Molecular Structures of trans-1,2-Bis(arylsulfonate)Cyclohexanes: What Determines the Conformation of Cyclohexane?

Abstract

With the aim of establishing a basic theory for the conformation of substituted cyclohexanes, arylsulfonate-substituted cyclohexanes were synthesized and structurally characterized. In marked contrast to the reported axial–axial (AA) conformation of tosyl-substituted cyclohexane, the cyclohexanes studied here exhibited an equatorial–equatorial (EE) conformation. Theoretical studies, specifically natural bond orbitals (NBO) analysis, suggest that the AA conformation is stabilized by an intramolecular donor–acceptor interaction between the σ_CO orbital and the σ_CH orbital of the adjacent CH bond. In the EE conformation, there is an orbital interaction between the σ_CO orbital of the sulfonate and the σ_CC orbital of the cyclohexane skeleton. Because of these interactions, the energy of the AA and EE conformations are comparable. Therefore, we can conclude that whether arylsulfonate cyclohexane exhibits an equatorial or axial conformation is coincidental, depending on crystallization conditions. Furthermore, we propose that the A-value, which estimates the size of substituents based on the axial–equatorial equilibrium of substituted cyclohexanes, should be reconsidered because previous considerations only estimated the size of the molecule without accounting for orbital interactions.