Buchwald and co-workers first reported the use of (CuOTf) 2-benzene as catalyst, with addition of 1,10-phenanthroline (Phen) 213 or dibenzylideneacetone (dba) 217 ( Figure 38) as ligands, to facilitate the N-arylation of imidazoles or benzimidazoles with aryl bromides or iodides ( Scheme 51). Due to its eco-friendly nature and low cost, relative to most other transition metals, copper-catalyzed reactions have received great attention since 1995. Classical Ullmann reactions use stoichiometric amounts of copper metal to mediate the N-arylation of an amine. The century-old Ullmann reaction is usually carried out under rather harsh conditions.
