Nucleophilicity of Neutral versus Cationic Magnesium Silyl Compounds

Abstract

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. KeywordsAncillary ligands, Electrophiles, elimination reaction, neutral complexes, nucleophilicities, Si-C bond formation, Tetramethylethylenediamine, Trimethylsilyl, magnesium Disciplines Chemistry CommentsReprinted (adapted) with permission from Organometallics 32 (2013) ABSTRACT: Charge and ancillary ligands affect the reactivity of monomeric tris(trimethylsilyl)silyl magnesium compounds. Diamine-coordinated (tmeda)-Mg{Si(SiMe 3 ) 3 }Me (tmeda = tetramethylethylenediamine; 2-tmeda) and (dpe)Mg{Si(SiMe 3 ) 3 }Me (dpe =1,2-N,N-dipyrrolidenylethane; 2-dpe) are synthesized by salt elimination reactions of L 2 MgMeBr and KSi(SiMe 3 ) 3 . Compounds 2-tmeda or 2-dpe react with MeI or MeOTf to give MeSi(SiMe 3 ) 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 Me 3 SiI, B(C 6 F 5 ) 3 , HB(C 6 F 5 ) 2 , and [Ph 3 C][B(C 6 F 5 ) 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.

Silyl–Hydrosilane Exchange at a Magnesium Triphenylsilyl Complex Supported by a Cyclen-Derived NNNN-Type Macrocyclic Ligand

“…The angles Si1–Mg1–O1 of 111.69(9)°, Si1–Mg1–O1′ of 105.92(9)°, and Si1–Mg1–Si1′ of 123.72(9)° indicate tetrahedral coordination geometry at the magnesium atom. The Mg1–Si1 bond length of 2.6737(15) Å compares to those in [Mg­(SiMe 3 )­Br­(TMEDA)] 2 (2.603(4) Å), [Mg­{Si­(SiMe 3 ) 3 } 2 ­(THF) 2 ] (2.682(2) Å), [Mg­{Si­(SiMe 3 ) 3 } 2 ­(TMEDA)] (2.708(3) Å), and [Mg­{Si­(SiMe 3 ) 3 }­Me­(TMEDA)] (2.6414­(9) Å) …”

“…The Mg1−Si1 bond length of 2.6737(15) Å compares to those in [Mg(SiMe 3 )Br(TMEDA)] 2 (2.603(4) Å), 15 [Mg{Si-(SiMe 3 ) 3 } 2 (THF) 2 ] (2.682(2) Å), 8 [Mg{Si(SiMe 3 ) 3 } 2 -(TMEDA)] (2.708(3) Å), 7 and [Mg{Si(SiMe 3 ) 3 }Me-(TMEDA)] (2.6414(9) Å). 21 [(Me 3 TACD)Mg(SiPh 3 )] (2) was synthesized from [Mg-(SiPh 3 ) 2 (THF) 2 ]•THF (1) and (Me 3 TACD)H in benzene at room temperature (rt) and was isolated as a colorless powder in 83% yield (Scheme 1). Complete removal of the byproduct triphenylsilane was only possible in benzene after a reaction time of 2 h. Triphenylsilane could not be fully separated by washing with n-pentane, if THF was used as the solvent or if the reaction time was not long enough.…”

“…Chelating ligands allowed the isolation of structurally characterized magnesium silyl compounds [Mg­(SiMe 3 ) 2 (L)] (L: DME = 1,2-dimethoxyethane ( A ); TMEDA = N,N,N ′ ,N ′-tetramethylethylenediamine; TMDAP = 1,3-bis­( N , N -dimethylamino)­propane). − Sterically bulky silyl groups led to the synthesis of bis­(silyl)magnesium compounds, such as [Mg­{Si­(SiMe 3 ) 3 } 2 ­(TMEDA)], [Mg­{Si­(SiMe 3 ) 3 } 2 ­(THF) 2 ] ( B ; THF = tetrahydrofuran), [Mg­(Si t Bu 3 ) 2 ­(THF) 2 ], [Mg­{Si­(SiMe 3 ) 2 Me} 2 (L) 2 ] (L = THF; 1,4-dioxane), [Mg­{Si­(SiMe 3 ) 2 Ph} 2 (THF) 2 ], and [{Fe­[C 5 H 4 ­{Si­(SiMe 3 ) 2 }] 2 }­Mg­(THF) 2 ] . In addition, magnesium mono­(silyl)­s such as [(To M )­Mg­{Si­(SiRMe 2 ) 3 }] ( C ) (To M = tris­(4,4-dimethyl-2-oxazolinyl)­phenylborate; R = H; Me), [(DIPPnacnac)­Mg­(SiMe 2 Ph)] ( D ) (DIPPnacnac = HC­[C­(Me)­N­(2,6- i Pr-C 6 H 3 )] 2 ), and [(Me 3 TACD·Al i Bu 3 )­Mg­(SiH 2 Ph)] , as well as mixed magnesium silyl halides − ,− and magnesium silyl alkyl compounds , became known (Chart ).…”

Preparations of Silyl Anions

“…The reaction between L 2 Mg-MeBr and KSi(SiMe 3 ) 3 in benzene at room temperature for 4 h gave diamine coordinated (TMEDA)Mg[Si(SiMe 3 ) 3 ]Me 144 and (dpe)Mg[Si(SiMe 3 ) 3 ]Me 145 (Scheme 49). 135 Scheme 49 Synthesis of ligand-coordinated mixed silyl-Grignard…”

Nucleophilic Magnesium Silanide and Silaamidinate Derivatives

“…While much of this chemistry has historically focused on their use as stoichiometric reagents, more recent research has also begun to delineate a broadly based catalytic reactivity. − In contrast, the chemistry of magnesium silanide reagents has attracted comparatively limited interest despite the clear utility of such species as sources of triorganosilyl anions. − While Oestreich and co-workers have recently demonstrated that triorganosilylmagnesium halides are accessible by in situ oxidation of magnesium metal to MgBr 2 and lithium silanide transmetalation, , true silicon-centered Grignard analogues, which are prepared from elemental magnesium and a triorganosilyl halide, are limited to Ritter’s synthesis of Me 3 SiMgBr·L and Me 3 SiMgI·L (L = tetramethylethylenediamine or pentamethyldiethylenetriamine) . More commonly, the synthesis of silylmagnesium compounds has been achieved either by redox transmetalation of (R 3 Si) 2 Hg and magnesium − or by salt metathesis between a sterically encumbered silylpotassium reagent and a magnesium halide. − As part of a broader exploration of magnesium compounds in which the alkaline-earth metal is directly bonded to an element of group 13 or 14, − we have previously reported that reaction of the β-diketiminato organomagnesium complex [(BDI)­MgBu] ( 1 ; BDI = HC­{(Me)­CN-Dipp} 2 , where Dipp = 2,6-diisopropylphenyl), with the silaborane Me 2 PhSi-Bpin provides facile access to the heteroleptic magnesium silanide [(BDI)­MgSiMe 2 Ph] ( 2 ; Scheme ). …”