Carbon-13 nuclear magnetic resonance examination of benzonorbornene derivatives. Assignment of site of aromatic ring substitution in benzonorbornen-2-ones

¹³C nuclear magnetic resonance spectra were collected for a series of 5- and 6-monoaromatic ring-substituted benzonorbornadienes and 5-, 6-, 7-, and 8-monosubstituted benzonorbornen-2-ones. ¹³C chemical shifts values were found to be useful in the differentiation of the four monosubstituted aromatic ring isomers of benzonorbornen-2-one (1). We found that the ¹³C NMR spectrum of a substituted benzonorbornen-2-one (A) could be predicted with good accuracy (R² for each aromatic carbon ≥ 0.925) from a knowledge of the ¹³C NMR spectrum of 1 and the appropriately substituted benzene. The substituents studied were NO₂, NH₂, I, CF₃, CN, OCH₃, and H. Correlation analysis showed that the carbonyl in A was effectively insulated from the ring π-system.