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

Abstract

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-2 H-pyrans (tetrahydropyrans, oxacyclohexanes, oxanes; CH 3, C 2H 5, i-C 3H 7, t-C 4H 9, neo-C 5H 11) and 3-(trimethylsilyl)tetrahydro-2 H-pyran; Si(CH 3) 3). The MP2/6-31G ∗//HF/6-31G ∗ conformational energies (−Δ G 0 or A values, kcal/mol) of 3-alkyltetrahydro-2 H-pyrans (CH 3=0.97; C 2H 5=0.74; i-C 3H 7=0.59; t-C 4H 9=2.42; neo-C 5H 11=0.27; Si(CH 3) 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 ∗).