Variable temperature 1H and 13C NMR spectroscopic investigation of the enol-enethiol tautomerism of β-thioxoketones.: Isotope effects due to deuteron chelation

A chemical shift vs. temperature analysis of beta-thioxoketones has been performed for the four beta-thioxoketones, thioacetylacetone (1), benzoylthioacetone (2), thiobenzoylacetone (3) and monothiodibenzoylmethane (4), to test this method as a general way of obtaining the individual chemical shifts of tautomers involved in tautomeric equilibria. Both C-13 and chelate H-1 resonances far 1 and 2 showed a coalescence point subsequently followed by observation of two sets of resonances when lowering the temperature. Analysis of chemical shifts and isotope effects on these reveals that a three component system is involved in the tautomeric equilibria for 1 and 2. The three components are the intramolecularly hydrogen-bonded (Z)-enol form (A), the intramolecularly hydrogen-bonded (Z)-enethiol form (B) (which are interconverting rapidly by intramolecular proton transfer/electron redistribution) and the non-proton chelated (Z)-enethiol form (C). This third species is observable at low temperature in CD2Cl2 as well as in mixtures of freons. The hydrogen-bonded (Z)-enol and (Z)-enethiol forms A and B appear to be in equilibrium at all obtainable temperatures. The analysis of the data for 3 and 4 leads to Delta H degrees and Delta S degrees values as well as chemical shifts for the individual tautomers. It is demonstrated how deuteriation of the chelate proton may lead to analysis of a complex three species equilibrium system of which only one component can be observed directly. The large negative isotope effects observed are due to large equilibrium isotope effect contributions. A very large shift in the equilibrium is observed upon deuteriation. The negative primary isotope effects found for the chelate protons are resolved into intrinsic and equilibrium parts. The large positive intrinsic effects clearly point to a two-potential well in agreement with results from UV and IR measurements and indicate strong hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved.