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

Abstract

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.