CONFORMATIONAL ANALYSIS .18. 1,3-DITHIANES . CONFORMATIONAL PREFERENCES OF ALKYL SUBSTITUENTS AND CHAIR-BOAT ENERGY DIFFERENCE

Conformational preferences of alkyl substituents at various positions in the 1,3-dithiane system have been determined by acid-catalyzed equilibration of diastereoisomeric di- and trialkyldithianes. The results are supported by evidence from nuclear magnetic resonance spectroscopy. For alkyl groups at positions 2 and 4, free-energy differences (–∆G° values) between equatorial and axial locations are of the same magnitude as in cyclohexane. In contrast, the preference of alkyl substituents at position 5 for the equatorial orientation is much less than in cyclohexane. The equatorial-axial free-energy differences (in kilocalories per mole) are Me-2, 1.77; Me-4, 1.69; Me-5, 1.04; Et-2, 1.54; Et-5,0.77; i-Pr-2, 1.95; i-Pr-5, 0.85; t-Bu-2,⋝ 2.7. The chair-skew boat enthalpy and entropy differences for 2,5-di-t-butyl-1,3-dithiane have been determined to be 3.42 kcal/mol and 5.3 cal/deg mol, respectively, by a study of the dependence of equilibrium upon temperature. The results are interpreted in terms of the known geometry of the 1,3-dithiane ring, the relative space requirement of sulfur compared with a methylene group and the known barrier to rotation about C-S bonds. © 1969, American Chemical Society. All rights reserved.