The effect of carbonyl group in the asymmetry of ^{3, 4}J_CH coupling constants in norbornanones

Abstract

A rationalization of the known difference between the 3,4JC4H1 and 3,4JC1H4 couplings transmitted mainly through the 7-bridge in norbornanone is presented in terms of the effects of hyperconjugative interactions involving the carbonyl group. Theoretical and experimental studies of 3,4JCH couplings were carried out in 3-endo- and 3-exo-X-2-norbornanone derivatives (X = Cl, Br) and in exo- and endo-2-noborneol compounds. Hyperconjugative interactions were studied with the natural bond orbital (NBO) method. Hyperconjugative interactions involving the carbonyl pi*(C2=O) and sigma*(C2=O) antibonding orbitals produce a decrease of three-bond contribution to both 3,4JC4H1 and 3,4JC1H4 couplings. However, the latter antibonding orbital also undergoes a strong sigmaC3--C4 --> sigma*(C2=O) interaction, which defines an additional coupling pathway for 3,4JC4H1 but not for 3,4JC1H4. This pathway is similar to that known for homoallylic couplings, the only difference being the nature of the intermediate antibonding orbital; i.e. for 3,4JC4H1 it is of sigma*-type, while in homoallylic couplings it is of pi*-type.

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

“…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. It is noted that a somewhat similar effect was reported recently,36 where the carbonyl group in norbornanones yielded a difference in 3, 4 J CH SSCCs.…”

On the 4JHH long‐range coupling in 2‐bromocyclohexanone: conformational insights

“…The carbonyl group has shown to play a driving role in the through-the-bridge 3 J C,H couplings (actually 3,4 J C,H couplings) in norbornanone, in which a σ C3 -C4 → σ * C2=O interaction defines an additional coupling pathway for 3,4 J C4,H1 , but not for 3,4 J C1,H4 ; consequently, the former coupling constant was found to be significantly larger than 3,4 J C1,H4 . [12] Since the conformational isomerism of 2-bromocyclohexanone is expected to be governed by orbital interactions, especially those involving the π * C=O orbital, in addition to steric and electrostatic interactions, the investigation of the origins of the through-carbonyl long-range 4 J H2,H6 coupling must provide an experimental insight about the hyperconjugation role in this model system. To corroborate the 1 H NMR findings, theoretical approaches were applied to rationalize the results.…”

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

“…We have previously demonstrated that in mono-and di-fluoro-1,1,2,2,9,9,10,10-octafluoro [2.2] paracyclophane (fluoro-AF4s), a bridge fluorine couples with a large coupling constant (20)(21)(22)(23)(24)(25)(26)(27)(28)(29)(30) Hz) with the aromatic fluorine ortho and syn, and with a somehow smaller constant (20-10 Hz) with the aromatic fluorine which is pseudo-gem to the fluorine ortho and syn. [2] The other large couplings used for the assignment were the ortho coupling of the aromatic fluorines, 28-18 Hz, significantly larger than the meta or para couplings, 12-0 Hz.…”