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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)

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Abstract

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.

Graphical abstract: 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)

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Publication details

The article was received on 12 Jun 2018, accepted on 31 Jul 2018 and first published on 01 Aug 2018


Article type: Edge Article
DOI: 10.1039/C8SC02591F
Citation: Chem. Sci., 2018, Advance Article
  • Open access: Creative Commons BY-NC license
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    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)

    A. Martens, P. Weis, M. C. Krummer, M. Kreuzer, A. Meierhöfer, S. C. Meier, J. Bohnenberger, H. Scherer, I. Riddlestone and I. Krossing, Chem. Sci., 2018, Advance Article , DOI: 10.1039/C8SC02591F

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