On the 4JHH long-range couplings in 2-bromocyclohexanone: conformational insights

Abstract

2‐Bromocyclohexanone is a model compound in which a 4J H2, H6 coupling constant is observed, whereas the corresponding 4J H2, H4 is absent. The observed long‐range coupling is not only a result of the known W ‐type coupling, in the axial conformation, but also because of the less usual diaxial spin–spin coupling in the equatorial conformer. The carbonyl group plays a determining role in describing the coupling pathway, as concluded by natural bond orbital (NBO) analysis; although the equation image and equation image interactions in the axial conformer contribute for transmitting the spin information associated with the W ‐type coupling, the strong equation image and equation image hyperconjugations in the equatorial conformer define an enhanced coupling pathway for 4J H2, H6, despite the inhibition of this coupling because of equation image interaction and the large carbonyl angle. These findings provide the experimental evidence that orbital interactions contribute for the conformational isomerism of 2‐bromocyclohexanone.