Basicity of 1‐nitroaryl‐4,5‐dihydropyrazoles: p K a ­measurements and theoretical calculations

The basicity of simple 4,5‐dihydropyrazoles (Δ 2 ‐pyrazolines) was previously discussed on the basis of protonation at position 1 in the case of 1‐unsubstituted, 1‐methyl and 1‐phenyl derivatives. The p K a of 15 4,5‐dihydropyrazoles substituted at position 1 by p ‐nitrophenyl, 2,4‐dinitrophenyl and 2,4,6‐trinitrophenyl groups have now been determined. After examining some linear free energy relationships, to discuss these p K a values further, DFT theoretical calculations, including temperature effects, were carried out on the parent compounds (no C ‐substituents) for the 1‐unsubstituted, 1‐methyl, 1‐phenyl, 1‐ p ‐nitrophenyl and 1‐(2′,4′,6′)trinitrophenyl series. These calculations predict an inversion of N‐1 and N‐2 basicity between 1‐phenyl and 1‐ p ‐nitrophenyl‐4,5‐dihydropyrazoles. Since there were no experimental data for the protonation of 4,5‐dihydropyrazoles in the gas phase, chemical ionization mass spectrometry was used to try to determine the structure of protonated 1‐methyl‐ and 1,3‐dimethyl‐5‐phenyl‐4,5‐dihydropyrazoles ( 3 and 7 , respectively). In both cases, it appears that these pyrazoles are protonated on N1, but the production of another isomeric species cannot be completely ruled out for 7H + . Copyright © 2000 John Wiley & Sons, Ltd.