Nature of bonding in the thermal cyclization of (Z)-1,2,4,6-heptatetraene and its heterosubstituted analogues

The topological analysis of the electron localization function (ELF), a measure of local Pauli repulsion and an orbital-independent technique for bonding analysis, has been applied to explore further the intriguing nature of bonding at the transition states (TSs) of the thermal electrocyclization of (Z)-1,2,4,6-heptatetraene and its heterosubstituted analogues, (2Z)-2,4,5-hexatrienal and (2Z)-2,4,5-hexatrien-1-imine. In the context of the relevant controversy concerning the pericyclic or pseudopericyclic intimate nature of bonding at these TSs (De Lera, A. R.; Alvarez, R.; Lecea, B.; Torrado, A.; Cossio, F. P. Angew. Chem., Int. Ed. 2001, 40, 557; Rodriguez-Otero, J.; Cabaleiro-Lago, E. M. Angew. Chem., Int. Ed. 2002, 41, 1147; De Lera, A. R.; Cossio, F. P. Angew. Chem., Int. Ed. 2002, 41, 1150; Rodriguez-Otero, J.; Cabaleiro-Lago, E. M. Chem. -Eur. J. 2003, 9, 1837), we show for the first time that the analysis based on ELF provides further evidence in support of a single disrotatory pericyclic bond interaction in the three cases. This conclusion arises from an examination of the fluctuation of electron density at the cyclic reaction center.