Aromaticity and curvature in heteroacepentalenes.

Heteroatom-substituted acepentalene derivatives which are isoelectronic with the known acepentalenediide dianion are nonplanar, fused aromatic tricycles which are hemifullerenes of the corresponding C20 heteroanalogue. Depending on the number and position of heteroatoms, they may be anionic, neutral, or cationic. A nucleus-independent chemical shift study indicates that substitution of the central carbon of the acepentalenediide system with N or O results in a substantial increase in aromaticity. Peripheral aza substitution on the other hand tends to increase curvature and decrease aromaticity. Alkylation or protonation at the central position of asymmetrically substituted heteroacepentalenides leads to chiral, bowl-shaped, 10 pi aromatic species.