Mechanochemical aerobic oxidative Heck coupling by polymer-assisted grinding: cyclodextrin additive facilitating regioselectivity control

Abstract

The oxidative Heck reaction is a well-known organic transformation. However, previous reactions have been limited by the need for sophisticated directing or auxiliary groups, reliance on bulk solvents, lack of catalyst recycling, and the formation of a mixture of isomeric products, thereby hindering its broader application. Herein, we present a novel approach utilizing the polymer-assisted grinding (POLAG) technique to facilitate the regioselective oxidative Heck coupling of aryl boronic acids with electronically unbiased olefins under solvent-free conditions. This catalytic reaction is facilitated by a ligand-free Pd(TFA)₂ catalyst in conjunction with cyclodextrin (CD) serving as a POLAG additive and molecular oxygen as the green oxidant. Mechanistic studies have indicated that cyclodextrin not only stabilizes the palladium catalyst but also influences the reaction selectivity by imposing steric hindrance on the substrates. Recovery experiments and electron microscopy images demonstrate an effective recovery capability of Pd/CD, with the catalyst morphology largely preserved after five cycles. XPS experiments confirm the formation of a new palladium catalytic species during the ball-milling process. Remarkably, this reaction takes place at ambient temperature, obviating the need for additional stoichiometric amounts of hazardous oxidants and toxic solvents, aligning with environmentally friendly practices upon green metrics evaluation.