Ab initio molecular orbital study of the conformational energies of 3- alkyltetrahydro-2H-pyrans

Ab initio HF/6-31G* and MP2/6-31G*//HF/6-31G* methods were used to calculate the relative energies of the rotamers in the chair conformations of 3-alkyltetrahydro-2H-pyrans (tetrahydropyrans, oxacyclohexanes, oxanes; CH₃, C₂H₅, i-C₃H₇, t-C₄H₉, neo-C₅H₁₁) and 3-(trimethylsilyl)tetrahydro- 2H-pyran; Si(CH₃)₃). The MP2/6-31G*//HF/6-31G* conformational energies (- ΔG⁰ or A values, kcal/mol) of 3-alkyltetrahydro-2H-pyrans (CH₃ = 0.97; C₂H₅ = 0.74; i-C₃H₇ = 0.59; t-C₄H₉ = 2.42; neo-C₅H₁₁ = 0.27; Si(CH₃)₃ = 0.86) are smaller than those calculated for alkylcyclohexanes and 2-alkyloxacyclohexanes. The carbon-oxygen bond lengths are in the narrow range of 1.399-1.403 Å with the C(2)-O(1) bond length essentially the same as the C(6)-O(1) bond length. The C(3)-H bond lengths range from 1.086 to 1.092 Å for the respective axial and equatorial conformers. The C(3)-C(7) bond length shows greater variability (1.528 to 1.640 Å) with the axial C(3)-C(7) bond generally ~0.004 Å longer than the corresponding equatorial bond. The C(2)-O(1)-C(6) bond angles range from 112.9°to 114.2°and the C(3)-C(4)-C(5) angles vary from 110.7 to 114.6°. Plots of the calculated conformational energies for the 3-alkyloxacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 0.54 and r = 0.992 for HF/6-31G* and slope = 0.45 and r = 0.977 for MP2/6-31G*//HF/6-31G*).