Showing papers by "José Elguero published in 1986"

Electrostatic proximity effects in the relative basicities and acidities of pyrazole, imidazole, pyridazine, and pyrimidine

Abstract: The acid/base behavior of above titled compounds is widely recognized for its importance to the life sciences. The gas- and aqueous-phase basicities of pyrimidine (1,3-diazine, I) are known to be distinctly smaller by 3.4 and 1.1 pK/sub b/ units, respectively, than that of pyridazine (1,2-diazine, II). The author have found that in contrast the gas-phase basicity of imidazole (1,3-diazole, III) is larger (as in also the aqueous basicity) by significantly greater amounts (8.0 and 4.6 pK/sub b/ units, respectively) than that of pyrazole (1,2-diazole, IV). This very large basicity difference (in either phase) has not previously been satisfactorily explained. The relative gas-phase and solution basicities of pyridazine and pyrimidine and of pyrazole and imidazole, as well as the relative gas-phase and solution acidities of the latter diazoles, have been accounted for approximately by considering important NH/sup +/ and lone pair electrostatic interactions that act from the 2-position.

The structure of indazolinone and derivatives in the solid state and in solution: an X-ray and nuclear magnetic resonance study

Abstract: The structure of indazolinone and some of its derivatives was obtained by the combined use of crystallography, 13C CP/MAS, and 15N n.m.r. It was concluded that indazolinone exists as such in the solid state but only as a minor tautomer (15%) in DMSO solution, where the 3-hydroxyindazole tautomer predominates (85%). The result of bromination of indazolinone is reported. Two acylhydrazines, as open analogues of indazolinone, were studied as model compounds.

Diazafulvenones. Thermal isomerizations and eliminations in alkoxycarbonyl and anilinocarbonyl derivatives of imidazole

Abstract: 4-Carbonyl-4H-imidazole (10) and 2-carbonyl-2H-imidazole (11) are formed by flash vacuum pyrolysis of methyl 4- and 2-imidazolecarboxylates, respectively. 10 and 11 dimerize to diketopiperazines 14 and 16, respectively. The same products are also obtained from 4- and 2-(anilinocarbonyl)imidazoles, respectively. Methyl imidazole-1-carboxylate (4) on pyrolysis gives a ca. 1:1 mixture of the same ketenes 10 and 11, which dimerizes to a 1:2:1 ratio of diketopiperazines 14–16. In contrast, ethyl imidazole-1-carboxylate gave CO, ethylene, and imidazole as the major products. The pyrolysis reactions were monitored by low-temperature infrared and high-temperature mass spectrometry.

Synthesis of highly dipolar betaines: pyridinium azolates

Abstract: Theoretical calculations have been carried out and experimental dipole moments measured for several pyridinium azolate inner salts; depending on the substituent on the pyridinium ring the compounds are either planar, almost freely rotating molecules, or are hindered with the rings perpendicular to each other, and in all cases the dipole moments are very large (10–18 debyel).