Dications of Fluorenylidenes:  Conformational and Electronic Effects on the Paratropicity/Antiaromaticity of Fluorenyl Cations with Cyclic Substituents

Abstract

Oxidation of tetrabenzofulvalene derivatives 1-5 containing fluorenylidene and 5-7-membered rings resulted in the formation of dications which are effectively fluorenyl cations with perpendicular cyclic substituents. The observed paratropicity of the fluorenyl cation in these systems is attributed to an antiaromatic ring current, after evaluation of the effects of geometry, charge density, and polarity of the medium, and is dependent on the geometry and electronic character of the cyclic substituent. A linear relationship between the 1 H shifts of appropriate protons of the fluorenyl system and 13 C shift of carbon a suggests that the effect of the substituent is transmitted through cross-hyperconjugation.

Calculated geometries of dications of bis odd‐membered π‐ring systems containing a NCN fragment and related π‐systems. An opposite out‐of‐plane rotation of the 4nπ‐ring subsystems

“…The lowering of the S LUMO results in electron‐density transfer to C 9 that involves an increase of positive charge delocalization over the fluorenyl parts. A similar paratropicity in the fluorenyl part of the dication of tetrabenzo[5.7]fulvalene has been established with the 1 H NMR spectrum 1, 2. In this dication also both halves are in a perpendicular position to each other.…”

“…It could be established that in the dication the fluorenyl parts are orientated in a perpendicular position. This conclusion was based on the observed paratropicity of the fluorenyl cation as the result of an antiaromatic ring current 1, 2. It should be noticed that the 9‐fluorenyl cation itself is not antiaromatic 3.…”

Odd-memberedC2? ?-cyclic mono- and dications with internal cross-linking. A H�ckel approach for the description of antiaromaticity

“…Recently an interesting observation was made for the dication of tetrabenzo[5.5]fulvalene: This dication, which exists in a geometry in which the fluorenyl monocations are in a perpendicular orientation to each other, is indeed antiaromatic as indicated by a paratropic shift in the 1 H NMR spectrum 5, 6. The center of gravity for the proton shift of the dication of tetrabenzo[5.5]fulvalene is 5.41 ppm, which is substantially upfield from the center of gravity for the substituted fluorenyl cations, which is ≈7.4 ppm.…”

Antiaromatic Dianions: Dianions of Dixanthylidene by Reduction and Attempted Excited-State Deprotonation

“…W e have been involved in the preparation of antiaromatic dications via oxidation of unsaturated precursors [1][2][3][4][5][6][7][8] and ionization of diols for some time. 9,10 The antiaromaticity of these species can be evaluated by the same type of criteria used in the evaluation of aromatic compounds.…”