Photocyclization of aryl halides. Part 2. The upper excited state reactivity of 3-(2-halogenoanilino)-1,3-diphenylprop-2-enones; hydrogen-bonded configurational lock in photosynthesis

Abstract

Photocyclization of the internally hydrogen-bonded dibenzoylmethane 2-bromo- and 2-iodo-anils proceeds cleanly, though with low quantum yield, to afford a phenanthridine derivative directly and with very little competition from hydrogen abstraction reactions involving solvent molecules. In contrast, the photoexcited copper complexes of the above anils undergo demetallation as the principal reaction. The cyclization quantum yield from the anils is strongly dependent on the halogen group and is also influenced by the viscosity, but not the polarity, of the solvent. The effects of added air and change of the irradiation wavelength indicate that cyclization proceeds by carbon–halogen bond fission from an upper excited state.