1H‐and 13C‐NMR Investigation of Pentafulvenes

Abstract 1H‐ and 13C‐NMR spectra of a series of 6‐(p‐X‐phenyl)pentafulvenes 1–9 as well as of 6‐R‐substituted and 6,6‐R1,R2‐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 or MNDO‐calculated C–C bond lengths and 3J values are observed. On the other hand, a systematic high‐field shift of 13C 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.