Fritz Duus

β-Thioxoketones. Part 9. A dynamic 1H nuclear magnetic resonance spectroscopic study of thioacetylacetone and related β-thioxoketones. Direct observation of the enol and enethiol tautomeric constituents and their interconversion

Thioacetylacetone (I), 2-acetylcyclohexanethione (II), and 2-thioacetylcyclohexanone (III) have been studied by dynamic 1H n.m.r. spectroscopy from ambient temperature down to ca. 130 K. The general co-existence of two tautomeric constituents has been established, and the constituents, in the cases of (I) and (III), directly observed. These were identified as the strongly chelated (Z)-enol form and the weakly chelated (Z)-enethiol form. Band-shape analysis provided ΔG‡(enol → enethiol) 8.9 ± 0.1 kcal mol–1 for (I) and 7.6 ± 0.1 kcal mol–1 for (III).

β-Thioxo-ketones. Part 5. Photo-induced enol–enethiol interconversion of β-thioxo-ketones

Thioacetylacetone, which exists in 2-methylbutane–methylcyclohexane (5 : 1) solution at 95 K exclusively as the intramolecularly hydrogen-bonded enol tautomer, is converted upon irradiation at 353 nm into the corresponding enethiol tautomer, characterized by its absorption at 288 nm. The reverse process takes place upon irradiation of the enethiol tautomer at 288 nm. Both processes are successively repeatable. Other isomeric and/or tautomeric forms have not been observed. Monothiodibenzoylmethane behaves similarly upon photolysis at 95 K.

Definitive evidence for the existence of the hydrogen-bonding enol form of non-aromatic β-thioxoketones. X-Ray crystal structure of 1-(1-methylcyclopropyl)-3-thioxobutan-1-one

1-(1-Methylcyclopropyl)-3-thioxobutan-1-one, a simple non-aromatic β-thioxoketone, has been found by X-ray diffractometry to exist exclusively in the hydrogen-chelating (Z)-enol form in the crystalline state.

β-Thioxoketones. Part 6. Electronic absorption spectra of aromatic β-thioxoketones. A study of enol–enethiol tautomerism

Aromatic β-thioxoketones exist in solution as mixtures of rapidly interconverting Z-enol and Z-enethiol tautomers. The electronic absorption spectra exhibit in general four absorption bands in the u.v.–visible region at ca. 265 (ArCO, π,π*; ArCC π,π*), 330 (ArCS π,π*; OCCCS π,π*; CO n,π*), 415 (OCCCS π,π*), and 520 nm (CS n,π*), respectively. The β-thioxoketones are converted by sodium hydroxide into the corresponding anions. CNDO/B Calculations predict that the negative charge in the β-thioxoketonates is delocalized over the OCCCS system, suggesting simultaneously sickles or W shaped conformations. Two characteristic absorption bands found for the β-thioxoketonates at ca. 275 and 400 nm are assigned to π,π* transitions involving the Ar–CCC–Ar′ and SCCCO chromophores, respectively. The enol–enethiol tautomeric equilibrium has been studied by means of low temperature spectroscopy. At room temperature equilibrium constants (K293) of 3–5 have been found corresponding to a 4 : 1 enol–enethiol concentration ratio. The reaction entropy (ΔSr) has been found to be negative for the enethiol→enol conversion, reflecting the intramolecular O–H ⋯ S hydrogen bond to be considerably stronger than the corresponding O ⋯ H–S hydrogen bond. Variations in ΔSr and K293 as functions of substitution in the aryl group next to oxygen are discussed.

On hydrogen bonding in the intramolecularly chelated tautomers of enolic malondialdehyde and its mono- and dithio-analogues

The intramolecular hydrogen bondings in enolic malondialdehyde and it mono- and dithio-analogues have been evaluated by a semiempricial SCF–MO–CNDO method. The calculations predict that the hydrogen bonds play an important part in the stabilities of malondialdehyde and monothiomalondialdehyde, whereas dithiomalondialdehyde hardly exists as a hydrogen-chelated tautomeric form.