The Chemistry of 5-Oxodihydroisoxazoles. XX Photolysis of 2,4-Diphenylisoxazol-5(2H)-one: Evidence for Singlet and Triplet Pathways

2,4-Diphenylisoxazol-5(2H)-one (2) has been photolysed in the presence of alcohols, amines and in inert solvents, and the products are shown to arise by two competitive singlet state photolytic processes. The minor pathway involves loss of carbon dioxide to give an imino carbene which is captured by nucleophiles: the major pathway involves isomerization to a ketene which is rapidly decarbonylated, and the resultant carbene captured by solvent. The presence of acetone or other triplet sensitizers induces a third competitive pathway involving triplet states.