No abstract

On the basis of improved calculation models, of new NMR measurements with bicyclo[2.2.1]heptyl and cyclohexyl compounds, and of literature data it is shown that classical mechanisms of sterically induced charge polarizations, of linear electric field, and of anisotropy effects can account for many substituent‐induced shielding (SIS) differences. The rǒle of steric distortions on α‐and β‐SIS is discussed; bond angle variations generated by a methyl group introduction at C‐α and subsequent hybridization changes are correlated with Cα‐Me SIS values as well as with the eq/ax shielding in cyclohexanes.

Principal component (PC) or factor analysis was applied to "C substituent effects (literature SCS values) in a to 6 positions of 47 aliphatic frameworks with 4-13 different substituents X. Two PC terms for each substituent X are sufficient to describe 99.3% of all C-a shifts within < f0.6 ppm, even in the largest data matrix containing 13 x 47 = 611 shifts; shifts in /?. y and 6 positions require a third PC term for a similar percentage SCS accuracy. Scaling problems with the different order of magnitudes between a to 6 SCS variations are avoided by analysing the corresponding data sets separately; this approach, together with the large data base, leads to a dominating first PC term for C-a which unambiguously correlates with substituent electronegativities (EN), in contrast to an earlier factor analysis. The same result is obtained by direst comparison of aSCS with different EN scales, including for the first time group electronegativities. PCs for p to y SCS indicate only less consistent trends, which again agree partially with results from direst correlatious, but not with EN values. The present PC analysis did not include stereochemical descriptors, but furnishes clear distinctions between clusters of (pseudo-) equatorially or axially substituted compounds. The PC loadings for C-a also lead to the unambiguous distinction of primary, secondary, tertiary and quaternary carbon atoms.

On the basis of improved calculation models, of new NMR measurements with bicyclo[2.2.1]heptyl and cyclohexyl compounds, and of literature data it is shown that classical mechanisms of sterically induced charge polarizations, of linear electric field, and of anisotropy effects can account for many substituent-induced shielding (SIS) differences. The r61e of steric distortions on a-and PSIS is discussed; bond angle variations generated by a methyl group introduction at C-a and subsequent hybridization changes are correlated with Ca-Me SIS values as well as with the eq/ax shielding in cyclohexanes.The significant shielding differences between gauche and [runs orientations in aliphatic frameworks have initiated many publications. particularly in the field of I3C-NMR spectroscopy". The advent of high-field and of 2D NMR makes 'H-NMR shifts a similarly promising tool even for complicated frameworks such as saturated steroids", and allows for the first time a mechanistic comparison of complementary proton and carbon shieldings.Stereochemical applications of 13C-NMR shifts have been largely based on the shielding of y-carbon atoms by a syn (gauche) alkyl substitutent I), which has been rationalized by Grant and Cheney3' with sterically induced charge polarization along a syn-axial C -H bond. On the basis of more realistic model geometries and of a different equation derived a recognized force-field parametrization we have shown4a' that these steric effects not only correctly predict the substituent induced shieldings (SIS) by X = Me on syny atoms such as c -3 in l a / l b, Id, c-7 in 2a (R = R' = H), or C-6 in 2b, or spn-methyl carbons in 3a, 3b (Table 1) and similarly in steroids4b), but also do allow for deshielding effects on 9-carbon atom^]^.^,'), such as on C-10 in 1c6' or C-8 in 2a7' (R = Me, Table 1). As a necessary consequence of the angular dependence of the polarizing gradient, deshielding at y-carbon atoms will occur for 6 < 90 , which is the case for several interactions in 9 groups such C-8 in 2a (R' = R = Me)4d). If one places the point of action for the steric forces not arbitrarily at the hydrogen end of the polarized C -H bond as it has been done earlier3,4d' (model 1 a), but more towards carbon (model 1 b) -which is a better representation of the polarized electron cloud -it becomes clear, why with Chem. Ber. 122 (1989) 321 -326

No abstract