1,2-DIACYLCYCLOPENTADIENES (2-ACYL-6-HYDROXYFULVENES)

1,2-Diacylcyclopentadienes exist in the mono-enol form. They readily undergo electrophilic substitution. Protonation, mononitration, and monobromination take place at the 4-position, but they, or their anions, couple at the 3-position with diazonium salts, possibly via an initial attack at an oxygen atom. Both the diacylcyclopentadienes and their anions readily form mono- and tribromo-derivatives; no dibromo-compounds were obtained however. Triphenylarsine oxide condenses at the 4-position of dibenzoyl- and dipivaloyl-cyclopentadienes to give arsonium cyclopentadienylides. Aniline reacts with one of the carbonyl groups to form mono-anils. The dibenzoylcyclopentadienide anion reacts at an oxygen atom with methyl chloroformate, to give a fulven-6-yl-carbonate.