Dimethoxymethane model

Fig. 3. Variations of the dihedral angles in dimethoxymethane to model the a- or P-anomeric center of an oligosaccharide...

Fig. 4. Modelling of the torsion potential of the exo anomeric effect using the results of the ab initio calculation of dimethoxymethane. The difference between the non bonded terms and the ab initio calculation is fitted with the general function of torsion potentials to yield the equations for the oc-and the P-anomers discussed in Sect. 2.5.3 for the HSEA method...

Under typical polymerization conditions, the total concentration of growing species is e.g. 10 mole F and the concentration of polymer is equal to e.g. 2.5 mole 1. Under these conditions, taking into account the value of Kg = 3 10 mole", we would have 10" mole 1" of oxonium ions (assuming that dimethoxymethane is a suitable model fmonomer molecules converted into polymer, only 10 wouU be added through carbenium ions and the rest throu oxonium ions. The actual data for 1,3-dioxolane are not known at pr nt and may differ from values found for dimethoxymethane. Nevertheless, of primary importance is the finding that the reactivities of alkoxycarbenium ions and tertiary oxonium ions toward linear acetals, expressed through the corresponding rate constants, differ for the discussed above conditions only by 10 times. 

Figure 2. Dimethoxymethane as a model compound for methyl -d and methyl...

The anomeric effect is readily rationalized in PMO terms, using the hybrid model for the lone pairs (see p. 27 and Deslongchamps, 1983). In the g g form of dimethoxymethane [21], one lone pair on each oxygen atom is anti-periplanar to the cr c o orbital involving the other oxygen atom. Thus two favourable n-o interactions exist in this conformation. The generalized anomeric effect was rationalized in similar terms by David et al. (1973) who used the canonical model for the lone pairs. Calculations for the CHCl—O—C system predicted stabilization of conformer [23a] due to the n-cf c-ci interaction by 3.3kcalmol with respect to conformer [23b], a... 

To reproduce an exo-anomeric effect, separate energy functions, Eqs. [3] and [4], were incorporated for a- and p-anomers. The parameters for these equations were derived by fitting to the rotational energy curves for dimethox-ymethane computed with nonoptimized geometries at the HF/4-31G ab initio level.Dimethoxymethane is the simplest chemical model system for a gly-cosidic linkage. 

Figure 6.62 (a) Stereoelectronic basis for the anomeric effect illustrated In dimethoxymethane. (b) Contrasting shapes of pentane and dimethoxymethane ("2,3-dioxapentane"). Both p/sp and sp /sp representations of oxygen lone pairs are shown (in the panels a and b, respectively). NBO analysis finds that the p/sp model for the oxygen lone pairs is preferred. 

The most simple compound that shows the classical anomeric effect is dihydroxymeth-ane. This compound exerts hydrogen bonding characteristics that may complicate the situation, so we will here examine the corresponding ether, dimethoxymethane, as our parent model compound. We will take up the energetic effects first, as historically these were the first indication of the anomeric effect. 

Calculated rotamer states for the acetoxymethyl group at C-5 of gluco-and galacto-pyranosyl rings have been compared with those obtained from A"-ray crystallography and found to be in good agreement. Molecular orbital calculations on dimethoxymethane, a model for the anomeric centre of methyl pyranosides, have produced results consistent with the anomeric and the exn-anomeric effects. The results were compared with evidence obtained by neutron diffraction. Dynamic H n.m.r. methods applied to the... 

Electrochemical oxidation of complex 1 in die presence of methanol leads to considerable enhancement of the oxidative currents (Figure 2), consistent with an electrocatalytic oxidation process. The onset of this catalytic current coincides with the irreversible Pt(II/IV) oxidative wave at 1.70 V. The bulk electrolysis of 1 and dry methanol were performed at 1.70 V (onset of catalytic current) in 0.7 M TBAT/DCE. Gas chromatographic analysis of the solution indicated that dimethoxymethane (DMM, formaldehyde dimethyl acetal) and methyl formate (MF) are formed (Scheme 1). This result is consistent with the electrooxidation of dry methanol on PtRu anodes, which yields DMM after acid-catalyzed condensation of the formaldehyde product with excess methanol (36). Bulk electrolysis of methanol in the presence of heterobimetallic complex 1 resulted in higher current efficiencies than those obtained from the mononuclear model compound CpRu(PPh3)2Cl (2) (Table II), No oxidation products were found when the electrolysis was performed at 1.70 V in the absence of a Ru complex or in the presence of the Pt model compound (ri -dppm)PtCl2. These results suggest that Pt enhances the catalytic activity of the Ru metal center. 

Equilibrium constants for the model reaction between methoxymethylium cation and dimethoxymethane (simple linear model of acetal) have been determined by dynamic NMR studies and were found to be Kga = fea/ d = 3x10 mol (SO2, -70 °C). This value indicates that active species in cyclic acetal polymerization exist predominantly in the form of oxonium ions, although a small proportion exist in the form of alkoxycarbenium ions. 

Figure 1 Conformational relationships between dimethoxymethane (1), 2-methoxytetrahydropyran (2), and 2-ethyItetrahydropyran (3). The equatorial configuration (2e) is a model for -glycosides, the axial (2a) for o-glycoside.s. The labels. sc = synclinal, ap = antiperi-planar refer to the orientations of the C-O-C-0 (endocyclic) and O-C-O-C (exocyclic) dihedral angles...

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