Dications of Fluorenylidenes. Electronic Effects on the Paratropicity/Antiaromaticity of Diphenylmethyl-Substituted Fluorenyl Cations

Abstract

Oxidation of para-substituted diphenylmethylidenefluorenes results in the formation of antiaromatic fluorenyl cations, as evidenced by the paratropic shift in the 1 H NMR spectrum. The magnitude of the shift is affected by the nature of the remote para substituent. Correlations between the paratropic shift and substitutent parameters for field, inductive, and resonance effects were explored. The greatest correlation between substituent parameters and the paratropic shift is found with parameters based on electronegativity, which reflect inductive effects, rather than field effects, although the sample size is small enough for this conclusion to be viewed with some reservations. If inductive effects were the primary mode of transmission of the effect of the substituent, this would reflect the longest σ induction reported to date.

Dications of Fluorenylidenes. Use of Magnetic Properties To Evaluate the Antiaromaticity of the Dication of Tetrabenzo[5.5]fulvalene and Substituted Fluorenyl Cations

“…It remains to address the reasons for the difference in the antiaromaticity of the fluorenyl cation in 1 compared to the aromaticity/nonaromaticity of 3a − d . It appears that the presence of a positively charged substituent on C-9 of the fluorenyl cation is crucial for the antiaromaticity of the fluorenyl cation and we have observed this in other systems. − The calculations, particularly of NICS, give additional information. The assessment of aromaticity/antiaromaticity in polycyclic systems often involves a consideration of the pattern of delocalization of electron density. , For the fluorenyl monocations, the difference between the signs of the NICS in the five- and six-membered rings suggests that the delocalization of electron density occurs to maintain the aromaticity of the six-membered benzene ring.…”

“…We have been interested in this controversy because of our ongoing examination of the dications of tetrabenzo[5.5]fulvalene, , as well as other dications which contain the fluorenyl cation. , The dication of tetrabenzo[5.5]fulvalene ( 1 ) is essentially two fluorenyl cations linked by a single bond. The 1 H NMR shift of 1 shows a substantial upfield chemical shift, compared to its acyclic analogue, the tetraphenyl ethylene dication ( 2 ) (Table ).…”

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.…”

A Chemiluminescent Tetraaryl Diborane(4) Tetraanion

“… The two additional π charges are delocalized from the central boryl anions/carbanions into the phenylene rings. The additional shielding per phenylene ring amounts to Σ(Δ δ )=−68.2 ppm in Na 4 [ 3 ] and −66.1 ppm[ 53 , 54 ] in Li 2 [ B ]. Shielding of the two carbon atoms ortho to B or C‐9 in [ 3 ] 4− or [ B ] 2− , respectively, is distinctly different.…”

“…The two additional π charges are delocalized from the central boryl anions/carbanions into the phenylene rings. The additional shielding per phenylene ring amounts to Σ(Δ δ )=−68.2 ppm in Na 4 [ 3 ] and −66.1 ppm[ 53 , 54 ] in Li 2 [ B ].…”