3 J (H,H) Coupling Constants as a Simple Criterion for π Delocalization of Pentafulvenes and Pentafulvalenes

A simple criterion for estimating the extent of π delocalization in the five‐membered ring of pentafulvenes and pentafulvalenes is described. It is based on the fact that changes of bond lengths (induced by exocyclic substituents R 1 ‐R 2 of 1 ) are reflected by systematic changes of 3 J (H,H) values, so that linear correlations of σ vs , 3 J (H,H)are obtained. Plots of that type ( Fig. 1 ) are very useful for determining the extent of π delocalization of various pentafulvalenes 2 – 5 ( Fig. 3 ) which show a very similar behavior to pentafulvenes. In principle, these plots could additionally be used for estimating substituent constants σ or for approximating the extent of π overlap between exocyclic substituents and the π system of pentafulvenes. Charge‐density effects of pentafulvenes and pentafulvalenes are observed by substituen‐induced shifts of the ring C‐atoms ( Fig. 4 ).