Non-oxidative photoreaction of 1,3,5-triaryl-2-pyrazoline-4-ol (4-methoxy) derivatives

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• A suitable procedure for the synthesis of 2-pyrazoline-4-ol (-methoxy) derivatives is proposed. • Non-oxidative photochemical reaction of 2-pyrazoline-4-ol (-methoxy) derivatives in CHCl 3 is occurred. • The electronic nature of the substituent on the 4- and 5-positions on the photochemical behavior is studied. • An efficient mechanism for the electron-transfer induced photoelimination reaction in chloroform is proposed. • DFT computational studies (optimization, orbital contour maps, MEP, NBO) at the B3LYP/6-31++(d,p) level of theory on these compoundsare reported. A series of 4-hydroxy- or 4-methoxy-substituted 1,3,5-triaryl-2-pyrazolines were synthesized and their structures, especially trans -orientation of 4- and 5-hydrogens of the 2-pyrazoline ring were characterized by analysis of IR, 1 H NMR, 13 C NMR spectra as well as the single-crystal structure study. In order to elucidate the effect of 4-hydroxy- or 4-methoxy-substitution on the photochemical behavior of these compounds, they were exposed to the UV light. The results explain the occurrence of a non-oxidative photoreaction, by the expulsion of 4-substitution and elimination of a molecule of water or methanol under the formation of the aromatized pyrazole ring. Density functional theory (DFT) calculations were carried out to support the proposed electron-transfer induced photoreaction mechanism. Natural bond orbital (NBO) analysis explains the preferred electron detachment process from the N 1 -atom rather than the oxygen atom of the hydroxy or the methoxy groups. Proton detachment from the involved radical cation species leads to the formation of the radical intermediate centered at the C 5 -position of the heterocyclic ring. Elimination of the hydroxy or the methoxy group accomplished the photoreaction under the formation of the C 4 -unsubstituted pyrazole molecule.