Dimethoxymethane stabilization

Through a significant rc-backbonding interaction, the coordination of benzene to [Os] (1) serves both to activate the arene toward the electrophilic addition of dimethoxymethane (Table 1, entries 1-4) or 3-penten-2-one (entry 5) and to stabilize the resulting benzenium intermediate 8. If manipulated at low temperature (—40 °C), 8 can be trapped with either a silyl ketene acetal (entries 1, 4, and 5), 2-trimethylsiloxypropene (entry 2), or phenyllithium (entry 3) to yield the substituted 1,4-cyclohexadiene complex 9 [15]. This species can be oxi-... 

According to Gaudemar and Curd, dimethoxymethane is an especially useful solvent for two-stage reactions and they report yields of 70-80% for the Reformatsky reagents derived from a variety of a-bromo esters (equation 2) however, the procedure was unsatisfactory with ethyl a-bromopropionate, methyl a-bromophenylacetate and phenyl a-bromoisobutyrate. The zinc enolates were generally used shortly after preparation and no data on their stability in this solvent were reported. 

The calculation of A//j is now usually done by computational chemistry, but the success of the group equivalent approaches makes an important point. The properties of groups are very similar from molecule to molecule, similar enough to make additivity schemes workable. However, specific interactions, e.g., nonbonded interactions, that depend on the detailed structure of the molecule are not accounted for. Whenever interactions that are not accounted for by the group equivalents exist, there will be a discrepancy between the calculated and actual properties of the molecule. Analyses such as that of Leroy can provide valuable insights and concepts. In particular, they provide a means for recognizing stabilization effects present in reactants, as demonstrated by the calculations for 1,3-butadiene and dimethoxymethane. 

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... 

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... 

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