Publications [#233563] of Andrew T. McPhail
Journal Articles
- Eliel, E; Willer, RL; McPhail, AT; Onan, KD, "The Conformational Equilibrium in S-Methylthianium Perchlorate", J. Amer. Chem. Soc., 1974, 96(9), 3021-3022, American Chemical Society (ACS) [doi].
(last updated on 2026/01/21)Abstract:
The energy difference between equatorial and axial methyl in methyl-substituted cyclohexanes was determined very early in the development of conformational analysis and with high accuracy;1,2 the equatorial conformation is preferred by 1.7 ± 0.1 kcal/mol in the liquid phase. For some time thereafter it was believed that equatorial substituents were, quite generally, more stable than axial ones; one of the earliest findings to dispel this notion was the discovery3 that the preferred conformation in thiacyclohexane sulfoxide is the one with axial oxygen (ΔG° = 0.2-1.3 kcal/mol).3 In recent years, quite a number of cases of preferential stability of axial conformations have come to light;4 most of these involve heteroatoms as one of the interacting partners. One that does not and yet lacks equatorial preference of the methyl group is P-methylphosphacyclohexane in which ΔG° for the P-methyl group is near zero.5 In contrast, it has been reported6 that in thiacyclohexylmethylsulfonium salts the Smethyl group is largely equatorial; this claim is, however, based on possibly unsafe considerations of geminal coupling constants of the protons at C(2) and C(6). © 1974, American Chemical Society. All rights reserved.