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

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• 作者：Cecilia H. Marzabadi ; J. Edgar Anderson ; Jorge Gonzalez-Outeirino ; Piers R. J. Gaffney ; Christopher G. H. White ; Derek A. Tocher ; and Louis J. Todaro
• 刊名：Journal of the American Chemical Society
• 出版年：2003
• 出版时间：December 10, 2003
• 年：2003
• 卷：125
• 期：49
• 页码：15163 - 15173
• 全文大小：201K
• 年卷期：v.125,no.49(December 10, 2003)
• ISSN：1520-5126

文摘

An NMR study of the diaxial/diequatorial chair equilibrium in a range of silylated derivatives oftrans-1,4- and trans-1,2-dihydroxycyclohexane is reported and discussed with a view to explaining unusuallylarge 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 axialand equatorial tertiary silyl groups have 1,3-repulsive interactions with whatever substituents or hydrogenatoms are at the two adjacent equatorial positions, and these are relieved by rotation toward the eclipsedconformation of the exocyclic C-O bond. The three substituents on silicon interact attractively with thenine atoms at the 3, 4, and 5-positions of the cyclohexane ring and calculations suggest that these stabilizinginteractions are significantly greater in the axial than in the equatorial conformation. An equatorial C-OSiR₃bond with one or two equatorial neighbors has a restricted potential energy well that becomes much broaderwhen the bond is axial without any equatorial neighbors in the alternative chair. Adjacent silyl groups in the1,2-disubstituted series interact in a stabilizing way overall in all conformations, this being particularly markedin the diaxial conformation of the more complex ethers. These factors lead to unusually large axialpopulations.