Carbon-13 magnetic resonance study of solvent stabilized tautomerism in pyrazoles

Chenon, Marie-Thérèse; Coupry, Claude; Grant, David M.; Pugmire, Ronald J.

doi:10.1021/jo00424a017

Cited by 61 publications

(34 citation statements)

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“…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.…”

Section: Resultsmentioning

confidence: 99%

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

López¹,

Claramunt²,

Trofimenko³

et al. 1993

Can. J. Chem.

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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.

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Section: Resultsmentioning

confidence: 99%

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

López¹,

Claramunt²,

Trofimenko³

et al. 1993

Can. J. Chem.

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“…However, with a combination of low temperature and aprotic solvents of high polarity, it can usually be slowed down to the point that separate signals for the two tautomers can be observed by NMR spectroscopy. [15] Of course, for the two tautomers to be observed by NMR spectroscopy, they must be different. In symmetric systems with the same substituents at the 3-and 5-positions, such as pyrazole or 3,5-dimethylpyrazole, the two tautomers are identical (this tautomerism is termed autotrope or degenerate), [14] and only one species is observed even at low temperature.…”

Section: Supramolecular Behavior Of Free Pyrazolesmentioning

confidence: 99%

“…However, when the tautomerism is slowed down, separate signals can be seen, for instance, for the carbon atoms C3 and C5. [15] Despite the fact that tautomerism in pyrazoles is relatively fast, it is an intermolecular process. Several theoretical studies have indicated that this intramolecular tautomerism has a relatively high energy barrier (around 50 kcal mol -1 ).…”

Section: Supramolecular Behavior Of Free Pyrazolesmentioning

confidence: 99%

Pyrazole Complexes and Supramolecular Chemistry

2009

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This Microreview deals with the supramolecular chemistry of pyrazole metal complexes, in particular, complexes of pyrazoles with a nonsubstituted N–H group, which are able to engage in hydrogen bonding. The most pertinent general features of pyrazoles, in comparison with other heterocycles that are also popular ligands, and some aspects of the supramolecular chemistry of free pyrazoles are also included. In addition to the general supramolecular chemistry of pyrazole complexes, sections of this Microreview are devoted to liquid crystals, self‐assembled species, and hosts for anions. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

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“…However, if proton transfer between azole tautomers is to take place at a reasonable speed at the room temperature, the barrier should be \20 kcal/mol [8] and this condition can be facilitated by solvent molecules capable of creating hydrogen bonds with azole and acting as helpers. Indeed, the experimental studies in solution show that the actual activation barriers are in range of 10-14 kcal/mol [21][22][23][24][25]. As was pointed out by Alkorta and Elguero [26], the kinetic aspect of azoles' tautomerism was ''somewhat neglected'' in the past when the focus was on the thermodynamics.…”

Section: Introductionmentioning

confidence: 99%

The kinetics of water-assisted tautomeric 1,2-proton transfer in azoles: a computational approach

Ozimiński

2016

Struct Chem

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A complete set of azoles undergoing 1,2-proton transfer, consisting of pyrazole, 1,2,3-triazole, 1,2,4-triazole, tetrazole and pentazole, was computationally investigated regarding proton transfer mechanism in gas phase and water solution. Complexes of one azole molecule with 1-4 water molecules were employed to facilitate the proton transfer by lowering the activation energy, which for the isolated azole molecule is prohibitively large. The calculations were performed at the MP2/aug-cc-pVDZ and B3LYP/6-311??G(d,p) levels of theory which showed very good concordance. It follows that in the most probable transition state one azole molecule is accompanied by two water molecules. The activation barrier in water solution modelled by PCM method was lowered to 18.8 kcal/mol in the case of pyrazole and more for azoles containing more nitrogen atoms in the ring, reaching 6.8 kcal/mol in the case of pentazole. The analysis of the IRC reaction paths showed that proton transfer in the gas phase has more synchronous character and in the water solution is rather stepwise. It also follows from the analysis of bond lengths in the transition state that in the case of pyrazole the transition state is more cation-like and for other azoles, especially pentazole, is more anion-like. In the water environment, the initial step of proton transfer is moving the proton from azole to the water cluster in all cases except pyrazole, where the proton moves first from the water cluster to the azole molecule.

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Carbon-13 magnetic resonance study of solvent stabilized tautomerism in pyrazoles

Cited by 61 publications

References 5 publications

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

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

Pyrazole Complexes and Supramolecular Chemistry

The kinetics of water-assisted tautomeric 1,2-proton transfer in azoles: a computational approach

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Carbon-13 magnetic resonance study of solvent stabilized tautomerism in pyrazoles

Cited by 61 publications

References 5 publications

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

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

Pyrazole Complexes and Supramolecular Chemistry

The kinetics of water-assisted tautomeric 1,2-proton transfer in azoles: a computational approach

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Product

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

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