Direct C - H arene homocoupling over gold nanoparticles supported on metal oxides

The direct C - H/C - H bond coupling of dimethyl phthalate was performed successfully over supported gold nanoparticle catalysts. Gold on reducible metal oxides, such as Co₃O₄, and on inert oxides that have an oxygen-releasing capacity, such as ZrO₂, showed the highest catalytic activity for the production of biphenyl tetracarboxylate using O₂ as the sole oxidant. Supported Pd(OH)₂ also catalyzed the reaction, but the catalytic activity was inferior to that of gold. Moreover, the gold catalysts exhibited excellent regioselectivity for the synthesis of valuable 3,3′,4,4′-tetrasubstituted biphenyls by coupling with each other at the 4-position without the need for additional ligands. Gold catalysts also promoted the oxidative homocoupling of arenes including o-xylene to give symmetrical biaryls with high regioselectivity. X-ray absorption fine structure measurements revealed that the catalytically active species was Au⁰ and that the lattice oxygen of Co₃O₄ played an important role in the gold-catalyzed oxidative coupling. The results of the kinetic studies were consistent with an electrophilic aromatic substitution pathway. Regioselectivity is not controlled by directing groups or the electronic character of the substituents but by steric hindrance, which suggests that gold nanoparticles not only catalyze the oxidative coupling but also act as bulky ligands to control the regioselectivity.