Probing Hyperconjugative Aromaticity of Monosubstituted Cyclopentadienes

Hyperconjugation and aromaticity are two of the most important concepts in chemistry. Mulliken and co-workers combined both terms to explain the stability of cyclopentadiene. Here, we carried out DFT calculations on a series of mono- and disubstituted cyclopentadiene derivatives to investigate their hyperconjugative aromaticity. Our results revealed that one electropositive substituent can induce aromaticity, whereas one electronegative substituent prompts nonaromaticity rather than antiaromaticity. When an electronegative substituent and a transition metal as an electropositive substituent were considered simultaneously, a slightly aromatic cyclopentadiene ring could be achieved, whereas an electropositive substituent of the main group in combination with an electronegative one will produce a nonaromatic cyclopentadiene ring.