Alkaline-Earth Derivatives of the Reactive [HB(C6F5)3]- Anion

The beta-diketiminato magnesium amidoboranes [HC{(Me)CNDipp}(2)Mg(NMe2BH2NMe2 center dot BH3)] and [HC{(t-Bu)CNDipp}(2)Mg(NMe2 center dot BH3)] are readily converted to the corresponding derivatives of the [HB(C6F5)(3)](-) anion by treatment with B(C6F5)3. The bis(borohydride) derivatives of the heaviest alkaline-earth elements, strontium and barium, may be similarly synthesized by reaction of strontium or barium dimethylamidoboranes and B(C6F5)(3) and by metathesis reactions of either SrI2 or BaI2 and 2 molar equiv of K(HB(C6F5)(3)). The strontium and barium compounds have been fully characterized in solution and in the solid state as the respective tris(diethyl ether) and tetrakis(tetrahydrofuran) adducts. The magnesium compound [HC{(Me)CNDipp}(2)Mg(HB(C6F5)(3))] has been applied to the catalytic hydroboration of i-PrN-C-N-i-Pr with HBpin. In contrast to carbodiimide hydroboration catalyzed by the corresponding beta-diketiminato magnesium hydride, which results in the exclusive production of the monoborylated amidine, use of the [HB(C6F5)(3)](-) derivative provides the product of bis-borylation, the aminal H2C(N{Bpin}i-Pr)(2), under mild conditions. A series of stoichiometric reactions highlight that, while this reactivity is likely to be primarily magnesium mediated, B(C6F5)(3) plays a vital role both in the delivery of reactive hydride and through the Lewis acid activation of the heteroallene substrate and various reactive intermediates.