Dimethoxymethane conformation

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...

Even molecules as simple as dimethoxymethane give evidence of anomeric effects. The preferred conformation of dimethoxymethane aligns each C—O bond with a lone-pair orbital of the adjacent oxygen. ... 

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. 

Anet and collaborators (46) as well as Dale and co-workers (47) have also shown the 8-membered ring, 1,3-dioxacyclooctane and some derivatives, to exist only in the boat conformation 47. Conformer 47 has two anomeric effects and is virtually the same as that found in 40 or in dimethoxymethane. 

We will first examine what miqht occur when stereoelectronic effects are taken into account. An analysis of the conformations D, A, and (Fig. 17) of dimethoxymethane which have two, one, and zero anomeric effects respec-... 

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... 

Y. R. Huang, S. Knippenberg, B. Hajgato, J.-P. Francois, J. K. Deng, and M. S. Deleuze, Imaging momentum orbital densities of conformationally versatile molecules A benchmark theoretical study of the molecular and electronic structures of dimethoxymethane, J. Phys. Chem. A, 111 (2007) 5879-5897. 

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. 

Furthermore, there are four minima on the conformational surface representing the conformations of the type (ap, sc), and one minimum corresponding to the ap, ap) conformation. The map calculated for methanediol is similar, but, in contrast to that for dimethoxymethane, the ap, ap) con-former of methanediol is found to be a local maximum. A potential surface has also been reported for methoxymethanol, a structural intermediate between dimethoxymethane and methanediol, with lessened symmetry due to the presence of two different rotors. For example, conformations ap, sc) and sc, ap) can be distinguished in this case, and both are only doubly degenerate. 

XIII. A qualitative rationalization of the conformational preference in CHjOCHjX, where X = F, SCHj, CH3, NHj, and NHJ, can be advanced along the same lines as for dimethoxymethane and chloromethoxymethane, with the contributions of the dipolar, delocalization, and intrinsic torsional terms. 

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. 

Because dimethoxymethane has a 2-fold axis of symmetry in the (5C, jc) and (ap, ap) conformations, the two outer, and similarly, the two inner, C-O bonds are indistinguishable. In 2-methoxyoxane, the calculations suggest that all four C-O bond-lengths at the acetal center are also different in the... 

Dimethoxymethane prefers a conformation that allows alignment of an unshared pair on each oxygen (donor) with a C-0 a orbital on the other. This condition is met in the conformation labeled gauche, gauche. In contrast, the extended hydrocarbon-like anti, anti conformation does not permit this alignment. 

The idea of competition between the anomeric interactions is strongly supported by an ab initio study (67) on proton affinities of oxygens in the sc, sc and ap, sc conformers of dimethoxymethane (11) (Figure 3). Deslong-champs (7a) suggested that oxygens involved in o hyperconjugative... 

Praly and Lemieux (66) have estimated the exo anomeric effect to be 1.66 times as strong for j3-anomers ap, sc here competition between anomeric effects cannot operate) as for a ones (sc, sc). In order to evaluate the corresponding relation between anomeric effects in an acyclic system, let us return to dimethoxymethane (11, Scheme 9). The anomeric effect A for the ap, sc conformer of 11, calculated to be 21.1kJ/mol (cf. Scheme 6),... 

Let us focus on molecules I4a and 14b (Scheme 8) in which an anomeric effect (in AG° or AH° terms) is possible in the vapor phase. Usually, the dipole moment of 14a is less than that of 14b (125-128). Since the more polar form is stabilized in solution (114, 128) [as a rule, with several exceptions (29, 129) vide infra] it may happen that the anomeric effect (i.e., increased preference for 14a) disappears in solution or may even change its sense (from AG g > 0 to AG g < 0). Such a possibility has been supported by quantum chemical calculations for dimethoxymethane (128). It was shown that, while in CCI4 solution dimethoxymethane (11) exists almost exclusively as the most stable sc, sc conformer, in aqueous solution almost exclusive occurrence of the ap, ap conformation may be expected. Hence, all considerations regarding the presence, magnitude, or origin of the anomeric effect must take... 

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