3. ➢ Fulvenes are cyclic polyenes with odd number of C atoms in the ring with unsaturated exocyclic substituents
➢ According to the size of the ring skeleton: they are named triafulvenes, pentafulvenes, heptafulvenes
➢ Although pentafulvenes are isomers of benzene, they show a surprisingly high reactivity towards a variety of
reagents
➢ Bond length alteration indicate a localized structure
Introduction
DM 1.90D 0.42D 0.48D
4. ➢ Parent fulvenes basically non-aromatic compounds (ref. Neuenschwander): Reason- spectroscopic results,
strongly alternating bond lengths and only a small dipole moment
➢ Fulvenes provide the possibility of dipolar resonance structure that suggest aromatic character in the cyclic
structure (ref. Carey & Sundberg)
Triafulvenes
➢ Theoretical calculations suggest small resonance energy, and the contribution of the dipolar form was about 20%.
➢ It has differently been characterized as being aromatic, nonaromatic and antiaromatic (ref. Tidwell).
Aromaticity of parent Fulvenes
5. Pentafulvene (Fulvene)
➢ The measured dipole moment of 0.424 D indicates very modest negative charge in the five-membered ring with
small 6π electron character
➢ Experimental and theoretical investigations concluded that it is nonaromatic
➢ Electron donor substituents on the exocyclic methylene of pentafulvene enhance conjugation and aromaticity of this
species
Heptafulvene
➢ It has a modest dipole moment of 0.48 D indicating a small contribution by dipolar resonance structure
➢ It has opposite polarization compared to pentafulvene
➢ non-aromatic (ref. Neuenschwander)
Aromaticity of parent Fulvenes
6. Substituent Effects on π-Delocalization of Fulvenes
➢ For triafulvenes and heptafulvenes, EWG at the exocyclic C atom should increase delocalization in the ring as well
as charge separation.
➢ Compared with the planar heptafulvene, heptafulvenolate takes a boat conformation
7. PYQ
➢ For pentafulvenes delocalization and charge separation should be enhanced by electron-donating substituents
➢ unshared electron pair on nitrogen in the amine B is not at all delocalized by resonance because the charge
separated structures constitute an unstable antiaromatic system
CSE 2011
