Tautomerization of Phenols, Part III, The Anthrone−Anthrol Equilibrium in Aqueous Solution

Abstract

Dedicated to Prof. A. Jerry Kresge on the occasion of his 75th birthdayThe equilibrium between 10H-anthr-9-one and 9-anthrol favors the ketone, which ionizes as a carbon acid in aqueous base. Rates of equilibration were measured over the pH range 1 ± 13 in aqueous solution (258, ionic strength I 0. m). Five independent thermodynamic and kinetic parameters were determined by analysis of the pH-rate profile: the equilibrium constant of enolization, pK E 2.17, the ionization quotient of anthrol, pQ E a 7.84, and the rate constants of enolization catalyzed by acid, k E H 2.2 ¥ 10 À4 m À1 s À1 , base, k E HO À 51.0 m À1 s À1 , and water, k E 0 1.21 ¥ 10 À5 s À1 . Structure-reactivity relationships strongly support the view that pH-independent enolization of anthrone in water proceeds by rate-determining ionization of the C-acid.Introduction. ± KetoÀenol tautomerism is a cornerstone of organic reactivity. Ninety years ago, Meyer [1] achieved the separation of solid 9-anthrol 2 ) (E) from its more stable tautomer 10H-anthr-9-one 3 ) (K). He also determined the equilibrium constant of enolization in EtOH solution,. Subsequent studies have dealt with the solvent [3 ± 9] and temperature [10] dependence of the anthroneanthrol equilibrium. Apolar solvents favor the ketone, hydrogen-bond acceptor solvents such as DMSO favor the enol. Kinetic data were reported for the enolization of anthrone induced by base catalysts in acetonitrile solution [6] and in apolar solvents [5]. The photophysical properties and photochemical reactions of anthrone were also investigated [11 ± 18].Over the last two decades, accurate equilibrium constants of enolization covering thirty orders of magnitude have been determined kinetically, i.e., as ratios of enolization and ketonization rate constants in aqueous solution [19]. Few data are available for compounds with K E close to unity [20,21]. Such data are of interest for free-energy relationships and to determine the intrinsic barrier to proton transfer involving C-atoms. To fill the gap between ketones (pK E % 8) and carboxylic acids or esters (pK E % 18), which strongly favor the keto form, and phenol at the other extreme {pK E (cyclohexa-2,4-dienone) À 12.7 [22]}, we have determined the kinetic and thermodynamic parameters of the anthrone-anthrol equilibrium in H 2 O.

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.…”

Ultrafast proton coupled electron transfer (PCET) dynamics in 9-anthranol-aliphatic amine system

“…Figure 1A shows optical absorption spectra of 9-anthrone/9-anthranol in tetrachloroethylene (TCE, non-interacting solvent), DEA and TEA (electron donating solvents). Freiermuth et al [5] reported that 9-anthrone/9-anthranol exist in different tautomeric forms, namely like 9-anthrone (ANT) in TCE and acetonitrile, but 9-anthranol (ANTH) in aliphatic amine solvents, such as DEA and TEA, whereas both tautomers are present in alcoholic solvents. The presence of ANTH form in amine solvents is clearly confirmed by the appearance of a new band in 330 -450 nm region of the optical Figure 2B).…”

The energetics of the isomeric anthrols

“…The tautomerization enthalpy of 9-anthrol and 9-anthrone Although tautomerization reaction (4) is not isodesmic, we may nonetheless compare its enthalpy values, which are found from taking the difference of our quantum chemically calculated enthalpies of formation of the two isomers, and from experiment and associated analysis [16].…”