Use of Charge‐Charge Repulsion to Enhance π‐Electron Delocalization into Anti‐Aromatic and Aromatic Systems

Abstract

A series of 9-fluorenyl cations has been studied and it is shown that increasing charge on a heterocyclic substituent group enhances the anti-aromatic character of the carbocation system. Similarly, a series of dibenzosuberenyl cations has been studied and increasing charge on a substituent group is shown to enhance aromatic character in the carbocation system. These studies include the direct observations of dicationic and tricationic species using stable-ion conditions and low temperature NMR. The structures of these ions were further characterized using DFT calculations, confirming that highly charged organic ions may exhibit unusual distributions of π-electrons and delocalization of electrons in 4n or 4n+2 π-systems.

Functionalized fluorenes via dicationic electrophiles

“…Our previous study showed that dicationic fluorenyl cations could be generated conveniently through the condensation of diaryl ketones in superacid [8,9]. The required ketones (4) have been prepared via two routes (Fig.…”

“…The chemistry involves formation of the superelectrophilic fluorenyl cation 2 from condensation of the 2-pyridyl ketone in the superacid. Using stable ion conditions and low temperature 13 C NMR, the dicationic species (2) was directly observed [9]. We demonstrated that the dication 2 was a useful electrophile for reactions with carbon nucleophiles such as benzene.…”

Superelectrophiles in ring-forming reactions

“…This tricationic species has been directly observed by low temperature NMR using stable ion conditions. 17 Reaction with benzene gives 34 as the final product. The condensation chemistry was also adapted to a polymer synthesis wherein the fluorene ring system is part of the polymer backbone.…”

Superelectrophiles: Recent Advances

“…Dehydration through the oxonium ion 26 leads to the highly electrophilic fluorenyl ion 27 . This tricationic species has been directly observed by low temperature NMR using stable ion conditions [ 31 ]. Reaction with benzene gives 28 as the final product.…”