6, 6-Dimethyl-2, 4-diphenylcyclohexadienyl Anion and the Failure of Carbanion Homoaromaticity

The title carbanion was generated by methyllithium deprotonation of 6, 6-dimethyl-2, 4-diphenyl-1, 4-cyclohexadiene in tetrahydrofuran. Attempted generation of the identical carbanion using potassium (methylsulfinyl)methide in Me₂SO led to cyclopropanation only via isomerization to the 1, 3-diene followed by addition/elimination of (methylsulfinyl)methide anion. Except for modest changes in carbon chemical shifts, the title compound exhibits little thermodynamic driving force toward ring closure or homoaromaticity. The NMR spectra at all temperatures were consistent with a planar nonaromatic anion, again failing to establish the existence of homoaromaticity in such carbanions.