The anomeric effect revisited. A possible role of the CH/n hydrogen bond.
Ab initio MO calculations were carried out at the MP2/6-311++G(d,p) level to investigate the conformational energy of 2-substituted oxanes and 1,3-dioxanes. It has been found that the Gibbs free energies of the axial conformers are smaller than those of the corresponding equatorial conformers in every case when the 2-substituent Z is electron withdrawing (OCH(3), F, Cl, Br). The difference in Gibbs energy between the equatorial and axial conformers DeltaG(eq-ax) increases from Z=OCH(3) to F, Cl, and then to Br. In the axial conformers, the interatomic distance between Z and the axial C-H, separated by four covalent bonds, has been found to be appreciably shorter than the van der Waals distance, suggesting the importance of the five-membered CH/n (CH/O or CH/halogen) hydrogen bond in stabilizing these conformations. Natural bonding orbital (NBO) charges of the relevant atoms have been shown to be different between the two conformers: more positive for H and more negative for C in the axial conformers than in the corresponding equatorial conformers. In view of the above findings, we suggest that the CH/n hydrogen bond plays an important role in stabilizing the axial conformation in 2-substituted oxanes and 1,3-dioxanes, and by implication, in the anomeric effect in carbohydrate chemistry.
Takahashi
Osamu
O
Department of Chemistry, Graduate School of Science, Hiroshima University, Kagamiyama, Higashi-Hiroshima 739-8526, Japan. shu@hiroshima-u.ac.jp
Yamasaki
Katsuyoshi
K
Kohno
Yuji
Y
Ohtaki
Ryuta
R
Ueda
Kazuyoshi
K
Suezawa
Hiroko
H
Umezawa
Yoji
Y
Nishio
Motohiro
M
D004146:Dioxanes / Q000737:chemistry*; D006860:Hydrogen Bonding; D009682:Magnetic Resonance Spectroscopy; D008956:Models, Chemical; D008968:Molecular Conformation; D013237:Stereoisomerism; D013816:Thermodynamics
D004146:Dioxanes