The title of this post paraphrases E. J. Corey’s article in 1997 (DOI: 10.1016/S0040-4039(96)02248-4) which probed the origins of conformation restriction in aldehydes. The proposal was of (then) unusual hydrogen bonding between the O=C-H…F-B groups. Here I explore whether the NCI (non-covalent-interaction) method can be used to cast light on this famous example of how unusual interactions might mediate selectivity in organic reactions.
Crystal structure of RUGYEX. Click for 3D
The crystal structure revealed a fixed syn orientation between one B-F bond and the formyl C-H bond, with a distance of ~2.36Å (using un-normalised C-H lengths), compared with a value of 2.55Å for the vdW sum. Since hydrogen bonds to fluorine are quite rare, it seems worth applying the NCI method (
DOI for calculation).
NCI surfaces for RUGYEX. click for 3D.
The result indeed shows a strongly blue (= attractive) interaction surface for the CH…F region, and only a tiny one for the adjacent H…H region. A welcome surprise however is the O…O region, which also shows a distinct (strong) attraction. Perhaps this is due to the presence of the electron withdrawing BF3 group on one of the oxygens. This could also be regarded as a through-space anomeric effect, from one oxygen donating into the O-B σ* orbital. Such
through spaceanomeric effects (they are normally propagated through bonds, as in sugars) are probably more common than we believe.
This result nicely illustrates the utility of the NCI method; it brings an immediate visual impact, which serves to highlight interactions which otherwise might be ignored.