Photocyclization of aryl halides. Part 3. Phenanthridone photosynthesis from 2-halogenobenzanilides

Abstract

2-Chlorobenzanilides cyclize to phenanthridones in deaerated cyclohexane solution on 254 nm irradiation, whereas 2-bromo- and 2-iodo-benzanilides undergo dehalogenative reduction. These observations are understandable in terms of the assisted homolysis model as applied to configurationally flexible molecules. Cyclization in hydrocarbon media is retarded by triplet quenchers and fails in polar solvents. The cyclization can also be singlet and triplet sensitized. 4′-Substituents influence the cyclization quantum yield by altering the π-donor ability of the aniline ring and the rotational barrier about the amide bond.