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New Zwitterionic Pentacoordinate Silicates with SiONFC2, SiONF2C, and SiO2N2C Frameworks:  Synthesis, Structure, and Dynamic Stereochemistry

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journal contribution
posted on 13.03.2000, 00:00 by Daniel Kost, Inna Kalikhman, Sonia Krivonos, Rüdiger Bertermann, Christian Burschka, Ruth E. Neugebauer, Melanie Pülm, Reiner Willeke, Reinhold Tacke
The zwitterionic monocyclic λ5Si-monofluorosilicates 3ac were synthesized by reaction of the zwitterionic λ5Si-trifluorosilicates 1ac with 1 molar equiv of (Z)-Me3SiO(Ph)CNN(H)SiMe3 (2). The zwitterionic monocyclic λ5Si-difluorosilicate 8 and the zwitterionic spirocyclic λ5Si-silicate 9 (isolated as the isomer 9-I) were prepared in an analogous manner, starting from the zwitterionic λ5Si-tetrafluorosilicate 7 and using 1 (→ 8) or 2 (→ 9) molar equiv of 2. Single-crystal X-ray diffraction studies showed that the Si-coordination polyhedra of 1a, 3a, 8, 9-I·CHCl3, and 9-I·CH3CN are distorted trigonal bipyramids. The axial positions are occupied by two fluorine atoms (1a), one fluorine atom and one oxygen atom (3a, 8), or two oxygen atoms (9-I·CHCl3, 9-I·CH3CN). These results are in agreement with the NMR data obtained for these compounds in solution. The chiral monocyclic compounds (3ac and 8) undergo two distinct rate processes, inversion of absolute configuration of the chiral λ5Si-silicate skeleton (enantiomerization) and a novel “ammonium-nitrogen inversion”, measured by variable-temperature and selective inversion recovery NMR experiments (activation barrier for “ammonium-nitrogen inversion”:  3ac, 17.0 ± 0.3 kcal mol-1; 8, 20.8 ± 0.3 kcal mol-1; activation barrier for inversion of absolute configuration at the silicon atom:  3c, 20.7 ± 0.3 kcal mol-1; 8, 11.7 ± 0.3 kcal mol-1). In the spirocyclic λ5Si-silicate 9-I only one of these processes, the “ammonium-nitrogen inversion” (ΔG = 20.8 ± 0.3 kcal mol-1), could be observed, while inversion of absolute configuration at the silicon atom could not be detected by NMR spectroscopy. Furthermore, no interconversion of diastereomers 9-I9-II was observed.