Bond strength, anomeric effect

The main factor leading to the recent unpopularity of the electrostatic explanation is the failure to account for perceived patterns of bond length changes, even though with more data now available, it is clear that these are not simple. A further weakness of the electrostatic explanation was highlighted by Romers et al. (1969) who showed there were quantitative discrepancies between experiment and the strength of the anomeric effect... 

The importance of orientation of the lone electron pairs on nitrogen (involved in anomeric interaction in H—N—C—O systems) for the strength of the anomeric effect about the C—O bond is discussed for compound 83 in Section II.I. 

Figure 6.63 (a) Effects of anomeric interactions in CFJ- molecules on geometry and C-F/C-H bond strength. Data are from ref 105c. (b) Anomeric interactions in CF and CFfOR molecules. 

Since the l,2-dioxan-3-ol is a hemiketal structme, it is considered that it exists as an equiUbriiun mixture of an OH-axial and an On-equatorial diox-ane in solution (Scheme 28). hi fact, the phenomenon was observed in the case of the 4-carbamoyl-l,2-dioxan-3-ols (Schemes 11 and 12) [83]. It seems that the equiUbrium depends on the balance of the anomeric effect and the strength of the hydrogen bonding between the hydroxyl group and the carbonyl function. 

On the other side of the debate, a recent review, highlights that C-F bond is significantly polarized, has unusual stability (strength) and interacts electrostatically with its environment." " In addition, the principles which direct the conformational preferences of C F bonds in organic molecules are rationalized in terms of the gauche and anomeric effect. In general the introduction of... 

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