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

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)-Ge-q) 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 I 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. (C) 2009 Elsevier B.V. All rights reserved.