On the Aromatic Character of Electrocyclic and Pseudopericyclic Reactions: Thermal Cyclization of (2Z)-Hexa-2,4-5-trienals and Their Schiff Bases

Abstract

Pericyclic or pseudopericyclic? Although both mechanisms lead to the same product, they are deeply different in nature. The ring‐current model proves to be a useful tool to define different kinds of aromaticity and to distinguish between pericyclic and pseudopericyclic reactions (see scheme).

Studying Double Group Transfer Reactions by Means of Computational Methods

“…Interestingly, the FMO's of this D 2h -transition state do not include any contribution from the migrating hydrogen atoms (vide infra). It is noteworthy that the MO's pattern of this saddle point has clear resemblances with that of 2 -aromatic six-electron molecules [20]. This conclusion can be readily understood if we apply the previously discussed treatment to a D 2h -symmetric six-electron aromatic molecule such as pyrazine (Fig.…”

6 Reaction mechanisms : Part (ii) Pericyclic reactions

“…Calculations by de Lera et al at the B3LYP/6-31ϩG* level reveal that cyclization of the aldehyde and imine occurs in a planar, pseudopericyclic process, in which the terminal carbon atom becomes partially negatively charged at the transition state. 88 The transition state for the hydrocarbon substrate [X = CH 2 ] shows rotation consistent with the disrotatory, electrocyclic process. The aromatic character of the transition states was evaluated using nucleus-independent chemical shifts and calculated ring currents, and reveal that only the hydrocarbon rearranges through an aromatic transition state.…”

Nature of Bonding in the Thermal Cyclization of (Z)-1,2,4,6-Heptatetraene and Its Heterosubstituted Analogues

“…Rodriguez-Otero and Cabaleiro-Lago 2a have questioned the results of De Lera et al 1 concerning the pseudopericyclic nature of the cyclization of (2Z)-2,4,5-hexatrien-1-imine (A) and (2Z)-2,4,5-hexatrienal (B). Their analysis yields the conclusion that the three processes studied are essentially disrotatory pericyclic electrocyclizations even though they are assisted by the electron lone pair on the heteroatom.…”

“…The distinction among these orbital topologies becomes relevant because in pseudopericyclic reactions the orbital overlap around the ring of breaking and forming bonds is absent, allowing planar TS geometries with very low activation barriers along the complete reaction paths, an aspect of importance in the design of new synthetic routes. 3 De Lera et al 1 carried out the first theoretical examination of the mechanism of reactions for A, B, and C systems, selected as model reactions of the thermal cyclizations of (2Z)-2,4,5-hexatrienals and their Schiff bases. In work carried out on the basis of the examination of nuclear motion along the imaginary frequency at the TS and also from computed nucleusindependent chemical shifts (NICS) associated with the aromatic character of structures, 4 they conclude that there are fundamental differences among the processes studied: the thermal cyclization of C occurs via a disrotatory electrocyclization with a pericyclic and aromatic TS1, and the cyclizations of heterosubstituted analogues A and B involve a nucleophilic addition of the heteroatom lone pair to the sp-hybridized carbon atoms through a pseudopericyclic and not an aromatic TS2.…”