Cooperative Au/Ag Dual-Catalyzed Cross-Dehydrogenative Biaryl Coupling: Reaction Development and Mechanistic Insight

An operationally simple and highly selective Au/Ag bimetallic-catalyzed cross-dehydrogenative biaryl coupling between pyrazoles and fluoroarenes has been developed. With this reaction, a wide range of biheteroaryl products can be obtained in moderate to good yields with excellent functional group compatibility. The exact role of silver salts, previously overlooked in most gold-catalyzed transformations, has been carefully investigated in this biaryl coupling. Insightful experimental and theoretical studies indicate that silver acetate is the actual catalyst for C–H activation of electron-poor arenes, rather than the previously reported gold­(I)-catalyzed process. An unprecedented Au/Ag dual catalysis is proposed, in which silver­(I) is responsible for the activation of electron-poor fluoroarenes via a concerted metalation–deprotonation pathway, and gold­(III) is responsible for the activation of electron-rich pyrazoles via an electrophilic aromatic substitution process. Kinetic studies reveal that ArFnAu­(III)-mediated C–H activation of pyrazoles is most likely the rate-limiting step.