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

Abstract

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.