Transmission of polar substituent effects through the bicyclo[2.2.2]octane ring system as monitored by NMR shifts: A 13C NMR study of 4‐substituted‐1‐methylbicyclo[2.2.2]octanes

Abstract

AbstractCarbon‐13 substituent chemical shifts (SCS) are reported for the methyl group of a series of 4‐substituted 1‐methylbicyclo[2.2.2]octanes covering a wide range of electronic substituent effects. Unlike the 13C SCS of the α‐carbon centre in various unsaturated side‐chain probes attached to the bicyclo[2.2.2]octane ring, which are mainly proportional to substituent field effects (σF), the 13CH3, SCS are found to display a definitive statistical dependence on substituent electronegativity effects as well as the field parameter. Compelling support for the validity of the statistical dissection by multivariate analysis is provided by an independent measure of polar susceptibility parameters (ρF; CDCl3, and CCl4). The coefficient (A) of the Buckingham equation for linear electric field effects on the CH3, group is calculated for CCl4 as the solvent. The origin of the reverse substituent electronegativity effect is discussed, and compared with the similar phenomenon displayed by the resonance contribution to the 13C SCS of the β‐carbon of p‐substituted ethylbenzenes.