Aromaticity in Pericyclic Transition State Structures? A Critical Rationalisation Based on the Topological Analysis of Electron Density

Abstract

The nature of the electron delocalisation pattern within a cyclic structure, i. e. the aromatic character, is examined for sixmembered pseudocyclic transition state structures (TSs) involved in five representative examples of so-called pericyclic reactions. Results of the electron localisation function (ELF) and the quantum theory of atoms in molecules (QTAIM) analyses of the electron density evidence that in four of the cases, at least one pair of atoms are not bound at the TS configuration, thus precluding a possible cyclic conjugation. These findings make it possible to rule out the aromatic character of these TSs. High values of the synchronicity Sy index at the TSs contrast with the bonding changes evidenced by ELF topological analysis. The magnitude of the nucleus independent chemical shift (NICS) computed for the five TSs becomes much more negative than that of the reference system, benzene, with no obvious relation to the strong evidence of the pattern of delocalisation revealed by the topological analysis of ELF and QTAIM for each TS. The topology of the anisotropy of current induced density (ACID) at each TS reveals the actual existence of strong nonsymmetrical patterns of electron delocalisation of the five TSs.[a] Prof.

The Mysticism of Pericyclic Reactions: A Contemporary Rationalisation of Organic Reactivity Based on Electron Density Analysis

“…Very recently, the electron density delocalisation in the TSs associated with the five pseudocyclic reactions given in Scheme was topologically analysed by using QTAIM and ELF . That study evidenced that in four of the five cases, at least one pair of atoms is unbound in the TS, thus precluding a possible cyclic conjugation, and consequently, any aromatic character, such as was suggested in the last century …”

“…These QC topological tools allowed, for the first time, analysis of the electron density obtained from QC calculations. Although some authors have drawn attention to some of the discrepancies between ELF and QTAIM in the analysis of some bonding properties, ELF has shown to be fully consistent with other QC topological tools, including QTAIM, in the characterisation of chemical bonds …”

Unravelling the Mysteries of the [3+2] Cycloaddition Reactions

“…[52] In 2010, a topological analysis of the ELF of the simplest AY 7 highlighted that this TAC has a pseudodiradical electronic structure presenting two carbon pseudoradical centres (see Figure 4). [52] In 2010, a topological analysis of the ELF of the simplest AY 7 highlighted that this TAC has a pseudodiradical electronic structure presenting two carbon pseudoradical centres (see Figure 4).…”

Understanding the mechanism of the decomposition reaction of nitroethyl benzoate through the Molecular Electron Density Theory

“…This fact, together with the non-concerted nature of the bonding changes, also allows ruling out the proposed pericyclic mechanism [52,53] in which the bonding changes take place "in concert on a closed curve" assuming a bound cyclic structure; iv) as TS is found at the end of Phase III (see Fig. 4), the high activation energy associated with this decomposition reaction, 42.5 kcal mol −1 (see Table 1), can mainly be related to the disappearance of the mentioned V(O3, C4) and V(C5, H6) basins; v) TS consists of three different separated frameworks: the nitroethylene one, the benzoate one and a free pseudoradical hydrogen, precluding any conjugation and thus, any aromatic character of the TS [39]; vi) next, a new V(O1, H6) protonated basin, associated with an O1-H6 bond, is formed at Phase IV through the merger of two V(O1) and V(H6) monosynaptic basins at d O1-H6 = 1.08 Å, and further populated releasing 6.3 kcal mol −1 ; and finally, vii) the V(C5) monosynaptic basin disappears towards the V(C4, C5) disynaptic basin, whose population suddenly increases and causes the split into two V(C4, C5) and V′(C4, C5) disynaptic basins associated with the C4-C5 double bond of the nitroethylene framework.…”

“…Note that broken lines at TS within the breaking and forming regions do not refer to a conjugated cyclic pattern but only indicate the bonds that are going to break or form [19] evolution of the electron density along a reaction involving a cyclic arrangement of nuclei, i.e. pseudocyclic reactions [39], reveal that the pericyclic mechanism does not exist as the bonding changes are not concerted [40]. In this work, as a continuation of the quantum chemical study about the decomposition reaction of nitroethyl carboxylates [18,19], an MEDT study of the decomposition process of NEB 1 is performed in order to establish the molecular mechanism of this decomposition reaction and the nature of the electronic rearrangement along it.…”