Photochemistry of 1,2,4-oxadiazoles. A DFT study on photoinduced competitive rearrangements of 3-amino- and 3-N-methylamino-5-perfluoroalkyl-1,2,4-oxadiazoles

Abstract

The photoinduced competitive rearrangements of 5-perfluoroalkyl-3-amino(N-alkylamino)-1,2,4-oxadia- zoles have been investigated by DFT calculations and UV-vis spectroscopy. The observed product selectivity depends on the number of hydrogen atoms present in the amino moiety and involves two or three possible routes: (i) ring contraction-ring expansion (RCRE), (ii) internal-cyclization isomerization (ICI), or (iii) C(3)-N(2) migration-nucleophilic attack-cyclization (MNAC). UV absorption and fluorescence spectra of the reactants, and vertical excitation energy values, calculated by time dependent DFT, support the involvement of a neutral singlet excited state in the photoexcitation process. The values of the standard free energy of the most stable prototropic tautomers of reactant, products, proposed reaction intermediates, and deprotonated anionic transition states allowed us to rationalize the competition among the three rearrangements, in agreement with chemical trapping experiments, in terms of: (i) the evolution of the excited state toward three stable ground-state intermediates, (ii) tautomeric and deprotonation equilibria occurring in methanol solution for each intermediate, and (iii) relative stabilization of intermediates and transition states in the thermally driven section of the reaction.