Cu(I)/Cu(III) catalytic cycle involved in Ullmann-type cross-coupling reactions

Copper-catalyzed cross-coupling reactions for C-heteroatom bond formation have attracted numerous research groups in the past 15 years aiming at finding more efficient methodologies under milder conditions. The use of auxiliary ligands has tremendously improved Ullmann-type couplings although a general methodology for different heteroatom-nucleophiles is still lacking. Mechanistic insights are seen as a clue for designing new effective, broad-scope and general methodologies. In this review we describe the widely discussed mechanistic options for this reaction and the use of model compounds to unravel key mechanistic aspects for copper-catalyzed C-heteroatom transformations. Stable aryl-Cu(III) species in model systems are shown to be reliable active catalysts in the coupling of a broad nucleophile scope such as phenols, amides, sulfides, selenides, phosphites, halides and also activated methylene susbtrates for carbon-carbon couplings.