First direct observation of tautomerism of monohydroxynaphthazarins by IR-spectroscopy

Some substituted monohydroxylated naphthazarins (5,8-dihydroxy-1,4-naphthoquinones) were synthesized and studied by IR-spectroscopy in aprotic organic solvents at ambient temperature. Two narrow stretching mode bands in the high frequency range 3540-3410 cm(-1) due to a beta-hydroxy group were observed; it was established that this effect was caused by tautomerism. This allowed the creation of a convenient and accurate method for the measurement of a small amount of tautomer engaged in rapid exchange with the principal tautomer. Solvent and substituent effects were estimated. (C) 2002 Elsevier Science Ltd. All rights reserved.