Dependence of the γ Carbon-13 Shielding Effect on Electronegativity

Article abstract of DOI:10.1002/mrc.1270170410

Three γ effects on 13C shielding in 3,3-dimethyltetracyclohexanes as a function of the hetero-atom X have been examined.The γ-anti effect on the equatorial 3-methyl group is small in absolute magnitude but strongly dependent on the polar properties of X.The plot of the 13C shielding of this carbon vs the electronegativity of X is linear, with a slope of -5.8 ppm/electronegativity unit.The γ-gauche effects on the axial 3-methyl group and on the 4-carbon are large in absolute magnitude but have quite different dependences on the polar properties of X.Whereas the shielding of the 4-carbon exhibits a linear dependence on electronegativity (slope -3.5), the axial 3-methyl group shows little dependence (slope crudely -0.7), even though the geometric relationship between X and either carbon is almost the same.Neither gauche carbon shielding appears to be related to the steric properties of X.The polar component of both the γ-anti effect and the γ-gauche effect is interpreted as arising from overlap of appropriately positioned parallel orbitals.For the anti case, the pathway is the familiar zigzag arrangement of bonds.For the gauche case, the pathway may be either through space (the orbitals would be only on X and C-α; for the 4-carbon, this interaction would be through the center of the ring) or through bonds (there are parallel axial orbitals on all four atoms).The absence of a significant polar effect for the axial 3-methyl group suggests that the gauche interaction requires a rigid pathway.The polar component of the general γ-gauche effect is superimposed upon a larger contribution that is essentially independent of the nature of X and may be associated with the removal of the hydrogen on the β-carbon and replacement with the γ-X group.