Theoretical study of the structure and tautomerism of N 1-unsubstituted pyrazoles in the solid state

Paz, J. L. G. De; Elguero, José; Foces‐Foces, Concepción; Llamas-Saíz, Antonio L.; Aguilar‐Parrilla, Francisco; Klein, O.; Limbach, Hans−Heinrich

doi:10.1039/a603035a

Cited by 55 publications

(36 citation statements)

References 29 publications

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“…Ab initio calculations performed on pyrazole clusters reproduced these findings [78] and indicated a switch from concerted double and triple proton transfers in the cyclic dimer and trimer of pyrazole to a stepwise HH+HH mechanism for the [27]. The solid curves were calculated using the Bell-Limbach tunneling model.…”

Section: Applicationsmentioning

confidence: 99%

Single and Multiple Hydrogen/Deuterium Transfer Reactions in Liquids and Solids

Limbach

2006

Hydrogen‐Transfer Reactions

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OverviewIn this chapter, Arrhenius curves of selected single and multiple hydrogen transfer reactions for which kinetic data are available over a large temperature range are reviewed. The curves are described by a combination of formal kinetics of reaction networks and the one-dimensional Bell-Limbach tunneling model for each reaction step. The main parameters of this model are the barrier heights and barrier widths of the isotopic reactions, the tunneling masses, the pre-exponential factor and a minimum energy for tunneling to occur. This approach allows one to compare efficiently very different reactions studied in different environments and to prepare the kinetic data for higher-dimensional quantum-mechanical treatments. The first type of reactions discussed is concerned with those where the hydrogen bond geometries of the reacting molecules are well established and where kinetic data of the isotopic reactions are available over a large temperature range. Here, it is possible to study the relation between kinetic isotope effects and chemical structure. Examples are the tautomerism of porphyrin, of the porphyrin anion and related compounds exhibiting intramolecular hydrogen bonds of medium strength, and the solid state tautomerism of pyrazoles and of benzoic acid in cyclic associates. One main result is the finding of pre-exponential factors of the order of kT/h @ 10 13 s -1 , as expected by transition state theory for vanishing activation entropies. The barriers of multiple H-transfers are found to be larger than those of single H-transfers.The second type of reactions discussed refers mostly to liquid state solutions and involves major heavy atom reorganization. Here, equilibria between reactive and nonor less reactive molecular configurations may play a role. Several cases are discussed where the less reactive forms dominate at low or at high temperature, leading to unusual Arrhenius curves. These cases include examples from small molecule solution chemistry like the base-catalyzed intramolecular H-transfer in diaryltriazene, 2-(2¢-hydroxyphenyl)-benzoxazole, 2-hydroxy-phenoxyl radicals as well as an enzymatic system, thermophilic alcohol dehydrogenase. In the latter case, temperature dependent kinetic isotope effects are interpreted in terms of a transition between two regimes with different temperature independent kinetic isotope effects. 135Hydrogen-Transfer Reactions. Edited by

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Section: Applicationsmentioning

confidence: 99%

Single and Multiple Hydrogen/Deuterium Transfer Reactions in Liquids and Solids

Limbach

2006

Hydrogen‐Transfer Reactions

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“…Similar results have been obtained in the tautomeric processes (proton transfer) of an isolated pyrazole molecule when compared to that occurring in the presence of solvent molecules like water or to the dimer. 21 As can be seen in Figure 7a …”

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confidence: 67%

Theoretical Studies on Proton Transfer Reactions of 8‐Hydroxyquinoline Monomers and Dimers

Zhao

Zhou

et al. 2006

Chin. J. Chem.

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Density functional theory (DFT) of quantum chemistry method was employed to investigate proton transfer reactions of 8-hydroxyquinoline (8-HQ) monomers and dimers. By studying the potential energy curves of the isomerization, the most possible reaction pathway was found. The total energy of 8-hydroxyquinoline was lower than that of quinolin-8(1H)-one, whereas the order was reversed in dimers. The findings explained the contrary experimental phenomena. The minimum reaction barrier of intramolecular proton transfer was 47.3 kJ/mol while that in dimer was only 25.7 kJ/mol. Hence it is obvious that proton transfer reactions of 8-HQ monomer have a considerable rate but it is easier to proceed for 8-HQ dimer than monomers. It implied that the hydrogen bond played an important role in depressing the activation energy of reaction. The mechanism of the tautomerization was discussed on the basis of theoretical results.

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“…Currently, there is a particular interest in bipyrazoles substituted in studies on intermolecular interactions with the participation of pyrazoles and their derivatives. [3][4][5][6][7][8][9] The structure of pyrazole molecules, containing the fragment RN-N(H)-, is favorable in the formation of various hydrogen bonded complexes and associates in solid-cyclic dimers, trimers and tetramers, linear chains and helices, depending on the substituent. [9][10][11] A literature survey turned up a few reports on 1,4'-bipyrazole and isoxazole synthesis and structure.…”

Section: Introductionmentioning

confidence: 99%

Highly regioselective synthesis of novel 1,4'-bipyrazoles

Martins

Fiss

Frizzo

et al. 2010

J. Braz. Chem. Soc.

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Este trabalho descreve a síntese de 1,4'-bipirazóis inéditos pela da reação de ciclocondensação de novas b-dimetilaminoenonas com as monoidrato de hidrazina, tert-butil hidrazina, fenil hidrazina, carboximetilhidrazina. Os produtos foram obtidos com alta regiosseletividade em rendimentos de bons a moderados (50-80%). A técnica de cristalografia de raios-X foi utilizada para a elucidação da regiosseletividade da formação dos produtos. 1,4'-Bipirazolilisoxazóis também foram descritos.Novel 1,4'-bipyrazoles were synthesized via highly regioselective cyclocondensation reactions of b-dimethylaminoenones with hydrazine monohydrate, tert-butylhydrazine hydrochloride, phenylhydrazine hydrochloride or carboxymethylhydrazine. X-ray diffraction technique was used in the elucidation of the regiochemistry of the b-dimethylaminoenaminones and of the 1,4'-bipyrazoles. 1,4'-Bipyrazolylisoxazoles were also reported.

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Theoretical study of the structure and tautomerism of N 1-unsubstituted pyrazoles in the solid state

Cited by 55 publications

References 29 publications

Single and Multiple Hydrogen/Deuterium Transfer Reactions in Liquids and Solids

Single and Multiple Hydrogen/Deuterium Transfer Reactions in Liquids and Solids

Theoretical Studies on Proton Transfer Reactions of 8‐Hydroxyquinoline Monomers and Dimers

Highly regioselective synthesis of novel 1,4'-bipyrazoles

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