Abstract
A remarkable difference for 2JC2-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* c2-cc ← LP1(O 1)⋯ LP2(Oc) → σ* Cc-Hf (σ*C2 -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.
Original language | English |
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Pages (from-to) | 846-850 |
Number of pages | 5 |
Journal | Magnetic Resonance in Chemistry |
Volume | 46 |
Issue number | 9 |
DOIs | |
Publication status | Published - 1 Sept 2008 |
Keywords
- Fermi-contact transmission
- Furfural
- Geminal NMR couplings
- NBO
- Through space