Synthesis of 3-fluoro-3-methyl-3-silatetrahydropyran and its conformational preferences in gas and solution by GED, NMR and theoretical calculations

Abstract

The 3,3-disubstitued 3-silaheterocyclohexane with an electronegative substituent at silicon, 3-fluoro-3-methyl-3-silatetrahydropyran 1, was synthesized, and its molecular structure and conformational properties studied by gas-phase electron diffraction (GED) and low temperature 13C and 19F NMR spectroscopy. Quantum-chemical calculations were carried out both for the isolated species and H-complexes in gas and in polar medium. The predominance of the 1-FeqMeax conformer (1-Feq:1-Fax ratio of 65:35, ΔG° = 0.37 kcal/mol) determined from GED is close to the theoretically estimated conformational equilibrium, especially at the DFT level. In solution, low temperature NMR spectroscopy showed no decoalescence of the signals in 13C (down to 95 K) and 19F NMR spectra (down to 123 K). However, the calculated 19F chemical shift of −173.6 ppm for the 1-FeqMeax conformer practically coincides with the experimentally observed value (−173 to −175 ppm) as distinct from that for the 1-FaxMeeq conformer (−188.8 ppm), suggesting compound 1 to be anancomeric in solution, in compliance with its theoretical and experimental preference in the gas phase.