Molecular structure and conformational analysis of 3-methyl-3-phenyl-3-silatetrahydropyran. Gas-phase electron diffraction, low temperature NMR and quantum chemical calculations

The molecular structure and conformational behavior of 3-methyl-3-phenyl-3-silatetrahydropyran 1 was studied by gas-phase electron diffraction (GED-MS), low temperature 13C NMR spectroscopy (LT NMR) and theoretical calculations. The 1-Pheq and 1-Phax conformers were located on the potential energy surface. Rotation about the Si-CPh bond revealed the phenyl ring orthogonal to the averaged plane of the silatetrahydropyran ring for 1-Pheq and a twisted orientation for 1-Phax. Theoretical calculations and GED analysis indicate the predominance of 1-Phax in the gas phase with the ratio of conformers (GED) 1-Pheq:1-Phax=38:62 (ΔG°307=−0.29 kcal/mol). In solution, LT NMR spectroscopy gives almost the opposite ratio Pheq:1-Phax=68:32 (ΔG°103=0.16 kcal/mol). Simulation of solvent effects using the PCM continuum model or by calculation of the solvent-solute complexes allowed us to rationalize the experimentally observed opposite conformational predominance of the conformers of compound 1 in the gas phase and in solution.

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