Cu-Zeolite Catalysis for Biaryl Synthesis via Homocoupling Reactions of Phenols or Aryl Boronic Acids.

Due to the importance of biaryls as natural products, drugs, agrochemicals, dyes, or organic electronic materials, a green alternative biaryl synthesis has been developed based on easy-to-prepare and cheap copper(I)-exchanged zeolite catalysts. Cu-USY proved to efficiently catalyze the direct homocoupling of either phenols or aryl boronic acids under simple and practical conditions. The Cu-USY-catalyzed oxidative homocoupling of phenols could conveniently be performed under air either in warm methanol or water with good to high yields. In methanol, a small amount of CsCO was required, while none was necessary in water. The homocoupling of aryl boronic acids was best performed also in warm methanol, without an additive. These mild conditions showed good functional-group tolerance, leading to a variety of substituted (hetero)biaryls (28 examples). The heterogeneous Cu-USY catalyst could readily be recovered and reused. Interestingly, the homocoupling of vinyl boronic acids was successfully coupled to a Diels-Alder reaction, even in a one-pot process, allowing access to highly functionalized cyclohexenes.