Substituent Effect versus Aromaticity─A Curious Case of Fulvene Derivatives

Abstract

A computational study on amino- and nitro-substituted penta- and heptafulvenes reveals the interplay between the aromaticity and the substituent effect (SE). Ring substitution alone has little influence on the aromaticity, but in combination with an exo substituent of opposite properties, it substantially enhances the cyclic π-electron delocalization. Despite the SE being stronger for β substitution, only γ substitution leads to higher aromaticity. An explanation is provided by the electron density of delocalized bonds (EDDB) method, which proves to be a valuable tool in analyzing both cyclic delocalization and the SE.

Substituent effects and electron delocalization in five-membered N-heterocycles

“…It resembles the situation in heptafulvenes, which are non-aromatic conjugated molecules with exocyclic double bond and π-electron excess in the ring; similarly, strongly electron-withdrawing exocyclic substituent can enhance the cyclic delocalization. 63 The HOMO and LUMO orbital energies of all studied heterocycles correlate with the properties of substituents evaluated by cSAR(X) (Fig. S10, ESI†).…”