Photochemical substitution of amino- and hydroxy-anthraquinones

Irradiation of 1-aminoanthraquinone with visible light in the presence of an excess of either sodium sulphite or sulphide in 50% aqueous pyridine gives exclusively sodium 1-aminoanthraquinone-2-sulphonate and -2-thiolate, respectively, in good yield. Under similar conditions with sodium sulphite, 1-methylamino-, 1-amino-4-chloro-, and 1-amino-5-chloro-anthraquinone give the respective sodium 2-sulphonate only, while 2-aminoanthraquinone gives the sodium 3-sulphonate. In contrast, 1-hydroxyanthraquinone gives a mixture of the sodium 2- and 4-sulphonates and disodium 2,4-disulphonates. The different substitution pattern observed for 1-amino- and 1-hydroxy-anthraquinone has been rationalised by application of semi-empirical molecular orbital theory.