Kinetic and product studies on Ullmann amination of 1-halogenoanthraquinones catalysed by copper(I) salts in acetonitrile solution

Abstract

The kinetics and products of reactions of some primary amines (RNH₂) with 1-halogenoanthraquinones (AQX) promoted by copper salts, particularly tetrakis(acetonitrile) copper(I) tetrafluoroborate, have been investigated in acetonitrile solution at 70 °C. Provided that oxygen does not come into contact with solutions of the copper(I) salt and amine, the kinetics of the reaction have the simple form v=k[AQX][RNH₂][Cu¹], and the products consist almost entirely of the aminated anthraquinone, AQNHR, and dehalogenated material, AQH, their ratio being directly proportional to [RNH₂]. The reaction rate is dependent on the identity of the departing halogen X, decreasing in the sequence I > Br > Cl, but the product ratio is little affected. N-Deuteriation of the reactant amine gives rise to a small kinetic isotope effect, but the product ratio is unaffected. Conversely deuteriation on the α-carbon atom of the amine has little kinetic effect but leads to a four-fold increase in the ratio of aminated to dehalogenated product. The observations are interpreted in terms of (i) formation of a copper(I)–amine complex, (ii) activation of this species for attack on AQX by proton abstraction from an amine ligand by a free molecule (stepwise or concerted), and (iii) generation of an arylcopper(III) intermediate which is partitioned between formation of AQNHR by attack of an external amine molecule and formation of AQH by an intramolecular process involving hydrogen transfer from the α-carbon atom in the amide ligand generated in (ii). Exposure of the initial copper(I)–amine complex to oxygen leads to a new complex, itself capable of reacting with AQX, which on prolonged incubation at 70 °C in the presence of an excess of amine is transformed back to its original state.