Copper-Catalyzed Homocoupling of Boronic Acids: A Focus on B-to-Cu and Cu-to-Cu Transmetalations

Controlling and understanding the Cu-catalyzed homocoupling reaction is crucial to prompt the development of efficient Cu-catalyzed cross-coupling reactions. The presence of a coordinating base (hydroxide and methoxide) enables the B-to-Cu(II) transmetalation from aryl boronic acid to (CuCl2)-Cl-II in methanol, through the formation of mixed Cu-(mu-OH)-B intermediates. A second B-to-Cu transmetalation to form bis-aryl Cu(II) complexes is disfavored. Instead, organocopper(II) dimers undergo a coupled transmetalation-electron transfer (TET) allowing the formation of bis-organocopper(III) complexes readily promoting reductive elimination. Based on this mechanism some guidelines are suggested to control the undesired formation of homocoupling product in Cu-catalyzed cross-coupling reactions.

Automated summary

Understanding and controlling the Cu-catalyzed homocoupling reaction is essential for the development of efficient Cu-catalyzed cross-coupling reactions. The presence of coordinating bases facilitates transmetalation reactions, while organocopper(II) dimers undergo a coupled transmetalation-electron transfer process promoting the formation of bis-organocopper(III) complexes. Guiding principles are suggested to avoid the undesired formation of homocoupling products in Cu-catalyzed cross-coupling reactions.