Dications of Fluorenylidenes. Electronic Effects on the Paratropicity/Antiaromaticity of 2,7-Disubstituted Fluorenyl Cations

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.