Cyclic Voltammetric Study of 3,5-Diaryl-1-phenyl-2-pyrazolines.

Cyclic voltammetry is used to derive HOMO energies of the 1-phenyl-2-pyrazolines containing electron-donating or electron-withdrawing substituted phenyl rings and or naphthalenyl substitution on the C3- or C5-positions of the heterocyclic ring to investigate the steric and electronic effects of the aryl substitutions and the type of aryl system on their electrochemical behaviors. The optical HOMO-LUMO gaps needed for the calculation of LUMO (excited state) energies of these compounds are obtained from their UV-vis spectra. Results show that the substitution on the C3-aryl ring has significant effect via its π-donor/acceptor ability, compared to the σ-donor/acceptor ability of the C5-aryl ring, on the CV oxidation peak and onset potentials. Comparative analysis showed very good agreement between the experimentally obtained HOMO and (apparent) LUMO energies and the (TD)DFT/6-311++G(d,p) calculated ground and excited states energies. These computational results indicate also that for all chloro- and methoxy-substituted 2-pyrazolines, the HOMO → LUMO is the most intense transition. While the strong acceptor NO2 substitution on all positions of either C3- or C5-aryl rings, except for one compound, increases the intensity of the HOMO → LUMO+1 (LUMO+2) transition significantly, the first (the first two) transition(s) HOMO → LUMO (and HOMO → LUMO+1) has (have) much smaller or negligible intensity (intensities).

Thermal electron-transfer-induced oxidation of 2-pyrazolines.

Various 3,5-diaryl-1-phenyl-2-pyrazolines were synthesized, and their thermal oxidation to their corresponding 2-pyrazoles was investigated using tetrabutylammonium peroxydisulfate in acetonitrile solution. Compared to the reported oxidative methods, this oxidizing agent provides a clean and non-expensive oxidative reaction in a short reaction time. Based on the proposed reaction mechanism, the extent of co-planarity of the C3-aryl ring toward C3=N2 double bond of the heterocyclic ring affects the electron-donating ability of the heterocyclic ring and decreases the time of oxidative reaction. The experimental results are supported by cyclic voltammetric measurements.