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

Abstract

Recent research on the magnetic characterization of aromaticity [1] has, over the last few years, promoted the reexamination of the aromatic character of pericyclic reactions. [2] This idea stems from the pioneering work of Evans, [3a] Dewar, [3b] and Zimmerman [3c] and has been refined by Herges, Jiao, and Schleyer. [2] Recently, we reported [4] that the concept of in-plane (s) aromaticity can be extended to several thermal pericyclic reactions. Within this context, we herein report several ring closures involving (2Z)-hexa-2,4,5-trienals and their corresponding imines (Scheme 1). These compounds are engineered restriction sites were inserted into the cloning plasmid pJF119EH1 [20] both separately or in tandem. Cloning sites were BamHI and PstI for entB, and SacI and BamHI for entC, respectively. The E. coli strains W3110, DH5a, AN193, and H1882 were transformed with each of the resulting three plasmids.

Pseudopericyclic design drives antara‐antara [1,5] methylene sigmatropic shifts from a stepwise to a concerted mechanism

“…11 and 8 ppm, respectively). This feature has been previously associated to π 2 aromaticity (that of benzene, above and below the molecular plane) 23, 24. The delocalized density computed with the ACID also supports the idea of aromaticity through the peripheral atoms.…”

“…Actually the activation energy for the concerted rearrangement of 1F is half that of its Z isomer (13.82 and 26.99 kcal/mol, respectively). This important difference in activation energy led us to consider it arising from factors beyond the obvious steric effects as we have previously found in other pericyclic and pseudopericyclic ring closure reactions of E and Z polyunsaturated imines 7, 23. In previous work the analysis of the electron density at the concerted transition states helped to discern between processes exhibiting pericyclic and nonpericyclic behavior.…”

The nature of bonding in pericyclic and pseudopericyclic transition states: Thermal chelotropic decarbonylations

“…[1][2][3][4][5][6][7][8][9][10][11] In this context, it has been pointed out that the fundamental difference among these two topologies is the conservation or not of a cyclic orbital overlap between the orbitals involved in bonding changes at the transition state. Henceforth, the lack of orbital overlap in a pseudopericyclic reaction has significant implications in the global ͑i.e., ther-modynamic͒ and local ͑i.e., selectivity͒ chemistry of these systems.…”

“…[1][2][3][4][5][6][7][8][9][10][11] In the framework of consistent ways of characterization of chemical bonding beyond the scope of orbital-dependent treatments, topological analysis ͑i.e., gra-dient field analysis͒ of well-defined local functions provides a powerful approximation to a rigorous quantitative and qualitative analysis. The atoms-in-molecules ͑AIM͒ theory of Bader 13 and the Savin, Silvi et al 14 analysis of the electron localization function ͑ELF͒ of Becke and Edgecombe, 15 provide us with a useful and convenient partition of the molecular space into basins of attractors that can be related to the intuitive concepts of chemical bonding.…”

Cyclization of an α,β‐Unsaturated hydrazone catalyzed by a BINOL‐phosphoric acid: Pericyclic or not?

“…Pericyclic reactions are associated with an aromatic TS, and hence we tested the character of the computed TSs through a NICS analysis as originally introduced by Schleyer in 1996 . NICS values are a very suitable measure of aromaticity . They are computed as the negative of the absolute shielding constant, typically at the ring center.…”