Aryl Ynol Ethers: Stable, Readily Available and Useful Precursors of Highly Reactive Ketenium Ions

Background: Ketenium ions are rarely used in chemical synthesis, despite a strong potential and their high reactivity, which can be mostly attributed to their difficult generation. There is therefore an important need for efficient methods enabling their in situ generation from stable and readily available starting materials in order to fully exploit the unique reactivity of ketenium ions. Method: We report in this manuscript a simple and efficient process for the generation of these highly reactive intermediates by simple protonation of the corresponding stable and readily available aryl ynol ethers with bistriflimide. By simply mixing these two reagents in dichloromethane at -50°C for 30 minutes, a transient ketenium ion is formed and is further, quite unexpectedly, trapped by the poorly nucleophilic bistriflimidate. Results: The scope of the reaction was delineated by using various aryl ynol ethers which were shown to be excellent precursors of ketenium ions. Their trapping with bistriflimidate further highlights the exceptional electrophilicity of these unique intermediates. Conclusion: We have demonstrated that ketenium ions can be conveniently prepared by simple protonation of the corresponding aryl ynol ethers, bench-stable and readily available starting materials. The unique electrophilicity of these cationic heterocumulenes was showcased by their trapping with poor nucleophiles such as bistriflimidate and in an intramolecular Friedel-Crafts reaction.