The Non-Ancillary Nature of Trimethylsilylamide Substituents in Boranes and Borinium Cations.

The known boranes (R(Me Si)N) BF (R=Me Si 1, tBu 2, C F 3, o-tol 4, Mes 5, Dipp 6) and borinium salts (R(Me Si)N) B][B(C F ) ] (R=Me Si 7, tBu 8) are prepared and fully characterized. Compound 7 is shown to react with phosphines to generate [R PSiMe ] and [R PH] (R=Me, tBu). Efforts to generate related borinium cations via fluoride abstraction from (R(Me Si)N) BF (R=C F 3, o-tol 4, Mes 5) gave complex mixtures suggesting multiple reaction pathways. However for R=Dipp 6, the species [(μ-F)(SiMe N(Dipp)) BMe][B(C F ) ] was isolated as the major product, indicating methyl abstraction from silicon and F/Me exchange on boron. These observations together with state-of-the-art DFT mechanistic studies reveal that the trimethylsilyl-substituents do not behave as ancillary subsitutents but rather act as sources of proton, SiMe and methyl groups.