C−F Bond Activation by Aryl Carbocations: Conclusive Intramolecular Fluoride Shifts between Carbon Atoms in Solution and the First Examples of Intermolecular Fluoride Ion Abstractions

Abstract

The chemistry of selective C-F bond activation is undergoing a surge of interest due to the scientific and commercial importance of fluorocarbons.1 However, to date investigations of C-F bond activation in solution usually have involved transition metal complexes or alkali metals. As part of a new approach to C-F bond activation, we were interested in the possibility of using carbocations to abstract fluoride from organic molecules through a three-center transition state or intermediate. Although cationic fluoride transfers involving hypervalent [C-F-C]+ interactions are documented in the gas phase in work by Morton,2 reports of such intermolecular abstractions in solution are virtually nonexistent, and the published cases of intramolecular shifts are fraught with controversy.3 In fact, the question of whether such shifts of fluoride could ever occur in solution has been a source of continuing debate. In this communication, we describe intramolecular fluoride shifts between carbon atoms in solution (Scheme 1) and the first documented examples of analogous intermolecular fluoride abstractions between an aryl cation and its counterion. In the gas phase the reactivity of carbocations is enhanced by the absence of solvent;2 in solution, cations of comparably high reactivity must be generated to provide the driving force for fluoride transfers. It would also be best to generate the carbocations in nonnucleophilic media, employing substrates that contain nonsolvolytically active C-F bonds. Our approach was to use aryl diazonium salts, from which highly reactive aryl cations are known to be generated in solution through mild thermolysis.4 We chose to investigate biphenyl diazonium salts 1 in the belief that upon the formation of aryl cation 2, Ftransfer through a six-membered ring (structure 3) would be favorable (Scheme 1). To minimize trapping of counterion Xby the aryl cation, we felt a weakly coordinating anion would be necessary.5