Water-Soluble Copper (I) Hydroxide Catalysts in Ligand-Free Suzuki-Miyaura Cross-Coupling Reactions

Transition-metal catalyzed Suzuki-Miyaura (SM) cross-coupling is a powerful technique for constructing C-C bonds and has been widely used in the pharmaceutical, agrichemical, and natural product syntheses. Although Pd has been used as the major catalyst for SM reactions, Cu has recently emerged as an alternative to Pd because Cu is inexpensive, less toxic, and easily available. A survey of the literature shows that most Cu-catalyzed SM reactions were carried with organic ligand-stabilized Cu catalysts. For ligand-free reactions, reaction phases and active species of Cu catalysts remain under debate. In this work, we report findings of ligand-free Cu-catalyzed-SM reactions using CuI as a precatalyst and benzene iodide and phenylboronic acid as coupling agents in the presence of potassium hydroxide, water, and polyethylene glycol 300. The catalyst is water-soluble copper (I) hydroxide (CuOH) species in their singlet electronic states as identified by UV-Vis absorption spectroscopy and x-ray photoelectron spectroscopy. It is formed by leaching of Cu(I) nanoparticles, which serve as the reservoir of the Cu(I) catalyst. The Cu(I) nanoparticles are characterized using transmission electron microscopy and extended x-ray absorption fine structure spectroscopy. Catalytically active CuOH catalyst is in the solution rather than on the surface of the Cu precipitate. The soluble Cu(I) species is stable for at least four weeks under ambient conditions, a desirable feature for any catalyst.