Abstract
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.
Highlights
Carbocations are key intermediates in numerous organic reactions
This observation prompted us to attempt the synthesis of a kinetically-stabilized m-terphenylacylium ion, in which two flanking mesityl groups were supposed to prevent the coordination of the counterion
Motivated by the debate[8] that the antiaromaticity[9] might be compensated by the two benzene rings, serveral attempts were made to spectroscopically characterize or even isolate fluorenyl cations
Summary
Carbocations are key intermediates in numerous organic reactions. Groundbreaking work by Olah in the 1970s demonstrated that many of these transient carbocations can be detected at low temperatures under superacidic conditions. Fluorenyl cations comprise a central antiaromatic 4π electron five-membered ring with two annelated benzene rings (Scheme 1).
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