Basicity of C-Substituted Pyrazoles in the Gas Phase: An Experimental (ICR) and Theoretical Study

Article abstract of DOI:10.1021/jo00040a040

The experimental gas-phase proton affinities (PAs) of 32 N-H and N-methyl pyrazoles have been determined by means of Fourier transform ion cyclotron resonance spectroscopy (FTICR).Together with the previously reported PAs for 12 C-methyl-substituted pyrazoles, they provide a set of 57 data (counting each tautomer separately).The remarkable large spread of PAs, ca. 55 kcal*mol^-1, makes this set most suitable for structural analyses.In a few cases, ab initio 6-31 G//6-31G protonation energies were calculated and found to be linearly related to the experimental PAs to a very high degree of precision.A simple additive model of substituent effects on PAs (including substitutions at positions 3,4, and 5) was found to hold, even for very crowded derivatives such as 1,4-dimethyl-3,5-di-tert-butylpyrazole (27).The only significant interaction appears between phenyl groups at positions 3 and 5.The statistically averaged substituent effects on PAs were successfully analyzed in terms of polarizability and field and resonance contributions, according to the Taft-Topsom model.Both positions 3 and 5 behave in a way similar to that of position 2 in the pyridines.From this interesting result it follows that, with the exception of 3-aminopyrazole, the tautomerism of pyrazoles is not very dependent of the nature of the 3(5)-substituent.