Rigid, Fluoroarene-Containing Phosphonium Borates and Boranes: Syntheses and Reactivity Studies

The reaction of (C6F5)2B(OEt) with tBu2P(CH2Li) yields the cyclic phosphonium ethoxyborate (C6F5)(OEt)B(CH2)(C6F4)PtBu2 ([1]OEt) via B−C adduct formation and ortho-fluoride substitution. Treatment of [1]OEt with HCl in Et2O gives the chloroborate [1]Cl in almost quantitative yield. The reaction of [1]Cl with Li[AlH4] leads not only to the reduction of the B−Cl bond but also to the selective substitution of one fluorine atom on the tetrafluorophenylene bridge ([4]H). [1]Cl is readily transformed into [1]F, [1]OAc, and [1]OTf upon reaction with Tl[PF6], AgOAc, and AgOTf, respectively (OAc = acetate; OTf = triflate). [1]OTf represents a convenient storage form of [1]+, because its B−O bond is highly labile. The free Lewis acid [1]+ was prepared in the form of its aluminate salt [1]+[Al(OtBuF)4]−. The phenyl derivative of [1]OEt, (C6H5)(OEt)B(CH2)(C6F4)PtBu2 ([5]OEt), is also accessible and serves as starting material for the preparation of [5]Cl and [5]+[Al(OtBuF)4]−.