New zwitterionic pentacoordinate silicates with SiONFC2, SiONF2C, and SiO2N2C frameworks: synthesis, structure, and dynamic stereochemistry

The zwitterionic monocyclic lambda(2)Si-monofluorosilicates 3a-c were synth esized by reaction of the zwitterionic lambda(5)Si-trifluorosilicates 1a-c with 1 molar equiv of (Z)-Me3SiO(Ph)C=N-N(H)SiMe3 (2). The zwitterionic mon ocyclic lambda(5)Si-difluorosilicate 8 and the zwitterionic spirocyclic lam bda(5)Si-silicate 9 (isolated as the isomer 9-I) were prepared in an analog ous manner, starting from the zwitterionic lambda(5)Si-tetrafluorosilicate 7 and using 1 (--> 8) or 2 (--> 9) molar equiv of 2. Single-crystal X-ray d iffraction studies showed that the Si-coordination polyhedra of 1a, 3a, 8, 9-I . CHCl3, and 9-I . CH3CN are distorted trigonal bipyramids. The axial p ositions 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 (3a-c and 8) undergo two distinc t rate processes, inversion of absolute configuration of the chiral lambda( 5)Si-silicate skeleton (enantiomerization) and a novel "ammonium-nitrogen i nversion", measured by variable-temperature and selective inversion recover y NMR experiments (activation barrier for "ammonium-nitrogen inversion": 3a -c, 17.0 +/- 0.3 kcal mol(-1); 8, 20.8 +/- 0.3 kcal mol(-1); activation bar rier 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 lam bda(5)Si-silicate 9-I only one of these processes, the "ammonium-nitrogen i nversion" (Delta G(double dagger) = 20.8 +/- 0.3 kcal mol(-1)), could be ob served, while inversion of absolute configuration at the silicon atom could not be detected by NMR spectroscopy. Furthermore, no interconversion of di astereomers 9-I reversible arrow 9-II was observed.