On the Mechanism of Photodehydrogenation of Aryl(hetaryl)pyrazolines in the Presence of Perchloroalkanes

Abstract

New (1,5-diaryl-3-pyrazolinyl)coumarins have been synthesized. The compounds do not undergo keto-enol tautomeric transformations with changes in the solvent. (1,5-Diaryl-3-pyrazolinyl)coumarins provide dehydrogenation reaction under irradiation in the presence of perchloroalkanes and manifest themselves as effective photogenerators of acidity. Several aspects of photodehydrogenation mechanism have been studied. Oxygen is shown not be involved in the reaction. Polar solvents increase rate of the reaction. The measured rate constants of the photodehydrogenation reactions vary in a significant range according to the structure of pyrazoline. The data correlate with ionization potentials of pyrazolines available from DFT quantum chemical calculations. These results are discussed in terms of proposed scheme of mechanism of pyrazolines photodehydrogenation assuming formation of ion-radical and ion intermediates.

Efficient Photooxidation of Aryl(hetaryl)pyrazolines by Benzoquinone

“…Previously, we found that 3‐coumarinyl(aryl)pyrazolines undergo oxidation during irradiation in the presence of perchloroalkanes (CCl 4 , C 2 Cl 6 ). This photooxidation occurs upon increase in the medium acidity, in full agreement with the proposed pathway given below (Scheme ) .…”

“…Previously, we have found that oxygen does not participate in the photooxidation of pyrazolines in the presence of C 2 Cl 6 . However, different results have been found while studying the photooxidation of pyrazolines 1f,h and 2b,c by benzoquinone (Fig.…”

“…Due to the electron‐donating function of pyrazolines in the photooxidation (Scheme ), introduction of a dimethylamino group into the substrate (compounds 1b and 1k ) increases significantly the rate of photodehydrogenation. A distinct dependence of the photooxidation of aryl(hetaryl)pyrazolines 1 in the presence of C 2 Cl 6 on their ionization potentials IP 1 has been found :…”

“…As shown previously, the photooxidation of pyrazolines in the presence of C 2 Cl 6 is accompanied by acid generation (Scheme ). In particular, acid generation was detected by the opening of the lactone ring of Rhodamine laser dyes . It is seen in Fig.…”

Protonation of meso-Tetraphenylporphyrin and Its β-Functionalized Derivatives by Photogenerators of Acidity in Toluene and Polymer Film

“…[15][16][17] We have previously shown that the irradiation of some organic substrates in the presence of polychloroalkanes is accompanied by the generation of acidity that led to the corresponding acid-catalyzed reactions or to sensory effects. [18] Moreover, photodehydrogenation of aryl(hetaryl) pyrazolines in the presence of carbon tetrachloride or hexachloroethane has been successfully used for optical information recording. [19][20][21] Under such context, we decided to evaluate the behavior of porphyrin macrocycles, in the presence of acid photogenerators in toluene or embedded in polymer films.…”