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.

Oxidation of 2,7-disubstituted tetrabenzo[5.5]fulvalene derivatives 1a−d resulted in the formation of dications which are fluorenyl cations linked by a single bond. These fluorenyl cations exhibit significant paratropicity in the 1H NMR spectrum, which is attributed to an antiaromatic ring current. Interaction of the perpendicular ring systems is evident in the upfield shift of carbons a and a‘, presumably due to σ−p donation. The lack of variation in the upfield shift of carbons a and a‘, compared to previously reported systems, is attributed to the similarities of the geometries of 1a−d2+ . Substitution at a remote site affects the antiaromaticity of the unsubstituted fluorenyl cation, but the nature of the effect of the substituent is not understood. Direct substitution on the fluorenyl cation by substituents which increase electron density, either through inductive π polarization or through resonance, cause a paratropic shift in the probe proton.

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.

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