A computational study of 4-alkyl equatorial tetrahydro-2 H-thiopyran-1-oxides (tetrahydrothiopyran-1-oxides, thiacyclohexane-1-oxides, thiane-1-oxides)

Abstract

Ab initio theory and density functional theory (B3LYP) have been used to calculate the geometry optimized structures, configurational isomer energy differences (Δ E), and the configurational enthalpies (Δ H 0), entropies (Δ S 0), and free energies (Δ G 0) of 4-alkyl equatorial tetrahydro-2 H-thiopyran-1-oxides (tetrahydrothiopyran-1-oxides, thiacyclohexane-1-oxides, thiane-1-oxides (Me, Et, neo-Pent, i-Pr, tert-Bu) and 4-trimethylsilyl equatorial tetrahydro-2 H-thiopyran-1-oxide. The calculated structural data indicate that repulsive steric interactions between the axial alkyl group and the ring atoms are the major contributors to the equatorial preference. The configurational isomer energy differences (Δ E), configurational enthalpies (Δ H 0) and free energies (Δ G 0) of the 4-alkyl equatorial tetrahydro-2 H-thiopyran-1-oxides have been compared with the reported experimental conformational enthalpies (Δ H 0) and free energies (Δ G 0) of the corresponding alkylcyclohexanes and 4-alkyltetrahydro-2 H-thiopyrans. The use of the configurational isomer energy differences (Δ E) and/or configurational free energies (Δ G 0) of 4-alkyl equatorial tetrahydro-2 H-thiopyran-1-oxides as possible models for assigning the difference in the steric requirements of an alkyl substituent in axial and equatorial positions is also discussed.