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 1H 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.