Fluoromethanol, anomeric effect

This is simply the anomeric effect viewed in a different way Consider fluoromethanol (32). It has an HOCF dihedral angle of 60°,26 so the CF bond will be perfectly antiparallel to one of the lone pairs if the oxygen atom is represented by 22. However, care is needed if we depict the oxygen by 21. Conformation 33 is not the best the p-o CF... 

Thus the synclinal conformer of fluoromethanol (121) appears to be preferred due to overlap repulsion as well. This conclusion is of significance, since it provides additional proof for the role of destabilizing interactions in the origin of the anomeric effect. Though the importance of destabilizing interactions involving lone electron pairs seems to be well established even in the... 

If these two conformations are presented pictorially in a different perspective, it becomes clear that the most stable conformation corresponds to axial fluoro-substitution in a tetrahydropyran ring, and the least stable conformation to equatorial fluoro-substitution. The calculated conformational preferences for fluoromethanol then mimic the anomeric effect seen in heterocyclic systems, where electronegative substituents are more stable in the axial than in the equatorial orientation. 

If these two conformations are shown from a different perspective it becomes clear that fluoromethanol is another example of the anomeric (or gauche) effect discussed in Section 3.5. The gauche arrangement permits a lone pair on oxygen to be in an anri-periplanar relationship to the C-F bond. 

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