Structural and aromatic aspects of tautomeric equilibrium in hydroxy aryl Schiff bases

Abstract

The synthesis and X-ray measurements of four Schiff bases were carried out at 100 K. The HOMA and HOSE aromaticity indices were estimated on the basis of the experimental data. The aromaticity of the phenyl ring and the chelate chain was analysed. A comparison of the aromaticity of naphthalene and phenyl derivatives of hydroxy aryl Schiff bases is presented. The balance between the aromaticity of adjacent rings of the naphthalene fragment and its effect on proton transfer is defined. Research on the interrelations between aromaticity and the intramolecular proton transfer in hydroxy aryl Schiff bases is shown.The synthesis of the hydroxy aryl Schiff bases from stoichiometric mixtures of ketones and anilines in methanol was performed according to Reference [30] .The intensity data were collected at 100 K using a Kuma KM4CCD diffractometer and graphite-monochromated MoKa (0.71073 Å ) radiation generated by an X-ray tube operating at 50 kV and 35 mA. The images were indexed, integrated and (www.interscience.wiley.com)

Structural and aromatic aspects for tautomerism of (Z)-6-((4-bromophenylamino)methylene)-2,3-dihydroxycyclohexa-2,4-dienone

“…7, the most strength intramolecular H-bond of the compound is observed for tautomeric transition states (OÁÁÁ HÁÁÁN) rather than those in either O-HÁÁÁN or OÁÁÁH-N tautomeric forms. This inference is in agreement with those reported by Filarowski and his colleagues for distinct Schiff base species in [3,37,40].…”

“…The reason for that HOMA index (0.380) of the chelate ring in o-hydroxyaryl Schiff bases for incipient H-bond [38] is more than that (0.348) in the title compound can be explained by the absence of another aromatic ring to which a portion of phenol ring aromaticity is transferred, giving rise to the lack of p-electron coupling. Pseudoaromaticity of the chelate ring in the X-ray geometry of the compound is 0.749, slightly more than its corresponding value (0.703) in [37]. The difference between the levels of pseudo-aromaticity of chelate ring reported here and that in [37] is essentially related to covalent skeleton of p-donor fragments, phenol, and naphthalen-2-ol.…”

“…Pseudoaromaticity of the chelate ring in the X-ray geometry of the compound is 0.749, slightly more than its corresponding value (0.703) in [37]. The difference between the levels of pseudo-aromaticity of chelate ring reported here and that in [37] is essentially related to covalent skeleton of p-donor fragments, phenol, and naphthalen-2-ol. Aromaticity for one of the fused six membered rings in naphthalen-2-ol, which does not contain o-hydroxyl, is increased while that of the other one is decreased [37].…”

“…The difference between the levels of pseudo-aromaticity of chelate ring reported here and that in [37] is essentially related to covalent skeleton of p-donor fragments, phenol, and naphthalen-2-ol. Aromaticity for one of the fused six membered rings in naphthalen-2-ol, which does not contain o-hydroxyl, is increased while that of the other one is decreased [37]. For this reason, p-electron coupling between naphthalen-2-ol and the chelate ring is deficient as compared with that of the title compound.…”

“…Symmetry operation used to generate equivalent atomic positions: (i) 1 -x, 2 -y, -z ring to a large extent and into 4-bromophenyl ring to a little extent, indicating the presence of long-ranged p-electron coupling for the title compound. It is worth noting that aromaticity of the chelate ring in the NH form is approximately stable beyond the tautomeric transition state (OÁÁÁ HÁÁÁN) due to the electronic redistribution of the rate of NH tautomeric forms, zwitterionic and cis-keto [37].…”