Electron-topological, energetic and π-electron delocalization analysis of ketoenamine-enolimine tautomeric equilibrium

The ketoenamine-enolimine tautometic equilibrium has been studied by the analysis of aromaticity and electron-topological parameters. The influence of substituents on the energy of the transition state and of the tautomeric forms has been investigated for different positions of chelate chain. The quantum theory of atoms in molecules method (QTAIM) has been applied to study changes in the electron-topological parameters of the molecule with respect to the tautomeric equilibrium in intramolecular hydrogen bond. Dependencies of the HOMA aromaticity index and electron density at the critical points defining aromaticity and electronic state of the chelate chain on the transition state (TS), OH and HN tautomeric forms have been obtained

Arene-fused 1,2-oxazole N-oxides and derivatives. The impact of the N-O dipole and substitution on their aromatic character and reactivity profile. Can it be a useful structure in synthesis? A theoretical insight

DFT calculations have shown that the N-O dipole of benzene- and naphthalene-fused 1,2-oxazole N-oxides causes a distortion of their σ and π frame, concentrated on the 1,2-oxazole ring, such that it increases its susceptibility to opening. The distortion forces the benzene ring into some diene geometry, thus, reducing π delocalization over the bi- or tricyclic structure and ultimately their aromatic character. C-3 substitution has a marked influence mainly on the naphthalene-fused N-oxides. C-5 and particularly C-6 substitution, as the position of most extended interaction with the N-O dipole through the π ring density, contribute to the distortion of the 1,2-oxazole geometry and thereby to the decrease of aromaticity of the structure. Bond uniformity (IA), average bond order (ABO) and Harmonic Oscillator Model of Aromaticity (HOMA) indices have been recruited to measure aromaticity changes. IA and ABO appear to be more credible to 1,2-benzoxazole N-oxides and 1,2-naphthoxazole N-oxides, respectively, while HOMA has been found equally reliable to both. Hardness and dipole moments follow similar trends. Energies, localization and separation of the four frontiers orbitals, i.e. HO, HO-1, and LU, LU+1, indicate a rather notable aromatic character of the N-oxides. Their reactivity profile, portrayed by descriptors such as Fukui and electro(nucleo)philicity Parr functions, shows good agreement with experimental outcomes towards electrophiles but succumbs to discrepancies towards nucleophiles due to the susceptibility of the hetero-ring to opening. The "push-pull" character of the N-O dipole and more importantly the extent of its double bonding direct site selectivity.Peer reviewe