A Novel Insight into the Ullmann Homocoupling Reactions Performed in Heterogeneous Catalytic Systems

Abstract

The Ullmann reaction has been reported to be the first cross-coupling reaction performed by using a transition metal catalyst. This reaction has been initially considered as the copper-catalyzed homocoupling of aryl halides, leading to the formation of symmetrical biaryl compounds via the generation of novel C–C bonds. Although this reaction has been extensively studied in recent decades and valuable results have been achieved, there are still considerable efforts focused on the development of novel catalytic systems, mild reaction conditions, and extended substrate scope. The mechanistic aspects of the Ullmann homocoupling reaction have also been investigated, as related to the introduction of new sustainable strategies and green procedures. The application of recyclable heterogeneous catalysts has been found to overcome most of the limitations associated with the harsh reaction conditions of the original Ullmann reaction. More recently, copper-based catalytic systems have also been replaced by palladium nanoparticles, ionic palladium species, gold nanoparticles, and palladium–gold bimetallic systems. In this review, current results reported on the Ullmann homocoupling reaction are discussed, with an emphasis on the development of novel catalytic systems, which can be efficiently used under heterogeneous conditions.

On the Current Status of Ullmann-Type N-Arylation Reactions Promoted by Heterogeneous Catalysts

“…Whereas recent studies on the Ullmann homocoupling reactions clearly indicated the predominance of heterogeneous Pd catalysts [15], C-N heterocoupling reactions have still been preferably conducted by using more readily available and less toxic Cu-based catalysts [16][17][18][19][20][21]. The subject of the current review is the presentation of the recent progress achieved in the Ullmann-type N-arylation reactions performed in heterogeneous systems, including the utilization of Cu and Pd-based catalysts and magnetic nanoparticles.…”

“…Novel synthesis procedures based on bilization of catalytically active metal species or organometallic complexes on v port materials provided reusable heterogeneous catalysts with improved pe [14]. Whereas recent studies on the Ullmann homocoupling reactions clearly in predominance of heterogeneous Pd catalysts [15], C-N heterocoupling reaction been preferably conducted by using more readily available and less toxic Culysts [16][17][18][19][20][21]. The subject of the current review is the presentation of the rece achieved in the Ullmann-type N-arylation reactions performed in heterogeneo including the utilization of Cu and Pd-based catalysts and magnetic nanopart…”