Pd Nanoparticle Catalyzed Heck Arylation of 1,1-Disubstituted Alkenes in Ionic Liquids. Study on Factors Affecting the Regioselectivity of the Coupling Process

The Heck reaction of neutral or electron-rich aryl bromides with the 1,1-disubstituted olefins butyl methacrylate and R-methylstyrene catalyzed by Pd nanoparticles in tetrabutylammonium bromide as solvent and tetrabutylammonium acetate as base leads to a prevalent formation of the terminal olefin. In contrast, reaction of p-bromoacetophenone leads to the internal olefin. Whereas the solvent stabilizes the metal nanoclusters, the base is responsible for a fast neutralization of the PdH impeding the hydride readdition to reaction products and avoiding the olefin interconversion. The terminal olefins are efficiently converted into the more stable internal E isomers by using tetrabutylammonium pivalate as catalyst.