Understanding Thermal and Photochemical Aryl-Aryl Cross-Coupling by the AuI/AuIII Redox Couple

Systematic mechanistic investigations of the gold(I)/gold(III) redox-controlled aryl-aryl cross-coupling reaction have been performed by using both a thermal and photochemical approach. Electron-deficient and electron-rich arenes were considered as the coupling partners of the reaction. Based on transition-state modeling and distortion/interaction analyses, it is shown that Au-I prefers to react with electron-deficient arenes whereas Au-III likes to activate electron-rich arenes. This orthogonal reactivity of gold makes it an efficient catalyst for the aryl-aryl cross-coupling reaction. The crucial role of the carboxylate ligand in the reaction has been elucidated through analysis of the transition states. It is shown that due to the presence of two coordination sites, a carboxylate ligand can stabilize the transition state more efficiently than other monodentate ligands such as chloride (Cl-). Moreover, carbon-boron transmetalation is shown to be favorable over direct C-H metalation, hence reactions initialized by C-B transmetalation are expected to be much faster and selective. Additionally, a dual photoredox/gold catalyst was employed to access the Au-I/Au-III catalytic cycle for the cross-coupling reaction. [Ru(bpy)(3)](2+) was used as the photoredox catalyst for the reaction, which, on excitation, transfers an electron to one of the coupling partners, namely a diazonium salt (ArN2+), and initializes the cycle.