Aromatic and Antiaromatic Dehydroannulenes

Cyclocarbons make up an emerging family of carbon allotropes, but their instability precludes the study of their physicochemical properties. We report a synthetic route to cyclopentenone-annulated [18]- and [24]dehydroannulenes that are stable aromatic and antiaromatic models of cyclocarbon precursors. Combining NMR spectroscopy, X-ray crystallography, optical spectroscopy, electrochemistry, spectroelectrochemistry, and density functional theory (DFT) calculations, we address the aromatic and antiaromatic behavior of these electron-accepting molecules. Cyclopenteno[24]dehydroannulenes undergo two-electron reductions to form Hückel-aromatic 26 π-electron dianions with significant planarization. Replacing cyclopentenone corners with more electron-withdrawing cyclopentenediones lowers the reduction potentials, thereby enhancing the redox activity. Cyclopenteno[18/24]dehydroannulenes are designed so that each cyclopentenone corner bears a dimethylacetal group on the central carbonyl moiety. The chemical shift of the methyl groups acts as a probe for the aromaticity and antiaromaticity of the annulene system, confirming the diatropicity and paratropicity of the π-conjugated system. The combination of structural control and electroactivity makes these scaffolds promising candidates for organic electronics and topochemical polymerization.