Showing papers by "José Elguero published in 1989"
111 citations
TL;DR: A combination of magnetic resonance, quadrupole double resonance, and x-ray studies of solid 3,5-dimethylpyrazole between 270 and 350 K has shown that the NH...N hydrogen bond units present in the crystal are dynamically disordered, so that each nitrogen atom is on average attached to half a hydrogen atom as discussed by the authors.
Abstract: A combination of {sup 13}C, {sup 15}N magnetic resonance, {sup 14}N quadrupole double resonance, and x-ray studies of solid 3,5-dimethylpyrazole between 270 and 350 K has shown that the NH...N hydrogen bond units present in the crystal are dynamically disordered, so that each nitrogen atom is on average attached to half a hydrogen atom. The molecules form discrete hydrogen-bonded cyclic trimers, in which the hydrogen atoms move in a double minimum potential energy surface which is symmetrical, to within experimental error. The experimental evidence in this temperature range is consistent with disorder by means of correlated triple hydrogen jumps with an activation energy of 45 kJ mol{sup {minus}1}. There is a large kinetic hydrogen (HHH)/deuterium (DDD) isotope effort of >20 at 299 K and equal to 8 at 347 K.
103 citations
TL;DR: In this article, the annular tautomerism of 1,2,3-triazole and 3(5)methylpyrazole is discussed by means of a combination of theoretical calculations and experimental (ICR) gas phase basicities and acidities.
Abstract: The annular tautomerism of 1,2,3‐triazole and 3(5)‐methylpyrazole is discussed by means of a combination of theoretical calculations and experimental (ICR) gas‐phase basicities and acidities. In the gas phase 1,2,3‐triazole exists as the 2H‐tautomer, whereas both tautomers of 3(5)‐methylpyrazole are of similar energy. The solvent effects on these prototropic equilibria are discussed taking into account solvent properties as polarity/polarizability, acidity, and basicity. In nonhydrogen bonding solvents, the difference in dipole moments between both tautomers plays a role that has usually been underestimated.
52 citations
50 citations
TL;DR: In this paper, the reactions of iridium and rhodium complexes derived from tris(pyrazolyl)-methane (tpzm) and tris (pyrazyl)ethane(tpze) have been examined.
36 citations
TL;DR: In this article, a comparative ab initio study of amides and sulfonamides has been carried out using minimal, extended and polarization basis sets, where rotational barriers and tautomerism are discussed comparatively.
Abstract: A comparative ab initio study of amides and sulfonamides has been carried out using minimal, extended and polarization basis sets Rotational barriers and tautomerism are discussed comparatively The two main conclusions of this study are the absence in sulfonamides of a conjugation of the type present in planar amides and the relative insensitivity of the total energy to nitrogen hybridization in sulfonamides in contrast with amides
32 citations
TL;DR: In this paper, a semi-empirical analysis of the tautomeric equilibrium of 3-Methyl-2-phenacyl-4-pyrimidinones is presented.
Abstract: 3-Methyl-2-phenacyl-4-pyrimidinones 1, 2 have been synthesized using the sulfide contraction. According to the NMR data, the compounds 1, 2 exist exclusively as the benzoylmethylene tautomers a both in solution and in the solid state. AM1 calculations of the parent system are in agreement with the experimental observations. The study of the tautomeric equilibrium by this semiempirical method has been extended to other cases of enamino-ketone/enolimine tautomerism.
Zur Tautomerie von 2-Phenacyl-4-pyrimidinonen und verwandten Verbindungen
3-Methyl-2-phenacyl-4-pyrimidinone 1, 2 werden durch Sulfidkontraktion synthetisiert. Aufgrund der NMR-Daten existieren die Verbindungen ausschlieslich in der Benzoylmethylen-Form, sowohl in Losung als auch als Festkorper. Diese Ergebnisse werden durch AM1-Berechnungen fur die Stammverbindung bestatigt. Die Untersuchung des Tautomeren-Gleichgewichts mittels dieser semiempirischen Methode wurde auf andere Falle von Enaminoketon/Enolimin-Tautomerie erweitert.
19 citations
TL;DR: In this article, an ab initio MO analysis of the correlation diagrams of these systems was made for the nitrogen inversion and for the increase in the N-N internuclear distance.
Abstract: The aromaticity of the nitrogen heterocyclic compounds 1 H -azirine, 1,2-Δ 3 diazetine, pyrrole and 1,2-dihydropyridazine has been investigated. An ab initio MO analysis of the correlation diagrams of these systems was made for the nitrogen inversion and for the increase in the N-N internuclear distance. The analysis showed that π aromaticity is a consequence and not the origin of the planarity of this type of compound. The existence of ring strain, nitrogen lone-pair interactions, or both, seems to be crucial in the absence of aromaticity in this kind of system. The non-aromatic character of azirine and 1,2-Δ 3 -diazetine are confirmed by resonance energies of opposite sign to that predicted for benzene, as well by their high barriers to nitrogen inversion. The relative stabilities of the different conformers of these compounds are discussed.
18 citations
TL;DR: A series of 4-pyrazolylquinolines have been prepared from β-dicarbonyl compounds and the previously proposed diazepine structure for the reaction products has been ruled out.
18 citations
TL;DR: Theoretical calculations have been carried out on several conformations of bis-, tris-and tetrakispyrazolylmethane (3) as mentioned in this paper, and the potential surface thus obtained has been compared with experimental results both in solution (lanthanide shift reagents, dipole moments) and in the solid state (crystallography).
18 citations
TL;DR: In this paper, the conformational potential surface of sulfamide has been studied by considering the S-N bond rotations and the nitrogen inversion processes, and the lowest energy conformation is found for a cis-trans arrangement of the amino groups, although conformations with cis-cis (a), transtrans (c), and near staggered (c′) arrangements lie close in energy.
Abstract: Abinitio calculations have been used to study the conformational potential surface of sulfamide, by considering the S—N bond rotations and the nitrogen inversion processes. The lowest energy conformation (b) is found for a cis–trans arrangement of the amino groups, although conformations with cis–cis (a), trans–trans (c), and near staggered (c′) arrangements lie close in energy. Nitrogen inversion barriers are very low, and consequently one may expect forms b and c′ to be the only ones present in the gas phase. Conformer a is very polar, its dipole moment being twice that of b, so it may be favored in condensed media or in polar-solvent solutions. The relative stability of the different isomers is governed by interactions between the amino protons and between the nitrogen lone pairs. Our results show that d–π backbonding, involving the d orbitals on sulfur, is responsible for the multiple bond character of the S—O linkage, but is very small in the S—N interactions. The role of the sulfur d-orbital exponen...
TL;DR: In this article, the Ullmann Goldberg reaction was used to improve the synthesis of N-arylanthranilic acids (shorter reaction times, higher purity of the final products) through ultrasonic irradiation.
TL;DR: In this paper, the influence of ultrasound and water on the Diels-Alder reaction between cyclopentadiene and methyl and benzyl-2-acetamidoacrylates has been studied.
TL;DR: In this paper, the authors proposed N-Arylpyrazoles and N-aryl pyrazolium salts for chemical shifts in NMR NMR and chemical shifts.
Abstract: Keywords:
13C NMR;
Chemical shifts;
J(CH) and J(FC);
N-Arylpyrazoles;
N-Arylpyrazolium salts
TL;DR: In this article, the origin of the N-methylation effect in azoles dissolved in water and dimethyl sulphoxide was discussed using a series of equations connecting experimental and theoretical values, and the existence in the azoles studied of a linear relationship between the gas → solution transfer enthalpies and the charge on the pyrrole hydrogen atom was demonstrated.
TL;DR: In this article, a linear relationship has been found between the logarithms of the capacity factors and the energy values of the interaction of these compounds with this stationary phase, and the interaction energies have been calculated for two different spatial dispositions of the solutes in relation to the sorbent.
Abstract: Molecular modeling and energy calculations have been used to study the chromatographic separation of aromatic compounds on microcrystalline cellubose triacetate. A linear relationship has been found between the logarithms of the capacity factors and the energy values of the interaction of these compounds with this stationary phase. The interaction energies have been calculated for two different spatial dispositions of the solutes in relation to the sorbent.
Journal Article•
TL;DR: H-Mebendazole and some related heterocyclic methyl carbamates were synthesized and their anthelmintic activity against Caenorhabditis elegans was determined and a structure-activity relationship (SAR) was established.
Abstract: 5H-Mebendazole and some related heterocyclic methyl carbamates were synthesized and their anthelmintic activity against Caenorhabditis elegans was determined. In order to study the influence of the heteroaromatic region with regard to the carbamate moiety on biological activity, the molecular electrostatic potentials (MEP) of all structures were calculated and a structure-activity relationship (SAR) was established. The electrostatic pattern of activity includes two minima of the carbamate moiety, a third heterocyclic minimum, and a pi-electronic region.
TL;DR: A series of 4-pyrazolylquinolines have been prepared from β-dicarbonyl compounds and the previously proposed diazepine structure for the reaction products has been ruled out as discussed by the authors.
Abstract: A series of 4-pyrazolylquinolines have been prepared from 4-hydrazinoquinolines and β-dicarbonyl compounds and the previously proposed diazepine structure for the reaction products has been ruled out. A 13C nmr study established the 3-position of the methyl group when two isomers were possible. Finally, a careful 1H nmr study of 2- and 4-pyrazolylquinolines led to the conclusion that intramolecular hydrogen bonds between aromatic (or heteroaromatic) C-H and pyridine like nitrogens is the factor that determines the conformation of these compounds.
Journal Article•
TL;DR: Two hexapeptides related to the undecapeptide substance P (SP) Glu-Phe-phe-Gly-Leu-Met-NH2 and Glu/Phe/Gly/Pro/Leu/Met/NH2, have been synthesized and their selectivity for the SP receptors studied.
Abstract: Two hexapeptides related to the undecapeptide substance P (SP) Glu-Phe-Phe-Gly-Leu-Met-NH2 and Glu-Phe-Phe-Pro-Leu-Met-NH2, have been synthesized and their selectivity for the SP receptors studied. Conformational analyses of both peptides have been carried out using a molecular modeling program. Activity appears to be related to the adoption of a U-shape conformation since the Pro9 containing peptide in which this folding is favoured is much more active than the hexapeptide containing Gly9. Moreover, such a substitution induces a significant selectivity for the SP-P receptor compared to the SP-E receptor.
TL;DR: In this article, the influence of ultrasound and water on the Diels-Alder reaction between cyclopentadiene and methyl and benzyl-2-acetamidoacrylates has been studied.
Abstract: The influence of ultrasound and water, both separately and combined, on the Diels-Alder reaction between cyclopentadiene and methyl and benzyl-2-acetamidoacrylates has been studied. Ultrasonic irradiation does not improve the yield. Water is worse than toluene as solvent both in yield and in exo/endo selectivity.
TL;DR: In this article, a semi-empirical analysis of the tautomeric equilibrium of 3-Methyl-2-phenacyl-4-pyrimidinones is presented.
Abstract: 3-Methyl-2-phenacyl-4-pyrimidinones 1, 2 have been synthesized using the sulfide contraction. According to the NMR data, the compounds 1, 2 exist exclusively as the benzoylmethylene tautomers a both in solution and in the solid state. AM1 calculations of the parent system are in agreement with the experimental observations. The study of the tautomeric equilibrium by this semiempirical method has been extended to other cases of enamino-ketone/enolimine tautomerism.
Zur Tautomerie von 2-Phenacyl-4-pyrimidinonen und verwandten Verbindungen
3-Methyl-2-phenacyl-4-pyrimidinone 1, 2 werden durch Sulfidkontraktion synthetisiert. Aufgrund der NMR-Daten existieren die Verbindungen ausschlieslich in der Benzoylmethylen-Form, sowohl in Losung als auch als Festkorper. Diese Ergebnisse werden durch AM1-Berechnungen fur die Stammverbindung bestatigt. Die Untersuchung des Tautomeren-Gleichgewichts mittels dieser semiempirischen Methode wurde auf andere Falle von Enaminoketon/Enolimin-Tautomerie erweitert.