Photolysis of heptamethylbenzenonium cation in aqueous acids. Preparation of pentamethylcyclopentadiene

Irradiation of heptamethylbenzenium ion in a variety of aqueous acids using light of wavelength greater than 350 nm leads to the formation of pentamethylcyclopentadiene and acetone. In 50% HBF 4 and 70% HClO 4 this photolysis is very clean and can be used as a convenient preparation of pentamethylcyclopentadiene. A mechanism is proposed for the reaction.