine atoms and r represents hydrogen and the type of aromatic ring, namely: X(0) = hydrogen, X(1) = phenyl, X(2) = 1-naphthyl, and X(3) = 9-anthryl. The similarity between calculated and experimental Delta(acid)G(0)(OH) values for ethanol (1a), 2-fluoroethanol (1b), 2,2-difluoroethanol (1c), 2,2,2-trifluoroethanol (1d), and 1-phenylethanol (2a) was used to validate the right theoretical method for this study. Substituent partial contributions to hydroxyl-, methylene-, and methine-hydrogen acidities were evaluated by linear combination. Good parameter fittings of the primary and secondary alcohols were obtained and interpreted as additive contribution of the substituent effects. The nonlinear contributions were identified. Calculations prove that fluoroalcohols exhibit C-H acidity, which is usually lower than O-H acidity. In principle, the inversion of this acidity order is possible by the introduction of a large aromatic ring instead to increase the number of fluorine atoms.

Chemical reactivity of hypervalent silicon compounds: The local hard and soft acids and bases principle viewpoint

The silicon atom may increase its coordination number to values greater than four, to form pentacoordinated compounds. It has been observed experimentally that, in general, pentacoordinated compounds show greater reactivity than tetracoordinated compounds. In this work, density functional theory is used to calculate the global softness and the condensed softness of the silicon atom for SiHnF4−n and SiHnF5−n1−. The values obtained show that the global and condensed softness are greater in the pentacoordinated compounds than in the tetracoordinated compounds, a result that explains the enhanced reactivity. If the results are analysed through a local version of the hard and soft acids and bases principle, it is possible to suggest that in nucleophilic substitution reactions, soft nucleophiles preferably react with SiHnF5−n1−, and hard nucleophiles with SiHnF4−n.

Nucleophilic attacks on maleic anhydride: a density functional theory approach

The reactivity of maleic anhydride with respect to nucleophiles is studied by using some concepts defined within density functional theory formalism. It is shown that the hard and soft acids and bases principle in a local approach, is useful for distinguish the reactivity of the two type of carbon atoms in the molecule.

Growth of fullerene fragments using the Diels-Alder cycloaddition reaction: first step towards a C60 synthesis by dimerization.

Density Functional Theory has been used to model the Diels-Alder reactions of the fullerene fragments triindenetriphenilene and pentacyclopentacorannulene with ethylene and 1,3-butadiene. The purpose is to prove the feasibility of using Diels-Alder cycloaddition reactions to grow fullerene fragments step by step, and to dimerize fullerene fragments, as a way to obtain C60. The dienophile character of the fullerene fragments is dominant, and the reaction of butadiene with pentacyclopentacorannulene is favored.

The Fukui function of an atom in a molecule: A criterion to characterize the reactive sites of chemical species

The principle of hard and soft acids and bases is interpreted as the result of two opposing tendencies, one related to the charge transfer process (chemical potential equalization principle), and the other one related to the reshuffling of the electronic density (maximum hardness or minimum softness principle). A local version of the principle is elucidated by assuming that these tendencies are dominated by the local properties rather than by the global properties of the molecule. This principle is used together with the Fukui function of the atoms in the molecule to characterize the reactive sites. The results presented for the nucleophilic addition to the pyridinium ion, and for the electrophilic substitution on pyridine oxide show the usefulness of these concepts in describing the inherent reactivity of chemical species.

Understanding the nucleophilic character and stability of the carbanions and alkoxides of 1-(9-anthryl)ethanol and derivatives.

The nucleophilic character and stability of the carbanions vs. alkoxides derived from 2,2,2-trifluoro-1-(9-anthryl)ethanol and 1-(9-anthryl)ethanol containing X electron-releasing and X electron-acceptor substituents attached to C-10, have been studied at the B3LYP/6-31+G(d,p) level of theory. Results analyzed in terms of the absolute gas-phase acidity, Fukui function, the local hard and soft acids and bases principle, and the molecular electrostatic potential, show that the central ring of the 9-anthryl group confers an ambident nucleophilic character and stabilizes the conjugated carbanion by electron-acceptor delocalization.

Protophilicity index and protofelicity equalization principle: new measures of Brønsted-Lowry-Lewis acid–base interactions

The simultaneous contributions of proton and electron transfer to the Brønsted-Lowry and Lewis acid–base properties of a set of p-substituted phenols are reported in this work. As a result of the analysis, a novel protophilicity index considered as the second-order energy change of a Brønsted-Lowry base as it is saturated with protons, a combined Brønsted-Lowry-Lewis acidity index (with a corresponding basicity index), and a protofelicity equalization principle (a parallel of the electronegativity equalization principle) are presented.