title: Effect of remote aryl substituents on the conformational equilibria of 2,2-diaryl-1,3-dioxanes: importance of electrostatic interactions.
abstract: The conformational preference of a variety of 2,2-diaryl-1,3-dioxanes bearing remote substituents on the phenyl rings has been studied via equilibration of configurationally isomeric 2,2-diaryl-cis-4,6-dimethyl-1,3-dioxane epimers, X-ray crystallography, (1)H NOESY analysis, and B3LYP/6-311+G* calculations. When the aryl ring bears a remote electron-withdrawing substituent, the isomer having both the higher dipole moment and the electron-withdrawing group in the equatorial phenyl ring and/or an electron-donating group in the axial ring has the lower energy. These results differ from the conclusions reported in a previous study of similar systems. The conformational energy differences of para-substituted 2,2-diaryl-1,3-dioxanes are linearly related to the Hammett σ values with a slope (ρ) of 0.6. In addition, there is a trend toward longer bond lengths between the C(2) ketal center and the aryl ring as the electron-withdrawing nature of the para-substituent is increased. Electrostatic interactions, rather than a hyperconjugative anomeric effect, appear to be responsible for the conformational behavior of such molecules.
authors: †Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, United States.
authors: William F
authors: WF
authors: Bailey
authors: †Department of Chemistry, University of Connecticut, Storrs, Connecticut 06269-3060, United States.
authors: Kyle M
authors: KM
authors: Lambert
authors: ‡Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States.
authors: Kenneth B
authors: KB
authors: Wiberg
authors: ‡Department of Chemistry, Yale University, New Haven, Connecticut 06520-8107, United States.
authors: Brandon Q
authors: BQ
authors: Mercado
mesh_terms: D018360:Crystallography, X-Ray; D004146:Dioxanes / Q000737:chemistry*; D004583:Electrons; D008958:Models, Molecular; D008968:Molecular Conformation; D013816:Thermodynamics
chemical_list: D004146:Dioxanes