Basicity of 1-nitroaryl-4,5-dihydropyrazoles: pK(a) measurements and theoretical calculations

The basicity of simple 4,5-dihydropyrazoles (Delta(2)-pyrazolines) was prev iously discussed on the basis of protonation at position 1 in the case of l -unsubstituted, I-methyl and l-phenyl derivatives. The pK(a) of 15 4,5-dihy dropyrazoles substituted at position 1 by p-nitrophenyl, 2,4-dinitrophenyl and 2,4,6-trinitrophenyl groups have now been determined. After examining s ome linear free energy relationships, to discuss these pK(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. T hese calculations predict an inversion of N-1 and N-2 basicity between I-ph enyl and l-p-nitrophenyl-4,5-dihydropyrazoles. Since there were no experime ntal data for the protonation of 4,5-dihydropyrazoles in the gas phase, che mical ionization mass spectrometry was used to try to determine the structu re of protonated 1-methyl- and 1,3-dimethyl-5-pheny1-4,5-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 (C) 2000 John Wiley & Sons, Ltd.