Study of the Enol-Enaminone Tautomerism of α-Heterocyclic Ketones by Deuterium Effects on 13C Chemical Shifts.

Deuterium isotope effects on 13C chemical shifts have been measured for the enol and enaminone tautomers of a series of α-heterocyclic ketones. Partial deuteration of the exchangeable hydrogen bound to the oxygen atom of the enol or the nitrogen atom of the enaminone leads to deuterium induced shifts of the 13C frequencies (2DIS) which are distinctive for the two types of structures. Thus, 9-methyl-2-phenacyl-1,10-phenanthroline, 2-pyridylacyl- and 2-phenacyl-quinazolines and 2-pyridylacyl- and 2-phenacyl-quinolines, which are known from independent evidence to exist in the enaminone structures, display large and variable negative 4DIS values, –240, –93, –126, –437 and –375, respectively, at the carbon bearing the oxygen atom. By contrast, 2-pyridylacyl- and 2-phenacyl-pyrazines, which are known to exist in the enol form, show large positive 2DIS values, 527 and 479, respectively, for the oxygen bound carbon atom.