Direct Amination of Anthraquinone in Basic Solvents

Abstract

The direct amination of anthraquinone in the presence of butylamine and the rhodium (I)complex was examined in basic solvents. Some methyl-substituted pyridines as solvents enha. nced the yield of 1- (butylamino) anthraquinone [1 a] in the presence of [RhCl (PPh₃)₂]₂ equimolar to anthraquinone. In β- and γ-picolines and 3, 4- and 3, 5-lutidines, the amination product [1 a] was obtained in better yield than in diglyme, while pyridine, α-picoline, quinoline and isoquinoline gave. low yields. Higher basicity and smaller steric requirement of the solvents seem to be preferable for the formation of [1 a] (Table 2). The yield of [1 a] rose to about 50% in β-picoline (3.75 ml) under the following optimum conditions; anthraquinone: 25 mmol/l, butylamine: 2.6 mol/l, [RhCl (PPh₃)₂]₂: 40 mmol/l at 110°C for 10 h in air (Table 3and Figs.1, 2 and 3).
In the reaction of an thraquinone-d₈ with butylamine in β-picoline under N₂, [RhCl (PPh₃)₂]₂ catalyzed a D-H exchange reaction at the α-positions of anthraquinone to afford anthraquinone2, 3, 6, 7-d₄ in 91% yield (Table 4). From these results a tentative reaction mechanism for the amination was proposed (Fig.4).