Photoreduction of 2-methyl-1-nitro-9,10-anthraquinone in the presence of 1-phenylethanol

Abstract

The photoreactions of 1-nitro-9,10-anthraquinone (N1) and 2-methyl-1-nitro-9,10-anthraquinone (N2) were studied in benzene and acetonitrile in the presence of 1-phenylethanol. For N2, a short-lived 10 ns transient observed upon flash photolysis is attributed to a triplet state, which can be intercepted by 1-phenylethanol to form a monohydro radical of N2 and a spectroscopically not detectable donor-derived radical. The decay of radicals yields the corresponding nitroso compound and eventually 1-amino-2-methyl-9,10-anthraquinone (A2) as photoproduct. The final reduction step requires participation of the anthraquinone carbonyl groups. The yield of radicals and the quantum yield (Φ_NH2) of conversion to A2 are small in inert solvents and increase with the donor concentration, approaching Φ_NH2 = 0.2. No triplet state was observed in the flash photolysis of N1, but a N1-derived radical and 1-amino-9,10-anthraquinone (A1) as final products were found. Various mechanistic aspects of complete photoreduction of nitroarenes to aminoarenes are discussed.