Tautomerism in 4‐substituted 1‐phenyl‐3‐methyl‐pyrazolin‐5‐ones—a theoretical ab initio and ¹³C NMR study

Abstract

The tautomeric equilibria between the CH, OH and NH forms in a series of 4‐substituted 1‐phenyl‐3‐methyl‐pyrazolin‐5‐ones have been studied using ab initio calculations at various levels of theory and comparison made with the experimental results obtained from NMR measurements. Quantitative comparison of both the relative energies and the 13C chemical shifts of the tautomers constituting the tautomeric equilibria were made by calculation of both sets of parameters. The influence of the solvent was included by employing various models of the self‐consistent reaction field theory. Initially, the solvent was included in the calculations by applying a continuum of differing polarity over the surface of isolated tautomers (self‐consistent isodensity polarized continuum model method), then later by assuming formation of an adduct between the solute and the solvent (thereby simulating effectively the hydrogen bonding in the OH and NH tautomers) and finally by calculating dimer or trimer complexes of the various tautomers. In this manner, the agreement between the theoretically calculated and the experimentally determined tautomeric equilibria was improved significantly. The theoretically calculated 13C chemical shifts of the tautomers were found to be viable for the assignment of the tautomers, particularly the preferred tautomer in the OH/NH equilibrium, which remains fast on the NMR time scale even at low temperatures. Copyright © 2001 John Wiley & Sons, Ltd.

3‐Nitrocoumarin Amidines: A New Synthetic Strategy for Substituted [1]Benzopyrano[3,4‐d]imidazol‐4(3H)‐ones

“…Another possibility could be that 4dα-4dβ interconversion is somehow enhanced by minor steric hindrance of the C-2 substituent. On the other hand, the solvent model used in this study might not be sufficiently accurate to predict this tautomeric behaviour correctly; it has indeed been reported by Kleinpeter and Koch [28] that a correct description of tautomerism in 4-substituted 3-methyl-1-phenylpyrazolin-5-ones was possible only through consideration of various dimeric and trimeric complexes of the evaluated compounds or of explicit solvent interactions by a supramolecular approach. Investigation in such depth was not intended in this study, in which we mainly report the synthesis of novel benzopyranoimidazolones, but is the object of future research.…”

1,3-Diphenyl-4,5-dihydro-1H-pyrazol-5-one

“…Equilibrium involving tautomers of II in solution have been variously studied using NMR and IR spectroscopy and ab initio calculations. (Feeney et al, 1970;Hawkes et al, 1977;Freyer et al, 1983;Dardonville et al, 1998;Kleinpeter & Koch, 2001). Crystal structures of various pyrazone compounds of forms IIa, IIb and IIc have been reported (see for example, Bechtel et al, 1973a;Bechtel et al, 1973b;Chmutova et al, 2001;Wardell et al, 2007;Gallardo et al, 2009;Ding & Zhao, 2010).…”

“…For background to the synthesis, see: Nef (1891); Katritzky et al (1997); Wardell et al (2007); de Lima et al (2010). For evaluation of tautomeric forms using NMR MO calculations and crystallography, see : Feeney et al (1970); Hawkes et al (1977);Freyer et al (1983); Dardonville et al (1998); Kleinpeter & Koch (2001); Bechtel et al (1973a,b); Chmutova et al (2001);Wardell et al (2007); Gallardo et al (2009);Ding & Zhao (2010). For a previous synthesis, see : Kimata et al (2007).…”

Coordination modes of 5-pyrazolones: A solid-state overview

“…In solution these compounds exist in different tautomeric forms, the relative abundance of which depends on the structure of the compound (number and position of the substituents), the concentration, the nature of the solvent and the temperature [19,20].…”