Oxygen-17 Nuclear Magnetic Resonance Studies of the Equilibria between the Enol forms of β-Diketones

A method for determining the relative concentrations of the two enol tautomers A and B of asymmetric β-diketones (eq la and b) using ¹⁷O nmr spectroscopy is described. The ¹⁷O spectra of eight ¹⁷O-enriched asymmetric β-diketones were studied. Two sets of lines were observed: (a) one broad peak in the range — 500 to — 600 ppm (downfield from H₂¹⁷O) due to the keto tautomer, and (b) two peaks in the range 0 to — 500 ppm due to the two nonequivalent oxygens of the enol tautomers. From the chemical shift of the enol peaks the equilibrium constant, K = [B]/[A], is calculated. The results show a small but systematic effect of ring size and substituents on K: (i) six-membered rings favor endocyclic double bonds while five-membered rings favor exocyclic double bonds; (ii) structure A is increasingly stabilized when R is changed from methyl to hydrogen to phenyl.