Use of TFE and HFIP for cation-stabilizing solvents

The highly polar nature of TFE and HFIP enables dissolution of ionic species and ion pairs. Moreover, the low or nonnucleophilic nature as well as low-lying HOMO of the solvents stabilizes cation and radical cation intermediates in the solvents. 

The stabilization of a cation by an acidic solvent has been considered to be due to the protonation of nucleophilic impurities and thus suppresses their reactivity toward cation species. Therefore, the cation-stabilizing ability has been considered to be proportional to the strength as an acid of the solvent. Meanwhile, the half-life time of aromatic cation radical species in HFIP is a few ten or hundred times longer than that in trifluoroacetic acid (TFA) [52]. This is a paradoxical result because TFA is less nucleophilic and a much stronger acid than HFIP. (This would be a future problem to be solved.) 

In any event, TFE and H FIP are inert to oxidants and anodic oxidation [ 53 ]. This property of the fluorinated alcohol solvents has been widely used for mechanistic studies in oxidative reactions as well as organic syntheses [53-58]. 

Jeffery, G.A. (1997) An Introduction to Hydrogen Bonding. Oxford University Press, Oxford. 

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