Bond lengths anomeric effect

The role of steric effects is unclear but the anomeric effect could also contribute to an increase in electron density at nitrogen. X-ray data for the two TV-acyloxy-TV-alkoxyamides, a urea and a carbamate outlined above show clear evidence, both from bond lengths and conformations, of an anomeric interaction RO-N bonds are short when compared to alkoxyamines. This interaction is responsible for SN1, SN2, homolytic and rearrangement reactions of /V-acyloxy-TV-alkoxyamides (vide infra) and has also been supported computationally. Acyloxylation of the hydroxamic esters results in both pyramidalisation as well as anomeric donation from the... 

In a probe for the presence of stereoelectronic effects in nucleophilic addition to 12 sterically unbiased ketones, calculations have identified subtle bond length differences in the C-Nu bond of the diastereomeric alcohol products, where Nu- = H-or Me-.304 The calculated differences are weak (<1%) but consistent the bond is longer in the major product, acting as a fossil record of the TS. Using microscopic reversibility, the easier bond to cleave (the longer one) is the easier to form. The effect bears comparison with the kinetic anomeric effect in sugars, where such bond length differences in calculation are borne out in X-ray crystal structures. 

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... 

Whatever the epiphenomena that will eventually rationalise these results, it is clear that the simple frontier-orbital picture of the anomeric effect cannot account unaided for all the bond-length changes in a system carefully designed to test it. 

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