Tautomerism of 4-amino- and 4-arylamino-1,2-naphthoquinones.

Abstract

The 13C chemical shifts of 4-amino- and 4-arylamino-1, 2-naphthoquinones measured in DMSO-d6, pyridine-d5 and a D2O solution of NaOD indicated that the predominant tautomer in neutral solvents is the 1, 2-dioxo-4-amino-naphthalene (Ia) form, while that in weak basic solvents is the 1-oxo-2-hydroxy-4-imino-naphthalene (Ib) form and that in an aqueous solution of NaOH is the anion of the Ib form. The electronic spectrum of 4-anilino-1, 2-naphthoquinone measured in a mixture of EtOH and 0.05M H3SO4 (1 : 4 by volume) indicated that its tautomeric form in an aqueous strong acid solution is the Ia form and that the compound is hydrolyzed quickly to produce 2-hydroxy-1, 4-naphtho-quinone. The magnitudes of substituent effects observed in the electronic spectra of 4-(4'-substituted)-anilino-1, 2-naphthoquinones measured in EtOH were found to be smaller than those measured in pyridine. This difference was considered to provide confirmation that the predominant tautomer in neutral solvents is the Ia form. The theoretical π-π* transition energies of 4-arylamino-1, 2-naphthoquinones were calculated for both the Ia and the Ib forms by means of Pariser-Parr-Pople type molecular orbital calculations.