Antiaromaticity of Cycloheptatrienyl Anions: Structure, Acidity, and Magnetic Properties

Abstract

Investigations of the nature and degree of antiaromaticity of cycloheptatrienyl anion derivatives using both experimental and computational tools are presented. The ground state of cycloheptatrienyl anion in the gas phase is triplet, planar and Baird‐aromatic. In DMSO, it assumes a singlet distorted allylic form with a paratropic ring current. The other derivatives in both phases assume either allylic or diallylic conformations depending on the substituent pattern. A combination of experimental and computational methods was used to determine the pKa values of 16 derivatives in DMSO, which ranged from 36 to –10.7. We revealed that the stronger stabilization of the anionic system, which correlates with acidity, does not necessarily imply a lower degree of anti‐aromaticity in terms of magnetic properties. Conversely, the substitution pattern first affects the geometry of the ring through the bulkiness of the substituents and their better conjugation with a more distorted system. Consequently, the distortion reduces the cyclic conjugation in the π‐system and thereby decreases the paratropic current in a magnetic field, which manifests itself as a decrease in the NICS. The triplet‐state geometries and magnetic properties are nearly independent on the substitution pattern, which is typical for simple aromatic systems.

The Antiaromatic Nucleophilic Substitution Reaction (SNAAr) in Cycloheptatrienyl-Anion Containing Zwitterions with a Möbius-Aromatic Intermediate

“…The reactivity of cycloheptatrienes can be significantly altered by the attachment of multiple electron-withdrawing groups. In contrast to electron-sufficient cycloheptatrienes that yield aromatic tropylium ions, these modified frameworks give rise to thermodynamically stable cycloheptatrienyl anions . Our recent investigations have focused on exploring their intriguing antiaromatic properties dependent on the substitution pattern as well as determining the p K a values (in DMSO) of various cycloheptatriene derivatives, both through experimental measurements and theoretical calculations.…”

“…Our recent investigations have focused on exploring their intriguing antiaromatic properties dependent on the substitution pattern as well as determining the p K a values (in DMSO) of various cycloheptatriene derivatives, both through experimental measurements and theoretical calculations. Remarkably, several existing electron-deficient derivatives exhibit higher acidity than even 4-nitrobenzoic acid, with theoretical heptacyanocycloheptatriene displaying an exceptionally low p K a (DMSO) value of −11 . Notably, our previous works highlighted the reactivity of stable hepta­(methoxycarbonyl)­cycloheptatrienyl potassium toward both electrophilic − and nucleophilic , reagents, particularly amines, leading to the formation of fluorescent 5-hydroxyisoquinolones with significant Stokes shifts due to superphotoacidity (excited-state proton transfer to solvent). , While such reactive cycloheptatrienes were previously limited to hepta­(methoxycarbonyl)­cycloheptatriene, our recent studies have expanded this repertoire to include seven stable cycloheptatrienyl-anion containing compounds. − …”