On the unusual 2 J C 2 H f coupling dependence on syn / anti CHO conformation in 5‐X‐furan‐2‐carboxaldehydes

A remarkable difference for 2 J   C   2 H   fcoupling constant in syn and anti conformers of 5‐X‐furan‐2‐carboxaldehydes (X = CH 3 , Ph, NO 2 , Br) and a rationalization of this difference are reported. On the basis of the current knowledge of the Fermi‐contact term transmission, a rather unusual dual‐coupling pathway in the syn conformer is presented. The additional coupling pathway resembles somewhat that of the J HH in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the π CC electronic system. The homoallylic coupling pathway can be labeled as σ* CH ← π CC → σ* CH . In the present case, this additional coupling pathway, using an analogous notation, can be labeled as σ*   C   2 C   C← LP 1 (O 1 )··· LP 2 (O C ) →σ*   C   C H   f(σ*   C   2 C   C) where O 1 and O C stand for the ring and carbonyl O atoms, respectively. This additional coupling pathway is not activated in the anti conformers since both oxygen lone pairs do not overlap. Copyright © 2008 John Wiley & Sons, Ltd.