Catalyst-free photooxidation reaction from 1,4-dihydropyridazine to pyridazine under air

Abstract In the inverse electron-demand Diels–Alder (iEDDA) reactions between tetrazines and strained alkenes, a mixture of 1,4-dihydropyridazine isomers are formed first, and they are then oxidized to pyridazines. Although the products of these related oxidation processes converge as pyridazines, the oxidation rate is quite low with some substrates. In this study, we revealed that 1,4-dihydropyridazines formed in the iEDDA reactions were oxidized to pyridazines by simply irradiating with an ultraviolet light under an air atmosphere. Our experimental results implied that singlet oxygen was formed in the course of the reactions to oxidize the 1,4-dihydropyridazine molecules.