Colloidal Gold Nanoparticles as Catalyst for Carbon−Carbon Bond Formation:  Application to Aerobic Homocoupling of Phenylboronic Acid in Water

Gold nanoparticles (<2 nm) stabilized by poly(N-vinyl-2-pyrrolidone) (Au:PVP NPs) were prepared by reduction of AuCl4- with NaBH4 in the presence of PVP and characterized via an array of methods including optical absorption spectroscopy, transmission electron microscopy, X-ray diffraction, X-ray absorption near-edge structure, extended X-ray absorption fine structure, and X-ray photoelectron spectroscopy. We demonstrate for the first time that the Au:PVP NPs act as catalyst toward homocoupling of phenylboronic acid in water under aerobic conditions. Suppression of biphenyl formation under anaerobic conditions indicates that molecular oxygen dissolved in water is intimately involved in the coupling reactions. A mechanism of the aerobic homocoupling catalyzed by the Au:PVP NPs is proposed on the basis of a crucial role of dissolved oxygen, steric effects on the product yields, and the well-established mechanism for the Pd(II)-based catalysts.