The effect of carbonyl group in the asymmetry of 3, 4 J CH coupling constants in norbornanones

A rationalization of the known difference between the 3, 4 J   C   4 H   1and 3, 4 J   C   1 H   4couplings transmitted mainly through the 7‐bridge in norbornanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3, 4 J CH 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*   C   2 Oand σ*   C   2 Oantibonding orbitals produce a decrease of three‐bond contribution to both 3, 4 J   C   4 H   1and 3, 4 J   C   1 H   4couplings. However, the latter antibonding orbital also undergoes a strong σ   C   3 C   4→ σ*   C   2 Ointeraction, which defines an additional coupling pathway for 3, 4 J   C   4 H   1but not for 3, 4 J   C   1 H   4. 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, 4 J   C   4 H   1it is of σ*‐type, while in homoallylic couplings it is of π*‐type. Copyright © 2007 John Wiley & Sons, Ltd.