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

2018 
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 13 C and 19 F 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 -F eq Me ax conformer ( 1 -F eq : 1 -F ax 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 13 C (down to 95 K) and 19 F NMR spectra (down to 123 K). However, the calculated 19 F chemical shift of −173.6 ppm for the 1 -F eq Me ax conformer practically coincides with the experimentally observed value (−173 to −175 ppm) as distinct from that for the 1 -F ax Me eq conformer (−188.8 ppm), suggesting compound 1 to be anancomeric in solution, in compliance with its theoretical and experimental preference in the gas phase.
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