New insights about electronic mechanism of electrocyclic reactions: theoretical study about stereoselectivity in cyclobutenes

Abstract

This work presents the study of a series of electrocyclic reactions with the main aim of obtaining new insights into the reaction process along IRCs. The energy variation of the different reaction paths as well as the different transition states have been calculated. These trends are according to the experimental data. The natural bond orbitals have been obtained and the second order perturbational theory analysis has been carried out to determine the main charge transfers due to delocalization. Bond reactivity indexes have been used to describe the reactivity mechanism in a local way. These reactivity indexes are also based on NBOs and this has made it possible to connect the results of the indexes with the previous analysis. To determine quantitatively the bond structure, we used the quantum theory of atoms in molecules and we have hereby completed the information obtained from the NBO analysis. Finally, we used the Hirshfeld population analysis as an approximation to understand how the load density changes in the different reaction pathways, and we have connected these variations with the information obtained from the bond structure. The results has found that the reaction path with the lowest energy barrier Transition State Inward Conrotatory (TSIC) or Transition State Outward Conrotatory (TSOC) is determined by two magnitudes: the charge donations by delocalisation of the substituents (which we obtained from the Second Order Perturbational Theory Analysis of the NBOs) and in the case that these donations were very similar, the non-covalent interactions dominated (which we studied by means of the interaction energies of the Hirshfeld charges). Additionality, the most important factor influencing the lower energy reaction path was the interaction of lone pairs of the substituents with the σ∗(C–C) bond that is broken at the opening of the cycle. The alignment of these lone pairs with the C–C bond favours charge donation between them and, as can be seen in the discussion, this alignment varies depending on whether the structure is TSIC and TSOC.

An analysis of the torquoselectivity effect in a ring-opening reaction through Fermi–Dirac’s entropy: revealing the origin of the stereoselectivity.

“…In the case of the reaction studied 3-cyancyclobutene (3-CN-cyclobutene) [53] these electrocyclic reactions were performed thermally and are stereospecific. This reaction was studied by our research group in a previous study and the preferred way is by the Intrinsic Reaction Coordinate (IRC) through Transition State Outward Conrotatory (TSOC) [54]. It should be noted that the prediction of the activation energy agrees with the experimental data [52].…”

“…This reaction has been studied by the Frontier Molecular Orbital theory and the Transition States Theory, [55] but, occasionally, the application of classical theories disagrees with experimental results [56]. Our research group has published several papers where various methodologies have been applied that involve the Electron Localization Function (ELF), the main conclusion of this study indicated that the electronic reorganization in the thermal electrocyclic opening of trans-3,4-dimethylcyclobutene is not due to a pericyclic electronic reorganization postulated by Woodward-Hoffmann and therefore, could be considered as a pseudodiradical {2n + 2π} process [57], maximum hardness, minimum electrophilicity and minimum polarizability principles [48], the natural bond orbitals have been obtained and the second-order perturbational theory analysis has been carried out to determine the main charge transfers due to delocalization [59] and Quantum Theory of Atoms in Molecules (QTAIM) and stress tensor frameworks, among others [60]. In such works the electronic reorganization agrees with the experimental data.…”

Philicity scales using molecular quantum similarity and chemical reactivity indices within density functional theory

“…Finally, these 21 reactions were studied in our work entitled “New insights about electronic mechanism of electrocyclic reactions: theoretical study about stereoselectivity in cyclobutenes” [34]. In it study the same reaction set was studied with the main aim of obtaining new insights into the reaction process along IRCs.…”

Local reactivity descriptors of the important atoms in chelotropic reactions provide insight into their global variants along the reaction path

“…In density functional theory (DFT) [11–13], the electron density is the fundamental variable, and the orbitals used (e.g., in the Kohn–Sham method) are simply auxiliary functions for the construction of the said fundamental quantity. In DFT, the absence of orbital phases (formally) excludes the possibility of analogous reasoning to that using Woodward‐Hoffmann (WH), recent research developed by our research group has suggested that a paradigm equivalent to that of WH can be built directly using the electron density [14–18]. Supported on this interpretation model is this study will systematically rationalize the molecular mechanism of Chelotropic reactions.…”