Substituent Effect versus AromaticityA Curious Case of Fulvene Derivatives

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.