Photoaddition of ethylenes and acetylenes to hexafluorobenzene

Abstract

Both cis- and trans-cyclo-octene undergo stereospecific photoreactions with hexafluorobenzene to give various 1 : 1-cycloadducts which arise via 1,2- and 1,3-addition processes and subsequent isomerisations of the initial adducts. No products arising from 1,4-cycloaddition to the aromatic ring have been detected. Effects of irradiation wavelength, temperature, and concentration are discussed in connection with the involvement of free and complexed S₁ hexafluorobenzene.

In contrast, 2,3-dimethylbut-2-ene and hexafluorobenzene gave mainly an acyclic 1 : 1 adduct formed by substitutive addition, and its dehydrofluorination product. Although maleic anhydride and maleimide readily undergo photoaddition to benzene, neither gave photoadducts with hexafluorobenzene.

Propyne and but-2-yne give cyclo-octatetraenes by photoaddition to hexafluorobenzene, and in the case of but-2-yne the precursor 1,2-cycloadduct was isolated as the sole primary product.

In general, the photoadditions to hexafluorobenzene show some points of resemblance to the corresponding processes involving benzene, but various major differences have been noted.