Facile and systematic access to the least-coordinating WCA [(RFO)3Al–F–Al(ORF)3]− and its more Lewis-basic brother [F–Al(ORF)3]− (RF = C(CF3)3)
By reaction of the Lewis acid Me3Si–F–Al(ORF)3 with a series of [PF6]− salts, gaseous PF5 and Me3Si–F are liberated and salts of the anion [F–Al(ORF)3]− ([f–al]−; RF = C(CF3)3) can be obtained. By addition of another equivalent of Me3Si–F–Al(ORF)3 to [f–al]−, gaseous Me3Si–F is released and salts of the least coordinating anion [(RFO)3Al–F–Al(ORF)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(ORF)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(ORF)4]− or by halide abstraction reactions with Me3Si–F–Al(ORF)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(ORF)4]− and more stable against strong electrophiles. This bridged anion can be seen as an adduct of [f–al]− and Al(ORF)3. Thus, it is similarly Lewis acidic as BF3 and eventually reacts with nucleophiles (Nu) from the reaction environment to yield Nu–Al(ORF)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(ORF)3.
- This article is part of the themed collection: 2018 Chemical Science HOT Article Collection