Stereoelectronic interaction effects (associated with the anomeric effects) on the conformational properties of 2-methylaminotetrahydropyran, 2-methylaminotetrahydrothiopyran, 2-methylaminotetrahydroselenopyran and their analogous containing P and As atoms: An Ab initio study and NBO analysis

Abstract

NBO analysis, density functional theory (DFT: B3LYP/6-311+G∗∗//B3LYP/6-311+G∗∗) and ab initio molecular orbital (MO: MP2/6-311+G∗∗//B3LYP/6-311+G∗∗) based methods were used to study the anomeric effects ( AE) on the conformational properties of 2-methylaminotetrahydropyran ( 1) [its analogous containing P ( 2) and As ( 3) atoms], 2-methylaminotetrahydrothiopyran ( 4) [its analogous containing P ( 5) and As ( 6) atoms] and 2-methylaminotetrahydroselenopyran ( 7) [its analogous containing P ( 8) and As ( 9) atoms]. B3LYP/6-311+G∗∗//B3LYP/6-311+G∗∗ results revealed that the Gibbs-Free energy difference ( G ax– G eq) values (GFED ax–eq) between the axial and equatorial conformations decrease from compound 1 to compound 3. On the other hand, based on the optimized ground state geometries using B3LYP/6-311+G∗∗ level of theory, the NBO analysis of donor–acceptor (bond–antibond) interactions revealed that the AE for compounds 1– 3 are −8.16, −9.87 and −11.31 kcal mol −1, respectively. The increase of the AE could fairly explain the decrease of the calculated GFED ax–eq from compound 1 to compound 3. Also, the correlation between the AE and GFED ax–eq values for compounds 4–9 has investigated. It should be noted that contrary to the large AE values in compounds 1–9, the instability of the axial conformations of these compounds could be explained by the steric repulsions which arise essentially from the 1, 3-syn-axial repulsions between the attached groups to C-2 and 4, 6-methylene groups.