On the unusual²Jcoupling dependence onsyn/antiCHO conformation in 5‐X‐furan‐2‐carboxaldehydes

Abstract

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.

Difference between 2JC2H3 and 2JC3H2 spin-spin couplings in heterocyclic five- and six-membered rings as a probe for studying σ-ring currents: a quantum chemical analysis

“…It is seldom observed that a negative hyperconjugative interaction33 takes place for a synperiplanar configuration between a lone pair and an antibonding orbital. Similar interactions were discussed for 2 ‐CHO‐pyridine,34 and furfural 35. Doubtless, the ‘acetaldehyde effect’ is one of the main effects leading to the differences between 2 J C 3 H 2 and 2 J C 2 H 3 SSCCs in furan ( 1a ) and in pyridine ( 2b ), since in both cases only the former SSCC is contributed to by that effect.…”

19F chemical shifts, coupling constants and conformational preferences in monosubstituted perfluoroparacyclophanes

“…[20 -22] Since many years ago, it is known that the FC term can be transmitted by an intermediate moiety, [23,24] like for instance, that suggested by Mallory et al, [25] in which a coupling 6 J F -F = 6.4 Hz is transmitted through a phenyl ring. A recently reported unusual case is that of the stereo-chemical dependence of the 2 J CH coupling involving the formyl proton in several 5-X-furfurals [26] where an FC pathway involving the overlap between the lone-pairs of both oxygen atoms is activated for the syn but not for the anti conformations.…”

Dependencies of J-Couplings upon Dihedral Angles on Proteins

“…In fact, in his equation, he included an additional sin(2y) term to improve the fitting. It was later utilized also by Altona [14,16] and in more recent studies [87,[91][92][93][94][95][96][97] particularly devoted to carbohydrates. In ordinary proteins, asymmetry is expected in coupling constants 3 3 J(N,C g ), and 3 J(C 0 ,C g ), which are governed by the dihedral angle w related to the amino acid side-chain torsion [83].…”

“…In this sense, exploring the dependence of a given J-coupling over the entire range of values of the corresponding dihedral angle can be better achieved using ab initio calculations [24,80,[87][88][89][90][91][92][93][94][95][96][97][98][99][100][101][102][103][104]107,[111][112][113][114][115][116][117][118][119][120][121][122][123]. Whether or not the associated Karplus fit will improve over the empirical ones for structure determination critically depends on the ability of the computational model (i.e.…”