Diastereo- And Enantiomerically Pure Zwitterionic Spirocyclic Lambda-Si-5-[(Ammonio)Methyl]Silicates With An Sio2n2c Skeleton Containing Two Bidentate Chelate Ligands Derived From Alpha-Amino Acids

The zwitterionic lambda(5)Si-silicates (Lambda,S,S)-4(.)CH(2)Cl(2), (Lambda,S,S)-5(.)CH(2)Cl(2), (Lambda,S,S)-6(.)2CH(2)Cl(2), and (Lambda,S,S)-6(.)1/2CH(2)Cl(2), 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 lambda(5)Si-silicates (Lambda,S,S)-4, (Lambda,S,S)-5, and (Lambda,S,S)-6 were demonstrated to exist as diastereo- and enantiomerically pure zwitterions in solution as well (CD2Cl2, temperature range -100 to 23 degrees C). The experimental investigations were complemented by computational studies of related model species.