62-Pyrazolines, protonation site

The ambiguity of infrared criteria for the identification of protonation sites in imidazoles and 2-pyrazolines has been discussed by Elguero et al. (1967a), who have also provided new information on the infrared spectra of the hydrochloride of 3,5,5-trimethyl-2-pyrazoline in chloroform solution. There is a shift of the stretching vibration of the C=N bond from 1624 cm in the base to 1649 cm" in the hydrochloride salt in chloroform, i.e., in the N-1 protonated cation. This is analogous to similar shifts of the carbonyl frequency in some amide salts (see page 338). 

Pyrazolines [107] also offer three possible protonation sites, the two nitrogens and C-4. They [Pg.328]

Some other examples of application of 15N NMR spectroscopy include establishing the protonation site of A2-pyrazolines (87MI301-01) and assigning the structures to isomeric N-7 and N-9 substituted purines (86T5073). 

The protonation site of 2-pyrazolines has been determined as N(l) using 13C NMR spectroscopy. This behaviour is not followed for l-phenyl-3-aminopyrazolines which are protonated on N(2) <1995J(P2)1875>. 13C NMR spectroscopy easily differentiates the 1,1- and 1,2-disubstituted pyrazolinium cations 152 and 153... 

The basicity of simple 4,5-dihydropyrazoles (A -pyrazolines) has been discussed on the basis of protonation at N-1 in the case of 1-unsubstituted, 1-methyl, and 1-phenyl derivatives <2000JP0372>. The pA values of 15 4,5-dihydropyrazoles substituted at N-1 by /i-nitrophenyl, 2,4-dinitrophenyl, and 2,4,6-trinitrophenyl groups were determined. After examining some linear free energy relationships, to discuss these values further, DFT calculations, including temperature effects, were carried out on the parent compounds (no C-substituents) for the 1-unsubstituted, 1-methyl, 1-phenyl, l-/)-nitrophenyl, and 1-(2,4,6-trinitrophenyl) series. These calculations predicted an inversion of N-1 and N-2 basicities between 1-phenyl- and l-(/>-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 l,3-dimethyl-5-phenyl-4,5-dihydropyrazoles. The substituent effects and protonation sites of 1-phenyl-3-methyl-5-A-benzylideneaminopyrazoles were studied by NMR and ab initio (6-31G ) MO calculations <1996J(P2)2383>. 

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