Showing papers by "Rosa M. Claramunt published in 2004"

The tautomerism of Omeprazole in solution: a 1H and 13C NMR study

Abstract: The tautomerism of 5(6)-methoxy-2-([(4-methoxy-3,5-dimethyl-2-pyridinyl)methyl] sulfinyl)-1H-benzimidazole (omeprazole) was determined in solution, K(T) = 0.59 in THF at 195 K, in favor of the 6-methoxy tautomer. The assignment of the signals was made by comparison with its two N-methyl derivatives in acetone-d6 and through theoretical calculations of the absolute shieldings (GIAO/DFT/6-311++G**).

Multiple hydrogen bonds and tautomerism in naphthyridine derivatives

Abstract: The behaviour of three 2,7-disubstituted 1,8-naphthyridines able to exhibit tautomerism has been studied by NMR in solution and in two cases in the solid state. The three derivatives studied are 2,7-dihydroxy- (1), 2-acetamido-7-amino- (3) and 2,7-diacetamido-1,8-naphthyridine (4). To explore the problem of secondary interactions, a series of complexes, with up to four simultaneous hydrogen bonds, where the monomers are generated using pyridine and 4-pyridone as building blocks, have been theoretically studied. The calculated interaction energies have been correlated with the number of hydrogen bonds and with attractive and repulsive secondary interactions. Further analysis of the electron density and orbital interactions shows that the secondary interactions, both attractive and repulsive, have a purely electrostatic origin. The X-ray structure of compounds 3 and 4 have been determined. In the solid state these compounds exist in the “diamino” tautomers with the N–H proton of the amido groups pointing towards the naphthyridine nitrogen. DFT and GIAO calculations have been essential to disentangle the problem of the structure of these compounds.

Apparent Allyl Rotation in New Allylpalladium(II) Complexes with Pyrazolyl N-Donor Ligands

Abstract: The ligands 2,4,6-tris(pyrazol-1-yl)-1,3,5-triazine (TPzT), 2,4,6-tris(3,5-dimethyl-pyrazol-1-yl)-1,3,5-triazine (Me2-TPzT), and 2,4,6-tris(3,4,5-trimethylpyrazol-1-yl)-1,3,5-triazine (Me3-TPzT) have been synthesised and their reaction with [Pd(η3-C4H7)(THF)2]X was explored. The reaction in a 1:3 molar ratio (ligand/solvate) leads to the new derivatives [{Pd(η3-C4H7)}2(BPz′TO)]X (Pz′ = pyrazole, X = BF4, 2a; Pz′ = 3,5-dimethylpyrazole, X = PF6, 2c; Pz′ = 3,4,5-trimethylpyrazole, X = PF6, 2d) in which the N-donor ligand has been partially hydrolysed. The complexes exist in the form of two isomers, a meso form and a d,l pair. An apparent allyl rotation process leads to a syn-syn/anti-anti interconversion and also to an interchange between the two isomers. The values of ΔGc‡ at the coalescence temperature have been calculated. The results, which have also been compared with those previously obtained for the complex with the 4-methylpyrazole ligand, indicate that the activation barrier for the process is not affected by the change in the pyrazole group. Appreciable effects were not observed on changing the concentration or on the addition of water to the acetone solutions of 2c. However, the influence of the solvent (acetone versus chloroform) is worth noting. A negative activation entropy has been found and a mechanistic proposal for the process is included. The molecular structure of 2c has been determined by X-ray diffraction. The meso isomer, in which the two C−Me axes of the allylic groups are oriented in the same direction, is found in the solid state. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Substituent and ring effects on enthalpies of formation: 2-methyl- and 2-ethylbenzimidazoles versus benzene- and imidazole-derivatives

Abstract: The enthalpies of combustion, heat capacities, enthalpies of sublimation and enthalpies of formation of 2-methylbenzimidazole (2MeBIM) and 2-ethylbenzimidazole (2EtBIM) are reported and the results compared with those of benzimidazole itself (BIM). Theoretical estimates of the enthalpies of formation were obtained through the use of atom equivalent schemes. The necessary energies were obtained in single-point calculations at the B3LYP/6-311+G(d,p) on B3LYP/6-31G* optimized geometries. The comparison of experimental and calculated values of benzenes, imidazoles and benzimidazoles bearing H (unsubstituted), methyl and ethyl groups shows remarkable homogeneity. The energetic group contribution transferability is not followed, but either using it or adding an empirical interaction term, it is possible to generate an enormous collection of reasonably accurate data for different substituted heterocycles (pyrazole-derivatives, pyridine-derivatives, etc.) from the large amount of values available for substituted ...

Pyrazoles as molecular probes to study the properties of co-crystals by solid state NMR spectroscopy

Abstract: Equimolar mixtures of 3,5-dimethylpyrazole (1) with four NH-imidazoles (2–5) have been studied by13C and 15N CPMAS NMR and by DSC. In three cases, the solid mixture behaves as the sum of the individual components [imidazole (2), 2-methylimidazole (3) and 2,4(5)-dimethylimidazole (5)]. In one case [4,5-dimethylimidazole (4)], the mixture corresponds to a new species in which the dynamic behavior of1 no longer exists.

On photoinduced double-proton transfer reactions: the photophysics of the 9H-imidazo[1,2- a]benzimidazole dimer

Abstract: The proton transfer in the C 2h doubly H-bonded 9H-imidazo[1,2- a ]benzimidazole (9HIB) dimer has been investigated. From the theoretical point of view, with the aid of density functional theory (DFT) and Moller–Plesset second-order perturbation theory: (i) the dimer formation presents at 298 K a large free energy for dimerization of Δ G 0 = −8.92 kcal/mol; (ii) the double-proton transfer (DPT) tautomer of the 9HIB dimer in the ground electronic state (S 0 ) is only slightly less stable (Δ G 0 = 2.45 kcal/mol) than the normal tautomer dimer; and (iii) the DPT potential energy curve in S 0 exhibits double minima, and a large activation energy barrier of 8.2 kcal/mol for the reverse DPT process. However, the 9HIB dimer does not undergo an excited state DPT reaction, calculated at the time-dependent DFT level and experimentally checked with fluorescence spectroscopy, owing to the unusual decrease of basicity (−16.7 kcal/mol) of the N-imidazole group upon photoexcitation. The UV–Vis spectroscopic experimental evidence (from 298 to 213 K) confirms the ease to generate the 9HIB dimer, and the card-pack aggregates of 1-methylimidazo[1,2- a ]benzimidazole in 2-methylbutane and decalin.

Multidentate ligands from N-hydroxy-and N-aminopyrazole

Abstract: The reaction between hexafluorobenzene and the anion of 1-hydroxypyrazole affords a mixture of the products of bis-, tetrakis- and hexakis-substitution. On the other hand the anion of 1- aminopyrazole affords only the product of monosubstitution probably due to the acidity of the remaining NH. Finally, in the case of hexakis(bromomethyl)benzene, its reaction with 1- hydroxypyrazole lead to the hexakis-substituted product. All compounds have been characterized by NMR ( 1 H and 13 C) and mass spectrometry.

Spiro-Fused (C2)-Azirino-(C4)-pyrazolones, a New Heterocyclic System. Synthesis, Spectroscopic Studies and X-Ray Structure Analysis.

Abstract: Reaction of 1-substituted 4-acyl-5-hydroxy-3-methyl-1H-pyrazoles (2) with hydroxylamine gives the corresponding “oximes” 3, which are mainly present as (Z)-2,4-dihydro-4-[(hydroxyamino)methylene]-3H-pyrazol-3-ones. Treatment of compounds 3 with trichloroacetyl isocyanate/potassium carbonate in anhydrous diethyl ether affords 7-methyl-1,5,6-triazaspiro[2.4]hepta-1,6-dien-4-ones (4). The structure of compounds 4 was elucidated by means of single-crystal X-ray analysis (4f, 4h) and confirmed by NMR spectroscopic investigations (1H, 13C).