1-Phenyl-3-methyl-5-N-benzylideneaminopyrazoles. Substituent effects and protonation sites studied by NMR and ab initio(6-31G*) MO calculations

Abstract

1-Phenyl-3-methyl-5-N-benzylideneaminopyrazole and its derivatives 11 prepared by condensation of 1-phenyl-3-methyl-5-aminopyrazole and aromatic aldehydes have been studied by multinuclear (1H, 13C, 14/15N and 17O) magnetic resonance spectroscopy. The 13C NMR chemical shifts and the direct spin–spin coupling constants 1J(C,H) of the azomethine carbon of these Schiff bases (SB) correlate significantly with the Hammett substituent constants, σp, of the para-substituents in the aryl ring bound to the azomethine carbon. The assignments of the 15N NMR chemicals shifts of SBs in CDCl3 were based on 2J(N,H)s observed for the azomethine nitrogen as well as 1H, 15N HMBC experiments. Based on the present 1H, 13C and 15N NMR data these SBs can be transformed to single and double protonated forms in trifluoroacetic acid (TFA). The protonation sites (the first one at the unsubstituted nitrogen of the pyrazole ring and the second one at the azomethine nitrogen) deduced from the NMR data are supported by ab initio MO calculations at HF/6-31G* level with a full geometry optimization performed for a model compound, 1,3-dimethyl-5-N-benzylideneaminopyrazole.