Ab initio molecular orbital study of the conformational energies (−ΔG° or A values) of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes)

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

• Publisher: John Wiley and Sons
• Year: 1998
• Tongue: English
• Weight: 107 KB
• Volume: 11
• Category: Article
• ISSN: 0894-3230
• DOI: 10.1002/(sici)1099-1395(1998110)11:11<831::aid-poc75>3.0.co;2-p

✦ Synopsis

Ab initio 6-31G * and MP2/6-31G * //6-31G * methods were used to calculate the relative energies of the rotamers in the chair conformations of 4-alkyltetrahydro-2H-thiopyrans (tetrahydrothiopyrans, thiacyclohexanes, thianes; 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°o r A values, kcal mol -1 ) of the 4-alkylthiacyclohexanes (Me =1.94; Et =1.72; i-Pr =1.62; t-Bu =5.49; neo-Pent =1.39; SiMe 3 =2.76) are similar to those calculated for the corresponding alkylcyclohexanes. Plots of the calculated conformational energies for the 4-alkylthiacyclohexanes versus the calculated conformational energies for the corresponding alkylcyclohexanes are linear (slope =1.01 and r = 0.999 for 6-31G * and slope =1.09 and r = 0.990 for MP2/6-31G * //6-31G * ). The conformational energies are strongly influenced by the magnitude and number of repulsive nonbonded hydrogen-hydrogen interactions and the gauche (synclinal) effects in the conformers. The carbon-sulfur bond lengths are mostly in the narrow range of 1.815-1.818 A ˚. The C-S-C angles vary from 97.1°to 99.4°and the C(3)-C(4)-C(7) angle in the most stable axial conformer is larger that the corresponding angle in its most stable equatorial conformer.