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

Abstract

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.; Cossío, F. P. Cabaleiro-Lago, E. M. Chem.s Eur. J. 2003Eur. J. , 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.

Isoelectronic and isolobal O, CH2, CH3+and BH3as electron pairs; similarities between molecular and solid-state chemistry

“…Such a population analysis has been found to be a useful tool in rationalizing the electron delocalization in several molecular systems, providing deeper insight and understanding about the nature of the chemical bonding in a variety of both stationary and reacting systems. Examples include the analysis of pericyclic processes (Chamorro & Notario, 2004;Cá rdenas et al, 2005;Chamorro et al, 2007), cycloaddition processes (Berski et al, 2003;Domingo et al, 2008), radical systems (Melin & Fuentealba, 2003) and aromaticity (Santos et al, 2005).…”

Borazine: to be or not to be aromatic

“…The higher the separation value, the higher is the expected degree of localization of electron pairing between these domains [47][48][49]. Such a bifurcation analysis is a useful tool to rationalize the nature of chemical bonding of aromatic substitution and reactive sites [51,52], pericyclic reactions [53][54][55], radical systems (e.g., through the analysis of the a-b spin components) [56], and the definition of an aromaticity scale for organic molecules and clusters via the analysis of r and p contributions to density [26,57].…”

A Density Functional Theory Study on the Electrocyclization of 1,2,4,6‐Heptatetraene Analogues: Converting a Pericyclic to a Pseudopericyclic Reaction

“…[21][22][23][24] For reaction 2 the results suggest that, although the electron lone pair on the oxygen atom seemingly plays a crucial role in the reaction mechanism, it does not suffice to deprive the reaction from the essential features of a pericyclic disrotatory electrocyclization. [20,22,24] The stabilization of the transition state due to the interaction of the lone pair with the p system already has been shown by Houk et al for the ring opening of 1,2-dihydroazete. [38] Because of this interaction, in reaction 2 the molecule need not undergo as much geometrical distortion as in a prototypical pericyclic reaction.…”

“…Hence, pseudopericyclic reactions cannot be orbital symmetry forbidden. Recently, Birney et al [2][3][4][5][6][7][8][9][10] and others [11][12][13][14][15][16][17][18][19][20] showed that a number of organic syntheses involve this type of process. Although Lemals definition is seemingly quite clear, there is some ambiguity as the orbital description is not unique; thus, any unit transformation of canonical molecular orbitals can be used to reproduce molecular properties.…”