STUDY OF THE ENOL-ENAMINONE TAUTOMERISM OF ALPHA-HETEROCYCLIC KETONESBY DEUTERIUM EFFECTS ON C-13 CHEMICAL-SHIFTS

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