Electrophilicity of a 9-aryl-9-fluorenyl cation in water — Kinetic evidence for antiaromaticity

Abstract

The 9-(4-methoxyphenyl)-9-fluorenyl cation (2) has been generated in 100% water by laser flash photolysis of 9-(4-methoxyphenyl)-9-fluorenol (3), representing the first observation of a 9-fluorenyl cation in this solvent with lifetimes in the microsecond timescale. The relatively long lifetime permitted quenching studies with a number of anionic nucleophiles, and bimolecular rate constants for each were determined. For both bromide and iodide, rate data suggest that an equilibrium between the cation and trapped product is rapidly established, followed by slower, irreversible trapping of the cation by water. The bimolecular rate constants obtained show that the generated 9-fluorenyl cation is significantly more reactive towards nucleophilic attack, by two orders of magnitude, than related triarylmethyl cations that lack the 4n π-system, lending support to the characterization of fluorenyl cations as antiaromatic.