The contribution of non-classical CHOC hydrogen bonds to the anomeric effect in fluoro and oxa-methoxycyclohexanes.

In this theory study we demonstrate the dominance of non-classical 1,3-diaxial CHOC hydrogen bonds (NCHBs) dictating a 'pseudo' anomeric effect in selectively fluorinated methoxycyclohexanes and also influencing the axial preference in the classical anomeric exhibitor 2-methoxytetrahydropyran, a phenomenon which is most often described as a consequence of hyperconjugation. Analogues of methoxycyclohexane where ring CH's are replaced by CF can switch to an axial preference and theory methods (NBO, QTAIM, NCI) indicate the dominance of 1,3-CHOMe interactions over hyperconjugation. For 2-methoxytetrahydropyran, it is revealed that the global contribution to the anomeric effect is from electrostatic interactions including NCHBs, not hyperconjugation, although hyperconjugation (n→σ* or n→σ*) remains the main contributor to the exo-anomeric phenomenon. When two and three ether oxygens are introduced into the ring, then both the NCHB interactions and hyperconjugative contributions become weaker, not stronger as might have been anticipated, and the equatorial anomers progressively dominate.