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

Abstract

AbstractA 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 R1‐R2 of 1) are reflected by systematic changes of 3J(H,H) values, so that linear correlations of σ vs, 3J(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).