The Fluorenyl Cation

The fluorenyl cation is a textbook example for a 4π antiaromatic cation. However, contrasting results have been published on how the annelated benzene rings compensate the destabilizing effect of the 4π antiaromatic five‐membered ring in its core. Whereas previous attempts to synthesize this cation in superacidic media resulted in undefined polymeric material only, we herein report that it can be generated and isolated in amorphous water ice at temperatures below 30 K by photolysis of diazofluorene. Under these conditions, the fluorenylidene is protonated by water to give the fluorenyl cation, which could be characterized spectroscopically. Its absorption in the visible‐light range matches that previously obtained by ultrafast absorption spectroscopy, and furthermore, its IR spectrum could be recorded. The IR bands in amorphous ice very nicely match predictions from DFT and DFT/MM calculations, suggesting the absence of strong interactions between the cation and surrounding water molecules.     

Gold Difluorocarbenoid Complexes: Spectroscopic and Chemical Profiling

Gold carbenes of the general type [LAu=CR₂]⁺ are sufficiently long‐lived for spectroscopic inspection only if the substituents compensate for the largely missing stabilization of the carbene center by the [LAu]⁺ fragment. π‐Donation by two fluorine substituents (R=F) is insufficient; rather, difluorocarbene complexes are so deprived in electron density that they sequester even “weakly coordinating” anions such as triflate or triflimide. This particular bonding situation translates into unmistakable carbenium ion chemistry upon reaction with stilbene as a model substrate.     

Catalytic Friedel–Crafts Reactions of Highly Electronically Deactivated Benzylic Alcohols

Highly electronically deactivated benzylic alcohols, including those with a CF₃ group adjacent to the OH‐bearing carbon, undergo dehydrative Friedel–Crafts reactions upon exposure to catalytic Brønsted acid in 1,1,1,3,3,3‐hexafluoro‐2‐propanol (HFIP) solvent. Titration and kinetic experiments support the involvement of higher order solvent/acid clusters in catalysis.