Photochemistry of organo-halogenic molecules. Part 20. The effect of cycloalkene structure on the [2 + 2] photocycloaddition to hexafluorobenzene

Abstract

The stereochemistry of photocycloaddition reactions of various cycloalkenes to hexafluorobenzene in cyclohexane solution depends on the structure of the cycloalkene, and [2 + 2] cycloadducts are mainly observed. [1,3]-Photocycloaddition is observed only when a large excess of the cycloalkene is used (hexafluorobenzene: cyclopentene, 1 :8) and at higher concentrations. Photoreactions with indene and 1,2-dihydronaphthalene occur stereospecifically syn, and a cis–syn–cis[2 + 2] cycloadduct is isolated. The photocycloaddition reaction with norbornene proceed exclusively exo, with preferential syn addition accompanied by 5%anti-addition. The stereochemistry of the [2 + 2]-photocycloaddition reactions is changed completely in the case of cyclopentene, where 85%anti addition is observed. The course of anti-photocycloaddition diminishes with increasing ring size of the cycloalkene until in the case of cyclooctene, it is reduced to 53%. The quantum yields of [2 + 2]-photocycloadditions, depend on the structure of the cycloalkene and on the concentration of hexafluorobenzene, the greatest effect being observed during [2 + 2]-cycloaddition with indene.