Fulvalene
Fulvalene (bicyclopentadienylidene) is the member of the fulvalene family with the molecular formula CH. It is of theoretical interest as one of the simplest non-benzenoid conjugated hydrocarbons. Fulvalene is an unstable isomer of the more common benzenoid aromatic compounds naphthalene and azulene. Fulvalene consists of two 5-membered rings, each with two double bonds, joined by yet a fifth double bond. It has D symmetry. History. An earlier attempt at synthesis of fulvalene in 1951 by Pauson and Kealy resulted in the accidental discovery of ferrocene. Its synthesis was first reported in 1958 by E. A. Matzner, working  under William von Eggers Doering. In this method, cyclopentadienyl anion is coupled with iodine to the dihydrofulvalene. Double deprotonation of dihydrofulvalene with "n"-butyllithium gives the dilithio derivative, which oxidizes with oxygen. Fulvalene was spectroscopically observed at from photolysis of diazocyclopentadiene, which induces dimerization of cyclopentadiene-derived carbenes. The compound was isolated in 1986. It was found to be nonaromatic. Above it dimerizes by a Diels–Alder reaction. Derivatives. Perchlorofulvalene (CClC) is quite stable in contrast to fulvalene itself.