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

A chemical shift vs. temperature analysis of beta-thioxoketones has been pe rformed for the four beta-thioxoketones, thioacetylacetone (1), benzoylthio acetone (2), thiobenzoylacetone (3) and monothiodibenzoylmethane (4), to te st 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. Analys is of chemical shifts and isotope effects on these reveals that a three com ponent system is involved in the tautomeric equilibria for 1 and 2. The thr ee components are the intramolecularly hydrogen-bonded (Z)-enol form (A), t he intramolecularly hydrogen-bonded (Z)-enethiol form (B) (which are interc onverting 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. T he 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 d egrees values as well as chemical shifts for the individual tautomers. It is demonstrated how deuteriation of the chelate proton may lead to analy sis of a complex three species equilibrium system of which only one compone nt can be observed directly. The large negative isotope effects observed ar e due to large equilibrium isotope effect contributions. A very large shift in the equilibrium is observed upon deuteriation. The negative primary iso tope 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 an d indicate strong hydrogen bonds. (C) 2000 Elsevier Science B.V. All rights reserved.