Ullmann reaction
Abstract
Ullmann reaction is a biaryl coupling reaction promoted by copper. This reaction typically requires very high temperatures (around 200 °C), which constitutes one of its main limitations compared to modern transition-metal-catalyzed cross-coupling reactions.
Keywords
Aryl halide, Copper, Homo coupling
Category
Aromatic, Organometallic catalysis
Mechanism
The Ullmann reaction begins with an oxidative addition of the aryl halide to copper(0), generating a copper(II) aryl complex (1). A subsequent transmetalation step (2), facilitates the displacement of the halide ligand by a second aryl group with a second oxidative addition, producing a copper(III) complex (3). Finally, reductive elimination from this complex affords the symmetrical biaryl product (4).
