A ¹³C NMR spectroscopy study of the structure of N-H pyrazoles and indazoles

Abstract

The 13C chemical shifts and some 1H–13C coupling constants of 23 pyrazoles and indazoles are reported and discussed. The spectra were recorded in CDCl3 and in DMSO-d6 solutions and, in several cases, also in the solid state (CP/MAS technique). These data allow us to determine the tautomerism of these compounds in the solid state (usually only one tautomer) and in solution. The position of the tautomeric equilibrium is related to the Hammett σm value of the 3(5)-substituent.

The structure in the solid state and in solution of 3(5)-trifluoromethyl-4,5(3)-polymethylenepyrazoles

“…Note that the signal of the C7a(C8a) carbon atom is much more dependent on the saturated ring than C3. In general and by analogy with literature results [3,7,8], the signals of b tautomers, 2b, 4b and 6b, are expected at different positions.…”

“…J HH (in this kind of symmetrical spin-system, observation of a triplet for a methylene group does not imply necessarily that the coupling constants are equal). The assignments are based on literature results on compounds 1, 3 and 5 and their Nmethyl derivatives [7,8], on NMR studies of trifluoromethylpyrazoles [3] as well as on 2D NMR experiments. The 15 N signal is that of the NH, the other one is not observed.…”

“…The aim of this work is to determine if the structural information can be transferred from the well-known methyl pyrazoles, 1, 3, 5, to the less studied trifluoromethylpyrazoles, 2, 4, 6 [3][4][5]. Regarding 3(5),4-polymethylenepyrazoles (the IUPAC name of tetramethylene-pyrazoles is tetrahydroindazoles and pentamethylenepyrazoles are also named hexahydrocyclohepta[c] pyrazoles), they are known but not with perfluorinated substituents [6][7][8][9].…”

On the solvatochromism, dimerization and tautomerism of indazole

“…2 The fact that these two nitrogen sites are located in adjacent positions in the molecular structure (see Scheme 1) allows indazole at low concentrations in mildly acid or basic media to form 1H and 2H tautomers via acid-base interactions (see Scheme 1), [3][4][5][6][7][8][9][10] as well as symmetrical dimers by hydrogen bonding. …”

The structure of the agrochemical fungicidal 4-chloro-3-(3,5-dichlorophenyl)-1H-pyrazole (RPA 406194) and related compounds

“…In the solid state, several situations have been found: pure a, pure b, and 50:50 mixtures of a and b. 8,10,12,14 The 13 C chemical shifts of pyrazoles 1 -4 are gathered in Table 1.…”