Molecular orbital study of the conformational energies (−ΔG° or A values) of 2-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

✍ Scribed by Fillmore Freeman; Anna Phornvoranunt; Warren J. Hehre

• Publisher: John Wiley and Sons
• Year: 1999
• Tongue: English
• Weight: 128 KB
• Volume: 12
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(199903)12:3<176::aid-poc106>3.0.co;2-n

✦ Synopsis

Ab initio 6-31G* and MP2/6-31G*// 6-31G* methods and density functional (pBPDN**) theory were used to calculate the geometries and relative energies of the chair, boat, and twist-boat conformations of tetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes). The chair conformation of thiacyclohexane is 5.3 and 8.0 kcal mol À1 , respectively, (1 kcal = 4.184 kJ) more stable than the twist-boat and boat structures at the MP2/6-31G*// 6-31G* level. Ab initio methods were used to calculate the relative energies of the rotamers in the chair conformations of 2-alkylthiacyclohexanes (CH 3 , C 2 H 5 , i-C 3 H 7 , t-C 4 H 9 , neo-C 5 H 11 , SiMe 3 ). The MP2/6-31G*// 6-31G* conformational energies (ÀDG°or A values, kcal mol À1 ) of the 2-alkylthiacyclohexanes (Me = 1.00; Et = 1.16; i-Pr = 1.02; t-Bu = 3.56; neo-Pent = 1.04; SiMe 3 = 2.05) are smaller than those calculated for the corresponding alkylcyclohexanes and 4-alkylthiacyclohexanes. Plots of the calculated conformational energies (ÀDG°) for the 2alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope = 0.680 and r = 0.983 for 6-31G* and slope = 0.667 and r = 0.989 for MP2/6-31G*// 6-31G*). The C(2)-S(1) bond lengths are in the range 1.827-1.840 A ˚and the C(6)-S(1) bond lengths are in the range 1.815-1.819 A ˚. With the exception of the axial 2-t-Bu substituent (103.0°), the C-S-C angles vary from 98.6°to 100.4°. The S(1)-C(2)-C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer.