Electron-transfer induced photo-oxidation of 4,5-dihydro-1,2,4-oxadiazoles: Experimental and computational studies

Abstract

A series of 3,5-disubstituted 4,5-dihydro-1,2,4-oxadiazoles (DHOZs) were considered to elucidate electronic effects of the substitutions on their UV-light sensitivities. Based on the proposed electron-transfer induced photo-oxidation in this study, the rate of photoreaction is dependent more on the electronic nature of the substituent on the C5-position of the heterocyclic ring than on the C3-position. Also, the electron-accepting ability of solvent significantly facilitates this photo-induced electron-transfer process. In addition, results of the computational studies carried out on the species along the photochemical reaction path steps approve that the photo-oxidation reaction proceeds via tunneling electron transfer process from the excited state of DHOZs to the solvent molecule, forming a radical cation complex intermediate, followed by proton transfer resulting in the formation of the radical pair complex intermediate. Structural relaxation of this radical pair intermediate resulted in the formation of the final oxidized product in a single step, which shows that the last step of photo-oxidation reaction consists of the transfer of the second electron/proton (hydrogen) to the solvent. These computational results thus explain how the relative rate of the photo-oxidation reaction affected by the nature of the substitutions. In addition, photo-isomerization of oxadiazole (OZ) containing the trans-5-styryl substitution to the cis-5-styryl isomer and the following photo-cyclization reaction are studied experimentally and computationally.