Conformational preferences for some 3,3-bis[(4′-substituted phenylsulfanyl)]1-methyl-2-piperidinones through spectroscopic and theoretical studies

Abstract

The analysis of the IR carbonyl band of some 3,3-bis[(4′-substituted phenylsulfanyl)]1-methyl-2-piperidinones 1–5 bearing as substituents OMe 1, Me 2, H 3, Cl 4 and Br 5, supported by B3LYP/6-31G(d,p) calculations for 3, indicated the existence of three conformers in the gas phase and practically a single conformer in solution. In the gas phase, the c1 conformer is less polar and slightly more stable than the most polar c2 conformer. The c3 conformer is the least polar and least stable conformer. The summing up of the selected natural bond orbital delocalization orbital energies is practically the same (ca.136 kcal mol−1) for the c1, c2 and c3 conformers of 3. Therefore, the trend of the [O δ−_(CO)···H δ+o− Ph] (hydrogen bond) attractive electrostatic interactions along with the trend of the [O δ−_(CO)···S δ−] repulsive electrostatic interactions are the main factors which determine the observed computed relative populations for the c1 (44%), c2 (31%) and c3 (25%) conformers. Moreover, the IR single carbonyl stretching band found (for 1–5) in solvents of increasing relative permissivity (CCl4, CHCl3, CH2Cl2, CH3CN), in agreement with polarisable continuum model calculations (for 3), show that the most polar c2 conformer is practically unique in the solution (for 1–5), and the geometry is very close to that of the c2′ conformer in the solid state (for 1–4).