Hyperconjugative π-Aromaticity: How To Make Cyclopentadiene Aromatic

Cyclopentadienes, when 5,5-disubstituted with electropositive groups, have considerably enhanced cyclic conjugation in comparison with the parent molecule, C5H6. According to various aromaticity indices, 5,5-distannylcyclopentadiene is nearly as aromatic as furan. Previous observations on the reactivity were interpreted in the light of the observed aromatic stabilization.