A Novel Direct Amination of Anthraquinone catalyzed by Rhodium(1) Complexes

Abstract

Anthraquinone was found to react with an alkylamine in the presence of a rhodium( I)complex to give 1- (alkylamino anthraquinone ) along with a small amount of 1, 4-his (alkylamino)anthraquinoneC2 J. This direct amination occurred only at thecr-position of anthraquinone and gave a high selectivity for [1].
Though oxygen was necessary for the amination, it was found to be advisable to reflux the reaction mixture under nitrogen atmosphere, before the reaction was carried out in air (Table 1). Among the metal complexes examined under the reaction of anthraquinone with butylamine in diglyme, the dimeric complex, [RhCl(PPh₃)₂]₂, proved to be most effective. The result seems to show that the formation of a monomeric rhodium ( I ) complex, which is unsaturated in coordination, plays an important role in giving an active species for the amination. The yield of [1] based on the rhodium quantity was in excess of 100% when [RhCl(PPh₃)₂]₂was used as a catalyst, suggesting the efficient recycle of the rhodium ( I ) comlex. The highest yield of (I) was obtained at 120°C under the following conditions; the rhodium(I)complex 2.70 mmolg, an amine 2.6 mol/l and anthraquinone 51 mmolg. The yield was depressed by the oxidation of amine caused by rhodium( I ) complex (Tables 3 and 5). The electronic and steric effects of amines on the reaction were also examined (Table 4). When equi-gram rhodium in [RhCl(PPh₃)₂]₂ to anthraquinone was used, the yield of [1] reached to 40% in j9-picoline, contrary to the results in diglyme (Table 7).