On the unusual 2J(C2-H(f)) coupling dependence on syn/anti CHO conformation in 5-X-furan-2-carboxaldehydes.
A remarkable difference for (2)J(C(2)-H(f)) coupling 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(H-H) in homoallylic couplings, which are transmitted by hyperconjugative interactions involving the pi(C=C) electronic system. The homoallylic coupling pathway can be labeled as sigma*(C-H) <-- pi(C=C) --> sigma*(C-H). In the present case, this additional coupling pathway, using an analogous notation, can be labeled as sigma*(C(2)-C(C)) <-- LP(1)(O(1))...LP(2)(O(C)) --> sigma*(C(C)-H(f)) (sigma*(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.
Pérez
Carlos
C
Departamento de Química Física, Facultad de Química, Universidad de La Habana, La Habana 10400, Cuba. cp@fq.uh.cu
Suardíaz
Reynier
R
Ortiz
Pedro J
PJ
Crespo-Otero
Rachel
R
Bonetto
Gloria M
GM
Gavín
José A
JA
García de la Vega
José M
JM
San Fabián
Jesús
J
Contreras
Rubén H
RH
D000447:Aldehydes / Q000737:chemistry*; D004583:Electrons; D005663:Furans / Q000737:chemistry*; D009682:Magnetic Resonance Spectroscopy / Q000379:methods* / Q000592:standards*; D008956:Models, Chemical; D008968:Molecular Conformation; D011789:Quantum Theory; D012015:Reference Standards; D013237:Stereoisomerism
D000447:Aldehydes; D005663:Furans