Diastereo- and Enantiomerically Pure Zwitterionic Spirocyclic λ⁵Si-[(Ammonio)methyl]silicates with an SiO₂N₂C Skeleton Containing Two Bidentate Chelate Ligands Derived from α-Amino Acids

Abstract

The zwitterionic λ5 Si-silicates (Λ,S,S)-4·CH2Cl2, (Λ,S,S)-5·CH2Cl2, (Λ,S,S)-6·2CH2Cl2, and (Λ,S,S)-6·1/2CH2Cl2, with a (2,2,6,6-tetramethylpiperidinio)methyl group and two identical bidentate chelate ligands derived from (S)-valine, (S)-tert-leucine, or (S)-proline bound to the silicon(IV) coordination center, were synthesized and structurally characterized (solution and solid-state NMR spectroscopy; single-crystal X-ray diffraction). All compounds were isolated as diastereo- and enantiomerically pure crystalline solids, and the λ5 Si-silicates (Λ,S,S)-4, (Λ,S,S)-5, and (Λ,S,S)-6 were demonstrated to exist as diastereo- and enantiomerically pure zwitterions in solution as well (CD2Cl2, temperature range −100 to 23 °C). The experimental investigations were complemented by computational studies of related model species.

Neutral Penta‐ and Hexacoordinate Silicon(IV) Complexes Containing Two Bidentate Ligands Derived from the α‐Amino Acids (S)‐Alanine, (S)‐Phenylalanine, and (S)‐tert‐Leucine

“…Compound 6-11 were isolated as colorless crystalline solids (yields: 51 (6), 56 (7), 34 (8), 48 (9), 57 (10), and 49 % (11)). The moderate yields of compounds 6-11 can be explained by the occurrence of a side reaction of the respective disilylated amino acid [Me 3 SiOC(O)CHRNA C H T U N G T R E N N U N G (SiMe 3 )H] (R = Me, Bn, tBu), which leads to the formation of an oligopeptide of the type…”

“…The water formed in this side reaction may be the proton source that could explain the existence of the NH 2 group of the bidentate monoanionic O,N ligands of 6-11. It is of interest to note that the oligopeptide formation from Me 3 SiOC(O)CHRNA C H T U N G T R E N N U N G (SiMe 3 )H (R = Me, Bn, tBu) only occurs upon addition of the (cyanato-N)silanes (SiA C H T U N G T R E N N U N G (NCO) 4 , MeSiA C H T U N G T R E N N U N G (NCO) 3 , and PhSiA C H T U N G T R E N N U N G (NCO) 3 , respectively); NMR spectroscopy studies performed with trimethylsilyl (S)-N-(trimethylsilyl)alaninate that was stored without solvent or as a solution in dichloromethane at room temperature over a period of three weeks did not show any evidence for oligopeptide formation.…”

“…In this context, it should be mentioned that compounds 7 and 9 and other related silicon(IV) complexes with an SiMe moiety and two bidentate ligands derived from different a-amino acids might be of interest for practical applications. The hydrolysis of these compounds would lead to the formation of the respective free a-amino acids and methylsilanetriol (MeSi(OH) 3 ), a silicon food supplement of long-standing use in humans that is currently being discussed as a potential drug for the improvement of bone and other connective tissue health. [15] Compounds 7 and 9 and related silicon(IV) complexes might also serve as silicon food supplements or as pro-drugs for methylsilanetriol.…”

“…Some years ago, we reported the first examples of this type, compounds 1-5. [2,3] These zwitterionic l 5 Si-silicates contain two identical bidentate dianionic ligands derived from glycine, (S)-alanine, (S)-valine, (S)-tert-leucine, or (S)-phenylalanine. Compounds 1-5 have been synthesized by using "the zwitterion trick", [4] that is, the pentacoordinate (formally negatively charged) silicon atom has been incorporated into a molecular framework that also contains a tetracoordinate (formally positively charged) nitrogen atom.…”

“…The neutral hexacoordinate silicon(IV) complex 6 (SiO 2 N 4 skeleton) and the neutral pentacoordinate silicon(IV) complexes 7-11 (SiO 2 N 2 C skeletons) were synthesized from Si(NCO) 4 and RSiA C H T U N G T R E N N U N G (NCO) 3 (R = Me, Ph), respectively. The compounds were structurally characterized by solid-state NMR spectroscopy (6-11), solution NMR spectroscopy (6 and 10), and single-crystal X-ray diffraction (8 and 11 were studied as the solvates 8·CH 3 CN and 11·C 5 H 12 ·0.5 CH 3 CN, respectively).…”