Why Are Silyl Ethers Conformationally Different from Alkyl Ethers? Chair−Chair Conformational Equilibria in Silyloxycyclohexanes and Their Dependence on the Substituents on Silicon. The Wider Roles of Eclipsing, of 1,3-Repulsive Steric Interactions, and of Attractive Steric Interactions

An NMR study of the diaxial/diequatorial chair equilibrium in a range of silylated derivatives of
trans-1,4- and trans-1,2-dihydroxycyclohexane is reported and discussed with a view to explaining unusually
large populations of chair conformations with axial substituents, noted previously for some monosilyloxycyclohexanes and in some silylated sugars. X-ray diffraction studies of three bis-triphenylsilyloxycyclohexanes are reported and show both axial and equatorial silyloxy groups with the exocyclic bonds eclipsed.
Eclipsing is also suggested by molecular mechanics (MM3) calculations on such derivatives. Both axial
and equatorial tertiary silyl groups have 1,3-repulsive interactions with whatever substituents or hydrogen
atoms are at the two adjacent equatorial positions, and these are relieved by rotation toward the eclipsed
conformation of the exocyclic C−O bond. The three substituents on silicon interact attractively with the
nine atoms at the 3, 4, and 5-positions of the cyclohexane ring and calculations suggest that these stabilizing
interactions are significantly greater in the axial than in the equatorial conformation. An equatorial C−OSiR3
bond with one or two equatorial neighbors has a restricted potential energy well that becomes much broader
when the bond is axial without any equatorial neighbors in the alternative chair. Adjacent silyl groups in the
1,2-disubstituted series interact in a stabilizing way overall in all conformations, this being particularly marked
in the diaxial conformation of the more complex ethers. These factors lead to unusually large axial
populations.