Captodative Versus Push-Pull Aromatic Cyclopentadienyl Derivatives

The problem of whether and how the captodative aromatic systems with the donor and acceptor substituents at the same carbon atom of the C=C bond can be more stable than the π-conjugated push-pull counterparts with the two substituents in the vicinal position is analyzed for cyclopentadienyl derivatives possessing an aromatic cyclopentadienyl ring. The analysis of electronic, magnetic, and structural criteria of aromaticity showed that among typical organic donors, such as amines NR₃, ethers R₂O, guanidine (NH₂)₂C=NH, siloxane O(SiH₃)₂, silatrane, the captodative isomers [cyclopentadienylium]⁻C(X⁺) = CH₂ can be more stable than their push-pull isomeric counterparts [cyclopentadienylium]⁻—CH=CH—X⁺. The largest energy difference in favor of the former was found to be ca. 13 kcal/mol for X = NH=C(NH₂)₂.