Study of the enol–enaminone tautomerism of α-heterocyclic ketones by deuterium effects on 13C chemical shifts

Abstract

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.