Simultaneous determination of primary and secondary thermodynamic isotope effects in tautomeric equilibria

NMR determination of site-specific hydrogen isotope ratios at natural deuterium abundance (SNIF-NMR) provides the basis for simultaneous access to primary and secondary thermodynamic fractionation factors in exchange reactions and avoids the need for selective isotope labelling of the reagents. The method was applied to the measurement of fractionation parameters involving OH, CH2, CH3 and =CH groups in keto–enol tautomeric equilibria. The fractionation factors relating the =CH and OH sites of the enol species are simply derived from 2H NMR spectra whereas the determination of isotope parameters which relate keto and enol positions exploits a combination of 2H and 1H NMR experiments. Since only monolabelled isotopomers have to be considered at natural abundance, the method also offers the advantage of avoiding the occurrence of complex equilibria associated with multi-labelled species possibly introduced by deuterium enrichment. The primary equilibrium isotope effects illustrate a preference of deuterium for the methylene fragment of the keto form with respect to the ethylenic position of the enol tautomer. Since the enol species is itself engaged in a fast tautomeric equilibrium associated with a symmetric or unsymmetric double minimum potential, the thermodynamic parameters are averaged over the exchanging partners. It is shown that the average thermodynamic fractionation factor relating the OH and =CH hydrogens of the enol are significantly influenced by the nature of the substituents at both carbonyl positions of the β-diketones. Moreover, methyl and chlorine substitution increases by a factor of about 1·1 the thermodynamic isotopic fractionation factor relating the –COCHCO– position of the keto form to the hydroxyl position of the enol.