Spirocyclic Zwitterionic λ<sup>5</sup><i>Si</i>-Silicates with Two Bidentate Ligands Derived from α-Amino Acids or α-Hydroxycarboxylic Acids: Synthesis, Structure, and Stereodynamics
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-phenyl lactic 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-licates with ligands derived from optically active a.-amino acids or (alpha-hydroxycarboxylic acids were isolated as enantiomerically and diastereomerically pure compounds that undergo a (Delta)/(Delta)-epimerization in solution.
Pentacoordination of Silicon by Four Covalent Si−S Bonds and One Covalent Si−C Bond
compounds 1 and 2 are the first pentacoordinate silicates with covalent SiS4 C skeletons. They were characterized by crystal-structure analyses and NMR spectroscopic studies. Although pentacoordination of silicon is usually favored by ligand atoms of high electronegativity, the structural data of both 1 and 2 are consistent with the presence of five covalent bonds to the silicon atom.