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

• Published in: Magnetic resonance in chemistry 2008-09, Vol.46 (9), p.846-850
• Main Authors: Pérez, Carlos, Suardíaz, Reynier, Ortiz, Pedro J., Crespo-Otero, Rachel, Bonetto, Gloria M., Gavín, José A., García de la Vega, José M., Fabián, Jesús San, Contreras, Rubén H.
• Format: Article
• Language: English
• Subjects: Fermi-contact transmission; furfural; geminal NMR couplings; NBO; through space
• ISSN: 0749-1581; 1097-458X
• DOI: 10.1002/mrc.2268

A remarkable difference for 2J C 2H f coupling constant in syn and anti conformers of 5‐X‐furan‐2‐carboxaldehydes (X = CH3, Ph, NO2, 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 JHH in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the πCC electronic system. The homoallylic coupling pathway can be labeled as σ*CH ← πCC → σ*CH. In the present case, this additional coupling pathway, using an analogous notation, can be labeled as σ* C 2C C← LP1(O1)··· LP2(OC) →σ* C CH f(σ* C 2C C) where O1 and OC 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.