Experimental and theoretical study of tautomerism in 1,4-bis[methoxyamino]anthracene-9,10-diones and their reduced forms

Treatment of leucoquinizarin 4a with methoxylamine hydrochloride (O-methylhydroxylamine hydrochloride) in pyridine led to tautomer 4e. Oxidation of 4e in refluxing nitrobenzene led to 1,4-diaminoanthracene-9,10-dione 1e while oxidation with manganese dioxide yielded 1,4-bis(methoxyimino)-2,3-dihydroanthracene-9,10-dione 6. Attempted diplacement of the fluorides from 1,4-difluoroanthracene-9,10-dione by methoxylamine in dimethyl sulfoxide led to 9,10-dihydroxy-1,4-bis(methoxyimino)anthracene 2h instead of the anticipated bis(methoxyamino) tautomer 1h. Calculations have been carried out on some of these derivatives using the 3-21G basis set and the results for 2h compared with X-ray data from a single crystal. The calculated results show that that 1h is marginally preferred over 2h by 0.68 kcal mol–1. The reduced forms show the opposite trend with 4e preferred by 11.3 kcal mol–1 over 3e in line with NMR data recorded in chloroform. The presence of an oxygen atom adjacent to the nitrogen atom in the methoxyimine substituent forces a tetrahedral conformation at nitrogen resulting in a destabilization of the anthracene-9,10-dione tautomers 1h and 3e.