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 (Phi(NH(2))) of conversion to A2 are small in inert solvents and increase with the donor concentration, approaching Phi(NH(2)) = 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.

Direct and ketone-sensitized photoconversion of 1-nitro-9,10-anthraquinone to 1-amino-9,10-anthraquinone mediated by donor radicals

“…The synthesized (A2 and N2) and the purchased (A1, N1, N2: Aldrich, TCI) AQs are the same as used previously [27]. The other compounds and the solvents were used as commercially available (Aldrich, Fluka, Merck) and checked for impurities.…”

“…For various donor concentrations the quantum yield of conversion was obtained, keeping the initial absorbance at k irr = 313 nm constant. The quantum yields U H 2 and U NH 2 were determined using the aberchrome 540 actinometer and the identity of the products was checked by HPLC analysis [27]. These analyses were performed on a reverse phase ODS-3 (Perfect Target, 3 lm) column, 0.8 ml min À1 , with mobile phase gradient, composed of 0.5% trifluoroacetic acid and either acetonitrile-water (1:5) or neat acetonitrile as eluent.…”

“…The photoreduction of 1-nitro-2-R-AQ (R = H: N1, methyl: N2) to the corresponding 1-aminoAQs (A1, A2) in benzene and acetonitrile in the presence of phenylethylamine and 1-or 2-phenylethanol was the subject of a preceding study [27]. The triplet lifetime of N2 is s T = 5-20 ns at room temperature and that of N1 is shorter.…”

“…Radicals derived form 1-nitroAQ and the (H-atom or electron) donors are intermediates. An alternative pathway via the 1-nitrodihydroAQ (O 2 NAQH 2 ), pathway b in Scheme 2, had been rejected [27]. For conversion of 1-nitroAQ and 2-nitroAQ in 2-propanol to NHOHAQs quantum yields of 0.0001 and 0.047 for k irr = 254 nm and 0.0001 and 0.0015 for k irr = 365 nm have been reported, respectively [25].…”

“…0.01 mM acetophenone (as trace impurity) function as donor and sensitizer, respectively. The quantum yield (U NH 2 ) of full conversion of N2 to A2, determined by photochemical means, is up to 0.2 in 1-phenylethanol [27]. Methanol, ethanol or 2-propanol are attractive H-atom donors, whereas amines, e.g.…”