Quantitative measure of the aromatic stabilization energy of 2-pyridone and related compounds

Abstract

NOR 88C)Sztwmary Consideration of tautomeric equilibria allows the estimation of the aromatic stabilization energy of 2-pyridone as 25 kcal mol-l: the method is applied to related compounds.THE concept of aromatic character remains a cornerstone in the teaching and practice of organic chemistry, despite difficulties in its precise definition and quantitative measurement. The significant interest is in additional stabilization energy that aromatic compounds possess over and above that for hypothetical similar but non-conjugated systems.estimated by a comparison of the corresponding tautonieric equilibria [(l) t (2)] and [(3) + (4)J. This is a

On the Aromatic Character of the Heterocyclic Bases of DNA and RNA

“…The variability of the aromatic character of the rings in the bases in question may be interpreted in terms of a dearomatizing effect of the exocyclic substituents (or of any other chemical links) that are of a double-bond character. A similar effect was used by Beak et al , and by others − for approximate estimation of the resonance energy from the difference in energy between the aromatic 2-methoxypyridine and nonaromatic N -methyl-2-pyridone. Recently this effect has been described in porphyrine derivatives where the partially double CC bonds (1.394−1.400 Å) of the bridges between the pyrrole rings cause a substantial decrease of their aromatic character, as measured by the HOMA values ranging from 0.450 to 0.660, depending on the protonation state of the pyrrole nitrogen atoms.…”

“…As in the case of the whole molecules, the deviations from the linear trend for HOMA are observed only for the unstable tautomers of guanine (∆E ) 3.4 kcal/mol), thymine (∆E ) 18.7 kcal/mol), and uracyl (∆E ) 19.4 kcal/mol). In the case of NICS, the and by others [40][41][42] for approximate estimation of the resonance energy from the difference in energy between the aromatic 2-methoxypyridine and nonaromatic N-methyl-2-pyridone. Recently this effect has been described in porphyrine derivatives 43 where the partially double CC bonds (1.394-1.400 Å) of the bridges between the pyrrole rings cause a substantial decrease of their aromatic character, as measured by the HOMA values ranging from 0.450 to 0.660, depending on the protonation state of the pyrrole nitrogen atoms.…”

“…The differences are most signifi- cant for those two bases which have two double-bond substituents at the ring. Both these substituents decrease delocalization [38][39][40][41][42] and in consequence the HOMA values.…”

Aromaticity in catalysis: metal ligand cooperation via ligand dearomatization and rearomatization

“…In the key step of the catalytic cycle, the activation of the substrate (C−H and X−H) is dictated by the energetic balance of Here, it is worth to mention, that in many dearomatized pyridine structures aromatic resonance form is present. 113,114 Thus (pseudo)dearomatized concept applies for those molecules with weak aromatic stabilization energy. For historic reasons, herein, we chose symbol of the sun (sunflower) 85 to represent aromatic rings and moon as the counterpart to illustrated (pseudo)dearomatized (nonaromatic) rings.…”

Different coordination modes and supramolecular aggregations in copper(II) pentane-2,4-dionato compounds with 2-pyridone and 3-hydroxypyridine

“…Here we report the influence of 2-pyridone and 3-hydroxypyridine ligands on molecular and crystal structure. The tautomeric equilibrium between lactam (2-pyridone) and lactim (2-hydroxypyridine) forms enables different coordination of this ligand via a nitrogen and/or oxygen donor atom [14][15][16]. The observed metal complexes with 2-pyridone [14,[17][18][19][20][21][22][23][24][25][26][27][28][29][30][31][32][33] show predominantly the lactam form in the solid state and in solution [16,34], but examples with the lactim form are also known for Pt, Ir, and Ru [35][36][37][38][39][40].…”