The effect of carbonyl group in the asymmetry of3,4J CH coupling constants in norbornanones

Abstract

A rationalization of the known difference between the 3,4J C4H1 and 3,4JC1H4 couplings transmitted mainly through the 7-bridge in norbomanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3,4JCH couplings were carried out in 3-endo- and 3-exo-X-2-norbornanone derivatives (X = Cl, Br) and in exo- and endo-2-noborneol compounds. Hyperconjugative interactions were studied with the natural bond orbital (NBO) method. Hyperconjugative interactions involving the carbonyl π*C2 = O and σ*C2=O antibonding orbitais produce a decrease of three-bond contribution to both 3,4JC4H1 and 3,4JC1H4 couplings. However, the latter antibonding orbital also undergoes a strong σC3-C4 → σC2=O interaction, which defines an additional coupling pathway for 3,4JC4H1 but not for 3,4JC1H4. This pathway is similar to that known for homoallylic couplings, the only difference being the nature of the intermediate antibonding orbital; i.e. for 3,4JC4H1 it is of σ*-type, while in homoallylic couplings it is of π*-type. Copyright © 2007 John Wiley & Sons, Ltd.