Spectroscopic Studies of the Intermediates in the Conversion of 1,4,11,12-Tetrahydro-9,10-anthraquinone to 9,10-Anthraquinone by Reaction with Oxygen under Basic Conditions

Abstract

The intermediates of the 1,4,11,12-tetrahydro-9,10-anthraquinone (THAQ) to 9,10-anthraquinone (AQ) reaction are studied by various spectroscopic and computational methods. X-ray diffraction, two-dimensional nuclear magnetic resonance (NMR) spectroscopy, and geometry optimization calculations, by the UB1LYP hybrid density functional technique, show that THAQ initially exists in the keto form (cisTHAQK). Addition of very small amounts of NaOH to cisTHAQK in solution catalytically converts it into the corresponding enol form (THAQE). The THAQE is then oxidized, by dissolved O2 in solution, to give the novel 1,4-dihydro-9,10-anthraquinone (DHAQ) which is isolated, and its single-crystal structure is characterized by X-ray spectroscopy. If NaOH is added to THAQE, the 1,4-dihydro-9,10-anthrasemiquinone (DHASQ) radical anion is produced. It is detected by electron paramagnetic resonance (EPR) spectroscopy, and its experimental nuclear hyperfine coupling constants are correlated with those computed by the UB1LYP ...