1 H‐and 13 C‐NMR Investigation of Pentafulvenes

1 H‐ and 13 C‐NMR spectra of a series of 6‐( p ‐X‐phenyl)pentafulvenes 1 – 9 as well as of 6‐R‐substituted and 6,6‐R 1 ,R 2 ‐disubstituted pentafulvenes 10 – 23 have been analysed. It turns out that the π‐system of pentafulvenes is an attractive probe for the investigation of electronic substituent effects. Changes of vicinal H , H ‐ coupling constants with increasing electron‐donating capacity of the substituents X and R are interpreted in terms of an increasing π delocalisation in the 5‐membered ring, and linear correlations of Hammett Substituent constants σ   p +or MNDO‐calculated C–C bond lengths and 3 J values are observed. On the other hand, a systematic high‐field shift of 13 C chemical shifts of the ring C‐atoms is induced by electron‐releasing substituents R and X, which decreases in the series C(5) > C(2)/C(3) > C(1)/C(4), and which mainly reflects changes in π‐charge density.