Heteropentalenes. The oxidation of pyrazolo[1,2-a]benzotriazoles

Abstract

The dye-sensitized photo-oxidation of 1,3-dimethylpyrazolo[1,2-a]benzotriazole (3) affords 4-benzo-triazol-1-yl-3,4-epoxypentan-2-one (5) and 4-benzotriazol-1-ylpent-3-en-2-one (6), as well as the corresponding benzotriazol-2-yl derivatives (7) and (8), in inert solvents, while in methanol the methoxy-hydroperoxides (9) and (10) are obtained. The mechanism of the reaction as well as the effects of steric hindrance (substituting a t-butyl for a methyl group in the starting material), of the temperature, and of trapping agents, have been investigated. A comparison with other oxidative methods shows that under the conditions in which singlet oxygen is liberated from diphenylfuran endoperoxide, the ketones (6) and (8) are formed together with the nitroso-derivative (16), while peracid oxidation affords exclusively product (16).