The dye-sensitized photooxidation of imidazoles: Trapping of intermediates by nucleophiles

Abstract The reactions of a series of imidazoles with singlet oxygen have been investigated. While there is evidence that 2,5-endoperoxides or zwitterionic intermediates may be initially formed, the isolated products appear to arise from the decomposition of hydroperoxides or dioxetanes. Nucleophiles such as methanol and diphenyl sulfide serve to intercept unstable intermediates. Based on the photooxidation of model 5-substituted imidazoles, a rationale is presented for the destruction of histidine residues by two equivalents of oxygen in the “photodynamic” process.