Spirocyclic Zwitterionic λ⁵Si-Silicates with Two Bidentate Ligands Derived from α-Amino Acids or α-Hydroxycarboxylic Acids:  Synthesis, Structure, and Stereodynamics

Abstract

A series of zwitterionic lambda(5)Si-silicates with a (2,2,6,6-tetramethylpiperidinio)methyl group and two identical bidentate ligands derived from glycine, (S)-alanine, (S)-phenylalanine, glycolic acid, (S)-lactic acid, (S)-3-phenyllactic acid, or (S)-mandelic acid were synthesized and structurally characterized (solution and solid-state NMR spectroscopy; single-crystal X-ray diffraction). The chiral lambda(5)Si-silicates with ligands derived from optically active alpha-amino acids or alpha-hydroxycarboxylic acids were isolated as enantiomerically and diastereomerically pure compounds that undergo a (Lambda)/(Delta)-epimerization in solution.

Competitive Substitution Reactions at Extracoordinate Silicon during Asymmetric Hydrosilylation

“…A broad variety of pentacoordinate amino and hydroxy acid derivatives, exemplified by 2, have been isolated in crystalline form (Scheme 2B). [15][16][17] We reasoned that related extracoordinate alkoxysilicon species derived from more basic β-aminoalkoxides should form readily and have sufficiently long lifetimes to permit complexation with ketones, leading to subsequent reduction. To test this hypothesis, a series of experiments were undertaken with the catalytic system derived from a chiral bidentate pyrrolidine alcoholate (Scheme 3), which induces the hydrosilylation of ketones enantioselectively.…”

A Zwitterionic Spirocyclic Pentacoordinate Silicon Compound Synthesized in Water by SiO and SiC Bond Cleavage

“…We have contributed to this field by systematic studies on zwitterionic λ 5 Si ‐silicates that contain a pentacoordinate (formally negatively charged) silicon atom and a tetracoordinate (formally positively charged) nitrogen atom 1d. The zwitterions 1 ,2 2 ,3 3 ,4 4 ,5 5 ,6 6 ,7 7 ,8 8 ,9 and 9 10 are examples of this type of compound Generally, these zwitterionic λ 5 Si ‐silicates are quite sensitive to water and easily undergo hydrolytic SiOC, SiSC, and SiNC bond cleavage.…”

Synthesis and Optical Resolution of Configurationally Stable Zwitterionic Pentacoordinate Silicon Derivatives

“…Other strategies have been undertaken to tackle the synthesis of configurationally stable pentaorganosilicates. Whereas the group of Tacke used β‐hydroxyacid ligands, [8a,11] the group of Lammertsma focused on phenylpyrrole and naphthylpyrrole bidentate ligands [12] but despite their configurational stability at the NMR timescale, most of these species possess corresponding enantiomerisation barriers too low (less than 21.5 kcal mol −1 ) to expect any isolation and applications. Recently, the latter group reported on the first enantiomerically‐enriched pentaorganosilicates [12c] via the addition of nucleophiles to an optically pure tetravalent spirosilane featuring two all‐carbon bidentate ligands (Figure 1).…”