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

Abstract

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-OSiR(3) 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.

Conformational response of tartaric acid to derivatization: Role of 1,3-dipole-dipole interactions

“…3b, 15,16 Recent detailed study has shown that unusually large populations of the diaxial conformer are found for trans-1,2-cyclohexanediol SiMe 2 t-Bu (11%) and Si(i-Pr) 3 (95%) derivatives. 17 The authors postulate that the quaternary substituent at oxygen atom, such as SiR 3 or t-Bu, introduces significant 1,3 interactions with the adjacent equatorial hydrogen atoms in either diaxial or diequatorial conformation. This is relieved by rotation about the C -O bond toward the eclipsed conformation.…”

Achievement of ring inversion of myo-inositol derivatives due to silyloxy/silyloxy repulsion enhanced by the trans-substituents on both sides

“…2). 5 At first, we postulated that increasing the functional groups surrounding the trans-adjacent OTBS groups would reinforce the steric repulsion of the silyloxy groups and flip the multifunctionalized cyclohexane ring into the axial-rich chair form. Contrary to our expectation, however, all the synthesized bis-O-TBS-protected compounds, 1, 1p, 4, and 4p maintained the original equatorialrich chair form regardless of the introduced places (3,4-or 4,5-positions) and the presence of the benzyl groups.…”

“…2). 5 In the multifunctionalized case, the functionalities except for the silyloxy groups are likely to affect the ring conformation.…”

“…3,4 Recently, a conformational change in a cyclohexane ring was reported by Marzabadi and co-workers based on the NMR experiments of the trans-1,2-bis-silyloxycyclohexanes. 5 They demonstrated that the introduction of triisopropylsilyl (TIPS) and tert-butyldiphenylsilyl (TBDPS) groups into trans-1,2-cyclohexanediol favored the 1,2- . Conformational restrictions of acyclic, tetrahydropyrane, and pyranose derivatives due to the repulsion between adjacent silyloxy groups.…”

Intramolecular Interactions of Trityl Groups

“…[15] It appears that the larger the substituents on silicon, the greater the population of axially substituted conformers. [14,15] The diaxial structure was found by X-ray diffraction for a zirconium(IV) trichloride alkoxide derivative of trans-1,2dihydroxycyclohexane. [14,15] The diaxial structure was found by X-ray diffraction for a zirconium(IV) trichloride alkoxide derivative of trans-1,2dihydroxycyclohexane.…”