Room-temperature, ligand- and base-free Heck reactions of aryl diazonium salts at low palladium loading: sustainable preparation of substituted stilbene derivatives.

The Pd(OAc)(2)-catalyzed Heck reaction of aryl diazonium salts with 2-arylacrylates led to cis-stilbenes with good to excellent stereoselectivity. The environmentally friendly protocol developed in this work features low palladium loading in technical-grade methanol at room temperature under base-, additive-, and ligand-free conditions. The same protocol applied to simple Heck coupling of aryl diazonium salts with methyl acrylate allows astonishingly low palladium loading, down to 0.005 mol%. The stereoselectivity experimentally observed for the synthesis of cis-stilbenes has been rationalized by DFT calculations. Moreover, the role of methanol in promoting the reaction has been clarified by a computational study.