Isolation of an Antiaromatic 9‐Hydroxy Fluorenyl Cation

Fluorenyl cations are textbook examples of 4π electron antiaromatic five‐membered ring systems. So far, they were reported only as short‐lived intermediates generated under superacidic conditions or by flash photolysis. Attempts to prepare a m‐terphenyl acylium cation by fluoride abstraction from a benzoyl fluoride gave rise to an isolable 9‐hydroxy fluorenyl cation that formed by an intramolecular electrophilic attack at a flanking mesityl group prior to a 1,2‐methyl shift and proton transfer to oxygen.