Aryl-aryl bond formation is one of the most important tools in modern organic synthesis.
Therefore, there is a high level of interest to develop green, effective reaction system to obtain biaryls.
This review summarized the recent advances in the metal-catalyzed Ullmann reaction in which
the aryl-aryl bond was formed directly. Furthermore, different types of catalytic mechanisms, especially
the surface reaction, have been summarized to help the design of the catalyst.