3J(H,H) COUPLING-CONSTANTS AS A SIMPLE CRITERION FOR PI DELOCALIZATION OF PENTAFULVENES AND PENTAFULVALENES

A simple criterion for estimating the extent of pi-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 sigma-p+ vs.3J(H,H) are obtained. Plots of that type (Fig.1) are very useful for determining the extent of pi-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 sigma-p+ or for approximating the extent of pi-overlap between exocyclic substituents and the pi-system of pentafulvenes. Charge-density effects of pentafulvenes and pentafulvalenes are observed by substituent-induced shifts of the ring C-atoms (Fig.4).