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, σ_p, 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.