H-1 AND C-13 NMR-SPECTRA OF ALPHA-HETEROCYCLIC KETONES AND ASSIGNMENTOF KETO, ENOL AND ENAMINONE TAUTOMERIC STRUCTURES

A method for distinguishing enol and enaminone tautomers of acylmethyl heterocycles is described based on differences in C-13 chemical shift between the carbonyl carbon atom of the enaminone (C=O) and enolic ca rbon atom of the enol (=C-OH). For 2-, 3- and 4-acetylmethyl, -phenacy l. and -pyridacyl-pyridines, -pyrazines, -quinolines, -quinoxalines, - phenanthrolines, -benzoxazoles and -benzothiazoles (usually in CDCl3 a s solvent) measured values of delta(c) fall in the non-overlapping ran ges 179-191 ppm for the enaminones and 161-171 for the enols. Both set s of chemical shifts depend on the electronegativity of the acyl subst ituent and for strongly electron-withdrawing groups, such as pyruvate, delta(c) for the enaminone does overlap the range for the enol. Howev er the difference in chemical shifts (Delta delta(c) similar to 20) ap pears to be nearly independent of substituent, and in these cases the value for the enaminone can be predicted from a correlation of delta(c ) with sigma for the acyl substituent based on data of Greenhill, Log hmani-Khouzani and Maitland (J. Chem. Soc. Perkin Trans. 1, 1991, 2831 ) for substituted quinolines. No other proton or carbon chemical shift differentiates the tautomers, but the structural assignments are corr oborated by (a) comparisons with N-methyl enaminone models for the ena minone tautomers, (b) coupling constants (J(34)) between 3- and 4-hydr ogen atoms of pyridine and quinoline rings and (c) allylic coupling be tween CH3 and vinyl hydrogens in enols of methyl ketones. With the exc eption of pyruvates, enols are observed only for 2-substituted heteroc ycles, usually as mixtures (in CDCl3) with the easily distinguished ke to tautomers, but for (2-substituted) quinolines and quinoxalines the enaminone tautomer predominates. Structural assignments in aqueous med ia or other solvents where solubility precludes NMR measurements follo w from correlations of UV-VIS with NMR spectra. NMR and UV-VIS measure ments are considered in detail for the example of 2-phenacylpyridine.