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

✍ Scribed by Francisco P. dos Santos; Cláudio F. Tormena; Rubén H. Contreras; Roberto Rittner; Alvicler Magalhães

• Publisher: John Wiley and Sons
• Year: 2008
• Tongue: English
• Weight: 97 KB
• Volume: 46
• Category: Article
• ISSN: 0749-1581
• DOI: 10.1002/mrc.2154

✦ Synopsis

Abstract

A rationalization of the known difference between the ^3, 4^J and ^3, 4^J couplings 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* and σ* antibonding orbitals produce a decrease of three‐bond contribution to both ^3, 4^J and ^3, 4^J couplings. However, the latter antibonding orbital also undergoes a strong σ → σ* interaction, which defines an additional coupling pathway for ^3, 4^J but not for ^3, 4^J. 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 it is of σ*‐type, while in homoallylic couplings it is of π*‐type. Copyright © 2007 John Wiley & Sons, Ltd.