An ab Initio Theory and Density Functional Theory (DFT) Study of Conformers of Tetrahydro-2H-pyran

Abstract

Ab initio theory with the 3-21G, 6-31G(d), 6-31G(d,p), 6-311G(d,p), 6-31+G(d), and 6-311+G(d,p) basis sets and density functional theory (SVWN, pBP, BLYP), including the hybrid density functional methods B3LYP, B3PW91, and B3P86, have been used to calculate the energies of the chair, half-chair, sofa, twist, and boat conformers of tetrahydro-2H-pyran (oxacyclohexane, oxane, pentamethylene oxide, tetrahydropyran, THP). The enthalpies (ΔH°), entropies (ΔS°), and free energies (ΔG°) of the conformers were also determined. The energy difference (ΔE, kcal/mol) between the chair and the 2,5-twist conformer is 5.92 to 6.10 (HF), 5.78 to 6.10 (MP2), 6.71 to 6.82 (SVWN), 6.04 to 6.12 (pBP), and 5.84 to 5.95 (BLYP, B3LYP, B3PW91, B3P86). The energy difference (ΔE, kcal/mol) between the chair and the 1,4-boat conformer is 6.72 to 7.05 (HF), 6.76 to 7.16 (MP2), 6.97 to 7.20 (SVWN), 6.26 to 6.36 (pBP), and 6.23 to 6.46 (BLYP, B3LYP, B3PW91, B3P86). The transition state between the chair conformation and the 2,5-twist conformation is 11 kcal/mol higher in energy than the chair conformer.

A computational study of conformational interconversions in 1,4‐dithiacyclohexane (1,4‐dithiane)

“…Density functional theory (DFT) gave larger energy differences (⌬E) than HF, which gave larger energy differences than MP2. The chair conformer of 1,3-dioxane (8) is 50.3 and 5.4 kcal/mol, respectively, more stable than the respective chair conformers of 1,4-dioxane (1) and 1,2-dioxane (12). The greater stability of 1,3-dioxane is consistent with stereoelectronic hyperconjugative interactions (anomeric effect), the electronegativity of oxygen, the gem effect, and Coulombic attraction between carbon and oxygen.…”

“…56,57 Stereoelectronic hyperconjugative interactions (e.g. n X 3 * C-Hax , C-Hax 3 * C-Hax , n X 3 * C-Heq , the W-effect or homoanomeric effect) have been observed in tetrahydro-2H-pyran, 12 tetrahydro-2H-thiopyran (thiacyclohexane, thiane, 7, C s symmetry, Fig. 6), 58 -60 1,2-oxathiane, BLYP and B3LYP gave longer C-C and C-S bond lengths than B3PW91, HF, and MP2.…”

“…[12][13][14] To maintain the desired structural integrity during partial geometry optimizations in the preliminary screening process, torsional angles () were constrained in the 1,4-boat (3a S1-C2-C3-S4 ϭ S1-C6 -C5-S4 ϭ 0°), 2,5-boat (3b C2-S1-C6 -C5 ϭ C2-C3-S4 -C5 ϭ 0°), 1,4-half-chair (4a S1-C2-C3-S4 ϭ 0°, S1-C6 -C5-S4 ϭ 57°); 2,5-half-chair (4b C5-S4 -C3-C2 ϭ 0°, C2-S1-C6 -C5 ϭ 66°); 1,4-sofa (5a C2-C3-S4 -C5-C6 are coplanar); 2,5-sofa (5b S1-C2-C3-S4 -C5 are coplanar), and planar (6 all atoms are coplanar) structures. With the half-chair structures it is sometimes useful to adjust the larger torsional angle in order to obtain a more reasonable starting transition structure.…”

“…[1][2][3][4][5][6][7][8][9][10][11][12][13][14][15] Although the conformational analysis of cyclohexane and substituted cyclohexanes have been extensively studied, [5][6][7][8][9][10]16,17 comparatively few computational studies have been reported on the conformers and ring reversal mechanisms of unsubstituted diheterocyclohexanes. [11][12][13][14] There are computational studies on stereoelectronic hyperconjugative interactions in the chair conformers of diheterocyclohexanes, 9 -14,17-25 but no reports concerning the various other conformers and the ring interconversion mechanisms of 1,4-dithiacyclohexane [1,4-dithiane, 1; eqs. (1), (2) and (3)].…”

“…1) is not expected to be involved in the ring reversal mechanisms. By analogy with cyclohexane, 6,7 tetrahydro-2H-pyran (oxacyclohexane, THP), 12 1,3-dioxacyclohexane (1,3-dioxane), 13 and 1-oxa-2-thiacyclohexane (1,2-oxathiane), 14 paths for the chair-chair interconversion in 1,4-dithiane could occur via twist conformers and possible pathways for the twist-twist interconversions could involve boat transition states.…”