Dimethoxymethane conformational energies

Jeffrey et al. have calculated the energy of several conformations of dimethoxymethan as these are representative examples for the glycosidic bonds in a- or P-anomers (cf. Fig. 3) [59]-The calculations were carried out on the RHF/4-31G level. The obtained potential energies for the conformations of 0°, 90°, 180°, and 270° dihedral angles respectively revealed the presence of a strong exo anomeric effect in acetal... 

Fio. 15.—The Conformational Energy Map of Dimethoxymethane, With Energy Contour in kJ.mol.. Two Conformations ofDimethoxymethane Corresponding to a and e forms of 2-Methoxyoxane Are Also Shown. 

Figure 2 Structures of conformers of dimethoxymethane. The relative energies are given in kcal mol permission of the American Chemical Society...

As we have seen, the anomeric effect confers a double-bond character to each C—0 bond of conformer D the energy barrier for a C —0 bond rotation in acetals must therefore be higher than that observed in simple alkanes. Borgen and Dale (41) may have provided the first evidence for this point by observing that 1,3,7,9-tetraoxacyclododecane (37) has a much higher conformational barrier (11 kcal/mol) than comparable 12-membered rings such as cyclododecane (7.3 kcal/mol (42) or 1,4,7,10-tetraoxacyclododecane (5.5 and 6.8 kcal/mol (43)). It was also shown that the two 1,3-dioxa groupings in 37 exist in a conformation identical to that of dimethoxymethane, i.e. the conformation D. 

Calculations of the effect of the solvent upon the conformational properties of dimethoxymethane ° based on Eq. 7 indicate that, in highly polar solvents, the ap orientation about the C-0 bond might even be preferred. For example, the sc, sc) conformation still prevails when parameters of CCI4 are assumed in Eq. 7, with only the energy difference between sc, sc) and ap, sc) lowered by 2 kJ.moC in comparison to the isolated molecule. In water. 

Detailed information about the anomeric effect has generally come from ab initio MO calculations. The principal rotamers of dimethoxymethane and their relative energies calculated at the HF/6-31G level are shown in Figure 2. The -l-sc,- -sc conformation is the global minimum, and it is interesting to note that it has the smallest dipole moment. The next to lowest energy conformer is +sc,trans, with a relative energy of 10.0 kJ mol" and a dipole moment of 2D. The -Hsc,- -sc conformer corresponds to axial 2-methoxytetra-hydropyran, and +sc,trans corresponds to the equatorial methoxy counterpart. 

Figure 3 A contour plot of the energies of the dimethoxymethane conformers as a function of the two O-C-O-C torsional angles, ri and rj. The energies are given in kcal mol . Reproduced from Ref. 31 with the permission of the American Chemical Society...

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