5 years ago

Using the Lewis Acid Me3Si−F−Al(ORF)3 To Prepare Phosphino-Phosphonium Cations with the Least-Coordinating Anion [(RFO)3Al−F−Al(ORF)3]−

Using the Lewis Acid Me3Si−F−Al(ORF)3 To Prepare Phosphino-Phosphonium Cations with the Least-Coordinating Anion [(RFO)3Al−F−Al(ORF)3]−
Harald Scherer, Alexander Ripp, Ingo Krossing, Arthur Martens, Daniel Kratzert, Mario Schleep, Josephine Possart
By reaction of two equivalents of Me3Si−F−Al(ORF)3 1 with an equimolar amount of PPh2Cl, the salt [Ph2P−PPh2Cl]+[(RFO)3Al−F−Al(ORF)3]− 2 is prepared smoothly in 91 % yield (NMR, XRD). The synthesis of [Ph2P−PPh3]+[(RFO)3Al−F−Al(ORF)3]− 3 is best achieved by a two-step reaction: first, two equivalents of 1 react with one PPh3 to give [Me3Si−PPh3]+[(RFO)3Al−F−Al(ORF)3]− 4 (NMR, XRD), which, upon reaction with PPh2Cl, yields pure 3 and Me3SiCl (NMR, XRD). Typically, a stoichiometry of two equivalents of 1 with respect to one equivalent of the chloride donor should be used. Otherwise, the residual strong Lewis acidity of the [(RFO)3Al−F−Al(ORF)3]− anion in the presence of the [F−Al(ORF)3]− anion—that forms with less than two equivalents of 1—leads to further chloride exchange reactions that complicate work-up. This route presents the easiest way to introduce the least-coordinating [(RFO)3Al−F−Al(ORF)3]− anion into a system. We expect a wide use of this route in all areas, in which chloride-bond heterolysis in combination with very weakly coordinating anions is desirable. Additionally, we performed calculations on the bond dissociation mechanisms of [R2P−PMe3]+ and the isoelectronic Me2P−SiMe3 and Me2Si−PMe3 in dependence of the solvent permittivity. These calculations show, especially for the neutral reference compounds, a heavy influence of the solvent on the dissociation mechanism, which is why we suggest investigating these properties in solution instead of gas phase. Poorly coordinated: Phosphino-phosphonium cations [R2P−PR3]+ can be obtained from Me3Si−F−Al(ORF)3 and phosphines giving access to cations with the least-coordinating [(RFO)3Al-F-Al(ORF)3]− weakly coordinating anion and opening new synthetic routes.

Publisher URL: http://onlinelibrary.wiley.com/resolve/doi

DOI: 10.1002/chem.201701260

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