RESUMO

This paper generalizes very recent and unexpected findings [J. Phys. Chem. A, 2021, 125, 5152-5165] regarding the known "direct- and inverse-electron demand" Diels-Alder mechanisms. Application of bonding evolution theory indicates that the key electron rearrangement associated with significant chemical events (e. g., the breaking/forming processes of bonds) can be characterized via the simplest fold polynomial. For the CC bond formation, neither substituent position nor the type of electronic demand induces a measurable cusp-type signature. As opposed to the case of [4+2] cycloaddition between 1,3-butadiene and ethylene, where the two new CC single bonds occur beyond the transition state (TS) in the activated cases, the first CC bond occurs in the domain of structural stability featuring the TS, whereas the second one remains located in the deactivation path connecting the TS with the cycloadduct.

Assuntos

RESUMO

This work revisits the topological characterization of the Diels-Alder reaction between 1,3-butadiene and ethylene. In contrast to the currently accepted rationalization, we here provide strong evidence in support of a representation in terms of seven structural stability domains separated by a sequence of 10 elementary catastrophes, but all are only of the fold type (F and F†), that is, C4H6 + C2H4:1-7-[FF]F[F†F†][F†F†][FF]F†-0: C6H10. Such an unexpected finding provides fundamental new insights opening simplifying perspectives concerning the rationalization of the CC bond formation in pericyclic reactions in terms of the simplest Thom's elementary catastrophe, namely, the one-(state) variable, one-(control) parameter function.

RESUMO

The Wittig reaction between triphenylphosphine methylide and benzaldehyde has been studied both from conceptual and computational approaches. The supernucleophilic character of ylide accounts for the feasibility of the initial nucleophilic attack. The nature of bonding driving the formation of the first oxaphosphetane (OPA) intermediate in such a domino reaction is examined within a topological-based bonding evolution theory perspective. The sequence of the electronic flow associated to the changes in electron density supports a rationalization via two main electronic stages characterizing the single kinetic step: first, the C-C bond formation, which takes place via donation of electron density of the ylide carbon to the carbonyl carbon of benzaldehyde at a C-C distance of 2.02 Å, is formally associated to the transition state region; then, the P-O bond formation via the donation of electron density of the nonbonding region of the carbonyl oxygen to phosphorus at a P-O distance of 2.06 Å is located at the end of the reaction path. The detailed picture of bonding patterns suggests that the OPA formation in the Wittig mechanism can be better understood in terms of a two-stage one-step mechanism beyond molecular orbital considerations behind the traditionally accepted [2+2] cycloaddition proposal.

RESUMO

The sequence of the electronic flow driving the hydrometallation of acetylene by lithium hydride (and that of the opposite ß-hydride elimination reaction from the alkenyl metal intermediate), was examined within the perspective provided by the bonding evolution theory (BET). The analysis was based on the application of catastrophe theory to the changes of the electron localization function topology along the intrinsic reaction coordinate. The description of the electronic processes occurring on the process was represented in terms of topological structural stability domains (SSDs) and the associated elementary bifurcation catastrophes. Within such a framework of representation, the "evolution" of the system through the different SSDs reveals the key chemical events driving the transformation, including the large polarization effect as a consequence of Pauli repulsion between ions of the positive cationic metal on the hydride domain, the activation of the CC triple bond to attack the cationic center, and the agostic stabilizing interactions involving the hardest cationic metal, followed by the attack of the hydride center. These results contribute to emphasizing the intrinsic value and usefulness of using topological-based approaches and associated tools to increase our knowledge and understanding of the subtleties underlying the electronic flow as nuclei evolve along the reaction coordinate, providing detailed and complementary insights in comparison to other interpretative tool such those based on orbital-based representations, concerning the intimate nature of the electronic rearrangement of key mechanistic processes in chemistry. Graphical abstract The sequence of the electron flow (indicated by letters a and b) along the intrinsic reaction path for the hydrometallation of acetylene by lithium hydride to yield ethenyl lithium via a four-membered transition structure (TS), as determined within the bonding evolution theory to provide the key chemical events driven the changes in the key bonding patterns. Blue arrow Main event on the side HC ≡ CH + LiH → TS, red arrow the TS → HLiC=CH2 pathway, green arrows relative motion of nuclei along the imaginary frequency at the position of TS on the intrinsic reaction coordinate.

RESUMO

Registered by the World Health Organization (WHO), 3-chloro-4-(dichloromethyl)-5-hydroxy-2(5H)-furanone (MX) is one of the strongest bacterial mutagens ever tested, as highlighted by the Ames Salmonella typhimurium TA100 assay. We provide new insights concerning this mutagenic activity on the basis of global and local theoretically defined electrophilicity indices. Our results further support the idea that mutagenicity of MX and its analogues is related more closely to one-electron transfer processes from the electron-rich biological environment than to adduct formation processes. We also stress that, although the Z-open tautomers are intrinsically more electrophilic than furanone ring analogues, the observed mutagenic activity is significantly correlated only to the electrophilicity response of the ring forms. In that context, we also emphasize that it is electrophilicity at the C α in the α-ß unsaturated carbonyl moiety that exhibits a strong correlation with the observed mutagenic activity.

Assuntos

RESUMO

The effect of Ni(II), Cu(II) and Zn(II) association on the diketo/keto-enol tautomerism of thymine has been investigated through the use of B3LYP density functional theory calculations. Final energies were obtained at the B3LYP/6-311+G(2df,2p)//B3LYP/6-311+G(d,p) level of theory. Ni(II) and Cu(II) lead to an oxidation of thymine which for Zn(ii) is only partial and catalyze the tautomerization process, this catalytic effect being much larger for Ni(2+) and Zn(2+) than for Cu(2+). One of the most significant consequences of the oxidation of the base is that the calculated BDE's are primarily dictated by the value of the second ionization potential of the metal, and therefore follow the sequence Cu(2+) > Ni(2+) > Zn(2+). Also importantly, metal dication association leads to a stabilization of the keto-enol tautomer, which becomes the most stable form upon interaction with Ni(2+) and Zn(2+). This stabilization enhancement is the consequence of three concomitant factors, namely, (i) a stronger interaction of the metal cation with the carbonyl oxygen, (ii) the interaction of the metal with the dehydrogenated ring nitrogen, (iii) an aromatization of the six-membered ring.

Assuntos

RESUMO

The 1,3 intramolecular hydrogen transfer reaction in free thymine and in Mg(II)-thymine have been studied at the density functional theory level. The mechanism of intramolecular proton transfer in these systems emerges from the analysis of the reaction force profile along the reaction path; it is rationalized in terms of structural and electronic reorganizations that take place during the chemical transformation. Results show that the presence of Mg(II) monocoordinated to thymine activates the hydrogenic motion by inducing structural and electronic changes in the molecular backbone. In the metallic complex, it is found that the hydrogen transfer is followed by a relaxation process that facilitates the metal cation migration to form a bicoordinated complex.

RESUMO

En Venezuela, existe una importante cantidad de pacientes con infección por el virus de inmunodeficiencia humana, con enfermedad avanzada y en quienes se desarrollan infecciones oportunistas que se presentan en el cuadros clínicos muy diversos. Una forma de presentación común son los pacientes febriles con citopénias cuya etiología mapas frecuente, en el Hospital Universitario de Caracas, es la histiplasmosis diseminada. En ellos suele realizarse estudio de médula ósea donde es frecuente el hallazgo del síndrome hemofacocitario reactivo. Determinar el valor del hallazgo del síndrome hemofagocitario reactivo como factor para la predicción del diagnóstico final de histoplasmosis diseminada en pacientes de virus de inmuno deficiencia humana con bicitipenia o pancitopania. Estudio de corte transversal, descriptivo, se estudiaron 36 pacientes virus de inmunodeficiencia humana, naive, bicitopénicos o pancitopénicos febríles en los cuales se realizó estudio de médula osea en busca del síndrome hemofagocitario reactivo. 14 pacientes tuvieron el diagnóstico de histoplasmosis, el síndrome hemofagocitario se observó en 11 de ellos (78,57 por ciento); en los 22 pacientes con diagnóstico distinto a histoplasmosis, el síndrome hemofagocitario reactivo se reportó en 13 (59,09 por ciento) P=0,19. El hallazgo de síndorme hemofagocitario reactivo, aunque es un fenómeno muy prevalente en estos pacientes, no establece diferencias en la predicción del diagnóstico de histoplasmosis diseminada.

Assuntos