Synthesis, Characterization and Reactivity of Thermally Stable Anhydrous Quaternary Ammonium Fluorides

Anhydrous Quaternary ammonium fluorides have been widely documented as very strong nucleophiles/bases and are of significant interest in inorganic and organic synthesis. However, when trying to prepare such compounds by hydrate removal conditions (heating under dynamic vacuum), they show instability and easily undergo Hoffmann elimination (E2) by the fluoride counter ion. For example, anhydrous TBAF decomposes even at room temperature, and therefore, it can only be prepared and used “in situ” at low temperatures. The present work describes the synthesis and properties of a new class of anhydrous quaternary ammonium fluorides based on the rigid skeleton of [2.2.2] azabicyclooctane, in which the Hoffmann elimination is structurally prevented even at temperatures up to 120^oC. Four such structures were easily prepared by passing the corresponding ammonium iodides over a fluoride-based resin followed by drying under heating and reduced pressure. The stability (experimental and theoretical study), solubility, reactivity and characterization by solution and solid-state MAS NMR are discussed.