Investigation of the Correlation between Geometrical Relationship and the Anomeric effect on Conformational Reactivities and Analyze the 1, 4-elimination

Ab initio molecular orbital hybrid density functional theory (B3LYP/6-311++G**) based methods and natural bond orbital (NBO) interpretation were used to investigate the 1, 4-eliminations and the correlations between the global hardness, global electronegativity, anomeric effect, thermodynamic parameters of 3-chloro-8-methyl-8-azabicyclo[3.2.1]octane (1), 3-chloro-8-methyl-8-phosphabicyclo- [3.2.1]octane (2) and 3-chloro-8-methyl-8-arsabicyclo [3.2.1]octane (3). The calculated global electronegativity (χ) differences between the axial- and equatorial-stereoisomers (Δ [χ (eq) - χ (ax)]) decreased from compound 1 to compound 3. This fact justifies that with the increase of the Lewis acid from the equatorial-stereoisomers of compound 1 to compound 3, the energy difference between the axial- and equatorial-stereoisomers decreases. NBO results showed that the anomeric effect benefits the equatorial stereoisomers. The reactions shown in this work are illustrative of the power of the anomeric effect and the geometrical relationship of the participating bonds. If the rC-Cl bond is axial, the 1, 4-elimination is avoided altogether. Whereas, if the rC-Cl bond is equatorial, the 1, 4-elimination is performed. These eliminations are rendered possible by the antiperiplanar relationship of the breaking central rC-C bond with the electron pair orbital on the heteroatom and the rC-X bond, X being a leaving group such as a halogen.