Interpretation of substituent‐induced chemical shifts in 13C NMR spectra of 2‐substituted norbornadienes. Influence of homoconjugation

Complete signal assignments and an interpretation of the 13C NMR spectra of bicyclo[2.2.1]hepta‐2,5‐diene and of 12 of its 2‐substituted derivatives are presented. The substituent effects observed at C‐1, C‐2 and C‐3 are very similar to those reported for 2‐substituted propenes and monosubstituted benzenes. Transmission of those effects on C‐5 and C‐6 may be at least partially attributed to the homoconjugative interaction between the double bonds in the systems investigated. The relative importance of the field and the resonance effects at particular positions have been evaluated by the dual substituent parameter analysis of the chemical shift data. Relationships between substituent‐induced chemical shifts and electron densities, calculated by the PM3 method, have been found.