Nucleophilicity of Neutral versus Cationic Magnesium Silyl Compounds

Charge
and ancillary ligands affect the reactivity of monomeric tris­(trimethylsilyl)­silyl
magnesium compounds. Diamine-coordinated (tmeda)­Mg­{Si­(SiMe3)3}­Me (tmeda = tetramethylethylenediamine; 2-tmeda) and (dpe)­Mg­{Si­(SiMe3)3}­Me (dpe =1,2-N,N-dipyrrolidenylethane; 2-dpe) are synthesized by salt elimination reactions of L2MgMeBr
and KSi­(SiMe3)3. Compounds 2-tmeda or 2-dpe react with MeI or MeOTf to give MeSi­(SiMe3)3 as the product of Si–C bond formation.
In contrast, 2-tmeda and 2-dpe undergo exclusively
reaction at the magnesium methyl group with electrophiles such as
Me3SiI, B­(C6F5)3, HB­(C6F5)2, and [Ph3C]­[B­(C6F5)4]. These reactions provide a series
of neutral, zwitterionic, and cationic magnesium silyl compounds,
and from this series we have found that silyl group transfer is less
effective with cationic magnesium compounds than neutral complexes.