Photochemical reactions of chloranil with norbornene,† bicyclo[2.1.1]hex-2-ene and cyclopentene. A novel intermolecular photocycloaddition

Abstract

The irradiation of chloranil (CA), dissolved in benzene, in the presence of norbornene gave the α,β-unsaturated α,γ-dichloro-γ-lactone 1 in 67% yield. Heretofore, such a product had not been observed in a photochemical reaction of CA with an alkene. In addition, bicyclo[2.1.1]hex-2-ene and cyclopentene furnished compounds such as 1. However, conventional products of the reaction of ³CA with alkenes were formed in these cases to a considerable extent as well. As the first step en route to 1, a [4 + 2] cycloaddition of ³CA onto norbornene is proposed giving rise to a diradical of type 23, which undergoes the opening of the six-membered ring originating from CA with formation of a β-oxoketene of type 20. The ring closure of the latter with concomitant [1,2] migration of a chlorine atom completes the sequence. On treatment with methanol, the pseudoacid chloride 1 was converted into the pseudoesters 3 and the ester 4. The structures of 3 and the 2∶1 cycloadduct 12 of cyclopentene onto CA were analysed by X-ray crystallography.