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).