Preparation of “Si‐Centered” Chiral Silanes by Direct α‐Lithiation of Methylsilanes

Abstract

The direct alpha-lithiation of methyl-substituted silanes as an efficient method for the preparation and elaboration of Si-chiral compounds is reported. Deprotonation of chiral oligosilanes occurs selectively and with high yields at the methyl group of the stereogenic silicon center, even in the presence of multiple methylsilyl or methylgermyl substituents. Computational studies have confirmed this preference as a consequence of pre-coordination of the lithiating agent by the amino side-arm and repulsion effects in the corresponding transition state. This complexation is also obvious from X-ray structure analyses of the alpha-lithiated silanes, which exhibit intriguing structure formation patterns differing in the type of aggregation and the amount of alkyllithium used. An alternative route to Si-chiral compounds is also presented, which involves desymmetrization of dimethylsilanes mediated by a chiral side-arm. Structure analyses and computational studies have shown that the diastereoselectivity of this alpha-lithiation is influenced by the selectivity of the formation of the stereogenic nitrogen upon complexation of the alkyllithium.

Crystal structures of diaquadi-μ-hydroxido-tris[trimethyltin(IV)] diformatotrimethylstannate(IV) and di-μ-hydroxido-tris[trimethyltin(IV)] chloride monohydrate

“…2) shows structural analogies and is also discussed herein. Structures 1 and 2 were obtained as byproducts from trapping reactions with trimethyltin chloride (Dä schlein et al, 2010;Unkelbach et al, 2012;Koller et al, 2015). ISSN 2056-9890…”

Preparation of aminomethyl functionalised silanes via an α-lithiated amine: From their synthesis, stability and crystal structures to stereochemical issues

“…The dimethylphenyl compound (R,R)-6 has already been described before in context of the preparation of Si-chiral compounds by diastereoselective deprotonation of its diastereotopic methyl groups. 11 All compounds were characterised by multinuclear NMR spectroscopy, elemental analysis and mass spectrometry. It is noteworthy, that due to the chiral side-arm the phenyl substituents in (R,R)-7 are diastereotopic, so that two sets of signals are observed in the 1 H and 13 C NMR spectra.…”

Anorganische Chemie 2010

“…Verbindungen, die optisch reine, asymmetrische Silicium‐Zentren aufweisen, lassen sich zum einen aus enantiomerenreinen Silanolen über die entsprechenden Chlor‐ und Lithium‐Derivate in Form ihrer Carboxylate erhalten (17) 1_21. Zum anderen führt die a‐Lithiierung von methylierten, asymmetrischen Silicium‐Verbindungen mit tBuLi zu konfigurationsstabilen, Si‐zentriert‐chiralen, carb‐anionischen Bausteinen (18) 1_22…”