Dications of Fluorenylidenes:  Conformational and Electronic Effects on the Paratropicity/Antiaromaticity of Fluorenyl Cations with Cyclic Substituents

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 1H shifts of appropriate protons of the fluorenyl system and 13C shift of carbon a suggests that the effect of the substituent is transmitted through cross-hyperconjugation.