The tautomerism of 3(5)-phenylpyrazoles: an experimental (1H, 13C, 15N NMR and X-ray crystallography) study

Abstract

~~ 3(5)-Phenyl-and 5(3)-methyl-3(5)-phenylpyrazole have been studied using multinuclear NMR spectroscopy at low temperature to determine the tautomeric equilibrium constants in the slow proton exchange regime by simple signal integration. In order to compare the results in solution with those in the solid state, the X-ray structure of a derivative of the first, namely 4-bromo-3-phenylpyrazole was * Lists of thermal components, hydrogen parameters and bond distances and angles have been deposited at the Cambridge Crystallographic Data Centre (CCDC). For details of the deposition scheme see 'Instructions for Authors,' J. Chem. Soc., Perkin Trans. 2, 1992, issue 1. * 3-216//3-216 calculations on fully optimized geometries 36 also favours 3a over 3b by 4.2 kJ mol-'.

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

“…Note that compounds 3, 4, and 12 behave like "planar" compounds, a fact that is related to their tautomerism. When X-ray diffraction data are available (30), the conformation of the aryl group deduced from I3c CP/MAS is consistent with those found in the monocrystals.…”

“…The determination of the tautomer present in the solid state requires either the knowledge of "blocked" pyrazoles, i.e., those in which the N-H is replaced by an N-R ( 3 3 , or the slowing down of prototropic exchange in order to observe the signals of both tautomers in solution (29,30,32,43). Several examples of this last case are reported in the tables.…”

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.…”

“…Assignment was based on previous works. 9,10,14 These spectra were analyzed using the DNMR program (see Section 4). Barriers were estimated at 14.35 (294 K), 13.90 (273 K) and 13.2 kcal mol 21 (243 K), within the range of those determined for other pyrazoles.…”

Tautomerism in 3{5}-(dimethoxyphenyl)pyrazoles

“…The pyrazole ring, which contains both a proton-donor pyrrolic N--H group and proton-acceptor pyridinic N atom in adjacent positions, affords a complex physical chemistry for these molecules in the gas, solution and solid phases (Abboud et al, 1992;. This bifunctionality also often affords unusual crystal packing arrangements for 3 { 5 }-substituted pyrazoles via intermolecular hydrogen bonding, which can give rise to complex fluxionality in the solid state involving degenerate intermolecular proton transfer reactions, which may be coupled with other molecular motions and can be sensitively probed by 1H, 13C and 15N NMR spectroscopies (Smith et al, 1989;Aguilar-Parrilla, Cativiela et al, 1992;Aguilar-Parrilla, Scherer et aI., 1992;Llamas-Saiz, @ 1996 International Union of Crystallography Printed in Great Britain -all rights reserved Foces-Foces, Elguero & Meutermans, 1992; LlamasSaiz, Foces-Foces, Sobrados, Elguero & Meutermans, 1993;Elguero et al, 1994;Aguilar-Parrilla et al, 1995). Several 3{5 }-(2'-hydroxyaryl)pyrazoles are also good UV stabilizers, a property that is linked to the presence of an intramolecular N.-.H--O hydrogen bond in these compounds which facilitates intramolecular proton transfer upon excitation (Catalan et al, 1992).…”