RESUMEN
A computational investigation of the aerobic oxidative C-C bond cleavage reaction of glycol catalyzed by an Anderson-type heteropolyanion HPA [IMo6O24]5- in the presence of acetonitrile as solvent has been performed at the WB97XD/6-31G(d,p)/lanl2dz level. Two reaction pathways have been identified. The catalytic cycle of each pathway consists of three steps: oxidation cleavage of a glycol molecule by the HPA, oxidation of the HPA by one dioxygen molecule, and, finally, oxidation of a second glycol and regeneration of the catalyst. These reaction pathways have been thoroughly investigated in terms of energetic, natural bond orbital (NBO), natural charges, and geometrical parameters. It is found that (i) even though the top oxygen atoms of the Anderson heteropolyanion are not the most negatively charged ones, they are more likely to react with the diol hydroxyl groups, (ii) a direct relationship between the presence of the iodine ion I(VII) and the studied oxidation reaction could not be identified, and (iii) in terms of energy, the transfer of the two hydrogen atoms is the most energetic step.
RESUMEN
Conceptual density functional theory has been applied to study the Himbert intramolecular arene/allene Diels-Alder reaction. The effect of substitutions at different positions on the kinetics of these reactions has been analyzed. Therefore, from the calculation of the activation energies of more than 27 reactions involving concerted mechanisms, the selectivity of these reactions can be predicted and rationalized with the aid of conceptual DFT descriptors. An application of the two concepts, natural population analysis (NBO) and the state-specific dual descriptor (SSDD) for evaluating substituent effects, allows the investigation of the different interactions that promote a reaction compared to another. The SSDDs computed for the transition state structures provide important information about charge transfer interactions during the chemical reaction. In our case, the SSDD results show that the substituents promoting Himbert reaction have the lowest excitation energies, a fact which facilitates the allene/arene interaction. The NBO results show that according to the nature of the substituent, the Himbert reaction stands as a normal-electron demand or reverse. Thus, the interactions favoring each reaction are mentioned. The geometric deformation observed in the case of OCH3 is at the origin to the emergence of other low interactions between diene and dienophile as well as a strong electronic delocalization stabilizing the arene moiety. The calculated synchronicity indexes show that the Himbert intramolecular Diels-Alder reactions are very synchronous.
RESUMEN
The present work studies theoretically the mechanisms involved in the fluorine substituent effect on the stereochemistry of Diels-Alder reactions. The case of R-butenone with cyclopentadiene is used for the purpose of modelling more general α-fluoro-α,ß-unsaturated carbonyl compounds, in catalyzed and uncatalyzed cases. A thorough analysis of the mechanism is performed using energy decomposition analysis (EDA) and conceptual DFT tools. It is shown that the endo conformation is privileged in all the studied cases with the exception of the α-fluorinated ketone. It is found that the endo selectivity of the non-fluorinated reactions is only due to the decrease of dispersion energy. On the other hand, the presence of a fluorine atom in the dienophile moieties increases remarkably the magnitude not only of the interaction energy between the reactants but that of the strain energy as well. Moreover, it is the strong destabilization strain energy occurring at the transition state of the endo pathway of the reaction cyclopentadiene/3-fluorobutenone that is mainly responsible for the exo selectivity. The effect of a Lewis acid catalyst on these reactions is also studied. The Lewis acid catalyst affects the activation energy of the studied Diels-Alder reactions but not their stereoselectivity. Furthermore, the dual descriptor results shed light onto the mechanism. Besides, natural bond orbital analysis (NBO) and determination of the condensed values of the state-specific dual descriptors (SSDD) are carried out to evaluate the donor-acceptor properties in these reactions. For the first time, a semiquantitative prediction of stereoselectivity due to substitutions of dienophile is obtained, thus complementing the previous interpretations (R. Hoffmann and R. B. Woodward, J. Am. Chem. Soc., 1965, 87, 4388; I. Fernández and F. M. Bickelhaupt, Chem. Soc. Rev., 2014, 43, 4953). Finally, since the role of dispersion forces is evidenced in some cases, a comparison between some popular exchange-correlation functionals is presented, assessing the performance of some standard functionals besides functionals with explicit dispersion corrections.
RESUMEN
A set of supramolecular cage-structures--spherophanes--was studied at the density functional B3LYP level. Full geometrical structure optimisations were made with 6-31G and 6-31G(d) basis sets followed by frequency calculations, and electronic energies were evaluated at B3LYP/6-31++G(d,p). Three different symmetries were considered: C1, Ci, and Oh. It was found that the bonds between the benzene rings are very long to allow pi-electron delocalisation between them. These spherophanes show portal openings of 2.596 A in Spher1, 4.000 A in Meth2, 3.659 A in Oxa3, and 4.412 A in Thia4. From the point of view of potential host-guest interaction studies, it should also be noted that the atoms nearest to the centre of the cavities are carbons bonded to X groups. These supramolecules seem to exhibit relatively large gap HOMO-LUMO: 2.89 eV(Spher1), 5.26 eV(Meth2), 5.73 eV(Oxa3), and 4.82 eV(Thia4). The calculated Delta H degrees (f) (298.15 K) values at B3LYP/6-31G(d) are (in kcal mol(-1)) 750.98, 229.78, -10.97, and 482.49 for Spher1, Meth2, Oxa3, and Thia4, respectively. Using homodesmotic reactions, relative to Spher1, the spherophanes Meth2, Oxa3, and Thia4 are less strained by -399.13 kcal mol(-1), -390.40 kcal mol(-1), and -411.38 kcal mol(-1), respectively. Their infrared and (13)C NMR calculated spectra are reported.