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.
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). 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.