ELECTRONIC INTERACTIONS IMPLIED BY THE NONADDITIVITY OF C-13 SUBSTITUENT PARAMETERS IN 2-SUBSTITUTED CYCLOHEXANONES

The carbon-13 chemical shifts were assigned for all ring positions in cyclohexanone and 4-tert-butylcyclohexanone with H, F, Cl, Br, I, MeO, MeS, MeSe, Me(2)N, Me or tert-butyl in the 2-position. The substituent-induced shifts were calculated by difference from unsubstituted cyclohexanone or 4-tert-butylcyclohexanone. Both stereoisomers (cis and trans) were available for the 4-tert-butylcyclohexanones in all but one case. Comparison of the substituent-induced shifts for the cis (equatorial 2-substituent) and trans (axial 2-substituent) isomers provides stereochemical insight into the interactions between the 2-substituent and the carbonyl group that bring about non-additivity of the substituent effects. In the 2-equatorial isomer, the dipole-dipole interaction between the functional groups causes non-additivities for the C-2 carbon that depend largely on the electronegativity of the 2-substituent. In the 2-axial isomer, hyperconjugation or other orbital interactions between the groups cause non-additivities for the C-2 carbon that depend largely on the polarizability in addition to the electronegativity of the 2-substituent.