Facile and systematic access to the least-coordinating WCA [(R^FO)₃Al–F–Al(OR^F)₃]⁻ and its more Lewis-basic brother [F–Al(OR^F)₃]⁻ (R^F = C(CF₃)₃)

Abstract

By reaction of the Lewis acid Me₃Si-F-Al(OR^F)₃ with a series of [PF₆]^- salts, gaseous PF₅ and Me₃Si-F are liberated and salts of the anion [F-Al(OR^F)₃]^- ([f-al]^-; R^F = C(CF₃)₃) can be obtained. By addition of another equivalent of Me₃Si-F-Al(OR^F)₃ to [f-al]^-, gaseous Me₃Si-F is released and salts of the least coordinating anion [(R^FO)₃Al-F-Al(OR^F)₃]^- ([al-f-al]^-) are formed. Both procedures work for a series of synthetically useful cations including Ag⁺, [NO]⁺, [Ph₃C]⁺ and in very clean reactions with 5 g batch sizes giving excellent yields typically exceeding 90%. In addition, the synthesis of Me₃Si-F-Al(OR^F)₃ has been optimized and scaled up to 85 g batches in an one-pot procedure. These anions could previously only be obtained by difficult to control decomposition reactions of [Al(OR^F)₄]^- or by halide abstraction reactions with Me₃Si-F-Al(OR^F)₃, generating relatively large countercations that are unsuited for further use as universal starting materials. Especially [al-f-al]^- is of interest for the stabilization of reactive cations, since it is even weaker coordinating than [Al(OR^F)₄]^- and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f-al]^- and Al(OR^F)₃. Thus, it is similarly Lewis acidic as BF₃ and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu-Al(OR^F)₃ and [f-al]^-. This prevents working with [al-f-al]^- salts in ethereal or other donor solvents. By contrast, the [f-al]^- anion is no longer Lewis acidic and may therefore be used for reactions involving stronger nucleophiles than the [al-f-al]^- anion can withstand. Subsequently it may be transformed into the [al-f-al]^- salt by simple addition of one equivalent of Me₃Si-F-Al(OR^F)₃.