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.
Departamento de Química Física, Facultad de Química, Universidad de La Habana, La Habana 10400, Cuba. cp@fq.uh.cu
Carlos
C
Pérez
Reynier
R
Suardíaz
Pedro J
PJ
Ortiz
Rachel
R
Crespo-Otero
Gloria M
GM
Bonetto
José A
JA
Gavín
José M
JM
García de la Vega
Jesús
J
San Fabián
Rubén H
RH
Contreras
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