Twisted Polycyclic Aromatic Hydrocarbon with a Cyclooctatetraene Core via Formal [4+4] Dimerization of Indenofluorene

Abstract

A polycyclic aromatic hydrocarbon having a twisted cyclooctatetraene (COT) core was prepared by a formal [4+4] dimerization of an in situ generated indeno[2,1-a]fluorene derivative. The compound consists of two indenofluorene frameworks connected with two double bonds and adopts a twisted geometry to avoid intramolecular steric repulsion. It exhibits a long-wavelength absorption band with a maximum at 634 nm extending to about 850 nm, which is consistent with the moderate antiaromaticity of the twisted COT ring as indicated by the NICS value (+11.9). A mechanism for cycloaddition reaction is proposed in terms of stepwise bond formations due to the diradical character of the indenofluorene framework, on the basis of the completely different reaction product obtained from the extended benzo analogue.

Key words

twisted aromatic hydrocarbon - cyclooctatetraene - cyclo­dimerization - indenofluorene - diradical character

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