Dications of Fluorenylidenes. Relationship between Electrochemical Oxidation Potentials and Antiaromaticity in Diphenyl-Substituted Fluorenyl Cations

Abstract

The antiaromaticity of a series of dications of p-substituted diphenylmethylidene fluorenes was explored using three criteria attributed to aromaticity/antiaromaticity. The relative stability of the dications (energetic criterion) was measured via the redox potentials obtained by electrochemical oxidation under very fast sweep rates with microelectrodes. Comparison of redox potentials with those of a model system, p-substituted tetraphenylethylenes, shows relatively small destabilization of the potentially antiaromatic fluorenylidene dication. However, the amount of destabilization is comparable with the limited electrochemical data available for other antiaromatic systems. Nucleus independent chemical shifts (NICS) were calculated for these dications (magnetic criterion) and indicated their antiaromaticity. A good linear relationship between experimental and calculated (B3LYP/6-31G(d)) (1)H and (13)C NMR shifts for the three dications, 3c, 3e, and 3f, for which NMR data has been reported, validated the accuracy of the NICS values. Bond length alternation/elongation (structural criterion) was explored via the harmonic oscillator model of aromaticity (HOMA) using the geometries calculated with density functional theory, but there was insufficient variation to evaluate relative antiaromaticity. In addition, the presence of benzannulation appears to restrict bond length alternation to such an extent that the magnitude of the HOMA index is of little use in evaluating the antiaromaticity of many polycyclic hydrocarbons. Both NICS values and redox potentials for formation of the dication in these systems show a strong linear correlation with sigma(p)(+) values, with the more antiaromatic fluorenylidene dication possessing the more electron-withdrawing substituent. The correlation between NICS values and redox potentials is also good, as might be expected, suggesting a strong relationship between magnetic and energetic characteristics of antiaromaticity. However, magnetic characteristics appear to be a more sensitive probe than energetic characteristics evaluated through redox potentials or structural characteristics evaluated through HOMA calculations.

Dications of Fluorenylidenes. The Relationship between Redox Potentials and Antiaromaticity for Meta- and Para-Substituted Diphenylmethylidenefluorenes

“…That is, the more positive the potential for formation of the dication, the greater its instability. We demonstrated this approach with the oxidation of a series of para-substituted diphenylmethylidenefluorenes, 2 , which demonstrated a linear relationship between the antiaromaticity of their dications, as evaluated by NICS calculations and paratropic 1 H NMR shifts, and redox potential for formation of the dications, with species with the greatest positive potential showing the largest degree of antiaromaticity . We here report the extension of that work to meta-substituted diphenylmethylidenefluorenes, 3 .…”

“…By evaluating the criteria in systems whose antiaromaticity should make them as far removed from benzene and its aromaticity as possible, differences in the responses of the criteria should be enhanced. However, there is some dispute about the antiaromaticity of the fluorenyl cation. − We have entered this discussion through a suite of fluorenylidene dications 1 − that demonstrate antiaromatic behavior. These dications are characterized by a cationic fluorenyl system that is antiaromatic and a second cationic system that can be modified to systematically “adjust” the antiaromaticity of the fluorenyl ring system.…”

Using Antiaromaticity To Illuminate Aromaticity during a Research Career with Undergraduates

“…We also examined a structural measure of aromaticity, the harmonic oscillator model of aromaticity (HOMA), using the calculated geometries of 16 , considering only the term reflecting bond length alternation. By that measure we saw much less variation for the variously substituted dications than was apparent through NICS values or oxidation potentials, suggesting that this was a much less sensitive probe of antiaromaticity and potentially aromaticity.…”

Peterson Olefination: Unexpected Rearrangement in the Overcrowded Polycyclic Aromatic Ene Series

“…Peterson olefination reactions have been extensively applied by Mills et al in the synthesis of overcrowded bistricyclic aromatic enes (BAEs, 1 ), including heteromerous bifluorenylidenes ( 1 , X ≠ Y) and diphenylmethylidene fluorenes 13–15. The 3,6‐dimethyl derivative of the parent bifluorenylidene 2 , for example, was synthesized by addition of trimethylsilyl carbanion to 9 H ‐fluorene to give (9 H ‐fluoren‐9‐yl)trimethysilane, which upon treatment with n BuLi and 3,6‐methyl‐9 H ‐fluoren‐9‐one gave 3,6‐dimethylbifluorenylidene 15 …”