Ab initio investigation of phloroglucinol

Abstract

All-electron ab initio Hartree-Fock (RHF) calculations have been carried out to investigate the ketoienol equilibrium of phloroglucinol. The calculations predict that the enol form of phloroglucinol, 1,3,5-benzenetriol, is by far the most stable of the two. This is confirmed by NMR spectra taken on phloroglucinol.A comparison of the keto enol form transformation of phloroglucinol with that of the phenol system shows that the keto form of phloroglucinol, 1,3,5-cyclohexanetrion, is more abundant in the phloroglucinol system, and the keto form of phenol, 2,4-cyclohexadien-l-on, in the phenol system.

General and Theoretical Aspects of Phenols

“…Both cyclohexa-2,4-dien-1-one 27 and cyclohexa-2,5-dien-1-one 28 Let us remember that the energy difference between phenol 26 and both keto isomers 27 and 28 amount to 73 and 69 kJ mol −1 , respectively ( Figure 31). The contribution of entropy is small, amounting to S = −9 and −1 J mol −1 K −1 , for both ketonization reactions, respectively, and this also leads to an estimate for the equilibrium constant of the enolization, pK E , ranging from −12 to −13, of the same order of magnitude as the experimental results in aqueous solution 186,187,469 . It should be stressed that such similarity of values in both gaseous and condensed phases should not be considered as an 'agreement' and need to be treated with much caution, due to the fact that the solvent effect on the equilibrium has not been taken into account.…”

Computational study of keto–enol equilibria of tropolone in gas and aqueous solution phase

“…Differences in bond length are rarely greater than Table 1 Gas phase absolute electronic energy in atomic units (hartrees), and relative energies (kcal mol À1 ) for tropolone (species (1) and (2)) and 3,5-and 3,6-cycloheptadiene-1,2-dione (molecules (3) and (4), respectively), evaluated at HF and B3LYP levels of theory, with 6-311++G ** and aug-cc-pVDZ basis sets, as well as with the G3 method Table 2 Absolute and relative electronic energy in atomic units and kcal mol À1 , respectively, at HF and B3LYP levels of theory for the four molecules in aqueous solution using the Onsager dipole and polarizable continuum solvent reaction field models in combination with 6-311++G ** and augcc-pVDZ basis sets 0.02 Å while bond angles, for the most part, agreed to within a degree. The largest difference between aqueous and gas phase bond angles was found for: A (3,4,5) in (1) with value +3.3°, A (12,14,15) in (2) is À0.9°, A (2,13,11) in (3) is +1.2°, and A (5,7,9) in (4) is +1.5°. The B3LYP gas phase optimized geometry of tropolone agrees very well with previous B3LYP/6-31+G * [15] and MP2/6-31G ** [14] calculations.…”

“…In a number of chemical systems, however, this trend is not followed. Examples of recently studied [1][2][3][4][5][6][7][8] molecules in which the enol structure is observed to predominate include hydroxycyclopropenone [8] and phenol [6,7]. Ab initio calculations of the tautomeric equilibrium constant on the latter are found to be in excellent agreement with experiment, and confirm the exclusive presence of phenol, while high-level computations on the former species in both gas and aqueous solution phase predict that equilibrium is directed toward the enol rather than to 1,2-cyclopropanedione.…”

New theoretical insights on tautomerism of hyperforin—a prenylated phloroglucinol derivative which may be responsible for St. John’s wort ( Hypericum perforatum ) antidepressant activity

“…Reproducing this experimental result by theoretical calculations is not easy, due to large dependence of the keto-enol energy gap on the approximations of the theoretical model. According to an early calculation at the Hartree-Fock level of theory, the 3OH enol tautomer was predicted to be more stable than keto by as much as 35 kcal/mol [20]. On the other hand, some recent calculation using semi-empirical PM3, DFT and MP2 Single Point calculations resulted in predicting keto tautomer to be more stable by several kcal/ mol [14].…”