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

The known boranes (R(Me 3 Si)N) 2 BF (R=Me 3 Si 1, tBu 2, C 6 F 5 3, o-tol 4, Mes 5, Dipp 6) and borinium salts (R(Me 3 Si)N) 2 B][B(C 6 F 5 ) 4 ] (R=Me 3 Si 7, tBu 8) are prepared and fully characterized. Compound 7 is shown to react with phosphines to generate [R 3 PSiMe 3 ] + and [R 3 PH] + (R=Me, tBu). Efforts to generate related borinium cations via fluoride abstraction from (R(Me 3 Si)N) 2 BF (R=C 6 F 5 3, o-tol 4, Mes 5) gave complex mixtures suggesting multiple reaction pathways. However for R=Dipp 6, the species [(μ-F)(SiMe 2 N(Dipp)) 2 BMe][B(C 6 F 5 ) 4 ] 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 3 and methyl groups.